Patent Publication Number: US-2021163947-A1

Title: Ultrasensitive and multiplexed cell-free biosensors using cascaded amplification and positive feedback

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation-In-Part of International Application PCT/US2020/063133, filed Dec. 3, 2020, which claims the benefit of U.S. Provisional Application No. 62/943,094 filed on Dec. 3, 2019, and U.S. Provisional Application No. 63/003,724 filed on Apr. 1, 2020, the contents of which are incorporated herein by reference in their entireties. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under FA8650-15-2-5518 awarded by the Department of Defense, Air Force Research Laboratory. The government has certain rights in the invention. 
    
    
     SEQUENCE LISTING 
     A sequence listing, provided as an ASCII text file and submitted via EFS-Web as part of the present specification was created on Dec. 8, 2020 is named 702581_01883_ST25.txt, is 168 KB, and is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The field of the invention relates to cell-free protein synthesis (CFPS) systems. In particular, the field of the invention relates to the use of CFPS systems for in vitro detection of target molecules using cellular extracts. 
     Cell-free systems offer practical and technical advantages over whole-cell sensors for point-of-use detection of contaminants in aqueous environments like lead, arsenic, mercury, fluoride, and nitrate, and for detecting chemical markers of health and performance in human samples such as blood, urine and saliva. However, the diversity of sensors that can function in  E. coli  extracts is constrained by the scarcity of characterized strong promoters that can be regulated by allosteric transcription factors. Because engineering promoter strength without affecting inducibility remains an unsolved challenge in synthetic biology, the output signals from cell-free sensors are often undesirably low, particularly when detecting trace contaminants. 
     To address this problem, here we disclose a platform that utilizes CFPS for in vitro sensing of metabolites including small-molecule metabolites in which the output from a cell-free sensor is amplified using an intermediate RNA polymerase synthesized in situ. Positive feedback introduced through autocatalytic transcription and translation decreases the time required for a generating a detectable signal. By employing orthogonal polymerases in parallel, multiple key target chemicals can be detectable simultaneously in a single reaction vessel. The disclosed technology will have transformative impact toward the engineering of highly sensitive and field-deployable cell-free biosensors for monitoring metabolites and contaminants and may have wide applications including applications for monitoring global water quality. 
     SUMMARY 
     Disclosed are methods, devices, kits, components, and compositions for detecting a target molecule in a test sample using a cell-free protein synthesis (CFPS) reaction. The methods, devices, kits, components, and compositions may be utilized for detecting target molecules which may include small molecules and/or metabolites of small molecules. The components used in the disclosed methods, devices, and kits may be dried or lyophilized and may be present or immobilized on a paper substrate. 
     The disclosed methods, devices, kits, components, and compositions typically utilize one or more transcription templates that encode and conditionally express one or more exogenous RNA polymerases in the presence of the target molecule. The expressed RNA polymerases in turn induce expression of one or more reporter molecules from transcription templates comprising promoters for the RNA polymerases, thereby amplifying an output signal that is generated in the presence of a detected target molecule. 
     The disclosed methods may be performed to detect a target molecule in a biological or environmental sample and may include steps of: (i) obtaining a biological or environmental sample which may or may not contain the target molecule and optionally concentrating and/or solubilizing the target molecule in the sample if necessary; and (ii) adding the sample and/or the optionally concentrated and/or solubilized target molecule in the sample to a cell-free protein synthesis (CFPS) reaction, where, if the target molecule is present in the sample, then an output is generated and amplified using an intermediate RNA polymerase synthesized in situ. The disclosed methods utilized positive autocatalytic transcription and translation which decreases the time required for generating a detectable signal. 
     In some embodiments, the disclosed compositions, kits, systems, or methods include an inhibition scheme to minimize background production, in the absence of the target molecule, of one or more RNA polymerases employed in the compositions, kits, systems, or methods. In some embodiments, the inhibition scheme comprises an inhibitor, optionally wherein the inhibitor is selected from a T7 lysozyme, an RNA or DNA aptamer against T7 RNAP, a DNA mimic of the native T7 RNAP promoter recognition sequence, a sequence-responsive protease that selectively degrades tagged T7 RNAP, and combinations thereof. In some embodiments, the inhibitor comprises a protease, such as basal ClpX protein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A-B .  FIG. 1  provides a schematic related to the versatility and robustness of one embodiment of the cell free sensor of the present disclosure. (A) The components of the sensor can be freeze dried and provided in a reaction vessel, such as a microfuge tube. The freeze dried components are stable and can be easily transported. To use, the sensor components can be rehydrated with test substance, e.g., a liquid environmental sample, subject sample, etc. The presence of the target molecule initiates production of a detectable marker which can be detected by the user after a brief incubation. (B) Provides a schematic of one aspect of a detection platform as disclosed herein. An allosteric transcription factor is activated by its ligand (e.g., a metal, protein, small molecule, etc.), initiating transcription of the reporter molecule. 
         FIG. 2A-C .  FIG. 2 . (A) Illustrates the typical scheme for cell-free sensing. A sensor plasmid encodes an allosteric transcription factor and a second plasmid, a reporter plasmid, expresses a fluorescent report such as Green Fluorescent Protein (GFP), with the cognate promoter/operator sequence. (B) Illustrates the typical response function of a cell-free sensor where the regulated promoter drives expression of the reporter molecule. (C) Illustrates the goal response function using a cascaded sensor embodiment to enhance sensitivity of the system. In this embodiment, the regulated promoter drives expression of T7 RNA polymerase (RNAP) or a variant of T7 RNAP, and T7 RNAP then drives the expression of a reporter molecule from a corresponding promoter. 
         FIG. 3A-C .  FIG. 3  shows three different platforms for the biosensor system of the present disclosure. (A) Shows a platform comprising the expression of a reporter or signal molecule (e.g., GFP), in response to the target molecule activating its transcription factor and stimulating the promoter (e.g., the  E. coli  J23119 promoter) to transcribe the reporter molecule. In this platform the components of the sensor include transcription and translation components. (B) In the cascaded sensor embodiment, the regulated promoter drives expression of T7 RNA polymerase (RNAP) or a variant of T7 RNAP, and T7 RNAP then drives expression of the reporter molecule from a corresponding promoter. Again, the system includes transcription and translation components. (C) Illustrates a third embodiment of the sensor systems disclosed herein, utilizing signal amplification and positive feedback and termed “double cascade.” In this embodiment, T7 RNAP is made through the top, regulated layer of the cascade and is able to amplify itself autocatalytically. The top level of this cascade is termed the “source,” the mid-level is termed the “transducer” or “amplifier” and the third level is termed the “reporter.” In some embodiments, the DNA templates for one or more components of this system is prepared in vitro by, for example, isothermal assembly and the polymerase chain reaction (PCR). 
         FIG. 4A-D .  FIG. 4  illustrates various aspects of the biosensors of the present disclosure. (A) The “enriched” extract that contains the allosteric transcription factor can be mixed against a “blank” unenriched extract to modulate the concentration of the transcription factor in the reaction. (B) For tighter control and specificity of a cascaded system, if T7 RNA polymerase is used to drive expression of the allosteric transcription factor in the host strain of extract, an engineered variant of T7 RNAP may be used as the output of the regulated promoter. Exemplary T7 RNAP mutants are illustrated. (C) Shows the kinetics of a cascade amplifier. The  E. coli  RNAP is used to express four T7 RNAP variants from a mock sensor plasmid (5 nM) that contains the consensus  E. coli  promoter J23119. The corresponding reporter plasmid is added at 5 nM. The kinetics of T7 RNAP synthesis lead to a time delay of about 20-30 minutes relative to a reaction that uses purified WT T7 RNAP. Reaction conditions were as follows: triplicate 10 μL technical replicates for cell-free gene expression reaction at 30° C. (D) Orthogonality of T7 variants to the wild-type T7 RNAP. 5 nM of each orthogonal T7 RNAP reporter plasmid was supplied to a cell-free reaction in the presence of the WT T7 RNAP. The leak is lowest with the AKSIRV promoter. Reporter yields are from triplicates of four-hour sfGFP yields on a plate reader at 30° C. All cell-free reactions were prepared as previously described with the following composition: 30 v/v % total S12 extract prepared from the  E. coli  strain BL21 Star (DE3), grown to optical density 3.0 sonicated, and processed by ribosomal runoff reaction and dialysis; 8 mM magnesium glutamate, 10 mM ammonium glutamate, and 60 mM potassium glutamate; 1.2 mM ATP; 825 μM of CTP, GTP, and UTP; 34 mg/L folinic acid; 171 mg/L tRNA; 2.5 mM each amino acid; 30 mM phosphoenolpyruvate (PEP); 330 μM nicotinamide adenine dinucleotide (NAD); 270 μM coenzyme A; 4 mM potassium oxalate; 1 mM putrescine; 1.5 mM spermidine; 57 mM HEPES; midiprepped plasmid DNA to the requisite concentration; and the remainder water. 
         FIG. 5A-D . Experimental transcription, translation, and resource limitation kinetic parameters validate cascade models. (A)-(C) Parameterization of the kinetics of transcription and translation in the cell-free sensor. We assume a model of transcription and translation under finite resources, accounting for utilization of RNAPs and ribosomes as well as an exponential decay in transcription and translation rates caused by byproduct accumulation. (D) This model is backed up by experimental data where we simultaneously measure RNA and protein levels using a version of sfGFP that is tagged at the 3′ end with the sequence of the malachite green RNA aptamer. Experimental data are 4-hour endpoint reads measured in triplicate from a cell-free gene expression reaction supplied with 33% PhlF-containing extract by volume and 5 nM reporter plasmid. Reaction conditions were as follows: triplicate 10 μL technical replicates for cell-free gene expression reaction at 30° C. for four hours. 
         FIG. 6A-B .  FIG. 6  shows that cascades are predicted to improve the dose response more than noncascaded physiochemical optimizations, both for ON state (for most promoters) and Limit of Detection. Model prediction of the improved dose response behavior using a cascaded amplifier (blue) relative to the no-amplifier condition (black), with a strong bacterial promoter and low transcriptional leak. The cascade improves the dose response far more than can be achieved by tuning DNA concentration in the absence of the cascade. (A) Absolute signal of sfGFP using parameterized data in a 4-hour cell-free gene expression experiment. (B) Signal normalized between the minimum and maximum fluorescence 
