# Patent

## Title
Modified biosensors and biocatalysts and methods of use

## Abstract

Biosensors including a nucleic acid encoding a PcaU protein, a PobR protein, a CatM protein, a PcaR protein, or a TphR protein are provided. In some examples, the biosensors include a promoter regulated by the sensed molecule operably linked to a reporter gene. The biosensors may be included in a vector or in cells including one or more of the biosensors or vectors. Modified chorismate pyruvate lyase (UbiC) and modified paraoxonase (PON1) proteins including one or more amino acid substitutions are provided. Finally, methods of selecting biocatalysts with increased activity including transforming a library of cells expressing a biosensor with one or more nucleic acids encoding one or more mutations in a gene involved in a biosynthesis pathway, determining activity of the reporter protein; and selecting a cell with increased reporter protein activity as expressing a biocatalyst with increased activity are provided.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This claims the benefit of U.S. Provisional Application No. 62/607,697, filed Dec. 19, 2017, which is incorporated herein by reference in its entirety.


ACKNOWLEDGMENT OF GOVERNMENT SUPPORT
This invention was made with government support under Contract No. 89233218CNA000001 awarded by the U.S. Department of Energy/National Nuclear Security Administration and awards from the U.S. Department of Energy BioEnergy Technologies Office (Agile BioFoundry). The government has certain rights in the invention.


FIELD
This disclosure relates to biosynthesis of compounds in prokaryotic organisms, in particular biosensors and biocatalysts for compounds of interest and methods of developing biosensors and biocatalysts for use in prokaryotic organisms.
BACKGROUND
Biomanufacturing is a sustainable route towards “drop in” and “replacement” chemicals which currently are produced from petroleum-based products. Biomanufacturing utilizes renewable feedstocks and microbial cell factories and/or enzymatic reactions to produce chemicals that are industrial precursors and building blocks of a variety of polymers. Among them, adipic acid (ADA) and terephthalic acid (TPA) are polymer precursors with high global demand that are currently made via chemical processes with adverse environmental impact. β-ketoadipic acid (BKA) is another “replacement” polymer precursor with a high global demand. The biotransformation routes for these chemicals can be achieved via one or more of 4-hydroxybenzoic acid (4HB), protocatechuic acid (PCA), catechol, and cis,cis-muconic acid (ccMA).
For example, PCA is an important central intermediate in aromatic catabolism and siderophore biosynthesis, as well as an important hub molecule in conversion of sugars to ccMA and other shikimate-derived products. These products are currently produced on an industrial scale via chemical processes, which are frequently environmentally harmful and expensive. Biomanufacturing could provide renewable production of commodity chemicals that reduces energy consumption and environmental impact.
SUMMARY
For the development of robust industrial strains, model organisms such as E. coli that already have well developed synthetic biology tools, including multiple biosensors for molecules of interest, may offer only limited advantages for a given target. Instead, the use of diverse range of organisms that exhibit beneficial phenotypes (e.g., high flux through target pathways, thermal or pH tolerance, etc.) often enables more efficient and cost effective process design. However, the deployment of biosensor-based screening in such organisms commonly does not necessarily directly transfer from model organisms (e.g., E. coli), and loss of sensitivity and dynamic range can occur. Pseudomonas putida KT2440 is one such strain that has increasingly been investigated as a potential microbial cell factory for producing target chemicals due to its high toxicity tolerance and high natural flux in aromatic-catabolic pathways.
Biosensors for polymer precursors via one or more of 4-hydroxybenzoic acid (4HB), protocatechuic acid (PCA), catechol, and cis,cis-muconic acid (ccMA) are disclosed herein. In addition, methods to improve biosynthetic routes utilizing a product- or an intermediate-specific biosensor and targeting key bottleneck enzymes for a gain-of function are disclosed. The present disclosure includes: (i) gene circuits that consist of DNA sequences, which code for proteins (e.g., biosensors) that can interact with metabolites and regulate the expression of a reporter (for example, gfp encoding green fluorescent protein); (ii) establishment of the sensor-reporter system in industrially relevant organisms, for example Pseudomonas putida; (iii) use of a sensor-reporter system to optimize a metabolic pathway by evolving key bottleneck enzymes; and (iv) introducing modified or improved enzymes into production strains for enhanced biomanufacturing. The sensor-reporter system for modification (e.g., “evolution”) of an enzyme or a pathway or a strain addresses a “needle-in-a-haystack” problem, since a library of variants can be conveniently screened using methods such as flow cytometry or visualizing colonies (e.g., on a petri dish). Introduction of the sensor-reporter system in non-model organisms with beneficial phenotypes, enabling effective process design for bio-based products is described.
Disclosed herein are biosensors for PCA, 4HB, ccMA, BKA, and TPA. In some examples, the biosensors further include a promoter (e.g., a promoter regulated by the sensed molecule) operably linked to a reporter protein. In some examples, the biosensor includes a nucleic acid encoding a PcaU protein (such as a modified PcaU protein), a PobR protein (such as a PobR-DM protein), a CatM protein, a PcaR protein, or a TphR protein.
Also disclosed are vectors including the disclosed biosensors and cells (such as transformed cells) including one or more of the disclosed biosensors or vectors. In some embodiments, the vectors include SEQ ID NOs: 2, 3, 6, 7, 21, 22, 29, 30, and 31. In some examples, the cells are Pseudomonas putida, Escherichia coli, or Acinetobacter baylyi cells.
Disclosed herein is a modified biocatalyst for producing 4HB. The modified catalyst is a chorismate pyruvate lyase (UbiC) that includes one or more (such as 1, 2, 3, or more) amino acid substitutions at amino acid positions corresponding to amino acids 31, 34, 78, 80, 92, and 114 of SEQ ID NO: 18.
Finally, methods of selecting biocatalysts with increased activity or gain-of-function using the disclosed biosensors are provided. The methods include transforming a library of cells expressing a disclosed biosensor with one or more nucleic acids encoding one or more mutations in a gene involved in a biosynthesis pathway, determining the activity of the reporter protein; and selecting a microbial cell with increased reporter protein activity as expressing a biocatalyst with increased activity or gain-of-function. In some examples, the methods further include isolating a nucleic acid encoding the biocatalyst from the selected cell.
The foregoing and other features of the disclosure will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are schematics illustrating a “smart” microbial cell in two alternate exemplary schemes utilizing genetic sequences of a biocatalyst and a biosensor on two different plasmids (FIG. 1A) or on the same plasmid (FIG. 1B). In FIG. 1B, the biocatalyst is under a positive feedback regulation. Based on the biocatalyst activity, an intracellular substrate or exogenously provided substrate is converted to a product (molecule of interest or a suitable intermediate), which activates a transcription factor (TF) on the biosensor or biosensor/biocatalyst plasmid resulting in expression of the reporter gene (exemplified by gfp). This results in gain of fluorescence in the microbial cells. These exemplary schemes can be utilized to select for gene variants that encode for an improved activity in a biocatalyst (e.g., enzyme).
FIGS. 2A and 2B show an example for promoter and protein evolution in P. putida. FIG. 2A shows alignment of the evolved PcaU promoter (pPcaU1.1; SEQ ID NO: 11) with E. coli functional promoter (pPcaU1; SEQ ID NO: 10). Positions randomized to create the library are shown below the alignment. The PcaU operator is only partially shown. FIG. 2B is a homology model of PcaU inducer binding domain as a dimer and docked with protocatechuate (spheres), showing spatial position of T147 and D148 (sticks), which were randomized.
FIG. 3 is a graph showing fluorescence signal measured using flow cytometry (ex/em 488/530) in P. putida KT2440 mutant strain with an evolved sensor plasmid pPcaU1.2. Cells grown in the absence of PCA (uninduced) are shown in light gray and cells grown in the presence of 1 mM PCA (induced) are shown in dark gray.
FIGS. 4A-4D are a series of panels showing whole cell biosensing of PCA in P. putida. FIG. 4A is a response plot of different generations of a PcaU-based sensor in P. putida strain when exogenously dosed with PCA. FIG. 4B is a set of plots showing specificity evaluation of the evolved sensor pPcaU1.2, against similar aromatic molecules. FIG. 4C is a diagram showing the ability of 4-hydroxybenzoate to be metabolized to PCA and beyond, depending on the genotype of P. putida strains. FIG. 4D is a graph showing intracellular production of PCA and sensor response in P. putida strains. Error bars are standard deviations from two or more experiments. Only positive or negative error bars shown in some cases for clarity.
FIG. 5 is a graph showing PCA dose-response measured using flow cytometry with the indicated sensor plasmids. T147/D148 on PcaU dimer interface were mutated on PcaU1.1 sensor background. The legend shows the identified mutations in parentheses.
FIGS. 6A-6C are schematic diagrams showing natural and synthetic genetic circuits involving the PobR transcription factor (TF). FIG. 6A shows that the natural PobR activates a promoter for expression of PobA that converts 4HB to PCA (top panel). A synthetic construct was designed for 4HB sensing consisting of a double mutant of PobR (PobR-DM) that has a stronger activation of the promoter than the natural TF (depicted by a thick arrow), which results in a high expression of sfGFP (reporter) (middle panel). The Sensor+Enzyme is a synthetic construct for 4HB conversion and sensing consisting of the same PobR-DM sensor and promoter but resulting in an expression of sfGFP fused UbiC (reporter). Hence, the promoter is under positive feedback regulation of the UbiC activity, which catalyzes the conversion of chorismate into 4HB and pyruvate (bottom panel). FIG. 6B is the plasmid construct for Sensor. FIG. 6C is the plasmid construct for Sensor+Enzyme.
FIGS. 7A and 7B show P. putida transformed with pPobR (Sensor) or pPobR_ubiC (Sensor+Enzyme). FIG. 7A is a graph showing a dose-response plot of P. putida KT2440 strains (wild-type KT2440 and CJ182) transformed with the PobR-DM sensor plasmid (pPobR). Error bars are standard deviations from duplicate experiments. FIG. 7B shows colonies of CJ182, with the plasmid pPobR (Sensor), or with the UbiC plasmid pPobR_ubiC-wt (Sensor+Enzyme), grown in the presence or absence of 100 μM 4HB and/or 10 mM benzoate inhibitor.
FIGS. 8A-8D are a series of panels related to improvement of an industrially relevant enzyme. FIG. 8A shows the pathway from chorismate to 4HB and pyruvate. Chorismate pyruvate lyase (UbiC from E. coli (ecUbiC)) can convert chorismate (an intermediate in the shikimate pathway) to 4-hydroxybenzoate (4HB) and pyruvate. 4HB is a value-added chemical that can be converted PCA via an endogenous PobA enzyme in P. putida KT2440. FIG. 8B is a schematic of the crystal structure of product-inhibited state of ecUbiC (PDB Code 1JD3). FIG. 8C shows a plasmid construct for UbiC evolution. UbiC gene is in-frame with reporter gene sfGFP which is under the control of PobR-DM (the 4HB sensor). FIG. 8D is a schematic illustrating a flow scheme for screening of ecUbiC library in P. putida. 
FIG. 9 shows P. putida strain CJ182 (ΔpobAR) transformed with PobR-DM-based sensor plasmid (pPobR) and plasmid with sensor with wild-type E. coli UbiC expression (pPobR_ubiC_wt) plated in the presence (left) and absence (right) of exogenous 4HB. In the absence of any exogenous 4HB, only Sensor+Enzyme carrying cells showed fluorescence, confirming formation and accumulation of 4HB inside the cells.
FIGS. 10A-10C show UbiC library and screening. FIG. 10A shows structure-based selection of mutational positions in UbiC based on the product-bound structure from X-ray crystallography (PDB code 1JD3). Whole protein (left) and targeted residues (right). Residues R116 and R126 were not mutated but neighboring residues were mutated to make the “flap” (Res 29-34) flexible, promoting exit of the product. M34 interacts with L30 and E31 interacts with R116 and R126. The residue numbers are based in the numbering in the published UbiC structure. Protein structures were created using PyMOL (The PyMOL Molecular Graphics System, Version 2.2, Schrödinger, LLC). FIG. 10B shows the UbiC library plated on LB agar petri dishes with different supplementations. The colonies that were picked based on the fluorescence intensity in the presence of 10 mM benzoate are marked with white arrows. Only a small portion of petri dish is shown here for clarity. Colonies from crowded regions were avoided since P. putida is capable of metabolizing benzoate, which would decrease the local concentration, and artificially relieve the UbiC inhibition. FIG. 10C shows quantitative evaluation of selected clones in liquid culture supplemented with different concentrations of benzoate (0, 10, and 20 mM). Error bars are standard deviations from three independent experiments.
FIGS. 11A-11C show muconate production and growth for CJ200 P. putida strain transformed with pBTL-2 derived plasmids, containing genetic information for the sensor (PobR-DM) and different variants of UbiC enzyme. FIG. 11A shows muconate production in shake flask with 50 mM glucose. FIG. 11B shows growth curves of the same strains measured on a plate reader in a 96-well plate. FIG. 11C are HPLC chromatograms for muconate CJ200 P. putida strain transformed with pBTL-2 derived plasmids: pBTL-2 empty vector (left); pPobR_ubiC-wt (wild-type E. coli UbiC; middle); pPobR_ubiC-C22 (low product inhibition variant UbiC-C22; right).
FIGS. 12A and 12B show muconate production in ubiC integrated P. putida strain CJ200. Growth curve, glucose depletion, and muconate production at 50 mM glucose in CJ200 (FIG. 12A) and NP015 (FIG. 12B) where the ubiC-C22 gene was integrated into the P. putida genome. Error bars are standard deviation from triplicate shake flasks.
FIGS. 13A-13C are a series of panels showing 4HB monooxygenase (PobA) activity sensing and library screening. FIG. 13A is a schematic diagram of an exemplary plasmid construct for sensing PobA activity. P. putida PobA enzyme is expressed as sfGFP fusion and is under positive feedback regulation of PCA (scheme represented in FIG. 1B). FIG. 13B is a graph representing concentration-dependent 4HB response of P. putida KT2440 cells with native pobA deleted and pobA gene added to the plasmid (shown in FIG. 13A). FIG. 13C shows P. putida cells harboring the above plasmid construct and where pobA gene has been diversified (Theoretical library diversity>60,000). The library of cells was plated on 100 μM 4HB, scraped and analyzed using flow cytometry for fluorescence in the cells. The fluorescence distribution of the cells in the third round (middle panel) was similar to the distribution observed with cells harboring wild-type PobA (PobA-wt) (bottom panel).
FIGS. 14A-14D show promoter diversification, dose-response plots of sensitive cis, cis-muconic acid (ccMA) sensor, redesign of the ccMA binding pocket and dose-response plot of ccMA sensor with reduced sensitivity. FIG. 14A represents the Acinetobacter baylyi CatM promoter region used for selection of ccMA sensing in P. putida (SEQ ID NO: 12 and 13 (reverse complement). Partial amino acid sequences of CatM (SEQ ID NO: 14) and sfGFP (SEQ ID NO: 15) are also shown. Segments that were partially diversified or completely randomized are shown in red. FIG. 14B is a plot showing intracellular production and sensing of ccMA in P. putida using CatM transcription factor from Acinetobacter baylyi ADP1 and an optimal promoter sequence (CatM_C2). A strain capable of metabolizing PCA into ccMA as a final product (ccMA production strain) shows a clear dose-response with PCA. Another strain with a knockout mutation in PCA-ccMA pathway (Non-production strain) failed to show any dose-response with PCA. FIG. 14C shows the diversification of CatM inducer binding pocket. FIG. 14D shows a selected variant (CatM_C2.9) with low sensitivity for an ideal dose-response at high ccMA titer, that is >10 mM.
FIGS. 15A-15E are a series of panels related to pathways leading to (3-ketoadipic acid (BKA) and whole cell biosensing in P. putida strains. FIG. 15A shows the protocatechuate and catechol branches of the β-ketoadipate pathway in P. putida KT2440 and a heterologous shunt feed (green arrow) from shikimate pathway. Mutant P. putida (CJ390) consist of an added shunt (green arrow) and deleted genes of BKA metabolizing enzymes (PcaIJ) (red arrow). FIG. 15B shows low fluorescence in P. putida KT2440 with BKA sensor (pPcaR_promo that uses genomic PcaR expression in P. putida) and high fluorescence in a P. putida strain capable of accumulation of BKA when grown on LB medium (CJ390). FIG. 15C shows dose response of BKA sensor (pPcaR_promo) in P. putida. Native KT2440 cannot make BKA from glucose and BKA is transiently accumulated due to activity of PcaIJ. This results in significantly lower response compared to CJ390, which can convert glucose or spiked PCA and catechol into BKA and also accumulate BKA. FIG. 15D shows positions identified for diversification of putative BKA binding pocket in P. putida PcaR based on a homology model. FIG. 15E shows dose response plots of identified variants of PcaR that show low sensitivity and hence detection of high concentration of BKA titer.
FIGS. 16A-16D show ‘Catalyze’ and ‘sense’ system for hydrolysis reactions. FIG. 16A shows various substrates for phosphotriesterases, glucanases, esterases, and lipases with common leaving group, p-nitrophenol (pNP). FIG. 16B shows a hydrolytic reaction, breakdown of PXN to pNP. FIG. 16C shows flow cytometer histograms (y-axis population, x-axis: GFP fluorescence intensity) showing the E. coli cells (strain JW0451 with acrB gene deletion) (Baba et al., Mol. Syst. Biol. 2:2006.008, 2006) transformed with Biosensor/Biocatalyst plasmid (scheme shown in FIG. 1B) containing different PON1 sequences and grown under different induction conditions. UI: background fluorescence from cells without any pNP source to induce the TF; PXN: exogenous Paraoxon is converted intracellularly to pNP by the enzyme under study, activating the TF; pNP: TF induced by exogenously supplied pNP as a control/reference. FIG. 16D shows streaked LB-agar plates with E. coli JW0451 strain transformed with genetic sequences of PON1 variants. Relative activity levels are distinguishable by eye.
FIGS. 17A-17G show evolution of PON1. FIG. 17A shows PON1 structure with a bound competitive inhibitor 2HQ (PDB code 3SRG). Residue positions varied in library construction are indicated. FIG. 17B shows E. coli (DH5alpha, Thermo Fisher Scientific) cells transformed with Biosensor/Biocatalyst plasmid containing variants of PON1 gene based on Lib1 diversity and plated on LB+agar supplemented with 330 μM PXN. Several colonies (circled) showed brighter fluorescence than the surrounding colonies when visualized under an illuminator (ex/em 488/515). FIG. 17C shows Lib2 colonies showing a ‘rare’ bright colony in the whole population. The circled region is magnified (inset) for easy visualization of the bright colony. FIG. 17D shows total activity dose-response: Picked clones from the plates were grown in LB as liquid culture and supplemented with varying PXN concentration. The contrast ratio is defined as total fluorescence of the cells in the presence of PXN over the background fluorescence of the cells in the absence of PXN. FIG. 17E shows Specific Activity dose-response: In vitro paraoxonase spectrophotometric assay using clarified cell lysate, normalized for GFP fluorescence (equal GFP fused PON1 variant). FIG. 17F shows time accumulated activity of PON1 variant C3.3 (50 nM) and the scaffold (PON-H115W at 500 nM). For convenient illustration, the scaffold was made 10× higher in concentration to achieve a comparable timescale. The product, pNP, is measured by absorbance (410 nm). Error bars are standard deviation from two independent assays with different fractions from size exclusion chromatography. FIG. 17G shows Michaelis-Menten plot for PON1-C3.3 (50 nM) and PON1-H115W (500 nM). Error bars are the standard deviation of two independent assays using elution fractions from first half and second half of the chromatogram peak from size exclusion chromatography.



