WO2020089235A1

WO2020089235A1 - Assay for screening of recombinant cells and microbes

Info

Publication number: WO2020089235A1
Application number: PCT/EP2019/079542
Authority: WO
Inventors: Sven Panke; Tsvetan KARDASHLIEV; Jacqueline MOCK; Martin Held
Original assignee: ETH Zürich
Priority date: 2018-10-29
Filing date: 2019-10-29
Publication date: 2020-05-07

Abstract

The present invention relates to a method for rapid screening of recombinant cells for improved enzymes or pathways or the identification of molecular binders employing the taxis machinery of the clones, whereby the clones are separated based on a difference of their mobility as compared to a background.

Description

Assay for Screening of Recombinant Cells and Microbes

The present invention relates to a method for rapid screening of recombinant cells for improved enzymes or pathways or for the identification of molecular binders employing the taxis machinery of clones, whereby the clones are separated based on a difference of their mobility as compared to a background.

Description

The conditional assembly of taxis employed by bacteria in order to move into the direction of potentially favorable environments, or to move away from unfavorable ones, has previously been used for the detection of environmental signals or diagnostic markers. In currently known settings, the compound that leads to the induction of the system employed for the conditional reassembly of the taxis machinery of the clones is added from the outside to the cells while aiming for instance at the detection of compounds of potential value for diagnostic or environmental analytic purposes.

Based on the above-mentioned state of the art, the objective of the present invention is to provide means and methods for rapid screening of recombinant cells for enzymes or pathways or for the identification of molecular binders. This objective is attained by the subject-matter of the independent claims of the present specification.

Terms and definitions

The term polypeptide in the context of the present specification relates to a molecule consisting of 50 or more amino acids that form a linear chain of amino acids connected by peptide bonds. The amino acid sequence of a polypeptide may represent the amino acid sequence of a whole (as found physiologically) protein or fragments thereof.

The term peptide in the context of the present specification relates to a molecule consisting of up to 50 amino acids, in particular 6 to 30 amino acids, more particularly 8 to 15 amino acids that form a linear chain wherein the amino acids are connected by peptide bonds.

In the context of the present specification, the term aptamer refers to oligonucleotide capable of specifically binding to another molecule or target with high affinity. An aptamer binds to its target similarly to the specific binding of an antibody. According to the invention, an aptamer is coupled to an mRNA molecule capable of regulating gene expression on the translational or transcriptional level such as a ribozyme, a riboswitch or a hammerhead ribozyme.

The terms gene expression or expression, or alternatively the term gene product, may refer to either of, or both of, the processes - and products thereof - of generation of nucleic acids (RNA) or the generation of a peptide or polypeptide, also referred to transcription and translation, respectively, or any of the intermediate processes that regulate the processing of genetic information to yield polypeptide products. The term gene expression may also be applied to the transcription and processing of an RNA gene product, for example a regulatory RNA or a structural (e.g. ribosomal) RNA. If an expressed polynucleotide is derived from genomic DNA, expression may include splicing of the mRNA in a eukaryotic cell. Expression may be assayed both on the level of transcription and translation, in other words mRNA and/or protein product.

In the context of the present specification, the term promoter refers to a region of DNA that initiates transcription of a particular gene. A promoter is located upstream near the transcription start site of a gene. Upon activation, a promoter initiates the transcription of a or several genes. It is understood that the promoter is operable in a cell.

In the context of the present specification, the term taxis-enabling gene refers to a gene required for directional taxis of an organism. If the taxis-enabling gene is expressed to protein, the organism is able to move towards or away from a stimulus. The taxis-enabling gene encodes a protein necessary for sensing the stimulus, or for transmitting the signal of a stimulus, or for the formation of the flagellar apparatus.

The term variant refers to a polypeptide that differs from a reference polypeptide, but retains essential properties. A typical variant of a polypeptide differs in its primary amino acid sequence from another, reference polypeptide. Generally, differences are limited so that the sequences of the reference polypeptide and the variant are closely similar overall and, in many regions, identical. A variant and reference polypeptide may differ in amino acid sequence by one or more modifications (e.g., substitutions, additions, and/or deletions). A substituted or inserted amino acid residue may or may not be one encoded by the genetic code. A variant of a polypeptide may be naturally occurring such as an allelic variant, or it may be a variant that is not known to occur naturally.

Summary of the invention

The invention relates to a method for rapid identification and isolation of clones that display a certain phenotype, comprising the steps of:

A.) Coupling the expression of the desired phenotype to the amount of at least one enzyme required for the assembly of a functional taxis machinery in the cells;

B.) Transferring the cells to a device allowing the separation of cells that feature a degree of taxis above a certain threshold from those that do not;

C.) Adjusting conditions under which the phenotype is expressed and the taxis machinery proteins are synthesized in those cells that do express it; D.) Allowing the cells to separate;

E.) Isolating the cells that displayed the phenotype.

In other words, a gene product of interest is able to influence the taxis machinery of the cell, in a way that the taxis machinery is only functional when the gene product of interest has an activity or a certain property above a threshold. It can be assayed, whether the taxis machinery of the cell is functional, in a device allowing for the distinction between cells that move and cells that do not move. Moving cells are separated from non-moving cells and the gene corresponding to the gene product of interest is sequenced.

The method of the invention can be applied to recombinant prokaryotic or eukaryotic cells. It allows the qualitative or (semi-)quantitative detection of compounds synthesized within the cells. In one embodiment, the taxis-inducing phenotype is the formation of an unknown compound capable of interacting with a polynucleotide or polypeptide-based drug target. The drug target has been integrated into a regulatory system such that the taxis genes are only expressed in its presence. This embodiment is of particular interest for drug development efforts. In another embodiment, the compound is known and the taxis-inducing phenotype is the mere presence of the compound or its presence above a desired threshold. In this embodiment, the enzymes employed for formation of the compound are of interest especially for the development of strains for fermentation processes or enzymes for biocatalysis.

Detailed description of the invention

A first aspect of the invention relates to a method for selecting a variant of a polypeptide, wherein said polypeptide catalyses conversion of a substrate to a product.

The method comprises

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a nucleic acid sequence encoding a variant of said polypeptide under control of a first promoter operable in said cell, wherein each member of said plurality of cells encodes a different variant;

comprises a taxis-enabling gene, wherein the protein encoded by said taxis- enabling gene enables tactic behavior of said cell in reaction to a stimulus, wherein said taxis-enabling gene is under control of a second, inducible promoter, and

said second promotor is activated, directly or indirectly, by the product; b) exposing said plurality of cells to said stimulus; and c) selecting a cell capable of exceeding a threshold of tactic behavior from said plurality of cells and isolating the selected cell;

d) isolating said nucleic acid sequence encoding a variant of said polypeptide from said selected cell.

An alternative of this first aspect of the invention relates to a method for selecting a polypeptide, wherein the polypeptide catalyses the conversion of a substrate to a product.

Herein, the method comprises

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a nucleic acid sequence encoding for a polypeptide which is synthesized by said cell, wherein each member of said plurality of cells encodes a different variant;

comprises a taxis-enabling gene, wherein the gene-product encoded by said taxis-enabling gene enables tactic behaviour of said cell in reaction to a stimulus;

wherein the cellular content to which the gene-product is synthesized by the cell, is under control of the cellular content of the product;

wherein the cellular content of the product and the cellular content of the gene-product are positively or negatively correlated;

in other words, the concentration of the gene-product depends on the concentration of the product and either

i. the more product is produced, the more gene-product is produced; or ii. the more product is produced, the less gene-product is produced wherein the control is at the level of the transcription of the taxis-enabling gene into mRNA or at the level of the translation of the said mRNA into the gene-product;

wherein the control is either direct or indirect and

b) exposing said plurality of cells to said stimulus; and

c) selecting a cell capable of exceeding a threshold of tactic behavior from said plurality of cells, and isolating the selected cell;

d) isolating said nucleic acid sequence encoding a variant of said polypeptide from said selected cell. In other words, the invention provides a method for screening a library. The phenotype of the library member is read out via the interaction of the polypeptide’s product with an inducible promoter of a taxis-enabling gene.

Variants of a polypeptide comprise either one polypeptide with single or multiple amino acid substitutions, deletions, or insertions or different polypeptides encoded by a metagenomics library.

In certain embodiments, said polypeptide is an enzyme catalysing the conversion of a substrate to a product. Said product, directly or indirectly, activates a second, inducible promoter. Direct activation is achieved via binding of the product to an activator of gene expression, which upon binding the product initiates gene expression. Direct activation may also be achieved via binding of the product to a repressor of gene expression, which upon binding the product releases the promoter thereby initiating gene expression. Indirect activation is achieved by binding of the product to an initiator of a signalling cascade that eventually starts gene expression of the taxis-enabling gene.