         FIG. 7A-G . Development of a panel of uncascaded cell-free sensors that detect inorganic metabolites: (A) arsenic, (B) mercury, (C), (D) nitrate, (E) copper, (F) lead, and (G) cadmium. Optimization of the ratio of extract enriched with the relevant transcription factor (or sensor kinase and response regulator for the nitrate two-component system), with the balance of the extract ratio provided by a blank extract from BL21* (DE3)  E. coli . The optimal extract ratio (measured by the activation ratio, ON/OFF at saturating analyte concentration) is bolded on each plot. Reaction conditions were as follows: triplicate 10 μL technical replicates for cell-free gene expression reaction at 30° C. for four hours. The reporter plasmid was supplied at 20 nM in each case. The data are background-subtracted from a no-DNA control. 
         FIG. 8A-G . Development of a panel of cascaded cell-free sensors that detect inorganic metabolites: (A) arsenic, (B) mercury, (C) nitrate, (D) copper, (E) lead, (F) fluoride, and (G) cadmium. Optimization of the sensor plasmid (regulated promoter+T7 AKSIRV RNAP) concentration with 5 nM AKSIRV reporter plasmid in each case. The optimal concentration (measured by the activation ratio, ON/OFF at saturating analyte concentration) is bolded on each plot. Reaction conditions were as follows: triplicate 10 μL technical replicates for cell-free gene expression reaction at 30° C. for four hours. The reporter plasmid was supplied at 5 nM in each case and the data are background-subtracted from a no-DNA control. 
         FIG. 9A-G . Comparative dose responses for a panel of cascaded cell-free sensors that detect inorganic metabolites: (A) mercury, (B) copper, (C) lead, (D) cadmium, (E) arsenic, (F) fluoride, and (G) nitrate, black represents the optimized dose response curve for the noncascaded sensor (measured from experimental triplicate and normalized to a FITC standard after 4-hour reaction at 30° C.) and blue represents the optimized dose response curve for the cascaded sensor. In each case, the cascade improves the response function (increasing signal and/or shifting the curve to the left indicating enhancement of limit of detection). Dashed vertical line represents either the WHO legal limit (or, for mercury, the EPA limit, which is more stringent) in drinking water. Reaction conditions were as follows: triplicate 10 μL technical replicates for cell-free gene expression reaction at 30° C. for four hours. The reporter plasmid is supplied at 5 nM (cascade) or 20 nM (noncascaded) and the concentration of the cascaded sensor plasmid is the optimal concentration from  FIG. 8 . 
         FIG. 10A-G . Shows the same data as  FIG. 9  but with the data re-normalized to have “fraction of maximum fluorescence”, with normalization error propagated. (A) arsenic, (B) fluoride, (C) mercury, (D) cadmium, (E) copper, (F) lead, and (G) nitrate. 
         FIG. 11 . Shows results of a cascaded system detecting Hg at the legal limit from a freeze-dried sensor components. Kinetics of activation of a freeze-dried cell-free mercury sensor at the WHO legal limit (6 ppb). This represents the best dynamic range for a cell-free mercury sensor that has a fluorescent protein output in the literature at this limit. The freeze-dried sensor was prepared following the same physiochemical reaction conditions as before, prepared to 33 μL scale, then lyophilized at 0.04 mbar and −80 C overnight. The reactions were rehydrated with either water or 6 ppb HgCl 2  in water and incubated at 30 C for eight hours. 
         FIG. 12A-B .  FIG. 12  illustrates an autocatalytic amplification, double-cascade system. (A) is the same as  FIG. 3C  and illustrates a third embodiment of the sensor systems disclosed herein, utilizing signal amplification and positive feedback and termed “double cascade.” In this embodiment, T7 RNAP is made through the top, regulated layer of the cascade and is able to amplify itself autocatalytically. In some embodiments, the DNA templates for one or more components of this system is prepared in vitro through, for example, isothermal assembly and the polymerase chain reaction (PCR). (B) shows a predicted dose response behavior through the implementation of an autocatalytic cascade, in a system with a low transcription leak, shifting the effective response another order of magnitude to the left. 
         FIG. 13A-B . Proof of concept of a double-cascade amplifier which is not autocatalytic. (A) In this example, Hg-inducible expression of one variant of T7 RNAP leads to expression of a second, orthogonal T7 RNAP through a linear expression template. (B) At the WHO legal limit (30 nM=6 ppb), the double cascaded variant (fifth set of bars) improves the ON signal compared to either the uncascaded version (first set of bars) or either of the single cascades (second, third, and sixth set of bars). The fourth set of bars is a double cascade control, where the intermediate amplifier plasmid does not generate any additional polymerase that binds to the reporter. pMer: promoter that recognizes the allosteric transcription factors MerR which is activated by mercury; GFP: Green Fluorescent Protein; AKSIRV: T7 polymerase that bind the T7 promoter mutant pAKSIRV (TAATACCTGACACTATAGG; SEQ ID NO:3); pAKSIRV: promoter mutant for the AKSIRV polymerase; RV: polymerase that binds the T7 promoter mutant pRV (TAATAACCCTCACTATAGG; SEQ ID NO:2); pRV: promoter mutant for the RV polymerase; sfGFP: super-folded Green Fluorescent Protein. Reactions were performed as follows. triplicate 10 μL technical replicates for cell-free gene expression reaction at 30° C. for four hours. 
         FIG. 14 . Shows the results of optimization of double cascaded amplifier. As predicted from the resource-constrained model, optimal sensor response will occur at a small but finite concentration of the intermediate node of the cascade. Pictured are experimental data: when provided a small amount of AKSIRV promoter expressed under the strong constitutive bacterial promoter J23119 (sequence: TTGACAGCTAGCTCAGTCCTAGGTATAATACTAGT; SEQ ID NO:6) (0.1 nM) the double cascade amplifies the low signal only when the transducer is at around 0.5 nM. Reactions were provided with GamS (a nuclease inhibitor) to protect the linear expression templates. P reg : J23119 promoter driven by endogenous  E. coli  RNA polymerase and driving expression of the AKSIRV T7 polymerase mutant (i.e., the T7 RNA polymerase that binds the AKSIRV mutant promoter, pAKSIRV). Reactions were performed according to the same molecular compositions at 10 μL technical duplicates for cell-free gene expression reaction at 30° C. for four hours. RV: T7 RV mutant polymerase (i.e., the T7 RNA polymerase that binds the RV mutant promoter, pRV); sfGFP: super-folded Green Fluorescent Protein. In the graph, the first bar, “AKSIRV source, AKSIRV reporter” indicates that there was only a single plasmid in this system: AKSIRV polymerase was produced by a first plasmid, comprising an  E. coli  J23119 promoter and activated by endogenous  E. coli  RNA polymerase—the base case to be amplified. The next two bars, “no source, transducer, RV reporter” are a set of experimental controls and demonstrate that the transducer can leak at high concentration due to the production of RV polymerase that can drive reporter expression. The next 7 bars “AKSIRV source, transducer, RV reporter” are a titration of the transducer and demonstrate that as this construct&#39;s concentration increases, production of RV polymerase through the cascade leads to amplification of signal and resource limitations at high transducer concentration. 
         FIG. 15 . Proof-of-concept for autocatalytic amplification. The presence of a linear expression template (LET) allowing for AKSIRV autocatalytic amplification improves the kinetics and final yield of sfGFP for an unregulated sensor. The sensor reaction was prepared as previously described in technical triplicates at 10 μL scale and the reaction was run at 30 C for four hours. 
         FIG. 16 . Proof-of-concept for autocatalytic cascaded sensing at 10 nM HgCl2 (the most stringent limit). Implementing an AKSIRV autocatalytic cascade improves the signal to a visible threshold (&gt;1 μM FITC) without greatly increasing the leak, when compared against the single AKSIRV cascade. Sensor reaction conditions were as follows: technical triplicates at 10 μL scale at 30 C for four hours. 
         FIG. 17 . Kinetics of autocatalytic amplification. Even in the absence of a source of AKSIRV, an AKSIRV autocatalytic amplifier turns ON to high signal at very low concentrations of its linear expression template (LET), indicating that tuning will likely be necessary to ensure robustness. Reaction conditions were as follows: technical duplicates at 10 μL scale at 30 C for four hours. 
         FIG. 18A-G . Figures A-C show that orthogonal T7 RNA polymerase variants can enable one-pot sensor multiplexing. In this multiplex embodiment, the transcription factors MerR, AsR, NarX, NarL are pre-enriched in the extract(s) to sense (A) Hg, (B) As, and (C) nitrate. Figures (D)-(G) show an alternative platform for multiplexing cell-free outputs using the BioBits color palette. In (D) and (E), the first line is blue, the second is green (Pmer), and the third line is red (Pasr). Reactions shown in  FIG. 18G  were prepared as follows. Technical triplicates at 10 μL scale at 30 C for four hours. 
         FIG. 19A-B . De-sensitizing using tunable proteolysis. (A) Model for stoichiometric inhibition. A programmable protease (mf-lon) that targets only tagged proteins (in this case, the orthogonal T7 RNAP variant that is the output of the sensor) is included in the reaction and degrades is target with zeroth order kinetics. (B) Predicted dose response behavior. An inhibitor is expected to shift a DR curve down and to the right, with the goal of mitigating sensor leak. 
         FIG. 20A-C . Overexpressed mf-Lon, pdt tag, and ATP contribute to protein degradation. Proof of concept for stoichiometric inhibition using mf-Lon. An mf-Lon enriched extract was mixed with a cellular extract containing pdt-tagged sfGFP. Increasing the concentration of mf-Lon and supplying additional ATP (a co-substrate for the reaction) results in some signal decay. (A) 0% mf-Lon; (B) 10% mf-Lon; (C) 50% mf-Lon. Reactions were performed as follows: mf-Lon enriched cellular extract from a BL21 Star (DE3) strain was directly mixed with 50% cellular extract from a BL21 Star (DE3) strain overexpressing pdt-tagged sfGFP, varying the ratio of the two extracts and making up the additional volume with a blank extract, and supplying exogenous ATP. The additional reaction components (e.g., salts and buffers) were left out. 
         FIG. 21 . Design of mitigating cross-talk of sensors. Experimental measurement of crosstalk between four metal-sensing aTFs using cell-free response. Heatmap is colored more brightly to indicate stronger fluorescent signal. In this example, there is crosstalk for the lead sensor with cadmium, indicating that this strategy will be necessary to distinguish the two analytes. 
         FIG. 22A-E . Alternative options for reducing background in biosensors. (A) A “T7 lysozyme-enriched” extract dose not inhibit T7 RNAP. An alternative embodiment would be to expression of the lysozyme from the J23119 promoter in vitro. (B) An anti-T7 aptamer expressed in situ does not inhibit T7 RNAP. An alternative embodiment includes purifying the aptamer from sp6 RNAP and providing the aptamer to the biosensor reaction mixture at high concentrations. (C) A T7 promoter mimic may selectively inhibit low concentrations of wild-type T7 RNAP. An alternative embodiment includes higher concentrations of promoter and measurement of T7 dose responses. (D) While SsrA mediated degradation in basal CplX may be too potent, it can reduce leak. (E) Provides a schematic showing the implementation of protein level logic to address sensor promiscuity. Experiments were carried out in experimental technical replicates (N=2, generally) at 30 C for 4 hours. 
     
