Patent Publication Number: US-2020291445-A1

Title: Cell-free protein synthesis driven metabolic engineering

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is a continuation-in-part of U.S. application Ser. No. 15/175,497, filed on Jun. 7, 2016, which is pending and claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/173,818, filed on Jun. 10, 2015, the content of which are incorporated herein by reference in their entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under D14PC00005 awarded by the Defense Advanced Research Projects Agency (DARPA) (subcontract Agreement 10/24/14//D14PC00005/0001 Covitect Inc. STTR Award ST12B-003 to Northwestern University). The government has certain rights in the invention. 
    
    
     REFERENCE TO A SEQUENCE LISTING SUBMITTED VIA EFS-WEB 
     The content of the ASCII text file of the sequence listing named “702581_01623_ST25.txt” which is 159 kb in size was created on Apr. 6, 2020 and electronically submitted via EFS-Web herewith the application is incorporated herein by reference in its entirety. 
     FIELD 
     The invention relates to cell-free systems for metabolic engineering, methods for cell-free metabolic engineering, kits for preparing the systems, and kits for performing the methods. In particular, the invention relates to systems and methods for preparing a chemical product or a natural product in cell-free conditions and to optimizing conditions for preparing a chemical product or natural product in cell-free conditions. The invention also relates to combinatorial cell-free metabolism engineering. 
     BACKGROUND 
     For decades scientists and engineers have turned to engineering biological systems for energy, medicine, materials, and more (Guo et al., 2015; Bornscheuer et al., 2012; Curran and Alper, 2012; Rollie et al., 2012). This has been an attractive, sustainable way to produce small molecules, especially when chemical synthesis is insufficient (Brown et al., 2014). The ability to harness organisms that naturally produce molecules of interest has expanded the chemical palate available (Demain, 2013; Harvey et al., 2015). Often when natural producers are not sufficient for production at the optimal quality or quantity, engineers turn to the power to manipulate biology (Kern, 2007). Our ability to introduce heterologous pathways into model microorganisms and metabolically engineer them to maximize small molecule production has led to large scale production of 1,3-propanediol, farnesene, and artemisinin with many more on their way to market (Hodgman and Jewett, 2012; Kwok, 2010). Efforts to make these molecules have resulted in success, but not without a great deal of challenges. 
     Bringing a biosynthetic molecule to market usually involves tireless efforts and countless hours of design-build-test (DBT) cycles (Kwok, 2010). The production of n-butanol is a prime example of these challenges. A series of Clostridia species are natural producers of this small molecule, and  Clostridia acetobutylicum  and  Clostridia bejinjernickii  are two of which are used in commercial butanol plants (Green, 2011). However, these species are difficult to engineer because of a biphasic metabolism, unknown regulation, and a limited number of species-specific engineering tools (Lüke-Eversloh and Bahl, 2011). Heterologous expression in model microorganisms like  E. coli  and  S. Cerevisiae  of Clostridial metabolism allows butanol production to be more easily engineered (Atsumi et al., 2008; Steen et al., 2008). Starting from a proof-of-concept, scientists have been able to increase titers dramatically by knocking out genes from genomes (Atsumi et al., 2008), increasing redox driving forces by introducing pathway independent enzymes (Shen et al., 2011), and identifying homologous enzymes with better activities (Bond-Watts et al., 2011). Years of iterative metabolic engineering led to these advances and many others in butanol biosynthesis, as is the same for most biosynthetic pathways. 
     Metabolic engineering is costly and time-consuming (Keasling, 2012). The constraints of cell membranes requiring a complete balancing of fluxes into and out of the cell makes it difficult to express biosynthetic pathways without taking into account the entire metabolic network. While there are many technologies that allow the engineer to better manipulate cells such as MFA, genome engineering, etc., the complexity of cells remains a limitation (Lee et al., 2012; Yadav et al., 2012). Furthermore, the tools we do have to regulate transcription, translation, and the genome require many DBT cycles increasing the time and effort needed to optimize the biosynthesis of interest (Boyle and Silver, 2014). Current DBT cycles are extraordinarily expensive. One estimate of the effort to develop new products has indicated current costs to be ˜10 8 -10 10  total $*years to develop (Alicia Jackson, DARPA, personal communication). There is a clear need for speeding up and decreasing the cost of metabolic engineering DBT cycles. While techniques continue to be develop to engineer cells, in vitro systems show promise in speeding up DBT cycles because they bypass many in vivo limitations by having direct access to the cellular contents (Sun et al., 2013; You and Zhang, 2013; Siegal-Gaskins et al., 2014). 
     In vitro systems for biomolecular transformations have been established showing the potential for biomanufacturing and discovery (Dudley et. al, 2015). In particular, cell-free metabolic engineering (CFME) harnessing the power of crude  E. coli  extracts to express heterologous pathways was first spearheaded for the production 2,3-butanediol (Kay and Jewett, 2016). This work led to the development of an extract mixing approach to CFME whereby lysates containing selectively overexpressed heterologous enzymes are mixed together to construct a biosynthetic pathway that can be activated by the addition of simple substrates (Dudley and Jewett, in preparation). This previous work proved the utility of the extract mixing approach to CFME by carrying out a three-step from acetyl-CoA biosynthetic pathway. This approach has many advantages including only expressing one enzyme in each strain, not needing to fine-tune expression, and the lack of the cell membrane. 
     The approaches outlined above are, however, constrained by the need to express enzymes in cells. As a result, there still remains a need for systems and methods for cell-free metabolic engineering with in vitro expression of enzymes. Further, there still remains a need for systems and method for cell-free metabolic engineering amenable to combinatorial optimization to reduce the costs associated with DBT cycles. 
     SUMMARY 
     Disclosed are cell-free systems for metabolic engineering, methods for cell-free metabolic engineering, kits for preparing the systems, and kits for performing the methods. The disclosed systems, methods, and kits may be utilized to prepare a chemical product or a natural product and to optimize conditions for preparing a chemical product or a natural product. The disclosed systems, methods, and kits also may be utilized for combinatorial cell-free metabolism engineering. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting embodiments of the presently disclosed cell-free systems, methods, and kits may be described by way of example with reference to the accompanying figures, which are schematic and may not be intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated may be represented by a single numeral. For purposes of clarity, not every component may be labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. 
         FIG. 1 . A cell-free framework for pathway prototyping demonstrated with a 17-step n-butanol model pathway. (A) Methodology for cell-free metabolic engineering (CFME) and cell-free protein synthesis driven metabolic engineering (CFPS-ME). (B) Schematic (non-stoichiometric) representation of the constructed biosynthetic n-butanol pathway. Acetyl-CoA is generated through  E. coli &#39;s natural glycolysis and funneled into the  C. acetobutylicum -derived CoA-dependent pathway to produce n-butanol. The butyryl-CoA dehydrogenase (Ter) here is from  Treponema denticola . Four NADH molecules are needed to produce one molecule of n-butanol. 
         FIG. 2 . Biosynthesis of n-butanol achieved via CFME of a coupled  E. coli  and  C. acetobutylicum  metabolic pathway. (A) Via SDS-PAGE, the gel verifies the selective overexpression of pathway enzymes in  E. coli  BL21(DE3) crude cell lysates: AtoB ( Escherichia coli ), Hbd1 ( Clostridia acetobutylicum , CA), Hbd2 ( Clostridia beijerinckii , CB), Crt1 ( Clostridium acetobutylicum , CA), Crt2 ( Pseudomonas putida , PP), Ter ( Treponema denticola , TD), AdhE1 ( Clostridium acetobutylicum , CA), and AdhE2 ( Clostridium pasteurianum , CP). (B) CFME reactions for n-butanol production from glucose were carried out using five crude lysates mixed together (1:1:1:1:1 based on total protein quantification) with glutamate salts (Mg + , NH 4   + , K + ), phosphate (K 2 HPO 4 ), buffer (Bis Tris), and cofactors (ATP, CoA, NAD + ). These lysates individually contained AtoB (EC), Hbd1 (CA), Crt1 (CA), Ter1 (TD), and AdhE1 (CA) selectively overexpressed at 37° C. Error bars represent standard deviations with n≥3 independent reactions. 
         FIG. 3 . Enzyme and physiochemical optimizations lead to increased yields of CFME n-butanol production. (A) Reactions for n-butanol production from glucose were performed using different sets of five crude lysates mixed together to obtain unique combinations of selectively overexpressed enzymes with AtoB, Hbd, Crt, Ter, and AdhE activities. Lysate mixes were combined with glutamate salts (Mg + , NH 4   + , K + ), phosphate (K 2 HPO 4 ), buffer (Bis Tris), and cofactors (ATP, CoA, NAD + ) and incubated for 24 h at 37° C. (B) To enhance yields and optimize pathway performance, a physiochemical optimization was performed with or without glutamate salts (Mg + , NH 4   + , K+), phosphate (K 2 HPO 4 ), buffer (Bis Tris), and cofactors (ATP, CoA, NAD + )) of cell-free reactions producing n-butanol. Reactions incubated for 24 h at 37° C. The grey bars represent the same recipe in (A) and in (B). All error bars represent standard deviations with n≥3 independent reactions. 
         FIG. 4 . Cell-free protein synthesis of entry enzyme activates n-butanol production in vitro by CFPS-ME approach. (A) Diagram describing the CFPS-ME experimental design. (B) Cell-free protein synthesis titers of Hbd2 from pJL1-hbd2 in a crude lysate mixture containing AtoB (EC), Crt1 (CA), Ter1 (TD), and AdhE1 (CA) overexpressed as determined by radioactive  14 C-leucine incorporation. CFPS reactions incubated over a 24-hr period at 30° C. (C) n-butanol production in the same mixed lysate system activated by cell-free protein synthesis of Hbd2 run at 30° C. for 3 h. Glucose was added to activate the n-butanol pathway and CFME reactions were incubated over a 24-hr period at both 30° C. and 37° C. (D) Cofactor (ATP, CoA, NAD +) optimization of downstream (ME portion of the CFPS-ME approach) cell-free reactions producing n-butanol were performed. Minus (−) signs represent no cofactor added, plus (+) signs represent mM amounts of cofactor to match conditions in CFME-alone experiments, and plus-plus (++) reactions represent double the amount of that cofactor. Reactions incubated for 24 h at 30° C. All error bars represent standard deviations with n≥3 independent reactions. 
         FIG. 5 . Using cell-free protein synthesis to activate metabolism from any node in the biosynthetic pathway. (a) Cell-free protein synthesis titers of AtoB (EC), Hbd2 (CB), Crt1 (CA), Ter1 (CA), and AdhE (CA) off pJL1 constructs in separate reaction mixtures as determined by radioactive  14 C-leucine incorporation. Each reaction mixture contained crude lysates with all pathway enzymes except the one made by CFPS. CFPS reactions were incubated for 3 h at 30° C. (b) n-butanol production in the same mixed lysate system activated by CFPS of each enzyme run at 30° C. for 3 h. Glucose, CoA, and NAD were added to activate the n-butanol pathway and CFME reactions were incubated for 24 h at 30° C. (c) n-butanol production activated by CFPS of enzymes in combinations: (1) AtoB (EC); (2) AtoB (EC) and Hbd2 (CB); (3) AtoB (EC), Hbd2 (CB), and Crt1 (CA); (4) AtoB (EC), Hbd2 (CB), Crt1 (CA), and Ter1 (TD); and (5) AtoB (EC), Hbd2 (CB), Crt1 (CA), Ter (TD), and AdhE1 (CA). The CFPS reactions were run at 30° C. for 3 hrs. Glucose, CoA, and NAD +  were added to activate the in-butanol pathway and reactions were incubated for 24 h at 30° C. (d) A plasmid ratio optimization of pJL1-adhE1 vs. all other pJL1 constructs along with a test of three concentrations of T7 polymerase. For each, CFPS was run at 30° C. for 3 h. Glucose, CoA, and NAD +  were added to activate the n-butanol pathway and reactions were incubated for 24 h at 30° C. All error bars represent standard deviations with n≥3 independent reactions. 
         FIG. 6 . Using CFPS-ME to rapidly screen pathway enzymes. (A) n-butanol production activated by CFPS of unique Ter homologs and AdhE homologs from pJL1 constructs: Ter3 ( Fibrobacter succinogenes , FS). Ter4 ( Flavobacterium johnsoniae , FJ), Ter5 ( Spirochaeta bajacaliforniensis , SB), Ter6 ( Cytophaga hutchinsonii , CH), AdhE9 ( Thermosynechococcus  sp. NK55a, TN), AdhE10 ( Providencia burhodogranaricea , PB), and AdhE13 ( Serratia marcescens , SM). Ter homologs were expressed in crude lysate mixtures containing AtoB (EC), Hbd2 (CB), Crt1 (CA), and AdhE1 (CA) overexpressed, and AdhE homologs were expressed in lysates containing AtoB (EC), Hbd2 (CB), Crt1 (CA), and Ter1 (TD) overexpressed. (B) n-Butanol production activated by CFPS putative bifunctional enzymes for Hbd and Crt activity: Hbdcrt2 ( Aeropyrum camini , AC), Hbdcrt3 ( Pyrobaculum aerophilum , PA), Hbdcrt4 ( Sulfolobus islandicus , SI), and Hbdcrt6 ( Sulfolobus acidocaldarius , SA). CFPS reactions were performed from linear DNA in crude lysate mixtures containing: (1) AtoB (EC), Ter1 (TD), and AdhE1 (CA) overexpressed to test bifunctionality, (2), AtoB (EC), Crt1 (CA), Ter1 (TD), and AdhE1 (CA) overexpressed to test Hbd functionality alone, and (3) AtoB (EC), Hbd2 (CB), Ter1 (TD), and AdhE1 (CA) overexpressed to test Crt functionality alone. For each, CFPS was run at 30° C. for 3 h. Glucose, CoA, and NAD +  were added to activate the n-butanol pathway and reactions were incubated for 24 h at 30° C. All error bars represent standard deviations with n≥3 independent reactions. 
         FIG. 7 . Example Chromatograms for n-butanol quantification. Three chromatogram n-butanol peaks are overlaid with retention time on the x-axis and relative intensity units of the y-axis. Peak A represents an example cell-free reaction with no n-butanol produced from the reaction. Peak B is a cell-free reaction with ˜0.3 g/L of n-butanol produced. Peak C is a cell-free reaction with ˜1.5 g/L of n-butanol produced. 
         FIG. 8 . Adjusting extracts for relative concentrations of selectively overexpressed Hbd1 and Hbd2 does not affect overall n-butanol production by CFME. (A) BL21(DE3) extract containing no overexpressed proteins, overexpressed Hbd1 extract, and overexpressed Hbd2 extract were each separated by SDS-PAGE and stained with Coomassie blue. Using densitometry with ImageJ software, each lane was analyzed for band density to determine approximate, relative amounts of overexpressed protein. Hbd1 extract contained ˜4% Hbd1 protein, and Hbd2 extract contained ˜17% Hbd2 protein. (B) Each Hbd extract was mixed with extracts containing the other pathway enzymes (AtoB, Crt, Ter, and AdhE) and CFME reactions were run for 24 h (n=1) to make n-butanol. The cases include Hbd2 extract, Hbd1 extract, and Hbd1 extract adjusted to contain approximately the same amount of overexpressed Hbd protein as the Hbd2 extract. Total extract concentration was kept constant at 10 μg ml −1  using a BL21(DE3) extract containing no heterologously expressed proteins to adjust the Hbd extracts. The observed discrepancies in ‘Hbd1 Extract’ and ‘Hbd1 Adjusted’ are likely an artifact of sample size. 
         FIG. 9 . Inorganic glutamate salt solutions perform the best in CFME reactions. Two different sets of 5 crude lysates mixed together containing selectively overexpressed enzymes with AtoB, Hbd, Crt, Ter, and AdhE activities were used to produce n-butanol from glucose. Bhlue is the original set of extracts (containing Hbd1) and orange is the best enzyme set (containing Hbd2). Lysate mixes were combined with glutamate salts (Mg + , NH 4   + , K + ) (solid line), acetate salts (long dashed line), or chloride salts (short dashed line). To activate metabolism and start CFME reactions phosphates (K 2 HPO 4 ), buffer (Bis Tris), and cofactors (ATP, CoA, NAD + ) were added and incubated for 24 h at 37° C. All error bars represent 1 s.d. with n≥3. 
         FIG. 10 . Comparing initial NAD +  and NADH ratios show relatively little effect on n-butanol production in CFME. Reactions for n-butanol production from glucose using the best set of crude lysates mixed together containing selectively overexpressed enzymes with AtoB, Hbd, Crt, Ter, and AdhE activities (determined as best by previous experiments) were run for 24 h and incubated at 37° C. Extract mixes were combined with glutamate salts (Mg + , NH 4   + , K + ), phosphates (K 2 HPO 4 ), buffer (Bis Tris), and cofactors (ATP, CoA, NAD + , NADH) to initiate CFME reactions. In the key, plus signs (+) indicate the addition of each individual components at levels described in the methods section. The divisions in the circular graphics accompanying each bar with blue being NAD +  and black being NADH indicate ratios of NAD +  to NADH. The total NAD(H) concentration is 0.5 mM. All error bars represent 1 s.d. with n≥3. 
         FIG. 11 . SDS-PAGE and autoradiogram for CFPS of each individual enzyme show full-length protein formation. (A) SDS-PAGE was run with two replicates of each CFPS reaction performed at standard conditions listed in the methods section and for 3 h at 30° C. producing  14 C-Leu incorporated protein corresponding to each enzyme in the n-butanol pathway. Molecular weights are listed at the bottom of the lanes. (B) An autoradiogram of the same gel showing that each enzyme is expressed in vitro as full-length proteins. 
         FIG. 12 . Protein SDS-PAGE gel and autoradiogram for CFPS of multiple enzymes produced full length product of each protein. SDS-PAGE was run with CFPS reactions containing one, two, three, four, and five DNA plasmids, building up the pathway. Reactions were each run at standard conditions listed in the methods section and for 3 h at 30° C. producing  14 C-Leu incorporated protein corresponding to each enzyme in the n-butanol pathway. 
         FIG. 13 . Quantification of individual enzymes in multiple protein CFPS reactions from autoradiogram indicates a lower yield of downstream pathway enzymes. (A) BL21(DE3) extract containing overexpressed proteins indicated in the key, different DNA plasmid compositions, and standard CFPS reagents identified in the methods section were incubated at 30° C. for 3 h. Each sample was separated by SDS-PAGE and stained with Coomassie blue. The gel was exposed to autoradiogram showing full-length product of each enzyme resulting from the DNA combinations. (B) Using densitometry with ImageJ software, each lane from the gel in panel A was analyzed for band density to determine approximate, relative amounts of each pathway enzyme produced by CFPS. The bars represent the percent of each enzyme produced in vitro. After CFPS for 3 h at 30° C., each reaction was supplemented with glucose, NAD+, and CoA and incubated at 30° C. for an additional 24 h to measure n-butanol production from each mix. Orange circles indicate n-butanol titers with error bars representing 1 s.d. with n≥3. 
         FIG. 14 . Plasmid optimization for CFPS-ME of all pathway enzymes in vitro shows the ability to produce n-butanol. CFPS reactions were run in BL21(DE3) extract containing no overexpressed proteins. DNA plasmids encoding each heterologous enzyme were added in equal ratios with pJL1-adhE1 modulated as per the divisions in the circular graphics in the figure key. In the key, black represents equal amounts of plasmids encoding AtoB, Hbd, Crt, and Ter, and red represents the amount of pJL1-adhE1. CFPS reagents were added and incubated at 30° C. for 3 h. Glucose, NAD+, and CoA were added, and samples were further incubated at 30° C. for 24 h. n-Butanol production is measured and differs with varied concentrations of added T7 polymerase. All error bars represent 1 s.d. with n≥3. 
         FIG. 15 . Using CFPS-ME to rapidly screen pathway enzymes from linear DNA templates. Linear DNA of each enzyme variants were created with the regulatory elements from pJL1 and with a randomized −0.20 bp on each end and used as DNA template in each CFPS reaction. CFPS of each enzyme variant (Ter and AdhE) was used to activate n-butanol production in crude lysate mixtures containing AtoB (EC), Hbd2 (CB), Crt1 (CA), and AdhE1 (CA) overexpressed or AtoB (EC), Hbd2 (CB), Crt1 (CA), and Ter1 (TD) overexpressed, respectively. All error bars represent 1 s.d. with n≥3. 
         FIG. 16 . Expression of Ter homologs in cells versus cell-free. SDS-PAGE was run on samples expressing the Ter homologs screened in  FIG. 6 . The first lane is the SeeBlue Plus2 Pre-stained standard. The second lane is a no plasmid (negative) control in BL21(DE3). For each homolog, the WC lane is a whole cell sample taken 4 hours after induction, and the CF lane is a cell-free protein synthesis sample taken after 3 hours of expression. (A) Coomassie-stained gel. While expression of Ter5 in the WC lane in an un-optimized (generic RBS) single attempt is unobservable, the other Ter enzymes are expressed, which confirms that our approach holds promise for identifying good enzymes that can be expressed in cells. (B) An autoradiogram of the same gel showing that each enzyme is expressed in vitro. As the in vivo expression did not incorporate radioactive leucine, bands are not expected in the WC lanes in panel B. 
         FIG. 17 . Comparison of n-butanol production in genomically unmodified hosts. Final titers (g/L) of n-butanol are reported from recent studies using genomically unmodified host strains. The Krustakorn et al. 2013 study is the only other in vitro n-butanol production system (uses purified enzymes). The highest titer from this study is reported in orange. 
         FIG. 18 . Scale-up of CFPS-ME reactions. CFPS reactions were run in BL21(DE3) extract containing no overexpressed proteins in 1.5 mL Eppendorf tubes. DNA plasmids encoding each heterologous enzyme were added in equal ratios with pJL1-adhE1 representing 70% of the total DNA. CFPS reagents were added and incubated at 30° C. for 3 h. Glucose, NAD+, and CoA were added, and samples were further incubated at 30° C. for 24 h. The total reaction volumes are listed on the x-axis with the data normalized to 25 μL reactions. Increasing the reaction volume improved overall butanol yields. All error bars represent 1 s.d. with n≥3. 
         FIG. 19 . Overview of a system for cell-free production of natural products via nonribosomal peptide biosynthesis by using cell-free protein synthesis driven metabolic engineering. From left to right, exogenous DNA is used as the input information for the production of biosynthetic enzymes. In the center, nonribosomal peptide synthetase proteins function in concert to select substrates and catalyze the formation of peptide bonds, ultimately resulting in the production of a 2,5-diketopiperazine. Right panel, detection of a D-Phe-L-Pro diketopiperazine (DKP) natural product by LC-MS as the result of in situ production of biosynthesis proteins. 
         FIG. 20 . Experiments showing that GrsA and GrsB1 are present in their active (holo) forms. Panel A shows the fluorescent labeling of GrsA and GrsB1 on the thiolation domain active sites with a conjugated BODIPY-CoA fluorophore. Panel B top shows the MS 2  spectrum resulting from the fragmentation of a precursor peptide containing the GrsA phosphopantetheine modification (SEQ ID NO:3). Panel B bottom shows the MS 2  spectrum for the corresponding GrsB1 T-domain peptide (SEQ ID NO:4), indicating the mass of the observed pantetheine-derived ion. 
         FIG. 21 . Detection of D-Phe-L-Pro DKP by LC-MS/MS and comparison to synthetically prepared DKP. A) Retention time comparison of D-L, L-L and CFPS-produced DKP to determine the stereochemistry of the DKP produced by CFPS. This panel also shows that SFr is required for DKP production. B) Time dependent increase in m/z 245.128 signal after Sfp is added to the CFPS reaction. Data points are the average of two technical replicates. C) Comparison of the fragmentation pattern of the CFPS-produced (top) and synthetically prepared DKP (bottom). The spectrum at the bottom of panel C is annotated with predicted fragment ion structures. 
         FIG. 22 . Reconstitution of valinomycin biosynthetic pathway in vitro. (A) Valinomycin synthetase contains two distinct NRPS enzymes Vlm1 (374 kDa) and Vlm2 (284 kDa) that are composed of adenylation (A), ketoreductase (KR), thiolation (T), condensation (C), epimerase (E), and thioesterase (TE) domains. A domains select and activate three substrates (pyruvate, α-ketoisovalerate (α-Kiv), and L-valine (L-Val)). T domains are responsible for the translocation of the bound aminoacyl or peptidyl intermediate between adjacent catalytic positions. C domains catalyze the formation of peptide bond and elongate the peptide chain. The KR domain in Module 1 reduces α-Kiv to D-hydroxyisovalerate (D-Hiv). The E domain in Module 2 converts L-Val to D-valine (D-Val). The KR domain in Module 3 reduces pyruvate to L-lactate (L-Lac). The four modules of valinomycin synthetase are iteratively reused to assemble three tetradepsipeptide monomers, which are eventually oligomerized and macrolactonized to form the 36-membered cyclododecadepsipeptide valinomycin. (B) Regeneration of the functionality of T domains in NRPS catalyzed by type II thioesterase (TEII). (C) Cell-free biosynthesis of valinomycin with different strategies: cell-free protein synthesis (CFPS), cell-free metabolic engineering (CFME), and a coupled CFPS-MS approach. CE, cell extract. 
         FIG. 23 . In vitro biosynthesis of valinomycin using CFPS system. (A) SDS-PAGE (7.5% gel) analysis of Vlm1 and Vlm2 expressed individually or together. T, total protein. S, soluble protein. M, protein molecular weight marker. NC, negative control without plasmid in the reaction. (B) SDS-PAGE (4-12% gel) analysis of coexpressed proteins. 1, coexpression of Vlm1, Vlm2, and Sfp. 2, coexpression of Vlm1, Vlm2, Sfp, and TEII. (C) LC-MS analysis of valinomycin produced by the CFPS reaction. Insert, MS spectrum of valinomycin peak. EIC, extracted ion chromatogram. a, negative control without plasmid in the reaction. b, valinomycin standard. c, CFPS reaction sample. (D) Valinomycin yields from different CFPS reaction conditions. (1) valinomyicn synthesis by coexpression of Vlm1, Vlm2, and Sfp. Valinomyicn synthesis by coexpression of Vlm1, Vlm2, Sfp, and TEII for a total time of (2) 20 h reaction and (3) 24 h reaction, respectively. Values show means with error bars representing standard deviations (S.D.) of at least three independent experiments. 
         FIG. 24 . In vitro biosynthesis of valinomycin using CFME system. (A) SDS-PAGE (7.5% gel) analysis of Vlm1 and Vlm2 overexpressed individually in  E. coli  BAP1. M, protein molecular weight marker. (B) CFME reactions for valinomycin production by mixing two cell lysates. Error bar represents standard deviation (S.D.) of at least three independent experiments. 
         FIG. 25 . Optimization of CFME reactions for enhanced valinomycin production. (A) Effect of supplementary cofactors (CoA, NAD, and ATP) on valinomycin formation. −, no cofactor added, +, cofactors were supplemented with 0.1 mM. ++, cofactors were supplemented with 1 mM. (B) Effect of the mass ratio of two cell extracts (CE) on valinomycin production. Values show means with error bars representing standard deviations (S.D.) of at least three independent experiments. 
         FIG. 26 . Boosting valinomycin yields with a coupled CFPS-ME system. (A) Enhancement of valinomycin yields by CFPS of TEII in the upstream of CFME reactions. (B) Investigation of the glucose concentration on valinomycin production. Values show means with error bars representing standard deviations (S.D.) of at least three independent experiments. 
         FIG. 27 . Cell-free framework for pathway expression. Enzymes in a pathway are overexpressed lysates or made in CFPS to construct cell-free “units” that can then be used to recreate the pathway or combinatorially diversify it. 
         FIG. 28 . Andrimid synthesis. a) Andrimid is synthesized via a hybrid 21 gene NRPS/PKS pathway. b) We separately expressed each enzyme in the cluster in CFPS reactions, quantified by radiolabeled leucine incorporation. Upon mixing all 21 enriched lysates together and addition of substrates and cofactors (i.e., malonyl-CoA), we could observe andrimid biosynthesis (c). 
         FIG. 29 . Characterization of Enterobactin biosynthetic pathway. (A) Schematic representation of enterobactin biosynthetic pathway. (B) CFPS analysis of total and soluble yields of each enzyme in the enterobactin biosynthetic gene cluster. (C) CFPS reactions including EntE, EntB, and EntF were mixed with DHB, HEPES buffer, Mg 2+ , NH 4   + , and K +  salts (Positive). Salts and HEPES were also individually left out. A blank was included containing only  E. coli  extract. After 20 h reactions, samples were analyzed for DHB and enterobactin by LC-MS. (D) MS2 spectrum for a CFPS reaction including EntE, EntB, EntF, DHB, HEPES buffer, Mg 2+ , NH 4   + , and K +  salts (Positive) and a commercial standard are shown. 
         FIG. 30 . Enzymatic conversion of L-phenylalanine to styrene. (A) A two-step conversion is catalyzed by phenylalanine ammonia lyase (PAL2) and ferulic acid decarboxylase (FDC1) to produce styrene with ammonia and carbon dioxide as byproducts. (B) The enzymes were synthesized with  E. coli  cell extract and mixed with phenylalanine in quantifiable proportions. 
         FIG. 31 . Expression and activity of the styrene biosynthetic pathway. Expression of PAL2 (A) and FDC1 (B) was assessed by radioactive leucine incorporation, and soluble enzyme fractions were used for subsequent quantification of enzyme concentrations. (C) Combining 0.5 μM PAL2 and FDC1 enabled styrene production, and the maximum titer of 5.36 mM was produced after 48 hours at 30° C. Error bars represent standard deviation (A-B) or standard error (C) of 3 technical replicates. 
         FIG. 32 . Modulating enzyme concentrations enhances styrene biosynthesis. (A) Increasing the concentration of each enzyme increased the final titer, as expected. (B) All conditions producing more than 16 mM styrene from 25 mM L-Phe were run with 50 mM L-Phe to identify the optimal enzyme ratio—0.25 μM PAL2 with 1 μM FDC1. Error bars represent standard deviation of 3 technical replicates. 
         FIG. 33 . Styrene biosynthesis is highly pH dependent. Cell-free reactions containing 0.25 μM PAL2 and 1 μM FDC1 were set with several initial pH values and produced a large range of styrene titers. (A) Endpoint titers after 72 hours indicate a clear optimum for reactions starting at neutral pH. (B) The rate of styrene synthesis diminished as the pH deviated from neutral. (C) pH changed little over the course of the reactions. Error bars represent standard deviation of 3 technical replicates. 
         FIG. 34 . Styrene standard curve for GC-MS analysis. Dilutions of styrene dissolved in dodecane were mixed with mock cell-free reactions containing GFP in place of biosynthetic enzymes to emulate the composition of experimental samples. In standards and samples, styrene counts were normalized to the trans-caryophyllene internal standard in the ethyl acetate loading solvent. Error bars represent standard deviation of 3 technical replicates. 
         FIG. 35 . Verification of enzyme activity. (A) HPLC analysis indicated PAL2 activity in vitro is independent of cofactors, which included 1 mM ATP, NAD, and Coenzyme A. (B) FDC1 is active in the presence of PAL2, but styrene quantification in later experiments was performed using GC-MS for greater accuracy. Error bars represent standard deviation of 2 technical replicates. 
         FIG. 36 . Styrene biosynthesis requires exogenous enzymes. (A) Reactions without PAL2 and FDC1 or additional phenylalanine do not produce styrene. (B) The pH of these reactions changes little over time, as expected without active glycolysis. Error bars represent standard deviation of 3 technical replicates. 
         FIG. 37 . Capturing styrene with dodecane. (A) Due to the insolubility of styrene in aqueous solutions, a dodecane layer above the reactions was required to trap the product. Altering the reaction and overlay volumes impacted the amount of styrene captured relative to the internal standard, caryophyllene. (B) For 30 L reactions, larger overlays did not inhibit styrene synthesis and captured a greater proportion of styrene per microliter sampled. These reactions contained 0.5 μM PAL2 and FDC1. Error bars represent standard deviation of 3 technical replicates. 
         FIG. 38 . Assessing the addition of blank CFPS reactions. Reactions in  FIG. 33  contained various volumes of CFPS reactions without plasmid so that all reactions contained the same total volume of CFPS regardless of enzyme concentration. For reactions with 0.5 μM enzymes and 25 mM L-phenylalanine, additional CFPS volume increased final styrene titer, which accounts for the difference in titers achieved in  FIG. 32  (with 14% CFPS) and  FIG. 33  (with 28% CFPS). Error bars represent standard deviation of 3 technical replicates. 
         FIG. 39 . Provides a table showing the gene sequences for biosynthetic enzymes PAL2 (SEQ ID NO: 1) and FDC1 (SEQ ID NO:2). 
         FIG. 40 . Overview of a system for cell-free production of natural products via nonribosomal peptide biosynthesis. From left to right, exogenous DNA is used as the input information for the production of biosynthetic enzymes. In the center, nonribosomal peptide synthetase proteins function in concert to select substrates and catalyze the formation of peptide bonds, ultimately resulting in the production of a 2,5-diketopiperazine. Right panel, detection of a D-Phe-L-Pro diketopiperazine natural product by LC-MS as the result of in situ production of biosynthesis proteins. 
         FIG. 41 . Panel A (Top) summarizes the biosynthesis of gramicidin S as it occurs in  Brevibacillus brevis . By turning through the assembly line pathway two times, the complete cyclodecapeptide is produced. Panel B shows how the first two modules interact to form D-Phe-L-Pro DKP. 
         FIG. 42 . Experiments showing that GrsA and GrsB1 are present in their active (holo) forms. Panel A shows the fluorescent labeling of GrsA and GrsB1 on the thiolation domain active sites with a conjugated BODIPY-CoA fluorophore (See  FIG. 46  for complete gel image). Panel B top shows the MS 2  spectrum resulting from the fragmentation of a precursor peptide containing the GrsA phosphopantetheine modification (SEQ ID NO:3). Panel B bottom shows the MS 2  spectrum for the corresponding GrsB1 T-domain peptide (SEQ ID NO:4), indicating the mass of the observed pantetheine-derived ion. 
         FIG. 43 . Detection of D-Phe-L-Pro DKP by LC-MS/MS and comparison to synthetically prepared DKP. A) Retention time comparison of D-L, L-L and CFPS-produced DKP to determine the stereochemistry of the DKP produced by CFPS. This panel also shows that SFP is required for DKP production. B) Time dependent increase in m/z 245.128 signal after Sfp is added to the CFPS reaction. Data points are the average of two technical replicates. C) Comparison of the fragmentation pattern of the CFPS-produced (top) and synthetically prepared DKP (bottom). The spectrum at the bottom of panel C is annotated with predicted fragment ion structures. 
         FIG. 44 . Production of NRPS proteins GrsA and GrsB1 with lysates prepared from different strains of  E. coli . Y-axis shows the protein yields determined by incorporation of  14 C-leucine and scintillation counting. 
         FIG. 45 . Autoradiography image showing the production of full-length GrsA and GrsB1 after incorporation of  35 S-methionine. Panel A shows the coommassie stained gel with the expected GrsA and GrsB1 bands marked with a red triangle. Panel B shows the phosphor image of the same gel with GrsA and GrsB1 once again marked with the red triangle. The numbers indicate the following protein/ E. coli  lysate pairs: 1) GrsA, #705; 2) GrsA, BL21(DE3); 3) GrsA, BL21(DE3)star; 4) GrsB1, #705; 5) GrsB1, BL21(DE3); 6) GrsB1, BL21(DE3) star. 
         FIG. 46 . Labeling of NRPS proteins GrsA and GrsB1 with conjugated BODIPY-CoA. Panel A shows the coommassie stained gel, and panel B shows the fluorescence image acquired at 520 nm. The numbers above the lanes on both images indicate the following conditions: 1) no plasmid, no Bodipy-CoA, no Sfp; 2) no plasmid, 1 μL Bodipy-CoA, 1 μL Sfp; 3) GrsA, after 17 h added 1 μL Bodipy-CoA, 1 μL Sfp, at 30° C. for another 3 h; 4) GrsA, after 17 h, centrifuge at 12000 g, 10 min, 4° C., take out 13 μL supernatant, then add 1 μL Bodipy-CoA, 1 μL Sfp, at 30° C. for another 3 h; 5) GrsA, after 17 h, centrifuge at 12000 g, 10 min, 4° C., remove 13 μL supernatant, then add 1 μL Bodipy-CoA, 1 μL Sfp, at 37° C. for another 3 h; 6-8. GrsB1, the conditions are the same as 3-5; 9—Sfp; 10—Bodipy-CoA. 
         FIG. 47 . Is a schematic showing the preparation of synthetic D/L-L DKP. 
         FIG. 48 . Shows a graph presenting a standard curve for LC-MS-based quantitation of D-L-DKP. 
         FIG. 49 . Shows plasmid maps of grsA (A) and grsB1 (B) expression vectors. 
         FIG. 50 . A generalized approach for capturing CoA-bound metabolites. A) A peptide with an N-terminal cysteine readily reacts with the thioester of RCO-CoA, creating a stable amide bond with the acyl group. After capture, the thiol of the peptide can then be used to immobilize the analyte and peptide onto a maleimide presenting monolayer. B) 500 μM of CoA conjugates of acetyl, acetoacetyl, HMG, and succinyl were reacted with the peptide CAK(Me 3 )SA. The resulting SAMDI spectra show all analytes can be efficiently detected. 
         FIG. 51 . A cell-free metabolic pathway from glucose to isoprenoid intermediate HMG-CoA. A) Cellular overexpression of ACAT and HMGS and subsequent lysis produces enzyme-enriched lysates which can convert glucose to HMG-CoA, as well as acetate and glutamate. The pathway includes acetyl-CoA and acetoacetyl-CoA intermediates. SAMDI can capture metabolites from crude lysates. B) Cell-free reactions containing lysates, cofactors, salts, and substrate were performed in standard 384 microtiter well plates. Reactions were then quenched, and any CoA-bound products were captured by incubation with sensor peptide. Complex reaction mixtures were printed onto monolayer arrays using liquid handling robotics for isolation and detection. HMG-CoA accumulates in the reaction over time for both C) acetate salts and D) glutamate salts. The dominant carbon source used for HMG production was determined by feeding cell-free reactions  13 C-labeled glucose and  13 C-labeled acetate and monitoring isotopic incorporation into the HMG product. The concentration of cofactors ATP, NAD + , and CoA were set to 1 mM each. 
         FIG. 52 . G3P can be detected. The initial steps of glycolysis were reconstituted in vivo using purified enzymes and fed glucose. When sufficient enzymes are present, G3P is captured and detected. 
         FIG. 53 . Derivatives of non-CoA bound, G3P were captured by the sensor peptide. A) A possible mechanism for detection of G3P as N-lactyl-peptide is via pyruvaldehyde, which is known to be generated from G3P and can undergo rearrangement with thiols to form lactyl-thioesters. B) Pyruvaldehyde can also undergo pH-dependent, aldol condensation to yield a 6-carbon species that overlaps in mass with the desired HMG product. 
         FIG. 54 . Parallel measurement of Metabolites. Ac-CoA, HMG-CoA, and G3P production was profiled in 768 unique cofactor conditions. Each product was quantified by calculating percent conversion relative to the unreacted peptide sensor using the area under the curve for each species. All reactions proceeded for 2 hrs at 37° C. 
         FIG. 55 . HMG-CoA concentration and carbon source shifts in response to cofactor conditions A) To analyze HMG-CoA yield across all conditions, the dataset was normalized to an internal standard, a peptide of similar sequence to the sensor without an N-terminal Cys, present at a constant concentration across all reactions. C 13  labeled glucose was used to concurrently monitor the fraction of HMG-CoA product coming from glucose. HMG production was also visualized as 4D plots for both B) acetate and C) glutamate systems. For the three highest yielding conditions in each system, average [HMG] was determined. In these plots each point represents a specific concentration condition of cofactors with [ATP], [NAD + ], [CoA] on the x, y, z axis respectively. Color of each point represents yield with highest yield represented by red. From acetate to glutamate; the red region shifts from high CoA and high ATP, to moderate CoA and low ATP. 
         FIG. 56 . A) In SAMDI mass spectra, the entire monolayer is desorbed by the laser of the mass spectrometer and detected as a disulfide to a molecule of the tri(ethylene)glycol background. For the HMG adduct, this observed product has a M/Z of just under 1515 mass units. The natural isotopic splitting pattern can be calculated or determined experimentally, giving the relative area of each isotopic peak. B) When the cell-free reaction is provided isotopically labeled  13 C 6  glucose, the incorporation of this label is observed as a peak splitting pattern of +2, +4, and +6 mass units, relative to the unlabeled species. This is because a single 6-carbon molecule of HMG-CoA is synthesized from 3×Ac-CoA units containing 2 carbons that are either labeled or unlabeled, depending on the source of the Ac-CoA. The distribution of these possible HMG labeling states depends on the relative concentrations of unlabeled versus  13 C 2 -labeled Ac-CoA in the cell-free reaction. C) The four label states (0, +2, +4, or +6) can each be separately quantified from the convoluted spectra by accounting for the overlap from the natural isotopic splitting of the peaks in front of it. From the integrated area under the peak (AUP) of the four peaks A, B, C, and D, the AUP for each of the labeling states was calculated as shown. The deconvoluted AUP for each of the labeling states was then used to determine the fraction of  13 C label, f C13 , in the observed HMG product; this was done by weighting the AUP for each labeled state by the relative number of labeled carbons in it, then dividing by the sum of all four states. The fraction of unlabeled carbon in the HMG product, f C12  can be calculated from these values as well using a similar equation. 
         FIG. 57 . The pathway intermediate, acetoacetyl-CoA, is not observed. To explore why we do not observe the AA-CoA pathway intermediate, we incubated 1 mM of Ac-CoA or AA-CoA in cell-free lysate overexpressing the ACAT enzyme for 2 hours. The cell free reactions were given 1 mM each of CoA, NAD + , and ATP, but no glucose was provided. A) When this cell-free reaction was incubated with Ac-CoA, this acetyl species is the primary species we capture and detect. B) Conversely, when the cell-free lysate is incubated with AA-CoA, a relatively small amount of acetoacetyl species is detected and the acetyl species is by far the primary product. The ACAT enzyme is reversible and these results suggests that it strongly favors the reverse reaction. In the reactions with the full pathway, the small quantities of AA-CoA produced are likely rapidly converted to HMG-CoA, and so we do not observe any significant accumulation of AA-CoA. 
         FIG. 58 . Cell-free limonene production using enzymes sourced from a variety of organisms. (A) The metabolic pathway from glucose to limonene requires nine enzymes plus glycolysis activity present in the lysate. (B) Phylogenetic comparison of enzyme sequences to select diverse enzyme homologs for testing. AA-CoA, acetoacetyl-CoA; HMG-CoA, 3-hydroxy-3-methylglutaryl-CoA; Mev, mevalonate; Mev-P, mevalonate-5-phosphate; Mev-PP, mevalonate pyrophosphate; IPP, isopentenyl pyrophosphate; DMAPP, dimethylallyl pyrophosphate; GPP, geranyl pyrophosphate; ACAT, acetyl-CoA acetyltransferase; HMGS, hydroxymethylglutaryl-CoA synthase; HMGR, hydroxymethylglutaryl-CoA reductase; MK, mevalonate kinase; PMK, phosphomevalonate kinase; PMD, pyrophosphomevalonate decarboxylase; IDI, isopentenyl pyrophosphate isomerase; GPPS, geranyl diphosphate synthase; LS, limonene synthase. 
         FIG. 59 . Initial characterization of enzyme homologs for cell-free production of limonene. (A) Cell-free protein synthesis (CFPS) generates “CFPS-enriched reactions” where each individual reaction expresses a single enzyme homolog. Additionally, enzymes can be overexpressed in vivo and the cell subsequently lysed to generate “pre-enriched lysates”. Mixing of glucose substrate and cofactors with enzyme-enriched CFPS reactions and/or pre-enriched lysates produces limonene. (B) Cell-free protein synthesis yields for 54 different enzymes. CFPS reactions with soluble/total protein below 30% were incubated at 16° C. instead of 30° C.; the amount of soluble protein increased for all 14 proteins. (C-D) Cell-free limonene production. Using various enzyme homologs generated by CFPS, the productivity from 3-6 hours (C) and titer at 24 hours (D) of limonene were measured. The standard reaction includes 1.0 μM PmHMGR, 0.4 μM ScMK, 0.4 μM ScPMK, 0.4 μM ScPMD, 2.0 μM EcIDI, and 3.0 μM PaGPPS plus pre-enriched lysates for EcACAT, ScHMGS, and MsLS (enzyme set 1.0). Unless noted by (*, see the Table at  FIG. 65  for details), each enzyme homolog is substituted for the standard enzyme at the same concentration. LS homologs are generated via CFPS and compared at 1.0 LVM in a reaction lacking the MsLS pre-enriched lysate. (E) the TREE (Titer, Rate, and Enzyme Expression) score is an empirically generated aggregate value of enzyme solubility, initial productivity, and final titer that enables ranking of enzyme homologs. The (+) symbol represents homologs included in enzyme set 2.0. 
         FIG. 60 . Comparison of new enzymes homolog set and optimization of cofactors (A) Comparison of initial enzyme homologs (enzyme set 1.0) and an improved set 2.0 containing new homologs for HMGR, MK, PMK, and IDI. (B) Supplementation of cofactors NAD + , CoA, and ATP improves limonene productivity, titer, and TREE score ( FIG. 73 ) compared to no additional cofactors (blue box). Four reaction conditions (in blue, yellow, and green boxes) were selected as cofactor conditions for further experiments. (C) TREE scores of PaGPPS and PgGPPS (the best two GPPS homologs in  FIG. 59E ) under different enzyme concentrations. These data informed the reduction of GPPS to 1.0 μM for enzyme set 2.2. (D) Re-testing of LS homologs derived from CFPS at a higher concentration (1.8 μM) and using supplemented cofactors. 
         FIG. 61 . Iterative testing of enzyme homologs at four different cofactor conditions within multiple different enzyme sets. (A) Conceptual approach: Starting with enzyme set 2.2, active homologs of GPPS were substituted for the default GPPS (PaGPPS). Each GPPS homolog (orange) was tested under four different cofactor conditions and the resulting TREE scores averaged to rank each enzyme set. The top two sets were carried forward into the next iterative experiment in which all active homologs of IDI (yellow) were tested. The top two sets plus the best performing StGPPS sets were again carried forward and all active PMD homologs (green) were tested. Each subsequent iterative experiment used the top five conditions from the previous iteration to test active PMK (blue), PMD (purple), and HMGR (magenta) enzymes in sequence. (B) TREE scores of 408 reactions where each bar is a unique cofactor/enzyme set. (C) The TREE score of each enzyme set was averaged across all four cofactor conditions and the highest score plotted to compare each homolog as a single value. The black bar represents the starting enzyme set 2.1. (D) Production of limonene (mM) and other metabolites over the course of the reaction. The concentration of each enzyme is 0.2 μM HMGR, 0.1 μM MK, 0.2 μM PMK, 0.2 μM PMD, 0.2 μM IDI, and 1.0 μM GPPS plus pre-enriched lysates for EcACAT, ScHMGS, and MsLS; see the Table at  FIG. 65  for more detailed description of the reaction components of each enzyme set. 