The activation of the taxis-enabling gene leads to restoration of a functional taxis machinery of the cell. The taxis-enabling gene employed to read out the phenotype can be one of a great variety of functional components of the machinery that allows the cell to move towards -or away from- a stimulus. When the cell is exposed to said stimulus, only the cells having an active polypeptide that produces a product will move towards or away from the stimulus. These cells are selected and their nucleic acid sequence encoding the polypeptide is isolated and sequenced.

In certain embodiments, the polypeptide is selected from the classes of oxidoreductases (E.C.1 ), transferases (E.C.2.), hydrolases (E.C.3.), lyases (E.C.4.), isomerases (E.C.5.), and ligases (E.C.6.). In certain embodiments, the polypeptide is used for industrial purposes. In certain embodiments, the polypeptide is selected from glucose oxidase (E.C. 1.1.3.4.), laccase (E.C. 1.10.3.2), catalase (E.C. 1.1 1.1.6.), lignin peroxidase (E.C. 1 .1 1.1 .7), manganese peroxidase (E.C. 1 .1 1.1.13), lipoxygenase (E.C. 1 .13.1 1.12.), transglutaminase (E.C. 2.3.2.13), cyclodextrine glucanotransferase (E.C. 2.4.1.19), glucosyltransferase (E.C. 2.4.1.24), esterase (E.C. 3.1.1.1.), triacylglycerol-lipase (E.C. 3.1 .1.3), phospholipase A (E.C. 3.1.1.4), phospholipase B (E.C. 3.1.1 .5), pectin esterase (E.C. 3.1 .1.1 1 ), tannase (E.C. 3.1.1.20), monoacylglycerol-lipase (E.C. 3.1.1 .23), phosphatase (E.C. 3.1.3.2), phytase (E.C. 3.1.3.8.), phosphodiesterase (E.C. 3.1.4.1 ), amylase (E.C. 3.2.1.1 .), glucoamylase (E.C. 3.2.1.3.), cellulase (E.C. 3.2.1.4.), beta-glucanase (E.C. 3.2.1.6.), inulase (E.C. 3.2.1.7), xylanase (E.C. 3.2.1.8), dextranase (E.C. 3.2.1 .1 1 ), pectinase (E.C. 3.2.1 .15), alpha- glucosidase (E.C. 3.2.1.20), beta-glucosidase (E.C. 3.2.1.21 ), alpha-galactosidase (E.C. 3.2.1.22.), lactase (E.C. 3.2.1.23), invertase (E.C. 3.2.1.26), exo-xylanase (E.C. 3.2.1.37), alpha-rhamnosidase (E.C. 3.2.1.40), pullalanase (E.C. 3.2.1.41 ), arabinofuranosidase (E.C. 3.2.1.55), exo-1 ,3-beta glucosidase (E.C. 3.2.1.58), endo-1 ,4-beta-mannanase (E.C. 3.2.1. 78), cellobiohydrolase (E.C. 3.2.1.91 ), arabinanase (E.C. 3.2.1.99), metalogenic amylase (E.C. 3.2.1.133), cymosin (E.C. 3.4.23.4.), glutaminase (E.C. 3.5.1.2), penicillin amindase (E.C. 3.5.1.11 ), aminoacylase (3.5.1.14), aminopeptidase (E.C. 3.4.11.X), AMP deaminase (E.C. 3.5.4.6.), nitrilases (E.C. 3.5.5.1.), acetolactate decarboxylase (E.C. 4.1.1.5.). pectat lyase (E.C. 4.2.2.2), pectin lyase (E.C. 4.2.2.10), or glucose isomerase (E.C. 5.3.1.5.). In certain embodiments, the polypeptide is an enzyme encoded by metagenomics libraries.

A second aspect of the invention relates to a method for selecting a variant of a polypeptide, wherein said polypeptide is capable of binding to a transcriptional regulatory protein, thereby modulating the activity of an inducible promotor.

The method comprises

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a nucleic acid sequence encoding a variant of said polypeptide under a promoter operable in said cell, wherein each member of said plurality of cells encodes a different variant;

comprises a taxis-enabling gene wherein the protein encoded by said taxis- enabling gene enables tactic behavior of said cell in reaction to a stimulus, wherein said taxis-enabling gene is under control of a second, inducible promoter; and

said second promotor is activated, directly or indirectly, by the polypeptide; b) exposing said plurality of cells to said stimulus; and

c) selecting a cell capable of exceeding a threshold of tactic behavior from said plurality of cells and isolating the selected cell;

An alternative of this second aspect of the invention relates to a method for selecting a polypeptide, wherein said polypeptide is a binder.

Herein, the method comprises

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a nucleic acid sequence encoding for a polypeptide which is synthesized by said cell, wherein each member of said plurality of cells encodes a different variant, comprises a taxis-enabling gene, wherein the gene-product encoded by said taxis-enabling gene enables tactic behaviour of said cell in reaction to a stimulus,

wherein the level to which the gene-product is synthesized by the cell is determined by the affinity of the binder for a target,

wherein the affinity of the binder for the binding site and its cellular content and the cellular content of the gene-product are positively or negatively correlated,

in other words, the concentration of the gene-product depends on the affinity of the binder and either

i. the higher the affinity of the binder for the binding site, the more gene- product is produced; or

ii. the higher the affinity of the binder for the binding site, the less gene- product is produced

wherein the target regulates the level of the transcription of the taxis-enabling gene into mRNA or the level of the translation of the said mRNA into the gene- product,

wherein the control is either direct or indirect, and

b) exposing said plurality of cells to said stimulus; and

c) selecting a cell capable of exceeding a threshold of tactic behavior from said plurality of cells [and isolating the selected cell];

In other words, the invention provides a method for screening a library of the polypeptide of interest. The phenotype of the library member is read out via the interaction of the polypeptide with a transcriptional regulatory protein of a taxis-enabling gene.

Said polypeptide activates the second promoter by binding to a transcriptional regulatory protein. The transcriptional regulatory protein may directly bind to the promoter region and initiate gene expression, or the transcriptional regulatory protein may indirectly via binding other regulatory factors or by initiating a signalling cascade activate the promoter and initiate gene expression.

A third aspect of the invention relates to a method for selecting a variant of a polypeptide, wherein said polypeptide catalyses conversion of a substrate to a product, wherein said product is capable of binding to a translational regulator, thereby modulating the translation efficiency of

i. an mRNA encoded by a taxis-enabling gene or

ii. an mRNA encoded by a constitutively expressed taxis-enabling gene repressor, wherein the taxis-enabling gene repressor inhibits gene expression of the taxis-enabling gene.

The method comprises

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a taxis-enabling gene, wherein the protein encoded by said taxis- enabling gene enables tactic behavior of said cell in reaction to a stimulus, wherein

i. translation to protein of an mRNA encoded by said taxis-enabling gene is under control of a translational regulator, and said translational regulator is activated by the product of the polypeptide, or ii. translation to protein of an mRNA encoded by said taxis-enabling gene repressor is under control of a translational regulator, and said translational regulator is deactivated by the product of the polypeptide, resulting in reduced repression of said taxis-enabling gene and b) exposing said plurality of cells to said stimulus; and

A translational regulator is a functional moiety that controls translation of a certain mRNA. In certain embodiments, a translational regulator is an aptamer. In certain embodiments, the aptamer binds to the product of said polypeptide and thereby is activated or deactivated.

Direct activation is achieved by a polypeptide’s product binding to a region of the mRNA of a taxis-enabling gene, which leads to increased translation of said mRNA and thus, it leads to restoration of the taxis machinery of the cell. The taxis-enabling gene is transcribed into mRNA, which is under control of a translational regulator. In certain embodiments, the translational regulator influences the initiation of translation. Binding of the product of the polypeptide causes a processing of the mRNA, which in turn makes the Shine-Dalgarno sequence accessible to the ribosome. This causes the tactic behavior mRNA to be translated.

Indirect activation can be effected by the polypeptide’s product binding to a region of the mRNA of a taxis-enabling gene repressor, which leads to decreased translation of said taxis-enabling gene repressor mRNA. This in turn causes the taxis-enabling gene to be transcribed in absence of the repressor and thus, it leads to restoration of the taxis machinery of the cell.

There are several taxis-enabling gene repressors known in the art. The taxis-enabling gene repressor is a polypeptide, which represses the transcription of a taxis-enabling gene by binding to a region on or near the taxis-enabling gene DNA sequence.