    
    
     DETAILED DESCRIPTION 
     The presently disclosed subject matter is described herein using several definitions, as set forth below and throughout the application. 
     Definitions 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of skill in the art to which the invention pertains. Although any methods and materials similar to or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described herein. 
     Unless otherwise specified or indicated by context, the terms “a”, “an”, and “the” mean “one or more.” For example, “a component,” “a metabolite,” and “a contaminant,” should be interpreted to mean “one or more components,” “one or more metabolites,” and “one or more contaminants,” respectively. For example, “a composition,” “a system,” “a kit,” “a method,” “a protein,” “a vector,” “a domain,” “a binding site,” and “an RNA” should be interpreted to mean “one or more compositions,” “one or more systems,” “one or more kits,” “one or more methods,” “one or more proteins,” “one or more vectors,” “one or more domains,” “one or more binding sites,” and “one or more RNAs,” respectively. 
     As used herein, “about,” “approximately,” “substantially,” and “significantly” will be understood by persons of ordinary skill in the art and will vary to some extent on the context in which they are used. If there are uses of these terms which are not clear to persons of ordinary skill in the art given the context in which they are used, “about” and “approximately” will mean plus or minus ≤10% of the particular term and “substantially” and “significantly” will mean plus or minus &gt;10% of the particular term. 
     As used herein, the terms “include” and “including” have the same meaning as the terms “comprise” and “comprising” in that these latter terms are “open” transitional terms that do not limit claims only to the recited elements succeeding these transitional terms. The term “consisting of,” while encompassed by the term “comprising,” should be interpreted as a “closed” transitional term that limits claims only to the recited elements succeeding this transitional term. The term “consisting essentially of,” while encompassed by the term “comprising,” should be interpreted as a “partially closed” transitional term which permits additional elements succeeding this transitional term, but only if those additional elements do not materially affect the basic and novel characteristics of the claim. 
     Ranges recited herein include the defined boundary numerical values as well as sub-ranges encompassing any non-recited numerical values within the recited range. For example, a range from about 0.01 mM to about 10.0 mM includes both 0.01 mM and 10.0 mM. Non-recited numerical values within this exemplary recited range also contemplated include, for example, 0.05 mM, 0.10 mM, 0.20 mM, 0.51 mM, 1.0 mM, 1.75 mM, 2.5 mM 5.0 mM, 6.0 mM, 7.5 mM, 8.0 mM, 9.0 mM, and 9.9 mM, among others. Exemplary sub-ranges within this exemplary range include from about 0.01 mM to about 5.0 mM; from about 0.1 mM to about 2.5 mM; and from about 2.0 mM to about 6.0 mM, among others. 
     As used herein, the terms “regulation” and “modulation” may be utilized interchangeably and may include “promotion” and “induction.” For example, a transcription factor that regulates or modulates expression of a target gene may promote and/or induce expression of the target gene. In addition, the terms “regulation” and “modulation” may be utilized interchangeably and may include “inhibition” and “reduction.” For example, a transcription factor that regulates or modulates expression of a target gene may inhibit and/or reduce expression of the target gene. 
     As used herein, the term “sample” may include “biological samples” and “non-biological samples.” Biological samples may include samples obtained from a human or non-human subject. Biological samples may include but are not limited to, blood samples and blood product samples (e.g., serum or plasma), urine samples, saliva samples, fecal samples, perspiration samples, and tissue samples. Non-biological samples may include but are not limited to aqueous samples (e.g., watershed samples) and surface swab samples. 
     The term “target molecule” means any molecule of interest in a test sample and may include so-called “small molecules” or metabolites of small molecules. Target molecules may be referred to herein alternatively as “analytes,” “metabolites,” and “contaminants.” Exemplary target molecules include metabolites, chemical compounds, and nucleic acids. By way of example, but not by way of limitation, target molecules include phloroglucinol, mercury, arsenic or its oxides, nitrate, fluoride, cyanuric acid, lead, copper, zinc, chromium or its oxides, or atrazine. 
     The term “metabolite” means a molecule to which a target molecule is converted, for example, by one or more components such as enzymes that are present in a cell-free protein synthesis (CFPS) reaction mixture and/or that are added to a CFPS reaction mixture. 
     The term “transcription factor” refers to a protein that regulates transcription of another protein, typically by interacting by one or more cis-acting DNA sequence in or near the promoter for the other protein. A transcription factor may increase expression or decrease expression depending upon whether the transcription factor is activated or deactivated. A transcription factor may become activated or deactivated by an interaction with another molecule (e.g., a metabolite as described above). Such transcription factors are termed allosteric transcription factors. 
     The term “reporter molecule” refers to a molecule (e.g., a reporter protein or RNA) that can be detected in a reaction mixture, such as a CFPS reaction mixture, typically in response to the presence of a target molecule or a metabolite thereof being present in the reaction mixture. For example, a reporter molecule may be expressed and detected in a CFPS reaction mixture when a target molecule or a metabolite thereof activates a transcription factor which promotes expression of the reporter protein in the CFPS reaction mixture. Exemplary reporter molecules include fluorescent molecules, such as Green Fluorescent Protein and super-folded Green Fluorescent Protein. Any number of reporter molecules well known in the art (Yellow, Blue, and Red Fluorescent Proteins, mCherry, etc.) can be used in the methods, systems, compositions, and kits of the present disclosure. 
     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. 
     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,  Pyrococcus furiosus  (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, bacteriophage polymerases such as, but not limited to, 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, “expression template” refers to a nucleic acid that serves as substrate for transcribing at least one RNA that can be translated into a sequence defined biopolymer (e.g., 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. 
     As used herein, coupled transcription/translation (“Tx/Tl”), refers to the de novo synthesis of both RNA and a sequence defined biopolymer from the same extract. For example, coupled transcription/translation of a given sequence defined biopolymer can arise in an extract containing an expression template and a polymerase capable of generating a translation template from the expression template. Coupled transcription/translation can occur using a cognate expression template and polymerase from the organism used to prepare the extract. Coupled transcription/translation can also occur using exogenously-supplied expression template and polymerase from an orthogonal host organism different from the organism used to prepare the extract. In the case of an extract prepared from a yeast organism, an example of an exogenously-supplied expression template includes a translational open reading frame operably coupled a bacteriophage polymerase-specific promoter and an example of the polymerase from an orthogonal host organism includes the corresponding bacteriophage polymerase. 
     Polynucleotides and Uses Thereof 
     The terms “polynucleotide,” “polynucleotide sequence,” “nucleic acid” and “nucleic acid sequence” refer to a nucleotide, oligonucleotide, polynucleotide (which terms may be used interchangeably), or any fragment thereof. These phrases also refer to DNA or RNA of genomic, natural, or synthetic origin (which may be single-stranded or double-stranded and may represent the sense or the antisense strand). 
     The terms “nucleic acid” and “oligonucleotide,” as used herein, may 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 methods, 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. 
     Regarding polynucleotide sequences, the terms “percent identity” and “% identity” refer to the percentage of residue matches between at least two polynucleotide sequences aligned using a standardized algorithm. Such an algorithm may insert, in a standardized and reproducible way, gaps in the sequences being compared in order to optimize alignment between two sequences, and therefore achieve a more meaningful comparison of the two sequences. Percent identity for a nucleic acid sequence may be determined as understood in the art. (See, e.g., U.S. Pat. No. 7,396,664, which is incorporated herein by reference in its entirety). A suite of commonly used and freely available sequence comparison algorithms is provided by the National Center for Biotechnology Information (NCBI) Basic Local Alignment Search Tool (BLAST), which is available from several sources, including the NCBI, Bethesda, Md., at its website. The BLAST software suite includes various sequence analysis programs including “blastn,” that is used to align a known polynucleotide sequence with other polynucleotide sequences from a variety of databases. Also available is a tool called “BLAST 2 Sequences” that is used for direct pairwise comparison of two nucleotide sequences. “BLAST 2 Sequences” can be accessed and used interactively at the NCBI website. The “BLAST 2 Sequences” tool can be used for both blastn and blastp (discussed above). 
     Regarding polynucleotide sequences, percent identity may be measured over the length of an entire defined polynucleotide sequence, for example, as defined by a particular SEQ ID number, or may be measured over a shorter length, for example, over the length of a fragment taken from a larger, defined sequence, for instance, a fragment of at least 20, at least 30, at least 40, at least 50, at least 70, at least 100, or at least 200 contiguous nucleotides. Such lengths are exemplary only, and it is understood that any fragment length supported by the sequences shown herein, in the tables, figures, or Sequence Listing, may be used to describe a length over which percentage identity may be measured. 
     Regarding polynucleotide sequences, “variant,” “mutant,” or “derivative” may be defined as a nucleic acid sequence having at least 50% sequence identity to the particular nucleic acid sequence over a certain length of one of the nucleic acid sequences using blastn with the “BLAST 2 Sequences” tool available at the National Center for Biotechnology Information&#39;s website. (See Tatiana A. Tatusova, Thomas L. Madden (1999), “Blast 2 sequences—a new tool for comparing protein and nucleotide sequences”, FEMS Microbiol Lett. 174:247-250). Such a pair of nucleic acids may show, for example, at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% or greater sequence identity over a certain defined length. 
     Nucleic acid sequences that do not show a high degree of identity may nevertheless encode similar amino acid sequences due to the degeneracy of the genetic code where multiple codons may encode for a single amino acid. It is understood that changes in a nucleic acid sequence can be made using this degeneracy to produce multiple nucleic acid sequences that all encode substantially the same protein. For example, polynucleotide sequences as contemplated herein may encode a protein and may be codon-optimized for expression in a particular host. In the art, codon usage frequency tables have been prepared for a number of host organisms including humans, mouse, rat, pig,  E. coli , plants, and other host cells. 
     A “recombinant nucleic acid” is a sequence that is not naturally occurring or has a sequence that is made by an artificial combination of two or more otherwise separated segments of sequence. This artificial combination is often accomplished by chemical synthesis or, more commonly, by the artificial manipulation of isolated segments of nucleic acids, e.g., by genetic engineering techniques known in the art. The term recombinant includes nucleic acids that have been altered solely by addition, substitution, or deletion of a portion of the nucleic acid. Frequently, a recombinant nucleic acid may include a nucleic acid sequence operably linked to a promoter sequence. Such a recombinant nucleic acid may be part of a vector that is used, for example, to transform a cell. 
     The nucleic acids disclosed herein may be “substantially isolated or purified.” The term “substantially isolated or purified” refers to a nucleic acid that is removed from its natural environment, and is at least 60% free, preferably at least 75% free, and more preferably at least 90% free, even more preferably at least 95% free from other components with which it is naturally associated. 
     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. 
     The terms “target,” “target sequence,” “target region,” and “target nucleic acid,” as used herein, are synonymous and may refer to a region or sequence of a nucleic acid which is to be hybridized and/or bound by another nucleic acid. 