         FIG. 62 . Synthesis of additional terpenoids. (A) Various terpene synthases can produce different products from the common GPP precursor. (B) Comparison of limonene and pinene production from glucose using CFPS-derived terpene synthases. Pinene synthases (PS) were encoded by sequences from  Abies grandis  (Grand fir) and  Picea abies  (Norway spruce). (C) Production of bisabolene from glucose by CFPS-enriched reactions containing farnesyl diphosphate synthase (EcFPPS encoded by ispA) from  E. coli  and bisabolene synthase (BS) from  Abies grandis . Limonene, pinene, and bisabolene synthesis reactions use enzyme set 10.0 (see the Table at  FIG. 65 ) and are supplemented with 5 mM NAD + , 1 mM CoA, and 1 mM ATP.  FIG. 75  depicts the accumulation of terpenoids over time. 
         FIG. 63 . Provides a table listing the DNA sequences and plasmids used in Example 8. 
         FIG. 64 . Provides a table showing an estimation of overexpresesd protein concentration in pre-enriched lysates. To estimate the concentration of overexpressed proteins in pre-enriched (i.e. CFME) lysates, we used densitometry to estimate the band percentage in a Coomassie stained SDS-PAGE gel using ImageLab software. These percentages are multiplied by the concentration of pre-enriched lysates and converted to a molar concentration using the molecular weight of the protein in question. 
         FIG. 65 . Provides a table detailing the enzyme and cofactor concrntaration used in each experiment of Example 8. 
         FIG. 66 . Provides a table showing metabolic engineering for the production of monoterpenes in Example 8. 
         FIG. 67 . Addition of N-terminal expression tag encoding MEKKI (SEQ ID NO: 13) improves cell-free expression of monoterpene synthases. Each limonene synthase (LS) or pinene synthase (PS) was expressed in CFPS using three different DNA plasmids. The “RBS calculator” version (SEQ ID NO:5) does not contain an N-terminal expression tag but used the Salis RBS Calculator v1.1 to optimize the 27 base pair sequence (colored gray and containing a ribosome binding site; (SEQ ID NO:8) for maximum expression. The “pJL1” version (SEQ ID NO:6) does not contain a variable RBS or N-terminal expression tag but uses the RBS encoded by the pJL1 plasmid (SEQ ID NO:9; Addgene #69496, pJL1 is derived from pY71 (Bundy and Swartz, 2010; Swartz et al., 2004)). The “pJL1-MEKKI” version (SEQ ID NO:7) places the coding sequence of the protein into pJL1 behind an N-terminal tag (SEQ ID NO:10) known to improve in vitro expression. The N-terminal tag is a 15 nucleotide, AT-rich sequence which encodes the first five amino acids (Met-Glu-Lys-Lys-Ile, MEKKI (SEQ ID NO:13)) of chloramphenicol acetyl transferase. Chloramphenicol acetyl transferase has been previously used as a reporter protein during the development of the  E. coli  CFPS system (Jewett and Swartz, 2004; Swartz et al., 2004). 
         FIG. 68 . Two variants of the N-terminal expression tag give similar expression for five different enzymes. Gene sequences were unintentionally synthesized with two different N-terminal expression tags. The first tag sequence is catATGGAGAAAAAAATC (SEQ ID NO: 12) encoding MEKKI (SEQ ID NO: 13) and the second tag sequence is catATGCATATGGAGAAAAAAATC (SEQ ID NO: 14) encoding MHMEKKI (SEQ ID NO: 15). Comparison of the two N-terminal tags (using five different example enzymes) show very similar soluble protein concentrations and both tags ultimately used (see the Table in  FIG. 63 ). 
         FIG. 69 . Protein yields for ten representative proteins remain consistent when scaling up reaction volumes from 15 μL to 50 μL. The standard 15 μL reaction is typically incubated in a 1.5 mL microcentrifuge tube. Due to the large amounts of CFPS reactions required for this manuscript, we wanted to scale up the reaction to minimize pipetting. To keep the reaction environment consistent at a larger reaction volume, 2 mL microcentrifuge tubes are used to incubate 50 μL CFPS reactions. Enzyme expression levels are similar between 15 μL and 50 μL reactions. Alternatively, we tried incubating 15 μL reactions in a PCR plate covered with foil lid (to minimize plastic/reduce pipetting) but found reduced the relative protein expression for some enzymes (PaGPPS) compared to incubation in microcentrifuge tubes. Throughout Example 8, all CFPS reactions were run at the 50 μL scale. 
         FIG. 70 . Scaling volume of CFPS “blank” reactions (i.e. CFPS without a plasmid substrate) from 50 to 500 μL changes the metabolic products. (A) Analysis of metabolites in two CFPS reactions incubated for 20 hours with no DNA template. Phosphate is quantified by kit (Malachite Green Phosphate Assay POMG-25H, BioAssay Systems) while citrate, succinate, acetate, and pyruvate are measured on HPLC using methods previously described (Dudley et al., 2016). (B/C) Testing CFPS blank reaction volume using a glucose-to-limonene CFPS-ME reaction. Limonene titer is plotted (B) and the same data is converted to a TREE score (C). Each L CFPS-ME reaction includes 11.2 μL “blank” CFPS reaction, 3.8 μL CFPS of pathway enzymes, 2.5 μL pre-enriched extracts, and 12.5 μL substrates/cofactors/water. Cofactor conditions A, B, C, D are described in the Table at  FIG. 65 . Due the metabolic differences between 50 μL and 500 μL “blank” reactions, all “blank” reactions used in Example 8 were made at the 50 μL scale for consistency. 
         FIG. 71 . Mixing of CFPS reactions with pre-enriched lysates indicates minimum enzyme concentrations needed for limonene production. In order to compare the relative activity of pathway enzymes and determine appropriate concentrations for mixing CFPS-enriched reactions, we started with a mixture of 8 pre-enriched lysates capable of producing limonene (leftmost black bar, (Dudley et al., 2019)). We immediately observed that supplementing high concentrations of CFPS reaction expressing PaGPPS increased limonene titer (leftmost gray bar) and used 3.8 μM in subsequent tests. We then removed the pre-enriched lysate encoding MK, PMK, PMD, IDI, and LS and added increasing concentrations of CFPS reaction expressing these reactions. These results informed the initial enzyme concentrations for Enzyme Set 1.0 used in  FIG. 59  to be at 1.0 μM PmHMGR, 0.4 μM ScMK, 0.4 μM ScPMK, 0.4 μM ScPMD, 2.0 EcIDI, 3.0 μM PaGPPS. Finally we show that the initial enzyme homologs, mixed as nine CFPS-enriched reactions, can support limonene production. 
         FIG. 72 . Use of pre-enriched lysates for LS, ACAT, and HMGS improves limonene production and increases available space in the reaction for other CFPS-generated enzymes. (A) Titration of limonene synthase from  Mentha spicate  (MsLS) generated by CFPS (0.5-2 μM) or pre-enriched lysate (˜3.4 μM) suggests LS is a rate-limiting step for limonene production. (B) Titration of acetyl-CoA acetyltransferase from  Escherichia coli  (EcACAT) generated by CFPS (0.5-2.5 μM) or pre-enriched lysate (˜4.6 μM) suggests 0.5 LVM ACAT is the minimum enzyme concentration sufficient to produce 2 mM of limonene. (C) Titration of 3-hydroxy-3-methylglutaryl-CoA synthase from  Saccharomyces cerevisiae  (ScHMGS) generated by CFPS (0.2-4 μM) or pre-enriched lysate (˜5.5 μM) suggests 3 μM HMGS is the minimum enzyme concentration sufficient to produce 2 mM of limonene. The standard iPROBE reaction used throughout this work dictates that only 15 μL of the 30 μL reaction is available for CFPS reactions; using pre-enriched lysates for ACAT, HMGS, LS not only maximized limonene titer (in the case of LS) but also frees up space for other pathway enzymes generated by CFPS to be included at a higher concentration in the limonene synthesis reaction. Concentrations of ACAT, HMGS, LS in pre-enriched lysates are estimated by densitometry (Table at  FIG. 64 ). 
         FIG. 73 . Supplementation of cofactors NAD + , CoA, and ATP improves limonene productivity (A), titer (B), and TREE score (C). The improved set of enzyme homologs (enzyme set 2.0) has increased productivity and titer compared to the initial set of enzyme homologs (enzyme set 1.0) in nearly all cofactor conditions tested. Four reaction conditions (A, B, C, D) were selected as cofactor conditions for further experiments. Note that plot C is a bar chart representation of the data in  FIG. 60B . 
         FIG. 74 . Reducing enzyme concentrations to enable equivalent limonene production. (A-F) Plots of limonene TREE score as a function of enzyme concentration using cofactor condition D (5 mM NAD + , 1 mM CoA, 1 mM ATP). Pathway enzymes not being adjusted are held at the same concentrations used in enzymes set 2.0. Enzyme set 2.1 uses the same enzyme homologs as set 2.0 but reduces the concentrations of BpHMGR, MmMK, SpPMK, ScPMD, and SclIDI. Black arrows indicate the concentration of each enzyme in set 2.0 while gray arrows indicate concentration of each enzyme in set 2.1. (G) After testing the effect of concentration on each enzyme individually, the concentrations of HMGR, MK, PMK, PMD, and IDI were reduced to create enzyme set 2.1 which has a similar limonene production and TREE score to enzyme set 2.0. (H) Summary of enzyme set components used during cofactor and concentration optimization. 
         FIG. 75 . Limonene production using two different GPPS enzymes at varying concentration. The default homolog for GPPS throughout the study thus far has been PaGPPS; increasing the concentration of PaGPPS maximizes limonene titer (3 μM is one of the largest concentrations possible without exceeding the 15 μL of CFPS reaction included in a 30 μL limonene synthesis reaction). However, comparison with PgGPPS indicates that other GPPS homologs do not require such high enzyme concentrations to support the same limonene titer and that using 1 μM as the benchmark comparison is appropriate for future testing. Thus, enzyme set 2.2 (the starting point in  FIG. 61 ) reduces the concentration of GPPS from 3.0 μM to 1.0 μM in order to more accurately compare the specific activity of GPPS homologs in the next round of homolog screening. 
         FIG. 76 . Testing limonene synthases under improved cofactors and decreased enzyme concentrations. Comparison of MsLS (from  Mentha spicata ) and ClLS (from  Citrus limon ) paired with two differed GPPS enzymes; MsLS is known to produce (4S)-(−)-limonene (Colby et al., 1993) while ClLS typically produces (4R)-(+)-limoenene. (Lücker et al., 2002). 
         FIG. 77 . A more stringent selection condition differentiates similar homologs. (A) The TREE scores from  FIG. 59E  are replotted; * denoted the enzyme selected as the best candidate when transitioning from enzyme set 1.0 to enzyme set 2.0. (B) The average TREE scores from  FIG. 61C  are replotted. Homologs marked with “x” were inactive in  FIG. 59  and were not retested in  FIG. 61 . The boxed homologs (AgGPPS, EcIDI, ScPMD, ScPMK, ScMK, and SaHMGR/ScHMGR) indicate some of the commonly used enzyme homologs utilize for metabolic engineering in living cells, for example the production of limonene (Alonso-Gutierrez et al., 2013; Alonso-Gutierrez et al., 2015) and pinene (Sarria et al., 2014). 
         FIG. 78 . A-B. TREE scores of selected reactions depicted in  FIG. 61D . In particular, SaMK is inhibited by higher CoA concentrations compared to other MK homologs. 
         FIG. 79 . Production of limonene, pinene, and bisabolene over 96 hours. (A) Cell-free protein synthesis of tepenoid synthases; incubation at 16° C. improves expression of bisabolene synthase from  Abies grandis . (B) Production of limonene from glucose using Enzymes Sets 2.0, 9.0, and 10.0. Incubation of reactions in a 96-well plate covered in foil rather than a 1.5 mL microcentrifuge tube further increased titer. Set 9.0 uses a pre-enriched MsLS lysate while set 10.0 generates MsLS using CFPS (3.8 μM) (C) Production of bisabolene from glucose by CFPS-enriched reactions containing farnesyl diphosphate synthase (EcFPPS encoded by ispA) from  E. coli  and bisabolene synthase (BS) from  Abies grandis . (D) Production of pinene from glucose by CFPS-enriched reactions containing pinene synthases (PS) from  Abies grandis  (Grand fir) and  Picea abies  (Norway spruce). (E-F) Pinene exists as two isomers (α or β) that can be distinguished by GC-MS due to different fragmentation patterns and retention times. For biofuel applications, there is no preference for either isomer, however, β-pinene is a more valuable commodity chemical since it is less abundant than α-pinene in turpentine (Sarria et al., 2014). β-pinene is favored in this in vitro system. (G) Farnesol accumulates in all reactions conditions but especially in the bisabolene reaction. (H) The ratio of α or β pinene produced using purified pinene synthases ( A. grandis  (Bohlmann et al., 1997) and  P. abies  (Martin et al., 2004)), this study, and production in cells (Sarria et al., 2014). Unless noted, limonene, pinene, and bisabolene synthesis reactions use enzyme set 10.0 (see Table at  FIG. 65 ) and are supplemented with 5 mM NAD + , 1 mM CoA, and 1 mM ATP. 
         FIG. 80 . Simultaneous “one pot” CFPS-ME. The initial conception for Cell-Free Protein Synthesis-driven Metabolic Engineering (CFPS-ME) was developed for a butanol biosynthesis pathway (Karim and Jewett, 2016) and involved expressing multiple pathway enzymes simultaneously from multiple plasmids in a single “one pot” CFPS reaction. We initially tried this approach but found that the equal expression of six enzymes simultaneously was not possible despite adjusting the plasmid concentration of the lowest expressing enzyme. (A) Metabolic pathway from glucose to limonene. (B) Cell-free expression of pathway enzymes measured via  14 C-leucine incorporation after 3 hours. (C) Experimental approach used in panel D depicted for the MsLS enzyme: six pre-enriched lysates containing pathway enzymes 1-8 are mixed with CFPS-reagents and plasmid encoding enzyme 9 (MsLS). After three hours, glucose is added to initiate limonene biosynthesis. (D) The approach is repeated for each pathway enzyme in turn and limonene production measured. Each subsequent reaction synthesizes one pathway enzyme (labeled on the x-axis) via CFPS using a mixture of lysates pre-enriched in the other eight pathway enzymes. The “Control” reaction contains no plasmid but all nine limonene pathway enzymes from pre-enriched lysates (Dudley et al., 2019). (E) The CFPS reaction contains plasmids encoding: MsLS alone (1), MsLS and PaGPPS (2), MsLS, PaGPPS, and EcIDI (3), MsLS, PaGPPS, EcIDI, and ScPMD (4). MsLS, PaGPPS, EcIDI, ScPMD, and ScPMK (5). Reaction (6) contains MsLS, PaGPPS, EcIDI, ScPMD, ScPMK, and ScMK at three different ratios (6A, 6B, 6C). The lysates supporting CFPS are missing the corresponding enzymes being synthesized. As in panel C-D, glucose and buffer are added after 3 hr of protein expression and limonene measured 21 hr later. (F) Autoradiogram of CFPS reactions containing  14 C-leucine after 3 hr. Lanes 1, 2, and 3 uses the same ratios of six plasmids as in panel E where lanes 4-9 are control reactions each expressing only one plasmid template at a time. In conclusion, the two-pot approach detailed in iPROBE (Karim et al., 2020) offers several advantages relative to “one-pot” CFPS-ME (Karim and Jewett, 2016) or CFME (Dudley et al. 2018). For instance, the separation of catalyst synthesis (CFPS) from utilization (metabolite production) enables quantitative control of each pathway enzyme via pipetting and avoids the need to optimize enzyme expression in vivo (Dudley et al. 2018). Method details: To produce a given pathway enzyme, plasmid encoding the enzyme is added to a 20 μL CFPS reaction that is driven by a mixture of pre-enriched lysates at total protein concentration of 10 mg/mL. The standard pre-enriched lysate mix includes 3.5 mgmL −1  of a ACAT/HMGS/HMGR combined lysate, 1 mg/mL of MK, PMK, PMD, IDI, GPPS, and 1.5 mg/mL LS. When the pathway enzyme is synthesized by CFPS, the pre-enriched lysate is replaced by “blank” BL21(DE3) lysate containing no enriched enzyme. The reaction is incubated at 30° C. in 1.5 mL microcentrifuge tubes. After three hours, 5 μL of concentrated glucose and Bis-Tris buffer (final concentration of 200 mM and 100 mM, respectively) to make a 25 μL combined reaction. The reaction is incubated for another 21 hours under a 25 μL limonene overlay and the overlay is subsequently measured for limonene production using GC-MS as previously described (Dudley et al., 2019). Selected reactions were separated on an SDS-PAGE gel, exposed by autoradiography, and imaged with a Typhoon 7000 (GE Healthcare Life Sciences, Pittsburgh, Pa.). 
         FIG. 81 . Plots of the TREE score as a function of its component parameters. 
         FIG. 82 . The sensitivity of the TREE score to average enzyme solubility. The enzyme expression component has a small impact on the final TREE score (“TR” score=titer*rate) 
         FIG. 83 . Metabolite analysis of three enzyme sets. Limonene and mevalonate are detected by GC-MS (Dudley et al., 2016; Dudley et al., 2019) while the remaining metabolites are measured by HPLC (Dudley et al., 2016). Part C is reproduced from  FIG. 61D  here for comparison. Part D is a replotting of the data in  FIG. 60A  and  FIG. 79B  for comparison. 
         FIG. 84 . Comparison of ten enzyme sets at two enzyme ratios and four cofactor concentrations. Nine enzyme sets from  FIGS. 59 and 61  are replotted and/or run with the alternate enzyme concentration. For comparison, we also tested a commonly used set of enzyme homologs (Ctr) used to produce limonene (Alonso-Gutierrez et al., 2013; Alonso-Gutierrez et al., 2015) and pinene (Sarria et al., 2014). A higher concentration of enzymes enables limonene production using EcIDI, and ScMK; limonene is not produced when using the lowered (more stringent) enzyme concentrations utilized in  FIG. 61 . Both enzyme ratios indicate that ScPMK, SpPMK, and PzPMK are relatively similar homologs. The concentrations of enzymes used in “ FIG. 59  enzyme ratio” are 1.0 μM HMGR, 0.4 μM MK, 0.4 μM PMK, 0.4 μM PMD, 2.0 μM IDI, and 3.0 μM GPPS plus pre-enriched lysates for EcACAT, ScHMGS, and MsLS. The concentrations of enzymes used in “ FIG. 4  enzyme ratio” are 0.2 μM HMGR, 0.1 μM MK, 0.2 μM PMK, 0.2 μM PMD, 0.2 μM IDI, and 3.0 μM GPPS plus pre-enriched lysates for EcACAT, ScHMGS, and MsLS. Homolog source organism: Sc,  Saccharomyces cerevisiae ; Pm,  Pseudomonas mevalonii ; Bp,  Bordetella petrii ; Mm,  Methanosarcina mazei ; Sp,  Streptococcus pneumoniae ; Pz,  Paracoccus zeaxanthinifaciens ; Ec,  Escherichia coli ; Scl,  Streptomyces clavuligerus ; Sl,  Solanum lycopersicym  (tomato); Pa,  Picea abies ; Ag,  Abies grandis ; Pg,  Picea glauca . Although this figure is not referenced in the context of Example 8, it is provided as a benchmark for the in vivo experiments. 
         FIG. 85 . Gene sequences used in Example 8 (SEQ ID NOs:16-72). XbaI, ribosome binding site (RBS) from pJL1, NdeI (containing ATG start codon), SalI, stop codon, the N-terminal expression tag (MEKKI (SEQ ID NO: 13) or MHMEKKI (SEQ ID NO: 15)) sequence is underlined. 
         FIG. 86 . A two-pot cell-free framework for in vitro prototyping and rapid optimization of biosynthetic enzymes (iPROBE). A schematic overview of the iPROBE approach following a DBT and apply framework is depicted. In the design phase, reaction schemes and enzyme homologs are selected. In the build phase, lysates are enriched with pathway enzymes via overexpression prior to lysis 35  or by cell-free protein synthesis post lysis. Then, lysates are mixed to assemble enzymatic pathway combinations of interest. In the test phase, metabolites are quantified over time and data is reduced into a single quantitative metric for pathway combination ranking and selection. In the apply phase, cell-free pathway combinations are selected and implemented in cellular hosts. 
         FIG. 87 . Individual pathway enzymes can be tuned in pathway context and ranked using TREE scores with iPROBE. (A) The pathway to produce 3-HB from native metabolism (acetyl-CoA) is selected requiring two enzymes not natively present, CacThl and CklHbd. (B) Five pathway combinations are designed to be built and tested varying the concentration of CacThl low to high while maintaining CklHbd at one concentration. (C) The five pathway designs are built by enriching two  E. coli  lysates with CacThl and CklHbd, respectively, by CFPS (Pot #1). Then, the five pathway combinations are assembled by mixing CFPS reactions containing CacThl, CklHbd, and no enzyme (blank) with fresh  E. coli  lysate. Kanamycin, to stop further protein synthesis, glucose and cofactors are added to start biosynthesis of 3-HB. (D) 3-HB is measured at 0, 3, 4, 5, 6, and 24 h after the addition of glucose for each of the five pathway combinations. Error bars are shown at 24 h and represent technical triplicates. From these measurements, 3-HB titer at 24 h (E) and rate of production through 6 h (F) is quantified. Error bars shown for titer represent technical triplicates. Error bars shown for rate represent the standard error of the linear regression. Enzyme expression is quantified by adding the average soluble fraction of each enzyme (G) to the inverse of the total concentration of exogenous enzyme present (H). Error bars shown for enzyme concentrations represent technical triplicates. (I) The TREE is score is then calculated by multiplying the titer by the rate by the enzyme expression for each pathway combination with error bars representing the propagated error. 
         FIG. 88 . Enzymatic pathways can be screened with iPROBE to inform  Clostridium  expression for optimizing 3-hydroxybutyrate production. A reaction scheme for the production of 3-HB is presented in panel (a). Six homologs have been selected for each reaction step. The design in (b) includes the testing of six Thl homologs and six Hbd homologs at 0.5 μM each. We built each possible combination in cell-free systems (c) constituting 36 unique pathway combinations. We rapidly built these cell-free pathways by expressing each of the 12 enzyme variants in lysates by CFPS. We then mixed each to try all 36 possible combinations keeping enzyme concentration fixed. (d) 3-HB was measured, and TREE scores were calculated and plotted for each iPROBE pathway combination with propagated error. We then selected four pathway combinations to test in  C. autoethanogenum  (A, B, C, and D). These pathways were built in high copy plasmids with the highest strength promoters in single operons (e).  Clostridium  strains containing these pathway combinations were then fermented on gas and 3-HB was measured over the course of fermentation (f). Labels (A, B, C, and D) are used to signify corresponding cell-free combinations and TREE scores are listed. Error bars represent technical triplicates. 
         FIG. 89 . Cell-free pathway testing combined with data-driven design-of-experiments quickly screens 205 unique pathway combinations and selects pathways for cellular butanol production. A reaction scheme for the production of butanol is presented in panel (a). Six homologs have been selected for each of the first four reaction steps and are shown in panel (a). (b) The strategy for running an initial set of reactions is to test each homolog at 5 concentrations individually with the base case set of enzymes (blue). These 120 pathway combinations are run in cell-free reactions according to the two-pot iPROBE methodology and TREE scores are calculated from 24-h butanol time-courses. The dashed line is placed at the TREE score resulting from the base case set of enzymes. (c) Neural-network-based design of experiments is implemented using the data presented in (b) to predict enzyme sets and concentrations to be constructed. (d) These are then built in cell-free systems. TREE scores are calculated for all newly tested pathway combinations from 24-h butanol time-courses. The black dashed line is placed at the TREE score resulting from the base case set of enzymes, the grey dashed line corresponds to the TREE score of the best case from (b), and the orange dashed line represents the highest TREE score achieved through the data-driven iPROBE approach. (e) Nine pathway combinations (two high-performing pathways, two medium-performing pathways, and five low-performing pathways) were constructed and transformed in vivo in  C. autoethanogenum . End-point titers of butanol production (relative to the highest in vivo titer) are plotted against in vitro TREE scores for the corresponding pathway combination. Error bars for in vivo titers are relative standard deviations for four to six replicates. Error bars for all TREE scores are propagated error based on TREE score calculations. 
         FIG. 90 .  Clostridium  fermentations show improved production of 3-HB and identification of a new route to 1,3-butanediol. (A) The iPROBE-selected optimal pathway, CacThl and CklHbd1, for 3-HB production is built in a  C. autoethanogenum  strain and run in a 14-day continuous fermentation on CO/H2/CO2 gas as sole energy and carbon source. Fermentation parameter optimizations were performed for the new strain. Comparing fermentation of the control strain of  C. autoethanogenum  (white circles) to the initial (light grey circles) and optimized (dark grey circles) fermentations of the iPROBE-selected pathway expression strain, (B) 3-HB and (C) biomass were monitored. (D) 1,3-BDO was measured during steady-state fermentation and averages are shown for the control strain (−), the iPROBE-selected pathway expression strain (light green hexagon), and strain containing the PhaB homolog for Hbd (dark green hexagon). 
         FIG. 91 . A two-pot cell-free protein synthesis driven metabolic engineering (CFPS-ME) approach enables modular pathway construction in vitro. A schematic representation of the iPROBE approach (Two-Pot CFPS-ME) relative to previous works in crude lysate-based cell-free prototyping (One-Pot CFPS-ME) is shown. Pathways are assembled in two steps (i.e., 2 pots), where the first step is enzyme synthesis via cell-free protein synthesis (CFPS) and the second step is enzyme utilization via substrate and cofactor addition to activate small molecule synthesis. 
         FIG. 92 . Hbd can be tuned in pathway context and assessed using TREE scores with iPROBE. Five pathway combinations are designed, built, and tested varying the concentration of CklHbd low to high while maintaining CacThl at one concentration. (A) 3-hydroxybutyrate is measured at 0, 3, 4, 5, 6, and 24 h after the addition of glucose for each of the five pathway combinations. Error bars are shown at 24 h and represent technical triplicates. (B) The TREE is score is then calculated for each pathway combination. 
         FIG. 93 . Cell-free 3-HB titers, rates, and enzyme expression. Six homologs have been selected for each reaction step (Thl and Hbd) for 3-HB production and expressed by cell-free protein synthesis for all of the data represented in this study. Total and soluble yields of protein are displayed in panel (a). Error bars represent technical triplicates. iPROBE was run in 54 combinations listed in Supplemental File A found at Karim, A. S., et al, In vitro prototyping and rapid optimization of biosynthetic enzymes for cellular design. In revision. Nature Chemical Biology. Bio Riv 685768; doi; https://doi.org/10.110/685768 [doi.org], which reference and all supplemental data and files are incorporated herein by reference in their entireties. Titers at 24 h are shown in panel (b) with error bars representing technical triplicates. Rates determined by linear regression between 3 h and 6 h measurements are shown in panel (c) with error bars representing the standard error of the linear regression. The TREE score is plotted against (d) titer, (e) rate, and (f) enzyme expression. Error bars represent error calculation associated with the TREE score and error from titers, rates, and enzyme expression taken from (a), (b), and (c). The R 2  value associated with a linear regression and the Pearson correlation coefficient is represented on each plot. 
         FIG. 94 . Enzyme concentrations can be tuned with iPROBE to inform genetic design for  Clostridium  expression of 3-hydroxybutyrate. A reaction scheme for the production of 3-HB is presented in panel (a). The design in (b) includes the co-titration of CacThl and CklHbd at seven concentrations (0, 0.02, 0.05, 0.1, 0.25, 0.5, and 1 μM). We built these seven designs in cell-free systems (c) by CFPS of each enzyme in separate lysates (Pot #1) followed by mixing to assemble full pathways for 3-HB production (Pot #2). We measured 3-HB over the course of 24 h for each. We compared these results to  Clostridium -based expression by building eight genetic constructs with varying promoters and plasmid copy number (d). We measured final titer of 3-HB for each. Error bars represent technical triplicates. Error bars on enzyme concentrations are technical replicates with n&gt;3. 
         FIG. 95 . Cell-free butanol titers, rates, and enzyme expression. Six homologs have been selected for each reaction step (Thl, Hbd, Crt, and Ter) for n-butanol production and expressed by cell-free protein synthesis. Total and soluble yields of protein are displayed in panel (a) with error bars representing technical triplicates. iPROBE was run in 205 combinations listed in Supplemental File A of Karim et al., as referenced in  FIG. 93 . Titers at 24 h are shown in panel (b) with error bars representing technical triplicates. Rates determined by linear regression between 3 h and 6 h measurements are shown in panel (c) with error bars representing the standard error of the linear regression. 
         FIG. 96 . Cell-free experimental TREE scores for expert-selected and Neural Network-based design of experiments. TREE scores were calculated for pathway combinations experimentally tested with iPROBE for (a) pathway combinations selected from the neural network approach (43 combinations), (b) simultaneous changes in each enzyme&#39;s concentration using the base case set of enzymes (21 combinations), and (c) expert-selected pathway combinations based on data in  FIG. 90C  and understanding of biosynthesis (18 combinations). TREE scores were calculated based on 24 h time-course data of n-butanol production. 
         FIG. 97 . Analysis of iPROBE reaction compositions show enzyme homolog and concentration trends for butanol production. Each instance of each enzyme homolog of Thl, Hbd, Crt, and Ter tested in the 205 pathway combinations was counted. (A) The percentage of each homolog (six homologs total are represented for each enzymatic step) appearing in the ‘initial’ 120 combinations and in the ‘top’ 20 combinations (of all 205) is charted. (B) The concentrations used for each individual pathway enzyme (regardless of which homolog is used) in the assembly of all 205 combinations is plotted (A; grey) next to the concentrations used in assembling the top 20 combinations as ranked by TREE score (T; yellow for Thl, green for Hbd, blue for Crt, and purple for Ter). The median concentration for each enzyme and group is potted as a single black line. 
         FIG. 98 . Cellular butanol expression in  C. autoethanogenum  is significantly enhanced when using Bcd:EtfAB versus TdeTer. (a) Nine pathway combinations (two high-performing pathways, two medium-performing pathways, and five low-performing pathways) were constructed and transformed in vivo in  C. autoethanogenum . Two plasmids were constructed for each combinations Time-course measurements of butanol production were taken across six days and are plotted. Error bars for in vivo titers are standard deviations for four to six replicates. (b) Four pathway combinations were constructed for n-butanol production containing CacThl, CacHbd (left; traditional Hbd) or CklHbd (right; iPROBE-selected Hbd), CacCrt, and either TdeTer (purple square) or Bcd:EtfAB (purple ‘x’ shape) were built using the same plasmid architectures as the orange circle in (a). (c) Two pathways for n-butanol production were tested: one pathway contained CacThl, CacHbd, CacCrt, and TdeTer (purple square) built using the same plasmid architecture as the orange circle in (a), and one pathway contained CacThl, CacHbd, CacCrt, and Bcd:EtfAB (purple ‘x’ shape).  C. autoethanogenum  strains containing these two pathway combinations were then fermented on gas and n-butanol was measured at 4 days. Error bars represent one standard deviation of three biological replicates. 
         FIG. 99 . A positive correlation between cell-free pathway performance and cellular pathway expression. All 19 pathway combinations assembled via iPROBE also expressed in  C. autoethanogenum  are plotted. TREE scores are plotted on the x-axis and in vivo final titers are plotted in the y-axis. Cellular titers were normalized to the highest-producing strains of 3-HB and butanol. Error bars on TREE scores represent the corresponding propagated error. Error bars on in vivo titers are normalized standard deviations from biological triplicates. The Pearson correlation coefficient was calculated for all data points. 
         FIG. 100 .  Clostridium  fermentations show acetate production. The iPROBE-selected optimal pathway, CacThl and CklHbd1, for 3-HB production is built in a  C. autoethanogenum  strain and run in a 14-day continuous fermentation on CO/H 2 /CO 2  gas as sole energy and carbon source. Fermentation parameter optimizations were performed for the new strain. Comparing fermentation of the control strain of  C. autoethanogenum  (white circles) to the initial (light grey circles) and optimized (dark grey circles) fermentations of the iPROBE-selected pathway expression strain, acetate was monitored. Each circle represents a single data point. 
         FIG. 101 . Provides a table listing enzymes selected for use in Example 9. 
         FIG. 102 .  FIGS. 102A-102W  provide the codon-optimized gene sequences for  E. coli -based CFPS (SEQ ID NOs:73-95). See  FIG. 101  for information regarding the enzyme activity, organism, and source. 
         FIG. 103 .  FIGS. 103A-103F  provide a table showing the codon-optimized gene sequences for  C. autoethanogenum  (SEQ ID NOs:96-101). See  FIG. 101  for information regarding the enzyme activity, organism, and source. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein are systems and methods for cell-free protein synthesis driven metabolic engineering (CFPS-ME) that include cell-free protein synthesis as an integral step in the metabolic engineering process. The present invention allows for rapid prototyping and debugging of biosynthetic pathways. The present invention may also be implemented as a cell-free combinatorial method for optimization of biosynthetic pathways. 
     Cell-free protein synthesis driven metabolic engineering (CFPS-ME). In one aspect of the invention is CFPS-ME. In one embodiment of the invention is a method for the enzymatic preparation of a chemical product or a natural product in vitro. The method comprises providing a CFPS reaction mixture to a protein reaction vessel, expressing a translation template in the protein reaction vessel to prepare an enzyme, and providing the enzyme and a metabolic reaction mixture to a metabolic reaction vessel, wherein the feedstock reacts in the presence of the enzyme to prepare the chemical product or the natural product. In certain embodiments, the protein reaction vessel and the metabolic reaction vessel are the same vessel. The method is illustrated in  FIG. 1 . Methods for cell-free protein synthesis are provided in WO/2014/144583 to Jewett et al., the disclosure of which is incorporated by reference in its entirety. 
     The present invention is not limited to any certain enzyme or to any certain feedstock. As described and exemplified below, the enzymes and feedstsocks will be selected based on the chemical product or natural product to be made. 
     The present invention is not limited to any certain type of chemical product or natural product, but in certain embodiments the chemical product is 1-butanol or an intermediate in the 1-butanol biosynthetic pathway, styrene, 3-hydroxybutyrate, or specific isomers of butanediol. In other exemplary embodments, the natural product is valinomycin, andrimid, enterobactin, gramicidin S, HMG-CoA, or terpenes, such as limonene, pinene and bisabolene. 
     In certain embodiments, the method may include expressing one or more additional enzymes in a second protein reaction vessel. The second protein reaction vessel may be the same vessel as the protein reaction vessel. 
     In certain embodiments, the method may further comprise providing a transcription template to prepare the translation template. Where the transcription template is provided, one may additionally provide a polymerase and nucleoside triphosphates (NTPs): ATP, GTP, CTP, and UTP. Preparation of the translation template and preparation for the enzyme from the translation template may occur in the same reaction vessel or different reaction vessels. 
     An advantage of the present invention is that the cellular extract may provide natural enzyme metabolism from the host strain that may be exploited to perform desired chemical modifications. Natural enzyme metabolism means any process or chemical, including cellular extract enzymes, which may be necessary or beneficial for desired molecular transformations. Natural enzyme metabolism may provide energy, which may facilitate the desired molecular transformations. Natural enzyme metabolism may provide cofactor regeneration, which may facilitate the desired molecular transformation. Natural enzyme metabolism may also provide cellular extract enzymes. In certain embodiments, the cellular extract enzyme may be one or more heterologous enzymes expressed by the host. In certain embodiments, the cellular extract enzyme may be one or more native enzyme expressed by the host. In certain embodiments, the cellular extract enzyme may be a combination of one of more heterologous enzymes and native enzymes expressed by the host. In certain embodiments, the cellular extract enzyme is overexpressed by the host to enrich the extract with the cellular extract enzyme. In certain embodiments, the cellular extract enzyme may transform a chemical product or natural product into a feedstock. In other embodiments, the cellular extract enzyme may further transform the chemical product or the natural product that is formed by the reaction of the feedstock in the presence of the enzyme expressed by cell-free protein synthesis. 
     Combinatorial approach to CFPS-ME. The present invention, in certain embodiments, may be practiced in a combinatorial manor for rapid prototyping, design, and optimization of biosynthetic pathways. One step in the combinatorial method comprises providing N solutions. The N solutions may comprise one or more solutes. Solutes are typically chosen from those likely to be found in a CFPS reaction mixture and/or a metabolic reaction mixture. The solvent is typically buffer. It is possible that two or more of the N solutions are the same except for differences in the concentration of the solute. 
     Any of the solutions may be chosen to be combined, and as a result there are 2 N  possible combinations. Selection criteria may be employed to forbid certain combinations that would limit the number of combinations to something less than 2 N . One example of a selection criteria is to forbid the combination of two solutions that only differ in concentration of the solute. 
     Enzyme preparation may be initiated by combining the combinations of the N stock solutions with a CFPS reaction mixture in a cell-free protein synthesis reaction vessel to allow for the translation template to be expressed. It is not necessary for all of the combinations of the N stock salutation with a CFPS reaction mixture to result in the preparation of an enzyme because valuable information from negative results may aid the design and optimization of a biosynthetic pathway. 
     The method further includes providing the enzyme and a feedstock to a metabolic reaction vessel. The protein synthesis reaction vessel and the metabolic reaction vessel may be the same reaction vessel. 
     In some embodiments, a method for the enzymatic preparation of a chemical product or natural product in vitro, includes providing a cell-free protein synthesis reaction mixture to a protein reaction vessel, the cell-free protein synthesis reaction mixture comprising a cellular extract from a host strain, a translation template, and cell-free protein synthesis reagents, expressing the translation template in the protein reaction vessel to prepare an enzyme, providing the enzyme, the cellular extract, and a metabolic reaction mixture to a metabolic reaction vessel, the metabolic reaction mixture comprising a feedstock, wherein the feedstock reacts in the presence of the enzyme to prepare the chemical product or the natural product and wherein the cellular extract provides natural enzyme metabolism from the host strain. In some embodiments, the objective of the method is to guide the discovery, design, expression, and optimization of biosynthetic pathway production of valuable molecules and natural products. 
     In some embodiments, a method for combinatorial cell-free metabolism engineering comprises providing N solutions, combining between one and 2 N  combinations of the N solutions, providing a cell-free protein synthesis reaction mixture, the cell-free protein synthesis reaction mixture comprising a cellular extract and a translation template, combining the cell-free protein synthesis reaction mixture and at least one of the combinations of the N stock solutions in a cell-free protein synthesis reaction vessel, wherein the translation template is expressed to provide an enzyme; and providing the enzyme and a feedstock to a metabolic reaction vessel, wherein the feedstock is capable of reacting in the presence of the enzyme to form a product. In some embodiments, the objective of the method is to guide the discovery, design, expression, and optimization of biosynthetic pathway production of valuable molecules and natural products. 
     Kits 
     In one aspect of the invention, kits for CFPS-ME are disclosed. Kits for CFPS-ME comprising one or more components for the practice of the CFPS-ME method. In one aspect, the kits may comprise one or more components, individually or collectively, for the practice of CFPS. In certain embodiments, the kit may comprise a CFPS reaction mixture or the individual solutes or solutions that may be combined to form a CFPS reaction mixture. In one aspect, the kits may comprise one or more components, individually or collectively, for the practice of ME. In certain embodiments, the kit may comprise a ME reaction mixture or the individual solutes or solutions that may be combined to form a ME reaction mixture. In certain embodiments, the kit further comprises a feedstock. 
     Definitions 
     To aid in understanding the invention, several terms are defined below. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art. Although any methods and materials similar to or equivalent to those described herein can be used in the practice or testing of the claims, the exemplary methods and materials are described herein. 
     Moreover, reference to an element by the indefinite article “a” or “an” does not exclude the possibility that more than one element is present, unless the context clearly requires that there be one and only one element. The indefinite article “a” or “an” thus usually means “at least one.” 
     The term “about” means within a statistically meaningful range of a value or values such as a stated concentration, length, molecular weight, pH, time frame, temperature, pressure or volume. Such a value or range can be within an order of magnitude, typically within 20%, more typically within 10%, and even more typically within 5% of a given value or range. The allowable variation encompassed by “about” will depend upon the particular system under study. 
     The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, and includes the endpoint boundaries defining the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. 
     The terms “nucleic acid” and “oligonucleotide,” as used herein, refer to polydeoxyribonucleotides (containing 2-deoxy-D-ribose), polyribonucleotides (containing D-ribose), and to any other type of polynucleotide that is an N glycoside of a purine or pyrimidine base. There is no intended distinction in length between the terms “nucleic acid”, “oligonucleotide” and “polynucleotide”, and these terms will be used interchangeably. These terms refer only to the primary structure of the molecule. Thus, these terms include double- and single-stranded DNA, as well as double- and single-stranded RNA. For use in the present invention, an oligonucleotide also can comprise nucleotide analogs in which the base, sugar or phosphate backbone is modified as well as non-purine or non-pyrimidine nucleotide analogs. 
     Oligonucleotides can be prepared by any suitable method, including direct chemical synthesis by a method such as the phosphotriester method of Narang et al., 1979, Meth. Enzymol. 68:90-99; the phosphodiester method of Brown et al., 1979, Meth. Enzymol. 68:109-151; the diethylphosphoramidite method of Beaucage et al., 1981, Tetrahedron Letters 22:1859-1862; and the solid support method of U.S. Pat. No. 4,458,066, each incorporated herein by reference. A review of synthesis methods of conjugates of oligonucleotides and modified nucleotides is provided in Goodchild, 1990, Bioconjugate Chemistry 1(3): 165-187, incorporated herein by reference. 
     The term “primer,” as used herein, refers to an oligonucleotide capable of acting as a point of initiation of DNA synthesis under suitable conditions. Such conditions include those in which synthesis of a primer extension product complementary to a nucleic acid strand is induced in the presence of four different nucleoside triphosphates and an agent for extension (for example, a DNA polymerase or reverse transcriptase) in an appropriate buffer and at a suitable temperature. 
     A primer is preferably a single-stranded DNA. The appropriate length of a primer depends on the intended use of the primer but typically ranges from about 6 to about 225 nucleotides, including intermediate ranges, such as from 15 to 35 nucleotides, from 18 to 75 nucleotides and from 25 to 150 nucleotides. Short primer molecules generally require cooler temperatures to form sufficiently stable hybrid complexes with the template. A primer need not reflect the exact sequence of the template nucleic acid, but must be sufficiently complementary to hybridize with the template. The design of suitable primers for the amplification of a given target sequence is well known in the art and described in the literature cited herein. 
     Primers can incorporate additional features which allow for the detection or immobilization of the primer but do not alter the basic property of the primer, that of acting as a point of initiation of DNA synthesis. For example, primers may contain an additional nucleic acid sequence at the 5′ end which does not hybridize to the target nucleic acid, but which facilitates cloning or detection of the amplified product, or which enables transcription of RNA (for example, by inclusion of a promoter) or translation of protein (for example, by inclusion of a 5′-UTR, such as an Internal Ribosome Entry Site (IRES) or a 3′-UTR element, such as a poly(A)n sequence, where n is in the range from about 20 to about 200). The region of the primer that is sufficiently complementary to the template to hybridize is referred to herein as the hybridizing region. 
     The term “promoter” refers to a cis-acting DNA sequence that directs RNA polymerase and other trans-acting transcription factors to initiate RNA transcription from the DNA template that includes the cis-acting DNA sequence. 
     The terms “target, “target sequence”, “target region”, and “target nucleic acid,” as used herein, are synonymous and refer to a region or sequence of a nucleic acid which is to be amplified, sequenced or detected. 
     The term “hybridization,” as used herein, refers to the formation of a duplex structure by two single-stranded nucleic acids due to complementary base pairing. Hybridization can occur between fully complementary nucleic acid strands or between “substantially complementary” nucleic acid strands that contain minor regions of mismatch. Conditions under which hybridization of fully complementary nucleic acid strands is strongly preferred are referred to as “stringent hybridization conditions” or “sequence-specific hybridization conditions”. Stable duplexes of substantially complementary sequences can be achieved under less stringent hybridization conditions; the degree of mismatch tolerated can be controlled by suitable adjustment of the hybridization conditions. Those skilled in the art of nucleic acid technology can determine duplex stability empirically considering a number of variables including, for example, the length and base pair composition of the oligonucleotides, ionic strength, and incidence of mismatched base pairs, following the guidance provided by the art (see, e.g., Sambrook et al., 1989, Molecular Cloning-A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.; Wetmur, 1991, Critical Review in Biochem. and Mol. Biol. 26(3/4):227-259; and Owczarzy et al., 2008, Biochemistry, 47: 5336-5353, which are incorporated herein by reference). 