In certain embodiments, said taxis-enabling gene is a taxis-enabling gene selected from a prokaryote organism selected from any one of the genera Bacillus, Pseudomonas, Escherichia, and Salmonella. In certain embodiments, said taxis-enabling gene is a taxis- enabling gene selected from any one of the species Escherichia coli, Pseudomonas aeroguinosa, Pseudomonas fluorescence, Pseudomonas stutzeri, Pseudomonas pseudoalcaligenes, Pseudomonas spiringae, Pseudomonas putida, Pseudomonas oleovorans, Salmonella thyphimurium, Bacillus subtilis, or Bacillus licheniformis. In certain embodiments, said taxis-enabling gene is a taxis-enabling gene selected from the below list of bacterial genes encoding for

a) any one gene having a function in a prokaryote’s detection of an exogenous stimulus. In certain embodiments, the gene has a function in a prokaryote’s detection of an exogenous small chemical compound in the environment of the bacterium, or a gene having a function in intracellular signalling in response to the exogenous stimulus. In other words, the gene has a function in the machinery employed for sensing of compounds and signalling. In certain embodiments, the gene is selected from Escherichia coli and Salmonella thyphimurium chemoreceptors Tar, Tap, Trg, Tsr, the Bacillus subtilis and Bacillus licheniformis chemoreceptors HemAT, McpA, McpB, TlpA, TlpB, McpC, TlpC, YvaQ, YfmS, YoaH, the Pseudomonas oleovorans, Pseudomonas putida, Pseudomonas fluorescence, Pseudomonas aeroguinosa chemoreceptors McpA, McpB, PilJ, WspA, PctA, PctB, PctC, CtpH, CtpL, McpS, McpK, PA1251 , PA1423, Pa1561 , PA1608, PA1646, PA2573, PA2652, PA2654, PA2788, PA2867, PA2920, PA4290, AP4530, PA4633, PA4915, and conserved bacterial signalling proteins CheA, CheB, CheC, CheD, CheR, CheV, CheW, CheZ, or FliY. b) a protein involved in the formations of the flagellar apparatus. In certain embodiments, the protein is involved in assembly of the flagella filaments extending into the extracellular space, the base of the flagella-stand anchoring the flagella in the bacterial wall and extending into the cytosol, or the assembly of the machinery employed for generation of the torque enabling rotation of the flagella. In certain embodiments, the protein is selected from FliD, FliC, FlgL, FlgK, FliK, FlgD, FlgG, FlgF, MotY, MotX, FlgH, FlgL, MotB, MotA, FliE, FlgB, FlgC, FliF, FUG, FliM, FlhB, FlhA, FiH, FliL, FliO, FluQ, FliT, FliS, FliJ, FliN, FliA, FliP, FUR, FlhC, FlhD, FlgM, and FliA.

In certain embodiments, said stimulus is a concentration gradient of a chemical compound. Either the presence of a compound causes positive tactic behavior or the absence of a compound causes negative tactic behavior. In certain embodiments, the chemical compound is selected from:

a list of chemoattractants reported for Escherichia coli, Pseudomonas aeroguinosa, Pseudomonas fluorescence, Pseudomonas stutzeri, Pseudomonas pseudoalcaligenes, Pseudomonas spiringae, Pseudomonas putida, Pseudomonas oleovorans, Salmonella thyphimurium, Bacillus subtilis, or Bacillus licheniformis. In certain embodiments, the compound is selected from sugars, aminoacids, acids, di- or oligopeptides, sugars, di- or oligosaccharides, and environmental chemicals. In certain embodiments, the compound is selected from acetate, butyrate, citrate, glutaric acid, indole acetic acid, malate, succinate, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryoptophane, tyrosine, valine, 2- aminobenzoate, 3-aminobenzoate, 4-aminobenzoate, arogenate, benzene, benzoate, benzonitrile, benzoylformate, m-bromotoluene, p-bromotoluene, butylbenzole, catechol, 4-chloroaniline, 2-chlorobenzoate, 3-chlorobenzoate, 4-chlorobenzoate, o- chlorotoluene, m-chlorotoluene, p-chlorotoluene, 2,4-dichlorophenoxyacetate, 2,3- dimethylphenol, 3,4-dimethylphenol, ethylbenzene, p-ethyltoluene, fluorobenzene, o- fluoroptoluene, m-fluoroptoluene, p-fluoroptoluene, hydroquinone, p- hydroxymandelate, o-iodotoluene, m-iodorotoluene, p-iodorotoluene, isopropylbenzene, mandelate, 3-methylcatechol, 4-methylcatechol, 2-methylphenol, 3- methylphenol, nitrobenzene, 2-nitrobenzoate, 3-nitrobenzoate, 4-nitrobenzoate, 4- nitrocatechol, p-nitrophenol, o-nitrotoluene, m-nitrotoluene, phenol, phenylactic acid, phenoxyacetate, beta-phenylbutyrate, propylbenzene, salicylate, o-toluate, m-toluate, p-toluate, toluene, o-toluidine, m-toluidine, p-toluidine, 2,4,5-trichlorophenoxyacetate, 1 ,2,4-trimethylbenzene, 1 ,3,5-trimethylbenzene, o-xylene, m-xylene, biphenyl, 2- chlorobiphenyl, 3-chlorobiphenyl, 4-chlorobiphenyl, 2,3-dichlorobiphenyl, cis-1 ,2- dichloroethane, trans-1 ,2-dichloroethane, 1 ,1-dichloroethane, ethylene, trichloroethylene, tetrachloroethylene, vinyl chloride, furfural, furfural alcohol, furoic acid, 5-hydroxymethylfurfural, dichloromethane, chloroform, trichloroethane, cytidine, cytosine, thymidine, thymine, uracil, uridine, ametryn, atrazine, cyanuric acid, N- iospropylammelidine, gamma-aminobutyric acid, alpha-aminoisobutyrate, ammonium, ammonium chloride, fumarate, galactose, glucose, maleate, maltose, methylthioisocyanate, methylparathion, morphine, naphthalene, oxygen, phosphate, ribose, 1 ,2,3,4-tetrahydronaphthalene, thiocyanic esters, isothiocyanic esters, dilaurosylphosphatidylethanolamine, dioleoylphosphatidylethanolamine, phosphatidylethanolamine, palmitoleic acid, oleic acid, arachidonic acid, N-acetyl-D- glucosamine, D-fructose, 1-D-glycerol- beta,-D-galactoside, 6-deoxy-D-glucose, methyl-beta-D-glucoside, D-fucose, methyl-beta-D-galactoside, methyl-beta-D- glucoside, L-arabinose, D-xylose, L-sorbose, 2-deoxy-D-glucose, D-glucosamine, D- mannose, D-glucosamine, 2-deoxy-D-glucose, methyl-beta-D-glucoside, methyl- alpha-D-glucoside, D-mannitol, D-sorbitol, D-ribose, D-ribulose, trehalose and their halogenated derivatives or Ni²⁺, Cu²⁺, Cd²⁺, Zn²⁺.

In certain embodiments, the taxis-enabling gene is activated in response to a binding event of a polypeptide or polynucleotide based transcriptional or translational regulator, particularly a transcriptional regulatory protein;

a regulated promoter sequence;

an auxiliary regulatory protein component;

a riboswitch, or an siRNA.

An auxiliary regulatory protein component is a protein regulating gene expression by binding certain factors that allow the RNA polymerase to initiate mRNA production.

A riboswitch is a regulatory segment of an mRNA that binds a small molecule, which results in a change of production of the protein encoded by said mRNA.

An siRNA may regulate the second promoter by silencing a repressor of said promoter.

In certain embodiments, said cells capable of exceeding a threshold of directional tactic behavior are selected via

a disc diffusion assay,

a Boyden chamber,

a Zigmond chamber,

a Dunn chamber,

a thermotaxis or a magnetotaxis assay. In certain embodiments, a gel-like surface for cultivation of the bacteria is employed.

The disc diffusion assay is performed on agar plates of PP-chambers employing parallel or radial arranged wells and channels.

The Boyden chamber relies on at least two chambers, employing a porous membrane as a mobility barrier between two compartments.

The Zigmond chamber relies on a narrow bridge between two horizontal chambers comprised by a very thin layer of medium selectively allowing chemotactic cells to delocalize from one compartment to the other.

The Dunn chamber relies on a narrow bridge between concentric rings comprised by a very thin layer of medium selectively allowing chemotactic cells to delocalize from one compartment to the other.

In certain embodiments, the bridge between the two chambers is filled with agar and cells have to "glide" in this semisolid layer.

In certain embodiments, said threshold of directional, tactic behavior exceeds the average tactic behavior level by at least 10 % when being subjected to any of the assay conditions outlined in the last paragraph. In certain embodiments, said threshold of directional, tactic behavior exceeds the average tactic behavior level by at least 30 % when being subjected to any of the assay conditions outlined in the last paragraph. In certain embodiments, said threshold of directional, tactic behavior exceeds the average tactic behavior level by at least 80 % when being subjected to any of the assay conditions outlined in the last paragraph. The average tactic behavior level is the average of the directional, tactic movement performed by all cells employed.