     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 “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. 
     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, 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,  Pyrococcus furiosus  (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, RNA polymerases of bacteriophages (e.g. T3 RNA polymerase, T7 RNA polymerase, SP6 RNA polymerase, Syn5 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. 
     Also contemplated for us in the disclosed compositions, systems, kits, and methods are engineered RNA polymerase. For example, an engineered polymerase may be a non-naturally occurring RNA polymerase whose amino acid sequence has been engineered to include one or more of an insertion, a deletion, or a substitution relative to the amino acid sequence of a naturally occurring or wild-type RNA polymerase. 
     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. 
     As used herein, “an engineered transcription template” or “an engineered expression template” refers to a non-naturally occurring nucleic acid that serves as substrate for transcribing at least one RNA. As used herein, “expression template” and “transcription template” have the same meaning and are used interchangeably. Engineered include nucleic acids composed of DNA or RNA. Suitable sources of DNA for use in 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. 
     “Transformation” or “transfection” describes a process by which exogenous nucleic acid (e.g., DNA or RNA) is introduced into a recipient cell. Transformation or transfection may occur under natural or artificial conditions according to various methods well known in the art, and may rely on any known method for the insertion of foreign nucleic acid sequences into a prokaryotic or eukaryotic host cell. The method for transformation or transfection is selected based on the type of host cell being transformed and may include, but is not limited to, bacteriophage or viral infection or non-viral delivery. Methods of non-viral delivery of nucleic acids include lipofection, nucleofection, microinjection, electroporation, heat shock, particle bombardment, biolistics, virosomes, liposomes, immunoliposomes, polycation or lipid:nucleic acid conjugates, naked DNA, artificial virions, and agent-enhanced uptake of DNA. Lipofection is described in e.g., U.S. Pat. Nos. 5,049,386, 4,946,787; and 4,897,355) and lipofection reagents are sold commercially (e.g., Transfectam™ and Lipofectin™). Cationic and neutral lipids that are suitable for efficient receptor-recognition lipofection of polynucleotides include those of Felgner, WO 91/17424; WO 91/16024. Delivery can be to cells (e.g. in vitro or ex vivo administration) or target tissues (e.g. in vivo administration). The term “transformed cells” or “transfected cells” includes stably transformed or transfected cells in which the inserted DNA is capable of replication either as an autonomously replicating plasmid or as part of the host chromosome, as well as transiently transformed or transfected cells which express the inserted DNA or RNA for limited periods of time. 
     The polynucleotide sequences contemplated herein may be present in expression vectors. For example, the vectors may comprise a polynucleotide encoding an ORF of a protein operably linked to a promoter. “Operably linked” refers to the situation in which a first nucleic acid sequence is placed in a functional relationship with a second nucleic acid sequence. For instance, a promoter is operably linked to a coding sequence if the promoter affects the transcription or expression of the coding sequence. Operably linked DNA sequences may be in close proximity or contiguous and, where necessary to join two protein coding regions, in the same reading frame. Vectors contemplated herein may comprise a heterologous promoter operably linked to a polynucleotide that encodes a protein. A “heterologous promoter” refers to a promoter that is not the native or endogenous promoter for the protein or RNA that is being expressed. 
     As used herein, “expression” refers to the process by which a polynucleotide is transcribed from a DNA template (such as into mRNA or another RNA transcript) and/or the process by which a transcribed mRNA is subsequently translated into peptides, polypeptides, or proteins. Transcripts and encoded polypeptides may be collectively referred to as “gene product.” 
     The term “vector” refers to some means by which nucleic acid (e.g., DNA) can be introduced into a host organism or host tissue. There are various types of vectors including plasmid vector, bacteriophage vectors, cosmid vectors, bacterial vectors, and viral vectors. As used herein, a “vector” may refer to a recombinant nucleic acid that has been engineered to express a heterologous polypeptide (e.g., the fusion proteins disclosed herein). The recombinant nucleic acid typically includes cis-acting elements for expression of the heterologous polypeptide. 
     In the methods contemplated herein, a host cell may be transiently or non-transiently transfected (i.e., stably transfected) with one or more vectors described herein. A cell transfected with one or more vectors described herein may be used to establish a new cell line comprising one or more vector-derived sequences. In the methods contemplated herein, a cell may be transiently transfected with the components of a system as described herein (such as by transient transfection of one or more vectors), and modified through the activity of a complex, in order to establish a new cell line comprising cells containing the modification but lacking any other exogenous sequence. 
     Peptides, Polypeptides, and Proteins 
     As used herein, the terms “protein” or “polypeptide” or “peptide” may be used interchangeable to refer to a polymer of amino acids. Typically, a “polypeptide” or “protein” is defined as a longer polymer of amino acids, of a length typically of greater than 50, 60, 70, 80, 90, or 100 amino acids. A “peptide” is defined as a short polymer of amino acids, of a length typically of 50, 40, 30, 20 or less amino acids. 
     A “protein” as contemplated herein typically comprises a polymer of naturally or non-naturally occurring amino acids (e.g., alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine). The proteins contemplated herein may be further modified in vitro or in vivo to include non-amino acid moieties. These modifications may include but are not limited to acylation (e.g., O-acylation (esters), N-acylation (amides), S-acylation (thioesters)), acetylation (e.g., the addition of an acetyl group, either at the N-terminus of the protein or at lysine residues), formylation lipoylation (e.g., attachment of a lipoate, a C8 functional group), myristoylation (e.g., attachment of myristate, a C14 saturated acid), palmitoylation (e.g., attachment of palmitate, a C16 saturated acid), alkylation (e.g., the addition of an alkyl group, such as an methyl at a lysine or arginine residue), isoprenylation or prenylation (e.g., the addition of an isoprenoid group such as farnesol or geranylgeraniol), amidation at C-terminus, glycosylation (e.g., the addition of a glycosyl group to either asparagine, hydroxylysine, serine, or threonine, resulting in a glycoprotein). Distinct from glycation, which is regarded as a nonenzymatic attachment of sugars, polysialylation (e.g., the addition of polysialic acid), glypiation (e.g., glycosylphosphatidylinositol (GPI) anchor formation), hydroxylation, iodination (e.g., of thyroid hormones), and phosphorylation (e.g., the addition of a phosphate group, usually to serine, tyrosine, threonine or histidine). 
     The proteins disclosed herein may include “wild type” proteins and variants, mutants, and derivatives thereof. As used herein the term “wild type” is a term of the art understood by skilled persons and means the typical form of an organism, strain, gene or characteristic as it occurs in nature as distinguished from mutant or variant forms. As used herein, a “variant, “mutant,” or “derivative” refers to a protein molecule having an amino acid sequence that differs from a reference protein or polypeptide molecule. A variant or mutant may have one or more insertions, deletions, or substitutions of an amino acid residue relative to a reference molecule. A variant or mutant may include a fragment of a reference molecule. For example, a mutant or variant molecule may have one or more insertions, deletions, or substitution of at least one amino acid residue relative to a reference polypeptide. 
     Regarding proteins, a “deletion” refers to a change in the amino acid sequence that results in the absence of one or more amino acid residues. A deletion may remove at least 1, 2, 3, 4, 5, 10, 20, 50, 100, 200, or more amino acids residues. A deletion may include an internal deletion and/or a terminal deletion (e.g., an N-terminal truncation, a C-terminal truncation or both of a reference polypeptide). A “variant,” “mutant,” or “derivative” of a reference polypeptide sequence may include a deletion relative to the reference polypeptide sequence. 
     Regarding proteins, “fragment” is a portion of an amino acid sequence which is identical in sequence to but shorter in length than a reference sequence. A fragment may comprise up to the entire length of the reference sequence, minus at least one amino acid residue. For example, a fragment may comprise from 5 to 1000 contiguous amino acid residues of a reference polypeptide, respectively. In some embodiments, a fragment may comprise at least 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 150, 250, or 500 contiguous amino acid residues of a reference polypeptide. Fragments may be preferentially selected from certain regions of a molecule. The term “at least a fragment” encompasses the full-length polypeptide. A fragment may include an N-terminal truncation, a C-terminal truncation, or both truncations relative to the full-length protein. A “variant,” “mutant,” or “derivative” of a reference polypeptide sequence may include a fragment of the reference polypeptide sequence. 
     Regarding proteins, the words “insertion” and “addition” refer to changes in an amino acid sequence resulting in the addition of one or more amino acid residues. An insertion or addition may refer to 1, 2, 3, 4, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, or more amino acid residues. A “variant,” “mutant,” or “derivative” of a reference polypeptide sequence may include an insertion or addition relative to the reference polypeptide sequence. A variant of a protein may have N-terminal insertions, C-terminal insertions, internal insertions, or any combination of N-terminal insertions, C-terminal insertions, and internal insertions. 
     Regarding proteins, the phrases “percent identity” and “% identity,” refer to the percentage of residue matches between at least two amino acid sequences aligned using a standardized algorithm. Methods of amino acid sequence alignment are well-known. Some alignment methods take into account conservative amino acid substitutions. Such conservative substitutions, explained in more detail below, generally preserve the charge and hydrophobicity at the site of substitution, thus preserving the structure (and therefore function) of the polypeptide. Percent identity for amino acid sequences may be determined as understood in the art. (See, e.g., U.S. Pat. No. 7,396,664, which is incorporated herein by reference in its entirety). A suite of commonly used and freely available sequence comparison algorithms is provided by the National Center for Biotechnology Information (NCBI) Basic Local Alignment Search Tool (BLAST), which is available from several sources, including the NCBI, Bethesda, Md., at its website. The BLAST software suite includes various sequence analysis programs including “blastp,” that is used to align a known amino acid sequence with other amino acids sequences from a variety of databases. 
     Regarding proteins, percent identity may be measured over the length of an entire defined polypeptide sequence, for example, as defined by a particular SEQ ID number, or may be measured over a shorter length, for example, over the length of a fragment taken from a larger, defined polypeptide sequence, for instance, a fragment of at least 15, at least 20, at least 30, at least 40, at least 50, at least 70 or at least 150 contiguous residues. Such lengths are exemplary only, and it is understood that any fragment length supported by the sequences shown herein, in the tables, figures or Sequence Listing, may be used to describe a length over which percentage identity may be measured. 
     Regarding proteins, the amino acid sequences of variants, mutants, or derivatives as contemplated herein may include conservative amino acid substitutions relative to a reference amino acid sequence. For example, a variant, mutant, or derivative protein may include conservative amino acid substitutions relative to a reference molecule. “Conservative amino acid substitutions” are those substitutions that are a substitution of an amino acid for a different amino acid where the substitution is predicted to interfere least with the properties of the reference polypeptide. In other words, conservative amino acid substitutions substantially conserve the structure and the function of the reference polypeptide. The following table provides a list of exemplary conservative amino acid substitutions which are contemplated herein: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Original 
                   