     The term “amplification reaction” refers to any chemical reaction, including an enzymatic reaction, which results in increased copies of a template nucleic acid sequence or results in transcription of a template nucleic acid. Amplification reactions include reverse transcription, the polymerase chain reaction (PCR), including Real Time PCR (see U.S. Pat. Nos. 4,683,195 and 4,683,202; PCR Protocols: A Guide to Methods and Applications (Innis et al., eds, 1990)), and the ligase chain reaction (LCR) (see Barany et al., U.S. Pat. No. 5,494,810). Exemplary “amplification reactions conditions” or “amplification conditions” typically comprise either two or three step cycles. Two-step cycles have a high temperature denaturation step followed by a hybridization/elongation (or ligation) step. Three step cycles comprise a denaturation step followed by a hybridization step followed by a separate elongation step. 
     As used herein, a “polymerase” refers to an enzyme that catalyzes the polymerization of nucleotides. “DNA polymerase” catalyzes the polymerization of deoxyribonucleotides. Known DNA polymerases include, for example, Pyrococcusfuriosus (Pfu) DNA polymerase,  E. coli  DNA polymerase I, T7 DNA polymerase and  Thermus aquaticus  (Taq) DNA polymerase, among others. “RNA polymerase” catalyzes the polymerization of ribonucleotides. The foregoing examples of DNA polymerases are also known as DNA-dependent DNA polymerases. RNA-dependent DNA polymerases also fall within the scope of DNA polymerases. Reverse transcriptase, which includes viral polymerases encoded by retroviruses, is an example of an RNA-dependent DNA polymerase. Known examples of RNA polymerase (“RNAP”) include, for example, T3 RNA polymerase, T7 RNA polymerase, SP6 RNA polymerase and  E. coli  RNA polymerase, among others. The foregoing examples of RNA polymerases are also known as DNA-dependent RNA polymerase. The polymerase activity of any of the above enzymes can be determined by means well known in the art. 
     As used herein, a primer is “specific,” for a target sequence if, when used in an amplification reaction under sufficiently stringent conditions, the primer hybridizes primarily to the target nucleic acid. Typically, a primer is specific for a target sequence if the primer-target duplex stability is greater than the stability of a duplex formed between the primer and any other sequence found in the sample. One of skill in the art will recognize that various factors, such as salt conditions as well as base composition of the primer and the location of the mismatches, will affect the specificity of the primer, and that routine experimental confirmation of the primer specificity will be needed in many cases. Hybridization conditions can be chosen under which the primer can form stable duplexes only with a target sequence. Thus, the use of target-specific primers under suitably stringent amplification conditions enables the selective amplification of those target sequences that contain the target primer binding sites. 
     As used herein, “expression template” refers to a nucleic acid that serves as substrate for transcribing at least one RNA that can be translated into a polypeptide or protein. Expression templates include nucleic acids composed of DNA or RNA. Suitable sources of DNA for use a nucleic acid for an expression template include genomic DNA, cDNA and RNA that can be converted into cDNA. Genomic DNA, cDNA and RNA can be from any biological source, such as a tissue sample, a biopsy, a swab, sputum, a blood sample, a fecal sample, a urine sample, a scraping, among others. The genomic DNA, cDNA and RNA can be from host cell or virus origins and from any species, including extant and extinct organisms. As used herein, “expression template” and “transcription template” have the same meaning and are used interchangeably. 
     As used herein, “translation template” refers to an RNA product of transcription from an expression template that can be used by ribosomes to synthesize polypeptide or protein. 
     The term “reaction mixture,” as used herein, refers to a solution containing reagents necessary to carry out a given reaction. A reaction mixture is referred to as complete if it contains all reagents necessary to enable the reaction, and incomplete if it contains only a subset of the necessary reagents. 
     An “amplification reaction mixture”, which refers to a solution containing reagents necessary to carry out an amplification reaction, typically contains oligonucleotide primers and a DNA polymerase in a suitable buffer. 
     A “PCR reaction mixture”, which refers to a solution containing the reagents necessary to carry out a PCR reaction, typically contains DNA polymerase, dNTPs, and a divalent metal cation in a suitable buffer. 
     A “cell-free protein synthesis (CFPS) reaction mixture”, which refers to a solution containing the reagents necessary to carry out CFPS, typically contains a crude or partially-purified yeast extract, an RNA translation template, and a suitable reaction buffer for promoting cell-free protein synthesis from the RNA translation template. In some aspects, the CFPS reaction mixture can include exogenous RNA translation template. In other aspects, the CFPS reaction mixture can include a DNA expression template encoding an open reading frame operably linked to a promoter element for a DNA-dependent RNA polymerase. In these other aspects, the CFPS reaction mixture can also include a DNA-dependent RNA polymerase to direct transcription of an RNA translation template encoding the open reading frame. In these other aspects, additional NTP&#39;s and divalent cation cofactor can be included in the CFPS reaction mixture 
     A “metabolic reaction mixture”, which refers to a solution containing the reagents necessary to carry out an enzyme-mediated metabolic or biosynthetic step, typically includes a feedstock that reacts in the presence of the enzyme to produce a final or intermediate product in the metabolic or biosynthetic pathway. A metabolic reaction mixture may optionally contain a cofactor, e.g. coenzyme-A, NAD, or ATP, or a buffer. 
     It will be understood by one of ordinary skill in the art that reaction components are routinely stored as separate solutions, each containing a subset of the total components, for reasons of convenience, storage stability, or to allow for application-dependent adjustment of the component concentrations, and that reaction components are combined prior to the reaction to create a complete reaction mixture. Furthermore, it will be understood by one of ordinary skill in the art that reaction components are packaged separately for commercialization and that useful commercial kits may contain any subset of the reaction components of the invention. 
     Miscellaneous. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred aspects of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred aspects may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect a person having ordinary skill in the art to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 
     EXAMPLES 
     The following examples are illustrative and should not be interpreted to limit the scope of the claimed subject matter. 
     Example 1: Cell-Free Biosynthesis of n-Butanol 
     Reference is made to Karim and Jewett, “A cell-free framework for rapid biosynthetic pathway prototyping and enzyme discovery,” (Metab. Eng. March 17; 36:116-126, doi: 10.1016/j.ymben.2016.03.002. [Epub ahead of print], hereinafter “Karim 2016”)), the content of which is incorporated herein by reference in its entirety. 
     A. Abstract 
     Speeding up design-build-test (DBT) cycles is a fundamental challenge facing biochemical engineering. To address this challenge, we report a new cell-free protein synthesis driven metabolic engineering (CFPS-ME) framework for rapid biosynthetic pathway prototyping. In our framework, cell-free cocktails for synthesizing target small molecules are assembled in a mix-and-match fashion from crude cell lysates either containing selectively enriched pathway enzymes from heterologous overexpression or directly producing pathway enzymes in lysates by CFPS. As a model, we apply our approach to n-butanol biosynthesis showing that  Escherichia coli  lysates support a highly active 17-step CoA-dependent n-butanol pathway in vitro. The elevated degree of flexibility in the cell-free environment allows us to manipulate physiochemical conditions, access enzymatic nodes, discover new enzymes, and prototype enzyme sets with linear DNA templates to study pathway performance. We anticipate that CFPS-ME will facilitate efforts to define, manipulate, and understand metabolic pathways for accelerated DBT cycles without the need to reengineer organisms. Highlights of CFPS-ME include the following: Cell-free protein synthesis can reconstitute entire biosynthetic pathways (CFPS-ME); In vitro systems provide greater control and higher resolution in studying metabolism; CFPS-ME enables the in vitro study of pathways in context of native metabolism; Linear DNA with CFPS-ME can bypass in vivo cloning steps in pathway construction; and CFPS-ME allows rapid enzyme prototyping in vitro before putting designs into a host. 
     B. Introduction 
     For decades scientists and engineers have turned to engineering biological systems to help meet societal needs in energy, medicine, materials, and more 1-4 . This has been an attractive, sustainable way to produce small molecules, especially when chemical synthesis is untenable 5,6  The ability to harness organisms that naturally produce molecules of interest has expanded the available chemical palate 7,8 . Often when natural producers are insufficient for production at the optimal titer (g 1 −1 ), yield, or volumetric productivity (g 1 −1  h −1 ), engineers seek to design biosynthetic pathways and regulatory processes in cells to meet certain manufacturing criteria 9,10 . For example, introducing heterologous pathways into model microorganisms and engineering them to maximize a particular biosynthesis has led to large scale production of 1,3-propanediol, farnesene, and artemisinin with many more on their way to market 6,11 . Efforts to make these molecules have resulted in success, but not without a great deal of challenges. 
     Bringing a biosynthetic molecule to market usually involves countless hours of design-build-test (DBT) cycles 12 . The production of n-butanol is a prime example of these challenges. A series of  Clostridia  species are natural producers of n-butanol during acetone-butanol-ethanol fermentation, and  Clostridia acetobutylicum  and  Clostridia beijerinckii  are two of which are commonly used in commercial n-butanol plants 13 . However, these species are difficult to engineer because of a biphasic metabolism, unknown regulation patterns, and a limited number of species-specific engineering tools 14 . Heterologous expression of  Clostridia  metabolism in model microorganisms like  Escherichia coli  and  Saccharomyces cerevisiae  allows n-butanol production to be more easily engineered but can be accompanied by lower titers 15,16 . Starting with heterologous expression of the n-butanol pathway as a baseline, scientists have been able to increase titers dramatically by knocking out genes from genomes 15 , increasing redox driving forces by introducing pathway-independent enzymes 17 , and identifying homologous enzymes with better activities 18 . Years of iterative metabolic engineering led to these advances, but titers are still not high enough and scale-up is often too unpredictable to outcompete natural producers for commercial production 19 . As is the same for many biosynthetic pathways, we cannot quickly enough identify optimal biosynthetic systems and discover the best sets of enzymes that work together as a group. Therefore, metabolic engineering remains costly and time-consuming 20,21 . 
     A key challenge in metabolic engineering is balancing the tug-of-war that exists between the cell&#39;s physiological and evolutionary objectives on one side and the engineer&#39;s process objectives on the other. Put another way, it is very difficult to balance intracellular fluxes to optimally satisfy a very active synthetic pathway while the machinery of the cell is functioning to maintain reproductive viability. Other challenges include: (i) the need for reliable computational selection and design of enzyme homologs for pathway design, (ii) the limited number of feasible homologs and genetic constructs that can be searched in any one project, and (iii) the unknown effects of optimal pathway enzyme expression on the entire metabolic system 22-24 . 
     Many established and emerging technologies seek to address these challenges. For example, metabolic flux analysis and genome engineering offer generalized capabilities to modify living organisms for improving product titers 25,26 . In addition, coupling machine-learning algorithms to multiplexed designs can accelerate efforts to rationally engineer cells 27 . However, DBT cycle time remains a limitation 28 . In vitro systems offer a complementary, yet underutilized approach to speed up DBT cycles with some potential advantages 11,29-31 . For example, the open reaction environment allows for the addition of components such as cofactors and intermediates at any time during a cell-free reaction, which can be maintained at precise concentrations. In addition, cell-free systems have no cell viability constraints. Furthermore, the cell-free format permits DBT iterations without the need to reengineer organisms 30 , with the potential to reduce DBT cycle time 31 . Cell-free metabolic engineering (CFME), or using cell-free techniques to aid metabolic engineering efforts, is emerging as a complementary approach to existing strategies for carrying out biomolecular transformations of interest with in vitro ensembles of catalytic proteins, prepared from purified enzymes or crude lysates of cells 32-39 . 
     In this work, we develop a cell-free protein synthesis driven metabolic engineering (CFPS-ME) framework to accelerate DBT cycles for optimizing and debugging biosynthetic pathways ( FIG. 1A ). The foundational principle is that we can construct discrete metabolic pathways through combinatorial and modular assembly of lysates containing enzyme components produced by overexpression in the lysate chassis strain or by cell-free protein synthesis (CFPS). We focus on using CFPS because these systems can help address the growing demand for simple, inexpensive, and efficient protein production technologies for a wide array of applications 11,29,40-44 . In addition, processes that take days or weeks to design, prepare, and execute in cells can be done more rapidly in a cell-free system, because no time-consuming cloning steps are needed 45 . Three recent advances enable the use of CFPS for CFME. First, Jewett et al. demonstrated the ability to stimulate highly active energy and cofactor regeneration pathways in crude cell lysates 46 . Second, Kay and Jewett showed that crude cell lysate based cell-free systems from  E. coli  could fuel highly active heterologous metabolic transformations 36 . Third, Dudley and Jewett established the ability to build a heterologous biosynthetic pathway by mixing lysates each containing individually overexpressed heterologous enzymes (in preparation). The mix-and-match approach has many advantages including only needing to express one enzyme in each strain, not needing to fine-tune expression, and being able to directly monitor and sample the reaction environment. Here, we extend this approach by demonstrating modular assembly of pathways through the ability to enrich lysates with biosynthetic enzymes using well-defined experimental conditions and CFPS. It is important to note that our goal in this work was not to develop cell-free systems for the highest product titer, an engineered strain for best in vivo synthesis of n-butanol, or industrial applicability. However, we do show that CFPS-ME offers an even faster approach (hours rather than days) for building pathways directly in lysates for the purpose of enzyme selection and pathway design. 
     To demonstrate CFPS-ME, we selected the model n-butanol biosynthetic pathway derived from  Clostridia  metabolism involving CoA intermediates ( FIG. 1B ). Endogenous glycolytic enzymes convert glucose to acetyl-CoA, the starting intermediate for n-butanol synthesis, another  E. coli  enzyme takes acetyl-CoA to acetoacetyl-CoA, and heterologous enzymes convert acetoacetyl-CoA to n-butanol. We first show the ability to mix five crude lysates each with selectively overexpressed enzymes to activate the entire 17-step n-butanol production pathway in vitro with high yield and productivities. We then establish the CFPS-ME concept by modularly building the n-butanol pathway with lysates harboring heterologous pathway enzymes expressed by CFPS or having been overexpressed in the chassis source strain. We apply this framework to rapidly screen enzymes for optimal pathway operation and enzyme discovery. We expect that the CFPS-ME framework will increase the resolution at which we can manipulate biosynthetic pathways by examining enzyme kinetics, measuring metabolic flux, determining catalyst stability, studying redox effects, and prototyping metabolism. 
     C. Material and Methods 
     1. Bacterial Strains and Plasmids. 
       E. coli  NEB Turbo™ (NEB) was used in plasmid cloning transformations and for plasmid preparation.  E. coli  BL21(DE3) (NEB) was used for protein overexpression and for preparation of all extracts. (See Karim 2016, and Table 1 for strain details). A modified version of pET-22b (Novagen/EMI) Millipore), used in previous studies 36 , was used for all constructs for in vivo over-expression of proteins. For in viro expression of proteins, the pJL1 vector was used. Carbenicillin (100 μg ml −1 ) was used with the pET vector system and kanamycin (50 μg ml −1 ) was used with the pJL1 vector system. 
     Gibson assembly was used for seamless construction of plasmids. (See Karim 2016, and Table 1 for plasmid details). Each gene and vector was amplified via PCR using forward and reverse primers designed with NEB&#39;s Gibson Assembly Designer (New England Biolabs, Ipswich Mass., USA) and purchased from IDT and Phusion® High-Fidelity DNA polymerase (Finnzymes, Thermo Scientific Molecular Biology). (See Karim 2016, and Table 2 for genes and enzymes and Karim 2016 for primer details). Both PCR products were cleaned and mixed with Gibson assembly reactants and incubated at 50° C. for 60 min. Plasmid DNA from the Gibson assembly reactions were immediately transformed into  E. coli  NEB Turbo cells. Propagated constructs were purified using an EZNA Plasmid Mini Kit (Omega Bio-Tek). Completed constructs were used to transform  E. coli  BL21(DE3). 
     Codon optimized versions of each gene were identified using IDT&#39;s codon optimization online tool (Integrated DNA Technologies®, Coralville, USA) and NCBI&#39;s Basic Local Alignment Search Tool (National Center for Biotechnology Information, U.S. National Library of Medicine, Bethesda Md., USA). These genes were purchased from Gen9, Inc. (Cambridge Mass., USA). (See Karim 2016, and SEQ ID NOs:1-18). 
     2. Cell Extract Preparation. 
       E. coli  BL21(DE3) cells (see Karim 2016 and Table 1 for strains) were grown in 2×YTPG media (16 g l −1  tryptone, 10 g l −1  yeast extract, 5 g l −1  NaCl, 7 g l −1  potassium phosphate monobasic, 3 g l −1  potassium phosphate dibasic, 18 g l −1  glucose). These cells were cultured at the 50 ml scale in 250 ml baffled tunair shake flasks (IBI Scientific, Peosta, Iowa) in a 37° C. incubator with vigorous shaking at 250 rpm. The cultured cells were monitored by spectrophotometry (Genesys 10S UV-Vis, Thermo Fisher Scientific, Waltham, Mass.). When cells reached OD 600 =0.6-0.8, the cultures were induced with 0.1 mM IPTG. After induction cultures were grown for 4 h at 30° C. Antibiotics were not used during cell growth. The cells were harvested by centrifuging at 8,000 g at 4° C. for 15 min and were washed two times with cold S30 buffer (10 mM Tris-acetate (pH 8.2), 14 mM magnesium acetate, and 60 mM potassium glutamate). After final wash and centrifugation, the pelleted wet cells were weighed, flash frozen in liquid nitrogen, and stored at −80° C. The thawed cells were suspended in 0.8 ml of S30 buffer per 1 g of wet cell mass. In order to lyse cells by sonication, thawed and suspended cells were transferred into 1.5 ml microtube and placed in an ice-water bath to minimize heat damage during sonication. The cells were lysed using a Q125 Sonicator (Qsonica, Newtown, Conn.) with 3.175 mm diameter probe at frequency of 20 kHz and 50% of amplitude. The input energy (Joules) was monitored and 830 J was used for 1.4 ml of suspended cells. The lysate was then centrifuged twice at 21,100 g at 4° C. for 15 min. All of prepared cell extract was flash frozen in liquid nitrogen and stored at −80° C. until use. 
     3. Extract Protein Quantification. 
     The total protein concentration of the extracts was measured by Quick-Start Bradford protein assay kits (Bio-Rad) with a bovine serum albumin standard. The extracts were subsequently run on a Coomassie-blue stained NuPAGE Bis-Tris 12% SDS-PAGF gel with MOPS buffer (Life Technology, Grand Island, N.Y.). The SeeBlue Plus2 pre-stained ladder (Life Technology, Grand Island, N.Y.) was used and ˜10 μg of total protein for each sample was loaded on the gel. 
     4. CFME Reactions. 
     Reactions were carried out in 1.5 ml Eppendorf tubes at 37° C. in 25 μl volumes. Each reaction consisted of mixing five extracts, containing one enzyme overexpressed each, to complete the biosynthetic n-butanol pathway (2 mg ml −1 ) along with magnesium glutamate (8 mM), ammonium glutamate (10 mM), potassium glutamate (134 mM), glucose (200 mM), dipotassium phosphate (10 mM, pH 7.2), Bis Tris (100 mM), NAD (1 mM), ATP (1 mM), and CoA (0.5 mM), unless otherwise noted. Reactions were terminated by adding 5% w/v trichloroacetic acid in a 1:1 ratio. Precipitated proteins were pelleted by centrifugation at 15,000 g for 10 min. The supernatant was stored at −80° C. until analysis. 
     5. CFPS-ME Reactions. 
     CFPS reactions were performed to express enzymes involved in n-butanol production prior to starting the CFME portion of the reactions using a modified PANOx-SP system 47 . A 25 μl CFPS reaction in a 1.5 ml microcentrifuge tube was prepared by mixing the following components: ATP (1.2 mM); GTP, UTP, and CTP (0.85 mM each); folinic acid (34.0 μg ml −1 );  E. coli  tRNA mixture (170.0 μg ml −1 ); T7 RNA polymerase (100 μg ml −1 ); 20 standard amino acids (2 mM each); nicotinamide adenine dinucleotide (NAD; 0.33 mm); coenzyme-A (0.27 mM); spermidine (1.5 mM); putrescine (1 mM); potassium glutamate (130 mM); ammonium glutamate (10 mM); magnesium glutamate (12 mM); phosphoenolpyruvate (PEP; 33 mM), and cell extract (10 mg ml −1 ). For each reaction plasmid was added at ˜13.3 or ˜26.6 μg ml −1 . The n-butanol production portion of the reaction was initiated by spiking in glucose (200 mM) and additional reagents (NAD, CoA) noted throughout the manuscript. 
     6. Quantification of Protein Produced In Vitro. 
     Cell-free protein synthesis reactions were performed as noted above (Section 2.5) with radioactive  14 C-Leucine (10 μM) supplemented in addition to all 20 standard amino acids. We used trichloroacetic acid (TCA) to precipitate radioactive protein samples. Radioactivity of TCA-precipitated samples was measured by liquid scintillation counting to then quantify the protein produced as previously reported (MicroBeta2; PerkinElmer) 46,47 . These reactions were also run on a Coomassie-stained SDS-PAGE gel and exposed by autoradiography. Autoradiographs were imaged with a Typhoon 7000 (GE Healthcare Life Sciences, Pittsburgh, Pa.). Multiple proteins produced in vitro were further quantified by gel image intensity comparisons using ImageJ (NIH). 
     7. n-Butanol Quantification. 
     High-performance liquid chromatography (HPLC) was used to analyze the components in the reactions. n-Butanol was measured with an Agilent 1260 series HPLC system (Agilent, Santa Clara, Calif.) via a refractive index (RI) detector. Analytes were separated using the Aminex HPX-87H anion exchange column (Bio-Rad Laboratories) with a 5 mM sulfuric acid mobile phase at 55° C. and a flow rate of 0.6 ml min. Commercial standard of n-butanol was used for quantification of experimental samples by linear interpolation of external standard curves. An example chromatogram for n-butanol is given in  FIG. 7 . 
     Results 
     In developing a framework for biosynthetic pathway prototyping, we constructed a 17-step pathway for the production of n-butanol. n-butanol synthesis was selected as a model because of its importance as a potential biofuel, it is easily quantified by HPLC, and it has multiple heterologous steps. We sought to combine  E. coli &#39;s endogenous 11-step glycolytic pathway from glucose to acetyl-CoA (AcCoA) with the  Clostridia -derived six-step n-butanol pathway from AcCoA ( FIG. 1B ). The idea that natural energy and cofactor regeneration would be harnessed in the lysate to fuel n-butanol production is a distinct break from typical in vitro approaches, which use purified enzymes 32 . Complementary to those systems, our approach allows for studying pathway performance in a setting that better mimics the in vivo operation (e.g., from glucose rather than AcCoA). The crude lysate system also allows us to focus on expressing only the necessary heterologous enzymes to complete the entire pathway. These enzymes include a thiolase to merge two AcCoAs followed by a number of dehydrogenases to perform a series of reductions through CoA intermediates to obtain n-butanol (See Karim 2016, and Table 2 for Genes and Enzymes). 
     1. Cell-Free Metabolic Engineering for n-butanol Production. 
     To enable cell-free biosynthesis of n-butanol, we first introduced genes encoding the five enzymes needed to convert AcCoA to n-butanol individually into our extract source strains, in this case BL21(DE3) (See Karim 2016, Table 1 for Strains and Plasmids, and Karim 2016 for Primers). We selected two homologs each for hydroxybutyryl-CoA dehydrogenase (Hbd), crotonase (Crt), and bifunctional aldehyde/alcohol dehydrogenase functionalities. For the thiolase (AtoB) and butyryl-CoA dehydrogenase (Ter) we chose  E. coli &#39;s endogenous enzyme and a widely used enzyme from  Treponema denticola , respectively. Next, we selectively overexpressed each heterologous enzyme in separate strains using a tightly controlled T7 promoter and strong ribosome binding site. As expected, we observed that the heterologous proteins were overexpressed as the dominant bands, with the exception of Hbd1, on an SDS-PAGE gel ( FIG. 2A ). The low expression of Hbd1 is likely due to RBS used for expression. 
     After lysis and extract preparation, we then reconstituted the 17-step pathway from glucose to n-butanol by mixing equal total protein concentrations of five separate extracts containing each enzyme. Specifically, we started with the following enzyme set:  E. coli &#39;s AtoB,  C. acetobutylicum  &#39;s Hbd, Crt, and AdhE2, as well as Ter from  T. denticola . This set was chosen to include most of  C. acetobutylicum  &#39;s enzyme set, one of the most widely used sets for n-butanol production, along with previously identified best enzymes for thiolase and butyryl-CoA dehydrogenase functions 17,19 . Upon incubation with essential substrates, salts, and cofactors (e.g., magnesium, potassium, and ammonium salts, glucose, phosphate, buffer, NAD, CoA, ATP), we assessed n-butanol synthesis in 25 μl CFME batch reactions carried out for 24 h at 37° C. via high performance liquid chromatography (HPLC). We observed production of 0.51±0.04 g l −1  n-butanol (˜0.05 mol n-butanol/mol glucose) over the course of a 24 h reaction ( FIG. 2B ), without any optimization to improve titers. As expected, we also observed lactate, acetate, and ethanol as byproducts seen in previous reports of n-butanol production, which could be addressed through genome modifications (e.g., deletion of ldh gene in the source strain) 19 . Butanol production shows that both the heterologous pathway and endogenous glycolysis is activated with cofactors being regenerated. However, n-butanol production stops after ˜9 h. In previous work, substrate depletion was shown to be the most typical cause for reaction termination 36 . One way to avoid this limitation is to run reactions in fed-batch or continuous reactor set-ups or use substrates that are metabolized slower (e.g. polymeric sugars). Except in few instances 39,48 , limited cofactor regeneration has historically plagued in vitro synthetic enzymatic pathway conversions 32,34,35 . Here, however, native glycolytic enzymes in the lysate provide a simple route to fuel highly active heterologous metabolic conversions. For example, to produce ˜7 mM n-butanol we would need ˜56 NADH turnover events, exceeding typical turnover numbers of ˜5-20 for purified in vitro systems 32 . 
     Following demonstration of activating n-butanol synthesis, we next aimed to modularly build n-butanol synthesis pathways with different enzyme homologs to improve pathway performance. We cycled through multiple distinct ensembles of enzymes by mixing and matching lysates containing different versions of enzymes necessary to complete the biosynthetic n-butanol pathway. Trying out different homologs in this manner allowed us to quickly identify a better set of enzymes producing n-butanol at 0.84±0.19 g l −1  (0.09 mol n-butanol/mol glucose) ( FIG. 3A ). Specifically, we showed that Hbd2 from  C. beijerinckii  enabled a 65% increase in n-butanol synthesis titers over Hbd1 from  C. acetobutylicum . A follow-up experiment doubling the Hbd1 enzyme did not alter the amount of n-butanol produced, suggesting that this increase was not due to discrepancies in enzyme concentrations in the lysate ( FIG. 8 ). However, further studies of these enzymes would elucidate whether the observed n-butanol production was a result of using BL21(DE3) extract without heterologous genes expressed (used for normalization), which may have more active glycolytic and byproduct pathways that could divert flux away from n-butanol. 
     While the selection of enzymes is crucial to improving n-butanol production, the value of each physiochemical parameter of the cell-free system also affects n-butanol production and becomes key in further optimization and debugging of the pathway. To demonstrate the facile nature of combinatorial optimizations in our cell-free framework, we explored changes in the ionic composition because the composition of salts added to in vitro systems affects the systems&#39; performance 46,47,49,50 . Specifically, we tested the effect of using glutamate, acetate, and chloride salts on n-butanol production and found that glutamate salts perform more than 15% better than the other salt compositions ( FIG. 9 ). Our results are consistent with previous works, which have shown that glutamate salts better mimic the intracellular cytoplasmic conditions of  E. coli  to co-activate authentic biological processes such as the in vitro co-activation of central metabolism, oxidative phosphorylation, and protein synthesis 46 . 
     Beyond studying pathway performance by altering the ionic composition, the states of critical cofactors (organic molecules necessary for enzyme catalysis) can also be studied. The balance of cofactors, such as oxidized and reduced NAD, is critical to energy regeneration within the lysate by also the heterologous pathway under investigation. In our cell-free framework, the lack of a cell wall enables direct sample acquisition, reaction monitoring, and control. We used this flexibility to study the impact of the ratio of initial cofactors in the reaction to see the ratio&#39;s effect on n-butanol production. We found that the ratio of NAD(H) at the start of the reaction (e.g., NAD:NADH: 1:0, 1:0.5, 1:1, 0.5:1, 0:1), keeping the total cofactor concentration at 0.5 mM, plays a minimal role in how much n-butanol can be produced ( FIG. 10 ). This suggests that metabolism in the lysate may control the overall levels of reduced and oxidized cofactor, which is consistent with data from Kay and Jewett 36 . 
     Understanding that some components play more of a role in pathway performance than others, we next performed a number of reactions to identify which added components are necessary for n-butanol production with a particular interest in the three added cofactors (ATP, NAD, and CoA). The supplementation of cofactors to cell-free reactions would be costly and hinder industrial practicality of this technology if it were proposed as a biomanufacturing platform. In our study of cell-free systems as a prototyping framework, we surprisingly found that omitting ATP boosts n-butanol production by greater than 180% from 0.84±0.19 g l −1  to 1.43±0.12 g l −1  (0.11 mol n-butanol/mol glucose) ( FIG. 3B ). More unexpectedly, by just adding salts to mimic the cytoplasm and glucose as a starting substrate we are able to produce n-butanol at 0.28±0.12 g l −1 . In other words, if lysates are prepared without dialysis, as we have done, cofactors remaining in the lysate are sufficient for the cell-free transformation and do not need to be added. Collectively, our results here show that the cell-free framework offers a strategy to explore how enzyme variants, substrates, cofactors, ionic composition, etc. can be varied in unique combinations to influence pathway performance. While CFME (i.e., selective enriching or functionalizing the lysate with pathway enzymes prior to extract generation) provides us with a rather quick way to screen enzyme ensembles and reaction conditions, this approach is limited by the cell&#39;s ability to produce the enzymes individually in vivo, a limitation that we address below. 
     2. Cell-Free Protein Synthesis Driven Metabolic Engineering. 
     We next aimed to combine CFPS and CFME to modularly build the n-butanol pathway for forward engineering. This is dissimilar from previous works in which synthetic in vitro pathways have been built by purified enzymes or using lysates selectively enriched by heterologously expressed enzymes. Integration of CFPS enables one to speed up DBT cycle time for prospecting biosynthetic pathways. Indeed, using CFPS to express enzymes can reduce the time to build pathways to hours rather than days. As a proof-of-concept of this approach, we tested making Hbd2 (the non-native entry enzyme to the pathway) by CFPS to activate n-butanol production ( FIG. 4A ). The key idea of the experiment was that the pathway would remain inactive (as downstream enzymes will not have their substrates) until active Hbd2 was synthesized. We chose to validate CFPS-ME in a three-step process. First, we quantified our ability to express Hbd2 in a CFPS reaction comprised of a mixture of lysates harboring selectively enriched pathway enzymes lacking Hbd2. This was important because typical CFPS systems use lysates from cells harvested in mid-late exponential phase, where as our lysates were collected 4 h post-induction of pathway enzymes. Second, we studied the ability to activate the entire pathway by combining CFPS and CFME. Third, we carried out a series of optimizations to try to increase yields. 
     For CFPS, we used the tunable and well characterized PANOx-SP CFPS system developed by Jewett and Swartz 47  to quantitatively test the synthesis of Hbd2. CFPS reactions at 30° C. were allowed to run for 24 h in batch operation and the yields of cell-free synthesized Hbd2 was quantified by monitoring  14 C-leucine incorporation. We based the system on a mixture of lysates used above, except the lysate with Hbd2 was not included. Endogenous protein synthesis machinery should act to synthesize and fold desired protein products upon incubation with essential substrates (e.g., amino acids, nucleotides, DNA or mRNA, energy substrates, cofactors, and salts). In this case, we showed that when the DNA for the Hbd2 enzyme on a pJL1 vector was added, the mixed extract could produce 559±15 mg l −1  of Hbd2 over a 24-hour period ( FIG. 4B ). Based on this result and the fact that this reaction was over 50% complete by three hours, we chose to run all subsequent CFPS reactions for three hours, which should provide sufficient protein quantities for prototyping. 
     We next investigated the ability of the cell-free synthesized Hbd2 to activate the full n-butanol pathway. After three hours of CFPS, we initiated n-butanol metabolism by adding 200 mM glucose to the reactions. We showed that CFPS of Hbd2 could activate n-butanol metabolism reaching a titer of 0.92±0.13 g l −1  ( FIG. 4C ). Negative control reactions without synthesis of the Hbd2 did not produce n-butanol. Notably, the CFME portion resulted in the same n-butanol yields when carried out at either 30 or 37° C., so for ease we selected 30° C. for all future experiments to have the CFPS and CFME portions performed at the same temperature. As in the CFME system alone, we found that small molecules, cofactors, etc. can modulate pathway performance. For example, we found that adding both NAD and CoA with glucose to initiate n-butanol metabolism after CFPS gave us 1.22±0.22 g l −1  n-butanol ( FIG. 4D ). Collectively, our results prove for the first time to our knowledge the ability to combine CFPS and CFME to support a highly active biosynthetic pathway. 
     We further extended this proof-of-concept to activate n-butanol production using CFPS at any pathway node by producing each n-butanol pathway enzyme. Using mixed extracts with all but one necessary enzyme, we performed CFPS of the ‘missing enzyme’ and saw that each enzyme could be produced individually at more than 100 mg l −1  without optimization ( FIG. 5A ). We then proved that full product of each protein is made exclusively in each reaction by an autoradiogram ( FIG. 11 ). After validating expression of each enzyme, we then performed CFPS-ME reactions. We carried out three-hour CFPS reactions and then initiated the n-butanol pathway by adding glucose, NAD, and CoA, because supplementation of CFPS-ME reactions with both NAD and CoA increased n-butanol titers for Hbd2 ( FIG. 4D ). Strikingly, CFPS-ME could be used for each of the pathway enzymes to produce n-butanol at levels as high as 1.71±0.06 g l −1  ( FIG. 5B ). 
     We next set out to demonstrate we could build the entire pathway by CFPS of the pathway enzymes in our extracts. To this end, we extended the number of enzymes made in vitro one by one, by adding equal amounts of DNA of each, and saw that when we made one, two, three, and four of the five enzymes necessary in vitro we could produce n-butanol at levels between ˜0.6 and ˜1.4 g l −1  ( FIG. 5C ). Again, full-length product of each protein is made in each reaction as shown by autoradiogram ( FIG. 12 ). However, as we increase the number of enzymes produced by CFPS, the amount of n-butanol synthesized decreases. In fact, when we tried to produce all five enzymes in vitro we were initially unable to make any n-butanol. We attribute this drop in n-butanol production to there not being enough of the last enzyme in the pathway, AdhE, seen by quantification of the enzymes produced by CFPS ( FIG. 13 ). However, we were able to make all enzymes in vitro at sufficient levels necessary to make n-butanol at 0.75±0.12 g l −1  by increasing the plasmid DNA encoding AdhE to more than 50% of the total DNA added, ( FIG. 5D ;  FIG. 14 ). Reduced T7 polymerase added shows improvements in n-butanol production. Typical CFPS systems supplement T7 polymerase stored in glycerol, and increasing glycerol concentrations can be deleterious to the CFPS system. The extract used in this study contains T7 polymerase expressed in vivo prior to extract preparation, so T7 polymerase in the extract is expected to be sufficient without supplementation. Based on our result that added ATP was deleterious to n-butanol production by CFME ( FIG. 3B ), the ATP used in CFPS might be expected to inhibit CFPS-ME n-butanol titers if ATP is long-lived. We have previously shown that ATP concentrations are stable around 200 μM over a ˜6-8 hour batch CFPS reaction 47 . Though, a negative effect from ATP is expected, it is difficult to use the CFME optimization conditions for CFPS-ME, given the added complexity of protein synthesis. Our results importantly showed that we could build a five-step heterologous pathway to make n-butanol in vitro in three hours. 
     3. Rapid Prototyping and Enzyme Discovery with CFPS-ME. 
     The ability to use CFPS-ME to produce enzymes for n-butanol biosynthesis allows us to test pathway enzymes without expressing enzymes in the host cell. As a model case study, we decided to test for improved pathway performance (increased n-butanol production) by swapping out some of our initial Ter and AdhE enzymes for a variety of homologs. In less than a day, we studied 4 Ter and 3 AdhE homologs in a combined CFPS-ME reaction. In all cases, we observed synthesis of n-butanol, though lower than our previous best-performing enzymes ( FIG. 6A ). Five of these variants come from species never tested before. 
     Having demonstrated the ability to explore enzyme homologs using CFPS-ME, we then set out to demonstrate the potential for using linear DNA templates instead of plasmids. Using linear DNA molecules, i.e. PCR products, would expedite the process since the entire process could be done without cells and we could avoid laborious cloning steps. As a model system, we first repeated the experiments presented in  FIG. 6  with linear templates and observed that the linear DNA templates can successfully be expressed to complete the n-butanol biosynthesis pathway ( FIG. 15 ). Next, we chose to screen multifunctional enzymes that to our knowledge have never before used for n-butanol production. We selected four enzymes with proposed Hbd and Crt functionalities that were identified by NCBI-BLAST searches. By preparing reactions with three different enzyme mixtures (mixed extracts with overexpressed enzymes prior to lysis) (1) without Hbd, (2) without Crt, and (3) without Hbd and Crt, we could characterize each enzyme variant by their ability to perform each enzymatic function. We discovered that each of these enzymes could activate n-butanol synthesis, and the proposed Hbdcrt6 from  Sulfolobus acidocaldarius  only had Hbd functionality ( FIG. 6B ). The ability to use linear DNA templates for CFPS-ME makes possible the ability to rapidly screen individual and sets of enzymes completely in vitro. Here, we used this approach to parse out individual functionalities of multi-functional enzymes. 
     D. Discussion 
     In this study, we developed a new cell-free framework for prototyping biosynthetic pathways and screening enzymes. In one scenario, we overexpress individual pathway components in cells, lyse these cells, and mix and match lysates in cell-free cocktails to study biochemical pathway performance. In a distinct thrust from typical in vitro systems, our approach allows us to study heterologous pathways in the context of native metabolism. In another scenario, we bypass in vivo expression altogether by using CFPS to enrich lysates with different enzymes for combinatorial assembly of different pathways. The combination of CFPS to express homologs of individual biosynthetic enzymes for studying pathway performance is also a distinction of our workflow. In addition, the use of linear PCR templates, which could be improved by DNA stability techniques (e.g., the addition of purified GamS protein) 30 , allows us to avoid in vivo cloning steps altogether. Our CFPS-ME approach should therefore be faster than conventional approaches to select enzymes and pathway designs in cells (hours instead of days/weeks), and enables parallelized pathway construction of combinatorial designs to accelerate DBT cycles. 
     A key conceptual innovation of our work is that the DBT unit can be cell-free lysates rather than genetic constructs. Engineering large biosynthetic systems composed of many genes in microbes remains challenging 27 . One of the many obstacles is simply how many different genetic designs with beneficial chances are feasible to make. Cell-free systems have already been shown to screen genetic designs to improve enzyme performance at a rapid rate 51 . Our CFPS-ME framework should allow researchers to study more designs than previously possible by rapidly prototyping enzyme performance in vitro before putting designs into a host. As an example, a six-step biosynthetic pathway testing 5 homologs for each enzymatic step would require testing of 15,625 pathway combinations. While this set of combinations exceeds typical pipelines pursued in cells today, our CFPS-ME system could leverage robotic or automated liquid-handling systems to access such design space. 
     The goal of this manuscript was to provide a new approach to building biosynthetic pathways in a modular fashion in vitro. Now achieved, we plan to optimize a large-scale fermentation process with the CFPS-ME approach in the future. Towards this goal, we additionally carried out experiments to show that protein expression in the cell-free system translates to the in vivo system. Specifically, we took all Ter homologs screened in vitro by CFPS-ME ( FIG. 6 ) and expressed them in whole cells in vivo. All but one of the Ter homolog proteins can be expressed in cells on a first pass (as determined by SDS-page expression,  FIG. 16 ). These data show that protein expression in the cell-free system can translate to the in vivo system. Thus our approach holds promise for identifying good enzymes that can be expressed in cells, following a body of work that uses in vitro enzyme assays to identify enzymes with the best-performing biochemical characteristics for desired metabolic transformations prior to putting them into a host. For example, Liao and colleagues showed that in vitro reconstitution could be used to construct the non-oxidative glycolytic pathway prior to in vivo expression 52 , and Zhu et al. reconstituted the mevalonate pathway in vitro to study pathway kinetics before using the pathway in vivo for the production of farnesene 53    
     Our cell-free approach mimics the intracellular environment of  E. coli , where endogenous glycolytic enzymes from the cell extract convert glucose to AcCoA. Thus, our platform enables many different biosynthetic pathways to be studied in the context of central metabolism with enhanced control inherent to in vitro systems. Here, we were able to increase n-butanol production by ˜200% of our initial starting conditions (up to ˜1.5 g l −1 ) by simply testing the performance of different enzymes sets and adjusting the physicochemical environment. While it is be difficult to compare in a normalized fashion the in vitro process to the in vivo process, our results (given as final measured concentration) are higher than some published reports of n-butanol production in comparable genomically unmodified hosts ( FIG. 17 ) 37,54-56 . However, Bond-Watts et al. notably reported titers of 4.6 g l −1  in a genomically unmodified host by selecting a particular set of synergistic enzymes and taking advantage of their chemistries 18 . Given the reasonable yields, we were curious as to how the CFPS-ME reactions would perform at increased scale. We thus performed additional experiments of increasing size reactions to give confidence in our quantitative yields. Specifically, the reaction volume of CFPS-ME reactions was scaled from 25 to 250 μL, an order of magnitude increase ( FIG. 18 ). Our data shows that these reactions are scalable and are consistent with several previous works showing the ability of cell-free systems to scale linearly 36,57-60 . 
     Looking forward, specialty chemicals, natural products, and materials offer an extremely diverse set of compounds with a seemingly infinite set of structures and bioactivities. For example, we have applied our approach to reconstruct nonribosomal peptide biosynthesis directly from DNA ( FIG. 19 ). Specifically, we utilized the proteins involved in the first steps of gramicidin S biogenesis as a model to show the potential for making and prospecting natural products with our approach ( FIGS. 20 and 21 ). Our CFPS-ME approach offers a new discovery pipeline to leverage advances in DNA sequencing and DNA synthesis to optimize biosynthetic pathways, discover new enzymes, and test new hypotheses. Because it is an open system, cell-free reactors can be readily interrogated for intermediate product formation, such as by the on-line, high speed LC/MS approaches used by Panke and colleagues for optimization of glycolysis in cell-free extracts 61 . Cell-free systems in tandem with high-end metabolomics could offer a high degree of flexibility to model the kinetics and stability of individual enzymes, measure metabolite fluxes in multistep pathways, and experimentally isolate many other parameters confounded in living organisms. This has potential to speed up metabolic engineering DBT cycles. 
     E. References for Example 1 
     