Surprisingly, it was found that taxis proteins expressed in single cells of prokaryotic or eukaryotic origin can be used as marker for the identification of cells featuring a certain phenotype in a mixture of cells. Phenotypes such as the degree to which specifically engineered receptors serving as a model for a human drug target are influenced by potential binders, the activity of enzyme catalysts capable of catalyzing a certain biotransformation (e.g. hydrolysis of an ester, amination of a ketone or the hydroxylation of an alkane), or the successful de-bottlenecking of multistep pathways performed by microbial strains for synthesis of a product of interest can be coupled to the taxis machinery in such a way that a qualitative or quantitative relationship between the degree of mobility of the cells and the level to which the wanted phenotype is exerted can be obtained. According to the invention, cells are incubated under conditions under which tactic and non- tactic cells can be discriminated from each other. Such conditions can for instance be adjusted in a so called swarm plate assay (see e.g. Fig. 4). Generally, a gradient of an attractant is required in order to allow the cells to discriminate between increasingly favorable and unfavorable environments in the course of the tactic movement. In one embodiment, this gradient is of a chemical nature and is formed by compounds that either repel or attract the cells leading to the phenomenon of topotaxis or phobotaxis, respectively. Compounds such as nucleotides, amino acids, dipeptides, and monosaccharides for instance are known to attract certain members of the enterobacteriaceae such as E. coli or S. thyphimurium while melatonin, spermidine and phenols (above certain concentration threshold) rather efficiently repel them. If the gradient is made of an attractant, the cells will move towards those regions in which the abundance of the attractant is high. On the contrary, if the gradient is made of a repellent, the cells will move towards the regions in which the repellent is underrepresented. In any case, tactic cells follow the gradient with the aim of an improvement of their immediate environment. Thus, chemotactic cells, for instance, move towards compounds which either serve as a nutrient or are indicative for nutrient rich regions. Similarly, aerotactic cells will move towards oxygen-rich regions in order to take advantage of the oxygen as terminal electron acceptor enabling aerobic growth while thermotactic cells move along temperature gradients in order to grow faster or magnetotactic cells follow magnetic field gradients with the aim of entering nutrient rich sediments encountered for instance in fresh water ponds.

The machineries employed by cells in order to display tactic behavior comprise elements for sensing of the stimulus, hereinafter“sensor machinery”, and elements employed for movement of the cell, hereinafter“mobility machinery”. Each of these elements requires a plurality of cellular proteins to function in a cooperative and well-coordinated fashion. The chemotaxis machinery of E. coli, for instance, comprises at least 12 genes encoding for proteins required for assembly of the sensory machinery while more than 50 genes are encoding for gene products involved in the assembly and impulsion of the mobility machinery. The knockout of any of them leads to cells that do not display chemotactic behavior anymore. In the case of a deactivation of the parts of the mobility machinery, chemotaxis is abolished altogether. However, as most cells have more than one sensor system, deactivation of the parts of the sensor machinery may only lead to the deactivation of the tactic behavior towards a subset of the stimuli that may trigger tactic behavior (Fig. 2).

Deactivation of any of the two machineries required by the cells in order to enable tactic movement is rather simple and normally a single gene knockout suffices in order to destabilize the complex network to the extent that is required in order disable taxis behavior entirely or under a specifically defined condition. Interestingly, state of the art molecular engineering approaches also allow the heterologous transfer of the taxis machinery among species. As immediately apparent to those skilled in the art, attribution of tactic behavior to a cell hence can for instance be easily achieved by the introduction of a gene capable of complementing a previously introduced knockout or by the heterologous transfer of genes attributing taxis to cells that are otherwise foreign to them. In the following, the inventors will refer to the said genes as“taxis marker genes”. Different techniques are known in the art of genetic engineering in order to reach that goal but the most common ones are the expression of the missing taxis marker genes from plasmids that also express an antibiotic or auxotrophy marker and are added to the cells by transformation and selection protocols. If the promoter employed for the regulation of the expression of the said taxis marker gene can be regulated by a stimulus then tactic behavior also becomes an inducible feature.

Surprisingly, the inventors found that the regulation of the said taxis marker gene can be coupled to the display of a phenotype of interest. Cells are frequently used as tools for the rapid phenotypic assessment of large DNA sequence space in a process frequently referred to as screening. In the course of screenings larger fractions of DNA are sampled in order to identify those DNA fragments that encode for proteins that lead to the wanted phenotype. To the corresponding genes are herein referred to as“phenotype-implicating genes” and to the encoded proteins as“phenotype-implicating proteins”. According to the invention, phenotype- implicating proteins either directly or indirectly interact with the regulatory machinery employed for the regulation of the expression of the taxis marker gene, whereby to the regulatory machinery hereinafter is referred to as“taxis regulators”. In case of a direct interaction, the phenotype-implicating proteins may for instance bind directly to the taxis regulators and thereby upregulate the expression of the taxis marker gene. In this embodiment the taxis regulators preferably comprise at least one protein for which a ligand is sought (see Fig 1 -11). Examples for such proteins are disease associate targets such as the binding domain of G- protein coupled or tyrosine-kinase receptors or ion channels or transport channels or structural proteins or transcriptional regulator proteins or nucleic acids. Examples for regulator configurations enabling induction of the taxis gene upon binding of a ligand to the targets in bacteria are systems based on the fusion of the lambda repressor C1 and the RNA polymerase alpha or on the dimerization of LexA proteins. Both systems allow coupling of translation to protein binding and to gene expression.

In another embodiment, the phenotype-implicating protein synthesizes a compound that then induces taxis (Fig 1-1 and 3). In this case, the taxis regulator contains a binding domain with affinity for the said compound and the phenotype-implicating protein is an enzyme exerting a catalytic function. Examples of the said taxis regulators are NahR-LysR family regulators that are activated by amino acids (e.g. ArgP from E. coli, LysG from C. glutamicum), or XylR-NtrC- type regulators that respond to alcohols (e.g. BmoR from P. butanovora) or TetR-type regulators for flavonoids (e.g. QdoR from B. subtilis) or XylS-Ara family regulators responsive to organic acids (e.g. mutant AraC proteins from E. coli andS. typhimurium responsive to mevalonate and ectoine). In cases where the taxis regulators are translation repressors rather than inducers, the signal can also be reverted e.g., by first repressing the translation of another repressor (e.g. the TetR protein) which then discontinues to repress its cognate promoter which can then be used for regulation of the expression of the taxis marker gene (Fig 7).

The enzyme is preferably acting on metabolites formed by the cell or on compounds that are fed to the cell from the outside. In the case of metagenome screenings performed in order to identify enzymes capable of performing a specific conversion of a compound to a desired product, presence of any enzyme performing the conversions is the desired phenotype. In this case, preferable highly sensitive taxis regulators capable of detecting even a few molecules of the desired product are used as here marginal enzyme activities are already of interest. In the case of a screening for an improvement of the catalytic efficiency of enzymes, an elevated level of the compound over a background such as represented by the amount of a product formed by the parent enzyme is the desired phenotype. In this case, preferably rather insensitive taxis regulators are used as otherwise the background activity of the parent enzyme would already lead to the accumulation of sufficiently large amounts of the product so as to activate the taxis regulator and induce expression of the taxis marker gene. In yet another embodiment, the phenotype-implicating protein catalyzes a reaction that leads to a diversion of the metabolite flux in the cell. Whereby a flux reduction is of interest in cases where certain part of the metabolism needs to be silenced in order to for instance minimize the formation of an unwanted side product, an increase of the flux may allow boosting of certain pathways or reaction sequences required for synthesis of a fermentation product (e.g. an amino acid or a vitamin). In this case, taxis regulators are preferably sensitive for a metabolite or an end- product formed by the cell. This embodiment is particularly valuable for the screenings for improved strains employed for manufacturing purposes.