SEQUENCE LISTING
Any nucleic acid and amino acid sequences listed herein or in the accompanying sequence listing are shown using standard letter abbreviations for nucleotide bases and amino acids, as defined in 37 C.F.R. § 1.822. In at least some cases, only one strand of each nucleic acid sequence is shown, but the complementary strand is understood as included by any reference to the displayed strand.
The Sequence Listing is submitted as an ASCII text file in the form of the file named Sequence_Listing.txt, which was created on Oct. 13, 2021, and is 103,248 bytes, which is incorporated by reference herein.
SEQ ID NO: 1 is the nucleic acid sequence of pPcaU1. Nucleotides 990-1823, pcaU gene; 1824-2105, promoter sequence; 2106-2822, sfGFP gene.
SEQ ID NO: 2 is the nucleic acid sequence of pPcaU1.1. Nucleotides 990-1823, pcaU gene; 1824-2104, promoter sequence; 2105-2821, sfGFP gene.
SEQ ID NO: 3 is the nucleic acid sequence of pPcaU1.2. Nucleotides 990-1823, pcaU gene; 1824-2104, promoter sequence; 2105-2821, sfGFP gene.
SEQ ID NO: 4 is the nucleic acid sequence of PcaU1.2.
SEQ ID NO: 5 is the amino acid sequence of PcaU1.2.
SEQ ID NO: 6 is the nucleic acid sequence of an exemplary 4HB sensor plasmid with PobR-DM (pBTL2_PobR-DM or pPobR). Nucleotides 990-1799, PobR-DM gene; 1800-1935, promoter sequence; 1936-2652, sfGFP gene.
SEQ ID NO: 7 is the nucleic acid sequence of an exemplary cis,cis-muconic acid sensor plasmid with CatM_C2 (pBTL2_CatM_C2 or pCatM_C2). Nucleotides 990-1898, CatM gene; 1899-2021, promoter sequence; 2022-2738, sfGFP gene.
SEQ ID NO: 8 is the nucleic acid sequence of a PcaR-regulated promoter for (3-ketoadipic sensing.
SEQ ID NO: 9 is the nucleic acid sequence for a terephthalic acid sensor using TphR transcription factor from Comamonas. 
SEQ ID NO: 10 is a partial nucleic acid sequence of a PcaU1 promoter region.
SEQ ID NO: 11 is a partial nucleic acid sequence of a PcaU1.1 promoter region.
SEQ ID NOs: 12 and 13 are the nucleic acid sequence and reverse complement, respectively of a CatM promoter and partial catM and sfGFP coding regions.
SEQ ID NO: 14 is the amino acid sequence of a portion of a CatM protein.
SEQ ID NO: 15 is the amino acid sequence of a portion of a sfGFP protein.
SEQ ID NO: 16 is the amino acid sequence of an exemplary PobR double mutant protein.
SEQ ID NOs: 17 and 18 are the nucleic acid and amino acid sequences, respectively, of an exemplary wild type UbiC protein.
SEQ ID NO: 19 is the amino acid sequence of an exemplary PcaR protein.
SEQ ID NO: 20 is the nucleic acid sequence of a TphR promoter.
SEQ ID NO: 21 is the nucleic acid sequence of a β-ketoadipate sensor that utilizes genomic PcaR expression and an optimal promoter sequence with sfGFP as a reporter. Nucleotides 987-1211, PcaR-regulated promoter sequence; 1212-1928, sfGFP gene.
SEQ ID NO: 22 is the nucleic acid sequence of an exemplary β-ketoadipate sensor plasmid with PcaR sensor and sfGFP reporter (pBTL2_PcaR_sfGFP or pPcaR). Nucleotides 990-1862, PcaR sensor; 1863-2157, PcaR-regulated promoter; 2158-2874, sfGFP gene.
SEQ ID NO: 23 is the nucleic acid sequence of an exemplary CatM regulated promoter in ccMA sensor for reduced signal output to prevent saturation of fluorescence. The promoter (CatM_A9) can replace nucleotides 1899-2021 in CatM_C2 (SEQ ID NO: 7).
SEQ ID NO: 24 the nucleic acid sequence of CatM_C2.9 in ccMA sensor with weaker inducer sensitivity than the native sequence. The nucleotide sequence can replace nucleotides 990-1898 for CatM gene in SEQ ID NO: 7.
SEQ ID NO: 25 is the amino acid sequence of CatM_C2.9 which shows weaker inducer sensitivity to ccMA compared to native CatM.
SEQ ID NO: 26 is the amino acid sequence of PcaR-G5 for a low sensitivity BKA sensor.
SEQ ID NO: 27 is the amino acid sequence of PcaR-F6 for a low sensitivity BKA sensor.
SEQ ID NO: 28 is the amino acid sequence of PcaR-H6 for a low sensitivity BKA sensor.
SEQ ID NO: 29 is the nucleic acid sequence of an exemplary sensor+enzyme consisting of pPobR as the backbone and ubiC-wt gene in the same reading frame as the sfGFP reporter gene (pPobR_ubiC-wt). Nucleotides 990-1799, PobR-DM gene; 1800-1935, promoter sequence; 1936-2652, sfGFP gene; 2672-3162, ubiC-wt gene.
SEQ ID NO: 30 is the nucleic acid sequence of an exemplary sensor+enzyme plasmid consisting of pPcaU1.2 as the backbone and pobA-wt gene in the same reading frame as the sfGFP reporter gene (pPcaU1.2_pobA-wt). Nucleotides 990-1823, PcaU1.2 gene; 1824-2104, promoter sequence; 2105-2821, sfGFP gene; 2840-4024, pobA-wt gene.
SEQ ID NO: 31 is the nucleic acid sequence of an exemplary sensor+enzyme consisting of PobR-DM derived pNP sensor with PON1 enzyme gene in the same reading frame as the sfGFP reporter gene. Nucleotides 98-907, pNPmut1.1 gene; 908-1043, promoter region; 1044-1760, sfGFP gene; 1779-2840, PON1-G3C9 gene.
SEQ ID NO: 32 is the amino acid sequence of an exemplary PON1-G3C9.
SEQ ID NOs: 33-50 are nucleic acid sequences of oligonucleotide primers.
SEQ ID NOs: 51-55 are amino acid sequences of mutational library peptides.
DETAILED DESCRIPTION
Disclosed herein is development and improvement of biosensors for biocommodities, which in some examples, are useful for development of industrially relevant organisms such as P. putida. In one embodiment, a PcaU-based sensor optimized in P. putida KT2440 was developed that senses both PCA and catechol (with an appreciable contrast ratio), which are the key intermediates in the carbon flow via PCA or catechol branches of the 0-ketoadipate pathway.
In another embodiment, a PobR-based sensitive and specific sensor for 4HB was established in P. putida. 4HB is an important intermediate in the metabolic pathways for production of industrially important aromatic compounds in the manufacturing of bioplastics. The chorismate lyase UbiC that produces 4HB (e.g., E. coli UbiC) suffers from product inhibition and is a significant bottleneck in the shikimate pathway. Instead of the traditional methodology of site directed mutations in the product binding site to alleviate production inhibition, disclosed herein is use of a 4HB-binding PobR transcription factor to identify UbiC variants with reduced product inhibition. For this, the structurally similar benzoate that binds to UbiC was used as a surrogate product molecule to generate a high throughput library that was screened by fluorescence output from the 4HB biosensor. The enzyme kinetic parameters of the best selected variant had higher Kp, indicating lower product binding affinity, and increased kcat indicating higher turnover number. When the UbiC variant with reduced product inhibition was expressed in a muconate production strain, the product yield was improved by 50-200% (e.g., 130% when expressed on plasmid or 60% when genomically integrated).
In a further embodiment, a CatM regulon from Acinetobacter baylyi APD1 was optimized in the promoter for muconate sensing in P. putida. The sensor detected intracellular muconate production from glucose, PCA, benzoate and catechol. In a still further embodiment, the endogenous PcaR in P. putida was utilized for sensing β-ketoadipic acid (BKA) production inside a cell. In another embodiment, PcaR from P. putida was expressed via a plasmid and intracellular BKA production from glucose, PCA and catechol was detected in a PcaR ‘knock out’ P. putida strain.
With only a few examples of established sensor-reporter systems in P. putida (Garmendia et al., Microb. Biotechnol. 1:236-246, 2008), and limited efforts to establish sensor-reporter systems in non-model organisms (DeLorenzo et al., ACS Synth. Biol. doi:10.1021/acssynbio.7b00192), the present disclosure demonstrates detection of industrially important aromatic compounds and compounds from aromatic catabolic pathway in P. putida. 
I. Terms
Unless otherwise noted, technical terms are used according to conventional usage. Definitions of common terms in molecular biology may be found in Lewin's Genes X, ed. Krebs et al., Jones and Bartlett Publishers, 2009 (ISBN 0763766321); Kendrew et al. (eds.), The Encyclopedia of Molecular Biology, published by Blackwell Publishers, 1994 (ISBN 0632021829); Robert A. Meyers (ed.), Molecular Biology and Biotechnology: a Comprehensive Desk Reference, published by Wiley, John & Sons, Inc., 1995 (ISBN 0471186341); and George P. Rédei, Encyclopedic Dictionary of Genetics, Genomics, Proteomics and Informatics, 3rd Edition, Springer, 2008 (ISBN: 1402067534), and other similar references.
Unless otherwise explained, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The singular terms “a,” “an,” and “the” include plural referents unless context clearly indicates otherwise. Similarly, the word “or” is intended to include “and” unless the context clearly indicates otherwise. Hence “comprising A or B” means including A, or B, or A and B. It is further to be understood that all base sizes or amino acid sizes, and all molecular weight or molecular mass values, given for nucleic acids or polypeptides are approximate, and are provided for description.
Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. All database accession numbers (such as GenBank or UniProt accession numbers) are incorporated herein by reference in their entirety, as present in the database on Dec. 19, 2017. In case of conflict, the present specification, including explanations of terms, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
In order to facilitate review of the various embodiments of the disclosure, the following explanations of specific terms are provided:
4-hydroxybenzoic acid (4HB): A value-added chemical with the structure:




β-ketoadipic acid (BKA): A polymer precursor with the structure:




Biocatalyst: A biological molecule (such as a protein) that initiates a biochemical reaction and/or increases the rate of a biochemical reaction. In some examples, a biocatalyst is an enzyme.
Biomanufacturing: A process to convert renewable feedstocks into value-added chemicals using microbial cells and/or enzymes.
Biosensor: A biological molecule (such as a nucleic acid, peptide, or protein) that can detect a change in environment, for example, in a dose-dependent manner. In some examples, a biosensor is a protein (such as a transcription factor) that can sense a change in concentration of a small molecule in or around a cell. The biosensor may be coupled (directly or indirectly) to a reporter, including but not limited to an antibiotic resistance gene, a gene encoding a fluorescent protein (such as a green fluorescent protein), or a metabolic gene (such as lacZ). The reporter then indicates the presence and/or amount of the detected molecule, for example, by antibiotic resistance, fluorescence, or color change.
CatM: A transcription factor belonging to LysR family from Acinetobacter baylyi (e.g., A. baylyi ADP1). CatM binds to cis,cis-muconic acid and regulates the metabolism of the molecule by activating the cat gene expression. The GenBank Accession number P07774 is an exemplary wild-type CatM amino acid sequence.
Chorismate pyruvate lyase (UbiC): An enzyme that catalyzes removal of the pyruvyl group from chorismate, producing 4-hydroxybenzoate (4HB).
cis,cis-muconic acid (ccMA): Also referred to as muconate. A precursor of adipic acid, which is utilized for nylon 6,6 production. ccMA has the structure:




Expression control element: One or more sequences that control or regulate transcription and/or translation of a nucleic acid, such as operators, promoters, enhancers, leader sequences, transcription terminators, start and/or stop codons, internal ribosome entry sites (IRES), splicing signals, and polyadenylation signals.
Heterologous: Originating from a different genetic source or species. A gene that is heterologous to a prokaryotic cell originates from an organism or species other than the prokaryotic cell in which it is expressed. Methods for introducing a heterologous gene in a cell or organism are well known in the art, for example transformation with a nucleic acid, including electroporation, lipofection, particle gun acceleration, and homologous recombination.
Hydrolase: An enzyme that uses water molecule to catalyze the breakdown of substrates into smaller products. A labile chemical bond attaching the head group with a leaving group undergoes breaking when attacked by a water molecule to release the leaving group. Surrogate substrates for various hydrolases consist of p-nitrophenol (pNP) as a leaving group. A few anthropogenic molecules used as pesticides (for example Paraoxon) have pNP as a leaving group and can undergo hydrolysis in the presence of a hydrolase (for example a phosphotriesterase).
Isolated: An “isolated” biological component (such as a nucleic acid molecule, protein, or cell) has been substantially separated or purified away from other biological components in the cell of the organism, or the organism itself, in which the component occurs, such as other chromosomal and extra-chromosomal DNA and RNA, proteins and cells. Nucleic acid molecules and proteins that have been “isolated” include nucleic acid molecules and proteins purified by standard purification methods. The term also embraces nucleic acid molecules and proteins prepared by recombinant expression in a host cell as well as chemically synthesized nucleic acid molecules and proteins.
It is understood that the term “isolated” does not imply that the component is free of trace contamination, and can include molecules that are at least 50% isolated, such as at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, or even 100% isolated.
Modified: A “modified” nucleic acid or polypeptide is one that has 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. A modified nucleic acid or polypeptide is often produced by chemical synthesis or, more commonly, by the artificial manipulation of isolated segments of nucleic acids, e.g., by genetic engineering techniques.
Operably linked: A first nucleic acid is operably linked with a second nucleic acid when the first nucleic acid is placed in a functional relationship with the second nucleic acid. For instance, a regulatory region is operably linked to a coding sequence if the regulatory region affects the transcription or expression of the coding sequence. Generally, operably linked DNA sequences are contiguous and, where necessary to join two protein-coding regions, in the same reading frame.
PcaR: A transcription factor belonging to the IclR family from Pseudomonas putida KT2440. PcaR binds to β-ketoadipic acid (BKA) and regulates the expression of pca genes that divert BKA to TCA cycle. GenBank Accession No. Q88N41 is an exemplary amino acid sequence of PcaR.
PobR: A transcription factor involved in regulation of enzymes for catabolism of 4HB. PobR is a member of the IclR family. It binds to 4HB and regulates the expression of PobA enzyme that metabolizes 4HB into PCA. GenBank Accession No. Q43992 is an exemplary wild type (unmodified) PobR amino acid sequence. A double mutation version including deletion of L141 and L220V mutation has enhanced sensitivity and response to 4HB.
Production strain: Organisms that can be used for production of value-added chemicals from renewable feedstocks in a laboratory or an industrial environment. In some examples, a production strain is a model microbial strain such Escherichia coli or Saccharomyces cerevisiae. In other examples, a production strain is a non-model organism such as Pseudomonas putida or Acinetobacter baylyi.
Promoter: Promoters are sequences of DNA near the 5′ end of a gene that act as a binding site for RNA polymerase, and from which transcription is initiated. A promoter includes necessary nucleic acid sequences near the start site of transcription, such as, in the case of a polymerase II type promoter, a TATA element. In one embodiment, a promoter includes an enhancer. In another embodiment, a promoter includes a repressor element.
Protocatechuate (PCA): Also known as 3,4-dihydroxybenzoate (3,4-DHB) or protocatechuic acid (CAS Reg. No. 99-50-3). A compound having the structure:




PCA is utilized commercially in the production of food preservatives and pharmaceutical intermediates.

PcaU: A transcription factor belonging to the IclR family of transcription factors and involved in regulation of enzymes for catabolism of PCA. PcaU is a member of the Icl family that binds to a consensus operator sequence in the pca regulon. It binds PCA and activates the expression of pca genes. GenBank Accession No. 083046 is an exemplary wild type (unmodified) PcaU amino acid sequence.
Recombinant: A nucleic acid or protein that is not naturally occurring or has a sequence that is made by an artificial combination of two otherwise separated segments of nucleotides or amino acids. 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 such as those described in Sambrook et al. Molecular Cloning: A Laboratory Manual, 3rd ed., Cold Spring Harbor Laboratory Press, N Y, 2001. The term recombinant includes nucleic acids or proteins that have been altered solely by addition, substitution, or deletion of a portion of the nucleic acid sequence or amino acid sequence, respectively.
Terephthalic acid (TPA): A commodity chemical and polymer precursor with the structure:




TphR: A transcription factor belonging to the IclR family from Comamonas genus. TphR binds to terephthalic acid (TPA) to express downstream genes involved in transport of TPA into the cell. GenBank reference sequence WP_012837656.1 is an exemplary amino acid sequence of TphR.
Transduced and Transformed: A vector “transduces” a cell when it transfers nucleic acid into the cell. A cell is “transformed” by a nucleic acid transduced into the cell when the DNA becomes stably replicated by the cell, either by incorporation of the nucleic acid into the cellular genome, or by episomal replication. As used herein, the term transformation encompasses all techniques by which a nucleic acid molecule is introduced into such a cell, including transformation with plasmid vectors, and introduction of naked DNA by electroporation, lipofection, and particle gun acceleration.
Vector: A nucleic acid molecule that can be introduced into a host cell, thereby producing a transformed or transduced host cell. Recombinant DNA vectors are vectors including recombinant DNA. A vector can include nucleic acid sequences that permit it to replicate in a host cell, such as an origin of replication. A vector can also include one or more selectable marker genes, a cloning site for introduction of heterologous nucleic acids, a promoter (for example for expression of an operably linked nucleic acid), and/or other genetic elements known in the art. Vectors include plasmid vectors, including plasmids for expression in gram negative and gram positive bacterial cells. Exemplary vectors include those for use in E. coli or P. putida. 
II. Biosensors
Disclosed herein are biosensors for use in industrially relevant organisms (e.g., P. putida). In some examples, the biosensor is modified from a naturally occurring biosensor (e.g., transcription factor) and exhibits improved sensitivity and/or specificity or reduced sensitivity with broader applicability in a new host organism compared to the unmodified biosensor (such as a biosensor from a model organism, for example, from E. coli).
In particular embodiments disclosed herein a biosensor includes a transcription factor that is regulated by a molecule of interest “sensed” by the biosensor (such as PCA, 4HB, ccMA, BKA, TPA, or pNP). In some examples, the molecule of interest alters the activity of the transcription factor and alters metabolism or production of the sensed molecule. In some examples, the biosensor participates in positive or negative feedback regulation of the sensed molecule (see, e.g., FIG. 1B). Exemplary sensor/sensed molecule pairs are shown in Table 1.
Table 1. Exemplary biosensors and “sensed” molecules