         
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         60. Hong, S. H. et al. Improving cell-free protein synthesis through genome engineering of  Escherichia coli  lacking release factor 1. Chembiochem 16, 844-853, doi: 10.1002/cbic.201402708 (2015). 
         61. Bujara, M., Schumperli, M., Pellaux, R., Heinemann, M. &amp; Panke, S. Optimization of a blueprint for in vitro glycolysis by metabolic real-time analysis. Nat Chem Biol 7, 271-277, doi:10.1038/nchembio.541 (2011). 
       
    
     F. Tables for Example 1 
       
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Strains and Plasmids. 
               
            
           
           
               
               
               
            
               
                 Name 
                 Genotype/relevant characteristics 
                 Source 
               
               
                   
               
               
                 Strains 
                   
                   
               
               
                 NEB Turbo ™ 
                 F′ proA+ B+ lacIq ΔlacZM15/fhuA2 Δ(lac-proAB) glnV 
                 New England 
               
               
                   
                 galK16 galE15 R(zgb-210::Tn10)TetS endA1 thi-1 Δ(hsdS- 
                 Biolabs 
               
               
                   
                 mcrB)5 
               
               
                 BL21 (DE3) 
                 fhuA2 [lon] ompT gal (λ DE3) [dcm] ΔhsdS λ DE3 = λ 
                 New England 
               
               
                   
                 sBamHIo ΔEcoRI-B int::(lacI::PlacUV5::T7 gene1) i21 Δnin5 
                 Biolabs 
               
               
                 Strains &amp; Plasmids 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-atoB 
                 AtoB 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-hbd1 
                 Hbd1 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-hbd2 
                 Hbd2 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-crt1 
                 Crt1 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-crt2 
                 Crt2 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-ter1 
                 Ter1 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-adhE1 
                 AdhE1 
               
               
                 BL21 (DE3) 
                 Strain used for protein production and extract preparation of 
                 This Study 
               
               
                 pETBCS-rbsU-adhE2 
                 AdhE2 
               
               
                 pJL1-atoB 
                 Plasmid used for CFPS containing atoB 
                 This Study 
               
               
                 pJL1-hbd2 
                 Plasmid used for CFPS containing hbd2 
                 This Study 
               
               
                 pJL1-crt1 
                 Plasmid used for CFPS containing crt1 
                 This Study 
               
               
                 pJL1-ter1 
                 Plasmid used for CFPS containing ter1 
                 This Study 
               
               
                 pJL1-adhE1 
                 Plasmid used for CFPS containing adhE1 
                 This Study 
               
               
                 pJL1-hbdcrt2 
                 Plasmid used for CFPS containing hbdcrt2 
                 This Study 
               
               
                 pJL1-hbdcrt3 
                 Plasmid used for CFPS containing hbdcrt3 
                 This Study 
               
               
                 pJL1-hbdcrt4 
                 Plasmid used for CFPS containing hbdcrt4 
                 This Study 
               
               
                 pJL1-hbdcrt6 
                 Plasmid used for CFPS containing hbdcrt6 
                 This Study 
               
               
                 pJL1-ter3 
                 Plasmid used for CFPS containing ter3 
                 This Study 
               
               
                 pJL1-ter4 
                 Plasmid used for CFPS containing ter4 
                 This Study 
               
               
                 pJL1-ter5 
                 Plasmid used for CFPS containing ter5 
                 This Study 
               
               
                 pJL1-ter6 
                 Plasmid used for CFPS containing ter6 
                 This Study 
               
               
                 pJL1-adhE9 
                 Plasmid used for CFPS containing adhE9 
                 This Study 
               
               
                 pJL1-adhE10 
                 Plasmid used for CFPS containing adhE10 
                 This Study 
               
               
                 pJL1-adhE13 
                 Plasmid used for CFPS containing adhE13 
                 This Study 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Genes and Enzymes. 
               
            
           
           
               
               
               
               
            
               
                 Gene/ 
                   
                   
                   
               
               
                 Enzyme 
                 Enyme Activity 
                 Source Organism 
                 Source Database 
               
               
                   
               
               
                 atoB 
                 acetyl-CoA 
                 
                   Escherichia coli 
                 
                 GenBank 
               
               
                   
                 acetyltransferase/thiolase 
               
               
                 hbd1 
                 acyl-CoA dehydrogenase 
                 
                   Clostridium Acetobutylicum 
                 
                 GenBank 
               
               
                 hbd2 
                 acyl-CoA dehydrogenase 
                 
                   Clostridium beijerinckii 
                 
                 GenBank 
               
               
                 crt1 
                 acyl-CoA dehydrogenase 
                 
                   Clostridium Acetobutylicum 
                 
                 GenBank 
               
               
                 crt2 
                 acyl-CoA dehydrogenase 
                 
                   Pseudomonas putida 
                 
                 GenBank 
               
               
                 hbdcrt2 
                 3-hydroxyacyl-CoA 
                 
                   Aeropyrum camini 
                 
                 NCBI BLAST Search of 
               
               
                   
                 dehydrogenase 
                   
                 ADHE2 protein from  C.   
               
               
                   
                   
                   
                 
                   acetobutyllicum 
                 
               
               
                 hbdcrt3 
                 3-hydroxyacyl-CoA 
                 
                   Pyrobaculum aerophilum 
                 
                 NCBI BLAST Search of 
               
               
                   
                 dehydrogenase 
                   
                 ADHE2 protein from  C.   
               
               
                   
                   
                   
                 
                   acetobutyllicum 
                 
               
               
                 hbdcrt4 
                 3-hydroxyacyl-CoA 
                 
                   Sulfolobus islandicus 
                 
                 NCBI BLAST Search of 
               
               
                   
                 dehydrogenase 
                   
                 ADHE2 protein from  C.   
               
               
                   
                   
                   
                 
                   acetobutyllicum 
                 
               
               
                 hbdcrt5 
                 3-hydroxyacyl-CoA 
                 
                   Vulcanisaeta distributa 
                 
                 NCBI BLAST Search of TER 
               
               
                   
                 dehydrogenase 
                   
                 protein from  T. denticola   
               
               
                 hbdcrt6 
                 3-hydroxybutyryl-CoA 
                 
                   Sulfolobus acidocaldarius 
                 
                 NCBI BLAST Search of TER 
               
               
                   
                 dehydrogenase 
                   
                 protein from  T. denticola   
               
               
                 ter1 
                 trans-2-enoyl-CoA 
                 
                   Treponema denticola 
                 
                 GenBank 
               
               
                   
                 reductase 
               
               
                 ter3 
                 trans-2-enoyl-CoA 
                 
                   Fibrobacter succinogenes 
                 
                 NCBI BLAST Search of TER 
               
               
                   
                 reductase 
                   
                 protein from  T. denticola   
               
               
                 ter4 
                 trans-2-enoyl-CoA 
                 
                   Flavobacterium johnsoniae 
                 
                 NCBI BLAST Search of TER 
               
               
                   
                 reductase 
                   
                 protein from  T. denticola   
               
               
                 ter5 
                 trans-2-enoyl-CoA 
                 
                   Spirochaeta 
                 
                 NCBI BLAST Search of TER 
               
               
                   
                 reductase 
                 
                   bajacaliforniensis 
                 
                 protein from  T. denticola   
               
               
                 ter6 
                 trans-2-enoyl-CoA 
                 
                   Cytophaga hutchinsonii 
                 
                 NCBI BLAST Search of 
               
               
                   
                 reductase 
                   
                 HBD-CRT protein from  M.   
               
               
                   
                   
                   
                 
                   sedula 
                 
               
               
                 adhE1 
                 bifunctional acetaldehyde- 
                 
                   Clostridium acetobutylicum 
                 
                 GenBank 
               
               
                   
                 CoA/alcohol 
               
               
                   
                 dehydrogenase 
               
               
                 adhE2 
                 bifunctional acetaldehyde- 
                 
                   Clostridium pasteurianum 
                 
                 NCBI BLAST Search of 
               
               
                   
                 CoA/alcohol 
                   
                 HBD-CRT protein from  M.   
               
               
                   
                 dehydrogenase 
                   
                 
                   sedula 
                 
               
               
                 adhE8 
                 bifunctional acetaldehyde- 
                 
                   Chitinivibrio alkaliphilus 
                 
                 NCBI BLAST Search of 
               
               
                   
                 CoA/alcohol 
                   
                 HBD-CRT protein from  M.   
               
               
                   
                 dehydrogenase 
                   
                 
                   sedula 
                 
               
               
                 adhE9 
                 bifunctional acetaldehyde- 
                   Thermosynechococcus  sp. 
                 NCBI BLAST Search of 
               
               
                   
                 CoA/alcohol 
                 NK55a 
                 HBD-CRT protein from  M.   
               
               
                   
                 dehydrogenase 
                   
                 
                   sedula 
                 
               
               
                 adhE10 
                 bifunctional acetaldehyde- 
                 
                   Providencia 
                 
                 NCBI BLAST Search of 
               
               
                   
                 CoA/alcohol 
                 
                   burhodogranariea 
                 
                 HBD-CRT protein from  M.   
               
               
                   
                 dehydrogenase 
                   
                 
                   sedula 
                 
               
               
                 adhE13 
                 bifunctional acetaldehyde- 
                 
                   Serratia marcescens 
                 
                 NCBI BLAST Search of 
               
               
                   
                 CoA/alcohol 
                   
                 HBD-CRT protein from  M.   
               
               
                   
                 dehydrogenase 
                   
                 
                   sedula 
                 
               
               
                   
               
            
           
         
       
     
     Example 2: Cell-Free Biosynthesis of the Nonribosomal Macrolactone Peptide Valinomycin 
     A. Abstract 
     Natural products are important because of their significant pharmaceutical properties such as antiviral, antimicrobial, and anticancer activity. Recent breakthroughs in DNA sequencing reveal that a great number of cryptic natural product biosynthetic gene clusters are encoded in microbial genomes, for example, those of  Streptomyces  species. However, it is still challenging to access compounds from these clusters because many source organisms are uncultivable or the genes are silent during laboratory cultivations. To address this challenge, here we construct an efficient cell-free platform for the rapid, in vitro total biosynthesis of the nonribosomal peptide valinomycin as a model. We achieve this goal in two ways. First, we used a cell-free protein synthesis (CFPS) system to express the entire valinomycin biosynthetic gene cluster (&gt;19 kb) in a single-pot reaction, giving rise to approximately 37 tg/L of valinomycin after optimization. Second, we coupled CFPS with cell-free metabolic engineering system by mixing two enzyme-enriched cell lysates to perform a two-stage biosynthesis. This strategy improved valinomycin production ˜5,000-fold to nearly 30 mg/L. We expect that cell-free biosynthetic systems will provide a new avenue to express, discover, and characterize natural product gene clusters of interest in vitro. 
     B. Introduction 
     Natural products originating from living organisms (e.g., microbes, plants, and animals) have complex chemical structures and exhibit diverse biological activities (1). Historically, natural products have served as a rich source for pharmaceuticals like antibiotics to treat human diseases (2). During the so-called ‘Golden Era’ of antibiotic discovery from the 1940s to 1960s, many notable antibiotics were discovered. However, declining rates of new antibiotic discovery and rising rates of re-discovery through conventional low-throughput fermentation and chemical/biological screening approaches have limited the field (3). As a result, the accelerating problem of antibiotic resistance is projected to soon threaten up to 10 million lives annually (4). This motivates the need for new approaches to discover new antibiotics and natural products for human health. 
     Several approaches are already emerging. In one approach, the data available to mine for new biosynthesis machinery is rapidly expanding. This expansion results from next-generation DNA sequencing and synthesis (5), improved bioinformatic tools for genome mining and prediction (6), high-throughput mass spectrometry for metabolite analysis (7), and CRISPR-based technology for activation of silent biosynthetic gene clusters (8). Unfortunately, the rate at which new biosynthetic pathways are identified greatly outpaces the capacity to characterize the small molecules for which production is encoded (9). Current approaches are constrained by limitations in cultivating natural product producing organisms in the laboratory. Moreover, establishing heterologous expression systems is difficult. While several heterologous hosts like  Escherichia coli  and  Saccharomyces cerevisiae  have been used for production and discovery of natural products (10-13), engineering these cells remains a time-consuming and laborious process that often requires design-build-test-learn iterations to obtain the optimal cell performance (14). In addition, yields of most compounds are still not satisfactory in heterologous hosts (11). Therefore, a new generation of heterologous expression systems for efficient, high-yielding production of bioactive molecules is highly desirable. 
     Recently, cell-free systems have emerged as a complementary platform for biomanufacturing (15-18), with potential for natural product biosynthesis. For example, in vitro reconstitution strategies based on purified enzymes (19) have elegantly demonstrated the biosynthesis of polyketides (PKs) (20), nonribosomal peptides (NRPs) (21), and PK/NRP hybrids (22). Yet despite its success, this strategy often suffers from laborious purification process and enzyme instability. To circumvent these shortcomings, we recently demonstrated the use of cell-free protein synthesis (CFPS) systems derived from crude cell lysates to synthesize a NRP molecule (diketopiperazine, DKP) by in vitro coexpression of two large nonribosomal peptide synthetases (NRPS), namely, GrsA (126 kDa) and GrsB1 (121 kDa, the first module of GrsB) (23). While this work represented the first step to apply crude extract-based CFPS to synthesize natural products, DKP is only a shunt product and not the final pathway biosynthesis product gramicidin S. Thus, to date, total biosynthesis of a natural product has not been shown with crude extract-based CFPS systems. Here, we aim to address this gap, using the nonribosomal peptide valinomycin as our model system. 
     Valinomycin is a 36-membered cyclododecadepsipeptide (see  FIG. 22A  for the chemical structure), which possesses a broad spectrum of bioactivities such as antifungal (24), antimicrobial (25), insecticidal (26), antiviral (27), and anticancer efficacy (28). Naturally, valinomycin is synthesized by several  Streptomyces  strains via the NRPS enzyme valinomycin synthetase (29-31). Valinomycin synthetase consists of two distinct large NRPSs (Vlm1 370 kDa and Vlm2 284 kDa), and each of them has two modules for the assembly of their dedicated substrates pyruvate, α-ketoisovalerate, and L-valine ( FIG. 22A ) (32). Our previous studies have achieved the heterologous biosynthesis of valinomycin in vivo by reconstitution of the NRPS genes (vlm1 and vlm2) in  E. coli  (32). The yield of valinomycin reached a milligram per liter level after a systematic bioprocess optimization (33, 34). In the valinomycin biosynthetic gene cluster, a discrete type II thioesterase (TEII) was also identified (29). TEII is a repairing enzyme that usually is associated with NRPS enzymes, playing a key role in regenerating the functionality of NRPS through hydrolysis of either misacylated thiol groups (35) or incorrectly loaded substrates on the thiolation (T) domains ( FIG. 22B ) (36). Notably, coexpression of the cognate protein TEII with Vlm1 and Vlm2 resulted in the highest production of valinomycin (13 mg/L) in  E. coli  (37), demonstrating the dedicated function of TEII in heterologous valinomycin biosynthesis. 
     In this work, we demonstrate that simple cell-free systems enable the total biosynthesis of a complex NRP valinomycin ( FIG. 22C ). We show this in two ways. First, we use an  E. coli -based CFPS system to coexpress two enzymes Vlm1 (370 kDa) and Vlm2 (284 kDa) for valinomycin synthesis. Second, cell lysates are enriched with Vlm1 or Vlm2 by cellular expression and then the full metabolic pathway is assembled in vitro to enable valinomycin biosynthesis by mixing two enzyme-enriched cell lysates. In both strategies, TEII substantially improves valinomycin production. Taken together, our current work emphasizes three advancements. First, to our best knowledge, this is the first report to use CFPS for the coexpression of such large NRPS enzymes with catalytic activity. Second, in vitro total biosynthesis of valinomycin showcases the feasibility to produce other complex NRPs with cell-free systems. Third, fine tuning of cell-free reactions allows for enhanced production of target molecules. Looking forward, we envision that cell-free biosynthetic systems will provide a new avenue for the expression of complex natural product gene clusters (e.g., NRPs and PKs), enabling the rapid discovery and synthesis of novel natural products in vitro 
     C. Results 
     1. Cell-free synthesis of active Vlm1 and Vlm2 for valinomycin biosynthesis 
     To establish an in vitro platform for valinomycin biosynthesis, we used an  E. coli -based CFPS system (38) to synthesize the NRPS valinomycin synthetase. The successful expression of the large enzymes Vlm1 (370 kDa) and Vlm2 (284 kDa) was achieved in our cell-free system as shown clearly by SDS-PAGE with correct molecular weight bands ( FIG. 23A ). Most importantly, both enzymes can be coexpressed solubly in a single-pot cell-free reaction, suggesting the potential of valinomycin formation in vitro. 
     Once we had successfully expressed the two large Vlm1 and Vlm2 in vitro, we next needed to convert the enzymes to their functional (holo), active form. To be functional the apo-T (thiolation) domains of NRPS enzymes have to be posttranslationally modified by transfer of a phosphopantetheine moiety from coenzyme A (CoA) to a conserved serine residue in the T domain (39). This modification, called priming, of apo-T domains to functional, holo-T domains is carried out by phosphopantetheinyl transferases (PPTases) and consequently leads to active holo-NRPS (40). To enable priming, we chose the promiscuous PPTase Sfp from  Bacillus subtilis  (41), which has been frequently utilized for the phosphopantetheinylation of heterologous NRPS, to activate our Vlm1 and Vlm2 in vitro. 
     Previously, we added purified Sfp to cell-free reactions to modify NRPS (GrsA and GrsB1) (23). In this work, to avoid the laborious purification process and to demonstrate the robustness of cell-free system, we directly coexpressed the sfp gene with Vlm1 and Vlm2 in a one-pot reaction. As shown in  FIG. 23B , all three proteins were coexpressed and easily visible on an SDS-PAGE gel. Notably, cell-free expressed Sfp was able to activate Vlm1 and Vlm2 for the biosynthesis of valinomycin, which was successfully detected by LC-MS from the in vitro reaction mixture ( FIG. 23C ). While valinomycin was formed, the yield of 9.76±0.23 μg/L was relatively low ( FIG. 23D ). We next sought to enhance valinomycin biosynthesis yields. 
     2. Enhancing Valinomycin Biosynthesis by Coexpression of Type II Thioesterase (TEII) 
     TEII is a discrete protein that often encoded within NRPS gene clusters, playing a repairing (editing) role by removal of nonreactive moieties and aberrant substrates that block the NRP assembly lines (42). In this way, TEII restores the activity of NRPS that allows for further rounds of product biosynthesis. Since coexpression of TEII with its related NRPS in host cells can be used to improve product yields (37), we wondered if a similar benefit would hold true in vitro. Thus, we set out to investigate the effect of TEII on valinomycin production in our cell-free system. To do this, we added the entire gene cluster of valinomycin (vlm1, vlm2, and TEII) as well as the sfp gene, each independently cloned on a single plasmid, to a one-pot CFPS reaction. We carried out the reaction in two steps. In step one, we expressed Vlm1, Vlm2, and Sfp together for an initial 6 h period, allowing for the expression and modification of Vlm1 and Vlm2. In step two we added the TEII plasmid to the cell-free mixture, and allowed the reaction to continue for an addition 14 h. After the reaction, all four proteins were easily visible on an SDS protein gel ( FIG. 23B ). This, to the best of our knowledge, is the first example of expressing a whole natural product gene cluster (Vlm1, Vlm2, and TEII) and a heterologous modification enzyme (Sfp) together using an  E. coli -based CFPS system. Furthermore, as we expected the valinomycin yield was significantly improved to 17.59±0.29 μg/L, which is nearly two times higher than that of the reaction without coexpression of TEII ( FIG. 23D ). The yield was further increased by two-fold up to 37.11±0.84 g/L when the whole reaction time was extended from 20 to 24 h ( FIG. 23D ). Taken together, our data demonstrate the robustness (i.e., coexpression of four proteins including two large NRPSs, ˜300 kDa) and flexibility (i.e., adding plasmids sequentially for timing tunability of gene expression) of the  E. coli  CFPS system for the rapid, total biosynthesis of complex natural products like NRPs in vitro. However, the yield of valinomycin was still about three orders of magnitude lower than previous reports in  E. coli  cells. 
     3. In Vitro Biosynthesis of Valinomycin Using Cell-Free Metabolic Engineering (CFME) 
     While total biosynthesis was achieved in the cell-free biosynthesis approach above, we hypothesized that the low valinomycin yields resulted from resource limitations (e.g., energy) caused by the extended reaction duration and expression of four enzymes, especially the two large NRPSs (Vlm1 and Vlm2). In an attempt to bypass such a constraint, we next switched the expression of Vlm1 and Vlm2 from in vitro to in vivo. The key idea was to maximally express proteins in cells, and then mix lysates enriched with these enzymes to increase production of valinomycin in a setting without growth constraints ( FIG. 22C ). We previously used such an approach, which is called cell-free metabolic engineering (CFME), to efficiently synthesize small molecules (e.g., n-butanol, 2,3-butanediol) (43-46). For example, glycolysis in crude  E. coli  lysates powered the production of butanediol with near theoretical yields, high titers (&gt;80 g/L), and high productivities (&gt;10 g/L/h). 
     To demonstrate CFME synthesis of valinomycin, we first introduced genes encoding Vlm1 and Vlm2 individually into the source strain  E. coli  BAP1, which has the sfp gene chromosomally integrated (47). After heterologous expression of each enzyme in vivo, we lysed cell pellets to generate Vlm1 and Vlm2 enriched cell lysates, respectively. As expected, we observed that both enzymes were overexpressed in  E. coli  BAP1 as can be seen on the SDS-PAGE gel ( FIG. 24A ). In this case, Vlm1 and Vlm2 should be modified by Sfp that is encoded in the genome of BAP1 to be in their active holo form. We next initiated in vitro valinomycin biosynthesis by directly mixing two cell lysates (Vlm1 and Vlm2), acetate salts (Mg 2+ , NH 4   + , and K + ), cofactors (CoA, NAD, and ATP), and other components. The CFME reaction was carried out for 12 h at 30° C. and valinomycin was analyzed by LC-MS. However, the yield of 5.59±0.60 μg/L was unexpectedly low ( FIG. 24B ). We, therefore, next set out to optimize the CFME system for enhanced synthesis of valinomycin. 
     We began our optimization by investigating the effect of supplemental cofactors (CoA, NAD, and ATP) on valinomycin formation. Previous studies suggested that cofactors may play a positive role in the performance of cell-free metabolic pathways (43, 44). When the concentration of CoA, NAD, and ATP were reduced from initial 1 mM to 0.1 mM in our CFME system, the yield of valinomycin was increased by 1.5 times up to 8.77±1.37 μg/L ( FIG. 25A ). Then, CoA, NAD, and ATP (each at 0.1 mM) were removed one-at-a-time, two-at-a-time, and altogether from CFME reactions. We found the reaction without any supplementary cofactors produced the highest valinomycin of 19.99±1.30 μg/L, which is nearly 4-fold higher than the initial yield ( FIG. 25A ). Our data are in agreement with previous reports (45, 46) that cofactors from the cell lysate are sufficient to drive product synthesis, in our case, valinomycin. Therefore, we omitted costly cofactors from our CFME system in the following investigations. 
     A key conceptual shift of our approach is that the design element is a lysate rather than a gene. Once in hand, selectively enriched lysates can be mixed in different ratios to optimize the pathway as this will impact the enzyme concentration. In each 25 μL CFME reaction, we added two cell lysates with a total protein mass of 35 mg/mL. Since Vlm1 and Vlm2 were overexpressed in a similar level ( FIG. 24A ), we initially added two lysates at a mass ratio of 1:1, generating approximately 20 μg/L valinomycin ( FIG. 25B ). We next varied the mass ratio of the two lysates to find beneficial enzyme proportions. As shown in  FIG. 25B , valinomycin formation in CFME reactions was notably impacted by the mass ratio of cell lysates. It is clear that more Vlm1 enriched cell lysates in the reaction produced more valinomycin with the highest yield of 76.9±3.94 μg/L at a mass ratio of 3:1 (cell lysate-Vlm1:cell lysate-Vlm2). By contrast, valinomycin synthesis was almost abolished when the ratio was 1:3. The results demonstrate that the steps of valinomycin biosynthesis catalyzed by Vlm1 might be rate-limiting. 
     4. CFPS of TEII Improves Valinomycin Biosynthesis in Coupling with CFME 
     We next wondered if, as before, addition of TEII would enhance valinomycin yields. We therefore next used a coupled CFPS-CFME (CFPS-ME) system to perform a two-phase biosynthesis ( FIG. 26A ), whereby the repairing enzyme TEII was expressed by CFPS in the first reaction phase, which could be used to, as shown previously, regenerate the activity of Vlm1 and Vlm2 during the second CFME phase. We initially expressed TEII for 3 h and then added glucose (200 mM) to fuel the CFME process for another 12 h. With this strategy, the yield of valinomycin was dramatically improved to 29.32±1.37 mg/L ( FIG. 26A ), which is more than 5200 times higher than the initial CFME yield of 5.59±0.60 μg/L. This result demonstrates that (i) a dedicated enzyme can be expressed by the upstream CFPS to efficiently drive the downstream CFME reactions; (ii) in vitro expressed TEII is active to restore the function of in vivo heterologously expressed, but misprimed NRPS enzymes (Vlm1 and Vlm2); and (iii) the CFPS-ME system is robust for the synthesis of complex natural product (valinomycin) from a simple, cheap precursor glucose. Further increase of the TEII expression time (&gt;6 h) reduced the product yields ( FIG. 26A ), perhaps as a result of consuming more resources for TEII synthesis in the batch reaction that led to the decrease of valinomycin formation. 
     To fuel the reaction, glucose is converted to pyruvate in glycolysis by enzymes in the extract, which is further converted in the branched chain amino acid L-valine biosynthetic pathway to α-ketoisovalerate (α-Kiv) and L-valine (32)) ( FIG. 26B ). To further enhance valinomycin synthesis yields, we next assessed the impact of glucose concentration on valinomycin synthesis, noting that glucose is the key reaction substrate. Specifically, we tested starting glucose concentrations from 0-800 mM. We observed that the product yield was substantially enhanced from 1.3±0.10 mg/L without feeding glucose to 19.6±0.33 mg/L with addition of 50 mM glucose. The yield reached the highest of approximately 30 mg/L at the glucose concentration of 200 mM. When glucose concentrations were increased to 400-800 mM, valinomycin yields were slightly reduced to a stable level of around 25 mg/L. Overall, our data demonstrate the ability to use cheap glucose to fuel a highly active and productive natural product biosynthetic pathway that is constructed by CFPS and CFME in vitro. 
     D. Discussion 
     Natural products play, and will continue to play, a significant role in the drug discovery engine (1-3). DNA sequencing and genome mining have revealed that a far greater number of natural product biosynthetic gene clusters exist, but their products remain unknown (9). To access and harness these natural products, rapid and robust approaches need to be developed for their synthesis. In this report, we show that cell-free biology is one such approach. This approach shifts the design-build-test unit from a genetic construct and its associated cell-line to a cell-free lysate. The utility of our cell-free approach is demonstrated by the ability to enable total in vitro synthesis of the nonribosomal macrolactone peptide valinomycin in a single CFPS system and a coupled CFPS-ME system. 
     Our results have several key features. First, the use of cell-free systems allows for fine tuning of reaction conditions, easy monitoring, and multiple optimization. In recent years, cell-free systems have been utilized not only for the synthesis of various proteins (18), but also for the rapid and high-throughput prototyping of metabolic pathways (43, 48). Despite the wide applications of cell-free systems, total cell-free biosynthesis of both the enzymes and their natural product has not previously been reported. We show, somewhat surprisingly, that cell-free systems are capable large NRPS synthesis. Indeed, we succeeded in cell-free expression of the valinomycin biosynthetic gene cluster (&gt;19 kb), which contains two large NRPSs (Vlm1, 370 kDa and Vlm2, 284 kDa) and one associate repairing enzyme TEII, as well as a heterologous modification enzyme (Sfp). Notably, four enzymes were actively coexpressed in a single-pot CFPS reaction, giving rise to valinomycin formation with a yield of nearly 40 μg/L ( FIG. 23D ). 
     Second, cell-free systems enable high-level titers of natural products. We were able to leverage the high degree of flexibility of cell-free system to adjust the way enzymes were enriched in lysates (shifting from cell-free protein synthesis to heterologous overexpression) to facilitate increased natural product titers. Additionally, we could readily tune expression of the TEII enzyme to restore the activity of Vlm1 and Vlm2, as well as increase metabolic flux into pathway precursors (e.g., pyruvate, α-Kiv, and L-Val) by the addition of increased concentrations of glucose (200 mM). Of note, the optimized, coupled CFPS-ME system enabled synthesis of ˜30 mg/L valinomycin, which compares favorably to the best previous reports. Several native  Streptomyces  organisms produce 4 to 32 mg/L (31) and a heterologous  E. coli  host was shown to make 13 mg/L (37). However, there may be situations where  E. coli -based systems are not the best suited for expressing natural product biosynthetic gene clusters. To this point, new CFPS systems have recently been developed from  Streptomyces  species and others that have shown benefits for the soluble expression of high GC-content genes, which are often involved in natural product biosynthetic gene clusters (49, 50). 
     Third, our cell-free approach is fast. It requires only hours to obtain a target compound, whereas days or weeks may otherwise be needed to cultivate strains for product production. With the price of DNA synthesis declining and assembly of large gene clusters increasing, we anticipate that the ability to directly input DNA to cell-free systems for accessing biosynthetic pathways from clusters assembled using metagenomics for uncultivable organisms could enable new approaches for discovering and studying natural products. In addition, CFPS platforms are shown to work with linear PCR templates (51-53), highlighting the efficiency of cell-free systems for future high-throughput synthesis without laborious cloning work. As such, cell-free biosynthesis system offers a rapid and cost-effective way to synthesize molecules of interest. 
     In summary, this work, for the first time, demonstrates in vitro total biosynthesis of a complex NRP valinomyicn by cell-free expression of the entire gene cluster. Given the robustness and flexibility of cell-free systems, the reactions can be rationally optimized to increase the biosynthesis performance and thus give rise in high productivity. Looking forward, we believe that cell-free systems hold great potential to create an easy-to-use platform for studying and engineering natural product pathways. 
     E. Materials and Methods 
     1. Bacterial Strains and Plasmids 
       E. coli  BL21 Star (DE3) was used for preparation of cell extracts to perform CFPS reactions. The strain  E. coli  BAP1, a generous gift from Prof. Yong Wang (Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China), was used to overexpress Vlm1 and Vlm2 in vivo.  E. coli  BAP1 is a derivative of  E. coli  BL21 (DE3) with genomic integration of the sfp gene for posttranslational phosphopantetheinylation of NRPSs (47). Plasmids pCTUT7-Vlm1 and pKS01-Vlm2 harboring genes vlm1 (˜10 kb) and vlm2 (˜8 kb) (32), respectively, were kindly provided by Prof. Peter Neubauer (Technische Universitat Berlin, Germany). Cell-free expression of Sfp and TEII were achieved with plasmids pET28a-Sfp (23) and pJL1-TEII (49), respectively. 
     2. Preparation of Cell Extracts 
     All  E. coli  strains were grown in 2×YTPG medium, consisting of (per liter) 10 g yeast extract, 16 g tryptone, 5 g NaCl, 7 g K 2 HPO 4 , 3 g KH 2 PO 4 , and 18 g glucose. Cultivations of  E. coli  BL21 Star (DE3) were performed in 1 L of 2×YTPG in 2.5 L baffled Ultra Yield™ flasks (Thomson Instrument Company, USA). After inoculation (initial OD 600  of 0.05), cultures were incubated in the shaker at 220 rpm and 34° C. When OD 600  reached 0.6-0.8, cells were induced with 1 mM isopropyl-β-D-thiogalactopyranoside (IPTG) to express T7 RNA polymerase. Then, cells were grown until an OD 600  of 3.0 and harvested by centrifugation at 5,000 g and 4° C. for 15 min. Afterwards, cell pellets were washed three times with cold S30 Buffer (10 mM Tris-acetate, 14 mM magnesium acetate, 60 mM potassium acetate, and 2 mM dithiothreitol (DTT)). After the final wash and centrifugation, the pelleted cells were resuspended in S30 Buffer (1 mL per gram of wet cell mass) and lysed by sonication (10 s on/off, 50% of amplitude, input energy ˜600 Joules). The lysate was then centrifuged twice at 12,000 g and 4° C. for 10 min. The resulting supernatant was flash frozen in liquid nitrogen and stored at −80° C. until use. 
     To prepare enzyme-enriched lysates, two strains of  E. coli  BAP1 containing the plasmid pCTUT7-Vlm1 and pKS01-Vlm2, respectively, were grown in 1 L of 2×YTPG and incubated at 30° C. and 250 rpm. At OD 600  of 0.6-0.8, the cultures were induced with 20 μM IPTG, followed by 5 h cultivation for Vlm1 and Vlm2 expression. Afterwards, cell collection, disruption, and extracts were prepared the same as described above. The total amount of protein in cell extracts was quantified by the Quick-Start Bradford Protein Assay Kit (Bio-Rad). Typically, the total protein concentration of cell extracts was 30-40 mg/mL. Overexpression of Vlm1 and Vlm2 was confirmed by SDS-PAGE protein gels. 
     3. CFPS Reactions 
     CFPS reactions were performed at a total volume of 15 μL in 1.5 mL microcentrifuge tubes. Each reaction mixture contained the following components: 12 mM magnesium glutamate, 10 mM ammonium glutamate, 130 mM potassium glutamate, 1.2 mM ATP, 0.85 mM each of GTP, UTP, and CTP, 34 μg/mL folinic acid, 170 μg/mL of  E. coli  tRNA mixture, 2 mM each of 20 standard amino acids, 0.33 mM nicotinamide adenine dinucleotide (NAD), 0.27 mM coenzyme A (CoA), 1.5 mM spermidine, 1 mM putrescine, 4 mM sodium oxalate, 33 mM phosphoenolpyruvate (PEP), appropriate plasmids (see below), and 27% (v/v) of cell extract. In all 15 μL CFPS reactions, each plasmid was added individually or together as follows: 200 ng pCTUT7-Vlm1, 200 ng pKS01-Vlm2, 100 ng pET28a-Sfp, and 200 ng pJL1-TEII. In the case of coexpression of four proteins, pCTUT7-Vlm1, pKS01-Vlm2, and pET28a-Sfp were added together at the beginning for 6 h expression. Then, pJL1-TEII was added to the cell-free mixture for another 14 h reaction unless otherwise noted. Other CFPS reactions were incubated at 30° C. for 20 h. All cell-free expressed proteins were analyzed by SDS-PAGE gels. 
     4. CFME Reactions 
     Standard CFME reactions were carried out in 1.5 mL microcentrifuge tubes at 30° C. for 12 h in 25 μL volumes. Reaction components consisted of 8 mM magnesium acetate, 10 mM ammonium acetate, 134 mM potassium acetate, 200 mM glucose, 10 mM dipotassium phosphate (pH 7.2), 1 mM NAD, 1 mM ATP, 1 mM CoA, and 35 mg/mL total protein. The total protein concentration was maintained by adjusting the mass ratio of two lysates (Vlm1 and Vlm2). 
     5. CFPS-ME Reactions 
     CFPS synthesis of TEII was scaled up from 15 μL to a 25 μL reaction system. All reagent concentrations were the same as described in the section of “CFPS reactions”, except the plasmid (pJL1-TEII, 13.3 pg/mL) and cell extract (35 mg/mL, total protein of two mixed lysates). Unless otherwise noted, cell-free expression of TEII was performed at 30° C. for 3 h, followed by spiking in glucose (200 mM) for valinomycin biosynthesis for another 12 h. 
     6. Valinomycin Extraction and Quantification 
     Valinomycin was extracted with three-fold volumes of ethyl acetate from cell-free reactions. After centrifugation at 16,000 g for 5 min, the organic fraction was transferred to a fresh 1.5 mL microcentrifuge tube, air dried, and resuspended in methanol for valinomycin analysis. Valinomycin quantification was performed with an Agilent 6470 Triple Quadrupole LC/MS System equipped with an Eclipse Plus C18 column (2.1×50 mm, 1.8 μm). For valinomycin detection, 2 μL of each sample was injected and separated at a flow rate of 0.4 mL/min with elution buffers A (water+0.1% formic acid) and B (acetonitrile+0.1% formic acid) through a linear gradient elution from 80 to 100% B over 2.5 min, a 100% B wash for 7.5 min, and a post time wash for 10 min with 80% B. Valinomycin concentrations were calculated according to a calibration curve prepared with commercial valinomycin (Sigma) as a standard. All measurements were performed in triplicate. 
     F. References for Example 2 
     