               
               
                   
                 Residue 
                 Conservative Substitutions 
               
               
                   
                   
               
             
            
               
                   
                 Ala 
                 Gly, Ser 
               
               
                   
                 Arg 
                 His, Lys 
               
               
                   
                 Asn 
                 Asp, Gln, His 
               
               
                   
                 Asp 
                 Asn, Glu 
               
               
                   
                 Cys 
                 Ala, Ser 
               
               
                   
                 Gln 
                 Asn, Glu, His 
               
               
                   
                 Glu 
                 Asp, Gln, His 
               
               
                   
                 Gly 
                 Ala 
               
               
                   
                 His 
                 Asn, Arg, Gln, Glu 
               
               
                   
                 Ile 
                 Leu, Val 
               
               
                   
                 Leu 
                 Ile, Val 
               
               
                   
                 Lys 
                 Arg, Gln, Glu 
               
               
                   
                 Met 
                 Leu, Ile 
               
               
                   
                 Phe 
                 His, Met, Leu, Trp, Tye 
               
               
                   
                 Ser 
                 Cys, Thr 
               
               
                   
                 Thr 
                 Ser, Val 
               
               
                   
                 Trp 
                 Phe, Tyr 
               
               
                   
                 Tyr 
                 His, Phe, Trp 
               
               
                   
                 Val 
                 Ile, Leu, Thr 
               
               
                   
                   
               
            
           
         
       
     
     Conservative amino acid substitutions generally maintain (a) the structure of the polypeptide backbone in the area of the substitution, for example, as a beta sheet or alpha helical conformation, (b) the charge or hydrophobicity of the molecule at the site of the substitution, and/or (c) the bulk of the side chain. Non-conservative amino acids typically disrupt (a) the structure of the polypeptide backbone in the area of the substitution, for example, as a beta sheet or alpha helical conformation, (b) the charge or hydrophobicity of the molecule at the site of the substitution, and/or (c) the bulk of the side chain. 
     The disclosed proteins, mutants, variants, or described herein may have one or more functional or biological activities exhibited by a reference polypeptide (e.g., one or more functional or biological activities exhibited by wild-type protein). 
     In some embodiments of the disclosed compositions, systems, kits, and methods, the components may be substantially isolated or purified. The term “substantially isolated or purified” refers to components that are removed from their natural environment, and are at least 60% free, preferably at least 75% free, and more preferably at least 90% free, even more preferably at least 95% free from other components with which they are naturally associated. 
     Cell-Free Protein Synthesis (CFPS) 
     The disclosed subject matter relates in part to methods, devices, kits and components for cell-free protein synthesis. Cell-free protein synthesis (CFPS) is known and has been described in the art. (See, e.g., U.S. Pat. Nos. 6,548,276; 7,186,525; 8,734,856; 7,235,382; 7,273,615; 7,008,651; 6,994,986; 7,312,049; 7,776,535; 7,817,794; 8,298,759; 8,715,958; 9,005,920; U.S. Publication No. 2014/0349353, U.S. Publication No. 2016/0060301, U.S. Publication No. 2018/0016612, and U.S. Publication No. 2018/0016614, the contents of which are incorporated herein by reference in their entireties). A “CFPS reaction mixture” typically contains a crude or partially-purified bacterial extract (as used herein the terms “extract” and “lysate” are used interchangeably), 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 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. 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. For example, the cellular transcription and translational machinery may be provided in a lysate from an engineered bacterial strain, or the transcription and translational machinery may be purified separately and reconstituted to defined concentrations. In some embodiments, a lysate may be from an engineered bacterial strain, and include cellular transcriptional and translational machinery, and may also include other as other cellular proteins. 
     The disclosed cell-free protein synthesis systems may utilize components that are crude and/or that are at least partially isolated and/or purified. As used herein, the term “crude” may mean components obtained by disrupting and lysing cells and, at best, minimally purifying the crude components from the disrupted and lysed cells, for example by centrifuging the disrupted and lysed cells and collecting the crude components from the supernatant and/or pellet after centrifugation. The term “isolated or purified” refers to components that are removed from their natural environment, and are at least 60% free, preferably at least 75% free, and more preferably at least 90% free, even more preferably at least 95% free from other components with which they are naturally associated. 
     An aspect of the invention is a platform for preparing a sequence defined protein in vitro which may be utilized for detecting a target molecule or metabolite thereof. The platform for preparing a sequence defined polymer or protein in vitro comprises a cellular extract from a host strain. Because CFPS exploits an ensemble of catalytic proteins prepared from the crude lysate of cells, the cell extract (whose composition is sensitive to growth media, lysis method, and processing conditions) is the most critical component of extract-based CFPS reactions. A variety of methods exist for preparing an extract competent for cell-free protein synthesis, including U.S. patent application Ser. No. 14/213,390 to Michael C. Jewett et al., entitled METHODS FOR CELL-FREE PROTEIN SYNTHESIS, filed Mar. 14, 2014, and now published as U.S. Patent Application Publication No. 2014/0295492 on Oct. 2, 2014, and U.S. patent application Ser. No. 14/840,249 to Michael C. Jewett et al., entitled METHODS FOR IMPROVED IN VITRO PROTEIN SYNTHESIS WITH PROTEINS CONTAINING NON STANDARD AMINO ACIDS, filed Aug. 31, 2015, and now published as U.S. Patent Application Publication No. 2016/0060301, on Mar. 3, 2016, the contents of which are incorporated by reference. 
     The platform may comprise an expression template, a translation template, or both an expression template and a translation template. The expression template serves as a substrate for transcribing at least one RNA that can be translated into a sequence defined biopolymer (e.g., a polypeptide or protein). The translation template is an RNA product that can be used by ribosomes to synthesize the sequence defined biopolymer. In certain embodiments the platform comprises both the expression template and the translation template. In certain specific embodiments, the platform may be a coupled transcription/translation (“Tx/Tl”) system where synthesis of translation template and a sequence defined biopolymer from the same cellular extract. 
     The platform may comprise one or more polymerases capable of generating a translation template from an expression template. The polymerase may be supplied exogenously or may be supplied from the organism used to prepare the extract. In certain specific embodiments, the polymerase is expressed from a plasmid present in the organism used to prepare the extract and/or an integration site in the genome of the organism used to prepare the extract. 
     Altering the physicochemical environment of the CFPS reaction to better mimic the cytoplasm can improve protein synthesis activity. The following parameters can be considered alone or in combination with one or more other components to improve robust CFPS reaction platforms based upon crude cellular extracts (for examples, S12, S30 and S60 extracts). 
     The temperature may be any temperature suitable for CFPS. Temperature may be in the general range from about 10° C. to about 40° C., including intermediate specific ranges within this general range, include from about 15° C. to about 35° C., from about 15° C. to about 30° C., form about 15° C. to about 25° C. In certain aspects, the reaction temperature can be about 15° C. about 16° C., about 17° C., about 18° C., about 19° C., about 20° C., about 21° C., about 22° C., about 23° C., about 24° C., about 25° C. 
     The CFPS reaction can include any organic anion suitable for CFPS. In certain aspects, the organic anions can be glutamate, acetate, among others. In certain aspects, the concentration for the organic anions is independently in the general range from about 0 mM to about 200 mM, including intermediate specific values within this general range, such as about 0 mM, about 10 mM, about 20 mM, about 30 mM, about 40 mM, about 50 mM, about 60 mM, about 70 mM, about 80 mM, about 90 mM, about 100 mM, about 110 mM, about 120 mM, about 130 mM, about 140 mM, about 150 mM, about 160 mM, about 170 mM, about 180 mM, about 190 mM and about 200 mM, among others. 
     The CFPS reaction can also include any halide anion suitable for CFPS. In certain aspects the halide anion can be chloride, bromide, iodide, among others. A preferred halide anion is chloride. Generally, the concentration of halide anions, if present in the reaction, is within the general range from about 0 mM to about 200 mM, including intermediate specific values within this general range, such as those disclosed for organic anions generally herein. 
     The CFPS reaction may also include any organic cation suitable for CFPS. In certain aspects, the organic cation can be a polyamine, such as spermidine or putrescine, among others. Preferably polyamines are present in the CFPS reaction. In certain aspects, the concentration of organic cations in the reaction can be in the general about 0 mM to about 3 mM, about 0.5 mM to about 2.5 mM, about 1 mM to about 2 mM. In certain aspects, more than one organic cation can be present. 
     The CFPS reaction can include any inorganic cation suitable for CFPS. For example, suitable inorganic cations can include monovalent cations, such as sodium, potassium, lithium, among others; and divalent cations, such as magnesium, calcium, manganese, among others. In certain aspects, the inorganic cation is magnesium. In such aspects, the magnesium concentration can be within the general range from about 1 mM to about 50 mM, including intermediate specific values within this general range, such as about 1 mM, about 2 mM, about 3 mM, about 5 mM, about 6 mM, about 7 mM, about 8 mM, about 9 mM, about 10 mM, among others. In preferred aspects, the concentration of inorganic cations can be within the specific range from about 4 mM to about 9 mM and more preferably, within the range from about 5 mM to about 7 mM. 
     The CFPS reaction includes NTPs. In certain aspects, the reaction use ATP, GTP, CTP, and UTP. In certain aspects, the concentration of individual NTPs is within the range from about 0.1 mM to about 2 mM. 
     The CFPS reaction can also include any alcohol suitable for CFPS. In certain aspects, the alcohol may be a polyol, and more specifically glycerol. In certain aspects the alcohol is between the general range from about 0% (v/v) to about 25% (v/v), including specific intermediate values of about 5% (v/v), about 10% (v/v) and about 15% (v/v), and about 20% (v/v), among others. 
     Biosensors: Compositions, Kits, and Systems 
     The technology described herein relates generally to microbial-based biosensor compositions, systems, and kits for the detection of small molecules and analytes (e.g., metabolites, chemical compounds, nucleic acids), based on an analyte-responsive transcription factor-DNA binding mechanism, resulting in the expression of a detectable reporter protein. The biosensors employ one or more analyte-responsive transcription factor-DNA binding platforms for the cell free detection of target molecules. In some embodiments, the biosensor systems include one or more signal amplifiers to provide a cascade of polymerase expression, and/or include one or more inhibitors to decrease background (increase signal to noise ratio) of the reporter protein. 
     As used herein, the term “biosensor” refers to a reaction mixture comprising all of the components necessary to detect an analyte of interest. In some embodiments, the biosensor is provided as freeze-dried components contained in a vessel, such as a microfuge tube, test tube, or a multi-well plate. Upon rehydration of the components with a liquid or liquefied sample, the analyte of interest, if present, initiates a reaction in the vessel resulting in the expression of a reporter molecule. 
     The biosensor is designed to detect a ligand (the analyte) of an allosteric transcription factor. Once the ligand binds its transcription factor, a cascade of transcription and translation events occurs. Accordingly, the biosensors comprise the components for transcription and translation reactions and include the necessary enzymes, co-factors, nucleotides, amino acids, energy source, etc. These components can be provided individually, or can be provided as one or more extracts for CFPS as described above. 
     By way of example, a biosensor of the present disclosure comprises one or more lysates from engineered bacterial strains, the lysate comprising cellular transcriptional and translational machinery, and optionally other cellular proteins, co-factors, energy sources (e.g., ATP-based cellular energy or non-phosphate based energy); a biosensor molecule that modulates the expression of a target DNA sequence in a DNA transcription template (e.g., an ATF); a DNA transcription template whose expression is configured to be regulated by the biosensor molecule, and encoding the expression of additional RNA polymerase not present in the lysate (e.g., an exogenous or orthogonal RNA polymerase); and a second DNA transcription template encoding the expression of a reporter molecule (e.g., a reporter protein or RNA molecule) whose transcription is controlled by the expressed additional RNA polymerase (e.g., wherein the second DNA transcription template comprises a promoter for the exogenous or orthogonal RNA polymerase). 
     The biosensors comprise at least one biosensor molecule, such as an allosteric transcription factor (ATF). The term “allosteric transcription factor” as used herein refers to regulatory proteins that contain a DNA-binding domain as well as a ligand-binding domain that is able to recognize small molecules with high specificity and selectivity. In the presence of a target small molecule (i.e., the transcription factor ligand), transcription factor affinity for its DNA binding sequence is modulated, facilitating the repressor or derepressor regulation of downstream gene expression. In some embodiments, the biosensors disclosed herein comprise a plasmid (e.g., a sensor plasmid) that expresses the ATF either in the biosensor (e.g., transcription and translation of the ATF occurs upon rehydration of the biosensor components by adding liquid the sample), or in the host strain used for making the biosensor (e.g., as a component of a CFPS reaction). For example, in some embodiments, the biosensors comprise the ATF protein, and the biosensor molecule is overexpressed in the host strain prior to making the extract for cell-free protein synthesis. Additionally or alternatively, the biosensor molecule (e.g., an ATF) comprises an isolated protein and is added to the biosensor components. 
     As used herein, the term “sensor plasmid” refers to a plasmid comprising a promoter which drives the expression of an allosteric transcription factor. See e.g.,  FIG. 2A . In some embodiments, the sensor plasmid is provided in an extract (e.g., as a component of a CFPS reaction), and the promoter is driven by the host&#39;s transcription and translation components). In some embodiments, the sensor plasmid is provided to a host strain, and the ATF protein is purified and added to the biosensor. Exemplary plasmids include but are not limited to pT7-CueR, pT7-MerR, pT7-ArsR, pT7-NarX, pT7-NarL, pT7-CadR. 
     The biosensor disclosed herein also comprise a reporter plasmid, or a linear reporter DNA construct. The reporter molecule can be any detectable protein. In some embodiments, the reporter protein can be visualized without the use of additional equipment or reagents. While GFP and sfGFP are exemplified herein, the biosensors are not intended to be so limited, and any number of detectable protein markers can be employed and include, but are not limited to green fluorescent protein, red fluorescent protein, blue fluorescent protein or any derivatives thereof. In some embodiments, the reporter comprises a den enzyme that produces a visible signal, such as catechol 2,3-dioxygenase (C23D0) beta-galactosidase (LacZ), or glucuronidase (GusA). 
     The reporter plasmid or linear construct also comprises a promoter to drive the expression of the reporter molecule. The promoter may be reactive to the ATF and its polymerase (e.g., as shown in  FIG. 3A ), or it may be reactive to an unrelated polymerase (e.g., as shown in  FIGS. 3B and 3C ). 
     The biosensors disclosed herein may also include one or more signal amplification constructs in the form of plasmid or linear DNA constructs (see e.g.,  FIGS. 3B  and C). In the scheme outlined in  FIG. 3B , the biosensor comprises (1) a biosensor molecule (e.g., an ATF protein), (2) a signal amplification plasmid or linear DNA construct (“amplifier” or “transducer”), and (3) a reporter plasmid or linear DNA construct. The ATF can be incorporated in the biosensor as a plasmid, a protein, or both e.g., an enriched extract. Likewise, the amplification and reporter constructs may also be added individually, or as part of an extract. In this embodiment, the amplifier construct comprises a promoter linked to an orthogonal polymerase. The ATF and its polymerase bind to the promoter on the amplifier and drive the expression of the orthogonal polymerase. The reporter construct comprises the matching orthogonal promoter linked to the reporter molecule. Exemplary plasmids include but are not limited to: pMer-AKSIRV, pArs-AKSIRV, pNar-AKSIRV, pFluor-AKSIRV, pCue-AKSIRV, pPbr-AKSIRV, pAKSIRV-RV. 
     In the scheme outlined in  FIG. 3C , the biosensor comprises (1) a biosensor molecule (e.g., an ATF protein) (2) a “source” plasmid or linear DNA construct; (3) a signal amplification plasmid or linear DNA construct (“amplifier” or “transducer”), and (4) a reporter plasmid or linear DNA construct. Again, the ATF can be incorporated in the biosensor as a plasmid, a protein, or both e.g., an enriched extract. Likewise, the source, amplification, and reporter constructs may also be added individually, or as part of an extract. In this embodiment, the source construct comprises a promoter linked to a first orthogonal polymerase. The ATF and its polymerase bind to the promoter on the source construct and drive the expression of the first orthogonal polymerase. The amplifier construct comprises the matching first orthogonal promoter linked a second orthogonal polymerase. The first orthogonal polymerase binds to its promoter on the amplifier construct and drives expression of the second orthogonal polymerase. The reporter construct comprises the matching second orthogonal promoter linked to the reporter molecule. The second orthogonal polymerase binds to its promoter on the reporter construct and drives expression of the reporter molecule. In some embodiments, the first and second orthogonal polymerases are the same. Thus, in some embodiments, the presence of the target molecule increases the rate of transcription and translation of the additional RNA polymerase. 
     By way of example, allosteric transcription factors that are activated or deactivated by an interaction with another molecule include those shown below in Table 1. Their promoter sequences are also provided. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary Allosteric Transcription Factors 
               