A significant number of methods have been developed for investigating microbial taxis. Among them are swarm plate assays, capillary assays/plug assays, Transwell™ migration assay, and microfluidic assays (Fig. 4). Some of these assays are of preparative nature, i.e. allow not only the quantification of the degree to which the cells display taxis but also the isolation of individual fractions. The capillary/plug assay (Fig 4 II), for instance, is very well suited for visualization of migrating cells but can hardly be used for preparative purposes. On the other hand, the swarm plate assay (Fig. 4 I) allows the quantitative assessment of taxis but also the recovery of especially the highly tactic fraction which readily separates from the background of sessile or weakly tactic cells. This assay makes use of semi-solid growth media that contain one or more compounds capable of initiating taxis. If the swarm plate assay is used in accordance to the invention and for the purpose of enzyme screenings, then these plates may also contain a substrate for the phenotype-implicating protein. As the cells grow, they consume the attractants (naturally present in complex growth media) in their immediate vicinity which inevitably leads to the formation of a gradient of the attractant between the regions of the gel where the cells already grew and those that are not colonized yet. Cells that express the phenotype-implicating protein will reconstitute a functional taxis machinery and then start to migrate towards the gradient formed by the attractant. To this end, phenotype-implicating protein is synthesized by the cells that are located at the outer rim of the halo formed upon taxis. These calls can then simply be collected in a mechanical manipulation step.

The Transwell™ migration assay (Fig. 4 III), also known as the Boyden or modified Boyden chamber assay, is another example for a preparative assay. Here, the cultivation chamber is separated into an upper and a lower part by a membrane with a cut off in the order of the size of the tactic cells that ought to be assayed. The attractant is then added to the lower half of the device while the cells are added to the top. Following an incubation period, cells that have migrated through the membrane from the top to the bottom, from which they can be easily recovered, are the tactic ones.

Solutions for the problem of separating tactic from non-tactic cells are also provided by the art of microfluidics (Fig 4 IV). One class of devices relies on the establishment of gradients of the attractant in micro channels whereas hydrogels or porous membranes are optionally integrated in order to further stabilize the gradients. Other solution can be found with flow-based methods employing lateral flow regimes to generate attractant gradients across channels. Under these conditions, tactic cells can be isolated from attractant-rich streams while non-tactic ones remain in the sample liquid containing only very little attractant.

The invention is further illustrated by the following examples and figures, from which further embodiments and advantages can be drawn. These examples are meant to illustrate the invention but not to limit its scope.

Brief description of the figures

Fig. 1 Taxis platform for detection of enzyme activity or binders

I. A cell (i) that lost its capability of displaying tactic response due to the deactivation of one or more genes essential for correct functioning of its taxis machinery, expresses an enzyme (ii). This enzyme catalyzes the synthesis of a product (P) from a substrate or metabolite (S). The cell also contains a transcription regulator (iii) with affinity for the abovementioned product. Upon binding to the product, the transcriptional regulator (iv) directly or indirectly binds and activates a promoter (v). This promoter regulates the expression level of an open reading frame (vi) encoding for a taxis protein capable of reconstituting the taxis machinery. In this way, formation of the product is coupled to the tactic behavior of the cell.

II. In another setting, the cell (vii) expresses a substance (viii) with affinity for a transcriptional regulator (ix) for which a ligand is sought. If the transcriptional regulator displays affinity for the substance, it either (x) directly or indirectly binds and activates a promoter (xi) which then upregulates the expression level of an open reading frame (xii) encoding for a taxis protein capable of reconstituting the taxis machinery. In this way, the presence of a ligand for the transcriptional regulator is coupled to the tactic behavior of the cell.

Fig. 2 Separation of tactic and non-tactic E. coli cells

I. Phenotypic characterization of three E. coli strains on a semi-solid tryptone- agar plate (10 g/L tryptone, 5 g/L NaCI, and 0.2% agar). Noteworthy, the tryptone medium contains two attractants for E. coli, i.e. serine and aspartate. Serine and aspartate are consumed by E. coli in a diauxic way, i.e. serine first and an aspartate only once all serine has been used up. The wild type strain (i) displays tactic behavior to both attractants in the presence of a serine or an aspartate gradient as indicated by the formation of two distinct rings. Both, (ii) A motB strain and (iii) a Mar Msr strain does not display taxis anymore due to the deactivation of a key component of the flagellar motor or the knockout of the serine and aspartate receptor proteins. Interestingly, the Mar (iv) strain featuring a knockout of the aspartate sensor gene only indeed loses its tactic behavior in the presence of an aspartate gradient but still display it in the presence of a serine gradient.

II. Cells of a taxis proficient E. coli strain are mixed with a Mar Msr E. coli strain not displaying taxis at ratios of 1 : 10⁵ (v), 1 : 10⁶ (vi) and 1 : 10⁷ (vii), respectively. Approximately 10⁶ cells of each of the mixture were then spotted (diameter of the spot of approx. 1 - 2 mm) on a soft-agar plate (10 g/L tryptone, 5 g/L g NaCI and 0.2% agar) and incubated overnight (30°C). On the next day, cone- shamrock or conical films emerged from the spot.

Fig. 3 Conditional re-constitution of tactic behavior of E. coli knockout strains.

I. The motility protein B (MotB) is the stator element of the flagellar motor complex of E. coli. The gene was cloned under the control of a rhamnose- inducible promoter and introduced in the non-tactic E. coli knockout motb using plasmid (pBR322 origin of replication, bla gene encoding TEM-1 b- lactamase, inducible promoter Prha for motB induction by rhamnose). The cells were then spotted on semi-solid tryptone-agar plates (1 g tryptone, 0.5 g NaCI and 0.2% agar in 100 ml. distilled water supplemented with 100 pg/mL ampicillin) containing rhamnose at concentrations ranging between 0.01 and 0.2 % (w/v), respectively. Visual inspection of the plates after incubation (30°C, 12-14 h) indicate that the degree to which tactic behavior had been displayed positively correlated with the concentration of the rhamnose-inducer that was added to the plates.

II. A translational fusion comprised of a gene encoding for the serine chemoreceptor, tsr, and one encoding for a green fluorescence protein, sfGFP, was cloned under the control of an arabinose-inducible promoter (PBAD) using vector pBAD (pBR322 origin of replication, bla gene encoding encoding TEM-1 b-lactamase, inducible promoter PBAD for tsr induction by arabinose). E. coliAtar Atsr was transformed with the genetic construct and the cells were spotted on semi-solid tryptone-agar plates (1 g tryptone, 0.5 g NaCI and 2 g agar in 100 ml. distilled water) supplemented with concentrations of arabinose of 0 - 0.1 % (w/v). After incubation, arabinose concentration dependent taxis behavior was observed.

III. A translational fusion protein of a gene encoding for the serine chemoreceptor, tsr, and one encoding for a green fluorescence protein, sfGFP, was cloned under the control of a phenol-inducible promoter in pDmpR vector backbone (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols, constitutive Pr promoter for induction of dmpR). E. coli Atar Atsr was transformed with the resulting plasmid and cells were spotted on semi-solid tryptone-agar plates (1 g tryptone, 0.5 g NaCI, 2 g/L agar in 100 ml. distilled water) supplemented with increasing concentrations of phenol (0 - 0.1 mM). The size of the halos formed after overnight incubation (30°C) correlated with the amount of phenol present in the media. No swarming was observed in the absence of phenol.

Fig. 4 Assays for separation of tactic cells from a background

I. A Petri dish containing semi-solid media (e.g., 10 g/L tryptone, 5/L g NaCI and 0.2 % agar) is poured (i) and let to solidify for at least one hour before applying ca. 10⁴ CFU in the center of the plate (ii). The semi-solid media is pierced and cells are applied inside the agar, not on top. The inoculated plate is then incubated until a halo clearly visible by the naked eye is formed (iii). This typically requires between 8 - 16 hours of incubation at 30°C. Tactic cells can then be isolated from the rim of the resulting halo. II. A setup for resolution of tactic cells makes use of a microscopy slide (vi) onto which a drop of agarose enriched with a chemoattractant (vii) is dropped. The droplet is then immediately covered with a top microscopy slide (v). After the agarose drop (or plug) has solidified, cell suspension with OD(600) of 0.6 - 0.8 (vi) is applied in the crevice formed between the two microscopy slides. Within 30 to 60 minutes, the tactic cells will start to accumulate around the agarose plug (viii). This assay has a rather short analysis time but the separation and isolation of the tactic cells is not practical with this method.

III. The Transwell™ migration assay or Boyden chamber assay makes use of an insert (ix) with semi permeable membrane (x) which separates a vessel into top (xi) and bottom (xii) compartments allowing the migration of tactic bacteria (xiii) and therefore their separation from a background (xiv). Cells are seeded into the top compartment and the chemoattractant solution is placed in the bottom compartment. After incubation, counting the cells in the bottom compartment allows quantification of number of tactic bacteria as well as their isolation.

IV. Microfluidic devices allows for better control of cellular microenvironments compared to conventional cell migration assays. Many different microfluidic devices are reported in literature. In the presented example, the device consists of three channels with parallel flows with three inlets and two outlets. Buffered solution containing the chemoattractant is applied through the top inlet (xv) while cell suspension is pumped through the middle inlet (xvi) and buffer only solution is applied through the lower inlet (xvii). As cells enter the device in the middle of the central channel, they are immediately exposed to the attractant and tactic cells will preferentially migrate to the upper part of the device where the concentration of attractant is the highest. As the device maintains constant flow along all channels, the cells continuously washed out from the chip and exit it either via the upper outlet through which also most of chemoattractant is flushed out whereas (xviii) the background is preliminary found or at the lower one (xix).