         
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     Example 3—Cell-Free Biosynthesis of Andrimid 
     A. Abstract 
     Our platform&#39;s use of newly developed cell-free technologies overcomes many of the challenges of in vivo heterologous expression of natural products including cytotoxicity, silent gene clusters, and low yields. In our approach, the biosynthesis “units” are cell-free lysates or CFPS reactions rather than genetic constructs in live cells, which we can combinatorially assemble into desired pathways by mixing cell specific combinations of lysates enriched with pathway enzymes (1, 2) ( FIG. 27 ). We can mix lysates or CFPS reactions from different strains and different organisms, including for example high natural producers, to optimize the overall capabilities. A key advantage of this approach is we can build a library of these cell-free “units” only once but then can rapidly resample these many times using a DBT approach in different experimental designs to prepare multi-lysate cocktails with varying enzyme combinations and stoichiometry to build a pathway or generate variants, drastically lowering experimental costs for pathway discovery and optimization. 
     B. In Vitro Reconstruction of Andrimid Biosynthesis Directly from DNA Using Cell-Free Systems 
     In unpublished work, we have pursued the synthesis of andrimid ( FIG. 28A ), a broad-spectrum antibiotic (3). We considered the andrimid pathway because of the large number of genes involved and transcription of the native gene cluster is highly regulated. To enable cell-free biosynthesis of andrimid, we first expressed all of the 21 genes of the pathway each in multiple cell-free reactions in  E. coli , as detailed in the previous examples, and in the examples to follow ( FIG. 28B ). Then, we combined 21 enriched lysates with equal molar concentrations of each enzyme, supplemented the mixture with the necessary cofactors and substrates to convert malonyl-CoA to andrimid and observed biosynthesis by LC-MS ( FIG. 28C ). This work validates our unit operations framework to construct pathways to novel antibiotics. 
     C. References for Example 3 
     
         
         1. Dudley Q M, Anderson K C, Jewett M C. Cell-Free Mixing of  Escherichia coli  Crude Extracts to Prototype and Rationally Engineer High-Titer Mevalonate Synthesis. ACS synthetic biology. 2016; 5(12):1578-88. doi: 10.1021/acssynbio.6b00154. 
         2. Karim A S, Jewett M C. A cell-free framework for rapid biosynthetic pathway prototyping and enzyme discovery. Metabolic engineering. 2016; 36:116-26. doi: 10.1016/j.ymben.2016.03.002. 
         3. Jin M, Fischbach M A, Clardy J. A biosynthetic gene cluster for the acetyl-CoA carboxylase inhibitor andrimid. J Am Chem Soc. 2006; 128(33): 10660-1. doi: 10.1021/ja063194c. PubMed PMID: 16910643; PubMed Central PMCID: PMCPMC2529255. 
       
    
     Example 4—Cell-Free Synthesis of Enterobactin 
     A. Abstract 
     We studied cell-free production of enterobactin, a siderophore naturally produced by  E. coli  in iron limited environments but not in cell-free extracts. This pathway involves six proteins EntA-F to create the cyclic enterobactin ( FIG. 29A ). As described in the previous Examples and in the Examples that follow, we first expressed each of the six enzymes via CFPS and were able to produce near gram per liter quantities for many of these proteins ( FIG. 29B ). From here we validated the activity of the last steps of the pathway requiring EntB, EntE, and EntF (see  FIG. 29A ). We produced each of these enzymes separately by CFPS as describe previously, mixed them under different physiochemical conditions, and supplied dihydroxybenzoate (DHB) to initiate the reaction. After 20 h of incubation at 30° C. we observed enterobactin and DHB by LC-MS. Accordingly, enterobactin can be made in cell-free systems from in vitro-expressed EntB, EntE, and EntF ( FIG. 29C-D ), with yields in the range of ˜0.1 μg/L. 
     Example 5—Cell-Free Biosynthesis of Styrene 
     A. Abstract 
     Styrene is an important petroleum-derived molecule that is polymerized to make versatile plastics, including disposable silverware and foamed packaging materials. Finding more sustainable methods, such as biosynthesis, for producing styrene is essential due to the increasing severity of climate change as well as the limited supply of fossil fuels. Recent metabolic engineering efforts have enabled the biological production of styrene in  Escherichia coli , but styrene toxicity and volatility greatly limit biosynthesis in cells. To address these limitations, we have developed a cell-free styrene biosynthesis platform with the potential for biomanufacturing. Utilizing a cell-free system provides an open reaction environment without cell viability constraints, which allows exquisite control over reaction conditions and greater carbon flux toward product formation rather than cell growth. The two biosynthetic enzymes required for styrene production were generated via cell-free protein synthesis and mixed in defined ratios with supplemented L-phenylalanine and buffer. By altering the time, temperature, pH, and enzyme concentrations in the reaction, this approach increased the cell-free titer of styrene from 5.36 mM to 40.33 mM, an order of magnitude greater than cellular synthesis methods. Cell-free systems offer a complimentary approach to cellular synthesis of small molecules, which can provide particular benefits for producing toxic molecules. 
     B. Introduction 
     Metabolic engineering has enabled the production of commodity chemicals and valuable small molecules by genetically modifying microorganisms and overexpressing heterologous enzymes (Keasling, 2010; Nielsen, 2001; Stephanopoulos, 1994; Tyo et al., 2007; Tyo et al., 2010). Target biochemicals, such as butanol (Shen and Liao, 2008) and mevalonate (Martin et al., 2003), are often selected based on their utility for society. An important large-volume commodity chemical is styrene, which is produced globally on the scale of 30 million tons per year; 60% of the product is utilized for molded or foamed polystyrene and the remainder contributes to industrially important copolymers, such as styrene-acrylonitrile and styrene-butadiene (James and Castor, 2011). However, styrene production is an entirely petroleum-derived process that requires large excesses of steam and is responsible for over 100 million tons of greenhouse gas emissions each year (Wu et al., 1981; Zheng and Suh, 2019). Through metabolic engineering of  Escherichia coli , biosynthesis of styrene from glucose via the shikimate pathway was demonstrated as a potential, sustainable alternative to traditional styrene synthesis, albeit at low titers up to 2.59 mM styrene (McKenna and Nielsen, 2011). Recent efforts utilized genome editing and solvent extraction techniques to increase styrene titers in  E. coli  cultures, but the maximum concentration only reached 3.36 mM (Liu et al., 2018). Clearly, the cellular toxicity of styrene greatly limits biosynthesis titers and the feasibility of commercial styrene synthesis (Araya et al., 2000), so circumventing cellular toxicity could prove useful for the biochemical production of styrene. 
     Recent advances in cell-free technologies have showcased their utility for studying biological processes and engineering biological systems (Garenne and Noireaux, 2019; Jaroentomeechai et al., 2018; Martin et al., 2018; Silverman et al., 2019). For example, several studies have shown that crude extracts contain native metabolic enzymes and cofactor regeneration responsible for robust cell-free protein synthesis (Caschera and Noireaux, 2014; Jewett et al., 2008; Jewett and Swartz, 2004) and activation of key metabolic reactions in the cell-free environment (Dudley et al., 2015; Jewett and Swartz, 2004; Karim et al., 2018). In fact, cell-free systems have been used for biosynthesis of a wide variety of molecules, including 2,3-butanediol (Kay and Jewett, 2015), mevalonate (Dudley et al., 2016), n-butanol (Karim and Jewett, 2016), terpenes (Dudley et al., 2019; Korman et al., 2017), and polyhydroxyalkanoates (Kelwick et al., 2018). Cell-free systems provide an open reaction environment and rapid design-build-test cycles to reconstitute biosynthetic pathways in vitro to compliment and inform metabolic engineering efforts in cells (Bundy et al., 2018; Dudley et al., 2015; Gregorio et al., 2019; Hodgman and Jewett, 2012; Karim and Jewett, 2016). More importantly, cell-free systems have shown improved tolerance to toxic small molecules compared to living systems (Kay and Jewett, 2019), providing evidence that cell-free biomanufacturing platforms may be advantageous when cellular systems prove impractical. 
     In this work, we established a cell-free platform for styrene biosynthesis to increase the achievable titer in biological systems by circumventing the toxicity limits of styrene in vivo. We constructed this system in two parts. First, we used cell-free protein synthesis in  E. coli  crude extracts to express the two non-native enzymes required to convert L-phenylalanine (L-Phe) to styrene: phenylalanine ammonia lyase 2 (PAL2) from  Arabidopsis thaliana  and ferulic acid decarboxylase 1 (FDC1) from  Saccharomyces cerevisiae  (Liu et al., 2018; McKenna and Nielsen, 2011). Next, we combined these extracts enriched with biosynthetic enzymes with L-Phe and buffer to produce styrene in vitro ( FIG. 30 ). We further optimized the cell-free system by tuning enzyme ratios, reaction temperature, and reaction pH of these reactions to reach styrene titers over 40 mM, an order of magnitude greater than previous cellular efforts. We anticipate this work will expand the application space of cell-free systems and spur new research efforts in the metabolic engineering of toxic chemicals. 
     C. Materials and Methods 
     1. Bacterial Strains and Plasmids 
     Plasmid propagation was performed in  E. coli  DH5α (NEB), and in vitro enzyme expression was performed in cell extract prepared from  E. coli  BL21 Star (DE3) (Life Technologies). Plasmid pSpal2At, containing the PAL2 gene from  Arabadopsis thanliana , was a gift from David Nielsen (Addgene plasmid 78286). This gene was cloned into the pJL1 plasmid (Addgene plasmid 69496) using Gibson Assembly after PCR amplification with oligonucleotides from IDT (5′-tttaagaaggagatatacatATGGATCAAATCGAAGCAATG-3′ (SEQ ID NO: 102) and 5′-tttgttagcagccggtcgacTTAGCAAATCGGAATCGG-3′ (SEQ ID NO:103)). The FDC1 gene from  Saccharomyces cerevisiae  was synthesized and cloned into the pJL1 plasmid for expression by Twist Biosciences. Sequences of the PAL2 gene (SEQ ID NO:1) and FDC1 gene (SEQ ID NO:2)_are provided in the Table of  FIG. 39 . Propagated plasmids were purified using the ZymoPURE Plasmid Miniprep Kit (Zymo Research). 
     2. Cell Extract Preparation 
       E. coli  extracts were prepared as previously described. (Karim and Jewett, 2018; Kwon and Jewett, 2015) In brief, BL21 Star (DE3) cells (Life Technologies) grown in 1 L of 2×YTPG media in full-baffle shake flasks at 37° C. At an OD 600  of 0.4, 1 mM of IPTG was added to induce T7 RNA polymerase production. Cells were harvested at an OD 600  of 3.0. Cells were pelleted via centrifugation at 5,000 g for 10 minutes at 4° C., washed three times with cold S30 buffer (10 mM tris acetate, pH 8.2; 14 mM magnesium acetate; 60 mM potassium acetate; and 1 mM dithiothreitol), flash-frozen with liquid nitrogen, and stored at −80° C. For lysis, cells were thawed on ice and resuspended in 1 mL of S30 buffer per gram wet cell mass and then lysed in an EmulsiFlex-B15 homogenizer (Avestin) in a single pass at a pressure of 20,000-25,000 psi. Cellular debris was removed by two rounds of centrifugation at 12,000 g for 30 minutes at 4° C., and the final supernatant was flash-frozen with liquid nitrogen and stored at −80° C. until use. 
     3. Cell-Free Protein Synthesis (CFPS) Reactions 
     CFPS reactions for in vitro production of enzymes were assembled with 6 nM template DNA, 10 mg/mL  E. coli  extract, and the cofactors and crowding agents in 57 mM HEPES buffer. These reactions contained 8 mM magnesium glutamate; 10 mM ammonium glutamate; 130 mM potassium glutamate; 1.2 mM adenosine triphosphate; 0.85 mM each of guanosine, uridine, and cytidine triphosphates; 0.034 mg/mL folinic acid; 0.171 mg/mL transfer RNAs; 33.33 mM phosphoenolpyruvate; 2 mM of all 20 canonical amino acids; 0.40 mM nicotinamide adenine dinucleotide; 0.27 mM cofactor A; 1 mM putrescine; 1.5 mM spermidine. (Jewett and Swartz, 2004) The expression level of each enzyme was quantified using radioactive leucine incorporation assays as previously described. (Jewett et al., 2008) All reagents and chemicals were purchased from Sigma-Aldrich unless otherwise specified. 
     4. Cell-Free Metabolic Engineering (CFME) Reactions 
     Styrene biosynthesis reactions contained 8 mM magnesium glutamate, 10 mM ammonium glutamate, 134 mM potassium glutamate, 100 mM BisTris buffer, 0.5 mM kanamycin, varying concentrations of PAL2 and FDC1 enzymes from CFPS ranging from 0.05 to 1 μM, and 25 or 50 mM L-Phe. A layer of dodecane was placed atop the reaction to capture volatile styrene. (Dudley et al., 2019) 
     5. Metabolite Analysis 
     Styrene was quantified by diluting 20 μL of dodecane overlay into 200 μL of ethyl acetate containing 0.5 mM trans-caryophyllene (Sigma) as an internal standard. 1 L of this mixture was injected into an Agilent 7890A Gas Chromatograph with 5977A MSD (Agilent, Santa Clara, Calif.) using an Agilent HP-5MS (30 m length×0.25 mm i.d.×0.25 m film) column with helium carrier gas at constant flow of 1 mL-min-1. The inlet temperature was 70° C. and initial column temperature held at 70° C. for 1 minute, increased at 25° C. min-1 to 250° C., and maintained at 250° C. for 3 minutes. The injection volume was 1 L with a split ratio of 20:1. Extracted ion chromatograms (EIC) for 104 m/z (styrene, peak at 2.97 min) and 133 m/z (caryophyllene, peak at 6.34 min) were integrated using Agilent MassHunter Quantitation Analysis software. Concentrations were determined by use of a standard curve ( FIG. 34 ) generated by comparison to styrene (Sigma) standards mixed in dodecane with mock cell-free reactions containing green fluorescent protein in place of the biosynthetic enzymes that were incubated for 24 hours (Dudley et al., 2019). 
     6. pH Measurements 
     Samples were analyzed with a Thermo Scientific™ Orion™ ROSS Ultra™ Refillable pH/ATC Triode™. Reactions for which a pH was set prior to reaction start were measured with a mixture of all components except the enzyme-enriched CFPS reactions to avoid premature reaction initiation. Reaction pH was adjusted with glacial acetic acid or 5 N KOH as necessary. Measurements of pH over time were taken after sampling reactions for metabolite analysis. 
     7. Metabolite Quantification by HPLC 
     Reactions were quenched by adding 10% w/v tricholoacetic acid (Sigma) in a 1:1 ratio. Precipitated proteins were pelleted by centrifugation at 21,000 g for 10 minutes at 4° C., and 20 L of the supernatant was removed for analysis by high-performance liquid chromatography (HPLC). L-Phe was measured with an Agilent 1260 series HPLC system (Agilent, Santa Clara, Calif.) via a refractive index (RI) detector after passing through a reverse-phase Hypersil Gold column (4.6 mm×150 mm; Thermo Fisher, USA). 5 L from the prepared samples were injected at a total constant flow rate of 1 mL/min and temperature of 45° C. The column was operated with water (solvent 1) and methanol plus 0.1% trifluoroacetic acid (solvent 2). The eluent began as a mixture of 95% solvent 1 and 5% solvent 2 before a linear gradient was applied over 8 minutes to reach a mixture of 20% solvent 1 and 80% solvent 2. This composition was held constant for 2 minutes before a second linear gradient was applied over the course of 4 minutes to return to its initial composition of 95% solvent 1 and 5% solvent 2. 
     D. Results &amp; Discussion 
     To establish a cell-free platform for styrene biosynthesis, we took a two-pronged approach: first establishing enzyme synthesis and pathway assembly, and then optimizing physiochemical conditions for improved production. 
     1. Enzyme Synthesis and Styrene Pathway Assembly 
     We first demonstrated the ability to express functional enzymes for this biosynthetic pathway in vitro and to reliably capture the volatile styrene product. Cell-free protein synthesis (CFPS) enables rapid production of the enzymes for styrene biosynthesis, PAL2 and FDC1 ( FIG. 31A-B ). Using CFPS, we produced 4.99±0.36 μM soluble PAL2 and 5.93±0.47 μM soluble FDC1 over the course of a 20-hour reaction at 30° C. In an attempt to express greater soluble fractions of PAL2 and FDC1, we decreased the temperature of the reactions from the standard 30° C. to 16° C. The decreased temperature increased enzyme solubility at 6 h from ˜70% to ˜80% for PAL2 and from ˜60% to ˜67% for FDC1. For both temperatures, a majority (˜80%) of the soluble protein made during the reaction is produced by 6 h. Therefore, we chose to run CFPS reactions at 16° C. and stop reactions at 6 h to accelerate the workflow while still obtaining sufficient concentrations of soluble enzymes for all subsequent reactions. 
     To test for activity, we ran several reactions measuring phenylalanine conversion and styrene production. We confirmed that cell-free expressed PAL2 alone converts L-Phe to trans-cinnamic acid ( FIG. 35A ) and PAL2 combined with FDC1 produces styrene ( FIG. 35B ) as determined by HPLC. Additionally, no styrene is produced in cell-free reactions lacking the exogenous enzymes and supplemental L-Phe ( FIG. 36 ). Accurate quantification of styrene can be difficult due to its volatility and lack of solubility in aqueous media; therefore, we ran our reactions with a dodecane overlay to capture styrene and detect it by GC-MS. This approach is often used to extract volatile compounds from both in vivo (Liu et al., 2018) and in vitro (Dudley et al., 2019) systems. We found that larger ratios of overlay to reaction volume enabled greater styrene recovery relative to the trans-caryophyllene internal standard without inhibiting biosynthesis ( FIG. 37 ). 
     After demonstrating enzyme activity and the ability to measure styrene, we investigated the best temperature during the biosynthesis segment of the reaction for producing styrene. After cell-free expression of PAL2 and FDC1, we mixed the enzymes in a second pot reaction at a final concentration of 0.5 μM each. We then incubated reactions containing 25 mM L-Phe at 16, 22, 30, and 37° C. ( FIG. 31C ). The rate of styrene production was highest at 30° C. and produced a maximum titer of 5.36 mM styrene after 48 hours. This titer demonstrates a ˜1.5-fold increase over the observed inhibitory concentration of styrene for  E. coli  (Liu et al., 2018), confirming the potential for this cell-free platform to produce toxic compounds. 
     2. Optimization of Enzyme Ratio and Physiochemical Conditions 
     We stepped through series of optimizations to improve cell-free styrene synthesis by exploiting the open reaction environment which enables precise control over enzyme concentrations and physiochemical conditions (Karim et al., 2018; Karim and Jewett, 2016). Consistent with our previous work, we first normalized the total volume of CFPS added to a biosynthesis reaction by supplementing with CFPS mixtures without plasmid. By doing this, we minimize detrimental effects that additional CFPS volume, specifically the additional small molecules, tends to cause (e.g., decreased final titers of the desired product) (Karim et al., 2018). However, we also wanted test whether this is universally true or potentially specific to previously studied pathways. We found that biosynthesis reactions with increasing amounts of CFPS fraction result in increased styrene production ( FIG. 38 ). Our reactions are less inhibited by the CFPS mixtures likely because we are observing a two-step biosynthesis from L-Phe rather than longer pathways that take advantage of glycolysis that are known to have competition with several pathways branching from pyruvate and acetyl-CoA. These results suggest that the impact of CFPS reagents should be investigated for each new biosynthetic pathway tested in vitro. 
     We next decided to tune the biosynthetic enzyme ratio by mixing different volumes of cell-free expressed PAL2 and FDC1 in the reactions. We ran 36 unique reaction conditions varying the final PAL2 and FDC1 concentrations from 0 to 1 μM ( FIG. 32A ). The best condition produced up to 18.03+/−2.34 mM styrene from 25 mM of added L-Phe. As hypothesized, styrene titer generally increased with increasing enzyme concentrations. However, the best 8 enzyme ratios all produced 16-18 mM styrene, which suggested substrate limitation may prevent higher titers. We doubled the initial concentration of added L-Phe to 50 mM and ran reactions using the top eight enzyme ratio combinations in an attempt to further increase styrene yield. The best reaction condition produced 24.83+/−0.66 mM styrene with 0.25 μM PAL2 and 1 μM FDC1 ( FIG. 32B ). Increasing the substrate concentration enabled differentiation between the best conditions with 25 mM L-Phe in  FIG. 32A , but the modest 6-7 mM increase in product from 25 mM additional substrate indicated diminishing biosynthetic potential; thus, L-Phe concentrations greater than 50 mM were not examined. 
     3. Optimization of Physiochemical Conditions 
     To further optimize the reaction environment for styrene biosynthesis, we explored a range of acidic and alkaline conditions due to the dramatic changes that pH can cause in cell-free systems (Calhoun and Swartz, 2005; Karim et al., 2019). We tested six different initial pH conditions in reactions for styrene synthesis ranging from pH 5.6 to pH 9.5. Our standard condition began at pH 7.5-7.8 and ended near pH 8 ( FIG. 33 ; dark blue). We observed a strong pH dependence for styrene biosynthesis across the broad pH range, with a maximum titer of 40.33+/−1.03 mM styrene after 72 hours between pH 7 and pH 8 ( FIG. 33A ). The most alkaline reaction (pH 9.5) produced less than 1 mM styrene, whereas the most acidic reaction (pH 5.6) produced 3.27+/−2.85 mM styrene—nearly equivalent to the highest reported titer from in vivo biosynthesis (Liu et al., 2018). Reaction rates remained steady over 3 full days, which was considerably slower than the reported catalytic rates of purified PAL2 and FDC1 ( FIG. 33B ) (Cochrane et al., 2004; Payne et al., 2015). Although enzyme behavior can differ in vitro between purified systems and our crude cell extracts, the observed pH optimum for styrene biosynthesis lies within a reasonable range based on reported pH optima of 8.4-8.9 and 6.5 for PAL2 and FDC1, respectively (Cochrane et al., 2004; Lin et al., 2015). Additionally, the reaction pH changed very little over time without active glycolysis producing acetate and lactate, which provided a stable reaction environment without the need for a strong buffer ( FIG. 33C ). 
     E. Conclusion 
     In this study, we demonstrated the substantial increase in styrene titer possible in a cell-free system compared to in vivo biosynthesis through the optimization of reaction temperature, enzyme ratios, and pH. We first determined that PAL2 and FDC1 expressed by  E. coli  CFPS were soluble and active in vitro and that, in combination, these enzymes produced the most styrene at 30° C. Second, we found that cell-free styrene biosynthesis was maximized by combining a low concentration of PAL2 with a high concentration of FDC1, which would maximize conversion of trans-cinnamic acid to styrene. Third, we highlighted the pH sensitivity of in vitro styrene biosynthesis and found that reactions maintained at a neutral pH produced the highest concentration of monomer. These cell-free reactions achieved a maximum styrene titer of 40.33 +/−1.03 mM (4.20+/−0.11 g/L), which surpasses the highest published in vivo titer of 3.36 mM (0.350 g/L) by more than an order of magnitude (Liu et al., 2018). Such high titers of the toxic styrene monomer are possible due to the lack of viability constraints in a cell-free system and the ability to finely tune the reaction environment, which makes this system a powerful example of cell-free biosynthesis as an alternative to traditional methods. 
     Although the optimal pH for styrene biosynthesis was within a range consistent with the optima for the PAL2 and FDC1 homologs used, the overall reaction rate appeared much slower than expected (Cochrane et al., 2004; Payne et al., 2015). The observed rate of styrene synthesis could be slowed by a number of variables in the complex cell-free reactions, such as an altered chemical environment caused by the dodecane overlay and thermodynamic constraints for PAL2 due to the ammonium salts from CFPS, since ammonia is a byproduct of L-Phe to trans-cinnamic acid conversion. Regardless, the potential impact of this environmentally friendly styrene production method could be further increased by using glucose as the substrate in cell extract from an  E. coli  strain engineered with a highly active shikimate pathway to overproduce L-Phe (Averesch and Kromer, 2018). Enzymes constituting the shikimate pathway could also be produced via CFPS to allow greater control over metabolic flux. With glucose as the initial substrate, it may be necessary to balance styrene recovery in the dodecane with potential oxygen transfer limitations across the overlay, which could slow glycolysis. However, given that L-phenylalanine is an abundant and inexpensive feedstock for synthesizing value-added products, investigating a complete recreation of the styrene biosynthesis pathway is not critical at this time. 
     For increased potential as a biomanufacturing platform, a strategy for direct styrene polymerization could be investigated. Styrene readily polymerizes in organic solvents upon heating in the presence of a radical initiator, such as benzoyl peroxide, due to the vinyl group (Abere et al., 1945). Replacing the dodecane overlay used for styrene extraction in this study with toluene could enhance the solubility of biosynthesized styrene as the solvent is heated. Despite the substantial increase in titer achieved using a cell-free system, these microliter-scale reactions cannot feasibly produce styrene in the large quantities currently derived from petroleum (James and Castor, 2011). Fortunately, larger scales are possible with cell-free protein synthesis scaling linearly up to 100 L (Zawada et al., 2011), although increasing the scale of biosynthesis reactions with a hydrocarbon overlay presents technical challenges. Furthermore, reaching the capacity of petrochemical plants would not be necessary since smaller scale biomanufacturing facilities would still benefit from an economy of scale for feedstocks while simultaneously reducing capital risk and market saturation (Claypool et al., 2014). 
     At the laboratory scale, this example of high-titer styrene biosynthesis demonstrates the potential of cell-free systems for the production of toxic compounds that normally originate from petroleum-based processes. Expanding the cell-free approach to producing more value-added chemicals, such as other plastic precursors and biofuels, and increasing the scale of these reactions could spearhead the development of economically viable alternatives to fossil fuel-derived chemicals. 
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Technoeconomic evaluation of bio-based styrene production by engineered  Escherichia coli . J Ind Microbiol Biotechnol. 41, 1211-6.   Cochrane, F. C., Davin, L. B., Lewis, N. G., 2004. The  Arabidopsis  phenylalanine ammonia lyase gene family: kinetic characterization of the four PAL isoforms. Phytochemistry. 65, 1557-64.   Dudley, Q. M., Anderson, K. C., Jewett, M. C., 2016. Cell-Free Mixing of  Escherichia coli  Crude Extracts to Prototype and Rationally Engineer High-Titer Mevalonate Synthesis. ACS Synth Biol. 5, 1578-1588.   Dudley, Q. M., Karim, A. S., Jewett, M. C., 2015. Cell-free metabolic engineering: biomanufacturing beyond the cell. Biotechnol J. 10, 69-82.   Dudley, Q. M., Nash, C. J., Jewett, M. C., 2019. Cell-free biosynthesis of limonene using enzyme-enriched  Escherichia coli  lysates. Synth Biol (Oxf). 4, ysz003.   Garenne, D., Noireaux, V., 2019. Cell-free transcription-translation: engineering biology from the nanometer to the millimeter scale. 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Metabolic Engineering Communications.   Keasling, J. D., 2010. Manufacturing Molecules Through Metabolic Engineering. Science. 330, 1355-1358.   Kelwick, R., Ricci, L., Chee, S. M., Bell, D., Webb, A. J., Freemont, P. S., 2018. Cell-free prototyping strategies for enhancing the sustainable production of polyhydroxyalkanoates bioplastics. Synthetic Biology. 3.   Korman, T. P., Opgenorth, P. H., Bowie, J. U., 2017. A synthetic biochemistry platform for cell free production of monoterpenes from glucose. Nat Commun. 8, 15526.   Kwon, Y. C., Jewett, M. C., 2015. High-throughput preparation methods of crude extract for robust cell-free protein synthesis. Sci Rep. 5, 8663.   Lin, F., Ferguson, K. L., Boyer, D. R., Lin, X. N., Marsh, E. N., 2015. Isofunctional enzymes PAD1 and UbiX catalyze formation of a novel cofactor required by ferulic acid decarboxylase and 4-hydroxy-3-polyprenylbenzoic acid decarboxylase. ACS Chem Biol. 10, 1137-44.   Liu, C., Men, X., Chen, H., Li, M., Ding, Z., Chen, G., Wang, F., Liu, H., Wang, Q., Zhu, Y., Zhang, H., Xian, M., 2018. A systematic optimization of styrene biosynthesis in  Escherichia coli  BL21(DE3). Biotechnol Biofuels. 11, 14.   Martin, R. W., Des Soye, B. J., Kwon, Y. C., Kay, J., Davis, R. G., Thomas, P. M., Majewska, N. I., Chen, C. X., Marcum, R. D., Weiss, M. G., Stoddart, A. E., Amiram, M., Ranji Charna, A. K., Patel, J. R., Isaacs, F. J., Kelleher, N. L., Hong, S. H., Jewett, M. C., 2018. Cell-free protein synthesis from genomically recoded bacteria enables multisite incorporation of noncanonical amino acids. Nat Commun. 9, 1203.   Martin, V. J., Pitera, D. J., Withers, S. T., Newman, J. D., Keasling, J. D., 2003. Engineering a mevalonate pathway in  Escherichia coli  for production of terpenoids. Nature Biotechnology. 21, 796-802.   McKenna, R., Nielsen, D. R., 2011. Styrene biosynthesis from glucose by engineered  E. coli . Metab Eng. 13, 544-54.   Nielsen, J., 2001. Metabolic engineering. Applied Microbiology and Biotechnology. 55, 263-283.   Payne, K. A., White, M. D., Fisher, K., Khara, B., Bailey, S. S., Parker, D., Rattray, N. J., Trivedi, D. K., Goodacre, R., Beveridge, R., Barran, P., Rigby, S. E., Scrutton, N. S., Hay, S., Leys, D., 2015. New cofactor supports alpha,beta-unsaturated acid decarboxylation via 1,3-dipolar cycloaddition. Nature. 522, 497-501.   Shen, C. R., Liao, J. C., 2008. Metabolic engineering of  Escherichia coli  for 1-butanol and 1-propanol production via the keto-acid pathways. Metab Eng. 10, 312-20.   Silverman, A. D., Karim, A. S., Jewett, M. C., 2019. Cell-free gene expression: an expanded repertoire of applications. Nat Rev Genet.   Stephanopoulos, G. N., 1994. Metabolic Engineering. Current Opinion in Biotechnology. 5, 196-200.   Tyo, K. E., Alper, H. S., Stephanopoulos, G. N., 2007. Expanding the metabolic engineering toolbox: more options to engineer cells. Trends Biotechnol. 25, 132-7.   Tyo, K. E., Kocharin, K., Nielsen, J., 2010. Toward design-based engineering of industrial microbes. Curr Opin Microbiol. 13, 255-62.   Wu, C., Koylinski, T., Bozik, J., 1981. Preparation of styrene from ethylbenzene U.S. Pat. No. 4,255,599.   Zawada, J. F., Yin, G., Steiner, A. R., Yang, J., Naresh, A., Roy, S. M., Gold, D. S., Heinsohn, H. G., Murray, C. J., 2011. Microscale to manufacturing scale-up of cell-free cytokine production—a new approach for shortening protein production development timelines. Biotechnol Bioeng. 108, 1570-8.   Zheng, J., Suh, S., 2019. Strategies to reduce the global carbon footprint of plastics. Nature Climate Change. 9, 374-378.   