            
           
           
               
               
               
               
            
               
                   
                 Allosteric 
                 NCBI 
                   
               
               
                   
                 Transcrip- 
                 Refer- 
                   
               
               
                 Activator/ 
                 tion 
                 ence 
                 Promoter 
               
               
                 Ligand 
                 Factor 
                 Sequence 
                 Sequence 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 arsenic 
                 ArsR EP3 
                 562 
                 ACACATTCGTTA 
               
               
                   
                 mutant 
                   
                 AGTCATATATGT 
               
               
                   
                   
                   
                 TTTTGACTTATC 
               
               
                   
                   
                   
                 CGCTTCGAAGAG 
               
               
                   
                   
                   
                 ATATAATACCTG 
               
               
                   
                   
                   
                 CAA 
               
               
                   
                   
                   
                 (SEQ ID NO: 7) 
               
               
                   
               
               
                 mercury 
                 MerR 
                 83333 
                 ATCGCTTGACTCC 
               
               
                   
                   
                   
                 GTACATGAGTACG 
               
               
                   
                   
                   
                 GAAGTAAGGTTAC 
               
               
                   
                   
                   
                 GCTAT 
               
               
                   
                   
                   
                 (SEQ ID NO: 8) 
               
               
                   
               
               
                 nitrate 
                 NarX and 
                 83333 
                 (pydfJ115 hybrid 
               
               
                   
                 NarL 
                   
                 promoter from 
               
               
                   
                 (pydfJ115 
                   
                 Ekness, et. al. 
               
               
                   
                 hybrid) 
                   
                 2019 Nat. 
               
               
                   
                   
                   
                 Chem. Biol). 
               
               
                   
                   
                   
                 ACTGCATATTTGAAA 
               
               
                   
                   
                   
                 ATTGCCCAAACGTAC 
               
               
                   
                   
                   
                 ATGCCCGAATGTACGT 
               
               
                   
                   
                   
                 TTTTTTCATTTCATTG 
               
               
                   
                   
                   
                 TCAACTACAATGAGAA 
               
               
                   
                   
                   
                 AGAATGTGATCAAGCA 
               
               
                   
                   
                   
                 ATGTGTTGAAAGGAGA 
               
               
                   
                   
                   
                 TTATC 
               
               
                   
                   
                   
                 (SEQ ID NO: 9) 
               
               
                   
               
               
                 copper 
                 CueR 
                 83333 
                 TTCTTGACCTTCCCCT 
               
               
                   
                   
                   
                 TGCtGGAAGGTTTATC 
               
               
                   
                   
                   
                 CTCGGTT 
               
               
                   
                   
                   
                 (SEQ ID NO: 10) 
               
               
                   
               
               
                 lead 
                 PbrR 
                 470 
                 ATGTCTTGACTCTAT 
               
               
                   
                   
                   
                 AGTAACTAGAGGGTG 
               
               
                   
                   
                   
                 TTAAATCGGCA 
               
               
                   
                   
                   
                 (SEQ ID NO: 11) 
               
               
                   
               
               
                 fluoride 
                 crcB 
                 157 
                 TTGACAGCTAGCTCAG 
               
               
                   
                 riboswitch 
                   
                 TCCTAGGTATAATACT 
               
               
                   
                   
                   
                 AGTTTATAGGCGATGG 
               
               
                   
                   
                   
                 AGTTCGCCATAAACGC 
               
               
                   
                   
                   
                 TGCTTAGCTAATGACT 
               
               
                   
                   
                   
                 CCTACCAGTATCACTA 
               
               
                   
                   
                   
                 CTGGTAGGAGTCTATT 
               
               
                   
                   
                   
                 TTTTT 
               
               
                   
                   
                   
                 (SEQ ID NO: 12) 
               
               
                   
               
               
                 cadmium 
                 CadR 
                 384676 
                 ATAACTTGACTCTGtA 
               
               
                   
                   
                   
                 GttgCTaCAGgGTGTG 
               
               
                   
                   
                   
                 CAATCGGTT 
               
               
                   
                   
                   
                 (SEQ ID NO: 13) 
               
               
                   
               
               
                 chromate 
                 ChrB 
                 94626 
                 GTAGATCTTATCTCAT 
               
               
                   
                   
                   
                 TATTGTAGTAAtATCT 
               
               
                   