Fig. 5 Coupling of the catalytic activity of enzyme variants and taxis

I. E. coli Mar Msr is co-transformed with the plasmid harboring a phenol- inducible tsr and a second plasmid that either carry no gene (i.e., empty vector, i) or a gene encoding a tyrosine-phenol lyase under the control of a rhamnose inducible promoter (ii). The resulting E. coli strains are spotted into a soft tryptone-agar plate (1 g tryptone, 0.5 g NaCI and 2 g/L agar in 100 ml. distilled water) supplemented with 0.1 % rhamnose (for induction of enzyme production) and tyrosine (substrate of the tyrosine-phenol lyase enzyme). After overnight incubation at 30°C, the colony of the strain that express an active tyrosine phenol lyase and therefore converts tyrosine to ammonia, pyruvate and phenol forms a large halo and is unambiguously distinguishable from the strain that does not express an active enzyme variant.

II. E. coli Mar Msr equipped with plasmid pDmpR-fsr (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols, constitutive Pr promoter for induction of dmpR) was co-transformed with pCA24 empty vector (iii) or ybfF gene also expressed from plasmids pCA24N (pBR322 origin of replication, cam gene encoding chloramphenicol acetyltransferase and T5 promoter for ybfF induction by IPTG) (iv). The library expressing cells were spotted (approx. 10⁶ cells per spot) into soft-agar plates (10 g/L tryptone, 5 g/L NaCI and 0.2 g/L agar) containing 0.125 mM phenyl benzoate and incubated (30°C, 12-14 h). The next day, the size of the colonies varied considerably. Cells present at the outer rim of the halo of the large colonies are likely to display the strongest taxis response and therefore putatively liberate the highest amounts of phenol. These cells were isolated and subjected to further characterization.

III. £. coli Mar Msr is co-transformed with the plasmid harboring the phenol- inducible expression of Tsr and a second plasmid that either carry a gene encoding alkane monooxygenase with no activity towards toluene (v) or monooxygenase mutant (vi) with hydroxylation activity towards toluene as indicated by the results of a microscopic plug assay (see also Fig 4 II).

Fig. 6 Screening for identification of a variant of the esterase YbfF with activity for phenyl benzoate

I. Results of a swarm plate assay carried out with £. coli Mar Msr equipped with pDmpR-fsr (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols, constitutive Pr promoter for induction of dmpR) that also expresses the esterases YbfF (i), YjfP (ii), or a YbfF mutant library (iii) after 16 h of incubation at 30°C spotted into a soft tryptone- agar plate (10 g/L tryptone, 5 g/L NaCI and 2 g/L agar) supplemented with 0.25 mM IPTG (for induction of enzyme production) and 0.125 mM phenyl benzoate (esterase substrate). The radius of the halos formed from the YbfF mutant library is approximately 50 % larger than the halo of the spot for YbfF wildtype and comparable in size to the halo formed from the positive control (YjfP wildtype). II. Re-screening of 90 clones isolated from the halos formed around the library colony (iv) using 4-aminoantipyrine colorimetric assay for detection of phenol. Control reactions with YbfF wildtype (i) and YjfP (ii) were also included and served as reference points. The assay was performed with substrate (phenyl benzoate) load of 0.5 mM.

III. Swarm plate assays at two phenyl benzoate concentrations with the four isolates (iv) from YbfF library with the highest activity after 4-aminoantipyrine (4- AAP) colorimetric assay.

Fig. 7 Quantification of intracellular biotin by taxis

I. BirA regulon comprises the Pbiotin , which is repressed by transcriptional regulator, BirA, in the presence of biotin.

II. The BirA regulator suppresses expression of the TetR regulator and TetR the expression of the taxis-enabling gene, motB, being essential for tactic behavior of E. coli, from it’s the tetR promotor. In this way, motility of an E. coli motb knockout is only restored in the presence of an excess of biotin.

Fig. 8 Coupling of the intracellular B2 concentration to B. subtilis taxis

I. A riboswitch features a FMN binding pocket and a ribosome binding site (RBS) upstream of an ORF which encodes for the chemotaxis gene cheD gene. The construct is expressed in a B. subtilis strain which has lost its capabilities to display chemotactic response due to a cheD knockout. High intracellular concentrations of FMN leads to its binding to the riboswitch which then coils and thereby buries the RBS. Subsequently, the ORF is not translated, CheD is not synthesized and the cells do not display tactic behavior.

II. At low intracellular FMN concentrations, the FMN binding pocket of the riboswitch is unoccupied. In this configuration, the RBS is accessible, the CheD protein and tactic behavior of B. subtilis cheD knockouts is restored. The circuit can be transformed into B. subtilis strain libraries with the cheD gene being knocked-out in order to identify those mutants that synthesized only very small amounts of FMN being providing an undesired site product for riboflavin synthesis.

Fig. 9 Screening for transcriptional binder/regulator protein with novel binding capabilities

I. £. coli tar tsr was transformed with plasmid variants of pDmpR-fsr (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols, constitutive Pr promoter for induction of dmpR ) with point mutations specifically introduced into the dmpR gene encoding for the transcriptional regulator/binder protein, DmpR. The library consisted of at least 10⁵ variants. Approximately 10⁵ CFU were seeded in semi-solid agar plate (10 g/L tryptone, 5 g/L NaCI and 2 g/L agar) containing 1 mM benzyl alcohol not capable to induce the DmpR wildtype. After overnight incubation, halos were observed for the 4 spots containing the mutant library (i) but not for the spot comprising the DmpR wildtype (ii). Biomass was sampled with an inoculation loop from the outer rim of the halos of the colonies made from the library cells (iii) and monoseptic colonies were prepared on LB agar plates by streaking. Afterwards, single colonies were picked and spotted onto two semi-solid agar plates without (iv) or with 1 mM benzyl alcohol (v) each. While all isolates reacted strongly to the presence of benzyl alcohol, three strains displayed tactic behavior even in its absence. This indicates that some of the cells expressed a regulator with affinity for phenols while others just lost the capability of tightly regulating the expression of the marker gene.

II. The inducibility of three DmpR variants (vi, vii, viii) and DmpR wildtype (vii) was analyzed over a range of benzyl alcohol concentrations in E. coli Mar Msr using swarm plate assay as reporter and in £. coli DH5a using the expression of a fluorescence protein as a reporter.

Examples Example 1:

Screening of DNA Libraries for tyrosine-phenol lyase genes

The invention was applied for the isolation of tyrosine-phenol lyases from metagenomic libraries. For this, E. coli MG1665 knockout Mar Msr carry a recombinant DNA construct (plasmid vector pDmpR-fsr with a p15a origin of replication, tsr gene of E. coli, amp gene— TEM-1 b-lactamase, Po promoter for tsr induction via phenols, and a dmpR gene for activation of Po) for the conditional expression of the chemoreceptor Tsr, were used as a screening host, transformed with a metagenomic DNA library and subjected to a taxis assay.

A metagenomic library was isolated from fly compost employing standard protocols for DNA extraction and purification and transformed into the screening host. Semi-solid tryptone-agar plates were prepared (10 g/L tryptone, 5 g/L NaCI and 2 g/L agar), the solution was autoclaved and let to cool down to 40°C before 100 mg/L ampicillin and 50 mg/L kanamycin were added. Noteworthy, tryptone already contains substrates for tyrosine-phenol lyases (e.g. tryptophan and tyrosine) and needs thus not to be further supplemented. The entire library containing 10⁶ transformed cells was spotted in a volume of approx. 1 pl_ into the center of a plate and incubated (30°C, 14 h, see Fig. 6). Cells from the outer rim of the halos were sampled with a sterile loop and re-streaked on a LB agar plate (1.5 % agar, 10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCI, 100 mg/L ampicillin) such that individual colonies were obtained. A total of 96 of these colonies were picked and subjecting to a colorimetric assay for quantification of phenol liberated from tyrosine by tyrosine-phenol lyase activities. For this, all picked cells were propagated in a 96 deep-well microtiter plate in M9 minimal medium (3 g/L KH2PO4, 12.8 g/L Na₂HP0₄, 0.5 g/L NaCI, 1 g/L NhUCI, 2 mM MgS0₄, 0.1 mM CaCI₂, 0.4% glucose, 100 mg/L ampicillin, 50 mg/L kanamycin) supplemented with 1 mM tyrosine until early stationary phase. The plate is centrifuged in order to sediment the cells. Next, phenol was quantified using an 4- aminoantipyrine (4-AAP) assay. For this, the supernatant (120 pL) was transferred to a fresh 96-well flat bottom microtiter plate and mixed with 15 pL quenching solution (4 M urea in 0.1 M NaOH), 12 pL 4-aminoantipyrine solution (5 mg/mL) and 12 pL potassium peroxosulfate (5 mg/mL). The plate is then incubated at room temperature for 30 min at 750 rpm and the absorbance at 509 nm was measured. The increase of absorbance relative to a control (cells that do not express tyrosine-phenol lyase) indicates that the respective well contains cells that express an active tyrosine-phenol lyase enzyme.