     Example 6—Cell-Free Synthesis of Gramicidin S 
     A. Abstract 
     Genome sequencing has revealed that a far greater number of natural product biosynthetic pathways exist than there are known natural products. To access these molecules directly and deterministically, a new generation of heterologous expression methods is needed. Cell-free protein synthesis has not previously been used to study biosynthesis, and provides a tunable platform with advantages over conventional methods for protein expression. Here, we demonstrate the use of cell-free protein synthesis to biosynthesize a cyclic dipeptide with correct absolute stereochemistry. From a single-pot reaction, we measured the expression of two nonribosomal peptide synthetases larger than 100 kDa, and detected high-level production of a diketopiperazine. Using quantitative LC-MS and synthetically prepared standard, we observed production of this metabolite at levels higher than previously reported from cell-based recombinant expression, approximately 12 mg/mL. Overall, this work illuminates a path forward for the application of cell-free protein synthesis to discovery and characterization of natural product biosynthesis. 
     B. Introduction 
     Sequencing of over 10,000 microbial genomes has revealed that the diversity of secondary metabolism covers a largely unexplored region of chemical space 1, 2 . Currently, the rate at which new biosynthetic pathways are identified within biological sequence data greatly outpaces the capacity to characterize the small molecules for which production is encoded. This is at least in part due to the difficulty of cultivating natural product producing organisms in the laboratory, and therefore presents a major opportunity for synthetic biology to establish new methods for robust heterologous expression and characterization of secondary metabolism to unlock access to often bioactive secondary metabolites. Additionally, robust heterologous expression of biosynthetic proteins opens the possibility of rapid design-build-test cycles to re-engineer pathways to produce useful scaffolds beyond those found in nature 3 . 
     Proteins involved in the biosynthesis of complex natural products (NPs) provide a challenge for established heterologous expression platforms because of their sheer size, complex multi-domain structures and occurrence within even larger biosynthetic gene clusters (BGCs). Nonribosomal peptide synthetases (NRPSs) and polyketide synthases (PKSs) are often 100 to &gt;300 kDa proteins that operate like assembly lines, loading and passing covalently attached intermediates between catalytic domains ( FIG. 40 , center) 4 . Production of NPs by these classes of proteins frequently involves exotic precursors that may not be found in the cytosol of a typical heterologous host such as  Escherichia coli  or  Saccharomyces cerevisiae   5, 6    
     Cell-free protein synthesis (CFPS) offers an alternative protein expression platform with potential advantages for expressing natural product biosynthetic enzymes ( FIG. 40 ). CFPS is emerging as a robust tool for fundamental and applied research in the areas of synthetic biology and biotechnology 7-9 . Recently, cell-free systems have been shown to enable high yielding synthesis of, for example, active metalloenzymes 10-12 , and allow rapid prototyping of biological circuits and pathways 13-16 . For natural products research, CFPS offers flexibility over cell-based expression because the reaction conditions can be more easily controlled: for example, atypical precursors can simply be fed in during the course of a reaction 17  or made in situ using all enzymes from a biosynthetic gene cluster. Biosynthetic pathways could potentially be engineered for higher expression without the need to modify a native host strain s8 . Reactions can also be performed in the span of a few hours in a “single pot” and have a greater tolerance for producing potentially toxic proteins and metabolites than living cells 
     To investigate the potential of CFPS for studying natural product pathways, we utilized the proteins involved in the first steps of gramicidin S biogenesis as a model. Gramicidin S is biosynthesized by two NRPS proteins, GrsA and GrsB, that catalyze thiotemplated peptide bond formation and release by cyclodimerization to form the cyclic decapeptide ( FIG. 41A ). A natural shunt product of the gramicidin S pathway is the D-Phe-L-Pro diketopiperazine (DKP) formed by cyclization of the substrates of the first two NRPS modules in the pathway, GrsA and GrsB1 19 . Cyclic dipeptide DKPs have been studied for their diverse biological activity and are produced by many species of bacteria, fungi, and plants 20 . Bacterial DKPs have two known general biosynthetic origins: they are either synthesized by NRPS enzymes, as in the case of the D-Phe-L-Pro-DKP presented here 21 , or produced by aminoacyl-tRNA mediated cyclodipeptide synthase pathways as in the case of the antibiotic albonoursin 22    
     Here, we report in vitro expression of GrsA and GrsB1, encompassing the first two of five modules of gramicidin S biosynthesis in  Brevibacillus brevis . Each NRPS module typically consists of the three domains needed to incorporate an amino acid into the nonribosomal peptide: condensation (C), adenylation (A) and thiolation (T). GrsA, as a starter module, does not contain a C-domain; it contains an A and a T-domain, as well as an epimerization (E) domain that converts L-phenylalanine to D-phenylalanine. Previous reports have shown that isolated GrsA and GrsB1 can be harnessed to produce D-Phe-L-Pro DKP ( FIG. 41B ) 23 . While this diketopiperazine is not the primary product of this pathway in  Brevibacillus brevis , it has been found to be produced by other microbes 24 . In this report, we demonstrate high-level expression of GrsA and GrsB1, show that they are produced in their functional state when expressed using CFPS, and use these two proteins in a concerted reaction to produce D-Phe-L-Pro DKP with a final yield greater than 10 mg/L. 
     C. Results and Discussion 
     To establish an in vitro platform for D-Phe-L-Pro DKP biosynthesis, NRPS proteins GrsA and GrsB1 were expressed using an  E. coli -based cell-free protein expression system reported previously 25, 26 . GrsA and GrsB1 along with the functionally-equivalent tyrocidine synthetases TycA and TycB1 (often referred to as pheATE and proCAT) have served as the model thiotemplated biosynthetic systems for studying substrate specificity 27  and peptide bond formation 28  in NRPS pathways. 
     Individual plasmids containing the genes grsA and grsB1 were used to express these enzymes in the combined transcription-translation system. Transcription was driven using a T7 RNA polymerase. We tested protein synthesis yields in 17 h batch reactions across three different  E. coli  crude extracts (S30 extracts) 29 . Expression yields of GrsA and GrsB1 were markedly different across the different S30 extracts. For both enzymes,  E. coli  BL21 Star (DE3) extract synthesized the highest amount of NRPS yielding GrsA at 200±29 μg/mL and GrsB1 at 108±19 g/mL ( FIG. 44 ). Fully assembled GrsA (126 kDa) and GrsB1 (121 kDa) with correct molecular weight bands were observed by SDS-PAGE and further confirmed by autoradiogram analysis ( FIG. 45 ). Interestingly, protein titers of greater than 108±19 μg/mL were achieved without optimization of ribosome binding sites, codon usage, etc. 
     Once we had successfully expressed the two large NRPSs, GrsA and GrsB1 in vitro, we showed that the enzymes could be converted to their functional (holo) form using the 4′-phosphopantetheinyl transferase enzyme Sfp from  Bacillus subtilis . In order to be functional, NRPS proteins require modification by transfer of a phosphopantetheine group from coenzyme A (CoA) to a conserved serine residue in their thiolation (T) domain 30 . To verify that this essential modification is possible in our cell-free system, both GrsA and GrsB1 were labeled with a fluorescent BODIPY-CoA analog by the promiscuous action of Sfp ( FIG. 42A ). Expression of GrsA and GrsB1 continued for 17 hours in vitro, allowing nascent proteins to fold properly before labeling. After this incubation, BODIPY-CoA and Sfp were added directly to the cell-free expression system, followed by another 3 hour incubation at 30° C. or 37° C. 
     Phosphopantetheinylation was also detectable by LC-MS/MS using the method described in Miller et al. 2005 ( FIG. 42 ) 31 . GrsA and GrsB1 were prepared in vitro in separate reactions. Following phosphopantetheinylation by Sfp, GrsA with a 6×His tag was captured using cobalt-affinity resin, digested with trypsin, and analyzed by proteomic-style LC-MS/MS. In the GrsA sample, a peptide with the sequence DNFYALGGDSIK (SEQ ID NO:3) and m/z 820.357 was observed, representing the predicted mass of the tryptic peptide with the addition of phosphopantetheine. GrsB1 peptides were detected in a peptide sample prepared by digesting the total lysate with 1:5 trypsin. The T-domain active-site peptide (IWEEVLGISQIGIQDNFFSLGSLGGHSLK (SEQ ID NO:4), m/z 1076.533) was also observed for GrsB1 with attached phosphopantetheine. 
     Successful fluorescence labeling of GrsA and GrsB1 with Bodipy-CoA, and direct LC-MS/MS detection of the Ppant modification ( FIG. 46 ,  FIG. 42 ) demonstrated that, in the current in vitro system, i) both NRPSs were properly folded with accessible conserved serine residues located on T domains; ii) the purified recombinant Sfp is active for the phosphopantetheinylation; and iii) most importantly, the holo-NRPS could be functionally reconstituted for target molecule biosynthesis. In addition, GrsA and GrsB1 were expressed in soluble form and an extended incubation at 37° C. was not necessary for priming the NRPSs by Sfp ( FIG. 46 ). Therefore, we posited that the target compound could be biosynthesized via active holo-NRPSs without further optimization of the reaction conditions or incubation temperature. 
     We next sought to biosynthesize D-Phe-L-Pro DKP, and instead of expressing of GrsA and GrsB1 separately, the two enzymes were co-expressed in a single-pot mixture allowing reconstitution of the partial NRPS assembly line for product formation in situ. After carrying out the CFPS reaction, we added Sfp to the mixture to initiate biosynthesis, using the same reaction mixture without Sfp as a negative control. The reaction was stopped by extraction with n-butanol and chloroform (4:1, v/v) and the extract was analyzed by metabolomics-style LC-MS/MS. We detected a time-dependent increase in signal intensity of the ion corresponding to predicted DKP ion within 3 ppm mass error tolerance (m/z 245.129) ( FIGS. 43A  &amp; B). 
     To verify that the ion observed was the expected D-Phe-L-Pro DKP—and with correct D-L stereochemistry—we prepared both the D-L and L-L DKP as synthetic references ( FIG. 47 ). Based on earlier reporting, we expected the D-L stereoisomer to elute earlier than L-L on a reversed phase HPLC column 32 , and its elution profile matched with the metabolite produced with the CFPS reaction. Synthetic DKP was indistinguishable from the CFPS produced metabolite by MS 2  analysis ( FIG. 43C ). We used LC-MS to prepare a standard curve from the integrated intensity of synthetically prepared DKP measured at six different concentrations and used this curve to estimate the quantity of DKP produced by CFPS ( FIG. 48 ). Even without optimization of the system for high titers, we estimated our production of D-Phe-L-Pro DKP at ˜12 mg/L, a concentration higher than the 9 mg/L previously reported from recombinant protein expression in  E. coli   23 . 
     We believe this work to be the first example of natural product biosynthesis from NRPS enzymes produced using an  E. coli -based in vitro transcription and translation system. CFPS provides a feasible option for exploring natural product biosynthesis, at least for proteins ˜130 kDa such as the two presented here. CFPS offers freedom from many of the problematic processes present in cell-based expression systems (e.g., inclusion body formation, protein degradation), and may circumvent the issues of low-expressing or cryptic (non-expressing) BGCs encountered in native-producing organisms. While more investigation will be needed, the production of these two single-module NRPS proteins in vitro and the demonstration of their concerted function provides a groundwork for the study of increasingly complex natural product biosynthesis pathways using CFPS. Indeed, our discovery-centered cell-free approach sets the stage for high-throughput experimentation in a cell-free environment, where design-build-test iterations can be performed without the need to reengineer organisms, DNA for pathway enzymes is directly input with plasmid refactoring, and substrates and cofactors needed for secondary metabolism can be controlled and maintained at defined concentrations 33    
     As a resurgence of interest in natural products continues, and the number of sequenced biosynthetic gene clusters continues to grow, we expect that protein expression systems will play an increasingly important role in obtaining and studying new natural products. Especially as the price of DNA synthesis declines, direct expression of entirely synthetic gene clusters (typically 30-120 kilobases in length) will remove barriers to accessing biosynthetic pathways from clusters assembled using metagenomics for uncultivable organisms. By merging bottom-up design principles with innovative cell-free pathway engineering methodologies, our cell-free approach will create a greatly simplified framework for studying and engineering natural product pathways. 
     D. Methods 
     1. Preparation of S30 Cell Extracts 
       E. coli  cells were grown in 1 L of 2×YTPG (yeast extract 10 g/L, tryptone 16 g/L, NaCl 5 g/L, K 2 HPO 4  7 g/L, KH 2 PO 4  3 g/L and glucose 18 g/L, pH 7.2) in a 2.5-L Tunair flask (IBI Scientific, Peosta, Iowa) at 34° C. and 220 rpm with inoculation of 20 mL overnight cultures (initial OD 600  of ˜0.05). When the OD 600  reached 3.0, cells were collected by centrifugation at 5,000×g and 4° C. for 15 min. The pellets were washed thrice with cold S30 buffer (10 mM Tris-acetate pH 8.2, 14 mM magnesium acetate, 60 mM potassium acetate, 2 mM dithiothreitol (DTT)). Cells were suspended in 0.8 mL of S30 buffer per gram wet weight and lysed on ice using a Q125 Sonicator (Qsonica, Newtown, Conn.) for three pulses (50% amplitude, 45 s on and 59 s off). After sonication, 3 μL of DTT (1M) was added per mL of lysate, followed by centrifugation at 12,000×g and 4° C. for 10 min. The supernatant (S30 extract) was flash frozen in liquid nitrogen and stored at −80° C. until use. 
     2. Cell-Free Protein Synthesis (CFPS) Reactions 
     The standard CFPS reactions were performed in 1.5 mL microcentrifuge tubes with 15 L mixture composed of the following reagent concentrations: 12 mM magnesium glutamate, 10 mM ammonium glutamate, 130 mM potassium glutamate, 1.2 mM ATP, 0.85 mM each of GTP, UTP, and CTP, 34 μg/mL folinic acid, 170 μg/mL of  E. coli  tRNA mixture, 2 mM each of 20 standard amino acids, 10 μM of L-[ 14 C(U)]-leucine (used only in protein quantitation experiments, 11.1 GBq mmol −1 , PerkinElmer, Waltham, Mass.), 0.33 mM nicotinamide adenine dinucleotide (NAD), 0.27 mM coenzyme A (CoA), 1.5 mM spermidine, 1 mM putrescine, 4 mM sodium oxalate, 33 mM phosphoenolpyruvate (PEP), 26.7 μg/mL plasmid, 100 μg/mL T7 RNA polymerase, and 27% (v/v) of S30 cell extract. All CFPS reactions were incubated for 20 h at 30° C. unless otherwise described. 
     3. Fluorescence Labeling of GrsA and GrsB1 
     The coenzyme A analog BODIPY-CoA was prepared as previously described 34 . To label thiolation domains of GrsA and GrsB1, standard CFPS reactions with 26.7 μg/mL of each plasmid were incubated at 30° C. for 17 h. Afterwards, labeling reactions were performed following three strategies. Strategy #1): the CFPS system was directly supplemented with 1 μL BODIPY-CoA (1 mg/mL) and 1 μL Sfp (2 mg/mL) and incubated at 30° C. for another 3 h. Strategy #2): the CFPS reaction was first centrifuged at 12,000 g and 4° C. for 10 min. Then, 13 μL of supernatant was transferred to a new 1.5 mL microcentrifuge tube with addition of 1 μL BODIPY-CoA and 1 L Sfp. The mixture was incubated at 30° C. for 3 h. Strategy #3): the treatment of the CFPS reaction was the same as in Strategy #2, but was incubated at 37° C. for 3 h. After the labeling reaction, 3 μL of each sample was loaded on a 4-12% NuPAGE SDS-PAGE gel (Invitrogen). The BODIPY-labeled proteins were visualized by a fluorescence imaging system with 473 nm laser and 520 nm emission filter (Typhoon FLA7000, GE Healthcare Biosciences, Uppsala, Sweden). See supporting information for additional details and fluorescent gel images ( FIG. 46 ). 
     4. DKP Production In Vitro 
     D-Phe-L-Pro diketopiperazine (DKP) was biosynthesized by GrsA and GrsB1 expressed in situ. Cell-free co-expression of GrsA and GrsB1 was performed in the 15 μL reaction mixture as described above with addition of both plasmids (each of 26.7 pg/mL). The BL21 Star (DE3) S30 extract was used for the coupled transcription-translation. After incubation of the reaction mixture at 30° C. for 17 h, 1 μL Sfp (2 mg/mL); 1 μL Sfp (2 mg/mL) and 1 μL CoA (5 mM); or 1 μL Sfp (2 mg/mL), 1 μL CoA (5 mM), 1 μL Phe (1 mM) and 1 μL Pro (1 mM) were added directly to the reactions, followed by another 3 h incubation at 30° C. Reactions without plasmids and without addition of Sfp were carried out as negative controls. At the end of the production, all DKP samples were immediately extracted from the reaction mixtures for analysis by LC-MS/MS 
     5. Quantitation of Synthesized GrsA and GrsB1 
     The yields of synthesized GrsA and GrsB1 were quantified by the incorporation of  14 C-leucine into trichloroacetic acid (TCA)-measuring precipitated radioactivity using a liquid scintillation counter (MicroBeta2, PerkinElmer, Waltham, Mass.). After CFPS, 100 μL of 0.1 M NaOH was added to each 15 μL reaction mixture. The NaOH treated samples were incubated at 37° C. for 20 min. Then, 50 μL of the treated sample was loaded onto each of two separate pieces of Whatman 3MM chromatography paper and dried under a heat lamp for 1 h. One piece of filter paper in each pair was placed into a beaker on ice and covered with 5% (v/v) TCA at 4° C. to precipitate the proteins onto the filter paper. After 15 min, the solution was exchanged with fresh TCA. This incubation was performed a total of three times. Following the third precipitation, the filter papers were covered once with 100% ethanol for 10 min at room temperature, and then they were removed from the beaker and allowed to dry under a heat lamp for another 1 h. Radioactivity of both the TCA-precipitated (washed) and non-TCA-precipitated (unwashed) samples was measured using a liquid scintillation counter. The fraction of incorporated  14 C-leucine (washed/unwashed) was used to determine the amount of protein synthesized 
     6. Expression and Purification of Recombinant Sfp from  E. coli    
     pET28-Sfp l  was expressed and purified from BL21(DE3). Sfp was purified from the lysate of 250 mL culture over cobalt resin (Thermo Scientific), and washed in steps of 10, 25, 50 mM imidazole. Pure Sfp protein was eluted with 150 mM imidazole. Following desalting and buffer exchange using Zeba desalting spin columns (Thermo Scientific), the final concentration of Sfp was determined to be 2 mg/mL by the bicinchoninic acid assay. Sfp was stored in 10% glycerol at −80° C. prior to use. 
     7. Autoradiography Analysis 
     After the cell-free reaction, 3 μL of each sample was loaded on a 4-12% NuPAGE SDS-PAGE gel (Invitrogen). After running at 120 V for 1.5 h, the gel was stained using SimplyBlue™ SafeStain solution (Invitrogen) and destained in water. Then, the gel was fixed with cellophane films (Bio-Rad), dried overnight in a GelAir Dryer (Bio-Rad) without heating, and exposed for 48 h on a Storage Phosphor Screen (GE Healthcare Biosciences, Pittsburgh, Pa.). The autoradiogram was scanned using a Storm Imager (GE Healthcare Biosciences, Pittsburgh, Pa.) and analyzed using Quantity One software (Bio-Rad, Hercules, Calif.). 
     8. Synthetic Preparation of Racemic DKP 
     The scheme shown in  FIG. 47  illustrates the preparation of synthetic D/L-L DKP. Methyl (tert-butoxycarbonyl)phenylalanyl-L-prolinate. Boc-Phe-OH (0.1 mmol) and L-proline methyl ester (0.1 mmol) were dissolved in 5 mL of acetonitrile. PyBOP (0.12 mmol) was added to the reaction mixture and allowed to stir at room temperature. Triethylamine (0.2 mmol) was added to the flask. After 12 h, saturated NaCl solution (10 mL) was added to the mixture and extracted with ethyl acetate (3×10 mL). Organic layer was washed with 1 M HCl, 5% NaHCO 3 , and H 2 O sequentially. Organic layer was dried over sodium sulfate and solvent removed under reduced pressure to yield 75 mg of crude material, which was used without further purification. The Boc-protected dipeptide was dissolved in 5 mL of dichloromethane and 5 mL of trifluoroacetic acid and then stirred at room temperature for 1 hour. The solvent was removed under reduced pressure to yield a colorless oil, which was treated with 10 mL of 5% NaHCO 3  solution. After stirring for 12 hours, the solvent was removed under reduced pressure-resulting in 48 mg of crude DKP. A portion of this product was used for purification via HPLC. 
     9. Extraction and Identification of DKP 
     After DKP production, each 15 L reaction mixture was immediately extracted with 3 volumes of n-butanol/chloroform (4:1, v/v). The organic layer was separated by centrifugation at 16,000×g and 4° C. for 10 min and then transferred to a fresh microcentrifuge tube. After removal of the solvent under vacuum, samples were resuspended in 100 μL of 50% methanol for further analysis by LC-MS. 
     DKP samples were analyzed on a Q-exactive orbitrap instrument (Thermo) paired with an Agilent 1150 HPLC system. MS parameters were the following: 70 minute run-time, scan range 100-1000 m/z, resolution 35,000. After each full scan, the top five most intense ions selected for HCD fragmentation at 25% collisional energy. LC buffers were H 2 O+0.1% formic acid and acetonitrile+0.1% formic acid. For each sample 20 μL were injected into the LC system for a 70 minute gradient with a flow rate of 200 μL/min across a 150 mm×2.1 mm reversed-phase (C18) column (Phenomonex). SIC intensities were determined using Xcalibur software (Thermo Scientific) using the mass range of 245.127-245.129 m/z. 
     10. Bacterial Strains 
       Escherichia coli  stains C321.AA.705, BL21(DE3) and BL21 Star (DE3) were used for S30 cell extract preparation. C321.AA.705 was generated from a genetically encoded release factor 1 (RF1) deficient  E. coli  strain 2 . Both BL21(DE3) and BL21 Star (DE3) were purchased from Life Technologies (Grand Island, N.Y.).  E. coli  DH5α was used for cloning and plasmid maintenance. BL21(DE3) was also used for Sfp expression and purification. 
     11. Plasmid Construction 
     Plasmid construction was performed using a conventional restriction enzyme-ligation scheme. Primers were obtained from Integrated DNA technologies. GrsA and GrsB1 were first cloned from the genomic DNA of  Brevibacillus brevis . PCR was performed using GoTaq Green Master Mix (Promega, 60 L reaction volume). PCR products and pET28 plasmid were digested with the enzymes NcoI and BamHI (New England Biolabs) and gel extracted using the QIAquick Gel Extraction kit (Qiagen). Digested PCR products and plasmid backbones were ligated to form whole constructs using T4 DNA ligase. See plasmid maps at  FIG. 49 . 
     
       
         
           
               
             
               
                 TABLE 6.1 
               
             
            
               
                   
               
               
                 Primers used for amplification of NRPS 
               
               
                 encoding genes grsA and grsB1. 
               
            
           
           
               
               
            
               
                 Name 
                 Sequence (5′ -&gt; 3′) 
               
               
                   
               
               
                 NcoI-grsA-F 
                 Attattatccatggtgttaaacagttctaaaa 
               
               
                   
                 gtatattgattcatgc (SEQ ID NO: 104) 
               
               
                   
               
               
                 BamHI-grsA-R 
                 Aataatatggatccgttaatgaatcggccaac 
               
               
                   
                 aaatc (SEQ ID NO: 105) 
               
               
                   
               
               
                 NcoI-grsB1-F 
                 Attattatccatggtgagtacatttaaaaaag 
               
               
                   
                 aacatgttcagg (SEQ ID NO: 106) 
               
               
                   
               
               
                 BamHI-grsB1-R 
                 Aataatatggatccatataattagagatttcc 
               
               
                   
                 tgaatggttggtg (SEQ ID NO: 107) 
               
               
                   
               
            
           
         
       
     
     12. Quantitation of DKP Production 
     A standard curve was prepared using synthetic DKP (preparation described above) by performing injections of 1 μg, 500 ng, 250 ng, 125 ng, 50 ng, and 25 ng on an Agilent 1150 HPLC stack coupled to a Q-Exactive mass spectrometer. Signal intensity was determined from the integrated abundance value of DKP over the course of a 70 minute experiment. A linear equation was fit to the standard curve and used to estimate the abundance of DKP in the sample prepared from CFPS by calculating the total amount of DKP present in a 45 μL butanol/chloroform extraction. 
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         31. Miller, L. M., Mazur, M. T., McLoughlin, S. M., and Kelleher, N. L. (2005) Parallel interrogation of covalent intermediates in the biosynthesis of gramicidin S using high-resolution mass spectrometry, Protein Sci 14, 2702-2712. 
         32. Stachelhaus, T., and Walsh, C. T. (2000) Mutational analysis of the epimerization domain in the initiation module PheATE of gramicidin S synthetase, Biochemistry 39, 5775-5787. 
         33. Niederholtmeyer, H., Sun, Z. Z., Hori, Y., Yeung, E., Verpoorte, A., Murray, R. M., and Maerkl, S. J. (2015) Rapid cell-free forward engineering of novel genetic ring oscillators, Elife 4, e09771. 
         34. La Clair, J. J., Foley, T. L., Schegg, T. R., Regan, C. M., and Burkart, M. D. (2004) Manipulation of carrier proteins in antibiotic biosynthesis, Chem Biol 11, 195-201. 
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         2. Lajoie, M. J., Rovner, A. J., Goodman, D. B., Aerni, H. R., Haimovich, A. D., Kuznetsov, G., Mercer, J. A., Wang, H. H., Carr, P. A., Mosberg, J. A., Rohland, N., Schultz, P. G., Jacobson, J. M., Rinehart, J., Church, G. M., and Isaacs, F. J. (2013) Genomically recoded organisms expand biological functions, Science 342, 357-360 
         Dudley, Q. M., Karim, A. S. &amp; Jewett, M. C. Cell-free metabolic engineering: biomanufacturing beyond the cell. Biotechnol J 10, 69-82 (2015). 
         Chappell, J., Jensen, K. &amp; Freemont, P.S. Validation of an entirely in vitro approach for rapid prototyping of DNA regulatory elements for synthetic biology. Nucleic Acids Res 41, 3471-3481 (2013). 
         Takahashi, M. K. et al. Characterizing and prototyping genetic networks with cell-free transcription-translation reactions. Methods (2015). 
         Zemella A, Thoring L, Hoffmeister C, Kubick S. 2015. Cell-Free Protein Synthesis: Pros and Cons of Prokaryotic and Eukaryotic Systems. Chembiochem: a European journal of chemical biology. 
         Stech M, Brodel A K, Quast R B, Sachse R, Kubick S. 2013. Cell-free systems: functional modules for synthetic and chemical biology. Advances in biochemical engineering/biotechnology 137: 67-102. 
         Zawada, J. F. et al. Microscale to manufacturing scale-up of cell-free cytokine production—a new approach for shortening protein production development timelines. Biotechnol Bioeng 108, 1570-1578 (2011). 
         Hong, S. H. et al. Improving cell-free protein synthesis through genome engineering of  Escherichia coli  lacking release factor 1. Chembiochem 16, 844-853 (2015). 
         Kwon, Y. C. &amp; Jewett, M. C. High-throughput preparation methods of crude extract for robust cell-free protein synthesis. Sci Rep 5, 8663 (2015). 
         de Los Santos E L, Meyerowitz J T, Mayo S L, Murray R M. 2015. Engineering Transcriptional Regulator Effector Specificity Using Computational Design and In Vitro Rapid Prototyping: Developing a Vanillin Sensor. ACS synthetic biology. 
         Niederholtmeyer H, Sun Z Z, Hori Y, Yeung E, Verpoorte A, Murray R M, Maerkl S J. 2015. Rapid cell-free forward engineering of novel genetic ring oscillators. eLife 4: e09771. 
         Sun Z Z, Yeung E, Hayes C A, Noireaux V, Murray R M. 2014. Linear DNA for rapid prototyping of synthetic biological circuits in an  Escherichia coli  based TX-TL cell-free system. ACS synthetic biology 3: 387-397. 
         Jewett, M. C. &amp; Swartz, J. R. Mimicking the  Escherichia coli  cytoplasmic environment activates long-lived and efficient cell-free protein synthesis.  Biotechnol Bioeng  86, 19-26 (2004). 
       
    
     Example 7—Cell-Free Biosynthesis of HMG-CoA 
     A. Abstract 
     Metabolic engineering uses enzymes to produce small molecules with industrial, pharmaceutical, and energy applications. However, efforts to optimize enzymatic pathways for commercial production are limited by the throughput of assays for quantifying metabolic intermediates and end products. We developed a multiplexed method for profiling CoA-dependent pathways that utilizes a cysteine terminated peptide to covalently capture CoA bound metabolites. Captured metabolites are then rapidly separated from the complex mixture by immobilization onto arrays of self-assembled monolayers and directly quantified by SAMDI mass spectrometry. We demonstrate the throughput of the assay by characterizing the cell-free synthesis of HMG-CoA, a key intermediate in the biosynthesis of isoprenoids, collecting over 10,000 individual spectra to map more than 800 unique reaction conditions. We anticipate that our rapid and robust analytical method will accelerate efforts to engineer metabolic pathways. 
     B. Introduction 
     Biosynthetic pathways offer new opportunities for producing molecules with medical, energy, and industrial applications at commercial scales (1). Coenzyme A is a central molecule in metabolism and is required in over 100 distinct reactions, serving as an obligate cofactor for 4% of known enzymes (2). Acetyl-CoA serves as the universal precursor for numerous biosynthetic pathways including isoprenoids, fatty acids/alkanes, polyketides, bioplastics, and a number of biofuels (3). In yeast alone, acetyl-CoA is involved in 34 different metabolic reactions (1). A diverse range of bioengineering efforts have used CoA dependent pathways; these include the semi-synthetic production of the antimalarial drug artemisinin (4), the biosynthesis of methyl-butenol as a potential biofuel (5), and the bioengineering of brewer&#39;s yeast to produce the hop flavor compounds linalool and geraniol (6). The importance of CoA metabolism has motivated the engineering of organisms such as  S. cerevisiae  as “chassis” strains for overproducing acetyl-CoA (7) or other short chain acyl-CoA biosynthetic precursors (8).While several pathways have been successfully developed at the commercial scale (9), engineering cells is a time-consuming and non-intuitive process that can require hundreds of person years to tune enzymatic pathways (10). Many approaches use an iterative design-build-test (DBT) cycle to test numerous “weak hypotheses” and explore a large parameter space in which the underlying biology is not completely understood, or where the complexity of the system does not yet permit rational design. Recent advances in DNA synthesis and assembly highlight new capabilities to design and build biological systems (11, 12), while emphasizing the need for additional methods to rapidly test biosynthetic reaction systems. For CoA-bound metabolites, the current state-of-the-art detection methods rely on column-based separation, followed by mass spectrometry (13-17). These methods are highly sensitive (pmol of analyte per sample) (18) and can be adapted to measure multiple acyl-CoA compounds in a single analysis (19), but have low throughput, generally requiring greater than 15 minutes per analysis. Colorimetric screens and intracellular metabolite-sensing circuits offer increased throughput, but are typically specific for a single CoA-bound molecule and require laborious redesign to quantitate new target molecules (20). 
     In this work, we developed an assay based on SAMDI-MS (Self-Assembled Monolayers for MALDI-TOF mass spectrometry) that is capable of rapidly detecting CoA-bound metabolites in high-throughput. In SAMDI, biochemical assays are performed on self-assembled monolayers (SAMs) of alkanethiolates on gold, and the resulting immobilized reaction products are directly quantified using MALDI-TOF mass spectrometry (21). Previously, we have shown that SAMDI can be used as a general assay platform to profile a wide range of enzyme activities in situ and in complex lysates (22-26). Here, we demonstrate how SAMDI can characterize biosynthetic pathways by immobilizing a specific class of metabolites to the surface and obtaining mass spectra to quantitate each metabolite. Specifically, we use selective bioconjugation to capture all acyl intermediates on CoA followed by quantitative analysis by SAMDI-MS. The use of chemically defined monolayers enables the rapid isolation of all acyl-CoA species from complex lysates while simultaneously serving as the platform for detection. 
     A significant benefit of SAMDI is the compatibility with microtiter formats and laboratory automation to allow evaluation of tens of thousands of reactions per day (22, 24, 27). Here, we use this robust and high-throughput method to characterize a cell-free reaction system engineered to produce hydroxymethylglutaryl-CoA (HMG-CoA), the biosynthetic precursor to mevalonate and isoprenoid metabolites. Cell-free engineering is a powerful approach for accelerating the design and construction of biosynthetic pathways and has the further benefit that it is compatible with assay automation (28). In cell-free reactions, biosynthetic pathways are reconstituted in crude lysates, rather than living cells, bypassing the need for laborious metabolic engineering. These cell-free systems are liberated from the regulatory processes that, in vivo, support cell viability and growth, allowing for the design of synthetic pathways that may be difficult or impossible in living cells (28-30). We demonstrate the combination of cell-free reactions and high-throughput characterization with SAMDI to perform thousands of biosynthetic reactions in a single experiment. This strategy, applied here for the optimization of HMG-CoA biosynthesis, greatly increase the speed at which a large reaction space can be mapped for complex networks of enzymes. 
     C. Developing a Selective Assay for CoA Metabolites 
     We identified the thioester functional group, through which the CoA cofactor binds metabolites, as a unique chemical handle for bio-specific capture. Thioesters have been used extensively to ligate peptides via the native chemical ligation (NCL) chemistry pioneered by Kent and coworkers (31). We recognized that this ligation strategy could also be used to capture acyl moieties from CoA with the specificity required to operate in complex cell lysates. A peptide with an N-terminal cysteine was used for covalent capture, immobilization and detection of CoA metabolites. When the capture peptide is added to reactions containing CoA bound species, it readily scavenges the acyl groups from CoA by a trans-thioesterification reaction followed by a rapid internal rearrangement to irreversibly transfer the captured analyte to the N-terminal amine of the peptide. The reaction mixture is then applied to a monolayer presenting a maleimide group, where the free thiol of the cysteine undergoes immobilization to the monolayer and the lysate can be removed by rinsing ( FIG. 50A ). The resulting monolayers are covalently tethered to all of the intermediates and products from the CoA biosynthesis and can be directly analyzed by SAMDI mass spectrometry. 
     NCL has previously seen limited use for in vivo bioconjugation, with this chemical strategy used to label proteins containing N-terminal cysteines with fluorophores that possess thioester handles (32). However, the possibility of cross reactivity of NCL chemistry with biological thioesters, such as acyl-CoA species and intermediates of fatty acid synthesis, has been cited as a limitation of the chemistry for use as a bioconjugation technique (33). In this work, we take advantage of this potential shortcoming, using it to develop an assay targeting acyl-CoA metabolites. 
     We first validated this chemical strategy for capture and detection using a variety of commercially available, biologically relevant, CoA compounds. These CoA species were each reacted with the capture peptide, of sequence CAK(Me) 3 SA, in phosphate buffered saline (PBS) at pH 7 and 40° C., at concentrations of 500 μM of CoA species and 1 mM of capture peptide for 2 hrs. The reactions were then applied to self-assembled monolayers presenting maleimide at a density of 20% against a background of tri(ethylene glycol) groups and analyzed by SAMDI mass spectrometry to reveal peaks that correspond to capture of the acyl groups ( FIG. 50B ). For the purposes of this assay, the capture peptide requires an N-terminal cysteine, and the remaining sequence does not strongly impact the reaction. The peptide used in this work was chosen because of its high ionization efficiency, a characteristic that is important for maximizing the limit of detection in mass spectrometry. 
     D. Characterization of Pathway Kinetics and Flux 
     We applied this strategy to analyze the cell-free biosynthesis of HMG-CoA ( FIG. 51A )—a biosynthetic precursor to isoprenoids, which represent a diverse and useful class of molecules with applications in pharmaceuticals, flavorings, fragrances, commodity chemicals, and biofuels (4, 34-37). This pathway proceeds from intermediates acetyl-CoA (Ac-CoA) and acetoacetyl-CoA (AA-CoA) to HMG-CoA. We assembled the biosynthetic reactions by mixing crude  E. coli  lysates that separately overexpressed the necessary acetyl-CoA acetyltransferase and HMG synthase enzymes. The cell-free reactions also allowed precise control of the concentrations of glucose substrate, buffer, salts, and cofactors (38). All cell-free reactions were done in standard 384 microtiter well plates and were quenched by addition of formic acid. For capture of CoA metabolites, the pH was adjusted to 6.5 and the capture peptide was introduced. After capture, the reaction mixtures were diluted several fold and applied to arrays of monolayers presenting maleimide functional groups using liquid handling robotics, where captured analytes were selectively isolated from the reaction mixture ( FIG. 51B ). Analysis was performed using SAMDI arrays of 1536 spots, where each spot is 1 mm in diameter. The reactions from the 384 well plates were each spotted in quadruplicate so that 4 replicate spectra could be averaged for each reaction. For each spot, 0.5 μL of the reaction mixture was used for immobilization. 
     We first characterized the kinetics of HMG-CoA production in these cell free reaction systems with cofactors ATP, NAD + , and CoA each supplemented at 1 mM ( FIG. 51C-D ). We measured the rate of HMG-CoA production in two systems containing either acetate or glutamate salts, commonly used in cell-free reactions as necessary counterions for K + , NH 4   + , and Mg 2+  ions (39). Both systems reached similar yields of just over 600 μM of HMG-CoA, though HMG-CoA accumulated more rapidly in the acetate salts, where it was detectible at the earliest reaction times. The same reaction in the glutamate salts required approximately 30 min of reaction time to observe HMG-CoA. 
     While glucose is the intended primary carbon source for metabolism, acetate and glutamate act as secondary sources of carbon and can also be metabolized by enzyme pathways native to  E. coli  and converted to HMG-CoA (40) ( FIG. 51A ). We explored the utilization of these different carbon sources by repeating the experiments with  13 C 6 -glucose and quantifying the isotopic incorporation into the final product. Similarly,  13 C 2 -acetate was used to monitor metabolic flux from acetate. HMG-CoA is composed of three, 2-carbon acetyl subunits such that four possible label states (0, +2, +4, or +6 mass units) are observed. We quantified the relative abundance of each from the mass spectra and then used these values to determine the fraction of labeled carbon, f C13 , and the corresponding fraction of unlabeled carbon, f C1 2 , in the HMG-CoA produced ( FIG. 56 ). 
     We further used the label incorporation to calculate the concentration of HMG-CoA produced. A known amount of commercially available, unlabeled, HMG-CoA was spiked into each capture reaction and the fraction of label incorporation determined before and after. The change in measured label content can be used to calculate the original concentration of HMG-CoA, as described by equation 1 where f i   C12  represents the fraction of unlabeled carbon before the spike and f spike   C12  represents the fraction of unlabeled carbon after the spike. From these values, the initial concentration of HMG-CoA, [HMG] i , can be determined. 
     
       
         
           
             
               
                 
                   
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     This quantification method is robust to any variations in signal intensity across spectra, as it relies on the peak splitting pattern for the HMG product, and enabled the tracking of both HMG-CoA concentration and carbon usage simultaneously. 
     The cell-free reactions performed here contained 200 mM of glucose and 150 mM of either acetate or glutamate. We observed that acetate was the primary source of carbon in the acetate system, and for early timepoints it was the source for the bulk of the HMG-CoA produced ( FIG. 51C ). With longer reaction times, glucose utilization increased and leveled off at 30% of the incorporated carbon. In the glutamate system, glutamate was also a major source of carbon, but the carbon fraction from glucose reached the higher level of 55% after 6 hours ( FIG. 51C-D ). These results show both acetate and glutamate metabolism is active and both are significant carbon sources and not just passive components of the cell-free system, consistent with previous reports (40). 
     E. Identification of an Additional Metabolite 
     It is difficult to independently and quantitatively control the concentrations of cofactors in cell-based metabolic engineering. The cell-free approach used here enables straightforward, independent optimization of buffer composition, cofactor concentrations, and carbon sources to achieve the greatest yield of biosynthetic targets. In particular, supplementation of ATP, NAD + , and CoA can dramatically influence cell-free metabolism (38). As described below, we mapped the dependency of HMG-CoA synthesis over a wide range of these cofactors. However, during initial experiments, we observed an unidentified product bound to the peptide sensor that did not correspond to any of the expected CoA metabolites. This unidentified product gave a mass shift of +72 Da, and this peak increased by 3 mass units when the lysates were supplemented with  13 C 6 -glucose. This mass and carbon content was consistent with a lactate-functionalized peptide, but no enzyme in  E. coli  is known to produce lactyl-CoA. 
     We suspected this unidentified product was derived from glycolysis and identified the source as glyceraldehyde-3-phosphate (G3P) by biochemically reconstituting the initial steps of glycolysis using five purified enzymes ( FIG. 52 ). When the pathway was reconstructed stepwise and fed glucose, this species appeared only when all of the enzymes necessary to produce G3P were present. We hypothesize that the peak derives from a pyruvaldehyde intermediate, which is formed from G3P and dihydroxyacetone phosphate (DHAP) through elimination of the phosphate, and which is a known toxic byproduct of glycolysis (41). The enzyme that interconverts G3P and DHAP, triosephosphate isomerase, has also been shown to catalyze the formation of pyruvaldehyde from these species (42-44). Pyruvaldehyde reacts with thiols, through a thiohemiacetal rearrangement, to produce a lactyl-thioester (45, 46) ( FIG. 53A ). In this case, the resulting lactyl-thioester can further react to yield a N-lactyl-peptide. 
     We also observed a peak corresponding to the aldol-condensation product of two molecules of pyruvaldehyde ( FIG. 53B ). This dimeric adduct results in a product with the same mass shift as HMG-CoA. However, it can be differentiated from HMG because it derives from two 3-carbon subunits and possesses a characteristic +3, +6 labeling pattern when isolated from lysates that are supplemented with  13 C 6  glucose. We found that this aldol condensation product was only observed at a pH above 7. Therefore, to exclude capture of this adduct that can interfere with HMG-CoA detection, we performed all capture reactions at pH 6.5. The thioester rearrangement still proceeds at this pH, but the aldol condensation does not occur. 
     F. Mapping Metabolite Levels and Pathway Flux 
     To optimize HMG-CoA production and better understand the effect of cofactor concentrations on the reaction network, we performed a high-throughput experiment that tested 768 unique cofactor conditions—where the concentrations of NAD, CoA and ATP were systematically varied. For each condition three replicate cell-free reactions were performed, and for each reaction four spectra were collected and averaged, totaling over 9,000 individual spectra ( FIG. 54 ). All cell-free reactions were allowed to proceed for 2 hours, quenched, reacted with the capture peptide, and immobilized as described above. Once immobilized, each 1536-spot array can be read by MALDI-TOF MS in just 50 minutes. We did not observe AA-CoA under any conditions tested. In a separate control, we supplemented reactions with commercial AA-CoA and found that it was readily converted back to Ac-CoA, suggesting that the acetyl-CoA acetyltransferase favors the reverse reaction and so no significant amount of AA-CoA accumulates ( FIG. 57 ). For each spectrum, we determined the percent conversion for the three primary observed species (HMG-CoA, Ac-CoA, and G3P) using the integrated areas under the peaks (AUP) for the unreacted capture peptide and the products. Below, equation 2 shows how this calculation was done for HMG and analogous calculation were performed for acetyl and G3P products as well. 
     