                   
                   
                 AC 
               
               
                   
                   
                   
                 (SEQ ID NO: 14) 
               
               
                   
               
            
           
         
       
     
     As described above, the promoter sequence responsive to the activated ATF drives the expression of a reporter molecule or an orthogonal polymerase. In some embodiments, the promoter sequence responsive to the activated ATF comprises an  E. coli  promoter sequence. By way of example, variants of the  E. coli  J23119 promoter sequence is used. Plasmids were assembled using isothermal (Gibson) assembly and confirmed by Sanger sequencing. The sequences for the ArsR (#78635) and MerR (#123148) genes were obtained from Addgene from Dr. Baojun Wang&#39;s lab. The sequences for the NarX and NarL plasmids were a generous gift from Dr. Jeffrey Tabor&#39;s lab. Other constitutive promoters engineered to have aTF-binding sites (e.g., operators grafted into or after the entire Anderson promoter collection in the BioBricks catalog) would be appropriate. The promoter can essentially be any promoter, so long as it is responsive to the selected ATF and the biosensor includes an appropriately matched polymerase. 
     Orthogonal polymerases and matched promoters can be introduced in the biosensors to generate a cascade of polymerase transcription and translation (see e.g.,  FIGS. 3B  and C). Such a cascade can enhance the time to signal generation (e.g. decrease detection time), and enhance signal generation (e.g., improve limits of detection and increase signal strength). In some embodiments, the additional RNA polymerase comprises a bacteriophage polymerase, e.g., from a bacteriophage in the podovirus family, such as T7 RNA polymerase, SP6 RNA polymerase and T3 RNA polymerase. In some embodiments, the additional polymerase comprises an engineered or evolved variant of the natural RNA polymerase. By way of example, several orthogonal polymerase mutants and their matched promoters, based on the bacteriophage T7 polymerase and promoter, are exemplified herein (see e.g.,  FIG. 4B ), and are shown below in Table 2. Promoter variants were assembled by inverse PCR and blunt end ligation or Gibson assembly and confirmed by Sanger sequencing. The polymerases were assembled by Gibson assembly and were obtained as a generous gift from Dr. Ellington&#39;s lab on Addgene (#63627, 63628, 63629, and 63668). 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 T7 promoter mutants and polymerase 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Polymerase 
               
               
                   
                   
                   
                 [NCBI 
               
               
                   
                   
                   
                 Reference 
               
               
                   
                 T7 
                   
                 or other 
               
               
                   
                 Promoter 
                   
                 reference 
               
               
                   
                 Mutant 
                 Sequence 
                 to identify] 
               
               
                   
               
               
                   
                 WT 
                 TAATA CGA   
                 10760 
               
               
                   
                   
                   CT CACTAT 
                   
               
               
                   
                   
                 AGG 
                   
               
               
                   
                   
                 (SEQ ID 
                   
               
               
                   
                   
                 NO: 1) 
                   
               
               
                   
               
               
                   
                 RV 
                 TAATA ACC   
                 RV polymerase 
               
               
                   
                   
                   CT CACTAT 
                 from Meyer &amp; 
               
               
                   
                   
                 AGG 
                 Ellington, 
               
               
                   
                   
                 (SEQ ID 
                 ACS Synth. 
               
               
                   
                   
                 NO: 2) 
                 Biol. 2014. 
               
               
                   
               
               
                   
                 AKSIRH 
                 TAATA CCT   
                 AKSIRV 
               
               
                   
                   
                   GA CACTAT 
                 polymerase 
               
               
                   
                   
                 AGG 
                 from Meyer 
               
               
                   
                   
                 (SEQ ID 
                 &amp; Ellington, 
               
               
                   
                   
                 NO: 3) 
                 ACS Synth. 
               
               
                   
                   
                   
                 Biol. 2014. 
               
               
                   
               
               
                   
                 IRH 
                 TAATA ACT   
                 IRH polymerase 
               
               
                   
                   
                   AT CACTAT 
                 from Meyer &amp; 
               
               
                   
                   
                 AGG 
                 Ellington, 
               
               
                   
                   
                 (SEQ ID 
                 ACS Synth. 
               
               
                   
                   
                 NO: 4) 
                 Biol. 2014. 
               
               
                   
               
               
                   
                 KIRV 
                 TAATA CCG   
                 KIRV polymerase 
               
               
                   
                   
                   GT CACTAT 
                 from Meyer &amp; 
               
               
                   
                   
                 AGG 
                 Ellington, 
               
               
                   
                   
                 (SEQ ID 
                 ACS Synth. 
               
               
                   
                   
                 NO: 5) 
                 Biol 2014. 
               
               
                   
               
            
           
         
       
     
     By way of example but not by way of limitation, expression vectors that can be used in the methods and systems disclosed herein are provided in Table 3 below. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Exemplary Expression Constructs 
               
            
           
           
               
               
               
            
               
                 Construct 
                 Construct NAME 
                 SEQ ID NO: 
               
               
                   
               
               
                 aTFs 
                 pT7-ArsR-EP3 
                 SEQ ID NO: 15 
               
               
                   
                 pT7-CadR 
                 SEQ ID NO: 16 
               
               
                   
                 pT7-CueR 
                 SEQ ID NO: 17 
               
               
                   
                 pT7-MerR 
                 SEQ ID NO: 18 
               
               
                   
                 pT7-NarL-hybrid 
                 SEQ ID NO: 19 
               
               
                   
                 pT7-NarX 
                 SEQ ID NO: 20 
               
               
                   
                 pT7-PbrR 
                 SEQ ID NO: 21 
               
               
                 Cascade Reporters 
                 pAKSIRV-sfGFP 
                 SEQ ID NO: 22 
               
               
                   
                 pAKSIRV-XylE 
                 SEQ ID NO: 23 
               
               
                   
                 pIRH-sfGFP 
                 SEQ ID NO: 24 
               
               
                   
                 pKIRV-sfGFP 
                 SEQ ID NO: 25 
               
               
                   
                 pRV-sfGFP 
                 SEQ ID NO: 26 
               
               
                   
                 pT7-sfGFP 
                 SEQ ID NO: 27 
               
               
                 Cascade Sensors 
                 crcB-AKSIRV 
                 SEQ ID NO: 28 
               
               
                   
                 J23119- 
                 SEQ ID NO: 29 
               
               
                   
                 AKSIRV_low_copy 
               
               
                   
                 J23119-IRH 
                 SEQ ID NO: 30 
               
               
                   
                 J23119-KIRV 
                 SEQ ID NO: 31 
               
               
                   
                 J23119-RV 
                 SEQ ID NO: 32 
               
               
                   
                 pArs-AKSIRV 
                 SEQ ID NO: 33 
               
               
                   
                 pCad-AKSIRV 
                 SEQ ID NO: 34 
               
               
                   
                 pMer-AKSIRV 
                 SEQ ID NO: 35 
               
               
                   
                 pNar-AKSIRV 
                 SEQ ID NO: 36 
               
               
                 Multilayer Cascades 
                 pAKSIRV-AKSIRV 
                 SEQ ID NO: 37 
               
               
                   
                 pAKSIRV-AKSIRV- 
                 SEQ ID NO: 38 
               
               
                   
                 LET 
               
               
                   
                 pAKSIRV-RV 
                 SEQ ID NO: 39 
               
               
                 Non-Cascaded 
                 crcB-fGFP 
                 SEQ ID NO: 40 
               
               
                 Sensors 
                 pArs-sfGFP 
                 SEQ ID NO: 41 
               
               
                   
                 pCad-sfGFP 
                 SEQ ID NO: 42 
               
               
                   
                 pCue-sfGFP 
                 SEQ ID NO: 43 
               
               
                   
                 pMer-sfGFP 
                 SEQ ID NO: 44 
               
               
                   
                 pNar-sfGFP 
                 SEQ ID NO: 45 
               
               
                   
                 pPbr-sfGFP 
                 SEQ ID NO: 46 
               
               
                   
               
            
           
         
       
     