TABLE 1







Exemplary biosensors and “sensed” molecules










Sensor (Transcription factor)
Sensed molecule







PcaU
PCA



PobR
4HB



CatM
ccMA



PcaR
BKA



TphR
TPA



PobR-derived pNPmut
pNP










In one embodiment, the biosensor is a biosensor for PCA that is modified for use in P. putida. The biosensor may further include one or more expression control elements operably linked to a nucleic acid encoding a reporter (such as an antibiotic resistance gene, a fluorescent protein-encoding gene, or a metabolic gene). In some examples, the biosensor for PCA encodes a PcaU protein with one or more amino acid changes compared to a wild type PcaU protein (such as a PcaU protein from Acinetobacter baylyi, e.g., GenBank Accession No. 083046: SEQ ID NO: 56). For example, a modified PCA biosensor may include a nucleic acid encoding a PcaU protein with an amino acid substitution at amino acid position 147 and/or amino acid position 148 compared to the wild type PcaU protein. In non-limiting examples, the modified PCA biosensor encodes a PcaU protein with the amino acid glycine, serine, or proline at position 147 and/or the amino acid tyrosine or phenylalanine at position 148. Exemplary PcaU amino acid substitutions include T147G/D148Y, T147S/D148F or T147P/D148F.
In some examples, the PcaU protein is encoded by a nucleic acid sequence comprising or consisting of SEQ ID NO: 4 and comprises or consists of the amino acid sequence of SEQ ID NO: 5. In other examples, the PcaU protein is encoded by a nucleic acid sequence with at least about 85% sequence identity (such as at least about 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) to SEQ ID NO: 4, wherein the nucleic acid encodes a PcaU protein with an amino acid substitution at amino acid position 147 and/or amino acid position 148 compared to the wild type PcaU protein. In other examples, the PcaU protein has at least about 90% sequence identity (such as at least about 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) to SEQ ID NO: 5, wherein the PcaU protein has an amino acid substitution at amino acid position 147 and/or amino acid position 148 compared to the wild type PcaU protein.
In some embodiments, the modified PCA biosensor includes one or more modifications to expression control sequences operably linked to the nucleic acid encoding the reporter protein. In some examples, the modifications include one or more of those shown in FIG. 2A, for example, a one base deletion between the operator and the promoter, one or more nucleic acid changes in the −35 region (such as 1, 2, or more changes), and/or one or more nucleic acid changes in the −10 region (such as 1, 2, or more changes). In one non-limiting embodiment, the expression control sequence has the nucleic acid sequence shown in FIG. 2A (pPcaU1.1), such as the sequence of nucleotides 1930-1988 of SEQ ID NO: 2.
In additional embodiments, the modified PCA biosensor includes one or more modifications to expression control sequences (such as one or more of the changes shown in FIG. 2A) and a nucleic acid encoding a PcaU protein with an amino acid substitution at amino acid position 147 and/or amino acid position 148 compared to the wild-type PcaU protein. In a particular example, the modified PCA biosensor includes the sequence of nucleotides 1930-1988 of SEQ ID NO: 2 and the PcaU protein is encoded by a nucleic acid sequence comprising or consisting of SEQ ID NO: 4, wherein the nucleic acid encodes a PcaU protein with an amino acid substitution at amino acid position 147 and/or amino acid position 148 compared to the wild type PcaU protein (such as those described above). In further embodiments, the modified PCA biosensor includes a nucleic acid encoding a fluorescent protein reporter molecule. In some examples, the fluorescent protein is a green fluorescent protein (GFP), for example, a superfolder GFP (sfGFP).
In additional embodiments, the modified PCA biosensor is incorporated in a vector. The vector can be a plasmid vector for replication and/or expression in a bacterial cell. In one example, the vector is for expression in P. putida. In one non-limiting example, the vector includes or consists of the nucleic acid sequence of SEQ ID NO: 1 (pPcaU1.1), or a nucleic acid sequence with at least 90% sequence identity (such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) with SEQ ID NO: 2. In further examples, the vector includes or consists of the nucleic acid sequence of SEQ ID NO: 3 (pPcaU1.2), or a nucleic acid sequence with at least 90% sequence identity (such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) with SEQ ID NO: 3.
In other embodiments, the biosensor is a biosensor for 4HB, such as PobR. In some examples, the biosensor includes a nucleic acid encoding a PobR DoubleMut (PobR-DM) protein (Jha et al., Proteins Struct. Funct. Bioinforma 83:1327-1340, 2015) that carries two mutations (deletion of L141 and L220V mutation) on the wild-type sequence (GenBank Accession No. Q43992), along with the wild-type promoter region from Acinetobacter baylyi (such as A. baylyi ADP1). In some examples, the PobR-DM protein includes or consists of the amino acid sequence of SEQ ID NO: 16. In one embodiment, the reporter gene sequence (e.g., sfgfp) is added downstream of the promoter and the whole cassette is inserted in a vector (such as a pBTL-2 vector). In some examples, the promoter includes or consists of nucleotides 1800-1935 of SEQ ID NO: 6. The vector can be a plasmid vector for replication and/or expression in a bacterial cell. In one example, the vector is for expression in P. putida. In one non-limiting example, the vector includes or consists of the nucleic acid sequence of SEQ ID NO: 6, or a nucleic acid sequence with at least 90% sequence identity (such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) with SEQ ID NO: 6.
In additional embodiments, the biosensor includes a transcription factor for cis,cis-muconic acid (ccMA), such as CatM. In some examples, the biosensor includes a nucleic acid encoding a CatM polypeptide and may further include a wild-type CatM promoter optimized for activity in P. putida. The CatM promoter region includes the Shine Dalgarno RBS sequence AAGGAG and three mutations in −35 region (AGA 4 CCA) and two mutations in the −10 promoter proximal CatM operator region (TA 4 AC) (FIG. 13A). CatM which is a repressor, upon binding to ccMA regulates the expression of a downstream reporter gene (such as sfgfp). In some examples, a sensor plasmid for ccMA includes a pBTL-2 plasmid backbone and a nucleic acid encoding wild-type CatM from Acinetobacter baylyi (e.g., GenBank Accession No. P07774 or nucleotides 990-1898 of SEQ ID NO: 7). In one non-limiting example, the vector includes or consists of the nucleic acid sequence of SEQ ID NO: 7, or a nucleic acid sequence with at least 90% sequence identity (such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) with SEQ ID NO: 7.
In another embodiment, the biosensor is a biosensor for BKA, such as a PcaR operator region and −35/−10 regions based on PcaI promoter in P. putida and inclusion of P. putida compatible RBS sequence AAGGAG (same as E. coli Shine Dalgarno sequence). In some examples, a sensor plasmid for BKA includes a pBTL-2 plasmid backbone and a nucleic acid encoding wild-type PcaR from P. putida (e.g., UniProtKB Accession No. Q88N41; SEQ ID NO: 19). In some examples, the biosensor encodes an amino acid sequence with at least 90% (such as at least 95%) sequence identity with SEQ ID NO: 19 or including or consisting of the sequence of SEQ ID NO: 19, or nucleotides 990-1862 in SEQ ID NO: 22. In one non-limiting example, the promoter is a PcaR promoter that includes or consists of the nucleic acid sequence of SEQ ID NO: 8, or a nucleic acid sequence with at least 90% sequence identity (such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) with SEQ ID NO: 8 or nucleotides 1863-2157 of SEQ ID NO: 22.
In other embodiments, the biosensor is a biosensor for TPA. A sensor plasmid for terephthalic acid (TPA) includes the nucleotide sequence of TphR transcription factor and an optimized intergenic sequence from tphR-tphC regulon from Comamonas testosteroni. In the sensor plasmid, tphC promoter regulates the expression of a reporter gene (such as a sfgfp gene), which is also preceded by an optimal P. putida RBS site (AAGGAG). In one non-limiting example, the biosensor includes or consists of the nucleic acid sequence of SEQ ID NO: 9, or a nucleic acid sequence with at least 90% sequence identity (such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) with SEQ ID NO: 9. In some examples, the TphR promoter includes or consists of the nucleic acid sequence of SEQ ID NO: 20.
III. Modified Biocatalysts
Disclosed herein are biocatalysts modified for use in industrially relevant organisms (e.g., P. putida). In some examples, the modified biocatalyst exhibits improved activity in the organism compared to the unmodified biocatalyst (such as a native biocatalyst, for example, from E. coli).
In one embodiment, the modified biocatalyst is a modified chorismate pyruvate-lyase (UbiC) from E. coli that is modified for use in P. putida, for example, for enhanced productivity. A novel screening approach for relieved product inhibition included utilizing the product sensor with the product surrogate benzoate as an inhibitor. In some examples, the product surrogate at a concentration of 3 mM is capable of inhibiting the biocatalyst by 50% but at that concentration, it does not show any activation of the 4HB sensor.
In some embodiments, the modified UbiC includes of one or more (such as 1, 2, 3, or more) mutations at amino acid positions 31, 34, 76, 78, 80, 90, 92, and/or 114 of SEQ ID NO: 18. In some examples, combination of 2 or 3 mutations result in decreased product inhibition, which also contributes to enhanced enzyme turnover or kcat. Exemplary substitutions in UbiC include E31Q, M34V, I78V, L80V and T92A. In some examples, combinations of the substitutions, for example, E31Q/T92A, E31Q/I78V/L80V, E31Q/I78V/T92A, M34V/I78V, E31Q/M34V, show a gain-of-function.
In some examples, the UbiC protein is encoded by a nucleic acid sequence comprising or consisting of SEQ ID NO: 17 and comprises or consists of the amino acid sequence of SEQ ID NO: 18. In other examples, the UbiC protein is encoded by a nucleic acid sequence with at least about 85% sequence identity (such as at least about 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) to SEQ ID NO: 17, wherein the nucleic acid encodes one or more substitutions at amino acid positions 31, 34, 76, 78, 80, 90, 92, and/or 114 compared to the wild type UbiC protein. In other examples, the UbiC protein has at least about 90% sequence identity (such as at least about 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) to SEQ ID NO: 18, wherein the UbiC protein one or more substitutions at amino acid positions 31, 34, 76, 78, 80, 90, 92, and/or 114 compared to the wild type UbiC protein.
In another embodiment, the modified biocatalyst is a modified CatM polypeptide. In some embodiments, the modified CatM includes a mutation at one or more (such as 1, 2, 3, or more) of amino acid positions 97, 127, 128, and/or 147 of SEQ ID NO: 25. In some examples, the variant CatM_C2.9 includes the following mutations in the inducer binding pocket: V97I, G127A, T128A and L147V (SEQ ID NO 25).
In another embodiment, the modified biocatalyst is a modified PcaR polypeptide. Exemplary modified PcaR polypeptides include or consist of the amino acid sequences of PcaR-G5 (SEQ ID NO: 26), PcaR-F6 (SEQ ID NO: 27) and PcaR-H6 (SEQ ID NO: 28).
In another embodiment, the modified biocatalyst is a modified paraoxonase (PON1). In some embodiments, the modified PON1 includes a mutation at amino acid 115 and one or more (such as 1, 2, 3, or more) mutations at amino acid positions 69, 70, 71, and/or 73 of SEQ ID NO: 32. In some examples, combination of 2, 3, or 4 mutations in combination with H115W result in enhanced enzyme activity. Exemplary substitutions in PON1 include L69V, K70Y/Q/L/H/F, Y71V/I/L, G73/S/A. In some examples, combinations of the substitutions, for example, H115W/L69V/K70Y/Y71V; H115W/L69V/K70Q/Y71I; H115W/L69V/K70L/G73S; H115W/L69V/K70Q/Y71I/G73A; H115W/L69V/K70H/Y71V/G73S; H115W/L69V/K70L; H115W/L69V/K70Q/Y71V/G73A; H115W/L69V/K70Q/Y71L/G73A; and H115W/L69V/K70F/Y73V show enhanced activity.
In some examples, the PON1 protein is encoded by a nucleic acid sequence comprising or consisting of nucleotides 1779-2840 of SEQ ID NO: 31 and/or comprises or consists of the amino acid sequence of SEQ ID NO: 32. In other examples, the PON1 protein is encoded by a nucleic acid sequence with at least about 85% sequence identity (such as at least about 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) to SEQ ID NO:32, wherein the nucleic acid encodes an amino acid substitution at amino acid position 115 and one or more substitutions at amino acid positions 69, 70, 71, and/or 73 compared to the wild type PON1 protein. In other examples, the PON1 protein has at least about 90% sequence identity (such as at least about 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity) to SEQ ID NO: 32, wherein the PON1 protein includes an amino acid substitution at amino acid position 115 and one or more substitutions at amino acid positions 69, 70, 71, and/or 73 compared to the wild type PON1 protein.
IV. Methods of Using Biosensors
Also disclosed herein are methods of utilizing biosensors (e.g., a modified or improved biosensor) to identify and/or select for biocatalysts (e.g., enzymes) with improved activity or strains with gain-of-function. FIGS. 1A and 1B show two exemplary alternate methods to set up microbial cells for biocatalysis and biosensing. In the two plasmid system (FIG. 1A), a nucleic acid encoding the biocatalyst and the genetic sequences of sensor-reporter are on separate plasmids. A single plasmid system where the biocatalyst expression is under positive feedback regulation (FIG. 1B) offers convenience during transformation in microbial cells and can amplify the output signal difference (e.g., fluorescence or other visually detectable signal) in two microbial cells with differential activities (e.g., via positive feedback).
The sensor plasmid (FIG. 1A) can also be used alone in microbial cells where targeted or random mutations are performed for strain evolution. The optimization of a strain for increased product titer involves hundreds of mutagenesis loci, and the efficiency of the variant is coupled to the activation of the sensor in a correlated increase in fluorescence of the cell.
The screening of the microbial library (each cell containing certain variations at the genetic level) is possible via both flow cytometry (Jha et al., Nucl. Acids Res. 428150-8160, 2014) or on a solid growth media (e.g., petri dish) and selection based on the fluorescence of the individual cell (for flow cytometry) or fluorescence (or other visually detectable signal) of the colonies on a solid growth media. If the reporter gene in the sensor plasmid is a survival enhancing gene (for example the protein product of the gene provides resistance to a suitable antibiotic or the protein product of the gene provides a suitable nutrient to the auxotroph), the screening can be carried out based on the growth rate.
Exemplary methods of utilizing the disclosed biosensors are described in Examples 1-7. In some examples, bacterial cells (e.g., P. putida) are transformed with a plasmid encoding a disclosed biosensor and a reporter (such as a fluorescent protein). The cells can be transformed with variants of the biosensor (such as a library encoding variants of the biosensor). The cells can be screened for biosensors with improved function, such as increased sensitivity or specificity for the sensed molecule. In other examples, bacterial cells (e.g., P. putida) are transformed with a plasmid encoding a disclosed biosensor and a reporter (such as a fluorescent protein). The cells are also transformed with a nucleic acid encoding a biocatalyst, which may be on the same or a different plasmid (e.g., FIG. 1A or 1B). Exemplary biosensor/biocatalyst pairs are shown in Table 1. The cells can be transformed with variants of the biocatalyst and can be screened for biocatalysts with improved function, such as increased production of the product of the biocatalyst.
V. Bacterial Strains
Bacteria including nucleic acids for one or more biosensors (such as a PCA biosensor, 4HB biosensor, cis,cis muconic acid biosensor, β-ketoadipic acid sensor, terephthalic acid sensor, and/or pNP biosensor) and/or one or more improved enzymes (such as ecUbiC or PON1) are disclosed. In some examples, the bacteria are P. putida or E. coli. 
In one embodiment, a bacterial strain disclosed herein is a population of P. putida cells including a PCA biosensor or a vector including a PCA biosensor disclosed herein. In some non-limiting examples, the bacterial strain is P. putida including a pPcaU1.1 (SEQ ID NO: 2) or pPcaU1.2 plasmid (SEQ ID NO: 3). In other embodiments, a bacterial strain is a population of P. putida cells including 4HB biosensor or a vector including a 4HB biosensor disclosed herein, such as a plasmid including PobR-DM (e.g., pBTL2_PobR-DM, such as SEQ ID NO: 6). In further embodiments, a bacterial strain is a population of P. putida cells including a ccMA biosensor or a vector including a ccMA biosensor disclosed herein, such as a plasmid including CatM_C2 (e.g., pBTL2_CatM_C2, such as SEQ ID NO: 7) or wherein the promoter sequence is replaced by CatM_A9 (SEQ ID NO: 23) or CatM gene is replaced by CatM_C2.9 (SEQ ID NO: 24). In another embodiment, a bacterial strain is a population of P. putida cells including a BKA biosensor or a vector including a BKA biosensor disclosed herein, such as a plasmid including a modified PcaR promoter that uses endogenous PcaR expression in P. putida (e.g., SEQ ID NO: 8) or a plasmid including a PcaR gene, and modified promoter (e.g., SEQ ID NO: 22). In a further embodiment, a bacterial strain is a population of P. putida cells including a TPA biosensor or a vector including a TPA biosensor disclosed herein, such as a plasmid including a TphR from Comamonas (e.g., SEQ ID NO: 9). In another embodiment, a bacterial strain is a population of P. putida cells including a PobR-derived pNP mutant or a vector including a PobR-derived pNP mutant disclosed herein (e.g., SEQ ID NO: 31).
Exemplary bacterial strains include a biosensor (such as one of the modified biosensors disclosed herein) and one or more molecules involved in the pathway of interest for the biosensor. Thus, in some examples, the bacterial strains include one or more of the biocatalysts listed in Table 2 for each biosensor. In some examples, the one or more biocatalysts included in the bacterial strain with the biosensor are naturally occurring, modified (e.g., by introduction of one or more amino acid substitutions, deletions, and or insertions), heterologous (e.g., introduced from another strain or another species), or a combination of two or more thereof.
Table 2. Exemplary biosensor/biocatalysts combinations in bacterial strains Biosensor Biocatalyst(s)







TABLE 2







Exemplary biosensor/biocatalysts combinations in bacterial strains








Biosensor
Biocatalyst(s)





PCA (PcaU)
AsbF, PobA, 3-deoxy-D-arabino-heptulosonate-



7-phosphate synthase (DAHP synthase isozymes 



aroG, aroF, aroH), VanAB, Fcs, Ech, Vdh


4HB (PobR-DM)
DAHP synthases, UbiC, Fcs, Ech, Vdh,


cis,cis-muconic
DAHP synthases, AroY (ecdD, ecdB), PobA, 


acid (CatM)
AsbF, VanAB, CatA, BenABC, BenD, Dmp 



KMLNOP, Fcs, Ech, Vdh


β-ketoadipic acid
PobA, VanAB, CatA, BenABC, BenD, Dmp 


(PcaR)
KMLNOP, Fcs, Ech, Vdh, PcaHG, PcaBCD,



CatBC


Terephthalic acid
DAHP synthases, UbiC, Fcs, Ech, Vdh, PHA2, 


(TphR)
BsdBCD


p-nitrophenol
Hydrolases (glucanase, xylanase, lipase, esterase,


(PobR derived
phosphodiesterase, phosphotriesterase)


pNPmut)