       
         
           
             
               
                 
                   
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     These high throughput experiments revealed several notable trends ( FIG. 54 ). First, when low concentrations of CoA and high concentrations of ATP were used, G3P dominated as the major product. This is perhaps not unexpected as low CoA limits the total amount of HMG-CoA the system is capable of synthesizing and the high ATP accelerates the early, pay-in phase of glycolysis. Second, as the concentration of CoA was increased, the amount of G3P decreased; this trade-off was much more significant when reactions were performed in the system using acetate. Thus, G3P was prominent in conditions favoring early glycolysis, but where metabolic flux was unable to proceed to Ac-CoA. 
     We analyzed the same reaction dataset to monitor yield of and metabolic flux to HMG-CoA ( FIG. 55A ). As described earlier, we used  13 C labeled glucose to monitor metabolic flux from glucose across the array. For both acetate and glutamate systems, we identified the three reactions that produced the highest concentration of HMG-CoA among the set of 768 reactions. The averages of these highest yielding reactions were 6.4±0.5 mM and 2.6±0.1 mM for the acetate and glutamate systems, respectively. Across the array, the AUP for the HMG-CoA product was normalized to an internal standard present at a constant concentration across all reactions to control for variations in ionization across spectra. For this standard we used the peptide Ac-SK(Me) 3 GGC which possesses similar ionization efficiency to the capture peptide but lacks an N-terminal cysteine and does not overlap in mass with any species of interest. 
     Metabolic flux and HMG-CoA production complexly interplayed with cofactor concentrations and we observed significantly different reaction profiles for the systems using acetate and glutamate salts. For the acetate system, higher CoA concentrations improved HMG-CoA titers but also required an associated increase in ATP concentration to achieve this maximal concentration. At low ATP concentrations, increasing available CoA inhibited HMG-CoA production ( FIG. 55B ). Conversely, over the 2-hour reaction time, the glutamate system reached peak HMG-CoA production at 4 mM CoA. Increasing CoA concentration further to 6 and 8 mM did not increase production of HMG-CoA. However, this shifted the region where highest product concentrations were observed, requiring a shift to higher NAD +  concentration and somewhat higher ATP concentration to maintain the same titers ( FIG. 55C ). 
     When product concentration is highest, the fraction of carbon derived from glucose accounted for 5% of product in the acetate salts and 22% in the glutamate salts ( FIG. 55A ). As in  FIG. 52 , we see significant incorporation of non-glucose carbon (i.e. acetate and glutamate) in the final product for all cofactor conditions tested. At high CoA concentrations, the glucose incorporation decreased. Counterintuitively, in the acetate system, the best cofactor conditions limit glycolytic flux in favor of optimizing the four-step reaction from acetate to HMG-CoA exclusively. These results suggest that multiple substrates are an important consideration when optimizing cofactors for crude-lysate cell-free metabolic engineering. 
     Non-intuitive relationships underlie the optimization of complex biological reaction networks. In many cases, understanding the complex interdependency between system variables can only be ascertained by methodical testing of a large number of reaction conditions. In this work, we describe the design of an assay that enables the high-throughput analysis of CoA utilizing metabolic pathways. This assay combines a general strategy for biospecific capture of CoA bound metabolites with simple purification by covalent immobilization onto self-assembled monolayers, that serve as the platform for detection by SAMDI mass spectrometry. We applied this assay to the characterization of a cell-free reaction system for the biosynthesis of HMG-CoA, demonstrating the ability to perform and analyze thousands of individual biosynthetic reactions and to map the reaction space for this complex system with respect to multiple variables. 
     This assay represents a new, generalizable method for understanding and optimizing biosynthetic pathways. Technologies for rapid characterization can synergize with existing high capacity methods for design and construction of biological systems; such projects can also provide large experimental datasets to inform development of improved design tools. The method described can track multiple metabolites simultaneously and unambiguously using mass spectrometry and is compatible with tracking of metabolic flux using isotopic labeling. When compared to conventional column-based analysis of CoA metabolites, this strategy offers an orders-of-magnitude increase in scale and speed of analysis which should enable high throughput metabolic engineering and decrease the time needed to develop high yielding biosynthetic systems. 
     G. Materials and Methods 
     1. Preparation of Enzyme-Enriched Lysates 
     Two strains of  E. coli  BL21(DE3) containing plasmids pETBCS-ACAT(Eco) and pETBCS-HMGS(Sce) were grown in a 1 liter fermenter at 37° C. (250 rpm) at constant pH (6.95) in rich media (18 g·L −1  glucose, 16 g·L −1  yeast extract, 10 g·L −1  tryptone, 5 g·L −1  NaCl, 7 g·L −1  potassium phosphate dibasic (K 2 HPO 4 ), 3 g·L −1  potassium phosphate monobasic (KH 2 PO 4 )) containing 100 μg·mL −1  carbenicillin (IBI Scientific, Peosta, Iowa). After induction with 0.1 mM IPTG at OD 600  0.6, growth continued for four hours at 30° C. until harvest by centrifugation. See (38) for plasmid sequences and expression characterization.  E. coli  cells were lysed and crude lysates were generated using methods previously described (38). 
     2. Cell Free Reactions 
     Cell free reactions were performed at a volume of 15 L in 384-well plates and incubated at 37° C. The standard reaction contained the following components: 200 mM glucose, 100 mM Bis Tris buffer, acetate or glutamate salts (8 mM magnesium, 10 mM ammonium, 134 mM potassium), and 10 mM potassium phosphate (K 2 HPO 4 , pH 7.2). Unless specified, reactions also included 1 mM NAD + , 1 mM ATP, and 1 mM CoA (38). All reagents and chemicals were purchased from Sigma Aldrich (St. Louis, Mo.). Two lysates, enriched in ACAT(Eco) and HMGS(Sce) respectively, were mixed together at a total protein concentration of 5 mg·mL −1  each. Reactions were quenched by precipitating proteins using 2.25 μL of 10% formic acid and immediately stored at −80° C. until peptide incubation. Acetic acid-2- 13 C was neutralized to acetate (pH 7.00) by titration with 45% w/w potassium hydroxide and diluted to 5 M stock concentration. 
     3. Preparation of Monolayer Arrays 
     Array plates were prepared by patterning 1536 gold spots, in a standard microtiter format, on steel plates using electron-beam metal evaporation to deposit 5 nM of titanium, followed by 30 nM of gold. These plates were soaked in a solution of disulfide molecules in ethanol for 24-48 hours to form a self-assembled monolayer on the gold surfaces. The solution consisted of a mixture of EG3-alkanethiol disulfide and a mixed disulfide of EG3-alkanethiol and maleimide-terminated EG3-alkanethiol (47). The two disulfide molecules were present in a stoichiometric ratio to yield a 20% maleimide surface density, with an overall concentration of 1 mM. After formation of this primary monolayer on the gold spots, the plates were rinsed with ethanol and dried. The array plates were then soaked in a 10 mM hexadecylphosphonic acid solution in ethanol for 15 minutes. The phosphonic acid terminated hydrocarbon molecules react selectively with the steel background giving it hydrophobic properties that aid with delivery of sub-microliter reaction volumes to the spots. 
     4. Synthesis of Peptide Reagents 
     The peptide of the sequence CAK(Me) 3 SA (SEQ ID NO:108) was used in this work for capture of acyl-CoA species. The peptide Ac-SK(Me) 3 GGC (SEQ ID NO:109) was used as an internal standard for normalization of signal across reactions, as it possesses similar sequence and ionization efficiency to the capture peptide but is mass-resolved and lacks an N-terminal cysteine. Standard FMOC solid phase peptide synthesis on rink-amide resin was used to synthesize both peptides. To introduce the non-natural trimethyl-lysine residue, FMOC-Lys(Me) 3 Cl was purchased from Novabiochem and used along with standard amino acid coupling conditions. 
     5. Capture of CoA Bound Moieties in Lysates 
     After completion, cell-free reactions were quenched with formic acid to denature the proteins and stop the reactions. The reactions, in 384 well plates, were centrifuged at 3,500×g for 15 minutes to pellet any precipitated protein. For the acyl-CoA capture reactions, 3 μL of this cell-free reaction was transferred to a new 384-well plate and the following species were added, bringing the final reaction volume to 8 μL with the final concentrations as follows: 100 mM phosphate buffer at pH 6.5, 40 mM EDTA, 0.9 mM capture peptide, and 0.1 mM normalization peptide. The well plates were sealed, and the reaction mixtures were incubated at 42° C. for 3 hours. It is important to choose and appropriate concentration of capture peptide. If the concentration is too high, the signal from unreacted sensor peptide may overwhelm the signal from any captured species. In this work the total added peptide concentration was chosen to be 1 mM across all reactions, a reasonable concentration relative to the expected yield from the best cell-free reactions, while also giving good dynamic range for detection of acyl-species in low-yield conditions. 
     6. Immobilization of Captured Products 
     The 8 μL capture reactions in 384 well plates were diluted 3-fold to 24 L with 100 mM phosphate buffer at pH 7.2. This serves to dilute the concentration of peptide, which will immobilize efficiently at concentrations as low as 10 micromolar, and adjust the pH to an optimal range for the reaction with maleimide. A TECAN liquid handling robot equipped with a 384-tipped head was used to print the reaction mixtures onto 1536-spot array surfaces, generating 4 replicates per reaction, at a volume of 0.5 L per spot. The surfaces were placed in a humidified chamber and incubated at 37° C. for 60 minutes to allow the cysteine-containing peptides to react with the maleimide functionalized SAM. After reaction, the surfaces were rinsed with 1% SDS detergent, then rinsed with distilled water and dried under a stream of nitrogen gas. 
     7. Analysis of Reactions 
     Matrix of THAP in acetonitrile (20 mg/mL) was applied directly to the 1536 spot-surface. The matrix was allowed to dry, and each spot was analyzed by MALDI-TOF mass spectrometry using an AB Sciex 5800 series instrument. Captured metabolites of the cell free reactions were identified by their mass shifts relative to the unreacted capture peptide and quantified by integration of the corresponding peaks. 
     H. References for Example 7 
     
         
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     Example 8: Cell-Free Biosynthesis of Limonene, Pinene and Bisabolene 
     A. Abstract 
     Metabolic engineering of microorganisms to produce useful compounds from renewable substrates is a promising means for sustainable, on-demand production of chemicals. However, efforts to design and engineer microbial cell factories are constrained by costly and slow “build” times in which each genetic variation requires re-engineering a new strain. To alleviate this challenge, we have utilized a cell-free approach (termed iPROBE) which uses cell-free protein synthesis (CFPS) to manufacture pathway enzymes in separate reactions that are then mixed to assemble a cell-free, plug-and-play prototyping system for limonene biosynthesis. This approach shortens the time from receiving genes to characterizing active enzymes to ˜3 days and allows precise measurement and control of enzyme concentration. We compared 54 enzyme homologs which support nine pathway steps in an initial screen identifying 12 low-activity enzymes. The remaining 42 enzymes were iteratively screened in multiple enzymes sets and in multiple physiochemical conditions to identify enzyme sets which performed robustly. In total, we screened over 590 unique pathway conditions with the best condition producing 4.5 mM (610 mg/L) of limonene in 24 h. Finally, to demonstrate the modularity of this pathway, we also synthesized high titers of the biofuel precursors pinene and bisabolene. We anticipate this prototyping system will provide a controllable and flexible platform for high-throughput screening of enzyme combinations that inform design of microbial cell factories. 
     B. Introduction 
     Isoprenoids are a promising class of molecules with over 40,000 known structures and potential uses as pharmaceuticals, flavors, fragrances, pesticides, disinfectants, and chemical feedstocks (Bohlmann and Keeling, 2008; George et al., 2015; Jongedijk et al., 2016). While there have been demonstrations of cellular isoprenoid production at commercial scales (Benjamin et al., 2016; Paddon et al., 2013), efforts to engineer strains for new products have proved challenging. In particular, efforts to explore large sets of heterologous expression conditions are constrained by the need to re-engineer the cell in each iteration. This constraint has limited cellular approaches to 20-40 unique strains in each effort, modifying expression conditions such as ribosome binding site strength (Li et al., 2019; Nowroozi et al., 2014), enzyme expression timing (Alonso-Gutierrez et al., 2015), and plasmid architecture (Yang et al., 2016). New developments in parallelized DNA assembly and robotic liquid handling have enabled the testing of 122 plasmid architectures for the 16-gene refactored nitrogen fixation gene cluster from  Klebsiella oxytoca  (Smanski et al., 2014), the construction of “Marionette” strains for prototyping 243 different expression profiles for lycopene pathway enzymes (Meyer et al., 2019), and the characterization of thousands of ribosome binding site combinations for tuning the production of limonene (Jervis et al., 2019) and naringenin (Zhou et al., 2019). However, these efforts have not been adapted to characterizing enzyme homologs which can significantly enhance performance (Ma et al., 2011; Tsuruta et al., 2009). 
     Cell-free metabolic engineering offers tremendous flexibility to quickly tune reaction components, substrates, and cofactors (Dudley et al., 2015), giving access to test hundreds of unique pathway expression conditions. Cell-free systems using purified enzymes in particular have shown utility in prototyping metabolic pathways for a range of compounds including fatty acids (Liu et al., 2010), farnesene (Zhu et al., 2014), phenylalanine (Ding et al., 2016), and non-oxidative glycolysis (Bogorad et al., 2013). Crude lysates are becoming increasingly popular for prototyping metabolism because lysates contain competing pathways, alternate substrates, and cofactors (Miguez et al., 2019; Schuh et al., 2019). Additionally, when provided with an energy source, amino acids, NTPs, and excess cofactors, crude lysates contain the translational machinery for cell-free protein synthesis (CFPS) which enables rapid production of proteins (Carlson et al., 2012; Silverman et al., 2019). CFPS decreases the time from DNA to soluble protein and can be used to synthesize functional catalytic enzymes (Karim and Jewett, 2016). The hybrid approach of cell-free protein synthesis metabolic engineering (CFPS-ME) has been successfully adapted to prototype polyhydroxyalkanoate (Kelwick et al., 2018) and 1,4-butanediol (Wu et al., 2015; Wu et al., 2017) production. Yet, even with the rapid ability to synthesize and test enzymes in vitro, these examples have utilized only a small set of enzyme homologs in their optimization strategies. 
     In this work, we describe a modular, high-throughput isoprenoid production platform for quickly prototyping enzyme homologs, concentrations, and reaction conditions. By expressing pathway enzymes using CFPS in separate reactions and then mixing them together in known concentrations, we assemble pathway combinations for production of the monoterpenoid limonene. This conceptual approach, deemed in vitro Prototyping and Rapid Optimization of Biosynthetic Enzymes (iPROBE), dramatically shortened the time to prototype 3-hydroxybutyrate and n-butanol production for improving in vivo biosynthesis in  Clostridium  (Karim et al., 2020). Here we expand this approach to longer isoprenoid pathways. We use the 9-heterologous enzyme pathway to limonene as a model pathway ( FIG. 58A ). We screened 590 unique pathway combinations testing 45 different enzyme variants in several reaction (cofactor) conditions. By screening hundreds of enzyme sets and various reaction formats, we were able to improve production 25-fold from our initial setup. We also demonstrated pathway modularity by swapping out the isoprenoid synthetase to produce pinene and bisabolene. Our results suggest that previous cell-free isoprenoid systems, which have reported fewer than 20 enzyme and reaction combinations (Chen et al., 2013; Dirkmann et al., 2018; Korman et al., 2017; Korman et al., 2014; Rodriguez and Leyh, 2014; Zhu et al., 2014), could benefit from screening more enzyme variants. With the ability to test dozens of enzyme homologs in hundreds of combinations without needing to re-engineer a cell or re-assemble DNA, we expect iPROBE to enhance efforts to prototype isoprenoid and other biosynthetic pathways for cellular or cell-free biomanufacturing. 
     C. Materials and Methods 
     1. Strain, Plasmid, and Lysate Preparation 
     All enzyme sequences tested in CFPS were cloned into the pJL1 backbone (Addgene #69496). To assemble plasmids encoding previously tested enzyme homologs (Dudley et al., 2019), the coding sequence of each enzyme was PCR-amplified using forward primer [ttaactttaagaaggagatatacatatggagaaaaaaatcNNNNNNNNNNNNNNNNNNNN (SEQ ID NO:110) where the N-region encodes the gene sequence starting at the second codon (i.e. right after the ATG)] and reverse primer [ttcctttcgggctttgttagcagccggtcgacNNNNNNNNNNN (SEQ ID NO: 111) where the N-region encodes the C-terminus of the gene sequence including the stop codon]. The forward primer adds an N-terminal expression tag to improve cell-free expression which consists of a 15 nucleotide, AT-rich sequence encoding the first five amino acids (Met-Glu-Lys-Lys-Ile, MEKKI (SEQ ID NO: 13)) of chloramphenicol acetyl transferase which was used as a reporter plasmid during development of the  E. coli  CFPS system (Jewett and Swartz, 2004; Swartz et al., 2004). Addition of the MEKKI (SEQ ID NO: 13) sequence demonstrably improved in vitro expression ( FIG. 67 ). The PCR fragment was then mixed with pJL1 backbone digested with the restriction enzymes NdeI and SalI along with reagents for Gibson assembly (Gibson et al., 2009). 
     To obtain additional sequences for testing, we first generated phylogenetic trees of pathway homologs were generated using Geneious bioinformatics software (Auckland, New Zealand). Sequences for reactions 2.7.1.36 (MK), 2.7.4.2 (PMK), 4.1.1.33 (PMD) 5.3.3.2 (IDI), and 4.3.2.3.16/19/20 (LS) were downloaded from the BRENDA database (Scheer et al., 2010) and aligned in Geneious using a Jukes-Cantor genetic distance model and a Neighbor-Joining tree build method. To choose uncharacterized homologs for testing, we selected 15 sequences from branches of the phylogenetic tree not represented by 30 previously characterized homologs (Table at  FIG. 63 ). These 45 sequences were then codon optimized for expression in  E. coli  synthesized by Gen9 (Cambridge, Mass.) or Twist Bioscience (San Francisco, Calif.). Sequences from Twist were delivered cloned into the pJL1 backbone. Gene sequences contained two sequential NdeI recognition sequences at the start of each gene resulting in catATGCATATGGAGAAAAAAATC (SEQ ID NO:14) (encoding MHMEKKI (SEQ ID NO:15)) instead of catATGGAGAAAAAAATC (SEQ ID NO:12) (encoding MEKKI (SEQ ID NO:13)). Upon comparison, the CFPS expression was equivalent for both N-terminal expression tags ( FIG. 68 ) and both tags ultimately used (see Table at  FIG. 63 ). 
     Lysates pre-enriched with a pathway enzyme were generated as described previously (Dudley et al., 2019). To generate CFPS S30 lysate for cell-free protein synthesis, BL21 Star(DE3)  E. coli  was grown in 1 L of 2×TYPG media in Tunair™ shake flasks at 37° C. (250 rpm). Expression of T7 TRNA polymerase was induced at OD 600 =0.6 by addition of 0.1 mM IPTG and cells were harvested by centrifugation at OD 600 =3.0. Pellets were washed twice in S30 buffer (10 mM tris acetate pH 8.2, 14 mM magnesium acetate, 60 mM potassium acetate, no dithiothreitol (DTT)), flash frozen, and stored at −80° C. Cell pellets were then thawed on ice, resuspended in S30 buffer without DTT (0.8 mL per gram cell pellet), lysed via pressure homogenization with one pass at 20,000 psi (Avestin EmulsiFlex-B15), and centrifuged twice at 30,000×g for 30 minutes. The supernatant (i.e. lysate) was transferred to a new container without disturbing the pellet, aliquoted, and flash frozen for storage at −80° C. Optimal magnesium concentration was determined to be 8 mM based on expression of the plasmid pJL1-sfGFP (Addgene #69496) encoding superfolder Green Fluorescent Protein (sfGFP). 
     2. Cell-Free Protein Synthesis Reactions 
     All CFPS reactions used a modified PANOx-SP formula (Jewett and Swartz, 2004). 
     Each reaction contains 13.3 μL of S30 extract for a 50 μL CFPS reaction ( FIG. 69 ) in addition to ATP (1.2 mM), GTP, UTP, CTP (0.85 mM each), folinic acid (34 μg/mL),  E. coli  tRNA mixture (170 μg/mL), 20 standard amino acids (2 mM each), NAD +  (33 mM), coenzyme A (CoA) (0.27 mM), oxalic acid (4 mM), spermidine (1.5 mM), putrescine (1 mM), HEPES (57 mM), potassium glutamate (134 mM), ammonium glutamate (10 mM), magnesium glutamate (8 mM), phosphoenolpyruvate (PEP) (33 mM), and plasmid (13.3 μg/mL) encoding the metabolic pathway enzyme. T7 RNA polymerase was not added to CFPS reactions since extracts were induced with IPTG during cell growth. CFPS reactions of pathway enzymes are incubated for 20 hours at 30° C. or 16° C. (Table at  FIG. 63 ), flash frozen on liquid nitrogen, and stored at −80° C. 
     3. Quantification of CFPS Protein Production Using Radioactive Amino Acid Incorporation 
     To measure the amount of protein produced in a CFPS reaction,  14 C-leucine (10 μM) was supplemented in addition to the 20 standard amino acids. Reactions were centrifuge at 21,000×g for 10 minutes to pellet insoluble proteins for measurement of soluble protein. Reactions were quenched by addition of equal volume 0.5 M potassium hydroxide and pipetted onto two separate 96-well fiberglass papers (PerkinElmer Printer Filtermat A 1450-421, 90×120 mm) and dried. One paper was subjected to three trichloroacetic acid (TCA) washes (15 min each at 4° C.) to precipitated proteins and dried after rinsing with 100% ethanol. Scintillation wax (PerkinElmer MeltiLex A 1450-441 73×109 mm) was applied and radioactivity was measured using liquid scintillation counting via a MicroBeta2 (PerkinElmer, Waltham, Mass.). Protein concentration was determined as previously described (Jewett, 2004; Jewett et al., 2008; Jewett and Swartz, 2004) by comparing radioactivity in the total reaction to that of precipitated protein using Equation 1 below. 
     
       
         
           
             
               
                 
                   
                     
                       
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     4. Mixing of CFPS Reactions and Pre-Enriched Lysates to Produce Limonene 
     All limonene synthesis reactions are 30 tL in total volume and can be divided into the “CFPS fraction” and the “substrate/lysate/cofactor fraction”. The 15 μL CFPS fraction contains six to nine CFPS reactions (thawed on ice) and mixed at concentrations dictated by the experiment. “Blank” CFPS reaction (50 μL volume containing all CFPS reagents, no plasmid, and incubated for 20 h at 30° C.,  FIG. 70 ) is added until the total volume of CFPS fraction is 15 μL. The 15 μL “substrate/lysate/cofactor fraction” includes the following components (concentrations are given with respect to the fully assembled 30 plL limonene synthesis reaction): 4 mM magnesium glutamate, 5 mM ammonium glutamate, 65 mM potassium glutamate, 200 mM glucose, 50 μg/mL kanamycin, and 100 mM Bis-Tris (pH 7.4). Since “CFPS fraction” already contains glutamate salts, supplementation of glutamate salts in the “substrate/lysate/cofactor fraction” maintains the overall cation concentrations at 8 mM magnesium, 10 mM ammonium, and 130 mM potassium. Optionally, cofactors ATP, NAD + , and CoA can be supplemented as well (concentrations are given with respect to the fully assembled 30 μL limonene synthesis reaction). The “substrate/lysate/cofactor fraction” also includes fresh S30 lysate(s) at a total protein concentration of 4 mg/mL. Total protein is measured Bradford assay with bovine serum albumin (BSA) as the standard using a microplate protocol (Bio-Rad, Hercules, Calif.). Typically, three S30 lysates are added each pre-enriched for a single enzyme (EcACAT, ScHMGS, and MsLS) (Dudley et al., 2016; Dudley et al., 2019); each is included at a total protein concentration of 1.33 mg/mL. Based on the level of protein overexpression measured by densitometry (Table at  FIG. 64 ), the final concentration of enriched enzyme in the limonene synthesis reaction is estimated to be ˜4.7 μM for EcACAT, ˜5.5 μM for ScHMGS, and ˜3.4 μM for MsLS, respectively. “Blank lysate” (generated from BL21(DE3) containing no expression plasmid) is added in place of an enzyme-enriched lysate if the given enzyme is included as a CFPS reaction. Limonene synthesis reactions are incubated at 30° C. with a 30 μL dodecane overlay. Limonene in the overlay was quantified at 3, 4, 5, 6, and 24 hours using GC-MS as previously described (Dudley et al., 2019). See Table at  FIG. 65  for systematic description of the variable reaction components in each enzyme set tested. The TREE score is calculated using Equations 2 and 3, described below. 
       TREE Score=Titer·Rate·(Average enzyme solubility)  Equation 2
 
     Rationale for the TREE score. One of the perennial challenges of metabolic engineering are appropriate metrics for evaluating enzyme performance. Enzyme pathways in cells generally report titer, rate, and yield (TRY) as well as K m  and K cat  values for individual enzyme steps yet there is no consensus value to characterize pathway performance to a single value for ranking pathways or utilizing machine learning algorithms. To address this challenge, we developed the TREE (Titer, Rate, and Enzyme Expression) score which is an aggregate value of enzyme solubility, initial productivity, and final titer. We measured kinetics of limonene production using the baseline enzyme set 1.0 and saw that production is mostly done by 24 hours and the highest productivity is typically between 3-6 hours ( FIG. 60 ). Thus, the TREE score is the product of initial productivity (mM/hr), final titer at 24 hr (mM), and average solubility of 9 overexpressed proteins (%). Error from each component value is propagated to obtain an estimate of TREE score uncertainty. Incorporating the average solubility as a proxy for enzyme expression difficulty allows us to decrease (i.e. penalize) scores of pathways that are likely not to express well in cells. 
     
       
         
           
             
               
                 