Example 2:

Screening of a YbfF esterase variants with improved properties

Esterases catalyze the cleavage of ester bonds under mild conditions and frequently even at high stereo- and regioselectivity. These features make them interesting catalysts for industrial applications, e.g. for production of chemicals, food processing, or as catalysts in laundry chemicals. In this example, a taxis marker is used for identification of variants of the esterase YbfF with activity for cleavage of phenyl benzoate to benzoate and phenol.

In order to characterize esterases generated by directed evolution, a screening protocol employing bacterial chemotaxis as a reporter for the enzyme activity was employed. The polynucleotide sequence of the ybfF gene of E. coli encoding for its carboxyesterase, YbfF, was randomized by simultaneous site-directed mutagenesis of four amino acids all located in the active site of the enzyme. The mutagenized gene was then cloned into plasmid pCA24N (pBR322 origin of replication, cam gene encoding chloramphenicol acetyltransferase and T5 promoter for ybfF induction by IPTG) and transformed by electroporation into an E. coli host with the two housekeeping chemoreceptor genes tar and tsr naturally employed by E. coli for sensing of the chemoattractant aspartate and serine, respectively, being knocked out. The strain also contained a pDmpR-fsr vector plasmid (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols) for conditional expression of the chemoreceptor Tsr. Approximately 10⁶ CFU of the thus prepared cells were spotted into semi-solid tryptone agar (10 g/L tryptone, 5 g/L g NaCI and 0.2 % agar) supplemented with ampicillin (100 pg/mL), chloramphenicol (25 pg/mL) and phenyl benzoate (0.05 - 0.5 mM) and incubated (14 h, 30°C). Due to expression of a low basal housekeeping phenyl benzoate hydrolyzing activity at substrate concentration higher than 0.15 mM, all cells weakly displayed tactic behavior and migrate towards the nutrient rich region of the agar not yet depleted from the chemoattractant serine. However, some spots were considerably larger than others which indicates presence of cells capable of migrating faster than their peers which thereby indicates high expression of tsr, therefore high activity of the promoter Po, therefore a high intracellular phenol concentration, and therefore presence of a ybfF gene encoding for a highly active YbfF variant. Cells localized at the outer rim of the halos of those colonies that clearly exceeded the size of the background were collected with a sterile loop, re-streaked on LB agar plates (1.5% agar, 10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCI, 100 mg/L ampicillin, 25 mg/L chloramphenicol ) and incubated overnight (30°C). The next day, individual clones were picked and transferred to a 96-well microtiter plate filled with LB medium (10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCI, 100 mg/L ampicillin, 25 mg/L chloramphenicol) and propagated (14 h, 30°C ) until the early stationary phase was reached. The cells were diluted to an OD(600) of 0.2 in M9 glucose medium into a microtiter plate with the appropriate antibiotics (3 g/L KH₂P0₄, 12.8 g/L Na₂HP0₄, 0.5 g/L NaCI, 1 g/L NH₄CI, 2 mM MgS0₄, 0.1 mM CaCI₂, 0.4 % glucose, 100 mg/L ampicillin, 25 mg/L chloramphenicol) supplemented with 0.5 mM phenyl benzoate and incubated until the early stationary phase was reached (6 h, 30°C). The amount of phenol in the media formed was assayed for 90 strains using a colorimetric assay for phenol (see 4-AAP assay in example 1 ). The activity of nine of the thus prepared strains clearly exceeded the background as represented by the non-mutated ybfF gene expressed in the E. coli screening host. These clones were selected for further characterization by sequencing. For this, the plasmids were isolated (MiniPrep Kit by Qiagen following Supplier's protocol) and sequenced. Additionally, the YbfF variants were purified to homogeneity via affinity chromatography and the hydrolysis of phenyl benzoate was investigated in vitro. Unlike the parent enzyme YbfF WT, all mutants exhibited activity towards the said substrate ranging with catalytic rates (kcat) ranging from 2 to 9 s ¹.

Example 3:

Screening for a BioB variant with improved activity

Biotin production by microorganisms suffers from low productivity due to low turnover numbers of the biotin-syntheses pathway enzyme, biotin synthase or BioB, which is responsible for the formation of the sulfur ring of biotin from the precursor desthiobiotin. In order to isolate a new BioB variant with improved activity, the bioB gene of an E. coli K-12 derivative is isolated and amplified by error-prone PCR at an error frequency of approx. 15 nucleotides per 1000 bp and cloned in standard expression vectors.

As a screening host, E. coli motB not forming a functional flagellum anymore was used. The strain also constitutively expressed the biotin-sensitive regulator birA from a plasmid (p15a origin of replication, amp gene— TEM-1 b-lactamase, P_biotin promoter for motB induction via biotin, birA gene for activation of Pbiotin). BirA binds to the biotin promoter naturally used by £. coli for suppression of the biotin biosynthesis operon at high intracellular biotin concentrations. A screening vector (p15a origin of replication, bla gene— TEM-1 b-lactamase) was constructed in which the biotin promoter, Pbiotin, was cloned upstream of an open reading frame encoding for the tetR repressor with the cognate appertaining promoter cloned upstream of a tsr gene. In this way, high biotin levels lead to the repression of BirA which leads to the downregulation of TetR which then leads to the expression of motB. Subsequently, the MotB protein required in order to allow the cells to display taxis is only expressed at high intracellular biotin concentrations. The screening vectors also expressed variants of the bioB gene from an IPTG- inducible Plac-promoter. The ribosome-binding site employed in the course of the translation of the BioB variants was intentionally weakened to the extent required in order to avoid that the activity of the parent BioB protein already lead to tactic behavior of the screening host. The screening plasmid was then co-transformed into £. coli MG1665 motB already equipped with a pBirA-motB plasmid (p15a origin of replication, amp gene— TEM-1 b-lactamase, P_biotin promoter for tsr induction via biotin, birA gene for activation of Pbiotin) and approximately 10⁵- 10⁶ CFU are spotted into two semi-solid tryptone-agar screening plates (10 g/L tryptone, 5 g/L NaCI, 0.2% agar supplemented with 100 mg/L ampicillin, 50 mg/L kanamycin, 96 spots containing approx. 10³-10⁴ cells per spot on average) and incubated overnight (30°C). Inspection of the plates the following day indicated that for the 12 spots the halo size clearly exceeded that of the background. Cells from the other rim of these halos were scratched of the plate and pooled in test tube containing 3 ml. of brine (0.9 % NaCI). The cell concentration was determined spectrophotometrically and approximately 1000 cells were spotted into 10 semi-solid tryptone-agar screening plates (see above) and incubated overnight (28°C). On the next day, approximately 800 of all 1000 colonies that were grown over night displayed tactic behavior as indicated by a clearly enlarged colony diameter as compared to the background. 96 of the large colonies were picked and grown in an microtiter plate on LB (10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCI, 100 mg/L ampicillin, 50 mg/L kanamycin). Crude extracts were prepared by sonication (cell suspension of OD(600) of 15 in PBS, 10 cycles, 40 % amplitude, 30 seconds with 30 seconds pause between cycles) and the specific BioB activity was measured in all extracts whereas the product of the expected mass (m/z) was monitored by HPLC-MS. The results were compared to the specific biotin activity determined in extracts prepared from the same strain expressing the parent BioB protein indicating that in 49 extracts the BioB activity was within the experimental error of the assay (+/- 15 % of the activity measured with the parent strain) and in 42 cases the increase was higher than 15 % but lower than 50 % indicating only a modest improvement. The best performing strains yielded a BioB activity in the extracts two-fold and two- to three-fold over background which suggest presence of BioB variants featuring improved activity.

Example 4:

Screening for a cytochrome P450 variant in Pseudomonas putida

The XylMA system of P. putida highly efficiently hydroxylates terminal methyl groups in substituted benzenes (e.g. toluene or xylene). 4-Hydroxybenzyl alcohol (gastrodigenin) is of potential interest as precursors for the polymer industry but the chemical synthesis of the substance is too costly in order to allow it production at competitive prices. Here the inventors described a screening process using taxis as a readout in order to identify XylMA mutants that catalyze the hydroxylation of toluene to gastrodigenin.