Bacterial cells are available from numerous sources, including commercial sources known to those skilled in the art, such as the American Type Culture Collection (ATCC; Manassas, Va.). Commercial sources of cells used for recombinant protein expression also provide instructions for usage of such cells. Suitable bacteria for use in the methods disclosed herein include but are not limited to Pseudomonas (e.g., P. putida) or Escherichia (e.g., E. coli). The modified biosensors can also be used to modify biocatalysts in other genera, including but not limited to Acinetobacter (e.g., Acinetobacter baylyi) and Rhodococcus. Bacterial cells are available commercially, for example from American Type Culture Collection (Manassas, Va.).
One representative bacterial system for replication and/or expression of one or more of the disclosed biosensors or biocatalysts is P. putida. In some examples, the vector is a broad host range vector, such as pBTBX vectors (Prior et al., Biotechnol. Bioeng. 106:326-332, 2010) or pBTL vectors (Lynch et al., Biotechnol. Bioeng. 94:151-158, 2006). In one non-limiting example, the vector is based on vector pBTL-2 (Addgene plasmid #22806).
In some examples, the plasmid introduced is extrachromosomally and replicated within the host. In other examples, after introduction of the plasmid, a double homologous recombination event occurs and the one or more genes are inserted into the genome.
Transformation of a bacterial cell with recombinant DNA can be carried out by conventional techniques as are well known to those skilled in the art. Where the host is bacterial, such as, but not limited to, P. putida, competent cells which are capable of DNA uptake can be prepared from cells harvested after exponential growth phase and subsequently treated by the CaCl2) method using procedures well known in the art. Alternatively, MgCl2 or RbCl can be used. Bacteria can also be transformed by electroporation, conjugation, or transduction.
EXAMPLES
The following examples are provided to illustrate certain particular features and/or embodiments. These examples should not be construed to limit the disclosure to the particular features or embodiments described.
Example 1
Evolution of PCA Sensor in Pseudomonas putida 
A plasmid construct was created using a broad host range vector, pBTL-2 (Addgene plasmid #22806), genetic sequences encoding the transcription factor based sensor, PcaU, and a reporter, superfolder GFP (sfGFP). The regulatory region from a previously constructed E. coli-specific sensor-reporter system (Jha et al., Nucl. Acids Res. 42:8150-8160, 2014) was incorporated between the sensor and reporter genetic sequences, and the whole cassette was assembled between two bidirectional transcriptional terminators using NEBuilder Hi-Fi assembly kit (New England Biolabs). The new plasmid construct, pPcaU1 (SEQ ID NO: 1) was transformed into P. putida and grown to a mid-log phase at 30° C. before induction with 10 mM or lower PCA, and further grown overnight at 30° C. Cell fluorescence measurements using flow cytometry (ex/em 488/530) failed to show any dose-dependent response.
Four rounds of sorting using a FACSAria III flow cytometer (BD Biosciences) were performed, with one round of negative selection (bottom 50% population of culture grown in the absence of PCA or uninduced) to eliminate constitutively active variants after two rounds of sorting out the top 5% of induced population (grown in 10 mM PCA). The fourth round of sorting of top 1% population grown in 10 mM PCA, and subsequent plating and testing monoclonal isolates resulted in identification of a variant that showed a distinct response when dosed with 1 mM PCA. The isolated variant showed mutations in both −35 and −10 sites along with the intended deletion between the promoter and the operator (FIG. 2A). This version of sensor plasmid was named pPcaU1.1 (SEQ ID NO: 2).
Example 2
Further Evolution of PCA Sensor in Pseudomonas putida 
To achieve further enhancement of sensitivity and amplitude in a dose-response curve, saturation mutagenesis was carried out at the putative dimer interface as predicted from symmetric homology modeling (FIG. 2B) (Raman et al., Proteins Struct. Funct. Bioinforma 77:89-99, 2009; Jha et al., Proteins Struct. Funct. Bioinforma 83:1327-1340, 2015). Complete randomization of the PcaU sequence positions 147 and 148 using ‘NNK’ codons that code for all twenty amino acids and a stop codon, yielded a library with a theoretical diversity of approximately 440. P. putida strain CJ072 was transformed with the library and grown (uninduced) and sorted for the bottom 40% of the population to eliminate high background and constitutive variants. Subsequently, the sorted population was grown in 0.01-10 mM PCA, and the top 1% of the population was collected from the 0.01 mM induction based on fluorescence intensity. The second round of sorted populations yielded several monoclonal isolates that showed comparable dynamic range with the E. coli protocatechuate biosensor (Jha et al., Nucl. Acids Res. 42:8150-8160, 2014). The most sensitive and responsive variant with T147G/D148Y mutations was named pPcaU1.2 (SEQ ID NO: 3) and is able to detect exogenously supplemented PCA at concentrations below 0.003 mM and a contrast ratio [(Induced signal)/(Uninduced fluorescence signal)] of over twelve-fold (FIG. 3 and FIG. 4A). Other variants in the second round of sorting, for example T147S/D148F or T147P/D148F, showed reduced sensitivity but a sharp gain in signal beyond 1 mM exogenous PCA concentration (FIG. 5).
For specificity evaluation of the biosensor, PcaU1.2 was tested against molecules with similar functional groups, namely benzoate, 4-hydroxybenzoate (4HB), and catechol. The tested molecules were relatively smaller than PCA, to eliminate any rejection of the molecule from the binding pocket due to steric hindrance.
Low but measurable activation of the sensor was observed with benzoate and 4HB (FIG. 4B), while catechol showed a signal that was an appreciable fraction of PCA response (FIG. 4B).
In addition, the PcaU1.2 sensor was tested in a dynamic environment with different knockout strains of P. putida (FIG. 4C). Depending on the mutations in the strain, 4HB would either accumulate or be converted to PCA as an end product, or be further metabolized via the β-ketoadipate pathway (native P. putida KT2440) (FIG. 4C). The response to 4HB feeding in different strains harboring the sensor was consistent with the expected PCA level in the cell. The PcaU1.2 response was highest in CJ072 (due to PCA accumulation), followed by native (PCA metabolized into β-carboxymuconate) with the lowest response observed in CJ182 (where 4HB is not converted into PCA) (FIG. 4D).
Example 3
Alleviation of Enzyme Product Inhibition by Transcription Factor Based 4-Hydroxybenzoate Biosensor for Muconate Production
Materials and Methods
Growth medium, conditions, small molecules: Unless specified, all cultures of P. putida were grown in Lysogeny Broth (LB) medium and supplemented with 50 g/mL kanamycin (Kan50) when necessary to maintain plasmids. LB agar plates were prepared with 1-1.5% agar. The cultures were grown in 14 mL round bottom culture tubes (BD falcon) in a volume of 3-5 mL at 30° C. with shaking at 225-250 rpm. 4-HB (Acros), benzoic acid (Fluka), and cis,cis-muconic acid (Acros) were prepared as sodium salts by dissolving in an equimolar amount of sodium hydroxide. Hence, the actual forms of 4HB, benzoate, and muconate will be interchangeably used for their respective acid forms.
Plasmids and strains: Tables 3 and 4 summarize the oligonucleotides, plasmids, and strains used in this study. P. putida KT2440 (ATCC #47054) derivatives were used for cloning, for whole cell biosensing, UbiC library preparation, and screening of various UbiC constructs. P. putida KT2440 and the engineered derivative CJ182 (Jha et al., Metab. Eng. Commun. 6:33-38, 2018) were used to test the 4HB biosensor construct. CJ182 was used for screening of the UbiC library. P. putida KT2440 derivative CJ200 (Johnson et al., Metab. Eng. Commun. 3:111-119, 2016) was used for the muconate production experiments. The DNA fragment encoding PobR-DM and the native promoter were PCR amplified from a previous E. coli adapted biosensor (Jha et al., Proteins Struct. Funct. Bioinforma 83:1327-1340, 2015) using oligonucleotides described in Table 4 that amplified products to include overlap with a broad host range vector backbone and the sfgfp gene that encodes the superfolder GFP (sfGFP) reporter. For the backbone, the pBTL-2 vector (Addgene plasmid #22806), was PCR amplified using the oligonucleotides pBTL-2_Rev and pBTL-2_Fwd (Table 4). The two PCR products along with sfgfp gene were assembled using NEBuilder HiFi Assembly kit (New England Biolabs) to create the pBTL-2_PobR-DM_sfGFP plasmid (pPobR; SEQ ID NO: 6). The E. coli ubiC gene was PCR amplified from genomic DNA of E. coli type B cells (ATCC strain #11303) using oligonucleotides ecUbiC_EcoR1_f and ecUbiC_Avr2_r (Table 4), and cloned into the pPobR plasmid between EcoRI and AvrII sites, such that the ubiC gene was in frame with the sfgfp gene. The mutation corresponding to the previously published L30A UbiC mutant (Han et al., ACS Catal. 6:8440-8445, 2016) was introduced by first creating two PCR fragments using oligonucleotides ecUbiC_EcoR1_f and ecUbiC_L30A_R and ecUbiC_F1 and ecUbiC_Avr2_r and the ubiC gene as the template and then assembling them with an overlap oligonucleotide method. The mutant gene was cloned into the pPobR as described above. C-terminal 6×Histag versions of the UbiC variants were created using PCR amplification of the gene with oligonucleotides ecUbiC_EcoR1_f and ecUbiC_GGlink_6His_Avr2_r (Table 4). The PCR products were cloned into pPobR in the same way as described earlier. The new gene cassettes encode N-terminal sfGFP-fused UbiC variants with a glycine-glycine linker followed by a 6×Histag in the C-terminus. The electrocompetent cells of P. putida were prepared and transformations were performed according to an established protocol (Choi et al., J. Microbiol. Methods 64:391-397, 2006). The pBTL-2 derived plasmids were transformed in P. putida strains using electroporation (BioRad) in a 1 mm cuvette at 1.6 kV, 25 μF and 200 ohms. A typical time constant of 4.7-5.1 ms was observed for a successful transformation. The transformed cells were selected on LB agar plates with Kan50. A small scoop of cells (using 1 μL inoculation loop) from the transformation plates was grown overnight, mixed with glycerol at a final concentration of 20%, and stored in −80° C. as glycerol stocks.
Vector construction for genomic integration: The 5′ and 3′ homology arms flanking the intergenic region between the genes PP_1642 and PP_1643 were PCR amplified from CJ200 strain using KOD Hot Start polymerase (Millipore). The homology arms consisted of 1009 bp of PP_1642 and 947 bp of PP_1643. The pobR-DM_ubiC-C22 gene cassette was PCR amplified from the pPobR_ubiC-C22 plasmid. The homology arms and the pobR-DM_ubiC-C22 PCR product were Gibson assembled (Merryman and Gibson, Metab. Eng. 14:196-204, 2012) into the suicide integration vector pk18mobsacB (Johnson et al., Metab. Eng. 28:250-247, 2015; Marx, BMC Res. Notes 1:1, 2008; Schafer et al., Gene 145:69-73, 1994) using the NEBuilder HiFi Assembly kit (New England Biolabs). This created the pk18mobsacB_ubiC-C22 plasmid with the PobR regulated promoter driving ubic-C22 gene expression for genomic integration.
Genomic integration: Gene integration in the P. putida KT2440-derived strain CJ200 was accomplished by electroporation of the pk18mobsacB_ubiC-C22 plasmid as described above. Briefly, chromosomal integration by homologous recombination was selected on LB agar plates supplemented with 100 μg/mL kanamycin. Colonies were then counter-selected for a second cross-over event to remove the plasmid from the genome on YT (yeast extract+tryptone) agar plates supplemented with 25% sucrose, as described previously. Insertion was confirmed by PCR amplification using primers (F_100up_1642_UbiC and R_100up_1642_UbiC) that bind outside of the homology regions used to target integration.
Table 3. Strains and plasmids







TABLE 3







Strains and plasmids











Reference 



Description
or Source





Strain




Pseudomonas putida
P. putida ATCC # 47054
ATCC


KT2440




CJ182
P. putida KT2440 ΔpobAR
Jha et al., Metab.




Eng. Commu.




6:33-38, 2018


CJ072
P. putida KT2440 ΔpcaHG
Jha et al., Metab.




Eng. Commu.




6:33-38, 2018


CJ200
P. putida KT2440 ΔcatRBC::Ptac:catA
Johnson et al.,



ΔpcaHG::Ptac:aroY:ecdB:asbF
Metab. Eng.




Commun.




3:111-119, 2016


NP015
P. putida KT2440 ΔcatRBCA::Ptac:catA
This study



ΔpcaHG::Ptac:aroY:ecdB:asbF::pobR-




DM-UbiC-C22



CJ390
P. putida KT2440 ΔpcaIJ::Ptac:asbF
NREL


CJ184
P. putida KT2440
Vardon et al.,



ΔcatRBC::Ptac:catA
Energy Env Sc,



ΔpcaHG::Ptac:aroY:ecdB:ecdD
8:617-628, 2015


NN7
P. putida KT2440
This study (SN04



ΔpykA::Ptac:aroGD146N:asbF ΔpykF
received from



Δppc Δpgi-1 Δpgi-2 ΔpcaIJ ΔpcaR
NREL)



(or SN04 ΔpcaR)



JW0451
E. coli K12 F-, Δ(araD-araB)567,
CGSC#8609; Baba



ΔlacZ4787(::rrnB-3), ΔacrB747::kan, λ−,
et al., Mol. Syst.



rph-1, Δ(rhaD-rhaB)568, hsdR514
Biol. 2:2006.008,




2006


Plasmid




pBTL-2
Kanamycin resistance, pBR322 ori,
Lynch and Gill,



Plac promoter between soxR and tonB
Biotechnol. Bioeng.



terminators
94:151-158, 2006


pBTL-2_pobR-DM_sfGFP
pobR-DM, PobR promoter, sfgfp
This study


(pPobR)
sequences cloned between soxR and




tonB terminators



pPobR_ubiC-wt 
pPobR with E. coli ubiC gene cloned in
This study



frame with sfgfp using EcoR1/Avr2 sites



 pPobR_ubiC-L30A
pPobR with ubiC gene containing
This study



mutations encoding L30A



pPobR_ubiC-C2 
pPobR with ubiC gene containing
This study



mutations encoding E31Q, T92A



pPobR_ubiC-C11
pPobR with ubiC gene containing
This study



mutations encoding E31Q, I78V, L80V



pPobR_ubiC-C14
pPobR with ubiC gene containing
This study



mutations encoding E31Q, I78V, T92A



pPobR_ubiC-C21
pPobR with ubiC gene containing
This study



mutations encoding M34V, I78V



pPobR_ubiC-C22
pPobR with ubiC gene containing
This study



mutations encoding E31Q, M34V
















TABLE 4







Oligonucleotides











SEQ ID


Primer Name
Sequence
NO:





pBTL2_pobR_overlap_F1
TGCTATGGAGGTCAGGTATGATTTTATAC
33



CAGATTGCGCAGTTCG



 


PobRpromo_sfGFP_overlap_R1
GTTCTTCTCCTTTGCTAGCCATATGTATAT
34



CTCCTTGCTATTTTC



 


sfGFP_Fwd
ATGGCTAGCAAAGGAGAAGAAC
35


 


pBTL-2_sfGFP_overlap_Rev
GAGGCTCGTCCTGAATGATATCTTACCTA
36



GGTGTGAATTCAGAAC



 


pBTL-2_Fwd
GATATCATTCAGGACGAGCCTCAGACTCC
37


 


pBTL-2_Rev
AATCATACCTGACCTCCATAGCAGAAAGT
38



CAAAAG



 


ecUbiC_EcoR1_f
AGTCGAATTCTCACACCCCGCGTTAACGC
39



AAC



 


ecUbiC_Avr2_r
AGTCCCTAGGTTAGTACAACGGTGACGCC
40



GG



 


ecUbiC_F1
ACAAAACGTTTTGAACAGCAG
41


 


ecUbiC_31_34_R
CTGCTGTTCAAAACGTTTTGTCAYGGAAT
42



CCTSCAGCAACAGCCAGTCGAGCAG



 


ecUbiC_L30A_R
CTGCTGTTCAAAACGTTTTGTCATGGAAT
43



CCTCCGCCAACAGCCAGTCGAGCAG



 


ecUbiC_76_78_80_R
GCCACGGTTCACCATCGGCACATAMCAA
44



AAYTTCCHTTAACCAGTAACGAGACTCTT




TCG



 


ecUbiC_90_92_F
CCGATGGTGAACCGTGGCTTGCCRGTCGT
45



RCCGTCGTTCCTGTGTCAACGTTA



 


ecUbiC_114_R
GATGTGAACAGATAGCGTCCTAMCGGCG
46



TTTTACCCAATTTTTG



 


ecUbiC_F2
GGACGCTATCTGTTCACATC
47


 


ecUbic_GGlink_6His_Avr2_r
AGTCCCTAGGTTAGTGATGGTGATGGTGA
48



TGGCCACCGTACAACGGTGACGCCGG



 


F_100up_1642_UbiC
GGCATCATCGCGACGACATCGTCGAAA
49


 