                   
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     The titer (mM) is the limonene concentration in the cell-free reaction at 24 hours and Otiter=standard deviation of the three experimental replicates. The rate (mMlhr) is an estimate of initial productivity and is the slope of a linear regression of four single replicate limonene concentrations measured at 3, 4, 5, and 6 hours. Grate is the standard error for the mi slope term of the LINEST regression calculation and is determined using the following formula in Microsoft Excel: =INDEX(LINEST([3 hr titer, 4 hr titer, 5 hr titer, 6 hr titer],[3,4,5,6],1,1),2,1). 
     D. Results 
     iPROBE enables rapid and controlled assembly of active metabolic pathways in vitro. In our previous work, we demonstrated that nine mixed lysates, each pre-enriched with a pathway enzyme, could produce limonene from glucose with native  E. coli  enzymes supporting the glycolysis part of the pathway (Dudley et al., 2019) ( FIG. 58A ). This gave a starting point for rebuilding the pathway using lysates enriched, not by heterologous protein expression prior to lysis, but by expressing the protein from a DNA template in the reaction via cell-free protein synthesis (CFPS). In this work our goal was to (1) apply the iPROBE framework to build the nine-enzyme pathway to produce limonene, (2) assess the impact of cofactors on cell-free enzyme performance, (3) iteratively optimize the enzymatic production of limonene, and (4) extend the pathway to additional isoprenoids. 
     1. Application of the iPROBE Prototyping Paradigm to a Nine-Enzyme Pathway to Produce Limonene 
     We established a cell-free pathway to produce limonene and identified candidate sets of enzyme homologs to improve production. First, we built a pathway to produce limonene using a previously characterized set of nine pathway enzymes with the iPROBE approach, where each enzyme is produced individually via CFPS and the resulting “CFPS-enriched reactions” are mixed with glucose substrate and cofactors to produce the target molecule limonene ( FIG. 59A ). We find that this initial enzyme set produces 0.17 mM limonene ( FIG. 71 ). We next wanted to sample a diverse range of enzyme sequences because catalytic rate, substrate specificity, and feedback inhibition can vary widely across related enzymes in both primary (Maeda, 2019) and secondary (Schmidt et al., 2018) metabolism. Therefore, we generated phylogenetic trees for several pathway steps using all available sequences in the BRENDA enzyme database (Scheer et al., 2010) ( FIG. 58B ). We then selected 45 sequences encoding homologs of the nine enzymes of the mevalonate pathway to limonene (SEQ ID NOs:16-72; see sequences at  FIG. 85 ) and assembled them into plasmids for cell-free expression. Half of the selected homologs included uncharacterized enzymes randomly selected from diverse clades while biasing the selection towards homologs suspected to be interesting (e.g., the MK, PMK, PMD, and IDI sequences from  Paracoccus zeaxanthinifaciens  ATCC 21588, a marine bacteria that produces high levels of the isoprenoid zeaxanthin (Berry et al., 2009; Berry et al., 2003)). The other half were kinetically characterized or used in previous metabolic engineering efforts (Table at  FIG. 63 ). 
     Fifty-one different pathway combinations of the candidate enzymes selected were assembled using iPROBE and screened for limonene production. CFPS generated soluble protein (&gt;30% of total protein) for 40 of the 54 enzymes ( FIG. 59B ). By decreasing the CFPS incubation temperature from 30° C. to 16° C., we increased soluble protein yields for the remaining 14 enzymes. To test metabolic activity, iPROBE reactions were assembled containing the enzyme homolog of interest plus the remaining set of base-case pathway enzymes (along with pre-enriched lysates of EcACAT, ScHMGS, and MsLS ( FIG. 72 )). We measured limonene productivity ( FIG. 59C ) and final titer at 24 hours ( FIG. 59D ) and then ranked each of these 51 different pathway combinations based on a combined Titer, Rate, and Enzyme Expression (TREE) score ( FIG. 59E ). This score is a simple, empirical, aggregate value of initial productivity (mM/h), final titer at 24 h (mM), and average solubility of 9 overexpressed proteins (%) (Equations 2 and 3, above; (Karim et al., 2020)). From these data, we observed equivalent or improved productivity relative to the baseline homolog for 2 HMGR, 5 MK, 4 PMK, 4 PMD, 7 IDI, 5 GPPS, and 3 LS homologs. This approach also served as a quick way to rule out several homologs of MK, PMK, PMD, and LS with much lower (or zero) activity; these homologs were excluded from further testing. The relative activity of different HMGRs is very similar to our previous study using pre-enriched lysates (Dudley et al., 2016), as well as in vivo pathway testing (Ma et al., 2011) which suggests that expression of enzymes via CFPS produces proteins with similar properties to those generated with these other methods. 
     2. Cofactors are a Key Parameter in Testing Multi-Enzyme Pathways 
     We next wanted to compare the best enzymes from the initial screen to our enzyme set 1.0. To do this, we selected six homologs as best candidates using the TREE score metric ( FIG. 59E ) for enzyme set 2.0: HMGR from  Bordtella petrii , MK from  Methanosarcina mazei , PMK from  Enterococcus faecalis , PMD from  S. cerevisiae , IDI from  Streptomyces clavuligerus , and GPPS from  Picea abies  (Norway spruce). Each selected homolog was highest performing in the TREE score with the exception of MmMK which was chosen over similarly performing homologs due to its known lack of inhibition from downstream isoprenoid metabolites (Primak et al., 2011). We then compared the initial enzyme set 1.0 to the improved set 2.0 containing new homologs for HMGR, MK, PMK, and IDI over 96 hours and found that enzyme set 2.0 did not improve limonene production ( FIG. 60A ). While the first round of enzyme screening was successfully able to identify active and inactive enzyme homologs, there were only small differences between active homologs ( FIG. 59E ;  FIG. 60A ). 
     We next endeavored to increase the sensitivity of the cell-free system by optimizing cofactors and tuning enzyme concentrations. We know that cofactors are important parameter in optimizing the performance of in vitro enzyme pathways (Dudley et al., 2016; Karim et al., 2018; O&#39;Kane et al., 2019), having previously shown 100-fold differences in metabolite concentrations across 768 unique cofactor conditions of ATP, NAD + , and CoA (O&#39;Kane et al., 2019). Therefore, we tested enzyme sets 1.0 and 2.0 in 18 different cofactor conditions ( FIG. 60B ) and increased both limonene productivity and titer by two-fold ( FIG. 73 ). We found that enzyme set 2.0 is better than or equivalent to enzyme set 1.0 in all cofactor conditions but the differences are more pronounced at optimized cofactor conditions. Next, we wanted to reduce the concentration of enzymes that are not rate-limiting to leave additional space in the reaction for other CFPS-derived enzymes and potentially differentiate less-active homologs from more-active ones. We titrated the concentration of each CFPS-derived pathway enzyme individually using enzyme set 2.0 ( FIG. 74 ) and found that we could reduce the concentrations of HMGR, MK, PMK, PMD, and IDI (enzyme set 2.1) to produce equivalent levels of limonene ( FIG. 74G ). 
     We next examined GPPS, a key enzyme that directs metabolic flux towards ten-carbon GPP and competes with native metabolism which typically generates fifteen-carbon farnesyl pyrophosphate. At high protein concentrations (3 μM), PaGPPS from  Picea abies  produces the highest limonene final titer, however PgGPPS from  Picea glauca  has a higher productivity from 3-6 hours and produces the best TREE score at a lower concentration (1 μM) ( FIG. 60C ;  FIG. 75 ). Thus, PgGPPS appears to have a higher catalytic rate than PaGPPS but could be less stable and may be inactivated as the reaction proceeds. We therefore reduced the concentration of GPPS from 3.0 μM (Set 2.1) to 1.0 μM (Set 2.2) for further experiments. At this point, the reduced concentrations of upstream pathway enzymes enabled more space for higher concentrations of the final enzyme (limonene synthase). We retested all LS homologs at a higher concentration (particularly those that did not express well in the cell-free system). The pre-enriched lysate of MsLS from  Mentha spicata  again produces the highest TREE score but the difference was far more pronounced when using PgGPPS rather than PaGPPS ( FIG. 60D ;  FIG. 76 ). This result highlights the importance of testing enzyme homologs in a variety of pathway contexts. 
     3. Iterative Screening of Active Enzyme Homologs Under Multiple Cofactor Conditions 
     To demonstrate the full potential of our cell-free approach, we tested all active enzyme homologs in an iterative experimental approach that characterized 102 enzyme sets tested in four different cofactor concentrations for a total of 408 unique combinations ( FIG. 61 ). At this point, the primary experimental limitation was not enzyme expression or pathway assembly but product quantitation via GC-MS; this prohibited testing the full combinatorial space of 6*8*5*5*6*4*2=57,600 enzyme sets. The iterative testing approach started by substituting each the six active GPPS homologs and measuring the TREE score under four different cofactor conditions (24 unique conditions; Round 3.x) ( FIG. 61A ). The two enzyme sets with the highest average TREE score over four cofactor concentrations ( FIG. 61B ) were selected to move forward in our screens using the logic that the “most robust” enzymes will be active under multiple cofactor conditions. These sets, differing in GPPS homolog, were then used to test the eight active IDI homologs combinatorially. We assembled 64 unique pathway conditions (enzyme sets and cofactor conditions; Round 4.x) and measured TREE scores for each ( FIG. 61B ). Four enzyme sets were selected and carried forward to be the context for combinatorial testing of all PMD homologs (80 unique conditions; Round 5.x). Thereafter, each experiment used the best scoring five conditions from the previous iteration to test active PMK (Round 6.x), PMD (Round 7.x), and HMGR (Round 8.x) homologs sequentially. 
     The higher limonene production and reduced enzyme concentrations proved to be a more stringent screen for comparing enzyme homologs relative to  FIG. 59 . The iterative experiment found clear differences between MK and PMD homologs that produced the same amount of limonene in earlier experiments ( FIG. 61C ,  FIG. 77 ). Additionally, we found that EcIDI from  Escherichia coli  (one of the most common enzymes used for in vivo terpenoid production) is far less active at reduced concentration compared to other IDI homologs. Finally, we wish to highlight that some enzyme sets showed a variable ranking depending on which cofactor concentration was used. For example, SaHMGR, SaMK, and PzPMK prefer no supplemental CoA while StGPPS is highly active under optimal cofactors but quite low without cofactor supplementation ( FIG. 78 ). The final enzyme set 9.0 included EcACAT, ScHMGS, BpHMGR, MmMK, PzPMK, ScPMD, SIIDI, PgGPPS, and MsLS ( FIG. 61D ). 
     4. Bioproduction of Additional Terpenoids 
     To demonstrate the modularity and flexibility of the cell-free system, we substituted the final enzyme to generate the biofuel precursors pinene and bisabolene ( FIG. 62A ). Pinene is a useful precursor molecule for various perfumes and bisabolene can be chemically hydrogenated to bisabolane which has nearly identical properties to D2 diesel fuel (Peralta-Yahya et al., 2011). We compared two pinene synthase homologs which produce similar levels of pinene while producing more 3-pinene compared to previous in vitro and in vivo studies ( FIG. 79 ). When using 3.8 μM of CFPS-derived monoterpene synthase, the limonene synthase produces higher amounts of product compared to the pinene synthases ( FIG. 62B ). A cell-free system composed of two pre-enriched lysates and seven CFPS-enriched reactions (including 1.0 μM EcGPPS/FPPS (ispA) and 1.6 μM AgBS) produces 4.94±0.73 mM (1010 mg/L) bisabolene after 72 hours ( FIG. 62C ,  FIG. 79 ). This is similar to the best bisabolene titer achieved in cells (912 mg/L) (Peralta-Yahya et al., 2011). 
     E. Discussion 
     Cell-free prototyping has tremendous potential to accelerate the Design-Build-Test cycles for metabolic engineering. In this work, we tested over 590 conditions of different enzyme homologs, enzyme concentrations, and cofactor conditions. Cell-free protein synthesis enables the production of active enzyme in just days without requiring protein purification or extensive expression optimization. In the initial conceptions of CFPS-ME, our group utilized a one-pot strategy in which multiple plasmids were added to a single CFPS reaction (Karim and Jewett, 2016), this approach proved challenging to implement to produce limonene ( FIG. 80 ). The two-pot method utilized by iPROBE in which the concentrations of each enzyme are explicitly controlled ultimately proved effective in the nine-step limonene pathway. The final enzyme set 9.0 in a 96-well plate reaction configuration (rather than 1.5 mL microcentrifuge tube) produces 4.49±0.14 mM (610 mg/L) limonene in 24 h ( FIG. 79 ). These rates and titers are similar to recent in vivo efforts (Table at  FIG. 66 ) though lower than a cell-free system utilizing purified enzymes (Korman et al., 2017). 
     Cell-free prototyping has proved useful for several enzymatic pathways including fatty acids (Liu et al., 2010), farnesene (Zhu et al., 2014), phenylalanine (Ding et al., 2016), non-oxidative glycolysis (Bogorad et al., 2013), polyhydroxyalkanoates (Kelwick et al., 2018), 1,4-butanediol (Wu et al., 2015; Wu et al., 2017), and 3-hydroxybutyrate/n-butanol (Karim et al., 2020). In this work, we found that several enzyme steps including GPPS, IDI, and MK showed strong differences between homologs and PgGPPS, SIIDI, and MmMK ( FIG. 79 ) are promising candidates for further efforts to improve in vivo isoprenoid titers. Two recent efforts utilized MmMK in place of ScMK to produce 1.29 g/L limonene (Wu et al., 2019) and to improve isoprene titers 1.4-fold (Li et al., 2019); however, promoter swapping or ribosome binding site tuning was needed to realize these improvements. In general, this highlights the substantial efforts required to achieve high production in cells and the difficulty of deconvoluting the interacting effects of enzyme activity, expression level, and dynamic responses from the chassis organism. Recently, an exciting approach combining machine learning with ribosome site optimization produced limonene in  E. coli  at high titers using a standard set of enzyme homologs (EcACAT, SaHMG, SaHMGR, ScMK, ScPMK, ScPMD, EcIDI, AgGGPS, MsLS) (Jervis et al., 2019). 
     We developed the TREE score as a method for consolidating three performance parameters (final titer, initial rate, and enzyme solubility) into a single metric. Plotting the TREE score as a function of its component parts suggests that initial productivity has the strongest influence on the score ( FIG. 81 ) and that the effect of enzyme solubility is relatively small ( FIG. 82 ). Furthermore, our group has shown it useful in predicting pathway performance in  Clostridium  (Karim et al., 2020). Ultimately, the TREE score will require additional in vivo data to validate its effectiveness and potential improvements. The TREE score represents an important first step in developing a ranking system for high-throughput pathway prototyping. 
     Our study also highlights the importance of enzyme and cofactor concentrations on cell-free pathway activity. In the first round of homolog screening (higher enzyme concentration and non-optimal cofactors ( FIG. 59 )), the “active” homologs generally produced similar amounts limonene compared to the default homolog. After reducing each enzyme closer to its rate-liming concentration ( FIG. 74 ) and increasing limonene titer via cofactor tuning, re-testing the same homologs ( FIG. 61 ) illuminated differences in activity not seen in the first round. In general, trends of relative homolog activity are similar across different cofactor conditions ( FIG. 61 ) though there are a small number of exceptions such as PzPMK, SaMK, SaHMGR, and StGPPS ( FIG. 78 ). To better understand the correlation between cofactor condition and enzyme performance, we measured a panel of metabolites over time for enzyme sets 1.0, 2.0, and 9.0. The supplementation of NAD +  in enzyme set 9.0 increases lactate production and glycolytic turnover as lactate recycles excess NADH back to NAD +  ( FIG. 61D ,  FIG. 83 ) (Dudley et al., 2016; Dudley et al., 2019). This, in turn, increases glucose consumption and provides more substrates for downstream components of the pathway. While we had previously known that cofactors could improve pathway performance 100-fold (O&#39;Kane et al., 2019), the performance difference between sets of enzymes at different cofactor conditions had never been described until this study. 
     Looking forward, there are a number of promising strategies for improving the production of industrially relevant monoterpenoids in vitro and in cells. New pathway routes to utilize neryl pyrophosphate as a substrate for monoterpene synthases rather than GPP have helped decouple pathway production from competition by enzymes essential for growth such as EcGPPS/FPPS (ispA) in  E. coli  (Cheng et al., 2019; Ignea et al., 2019; Wu et al., 2019). Additionally, alternate enzymatic pathways for producing IPP and DMAPP precursors using prenol or isoprenol as an intermediate have been successful (Chatzivasileiou et al., 2019; Clomburg et al., 2019; Lund et al., 2019) and have already been adapted for cell-free isoprenoid production (Ward et al., 2019). 
     In summary, we anticipate that cell-free prototyping will facilitate the rapid design-build-test cycles of biosynthetic pathways and decrease the amount of time needed for cellular pathway tuning. As the iPROBE framework facilitates large numbers (500+) of enzyme combinations and cofactor conditions without needing requiring large-scale DNA assembly or metabolic engineering of living cells, this approach could be adapted to other pathways for screening enzyme activity, testing for interacting effects between component parts, and analysis of byproduct pathways. These abilities will be synergistically enhanced as new methods are developed for measuring target metabolites at high throughput (O&#39;Kane et al., 2019). This work demonstrates (1) the longest pathway utilized by iPROBE (nine steps) to date, (2) the most unique reaction conditions tested (590+), and (3) the importance of cofactors and enzyme concentration when comparing the activity of enzyme homologs. In future efforts, we look forward to more comparisons of enzyme performance in cell-free versus living cells which will provide additional insights for incorporating iPROBE into the standard design-build-test cycle of cellular metabolic engineering. 
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     Example 9: Cell-Free Biosynthesis of 3-hydroxybutyrate, n-butanol and (s)-(+)-1,3-butanediol 
     A. Abstract 
     The design and optimization of biosynthetic pathways for industrially relevant, non-model organisms is a challenge. Here we describe a platform for in vitro Prototyping and Rapid Optimization of Biosynthetic Enzymes (iPROBE) to accelerate this process. In iPROBE, cell lysates are enriched with biosynthetic enzymes by cell-free protein synthesis and then metabolic pathways are assembled in a mix-and-match fashion to assess pathway performance. We demonstrate iPROBE by assessing 54 different cell-free pathways for 3-hydroxybutyrate production and optimizing a 6-step n-butanol pathway across 205 permutations using data-driven design. After screening a selection of these pathways in cells, we observed a strong correlation (r=0.79) between cell-free and cellular performance. We then scaled up our highest performing pathway, improving in vivo 3-HB production in  Clostridium  by 20-fold to 14.63±0.48 g/L. We also identified a new biosynthetic route to (S)-(+)-1,3-butanediol. We expect iPROBE to accelerate design-build-test cycles for industrial biotechnology. 
     B. Introduction 
     For decades, scientists and engineers have turned to biological systems for making energy, medicines, and materials-especially when chemical synthesis is untenable. Unfortunately, biological production of small molecules does not often provide the desired titer, rate, or yield because natural sources (e.g., plants) are difficult to optimize and may not scale easily. Thus, engineers seek to design enzymatic reaction schemes in model microorganisms to meet manufacturing criteria. 1  Success in these endeavors depends upon identifying sets of enzymes that can convert readily available molecules (e.g., glucose) to target products, with each enzyme performing one of a series of chemical modifications. Unfortunately, this is difficult because design-build-test (DBT) cycles-iterations of re-engineering organisms to test new sets of enzymes—are detrimentally slow. 2,3  As a result, a typical project today might only initially explore dozens of variants of an enzymatic reaction pathway. This is often insufficient to identify a commercially relevant solution because selecting productive enzymes using existing single-enzyme kinetic data has limited applicability in multi-enzyme pathways and consequently requires more DBT iterations. 4  This challenge is exacerbated in industrially-relevant, non-model organisms (such as clostridia) for which genetic tools are not as sophisticated, high-throughput workflows are often lacking, there exist transformation idiosyncrasies, and there is reduced availability of validated genetic parts. 
     Yet, many industrial bioprocesses (e.g., synthesis of amino acids, 5  organic acids, 6-8  solvents 9,10 ) rely on non-model organisms as they offer exceptional substrate and metabolite diversity, as well as tolerance to metabolic end-products and contaminants, making them excellent chassis for biochemical production of exotic molecules from an array of possible feedstocks.  Clostridia  specifically were used industrially in acetone-butanol-ethanol (ABE) fermentations in the early-to-mid 20 th  century because of their unique solventogenic metabolism to produce large amounts of solvents (e.g., acetone and butanol) but were eventually phased out of use due to the success of petroleum until recently. 11  In addition, cellulolytic clostridia can degrade lignocellulosic biomass and acetogenic clostridia can robustly ferment on a variety of abundant, low-cost C1 gases including waste gases from industrial sources (e.g., steel mills, processing plants or refineries) or syngas generated from any biomass resource (e.g., municipal solid waste or agricultural waste). 12  Acetogenic clostridia have recently proven industrially relevant for full commercial scale ethanol production using emissions from the steel making process. 13  However, these strains tend to lack the natural machinery to produce such solvents or other more complex products, and the tools to engineer them are underdeveloped with the solutions established for  E. coli  and yeast not being directly transferrable. 14  In fact, until a few years ago,  Clostridium  organisms were considered genetically inaccessible with only a handful of genomic modifications being reported. 15,16  While developing tools for engineering clostridia is ongoing and promising progress has been made, 17,18  discovering methods to speed up metabolic engineering DBT cycles for these and other non-model organisms would accelerate the re-industrialization of such organisms. 14,19    
     Cell-free systems provide many advantages for accelerating DBT cycles. 20-22  For example, the open reaction environment allows direct monitoring and manipulation of the system to study pathway performance. As a result, many groups have used purified enzyme systems to study enzyme kinetics and inform cellular expression: testing enzymatic pathway performance in vitro, down-selecting promising pathway combinations, and implementing those in cells. 20,23-26  Crude lysates are becoming an increasingly popular alternative to purified systems to build biosynthetic pathways because they inherently provide the context of native-like metabolic networks as well as negate the need for purification. 27-29  For instance, pioneering work demonstrated that DHAP can be made in crude lysates and real-time monitoring can optimize production. 29  In addition, our group has shown that 2,3-butanediol, 30  mevalonate, 28  n-butanol, 27,31  limonene, 32,33  and more complex products 34  can be constructed in crude lysates with high productivities (&gt;g/L/h). However, to our knowledge, no attempts have been made using cell-free prototyping to improve engineering of industrially relevant, non-model organisms. 
     To address this opportunity, we report a new in vitro Prototyping and Rapid Optimization of Biosynthetic Enzymes approach (termed iPROBE) to inform cellular metabolic engineering. The foundational principle is that we can construct discrete enzymatic pathways through modular assembly of cell lysates containing pathway enzymes produced by cell-free protein synthesis rather than by living organisms ( FIG. 86 ). This reduces the overall time to build pathways from weeks (or usually months) to a few days, providing an increased capability to test numerous pathways by avoiding inherent limitations of cell growth and thus diminishing the reliance on single-enzyme kinetic data. A key conceptual innovation is that the DBT unit can be cellular lysates rather than genetic constructs, allowing us to perform DBT iterations without the need to re-engineer organisms. The rapid ability to build pathways in vitro using iPROBE allows us to generate large amounts of data describing pathway operation under several operating conditions. We demonstrate iPROBE in two ways. First, we use a new quantitative ranking system to bridge cell-free data and cellular metabolic engineering for the production of 3-hydroxybutyrate (3-HB) in  Clostridium autoethanogenum  from C1 gas. Specifically, we tested 54 different enzyme combinations for 3-HB in vitro, identify pathway combinations that produce at high-titers in vivo, and we show that cell-free and in vivo pathway performance is correlated (r&gt;0.7). Second, we show the utility of iPROBE by increasing cell-free n-butanol production ˜4-fold in less than two weeks by assessing performance of 205 pathways using data-driven design-of-experiments. We then selected 9 pathway combinations from the iPROBE screen to assess butanol production in  C. autoethanogenum  strains, observing a strong correlation (r&gt;0.9) between in cell and cell-free pathway performance. Finally, we show that we can scale up the best 3-HB-producing  C. autoethanogenum  strain containing an iPROBE-selected pathway to achieve the highest reported titers of 3-HB in clostridia at rates of &gt;1.5 g/L/h in a continuous system. This work also led to identification of a new route to (S)-(+)-1,3-butanediol (1,3-BDO) in vivo, both non-native products for acetogenic clostridia and to our knowledge the first demonstration for biological production of the (S)-(+)-isomer of 1,3-butanediol. 
     C. Results 
     1. Establishing a Two-Pot, Cell-Free Framework for Pathway Prototyping 
     We selected 3-hydroxybutyrate (3-HB) biosynthesis as our first example for pathway prototyping with iPROBE given it is non-native to clostridia and its importance as a high-value specialty chemical. 36,37  Our vision was to demonstrate modular assembly of the pathway by mixing multiple  E. coli  crude cell lysates each individually enriched with a pathway enzyme, identify best sets of enzymes and their ratios for pathway operation, and inform cellular design in an industrially proven, 12  non-model host organism, in this case acetogenic  Clostridium autoethanogenum  ( FIG. 86 ). A unique feature of the iPROBE approach, relative to previous work in crude lysate-based cell-free prototyping, 27,31-33,35  is that pathways are assembled in two steps (i.e., 2 pots), where the first step is enzyme synthesis via cell-free protein synthesis (CFPS) and the second step is enzyme utilization via substrate and cofactor addition to activate small molecule synthesis ( FIG. 91 ). While all approaches are crude lysate-based, previous methods relied on enzyme overexpression in living  E. coli  prior to cell-lysis (CFME) or one-pot CFPS and metabolic initiation (‘One Pot’ CFPS-ME). We believe this new two-pot workflow (iPROBE) used throughout this study is important for three reasons. First, it allows us to precisely quantify protein expression yields. With known protein expression yields at hand, we are able to control concentrations of enzymes in iPROBE pathways to assess pathway performance with different protein ratios, which we showed was important in optimizing the butanol pathway. Second, having exact enzyme concentrations enabled by the two-step approach allows us to ensure that biosynthetic pathways are not imbalanced (i.e., one enzyme 10× more or less than all of the others, which could skew interpretations of pathway performance). Third, negative physiochemical effects of the CFPS reaction 31  on small molecule biosynthesis can be reduced. 
     We first set out to use iPROBE to study the impact of enzyme ratios on pathway performance for the non-native pathway to 3-HB ( FIG. 87A ). From acetyl-CoA, a key intermediate in both  E. coli  and  Clostridium , three enzymes are required to make 3-HB: a thiolase (Thl), a hydroxybutyryl-CoA dehydrogenase (Hbd), and a thioesterase.  E. coli  and  C. autoethanogenum  have native thioesterases that convert 3-hydroxybutyryl-CoA to 3-hydroxybutyrate. 38  For optimal production, these native thioesterases or ones with a high-degree of specificity for 3-hydroxybutyryl-CoA will need to be overexpressed. However, for screening purposes these native enzymes were sufficient; only two, non-native enzymes (Thl and Hbd) were required to be overexpressed. We initially selected a Thl gene from  Clostridium acetobutylicum  (Cac) and a Hbd gene from  Clostridium kluyveri  (Ckl) (Table at  FIG. 101 ). We used the well-characterized  E. coli -based PANOx-SP CFPS system 39  to produce CacThl and CklHbd, with soluble concentrations of 5.85±0.82 μM and 19.31±3.65 M, respectively. Thioesterases were not produced via CFPS because they are natively present and active in  E. coli  lysates. Then, we designed five unique pathway combinations titrating different concentrations of thiolases (Thl) while maintaining a constant concentration of Hbd by mixing different ratios of CFPS reactions (keeping total CFPS reaction added constant using blank reactions containing no protein produced in vitro) ( FIG. 87B-C ). Upon incubation with essential substrates, salts, and cofactors (e.g., glucose, NAD, CoA, ATP), we assessed 3-HB synthesis at 0, 3, 4, 5, 6, and 24 h for each of the five pathway combinations via high performance liquid chromatography (HPLC) ( FIG. 87D ). The cell lysate contains endogenous enzymes for glycolysis that regenerate NADH 40  and convert glucose to acetyl-CoA, providing the starting intermediate for 3-HB biosynthesis. As expected, no 3-HB was produced in the absence of Thl. The highest 3-HB titers were observed for 0.5 μM CacThl and 0.5 μM CklHbd. We performed a similar titration of CklHbd while maintaining a constant concentration of CacThl ( FIG. 92 ). Taken together, our data show that crude lysates enriched by CFPS can be used to assemble metabolic reactions and sets the stage to optimize pathways using a two-pot cell-free approach 
     2. Developing a Metric to Quantify Biosynthetic Pathway Performance 
     To optimize pathways with iPROBE, we next defined a pathway ranking system that would enable assessment of activity in the cell-free environment and hold potential to inform cellular design. The basis of this ranking system is a single, quantitative metric for our cell-free experiments. We call this metric the TREE score (for titer, rate, and enzyme expression; important metrics for defining the success of cell-based metabolic engineering). The TREE score combines, through multiplication, titer at reaction completion, rate during the most productive phase of pathway operation, and enzyme expression as measured by soluble protein fraction and total enzyme amount. Using our initial set of data ( FIG. 87 ) as a guide, the TREE score is obtained by multiplying 3-HB titer at 24 h, the linear 3-HB production rate between 3 and 6 h, and the sum of the average soluble fraction of the pathway enzymes, Thl and Hbd, and the inverse of the total enzyme concentration for each of the five pathway combinations ( FIG. 87E-I ). While the TREE score rankings are not largely different from the titers (r=0.89 for all 3-HB data in this study) or rates (r=0.91 for all 3-HB data in this study) alone (all 3-HB pathway TREE scores shown in  FIG. 93 ), they exaggerate differences that might arise from each component of the score. For example, combining titer and rate enables use of both in ranking cell-free pathway performance which is helpful as it is unknown whether cell-free titer or rate is more or less important for informing cellular metabolic engineering. Additionally, we included the enzyme expression component to penalize a given pathway if in vitro expression is poor, decreasing its overall pathway rank. Typically, enzymes that are unable to be expressed (either lowly expressed or insoluble) in vitro are challenging to express in vivo. Thus, the average solubility of all the pathway enzymes overexpressed in the lysate was used to acquire a sense of how difficult the pathway might be to express. The inverse enzyme amount was used to penalize in vitro combinations that had high concentrations of enzyme which might improve a pathway&#39;s performance but would not be feasible in cells. While there are multiple ways one could imagine ranking pathways or weighting the TREE score factors, reducing the complexity of available cell-free data was important as it enabled a rapid approach to rank pathways for iPROBE. 
     3. iPROBE Informs Selection of Genetic Regulatory Architectures in  Clostridium    
     With a pathway ranking system at hand, we next aimed to validate that cell-free experiments could generate design parameters for DNA construction of biosynthetic pathways in cells, a difficult challenge since gene expression tools are often not available in non-model organisms. Gene expression involves designing a coding sequence, 5′ and 3′ regulatory elements, and vector maintenance components, among other parts. Selection of the promoter regulatory strengths (e.g., high, medium, low) for the expression of a coding sequence, in particular, is an essential factor for pathway tuning. Thus, we set out to develop a correlation between specific enzyme concentrations in iPROBE and specific strength regulatory architectures, relative promoter strengths and plasmid copy number for a single operon comprising the 3-HB pathway, for expression in  C. autoethanogenum . To achieve this goal, we built cell-free pathway combinations for 3-HB by co-titrating seven different enzyme concentrations of Thl and Hbd in our reactions ( FIG. 94 ). Specifically, we built seven cell-free reactions in increasing total concentration added, combining CacThl and CklHbd at equimolar amounts. We ran each cell-free reaction for 24 h and measured the titer of 3-HB produced ( FIG. 94C ). We observed that as the amount of enzyme added increases, the amount of 3-HB increased up to a threshold amount of 1 μM of each enzyme added. In parallel, we constructed plasmids expressing CacThl and CklHbd under eight regulatory architectures (change in promoter strength and plasmid copy number) of increasing strength and transformed them into separate strains of  C. autoethanogenum . We ran small-scale bottle fermentations of each strain under anaerobic conditions on carbon monoxide (CO), hydrogen (H 2 ), and carbon dioxide (CO 2 ) gas and measured stationary phase titers of 3-HB ( FIG. 94D ). In vivo, we found that increases in expression strength led to higher 3-HB titers, but did not saturate in 3-HB expression, whereas, the in vitro data show saturation with increasing enzyme amount ( FIG. 94 ). These data suggest a limitation of the dynamic range of expression possible with current, well-characterized genetic parts available for use in  C. autoethanogenum . However, given the trends observed, we used this data to build an initial cell-free to cell correlation that connects cell-free 3-HB production and corresponding enzyme concentrations to cellular 3-HB production and corresponding plasmid regulatory strength. We found that generally using &lt;0.1 μM enzyme in vitro corresponds to low regulatory strengths in vivo, using 0.1-0.3 μM enzyme in vitro corresponds to medium strengths in vivo, and using &gt;0.3 μM enzyme in vitro corresponds to high strengths in vivo. In principle, this allows us to screen many different pathway combinations in cell-free systems and provide a rational recommendation for plasmid construction of those pathway combinations in  Clostridium , which is what we did next. 
     4. iPROBE can Inform the Selection of Pathway Enzymes in  Clostridium    
     To showcase the iPROBE approach, we next screened several possible 3-HB pathway combinations using cell-free experiments, ranked a subset of candidate cellular pathway combinations using the TREE score, and showed cellular  C. autoethanogenum  3-HB biosynthesis from CO/H 2 /CO 2  gas correlates with cell-free experimental results. To do this, we tested six enzyme homologs of each Thl and Hbd originating from different  Clostridium  species as these would be best initial candidates for  Clostridium  expression ( FIG. 88A ; Table at  FIG. 101 ). We selected all pathway combinations of the 12 enzymes keeping a fixed total concentration of soluble enzyme added (high expression levels) to build in cell-free reactions ( FIG. 88B-C ). By measuring 3-HB production over the course of 24 h along with soluble enzyme expression for each of the enzymes ( FIG. 93 ), we are able to calculate TREE scores for each of the 36 pathway combinations ( FIG. 88D ). We found that a majority of pathway combinations perform poorly with only a handful achieving TREE scores above a value of 2. The top six pathways contained CklHbd1 for the second step. This indicated to us that iPROBE helped select a top-performing enzyme for cellular efforts in a single screen that took less than a week to build. Testing 36 pathway combinations in clostridia could have taken more than 6 months using our current workflows. Thus, we wanted to use this information to down-select the number of combinations to test in acetogenic clostridia due to the current limitations in high-throughput strain testing. We selected a subset of four pathway combinations from the iPROBE screening to test in  C. autoethanogenum  labeled A (CklThl1/CacHbd), B (CklThl2/CklHbd1), C (CklThl1/CklHbd1), and D (CacThl/CklHbd1). These pathways represent our highest-performing pathway, two that fell in the middle (C having a large degree of variability in performance), and one of our low-performers. We constructed and transformed DNA with strong regulatory architectures and each of the four-pathway enzyme sets into separate strains of  C. autoethanogenum  ( FIG. 88E ). We ran small-scale bottle fermentations of each strain under anaerobic conditions on CO/H 2 /CO 2  gas mixture and measured 3-HB titers at four time-points during the fermentation ( FIG. 88F ). We observed that the best cell-free pathway combination as determined by TREE score (D) also performs the best of the four in  Clostridium  cells, achieving 33.3±1.4 mM. The worst pathway combination in cell-free experiments (A) is also the worst performer in  C. autoethanogenum . The other two pathway combinations (B, C), which were not statistically different in the cell-free environment, fall in the middle in cellular experiments. Notably, we did not see detectable levels of non-specific products such as acetoacetate or acetone. The exact ranking of pathways B, C, and D differ between in vitro and in vivo construction, but all three of these designs are much better than a majority of the 36 combinations tested in the cell-free environment. At a high-level, these data suggest that the cell-free system is useful for guiding pathway selection in cells; while the  E. coli -based lysate conditions do not necessarily approximate the in vivo clostridia conditions (e.g., pH, small molecule concentrations, axillary proteins, aerobic vs. anaerobic), the cell-free environment remains a powerful prototyping environment for assessing biochemical information and informing design. This is especially true for down-selecting pathway combinations that should not be tested in cells (i.e., produce little to no product). In fact, the best pathway designs tested in two recent studies that explored autotrophic 3-HB production in acetogenic clostridia produced ˜4 mM and ˜1 mM 3-HB. 41,42  Their pathways correspond to TREE scores of 1.06±0.07 and 0.02±0.00, respectively. Based on our iPROBE screening we would have suggested not testing these combinations in vivo. For context, our best pathway had a TREE score of 17.76±3.08. In sum, iPROBE offers a framework to rapidly design, build, and test pathway combinations in cell-free experiments in a matter of days, bypassing DNA construction and transformation limitations, and to facilitate implementation of promising pathway combinations for engineering success in cells. 
     5. Cell-Free Pathway Prototyping for n-Butanol Biosynthesis 
     Having demonstrated the use of iPROBE to optimize the 3-HB pathway for informing cellular design in  C. autoethanogenum , we next aimed to show that iPROBE could be used to optimize longer pathways. We selected the 6-step pathway from acetyl-CoA to n-butanol as a model pathway for this because butanol is an important solvent and drop-in fuel with US$5 billion/year market ( FIG. 89A ). The key idea was to use iPROBE to optimize cell-free butanol production by constructing pathway variants with different enzyme homologs and enzyme ratios. The challenge with this optimization goal is the number of possible permutations. Indeed, testing just six homologs for each of the first four steps of the pathway at three different enzyme concentrations would alone require 314,928 pathway combinations, which exceeds typical HPLC analytical pipelines. To optimize enzyme homologs and concentrations simultaneously while also managing the landscape of testable hypotheses, we implemented a design-of-experiments using a neural-network-based algorithm to predict beneficial pathway combinations. While kinetic models are useful in making rational predictions, neural-network-based approaches can be used to make predictions independent of the molecular understanding of a biosynthetic pathway as well as with a small initial data set to train the models. 
     In creating the initial data set to guide improvements in titer, we chose six homologs of each Thl, Hbd, Crt, and Ter (Table at  FIG. 101 ;  FIG. 89A ). We tested five concentrations for each enzyme homolog in a pathway context consisting of our base set of enzymes ( FIG. 89A , highlighted in blue), totaling 120 pathway combinations. We built these combinations in cell-free reactions from enzymes produced by CFPS, measured butanol production over time, and calculated TREE scores for each ( FIG. 89B ;  FIG. 95 ). In total, we collected these additional data in five experimental sets of 20 pathway combinations with each set taking five days (three days of HPLC time). A majority of the enzyme homologs did not out-perform the original enzyme set (EcoThl/CbeHbd/CacCrt/TdeTer). This result is not surprising as this original enzyme set (our base case) has been extensively characterized and tested throughout the literature. 43-45  However, we find that CklHbd1 can double the TREE score at high concentrations and even out-performs the base case CbeHbd at lower concentrations. This agrees with a recent independent study that found a 1.6-fold improvement in ABE fermentation with  C. acetobutylicum  by replacing the native Hbd with the CklHbd1. 46    
     With this initial dataset (120 pathway combinations) collected ( FIG. 89B ), we identified ten neural network architectures based on a combination of heuristic search for model design and 10-fold cross validation (training and testing) for model scoring. We then optimized each of these models using a gradient-free optimization strategy to maximum butanol production. We used the ten best architectures (most accurate predictions and highest model entropy) to make pathway combination predictions (homolog set and enzyme ratios) and maximize the TREE score, which we could then build with the cell-free framework ( FIG. 89C ). Design predictions suggesting enzyme concentrations of &lt;0.01 μM (a majority &lt;2 nM) were ruled out due to experimental constraints, and we built the remaining 43 predictions in cell-free reactions ( FIG. 96A ). To evaluate our design-of-experiments approach, we compared the results with two additional sets of experiments: (i) a set varying enzyme ratios more thoroughly using only the base set enzymes (21 pathway combinations;  FIG. 96B ) and (ii) a hand-selected set of 18 pathway combinations based on our understanding of the biosynthesis ( FIG. 96C ). In total we tested 205 unique pathway combinations (base set combinations, initial round combinations, and data-driven designs) ( FIG. 89D ;  FIG. 95 ). Nearly 20% of the total pathway combinations screened have higher TREE scores than our base case. Without testing every possible combination, we were able to rapidly test a manageable subset achieving ˜4 times higher TREE scores (˜2.5 times higher titer and 58% increase in rate) over the base case pathway combination. The consensus enzyme set for top-performing pathways included EcoThl, CklHbd1, CacCrt, and TdeTer with variations in enzyme concentrations. Five of the top six TREE scores each arose from pathways predicted from the neural-network-based approach and were much better than our hand-selected set, highlighting that this approach can be used with a lack of apriori knowledge of the pathway. These results showcase the benefit from merging computational and experimental design. 
     Analysis of the iPROBE pathway combinations shows several key design parameters. First, we observed that there are specific enzyme homologs and concentrations that improve the TREE score. For example, the chosen thiolase does not seem to matter in the 20 top-performing combinations whereas the selection of Hbd does ( FIG. 97A ). Indeed, the iPROBE approach was used to discover that the  C. kluyveri  Hbd1 enzyme was superior to the rest. Second, iPROBE enables identification of enzymes to not test in clostridia. Specifically, ReuThl, CklHbd2, PpuCrt, FsuTer, FjoTer, and CpaTer all underperformed in the cell-free context. Being limited by throughput in non-model organisms, it is important to identify both promising and not promising enzyme candidates. Third, we noticed that in the 20 top-performing combinations Hbd is present at significantly (p&lt;0.001) higher concentrations and the median Crt concentration is lower, though not significantly, than the initial 0.3 μM ( FIG. 97B ). This suggests that higher concentrations of Hbd and Ter than Crt are optimal for effective pathway operation, which can be achieved in vivo by constructing plasmids with proper genetic architectures. However, this hypothesis should be further investigated by measuring enzyme abundance, metabolic fluxes, and metabolite concentrations with high-end metabolomics with high-end metabolomics. 
     We next assessed iPROBE&#39;s ability to inform cellular design by constructing representative pathway combinations from the iPROBE screening in  C. autoethanogenum  strains to produce butanol. While we tried to cover a wide range of TREE scores, challenges with transformation limited us to two pathway combinations scoring among the top five combinations (CacThl/CklHbd1/CacCrt/TdeTer and EcoThl/CklHbd1/CacCrt/TdeTer), two pathway combinations in the middle range of the data set, and five pathway combinations near the tail-end of all combinations tested. These combinations are listed in  FIG. 98A . To avoid diverting flux toward 3-HB, we identified and knocked-out a native thioesterase able to hydrolyze 3-HB-CoA from our screening strain. After monitoring butanol production over the course of six days ( FIG. 98A ), we see a promising correlation between in vivo expression in  C. autoethanogenum  and TREE scores from iPROBE ( FIG. 89E ). This further emphasizes that selecting top-performing pathways from iPROBE can improve production in  Clostridium  organisms and decreases the number of strains that need to be tested. Importantly, the iPROBE-derived hypothesis that keeping crotonyl-CoA at minimal concentrations by balancing Hbd and Ter expression, higher than Crt, improves pathway performance is corroborated by the combinations tested in vivo. We see lower butanol production (gray squares) when Hbd is expressed highly and Ter is expressed lowly but higher production (orange circles) when both are expressed highly. A mechanistic understanding of this perceived toxicity still needs to be elucidated, but iPROBE can generate probable hypotheses. 
     While overall butanol production was low in  C. autoethanogenum , we were able to increase production using the iPROBE-selected CklHbd from 0 mM to 0.2±0.0 mM. In addition, when comparing two butanol synthesis pathways in vivo-one with the standard CacHbd and one with the iPROBE-selected CklHbd-we increased butanol production 6-fold from 0.2±0.0 mM to 1.1±0.0 mM ( FIG. 98B ) by replacing the trans-2-enoyl-CoA reductase (Ter) enzyme with the ferredoxin-dependent electron bifurcating enzyme complex (Bcd-EtfA:EtfB) naturally used for these activities in clostridia. 47  This is not surprising in light of a recent study that showed Ter is detrimental to ABE fermentation when introduced in  C. acetobutylicum.   48  Using the Bcd-EtfA:EtfB complex, we were also able to increase production to 22.0±0.1 mM by manipulating the plasmid architecture ( FIG. 98C ). For comparison, the previously best reported butanol production in engineered acetogenic clostridia was ˜2 mM. 15  Moreover, the Bcd-EtfA:EtfB complex is a delicate complex that is extremely oxygen sensitive 49  and has so far been inactive in  E. coli  lysates (in alignment with previous reports that highlighted difficulties expressing Bcd in  E. coli ), 44  highlighting an area for potential improvement of iPROBE (i.e., compatibility of  E. coli  lysates with non-model organisms). Taken together, we observed that iPROBE strongly correlated with cellular performance ( FIG. 99 , r=0.79) for 20 pathways tested for both 3-HB production and n-butanol synthesis. Overall, this work demonstrates the power of coupling data-driven design-of-experiments with a cell-free prototyping framework to select feasible subsets of pathways worth testing in vivo for non-model organisms. 
     6. Scaled-Up Fermentations of iPROBE-Selected Pathway 
     As a final validation of our integrated iPROBE approach, we next set out to use in-house facilities to scale cell growth of an iPROBE-selected pathway from bench to continuous fermentation scales ( FIG. 90A ). Specifically, the best performing strain for 3-HB production selected by iPROBE was chosen for process scale-up from 0.1-L bottle fermentations to 1.5-L continuous fermentations using CO/H 2 /CO 2  gas as the sole carbon and energy source. Over a 2-week fermentation, we monitored 3-HB and biomass in a control strain and our iPROBE-selected strain with and without optimized fermentation conditions ( FIG. 90B-C ). In optimized fermentations we observed high-titers of 3-HB, −15 g/L (140 mM) at rates of &gt;1.5 g/L/h in a continuous system. This is not only higher than the previously reported concentration in acetogenic  Clostridium,   41,42  but to our knowledge also exceeds the previously highest-reported concentration for traditional model organisms like  E. coli  (titer of ˜12 g/L and rate of ˜0.25 g/L/h in fed-batch) 38,50  and yeast (titer of ˜12 g/L and rate of ˜0.05 g/L/h in fed-batch) 51  without any additional genomic modifications to optimize flux into the pathway. This shows the utility of iPROBE in identifying pathways for industrial strain development. As expected with an acetogenic host, we observe the production of acetate as a byproduct during fermentation ( FIG. 100 ). Implementing the heterologous production of 3-HB in this strain, however, results in less acetate produced over time. We anticipate that genome modifications to increase flux could further improve fermentation titers. For example, a recent study reported a 2.6-fold improvement in 3-HB production in a related engineered acetogenic  Clostridium  by downregulation of two native genes related to acetate production. 41    
     Surprisingly, we also observed production of a novel metabolite, 1,3-butanediol (1,3-BDO), at 3-5% of the 3-HB levels and up to 0.5 g/L ( FIG. 90D ). This is attributed to nonspecific activity of a native aldehyde:ferredoxin oxidoreductase (AOR) and alcohol dehydrogenase able to reduce 3-HB to 3-hydroxybutyraldehyde and further to 1,3-BDO. Indeed, no 1,3-BDO was observed when transforming the pathway into a previously generated AOR-knockout strain. 52  These enzymes have previously been shown to reduce a range of carboxylic acids to their corresponding aldehydes and alcohols through reduced ferredoxin. 15,53  While the (R)-(−)-form of 1,3-BDO has been produced via other routes, 54-56  when using the  C. kluyveri -derived Hbd we also detected the (S)-(+)-form of 1,3-BDO as determined by chiral analysis, which to our knowledge has never been produced in a biological system before. This chiral specificity is determined by the chosen 3-hydroxybutyryl-CoA dehydrogenase. Further investigation could elucidate the mechanism of the biosynthetic route. Given that 1,3-BDO is used in cosmetics and can also be converted to 1,3-butadiene used in nylon and rubber production with a US$20 billion/year market, 36,57  the discovery of this pathway is important. In sum, iPROBE provides a quick and powerful DBT framework to optimize and discover biosynthetic pathways for cellular metabolic engineering efforts, including those in non-model hosts. 
     D. Conclusion 
     We demonstrate a new, two-pot framework, iPROBE, that incorporates a modular cell-free platform for constructing biosynthetic pathways with a quantitative metric for pathway performance selection (the TREE score). We establish that iPROBE can be used to engineer and improve small molecule biosynthesis in non-model organisms that can be arduous to manipulate. In one example, we show that by screening 54 biosynthetic pathway combinations for the production of 3-HB in cell-free reactions, we can rationally select a handful of pathways to inform cellular metabolic engineering in clostridia. Specifically, iPROBE enabled the construction of a strain of  C. autoethanogenum  that produces high-titers and yields of 3-HB (˜20× higher than the previous highest reported concentration in the literature) in continuous fermentations with carbon monoxide/hydrogen/carbon dioxide gas as sole source of carbon and energy. The scale-up work also led to the identification of a new route to 1,3-BDO and the first production to our knowledge of the (S)-isomer of this molecule in a biological system. In another example, we show the ability to use iPROBE in conjunction with data-driven design-of-experiments to reduce an exceedingly large landscape of testable pathway designs, test a subset of 205 pathway combinations in vitro for the production of butanol, and show that by testing a further subset of designs in vivo we can improve butanol production in acetogenic clostridia. Importantly, we show, for the first time to our knowledge, the ability of  E. coli  crude lysate systems to prototype hundreds of biosynthetic pathway designs to inform cellular design and demonstrate a strong correlation with in vivo pathway performance ( FIG. 99 ). 
     Despite the inherent contextual differences in  E. coli  lysates and clostridia cells, including that the lysate context will likely not capture several important characteristics of the non-model organism (e.g., oxygen sensitivity), we have successfully demonstrated that iPROBE facilitates cellular design in three ways: (i) identifying sets of enzymes that work well together to produce a desired biochemical, (ii) down-selecting poor performing pathway/enzyme candidates, and (iii) evaluating optimal ratios of enzymes and potential synergy between enzymes prior to embarking on laborious experiments in these organisms. Highlighting the utility of iPROBE for accelerating DBT cycles, the 205 pathway combinations for butanol were built cumulatively in 12 days (excluding HPLC time) in cell-free reactions, which we estimate would have taken at least 6 months in clostridia. While not all issues with engineering non-model organism expression are mitigated by iPROBE, it complements and enhances in vivo strategies. 
     Future developments of iPROBE could seek to improve the ability to design and optimize biosynthetic pathways in non-model organisms using cell-free methods. For example, efforts to mimic physiochemical conditions of the organism of interest (e.g., cofactors) and various conditions that mimic the phase of fermentation used during product production (e.g., batch vs. semi-continuous, aerobic vs. anaerobic) could be explored. Additionally, creating productive cell-free gene systems directly from clostridia (or a non-model organisms of choice) could facilitate prototyping DNA elements (i.e., promoters, codon usage) as well as native metabolic and regulatory networks to better inform pathway design. In addition to enhancing the prototyping environment, refining the TREE score (e.g., by weighting each factor) with more in vitro to in vivo correlation data will help identify the minimal amount of cell-free data (e.g., rate, endpoint titer, enzyme expression) needed to accurately inform in vivo pathway performance. Indeed, we believe the TREE score metric only serves as a starting point and anticipate it evolving and improving in subsequent works. Finally, we note that while we focused on informing design in industrially relevant, non-model organisms many of our findings and tools could be applied to conventional hosts. 
     Looking forward, we anticipate that iPROBE will facilitate design-build-test cycles for biosynthetic pathways by enabling the rapid study of pathway enzyme ratios, tuning individual enzymes in the context of a multi-step pathway, screening enzyme variants for high-performance enzymes, and discovering enzyme functionalities. This in turn will decrease the number of strains that need to be engineered in vivo and time required to achieve desired process objectives. This will increase the flexibility of biological processes to adapt to new markets, expand the range of fossil-derived products that can be displaced with bio-derived alternatives, and enhance the economic benefits for co-produced fuels. 
     E. Methods 
     1. Bacterial Strains and Plasmids 
       Escherichia coli  BL21(DE3) (NEB) was used for preparation of cell extracts which were used to express all exogenous proteins in vitro. 35  A derivate of  Clostridium autoethanogenum  DSM10061 obtained from the German Collection of Microorganisms and Cell Cultures GmbH (DSMZ; Braunschweig, Germany) was used for in vivo characterization and fermentations. 58  For butanol production, this strain was used with a native thioesterase (CAETHG_1524) knockout made using Triple Cross recombination as described previously. 59    
     Twenty-three enzymes were examined in this study (Table at  FIG. 101 ). DNA for all enzyme homologs tested were codon adapted for  E. coli  using IDT codon optimizer. Non-clostridial sequences were codon adapted for  C. autoethanogenum  using a LanzaTech in-house codon optimizer, and all native clostridial genes were used as is.  E. coli  (SEQ ID NOs:73-95) and  C. autoethanogenum  (SEQ ID NOs:96-101) adapted sequences are listed in  FIG. 102 , A-W and  FIG. 103 , A-F, respectively. For the cell-free work, the pJL1 plasmid (Addgene #69496) was used. The modular pMTL80000 plasmid system 60  along with acsA 52 , fdx 52 , pta 61  and pfor 62  promoters were used for the  C. autoethanogenum  plasmid expression. 
     2. Cell Extract Preparation 
       E. coli  BL21(DE3) cells were grown, harvested, lysed, and prepared using previously described methods. 27,63    
     3. iPROBE Reactions 
     Cell-free protein synthesis (CFPS) reactions were performed to express each enzyme individually using a modified PANOx-SP system described in previous pubications. 39,64  Fifty-μL CFPS reactions were carried out for each individual enzyme in 2-mL microcentrifuge tubes. Enzyme concentrations in CFPS reactions were quantified by  14 C-leucine incorporation during in vitro translation. Then reactions performed for identical enzymes were pooled together when multiple reaction tube-volumes were needed to keep a consistent 50-μL reaction volume and geometry for every CFPS reaction. Based on molar quantities of exogenous enzymes in each CFPS reaction determined by radioactive measurement, CFPS reactions were mixed to assemble complete biosynthetic pathways in 1.5-mL microcentrifuge tubes. CFPS reactions constitute 15 L of a 30-L-total second reaction. When the total CFPS reaction mixture constituted less than 15 μL, ‘blank’ CFPS reaction was added to make the total amount of CFPS reaction up to 15 μL. The ‘blank’ reactions consist of a typical CFPS reaction with no DNA added. This 15 μL CFPS mixture was then added to fresh extract (8 mg/mL), kanamycin (50 μg/ml), glucose (120 mM), magnesium glutamate (8 mM), ammonium glutamate (10 mM), potassium glutamate (134 mM), glucose (200 mM), Bis Tris pH 7.8 (100 mM), NAD (3 mM), and CoA (3 mM); final reaction concentrations are listed. Reactions proceeded over 24 h at 30° C. Measurements from samples were taken at 0, 3, 4, 5, 6, and 24 h. 
     4. Quantification of Protein Produced In Vitro 
     CFPS reactions were performed with radioactive  14 C-Leucine (10 μM) supplemented in addition to all 20 standard amino acids. We used trichloroacetic acid (TCA) to precipitate radioactive protein samples. Radioactive counts from TCA-precipitated samples was measured by liquid scintillation to then quantify soluble and total yields of each protein produced as previously reported (MicroBeta2; PerkinElmer). 39,40    
     5. Metabolite Quantification 
     High-performance liquid chromatography (HPLC) was used to analyze 3-HB and n-butanol. We used an Agilent 1260 series HPLC system (Agilent, Santa Clara, Calif.) via a refractive index (RI) detector. 3-HB and n-butanol were separated with 5 mM sulfuric acid as the mobile phase and one of two column conditions: (1) an Aminex HPX-87H or Fast Acids anion exchange columns (Bio-Rad Laboratories) at 35 or 55° C. and a flow rate of 0.6 ml min −1  or (2) a Alltech IOA-2000 column (Hichrom Ltd, Reading, UK) at 35 or 65° C. and flow rate of 0.7 ml min −1  as described earlier. 65  1,3-butanediol was measured using gas chromatography (GC) analysis, employing an Agilent 6890N GC equipped a Agilent CP-SIL 5CB-MS (50 m×0.25 m×0.25 μm) column, autosampler and a flame ionization detector (FID) as described elsewhere. 65  For chiral analysis of (S)-(+)-1,3-Butanediol and (R)-(−)-1,3-Butanediol an Agilent 6890N GC equipped with a Restek Rt®-bDEXse 30m×0.25 mm ID×0.25 m df column and a flame ionization detector (FID) was used. Samples were prepared by heating for 5 minutes at 80° C., followed by a 3-minute centrifugation at 14,000 rpm. Exactly 400 μL of supernatant was then transferred to a 2-mL glass autosampler vial and 100 μL of and Internal Standard solution (5-methyl-1-hexanol and tetrahydrofuran in ethanol) was added. The capped vial was then briefly vortexed. Sample vials then were transferred to an autosampler for analysis using a 1.0 μL injection, a split ratio of 60 to 1, and an inlet temperature of 230° C. Chromatography was performed with an oven program of 50° C. with a 0.5 min hold to a ramp of 3° C./min to 70° C. to a ramp of 2° C./min to 100° C. with a final ramp at 15° C./min to 220° C. with a final 2-min hold. The column flow rate was 30 cm/sec using helium as the carrier gas. The FID was kept at 230° C. Quantitation was performed using a linear internal standard calibration. 
     6. TREE Score Calculations 
     The TREE Score is calculated by multiplying the titer by the rate by enzyme expression metric. 
       TREE Score=Titer·Rate·(Average Solubility+[Total Enzyme] −1 )
 
     The titer is the metabolite concentration (mM) in the cell-free reaction at 24 h, when the reaction is complete. The error associated with titer is the standard deviation of reaction triplicates. The rate is the slope of the linear regression of metabolite concentrations (mM/h) taken at 3, 4, 5, and 6 h time points. The rate-associated error is the standard error of the slope calculated by the linear regression. The average soluble fraction term is calculated by first determining the soluble fraction (soluble protein/total protein, n=3) for each individual enzyme via  14 C-leucine incorporation. The average soluble fraction is then the average value of soluble enzyme fractions (mM soluble/mM total protein) (in this case, five enzymes) and the error associated with the soluble fraction term is propagated error. The concentration of total enzyme is calculated by the addition of the final concentrations of each enzyme (mM soluble protein) in the reaction with propagated error. The final error on the TREE score is the propagated error of each individual component. 
     7. In Vivo Gas Fermentations 
     In vivo cultivation and small-scale bottle fermentation studies were carried out as described earlier using a synthetic gas blend consisting of 50% CO, 10% H 2 , 40% CO 2  (Airgas, Radnor, Pa.). 62  Continuous fermentations were carried out in 1.5 L continuous stirred tank reactors (CSTRs) with constant gas flow as described elsewhere. 65,66    
     8. Design-of-Experiments Using Neural Networks 
     A neural network-based approach was used to explore the vast landscape of possible experimental designs. We first processed the cell-free butanol dataset and then developed and optimized the neural network to provide cell-free pathway recommendations. Modeling enzymatic pathways requires a mix of continuous and categorical variables. Because many machine learning algorithms require numeric input and output variables, we used one-hot encoding, which is a process that converts categorical variables into a numerical form that machine learning algorithms can use. This method treats categorical variables as a multidimensional binary input that must sum to one. The concentration values were used as is, resulting in a 30 variable input matrix: 25 variables representing the categorical variation (i.e., different homologs) and 5 representing the concentration. We used these features to then build our deep neural network regressions. 
     We generated and evaluated neural network architectures based on a combination of heuristic search for model design and 10-fold cross validation for model scoring. We limited our model architecture search to fully connected layers but varied the number of hidden layers (between 5 and 15 layers) and the number of nodes in each layer (between 5 and 15 nodes). We first randomly generated hundreds of model architectures based on these criteria. Using a Genetic Algorithm (GA) we performed crossovers and mutations on the current model architectures, which were then trained using the back-projection method and scored using 10-fold cross validation. Although no direct regularization methods were used the cross-validation step reduces the chance of over fitting. We proceeded using the GA hundreds of times with thousands of iterations. Of the final 100 model architectures created, the top 10 models were chosen such that these models had the highest scores and highest design entropy. This ensures model diversity which highlights data ambiguity (i.e., model conclusions drawn from the same data set). 
     We then optimized each of these 10 models using the Nelder Mead Simplex which provides a gradient-free optimization strategy to find local minimum; in our case maximum butanol production. This method generated 10 recommendations per model yielding a total of 100 recommendations. From this we selected the top 10 recommendations that maximized both the average predicted butanol production (TREE score) and maximized the input entropy. This method ensured that we were not over sampling in an area and sets the basis for our hybrid exploration and exploitation-based sampling strategy. Each of these represents an exploitation-based recommendation, but through enforcing diversity in models and the recommendation vector we also were able to explore the sample space. From these optimized models, ten predictions were selected from each of the top 10 architectures to be constructed in the cell-free environment. We removed predictions that were impossible experimentally (i.e., concentrations too low to pipet accurate volumes). 
     9. Statistics 
     All error bars on metabolite and protein quantification represent one standard deviation derived from technical triplicates. All error bars on TREE score values are propagated error as described in the TREE score calculation. In comparing the significance of enzyme concentration on TREE scores for butanol production in  FIG. 98B  we used the Mann-Whitney test to determine whether enzyme concentrations of the enzyme combinations that produced the top 20 TREE scores are greater than the enzyme concentrations of the entire data set. 
     F. References for Example 9 
     
         
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     It will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, limitation or limitations which is not specifically disclosed herein. The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention. Thus, it should be understood that although the present invention has been illustrated by specific embodiments and optional features, modification and/or variation of the concepts herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention. 
     Citations to a number of patent and non-patent references are made herein. The cited references are incorporated by reference herein in their entireties. In the event that there is an inconsistency between a definition of a term in the specification as compared to a definition of the term in a cited reference, the term should be interpreted based on the definition in the specification.