     The biosensors disclosed herein may be multiplexed; that is, more than one target can be detected in a single reaction vessel. By way of example, as shown in  FIG. 18 , by using different combinations of ATFs, amplification, and detection polymerases, multiple targets can be detected in a single reaction. Thus, in some embodiments, the compositions, kits, and systems disclosed herein include (a) a lysate from an engineered bacterial strain, the lysate comprising cellular transcriptional and translational machinery, and optionally, other cellular proteins, cofactors, and energy sources; (b) two or more DNA transcription templates encoding an additional RNA polymerase not present in the lysate and configured to be conditionally expressed (e.g., in the presence of the a target molecule); and (c) two or more DNA transcriptional templates encoding the expression of a reporter molecule under control of transcription by an additional RNA polymerase not present in the lysate. Thus in some embodiments, multiple target molecules can be detected in a single tube by using orthogonal RNA polymerases. 
     In some embodiments, the biosensors are optimized, e.g., to provide detectable signals in a shorter time, and/or cleaner signal (e.g., with less background). Cascaded systems, as described above, provide one means of optimization. Another means of optimization includes regulating the T7 polymerase activity and/or expression. As shown in  FIGS. 19-22 , several options can be employed, including T7 lysozyme, aptamers, promoter mimics, and targeted protein degradation (e.g., using AAA+ proteases such as ClpX, Lon, ClpAP, HslUV, and FtsH). With regard to targeted protein degradation, the protein of interest is modified to include a specific tag (e.g., SsrA; Sul20C, etc.), recognized by its protease. In some embodiments, the amount of the protease can be tightly controlled, e.g., by adding the purified or partially purified protease to the extract/reaction mixture. 
     In some embodiments, optimization includes “additional” RNA polymerases that have been specifically evolved or engineered for specificity for only a single promoter to avoid crosstalk. 
     In some embodiments, optimization includes reporter protein comprising orthogonal fluorescence or absorbance spectra, or catalyze enzymatic reactions that produce different colors. 
     In some embodiments, the target molecule to be detected comprises one or more of phloroglucinol, mercury, arsenic or its oxides, nitrate, fluoride, cyanuric acid, lead, copper, zinc, chromium or its oxides or atrazine. In some embodiments, the target molecule to be detected comprises RNA or DNA. 
     In some embodiments, the nucleic acids provided as components of the biosensor are amplified using an isothermal strategy prior to sensor activation. By way of example, nucleic acid sequence-based amplification (NASBA) and recombinant polymerase amplification (RPA) may be used. 
     Methods employing the biosensors are also contemplated herein. For example, methods of detecting a target molecule (e.g., a metabolite, a chemical compound, a nucleic acid) in a biological or environmental sample may include: (i) obtaining a biological or environmental sample which may or may not contain the target molecule and optionally concentrating and/or solubilizing the target molecule in the sample if necessary; (ii) adding the sample and/or the optionally concentrated and/or solubilized target molecule in the sample to a cell-free protein synthesis (CFPS) reaction, wherein if the target molecule is present in the sample then an output is generated (e.g., a visual, electronic, or optical output); wherein the output is generated via steps that include: (i) the target molecule inducing expression of an RNA polymerase from a first DNA transcription template, wherein the expressed RNA polymerase is not present in the CFPS reaction prior to its expression, optionally wherein the expression of the RNA polymerase is induced via a biosensor molecule in the presence of the target molecule; (ii) the expressed RNA polymerase expresses a reporter molecule from a second DNA transcription template (e.g., wherein the second DNA transcription template comprises a promoter for the expressed RNA polymerase) and the reporter molecule generates an output either directly or indirectly. 
     Applications and Advantages 
     Applications of the disclosed technology may include but are not limited to: (i) improving the sensitivity of molecular diagnostics, such as field-deployable molecular diagnostics; (ii) improving the maximum detectable signal of molecular diagnostics; (iii) improving the response transfer curve of molecular diagnostics for more sensitive and sigmoidal switching behavior; and (iv) enabling one-pot multiplexing of several cell-free sensors. 
     Advantages of the disclosed technology may include but are not limited to: (i) the development of sensors having an improved limit of detection for arbitrary analytes (target molecules) which is enhanced compared to a no-signal amplification condition, as well as the reporter signal in the ON (i.e. target chemical present) state; (ii) the development of sensors having a response which is more “switchlike”, or sigmoidal, enabling better semi-quantitative determination of concerning concentrations of relevant analytes; and (iii) the development of sensors which are extremely modular and adapted to various reporter outputs, which also enables one-pot multiplexing of various detection schemes with different fluorescent proteins or enzymes. Point-of-care, field-deployable diagnostics could allow consumers to rapidly and inexpensively determine water quality, be used for personalized health monitoring, be used for point-of-use health monitoring. 
     EXAMPLES 
     The examples provided herein are not intended to be limiting, and are provided to demonstrate aspects of the present technology. 
     Example 1—Cell-Free Sensors as Point of Care Diagnostics 
     In general, a cascaded and noncascaded sensor requires three plasmids that are designed and assembled using standard molecular biology strategies (isothermal assembly, restriction cloning, blunt-end ligation, solid-phase oligonucleotide synthesis, etc.). One plasmid encodes the target allosteric transcription factor (e.g., CueR) under the control of the wild-type T7 RNAP promoter. The other two encode the responsive promoter sequence (e.g., pCue) upstream of a reporter, either the sfGFP coding sequence (the noncascaded sensor) or T7 RNAP (the cascaded sensor). The natural promoter sequences are typically used, although for promoters derived from non- E. coli  hosts, mutations to the consensus −10 and −35 sites for sigma-70 promoters (TTGACA, TATAAT) can be helpful to generate stronger promoters that are still functionally regulated. Multiple operator sites can also be placed in tandem in a promoter to improve the ability of the aTF to regulate gene expression. The same cascaded reporter plasmid (e.g., pAKSIRV-sfGFP) is used for all experiments described herein. 
     The optimized reporter plasmids are isolated and sequence-confirmed and the reporter plasmids are purified to high concentration by midiprep. The plasmid encoding the aTF is transformed into a protein expression strain of  E. coli  (e.g., BL21 (DE3) or its derivatives) and an extract is prepared using previously reported protocols (e.g., citation 18). Separately, a “blank” extract is prepared from the base protein production strain. Cell-free extracts for transcriptional sensing are typically prepared by lysis and post-lysis clarification including ribosomal runoff reaction and dialysis, followed by aliquoting and flash-freezing on liquid nitrogen. 
     A series of tuning experiments are next used to optimize the sensor. First, the optimal concentration of the aTF (CueR) is found for the non-cascaded sensor (e.g., pCue-sfGFP) through a series of ratiometric titrations between the aTF-enriched extract and the blank extract in both the presence and absence of analyte (see e.g., the data in  FIG. 7 ). This experiment is an isothermal cell-free gene expression reaction held at 30° C. in a plate reader for ˜4 hours using an established set of physiochemical conditions to maximize protein production in general. It may also be necessary to determine the concentration of analyte that saturates the cell-free sensor without inhibiting transcription and translation in vitro, which can be done with a simple titration of analyte against an unregulated reporter plasmid (e.g., pT7-sfGFP). For example, 100 μM CuSO 4  shuts down cell-free protein synthesis. 
     Next, the optimal ratio of aTF enriched- and blank extract is used to optimize the concentration of the sensor plasmid sequence for the cascade (e.g., pCue-AKSIRV) (see e.g., the data in  FIG. 8 ). In each case, the goal is to optimize the fold activation of the sensor. Typically the concentration of the reporter plasmid is held constant and saturating for both the noncascaded sensor (20 nM) and the cascaded sensor (5 nM), although this can also be separately optimized to maximize fold activation. Then the dose responses are measured (see e.g.,  FIG. 9 ). 
     The innovations disclosed herein, include but are not limited to: (i) deploying a cell-free sensor that produces a bacteriophage RNA polymerase in the presence of a target chemical or nucleic acid; (ii) catalytic amplification of that bacteriophage polymerase with a positive feedback template, and co-expression of a reporter protein from the bacteriophage polymerase&#39;s cognate promoter, in one pot; and (iii) deploying multiple engineered polymerase variants in a single pot which allows for multiplexed detection of several analytes at once. The disclosed innovations allow for overall improved signal of the sensor and also makes the sensor be more switchlike, greatly increase the limit of detection of the disclosed sensors; and allow for practical sensing of several contaminants using a single reaction, which simplifies the device and decreases overall cost. The exemplary models and designs presented herein uniquely demonstrated the ability of the cascaded amplifiers to be applied to and monitor lead, mercury, nitrate, cadmium, copper, fluoride, chromate, and arsenic. 
     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. 
     REFERENCES 
     Non-Patent References 
     
         
         1. Pardee, K. et al. Paper-Based Synthetic Gene Networks. Cell 1-22 (2014). doi:10.1016/j.cell.2014.10.004 
         2. Pardee, K. et al. Rapid, Low-Cost Detection of Zika Virus Using Programmable Biomolecular Components. Cell 165, 1-25 (2016). 
         3. Salehi, A. S. M. et al. Biosensing estrogenic endocrine disruptors in human blood and urine: A RAPID cell-free protein synthesis approach. Toxicol. Appl. Pharmacol. 345, 19-25 (2018). 
         4. Peter L Voyvodic, Amir Pandi, Mathilde Koch, Jean-Loup Faulon, Jerome Bonnet. Plug-and-Play Metabolic Transducers Expand the Chemical Detection Space of Cell-Free Biosensors. doi: https://doi.org/10.1101/397315 
         5. Gootenberg, J. S. et al. Nucleic acid detection with CRISPR-Cas13a/C2c2. Science 356, 438-442 (2017). 
         6. Pandi, A., et al., Optimizing Cell-Free Biosensors to Monitor Enzymatic Production. ACS Synthetic Biology, 2019. 8(8): p. 1952-1957. 
         7. Verosloff, M., et al., PLANT-Dx: A Molecular Diagnostic for Point-of-Use Detection of Plant Pathogens. ACS Synthetic Biology, 2019. 8(4): p. 902-905. 
         8. Pellinen, T., T. Huovinen, and M. Karp, A cell-free biosensor for the detection of transcriptional inducers using firefly luciferase as a reporter. Analytical Biochemistry, 2004. 330(1): p. 52-57. 
         9. Alam, K. K., et al., Rapid, Low-Cost Detection of Water Contaminants Using Regulated In Vitro Transcription. bioRxiv, 2019: p. 619296. 
         10. Thavarajah, W., et al., Point-of-Use Detection of Environmental Fluoride via a Cell-Free Riboswitch-Based Biosensor. bioRxiv, 2019: p. 712844. 
         11. McNerney, M. P., et al., Point-of-care biomarker quantification enabled by sample-specific calibration Science Advances, 2019. 
         12. Liu, X., et al., Design of a transcriptional biosensor for the portable, on-demand detection of cyanuric acid. bioRxiv, 2019: p. 736355. 
         13. Gräwe, A., et al., A paper-based, cell-free biosensor system for the detection of heavy metals and date rape drugs. PLOS ONE, 2019. 14(3): p. e0210940. 
         14. Takahashi, M. K., et al., A low-cost paper-based synthetic biology platform for analyzing gut microbiota and host biomarkers. Nature Communications, 2018. 9(1): p. 3347. 
         15. Garamella, J., et al., The All  E. coli  TX-TL Toolbox 2.0: A Platform for Cell-Free Synthetic Biology. ACS Synthetic Biology, 2016. 5(4): p. 344-355. 
         16. Meyer, A. J., J. W. Ellefson, and A. D. Ellington, Directed Evolution of a Panel of Orthogonal T7 RNA Polymerase Variants for in Vivo or in Vitro Synthetic Circuitry. ACS Synthetic Biology, 2015. 4(10): p. 1070-1076. 
         17. Pandi, A., et al., Metabolic perceptrons for neural computing in biological systems. Nature Communications, 2019. 10(1): p. 3880. 
         18. Silverman, A. D., et al., Design and optimization of a cell-free atrazine biosensor. bioRxiv, 2019: p. 779827. 
         19. Wan, X., et al., Cascaded amplifying circuits enable ultrasensitive cellular sensors for toxic metals. Nature Chemical Biology, 2019. 15(5): p. 540-548. 
         20. Temme, K., et al., Modular control of multiple pathways using engineered orthogonal T7 polymerases. Nucleic acids research, 2012. 40(17): p. 8773-8781. 
       
    
     PATENT REFERENCES 
     U.S. Pat. Nos.: U.S. Pat. Nos. 5,478,730; 5,556,769; 5,665,563; 6,168,931; 6,518,058; 6,783,957; 6,869,774; 6,994,986; 7,118,883; 7,189,528; 7,338,789; 7,387,884; 7,399,610; 8,357,529; 8,574,880; 8,703,471; 8,999,668; 9,410,170; and US952813; the contents of which are incorporated herein by reference in their entirety. 
     U.S. Patent Publications: US20040209321; US20050170452; US20060211085; US20060234345; US20060252672; US20060257399; US20060286637; US20070026485; US20070154983; US20070178551; US20080138857; US20140295492; US20160060301; US20180016612; US20180016614; US20160312312; and US20160362708; the contents of which are incorporated herein by reference in their entirety. 
     Published International Applications: WO2003056914A1; WO2004013151A2; WO2004035605A2; WO2006102652A2; WO2006119987A2; WO2007120932A2; WO2014144583; and WO2017117539; the contents of which are incorporated herein by reference in their entirety. 
     In the foregoing description, 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 that 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.