R_100up_1642_UbiC
TATCGAGCTGGCCGGCATGGAGGCG
50









Whole cell biosensing of 4HB and benzoate: For 4HB sensing activity of the whole cell biosensor, a small scoop of pPobR transformed P. putida cells from a streaked plate or a glycerol stock was grown overnight as a seed culture. The overnight saturated seed culture was then diluted 100-fold in fresh growth medium, grown for 5-6 h at 30° C. to achieve an OD600 of  ̃0.6, then distributed in deep 96-well blocks in a volume of 300-500 μL and induced with varying concentrations of 4HB or benzoate. The cultures were grown overnight (14-16 h) under vigorous shaking in a deep-well shaker (Taitec BioShakerM-BR-022UP). The cultures were diluted 100-fold in phosphate-buffered saline (PBS) with 1% sucrose and analyzed using an LSR II flow cytometer (BD Biosciences) or Accuri C6 flow cytometer (BD Biosciences) with standard settings for measurement of GFP fluorescence (ex 488 nm, em  ̃530/30 nm). The arithmetic mean fluorescence value of  ̃100,000 cells tightly gated based on forward and side scatter (FSC vs SSC) was used for a dose-response plot. Same protocol was followed for comparison of UbiC clones.
Selecting mutagenesis sites in UbiC and library creation: The UbiC crystal structure with a bound 4HB molecule (PDB code 1JD3) was used for the determination of appropriate sites for mutagenesis that could weaken the 4HB/UbiC interaction. Based on the first shell amino acids (4 Å radius from 4HB ligand atoms), seven positions were identified for mutagenesis in the binding pocket. A few positions on the loop referred to as the flap (Han et al., ACS Catal. 6:8440-8445, 2016) were also selected for mutagenesis. Mutations were selected such that only small changes in side chain properties (hydrophobic to hydrophobic, charged to polar, polar to hydrophobic or hydrophobic to polar) were achieved. The diversified library of ubiC was constructed using the oligonucleotides with wobble at appropriate positions and PCR amplification using E. coli genomic DNA containing the ubiC gene as a template. The fragments were created using forward and reverse primer pairs of ecUbiC_EcoR1_f and ecUbiC_31_34_R, ecUbiC_F1 and ecUbiC_76_78_80_R, ecUbiC_90_92_F and ecUbiC_114_R and finally ecUbiC_F2 and ecUbiC_Avr2_r with the wild-type ubiC gene as the template. The fragments were assembled using overlap extension PCR. Complete ubiC gene variants were double digested with restriction enzymes EcoRI/AvrII restriction endonucleases (NEB Biolabs) and cloned into the pPobR plasmid such that the ubiC gene was in the same reading frame as sfgfp. The gene library created consisted of zero to eight mutations in any gene, since all the mutations were represented in a small set of random clones as confirmed by Sanger sequencing. Transformation in P. putida strain CJ182 was carried out using the electroporation method as described above. Following transformation, total number of colonies on the plate representing approximately 40-fold the library diversity, were scraped and stored as glycerol stocks. Typical OD600 of the glycerol stock was  ̃1. As needed, a scoop of glycerol stock using 1 μL inoculation loop was sequentially diluted twice in 1 mL LB to reach approximately 104 cells/mL and 100-400 μL of the diluted sample spread on agar plates with suitable growth conditions to get a total of 1000 to 4000 well separated colonies on a 85 mm or 125 mm diameter petri dish, respectively.
Selection of UbiC variants with relieved product inhibition: To select clones with relieved product inhibition, P. putida cells containing variants of ubiC were grown on LB agar plates containing high concentrations (3 mM and 10 mM) of benzoate. Benzoate works as a proxy for 4HB because the two molecules are aromatic and carry similar charge. Benzoate is capable of inhibiting the enzyme UbiC with a Kp thousand-fold higher than 4HB (Holden et al., Biochim. Biophys. Acta BBA-Protein Struct. Mol. Enzymol. 1594:160-167, 2002), but does not activate the 4HB biosensor at those concentrations (this work). Colonies were selected visually based on the intensity of their fluorescence under an illuminator (excitation wavelength 488 nm, emission filter 515 nm). Selected colonies were grown and stored as glycerol stocks for later use. The clones were compared in liquid cultures as described above.
Enzyme expression and purification: 5 mL cultures were grown from the glycerol stocks of P. putida CJ182 containing plasmids encoding 6×His-tag UbiC variants. At an OD600 of  ̃0.6, the cultures were induced with 1 mM 4HB and further grown for 16 h at 30° C. and 225 rpm shaking. The cultures were centrifuged at 3500×g for 10 min and the cell pellets were lysed with 500 μL BugBuster (Novagen) under slow shaking for 30 min. The cell lysates were centrifuged at 15,000×g for 15 min at 4° C. and the sfGFP-UbiC fusion proteins were purified by Affinity Chromatography using Talon beads (Clontech). For purification, the clarified supernatant for each UbiC variant was mixed with 200 μL Talon beads in an equilibration buffer (50 mM Tris-HCl, 300 mM NaCl, pH 7.5) for 30 min. The mixture was centrifuged at 2000×g, the supernatant was discarded and the pellet consisting of Talon beads with bound protein was washed two times with equilibration buffer containing 50 mM imidazole. Finally, the beads were mixed with elution buffer (50 mM Tris-HCl, 300 mM NaCl, 300 mM Imidazole, pH 7.5) and filtered through a spin column with 0.2 m filter. The enzyme concentration and purity of the protein in the eluate were determined by absorbance at 280 nm using a Nanodrop (Thermo Scientific) and SDS polyacrylamide gel electrophoresis, respectively.
Determining enzyme kinetic parameters: The production of 4HB from chorismate catalyzed by UbiC is accompanied with release of a pyruvate molecule. Hence, this activity of the purified UbiC proteins was monitored in vitro by a second reaction of NADH oxidation at 340 nm during conversion of pyruvate to lactate in a reaction mixture containing 0.5 units of lactate dehydrogenase (Sigma Aldrich) and 200 μM NADH (Sigma Aldrich) dissolved in 50 mM Tris-HCl buffer (pH 7.5), 0.5 μg of purified UbiC variants, chorismic acid (Sigma Aldrich) (30-300 μM), and 4HB (0-200 μM) in a final volume of 100 μL. The reactions were conducted in 96-well plate at 30° C. in the Synergy H4 Hybrid Microplate Reader (Biotek). The kinetic parameters and product inhibition constants were determined by measuring the initial reaction rates with respect to varying substrate and product concentrations and calculated using GraphPad Prism software.
Shake flask experiments for muconate production: P. putida CJ200 transformed with pPobR_ubiC plasmids or with genomically integrated ubiC-C22 under PobR regulation (NP015) were grown in 125 mL baffled shake flasks containing 25 mL of 1×M9 salts (6.78 g/L Na2HPO4, 3 g/L KH2PO4, 0.5 g/L NaCl, 1 g/L NH4Cl), 30-50 mM glucose, 2 mM MgSO4, 100 μM CaCl2), 18 μM FeSO4, and 50 μg/mL kanamycin. Seed cultures were grown from glycerol stocks in LB media supplemented with 50 μg/mL kanamycin, for 16 hours, pelleted and washed with M9 growth medium and subcultured into flasks to a final OD600 of 0.05. Cultures were incubated at 30° C. and 225 rpm for up to 2 days. Samples were taken at regular intervals to quantify cell density, glucose consumption, and muconate concentration.
Quantification of glucose and muconate concentrations using High Performance Liquid Chromatography (HPLC): To measure muconate concentration, culture samples collected at various time points were centrifuged either at 3,500 rpm for 10 min or 16,000 rpm for 1 min. The supernatants were transferred to a 0.22 μm spin column (Corning Costar Spin-X with cellulose acetate membrane), centrifuged, and the filtrates were transferred to HPLC vials for analysis by Agilent 1100 series HPLC system. The samples were analyzed for 10 min on a Fast Acid column (Phenomenex Rezex RFQ-Fast Acid H+(8%)) using 0.01 N H2SO4 at a flow rate of 0.8 mL/min as a mobile phase. A Diode Array Detector (DAD) set at 258 nm wavelength for detection was used for estimating muconate concentration. Alternatively, the samples were analyzed for 40 min on a SUPELCOGEL H Column (SUPELCO) with 0.1% H2SO4 at 0.5 mL/min. The temperature of both column and RID detector were maintained at 45° C. Glucose and muconate were quantified using a Refractive Index Detector (RID) and peaks were integrated using Agilent Chemstation software. Appropriate standards were made using commercial glucose and muconate samples.
Results
Double mutant of Acinetobacter baylyi ADP1 PobR and activity in Pseudomonas putida KT2440 for 4HB biosensing: PobR-DM is a double mutant (ΔL141, L220V) of the Acinetobacter baylyi ADP1 PobR transcription factor. When paired with the native promoter, the transcription factor exhibits very high sensitivity to 4HB in E. coli (Jha et al., Proteins Struct. Funct. Bioinforma 83:1327-1340, 2015; Jha et al., Nucleic Acids Res. 44:8490-8500, 2016). In E. coli, the sensor does not respond to closely related molecules like p-nitrophenol and very weakly responds to protocatechuate, but only at 1000-fold greater concentration than the native inducer, 4HB. The genetic fragment encoding PobR-DM and its promoter, along with the sfgfp (coding for superfolder GFP) reporter, were cloned into pBTL-2, a broad host vector. The resulting sensor plasmid, pPobR, was transformed into the P. putida strains KT2440 and CJ182, a strain incapable of metabolizing 4HB due to deletion of the native pobA. The whole cell biosensor responded in a dose-dependent manner to 4HB, showed high sensitivity (<3 μM exogenous 4HB in CJ182 and <30 μM in wild-type KT2440), and had a contrast ratio of >12-fold at 30 mM inducer concentration. In the absence of PobA activity in CJ182, the sensor would always see higher inducer concentration inside the cell compared to the native strain, resulting in the observed difference in sensitivity. Importantly, the whole cell biosensor did not show any response to benzoate, a molecule similar to 4HB but lacking the hydroxyl group (FIG. 7A). These data confirmed that the 4HB sensor is functional, sensitive, and specific for detection of 4HB in P. putida. 
Intracellular 4HB production and sensing: The E. coli gene that encodes for UbiC was introduced into the pPobR sensor plasmid and in the same reading frame as the sfgfp reporter. In this positive feedback regulation set up, the enzymatic activity of UbiC regulated its own expression as a sfGFP fusion protein: specifically, higher UbiC activity should lead to more 4HB, which in turn increases activation of the sensor, which then leads to an increase in the expression of that ubiC gene (FIG. 6A). P. putida CJ182 cells showed high fluorescence colonies in the absence of exogenously supplemented 4HB only when harboring the sensor+enzyme construct pPobR_ubiC-wt (SEQ ID NO: 29), confirming that the expression of UbiC resulted in intracellular formation of 4HB, which, in turn, activated the PobR-DM sensor (FIG. 9). The plasmid containing the sensor alone (pPobR) showed dim colonies in the absence of exogenous 4HB (FIGS. 7B and 9), presumably from low background activity of the native UbiC in P. putida KT2440. When the growth media was supplemented with 10 mM benzoate, an inhibitor of UbiC with Ki>3 mM (Holden et al., Biochim. Biophys. Acta BBA-Protein Struct. Mol. Enzymol. 1594:160-167, 2002), the colonies showed diminished fluorescence, confirming inhibition of the UbiC enzyme and reduced production of the 4HB. Finally, when 100-fold less (100 μM) 4HB was supplemented with 10 mM benzoate, the colonies showed fluorescence. Together these data indicate that benzoate can inhibit enzyme activity but does not block the inducer binding pocket of the sensor (FIG. 7B).
Mutational library of UbiC to reduce product inhibition: Based on the product-bound UbiC structure (PDB Code 1JD3), the amino acid positions that could be mutated to weaken the product binding were determined (FIG. 10A). We employed a conservative approach to select residue positions for mutagenesis, since an earlier study showed that even a single mutation in the enzyme was frequently detrimental to its stability (Han et al., ACS Catal. 6:8440-8445, 2016). It was also shown in the same study that mutations in the loop (L29 to M34, referred as the flap) can perturb the product binding to the enzyme. Two mutations L30A and D32A (numbering based on the UbiC structure PDB code: 1JD3), were expected to weaken hydrophobic and polar interactions respectively with the rigid portions of the protein. While the two mutations independently reduced product inhibition, combining them failed to show an additive effect in terms of gain-of-functions, probably due to poor stability and low expression of the double mutant protein (Han et al., ACS Catal. 6:8440-8445, 2016). We built upon this study, and took a two-pronged approach, where, in one case, mutations were further explored in the L29-M34 loop and, in another case, mutations specifically around 4HB were pursued such that product-enzyme interactions are weakened (FIG. 10A). For the first set of mutations, we chose to investigate E31Q and M34V mutations within the loop. E31, a negatively charged amino acid, exhibits a charge interaction with R116 and R126. We hypothesized that a conservative mutation of E31→Q would mitigate that interaction, resulting in an effect similar to the D32A mutation on the same loop that weakens the charge-charge interaction between D32 and R116 (Han et al., ACS Catal. 6:8440-8445, 2016). Similarly, the effect of the L30A mutation to create a void between position 30 and M34 can be recapitulated by shortening the side chain at position 34 by mutating it to valine. The other set of mutations focused around the product 4HB in the crystal structure. The R764→K/M mutations were selected to weaken/disrupt the existing electrostatic interaction between the carboxylate group of 4HB and the arginine at position 76 of the protein. Mutations I78→V, L80→V, T92→A and L114→V were also selected for diversification of UbiC, as these mutations are ‘neutral drift’ mutations and can weaken the 4HB/protein interaction due to shortening of the side chain in each case (FIG. 10A). The G90A mutation, as in the product bound structure of UbiC (PDB code 1JD3), is expected to increase the affinity of the 4HB/protein interaction. Hence, we chose the G90→S mutation since introduction of a hydroxyl group in close proximity of the 4HB aromatic ring was expected to weaken the interaction in addition to causing steric hindrance to 4HB due to a larger side chain. A combinatorial library consisting of the wild-type amino acid and selected conservative mutations at these eight positions in UbiC was created with a theoretical diversity of  ̃400, transformed into the P. putida strain CJ182, and plated. Transformed colonies (40-fold the library diversity) were scraped and evaluated in subsequent experiments.
Benzoate as a surrogate molecule for high throughput screening of UbiC library using the 4HB sensor: The PobR-based sensor is specific to 4HB, and benzoate fails to activate the sensor. However, UbiC is sensitive to product inhibition from both 4HB (its product) and benzoate (with an affinity 1000-fold weaker than 4HB) (Holden et al., Biochim. Biophys. Acta BBA-Protein Struct. Mol. Enzymol. 1594:160-167, 2002). These observations were leveraged to relieve the product inhibition of UbiC, where UbiC variants with high activity were selected based on high fluorescence in the presence of the benzoate inhibitor. For selection, a CJ182-based UbiC library was plated in the presence and absence of 10 mM benzoate (FIG. 10B). In the absence of any inhibitor (benzoate) or inducer (4HB), the library showed varying fluorescence in the colonies, indicating that the variants were producing 4HB in vivo at different levels and inducing the biosensor to express sfGFP in a correlated fashion (FIG. 10B, left). When the library was cultured on 10 mM benzoate with 100 μM 4HB inducer, all of the colonies uniformly accumulated fluorescence (FIG. 10B, right), indicating again that the sensor activity is unaffected by the addition of benzoate, as observed above for the wild type UbiC+sensor strain (FIG. 7B). In the presence of 10 mM benzoate inhibitor alone, a small number of colonies were distinctly brighter than the rest, indicating that these colonies were able to make 4HB even in the presence of a high concentration of the benzoate inhibitor (FIG. 10B, middle). However, some of the brightest colonies were in a crowded region of the plate. These most crowded regions could have exhibited higher UbiC activity simply because the local benzoate concentration was reduced due to utilization by P. putida, which can metabolize it. Thus, bright colonies from such regions on the plate were not picked. Instead, several isolated colonies exhibiting fluorescence above the background from the benzoate-inhibited population were picked and evaluated in liquid culture supplemented with varying concentrations of benzoate. Five mutants of UbiC were identified that exhibited high retention of fluorescence (30-40%) even at 20 mM benzoate (FIG. 10C) while the wild-type exhibited very low fluorescence in the cells. The previously published UbiC variant with reduced product inhibition (UbiC-L30A) also showed high retention of cell fluorescence at 20 mM benzoate. Sequencing of the top clones revealed the following mutations: C2: E31Q/T92A; C11: E31Q/I78V/L80V; C14: E31Q/I78V/T92A; C21: M34V/I78V; C22: E31Q/M34V.
Kinetic analysis of the UbiC variants: As sfGFP fusions, expression and yields of the UbiC mutants from single step affinity purification were comparable with each other. We performed an in vitro kinetic assessment on a few UbiC variants to understand the mutational effects on the enzymatic properties. As a control for a variant with relieved product inhibition, the previously studied variant UbiC-L30A was also included in the study. Their kinetic parameters, Km, kcat and Kp, were determined (Table 5). The variants, UbiC-C21 and UbiC-C22 showed a 3-5-fold increase in Km for chorismate along with 7-8-fold increase in product inhibition constant, Kp, confirming that the new variants of UbiC have alleviated product inhibition. The catalytic turnover number, represented by kcat, showed an increase by 2-3-fold in UbiC-C21 and UbiC-C22 compared to the wild type. Catalytic efficiency (kcat/Km) of the mutant UbiC variants showed a marginal drop from the wild-type UbiC.
Table 5. Kinetic parameters of sfGFP fused UbiC variants with a C-terminal 6×His tag







TABLE 5







Kinetic parameters of sfGFP fused UbiC variants with a C-terminal 6xHis tag
















kcat/Km 



UbiC
Mutation(s)
Km (μM)
kcat (s−1)
(s−1M−1)
Kp (μM)





Wild Type
None
32.9 ± 2.8
0.37 ± 0.04
1.1 × 104
 2.4 ± 0.2


UbiC-L30A
L30A
74.1 ± 8.7
0.64 ± 0.07
8.6 × 103
13.2 ± 1.0


UbiC_C11
E31Q/I78V/L80V
54.7 ± 3.4
0.51 ± 0.04
9.3 × 103
 4.7 ± 0.6


UbiC_C21
M34V/I78V
90.2 ± 7.8
0.67 ± 0.06
7.4 × 103
17.8 ± 1.4


UbiC_C22
E31Q/M34V
147.7 ± 9.4 
1.04 ± 0.09
7.0 × 103
19.3 ± 2.5









UbiC as a metabolic shunt for cis,cis-muconic acid production in P. putida: We next investigated if the UbiC mutants with reduced product inhibition can be applied to enhance the metabolic pathways for biosynthesis of valuable products. To test this, we chose cis,cis-muconic acid (muconate) as an end-product as it can be derived from 4HB via the shikimate pathway. We transformed the biosensor/enzyme plasmid with UbiC variants into the P. putida strain CJ200, which has been previously engineered to produce muconate (Johnson et al., Metab. Eng. Commun. 3:111-119, 2016). In P. putida KT2440, the introduction of the heterologous genes for the DHS dehydratase and protocatechuate (PCA) decarboxylase enables the conversion of dehydroshikimate (DHS) from the shikimate pathway to catechol via PCA. The native catechol-1,2-dioxygenase in the β-ketoadipate pathway converts catechol into muconate. Deletion of the downstream genes catBC and the regulator encoded by catR prevents further metabolism of muconate in the culture. With the introduction of the E. coli ubiC gene in CJ200, we introduced a pathway in which UbiC converts chorismite to 4HB, which is then converted to protocatechuate via the native PobA for subsequent conversion to muconate. We hypothesized that this additional route from the shikimate pathway to muconate might increase the overall flux to this target molecule. Earlier studies showed that in a batch culture with 50 mM glucose as a carbon source, CJ200 produced approximately 2.5 mM of muconate giving a molar yield ([muconate]/[glucose]) of 5% (Johnson et al., Metab. Eng. Commun. 3:111-119, 2016). CJ200 transformed with the pBTL-2 empty vector or the pPobR plasmid (sensor plasmid) and grown in the presence of Kanamycin (50 μg/mL) exhibited a similar yield of  ̃5.5% (FIG. 11B), indicating that kanamycin and/or presence of sfgfp gene do not add to any stress or metabolic burden to P. putida cells. With the introduction of the wild-type ubiC gene using the pPobR_ubiC-wt plasmid expressing sfGFP-fused UbiC under positive feedback regulation, the muconate yield increased by >30%, giving a final muconate concentration of 3.7 mM (FIG. 11A). Moreover, plasmids expressing variants of UbiC with alleviated product inhibition, UbiC-L30A, UbiC-C21, and UbiC-C22, exhibited a final muconate concentration of >6 mM and a yield increase of  ̃130% over CJ200 with no heterologous UbiC expression (FIGS. 11B and 11C). At 50 mM starting glucose concentration, the UbiC expressing CJ200 P. putida strains showed a longer lag phase during growth, which may be attributed to the metabolic burden caused by the production of higher amount of muconate from the essential shikimate pathway (FIG. 11B).
Genomic integration of evolved E. coli ubiC gene in P. putida: We further investigated the effect of the E31Q/M34V double mutant of UbiC-C22 in the muconate production strain, where the mutant ubiC gene was integrated into the genome of P. putida CJ200 strain. Simulating the plasmid version of ubiC-C22 in CJ200, we kept the expression of the UbiC under the positive feedback regulation of PobR and integrated it into the intergenic region between PP_1642 and PP_1643 to create strain NP015. At 50 mM glucose in a shake flask, CJ200 and NP015 were compared for growth, glucose consumption rates, and muconate production. While the two strains showed comparable rates for glucose consumption and growth, with complete glucose utilization and saturation cell density reached in 18 h, an enhancement in muconate production was observed in NP015 over the parent strain CJ200 (FIGS. 12A and 12B). At 18 h, muconate concentration in the culture media saturated. Muconate concentrations of 2.7 mM and 4.3 mM were observed in CJ200 and NP015 strains, giving a molar yield of 5.4% and 8.6%, respectively. This gave a yield improvement of 60% in the muconate production strain due to the addition of new carbon flux directed from shikimate pathway towards the heterologous muconate pathway (FIGS. 12A and 12B). A small difference in the final OD of the two strains can be attributed to the difference in muconate yields.
Example 4
PobA Activity and Library Screening
FIG. 13A is a schematic diagram of an exemplary plasmid construct for sensing PobA activity. PobA enzyme in P. putida converts 4HB into PCA. In order to improve the catalytic efficiency of PobA, a plasmid construct consisting of pPcaU1.2 (SEQ ID NO: 3) and pobA gene from P. putida expressed as sfGFP fusion and under positive feedback regulation of PCA (scheme represented in FIG. 1B) was created (FIG. 13A) (SEQ ID NO: 30). FIG. 13B is a graph representing 4HB concentration-dependent fluorescence response of CJ182 cells (shown in FIG. 13A). With conversion of 4HB into PCA, sfGFP-PobA fusion is expressed and results in observed fluorescence of the cells. FIG. 13C shows a P. putida cell population harboring the above plasmid construct and where pobA gene has been diversified (Theoretical library diversity>20,000). Each cell is expected to have single sequence of pobA on the plasmid. The library of cells was plated on 100 μM 4HB, scraped and analyzed using flow cytometry for cell fluorescence. FIG. 13C (top panel) shows the fluorescence histogram confirming two distinct population with low and high cell fluorescence respectively. Top 3% fluorescent cells were sorted (using BD FACSAria III flow cytometer) and regrown in 100 μM 4HB on plate, scraped and analyzed again using flow cytometer. In this round, top 1% fluorescent cells were sorted and collected and the process was repeated for one more round. The fluorescence distribution of the cells in the third round (middle panel) was similar to the distribution observed with cells harboring wild-type PobA (PobA-wt) (bottom panel). Top 2% sorted population from the third round was plated and 10 colonies were sequenced using Sangar sequencing. The sequence confirmed the emergence of wild-type PobA sequence, confirming that a majority of the mutations in the library were deleterious to the PobA activity. The high throughput approach using PCA sensor helped to screen through the library efficiently, and the wild-type PobA sequence, which was represented as a needle-in-a-haystack in the diversified library, could be sorted in as low as three sorting rounds of top 3%, 1%, and 2%, based on cell fluorescence.
Example 5
Development of a Muconate Sensor
CatM belongs to a LysR family of transcription factor and is involved in metabolism of ccMA in Acinetobacter baylyi ADP1. The intergenic region of catM and catB in A. baylyi ADP1 consit of operator and promoter regions for the expression of CatM and downstream CatB. Utilizing the same format, where the regulated downstream catB gene was replaced with sfgfp gene as a reporter in pBTL2 backbone a synthetic construct was made which when evaluated in a ccMA production strain (for example CJ184) failed to show any fluorescence. Hence, optimization of the biosensor via mutating operator and promoter regions in the intergenic region, was pursued.
FIG. 14A represents the Acinetobacter baylyi CatM promoter region used for selection of ccMA sensing in P. putida KT2440. The regions that were partially diversified or completely randomized are marked ‘red’ and the mutations pursued are labeled. For example −10 and −35 sites were partially randomized for saturation, the ribosome binding site (RBS) was made compatible to the consensus sequence AAGGAG followed by eight bases to the start codon “ATG” of the reporter. Overall the library diversity was expected to be well above 60,000. Using a method similar to what has been described in Example 1-3, catM gene, diversified regulatory region, and sfgfp were inserted into the pBTL2 backbone. The completed circular plasmids, carrying Kanamycin resistance marker, were then transformed into electrocompetent cells of P. putida strain CJ184. The transformation gave well above 60,000 colonies to cover the library adequately. The colonies were scraped and grown LB and 50 μg/mL kanamycin (Kan50) and in the presence of 10 mM Benzoate or PCA, since CJ184 is capable of accumulating ccMA from both these substrates (Vardon et al, Energy Env Sc, 8:617-628, 2015). As a control, the library was also grown in LB+Kan50 but lacking the precursors for ccMA. Post 16 h of growth at 30° C., the samples were analyzed on a flow cytometer (BD FACSAria III). Three rounds of growth and sorting consisted of negative-positive-positive or positive-negative-positive selection where negative selection (sorting low fluorescence from no Benzoate/PCA culture) was required to eliminate constitutively active sensor, and positive selection (sorting high fluorescence from the Benzoate/PCA supplemented cultures) was needed to choose the most active sensor. While 10 mM of Benzoate or PCA was used in Rounds 1 and 2, the dosage was decreased to 1 mM in the final round. Post third round of sorted population, a fraction was plated on LB agar+Kan50 plates and a few colonies evaluated for PCA, Benzoate and Catechol response, considering CJ184 strain is capable of converting them all to final product ccMA.
pCatM_C2 (SEQ ID NO: 7) that consist of optimized promoter CatM_C2 (SEQ ID NO: 12 and 13 (reverse complement)), showing high response to PCA was identified. Another variant of the plasmid, with promoter CatM_A9, (SEQ ID NO: 23) showed weaker fluorescence but very similar dose-response as the CatM_C2 was also identified which could be useful in a few cases. FIG. 14B is a plot showing intracellular production and sensing of ccMA in P. putida (CJ184) using pCatM_C2 sensor. CJ184, which is capable of metabolizing PCA into ccMA as a final product (ccMA production strain) shows a clear dose-response with PCA. Another strain, CJ072, with a knockout mutation in PCA-ccMA pathway (Non-production strain) failed to show any dose-response with PCA. Similar dose-response was observed with other substrates such as Benzoate or catechol. As evident from the dose-response plot (FIG. 14B) the CatM_C2 sensor saturates at  ̃ 100 μM, the biosensor has limited application in certain high muconate producing strains.
In order to address that problem, we went ahead to create a diversified library of the CatM inducer binding pocket. With an availability of a ‘holo’ crystal structure (PDB code 2F7C) where amino acid side chains within 5 Å radius were selected for diversification. FIG. 14C shows the residue numbers that were selected for diversification in the CatM inducer binding pocket. With ‘natural drift’ mutation strategy, V97→I/L/A/T, S99→A, G127→A, T128→A, L147→V/I and P196→S were introduced into the CatM sequence in a fashion such that either native or one of the proposed mutations at each residue position appeared. This gave a library with theoretical diversity  ̃300. The library was created in the promoter background of pCatM_C2, which was then transformed into P. putida strain CJ184, and processed using flow cytometry in a manner as described previously for promoter evolution. FIG. 14D shows a selected variant (CatM_C2.9) with low sensitivity for an ideal dose-response at high ccMA titer, that is >10 mM. The variant CatM_C2.9 (SEQ ID NO: 25) includes the following mutations in the inducer binding pocket: V97I, G127A, T128A and L147V.
Example 6
Development of a BKA Sensor
For constructing and evaluating a BKA sensor, there were two approaches that were pursued. Since BKA responsive transcription factor (PcaR, SEQ ID NO: 19) is native to P. putida KT2440, we went ahead to design an optimal promoter which will be sufficient to respond to BKA accumulation via the native genomic pcaR expression. The construct, where partial intergenic region of PP_1374 gene and pcaR was used to in pBTL2 backbone to create a construct pPcaR_promo (SEQ ID NO: 21). This BKA sensor construct was transformed in both P. putida KT2440 and CJ390 and evaluated using precursors of BKA, such as PCA and catechol.
FIG. 15A shows the protocatechuate and catechol branches of the β-ketoadipate pathway in P. putida KT2440 and a heterologous shunt feed (green arrow) from shikimate pathway. P. putida variant (CJ390) consist of an added shunt (green arrow) and deleted genes of BKA metabolizing enzymes (PcaIJ) (red arrow). Hence, as expected, CJ390 is capable of making BKA from glucose or LB but at the same time spiking with immediate precursors, like PCA or catechol, would give spike in BKA and hence increase in fluorescence with increasing precursor concentration. FIG. 15B shows low fluorescence in P. putida KT2440 and high fluorescence in a CJ390, which is capable of making and accumulating BKA when grown on LB medium. FIG. 15C shows dose response of BKA sensor in P. putida. Native KT2440 cannot make BKA from glucose and BKA is transiently accumulated due to activity of PcaIJ. This results in significantly lower response compared to mutant P. putida, which can convert glucose or spiked PCA and catechol into BKA and also accumulate BKA. In high BKA producing strain from glucose, this BKA sensor failed to show any dose-dependent response to BKA precursors like catechol and PCA. Hence, modification of the BKA binding pocket in PcaR was necessary to reduce the sensitivity of the sensor.
Rosetta homology modeling, using a known structure of a transcription factor with a homologous sequence (PDB code 2G7U) was used to get a 3D estimate of the PcaR inducer binding domain. Based on the predicted structure, several amino acids in the pocket were identified to be suitable for mutagenesis. FIG. 15D shows highlighted positions which were chosen for diversification to reduce the sensitivity of BKA sensor. Positions N140, L161, L166, L172, M179, L234, L238, S240, N257 and S259 were mutated for diversification to achieve N/S/T/I at positions 140 and 257, L/M/V at positions 161, 166, 172, 179, 234 and 238, S/T/A at position 240 and 259. This produced a library of diversity>100,000. The library was transformed in high BKA producing strain where native pcaR gene was deleted (strain NN7) and screened for increase fluorescence response with increasing dose of precursors while in the LB mediated BKA production background. FIG. 15E shows dose response plots of identified variants that show low sensitivity and hence detection of high concentration of BKA titer. As seen in the right panel, the native PcaR (SEQ ID NO: 19) expressing plasmid pPcaR (SEQ ID NO: 22) showed saturation in fluorescence, while the variants PcaR-G5 (SEQ ID NO: 26), PcaR-F6 (SEQ ID NO: 27) and PcaR-H6 (SEQ ID NO: 28) showed dose dependent response with BKA precursors, catechol and PCA, assuming all the precursors are converted to BKA in the background of BKA production from glucose/LB growth media.
Example 7
High Throughput Selection of Paraoxonase 1 Enzyme with Enhanced Activity
Since p-nitrophenol (pNP) is a chromogenic chemical leaving group, it has become ubiquitous reporter in surrogate substrates for hydrolytic catalyst characterization. Yet a spectrophotometric monitoring format is limited to low-throughput bulk samples, typically a 96-well or 384-well plate with one well for each enzyme species. Moreover, this can lose sensitivity amidst background absorption in many media. Instead, we previously reported a ‘smart’ microbial cell technology (SMC) and single-cell monitoring of pNP using a computationally re-designed Acinetobacter baylyi PobR transcription factor (TF) regulating expression of a fluorescent protein reporter. We showed this sensor-reporter system could detect intracellular organophosphate hydrolysis by a phosphotriesterase (PTE) via the pNP leaving group. We showed the single cell fluorescence correlated with PTE enzymatic activity (Jha et al., Nucl. Acids Res. 44:8490-8500, 2016).
We extended this technology to library-based design of a mammalian serum paraoxonase 1 (PON1) enzyme (Harel et al., Nat. Struct. Mol. Biol. 11:412-419, 2004). The previous state-of-the-art in PON1 performance was achieved mainly by mergers of point mutations chosen in the vicinity of the catalytic pocket (Aharoni et al., Proc. Natl. Acad. Sci. USA 101:482-487, 2004). Individual mutations L69V, H115W and V346A enhanced hydrolysis of organophosphates (OPs) by four- to sixteen-fold higher catalytic efficiency (Amitai et al., FEBS J. 273:1906-1919, 2006). Its variants have been adapted for catalysis and stereoselectivity against multiple OP substrates. Directed evolution of PON1 via DNA shuffling using homologous proteins (Aharoni et al., Proc. Natl. Acad. Sci. USA 101:482-487, 2004) or error-prone-PCR libraries (Gupta and Tawfik, Nat. Methods 5:939-942, 2008) have discovered mutations improving expression, substrate specificity and catalytic efficiency. However, without a means for direct selection for enhanced activity, library size and throughput are bottlenecked by the need for activity assays with individual genotypes expressed and lysed in separate sample wells.
The technique described here (referred to in this example as “smart microbial cells” or SMC; e.g., FIGS. 1A and 1B) allows for catalysis and sensing in the same microbial cell. For a practical library application, we simplified and improved the signal to noise ratio of the biosensor. This was achieved by gene fusing the PON1 to the GFP so that the TF is also regulating the catalytic enzyme expression as well (scheme shown in FIG. 1B). This fusion is expected to have three distinct effects. First, it provides positive feedback, such that more the activity, the more PON1 is expressed, and in turn further ‘feeds’ the activity. The feedback, hence, would enhance the sensitivity to weak activity by raising the signal above both ambient background and leaky expression of the GFP reporter. Notably, when the PON1 is inactive there is no positive feedback, so basal expression will not be increased by the feedback. Second, feedback will amplify the already present correlation between activity and GFP expression. Third, for suitably chosen GFP, the fluorescence signal conveniently reports the amount of fused PON1 present, allowing normalization for specific activity.
A weakly folding GFP, placed at the C-terminal after a protein-of-interest is known to function as folding reporter since a misfolded (insoluble) N-terminal protein will interrupt the formation of the subsequently translated GFP chromophore (Gupta and Tawfik, Nat. Methods 5:939-942, 2008; Waldo et al., Nat. Biotechnol. 17:691-695, 1999). In contrast, our assay specifically suppressed that effect using 1) an N-terminal GFP and 2) a super-folding variant of GFP (Pedelacq et al., Nat. Biotechnol. 24:79-88, 2006). Thus, it directly reports enzyme activity and not misfolding. However, because of the positive feedback, we still get the benefit of sensing misfolding since enzyme inactivity means no feedback for enhanced GFP expression.
The sfGFP-PON1 fusion can catalyze the hydrolysis of paraoxon (PXN) where pNP is a leaving group (FIG. 16B). Based on the catalytic efficiency and the cytoplasmic concentration of the hydrolase, PXN hydrolysis results in a correlated pNP production inside the microbial cell that ultimately determines activation of the pNP sensor and accumulation of sfGFP in the cells. Comparison of SMCs with PON1-G3C9 (SEQ ID NO: 32) (native like) and PON1-H115W mutant grown in PXN showed a higher cell fluorescence in the latter (FIG. 16C), which is consistent with the higher catalytic efficiency observed for PON1-H115W (Amitai et al., FEBS J. 273:1906-1919, 2006). The other published variant PON1-V346A, failed to show higher cell fluorescence than G3C9 expressing cells, even though the variant has higher catalytic efficiency for PXN hydrolysis, possibly due to reduced protein expression and/or slower folding rate caused by the mutation. The PON1 version that combines the two mutations H115W/V346A showed partial recovery of the cell fluorescence in the presence of PXN substrate (FIG. 16C, Table 6). When pNP was used as an inducer, such that the expression of sfGFP-PON1 expression is independent of the catalytic efficiency of PON1, modest differences in cell fluorescence were still captured across the PON1 variants indicating V346A mutation could be detrimental to PON1 expression and stability (FIG. 16C, Table 6).
Table 6. Raw whole cell fluorescence intensitya with pNP sensor and sfGFP-PON1 expression under positive feedback regulation of pNP sensor and measured using a flow cytometer







TABLE 6







Raw whole cell fluorescence intensitya with pNP sensor and sfGFP-PON1


expression under positive feedback regulation of pNP sensor and 


measured using a flow cytometer










PON1 variant
UI
pNP (200 μM)
PXN (1.6 mM)





G3C9
168 ± 5 
7052 ± 124
1761 ± 158


H115W
301 ± 42
13934 ± 802 
10276 ± 386 


V346A
174 ± 29
6263 ± 57 
895 ± 39


H115W/V346A
248 ± 36
11484 ± 2032
2410 ± 669





aMean cell fluorescence intensity from cultures in duplicate. Errors depicted as standard deviation






aMean cell fluorescence intensity from cultures in duplicate. Errors depicted as standard deviation

The SMCs when plated on a solid LB-agar medium supplemented with different induction conditions, recaptured the fluorescence signal observed in the liquid culture (FIG. 16D). While at 100 μM PXN, fluorescent colonies were observed only in case of PON1-H115W and that further gained brightness at 300 μM PXN supplemented growth medium, SMCs expressing PON1-V346A continued to show weaker fluorescence than both PON1-H115W and PON1-G3C9 expressing cells. At 200 μM pNP, irrespective of the PON1 variant expressed, the SMCs were saturated with fluorescence under an illuminator, beyond the capability to distinguish them visually with the naked eye (FIG. 16D).
The next step was to move this technology to library screening where two separate libraries were created using PON1-H115W as a starting scaffold. The first library (Lib1) targeted peripheral mutations on loop 69-74 (based on the structural information of PON1 bound to a competitive inhibitor molecule 2-hydroxyquinoline (PDB code 3SRG) (Table 7). This loop is expected to play a critical role in interaction with the substrate/transition state, since the loop otherwise disordered in apo state, gained structure only in the presence of inhibitor in the pocket. The second library (Lib2) consisted of mutations around the catalytic pocket mostly based on the literature (Table 8). The goal of Lib2 was to discover any combinatorial solution of synergistic mutations, which earlier were tried mostly as single point mutations (FIG. 17A). Theoretical diversity of each library,  ̃20,000 was reasonably matched to the SMC based plate assay, where on a single large petri dish (135 mm diameter) one can have  ̃104 isolated colonies demanding two to four petri dishes for modest over coverage.
Table 7. Mutational library 1a







TABLE 7







Mutational library 1a














L69
K70
Y71
P72
G73
I74





Diversified
DTA
NWWb
NDT
NCN
RST
AHT


codon








Amino
LVI
YHNDQKEFL(3)
RNDCGHILF
S(4)P(4)T(4)A(4)
TSAG
ITN


acids

I(2)V(2)c*
SYV
(SEQ ID NO: 53)
(SEQ





(SEQ ID NO: 51)
(SEQ ID NO:

ID






52)

NO:








54)



Unique
3
12
12
4
4
3


variants











aTheoretical library diversity = 3 × 12 × 12 × 4 × 4 × 3 = 20736


bone-sixteenth variant has premature STOP codon


cNumber in parentheses denotes number of degenerate codons for a particular amino acid






aTheoretical library diversity=3×12×12×4×4×3=20736

bone-sixteenth variant has premature STOP codon

cNumber in parentheses denotes number of degenerate codons for a particular amino acid

Table 8. Mutational library 2a








TABLE 8







Mutational library 2a
















G116
N168
F222
N224
L267
N270
I291
F292





Diversified
RST
VAT
WWT
VAC
VTT
VAC
NTC
NTC


codon










Amino
TSAG
HND
INYF
HND
LIV
HND
FLIV
FLIV


acids
(SEQ ID

(SEQ ID








NO: 54)

NO: 55)







Unique
4
3
4
3
3
3
4
4


variants













aTheoretical library diversity = 4 × 3 × 4 × 3 × 3 × 3 × 4 × 4 = 20736






aTheoretical library diversity=4×3×4×3×3×3×4×4=20736

In order to efficiently screen the library, the biosensor/biocatalyst plasmid consisting of variants of PON1 was directly transferred to high transformation efficiency DH5α cells. The SMC libraries harboring two sets of genetic variations showed a very different display on the plate supplemented with approximately 330 μM PXN. While the peripheral mutations in PON1 incorporated in Lib1, showed multiple colonies distinctly brighter than the surrounding colonies (FIG. 17B), Lib2 consisting of mutations in the catalytic pocket proved to consist of sequences that were mostly detrimental to the activity of PON1. A single colony though stood out from the rest and was picked for further experimental verification (FIG. 17C). The colony picking can further be made more stringent by using lower PXN (for example 100 μM) in the growth media and colonies selected based on the fluorescence intensity under an illuminator.
Whole cell catalysis and sensing using the picked clones at varying concentration of the substrate PXN, showed enhanced contrast ratio (3-15 fold higher than the scaffold) for selected clones from Lib1, while the single picked clone from Lib2 showed a marginal 2-fold higher contrast ratio compared to the scaffold (FIG. 17D). The clones were further confirmed for improved activity by using clear cell lysates that were normalized for sfGFP fluorescence (the enzyme variants expressed as sfGFP-PON1 fusion by inducing the same clones with 200 μM pNP) and assayed for activity at different concentrations of PXN. Over H115W mutation, which was a part of the scaffold, the Lib1 clones showed 2-4 mutations (Table 9) while Lib2-D12 also showed only two mutations (F222I, F292I). Interestingly L69V mutation, which is present in all the Lib1 clones has also been extensively investigated for hydrolysis of many OPs where, in the absence of H115W, four-fold increase in catalytic efficiency was seen over the wild-type (Amitai et al., FEBS J. 273:1906-1919, 2006). The most notable aspect of comparison between the in vivo activity measurements (FIG. 17D) and the in vitro measurements (FIG. 17E) is the relative ranking of the different PON1 variants. While Lib1-C3.3 did show highest Vmax in both formats of experiment, the Lib2-D12 was the lowest in both cases. Lib1-C3.3 also had highest expression and solubility next only to the starting scaffold (showing 95% yield), Lib2-D12 showed a low 50% protein yield compared to the scaffold based on the fluorescence of the cell lysate. The solubility of other PON1 variants ranged between these two extreme values.
Table 9. Observed mutations in the improved colonies from Lib1







TABLE 9







Observed mutations in the improved colonies from Lib1













Position#
69
70
71
73







Native
L
K
Y
G



A3  
V
Y
V




A9  
V
Q
I




A12 
V
L

S



B7  
V
Q
I
A



B12 
V
H
V
S



D10 
V
L





C2.2
V
Q
V
A



C3.3
V
Q
L
A



C4.1
V
F
V










Finally, the gene for top PON1 variant Lib1-C3.3 (H115W, L69V, K70Q, Y71L, G73A) and the scaffold (H115W) were transferred to a pRSF-1b vector (Novagen) and expressed as a C-terminal 6×His-tagged protein. Post two-step purification using affinity column and a size exclusion chromatography, pure fractions were combined and tested for activity at varying concentrations of PXN. At a ten-fold lower protein concentration in an enzymatic assay using 400 μM of PXN for paraoxonase activity, the linear rate of Lib1-C3.3 variant exceeded that of PON1-H115W by three-fold, confirming that the turnover number of the new variant is approximately thirty-fold higher than the starting sequence (FIG. 17F). With some challenges in determining Km values of the PON1 variants, especially in case of weaker enzyme (PON1-H115W), one to two-orders of magnitude of enhancement in catalytic efficiency (kcat/Km) was observed in the new version of PON1 (FIG. 17G).
The work presented here summarizes a method to navigate through a large combinatorial space encountered during enzyme evolution. Using this method, we can screen multiple combinations of mutations which otherwise are difficult to explore with conventional methods of mutagenesis one at a time and later combining the mutations for an additive effect. We showed that our method could pull out the L69V mutation from the library and determined it to be beneficial along with as many as 1-3 mutations in its vicinity. Mutations at K70 observed in PON1 variants with gain-of-function would have been counter-intuitive in structure-based mutagenesis studies since the residue is pointing away from the catalytic pocket. The ability to screen multiple mutations all at once and visually select without need of any sophisticated instrumentation, allows for screening many mutations at a time for finding a synergistic set of alterations with large gain-of-function.
In view of the many possible embodiments to which the principles of the disclosure may be applied, it should be recognized that the illustrated embodiments are only examples and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.

## Claims

1. A biosensor comprising:
(i) a protocatechuic acid (PCA) biosensor comprising a nucleic acid encoding a PcaU protein comprising one or more amino acid substitutions at an amino acid corresponding to amino acid position 147, an amino acid corresponding to amino acid position 148 of SEQ ID NO: 56, or both,
wherein the PcaU protein comprises at least 95% sequence identity to SEQ ID NO: 5;
(ii) a cis,cis-muconic acid (ccMA) biosensor comprising a nucleic acid encoding a CatM protein comprising an amino acid substitution at two or more of amino acid positions corresponding to amino acids 97, 127, 128, and 147 of SEQ ID NO: 14;
a CatM-regulated promoter; and
a nucleic acid encoding a reporter protein operably linked to the promoter,

wherein the CatM protein is encoded by a nucleic acid sequence with at least 90% sequence identity to nucleotides 990-1898 of SEQ ID NO: 7; or
(iii) a β-ketoadipic acid (BKA) biosensor comprising a nucleic acid encoding a PcaR protein comprising an amino acid substitution at two or more of amino acid positions corresponding to amino acids 172, 238, 240, and 257 of SEQ ID NO: 19;
a PcaR regulated promoter; and
a nucleic acid encoding a reporter protein operably linked to the promoter,

wherein the PcaR protein is encoded by a nucleic acid sequence with at least 90% sequence identity to SEQ ID NO: 19.

2. The PCA biosensor of claim 1, wherein the nucleic acid encodes glycine, threonine, or proline at amino acid position 147, tyrosine or phenylalanine at amino acid position 148, or both.

3. The PCA biosensor of claim 1, wherein:
the PcaU protein is encoded by a nucleic acid sequence comprising at least 90% sequence identity to SEQ ID NO: 4.

4. A vector comprising:
the PCA biosensor of claim 1;
a promoter comprising or consisting of nucleotides 1824-2104 of SEQ ID NO: 2; and
a nucleic acid encoding a reporter protein operably linked to the promoter.

5. The vector of claim 4, wherein the vector comprises a nucleic acid sequence with at least 90% sequence identity to SEQ ID NO: 2 or SEQ ID NO: 3.

6. A cell comprising the PCA biosensor of claim 1.

7. The ccMA biosensor of claim 1, wherein the promoter comprises nucleotides 1899-2021 of SEQ ID NO: 7.

8. A vector or a cell comprising the ccMA biosensor of claim 7.

9. The vector of claim 8, wherein the vector comprises a nucleic acid sequence with at least 90% sequence identity to SEQ ID NO: 7.

10. The BKA biosensor of claim 1, wherein the promoter comprises the nucleic acid sequence of SEQ ID NO: 8.

11. A vector or a cell comprising the BKA biosensor of claim 10.