The mcpS gene from P. putida encoding methyl-accepting chemotaxis protein, McpS, is cloned in vector pDmpR (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols, constitutive Pr promoter for induction of dmpR) and reintroduced into a P. putida strain having mcpS deleted from its genome. Transforming pDmpR -mcpS and induction of the expression of mcpS by phenols allowed the recovery of tactic behavior of P. putida on succinate. Furthermore, a second plasmid, pXylMA (pBR322 origin of replication, cam gene encoding chloramphenicol acetyltransferase and P_aiks promoter for xylMA induction by DCPK), harboring a mutant library of the gene xylMA encoding a xylene monoxygenase, XylMA, is transformed. The XylMA wildtype converts toluene exclusively to benzyl alcohol but does not perform the desired para-hydroxylation of the benzene-ring. Thus, the plasmid library introduced in the P. putida strain equipped with pDmpR -mcpS is screened for XylMA variants with activity for hydroxylation of the benzene residue of toluene. The screening is performed on semi-solid media (10 g/L tryptone, 5 g/L NaCI and 0.2% Agar) supplemented with antibiotics for plasmid maintenance (100 mg/L ampicillin, 50 mg/L kanamycin) and the screening substrate (1 mM toluene). Following overnight incubation at 30°C, several of the seeded spots formed a halo that exceeded the diameter of the spot formed from the XylMA wild type by at least 50 %. 90 strains were isolated from the rim of these halos and the XylMA variants were synthesized in P. putida cultivated in microtiter plates in LB (10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCI, 100 mg/L ampicillin, 50 mg/L kanamycin, 0.1 % dicyclopropaneketone added as an inducer). Whole cell bioconversion of toluene is carried out and the product profile of individual biocatalysts is analysed by HPLC using a ReproSil-Pur® C18aq column and 60% acetonitrile as a mobile phase and a UV/Vis detector for spectrophotometric analysis. While all 90 strains still produced benzyl alcohol as a main product, 45 of these also formed phenols (2-methyl phenol and gastrodigenin, respectively) in amounts between 5 - 35 % of the main product. The XylMA variants that exhibited the highest activity for gastrodigenin formation were selected for further characterization by sequencing. For this, the plasmids were isolated (MiniPrep Kit by Qiagen following Supplier's protocol) and sequenced using standard Sanger sequencing service.

Example 5:

Isolation of a RibC variant in riboflavin secreting B. subtilis

B. subtilis is used industrially for production of riboflavin. Of key importance for riboflavin overproduction is the flavokinase/flavin adenine dinucleotide synthetase RibC which catalyzes the phosphorylation of riboflavin to flavin mononucleotide (FMN) and its conversion to flavin adenine dinucleotide (FAD). Industrial riboflavin production strains synthesize the attenuated variant RibC820, which channels less riboflavin to FMN and FAD due to an increased activity. Here, the inventors disclose a method for application of the invention for the identification of a novel RibC variant catalyzing the FMA formation reaction at such a low rate that no more than 1000 FMN molecules per cell are formed. This variant is free from any feed-back inhibitory effects of FMN on the cell metabolism and reproducibly displays an 8 to 10 % higher riboflavin yield in shaking flask experiments.

The industrial B. subtilis-based riboflavin production strain RB52 is transformed with plasmid pJF1 19HE (ampicillin resistance, colE1 ori, P_tac) containing the CheD gene encoding for a key activity for methylation of the chemotaxis receptors of B. subtilis with a FMN-sensitive riboswitch located in the 5’-untranslated region of a library of the RibC820 gene generated by error prone PCR and controlled by a tac-promoter. The riboswitch has been developed previously and featured an apparent kD for FMN in the order of 1 mM. A test of its activity using the parent RibC820 and a the mutant RibC^* bearing a point mutation in the active site of RibC that deactivated the protein indicated no mobility of the RibC820 mutant but mobility of strains expressing RibC^* thereby indicating that cheD was not translated in RibC820 but in RibC (see Fig. 8). This results suggests that the strain can be used for the discrimination of RibC820 variants of so low activity that the intracellular accumulation of FMN is kept at an absolute minimum even if massive amounts of riboflavin are synthesized. The library was transformed and the clones were spotted in the form 9 patches (diameter of a patch approx. 1 mm containing 10⁵ clones each) on a swarm plate (10 g/L tryptone, 5 g/L NaCI, 0.2% agar, 100 mg/L ampicillin) which had been bleached overnight under UV/Vis in a sterile bench so as to destroy and riboflavin potentially present in tryptone. The plates were incubated overnight at 30°C. Next day all colonies originating from the RibC820 libraries were enlarged as compared to a control expressing RibC820 only. The outer part of all colonies was scratched of the plates, suspended in sodium chloride solution (3 ml_, 0.9 %). Then aliquots of 10 pL each were plated (16 spots per plate, 50 swarm plates in total comprised of 1 g tryptone, 0.5 g NaCI and 0.2% agar in 100 ml. distilled water) and incubated overnight at a low temperature (27°C) in order to minimize swarming. Next day, the 100 largest colonies as identified by visual inspection were picked, pooled, lysed, and the RibC inserts were amplified using two primers complementary to the nucleotides to the left and the right of the multi cloning site of the vector. The PCR fragments were purified and subjected to lllumina miseq sequencing in order to identify potential mutational hot-spots.

Example 6:

Screening by taxis for a transcription regulator displaying altered effector specificity in E. coli

The DmpR protein is both a small-molecule binding protein and a transcriptional regulator. Here, the inventors describe the use of taxis as a reporter for a DmpR variants with modified binding specificity.

A mutant library of the dmpr gene was prepared by performing saturation mutagenesis at key positions on a plasmid template, pDmpR-tsr (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for tsr induction by phenols, constitutive Pr promoter for induction of dmpR) resulting in a mutant library comprised of at least 10⁵ unique variants. The plasmid library is transformed into E. coli Mar Msr and 10⁵ cells (in a total volume not exceeding 1 pl_) are seeded in semi-solid agar plates (10 g/L tryptone, 5 g/L NaCI and 0.2 % agar supplemented with 100 mg/ml_ ampicillin and 1 mM benzyl alcohol). The plates were incubated overnight at 30°C and then subjected to visual inspection. Since DmpR wildtype does not respond to benzyl alcohol, the cells that expressed the wildtype protein did not show any swarming. On the other hand, for several spots that contained the mutant library, a strong tactic response was observed. Single cells were isolated from the edge of these halos and strains were subjected to re-analysis on swarm plate assay in the presence and absence of benzyl alcohol. Interestingly, 3 of the 9 clones showed some swarming even in the absence of the inducer, benzyl alcohol, which indicates that not only the effector specificity but also the binding affinity of the variants had been altered. However, in the presence of benzyl alcohol the formed halos were at least 50 % larger than the halos formed in the absence of benzyl alcohol indicating that while leakier these variants are still inducible by benzyl alcohol. To further verify this result, the mutagenized sequences were cloned in plasmid pDmpR-gfp (p15A origin of replication, bla gene encoding TEM-1 b-lactamase, inducible promoter Po for gfp induction by phenols, constitutive Pr promoter for induction of dmpR) and the inducibility of the three unique DmpR variants by benzyl alcohol in E. coli DH5alpha strain was investigated using green fluorescent protein as reporter. Indeed, all 3 DmpR mutants were activated by benzyl alcohol as indicated by elevated specific fluorescence relative to both the same strain variant grown in the absence of benzyl alcohol or DmpR wildtype-containing strain grown in the presence of benzyl alcohol. Finally, the DNA sequence of the DmpR was determined by sequencing and confirmed that the novel DmpR variants harbored mutations at a key residue with the effector binding site of the protein.

Example 7:

Fishing of binding partners for a bacterial target by taxis

The protein phosphates I (CPI-17) synthesized in lung epithelium of humans is known to bind to fourteen other human proteins in bacterial two hybrid screenings. In this example the inventors describe the applications of the innovation for coupling of binding of CPI-17 to its reported binding partners in order to trigger the tactic behavior of E. coli.

The H/S3-aadA reporter cassette of commercially available strain E. coli XL1 -Blue MRF Kan (BacterioMatch II Two-Hybrid Vector Kit from Agilant Technologies) has been replaced with AreR gene directly behind its lamdaCI promotor. The lapB gene was used as the bait and was inserted at the N-terminus of the lamda repressor C1 in pBT vector. As preys served a cDNA library prepared from human lung tissue cloned at the N-terminus of the RNA-polymerase alpha. Both plasmids were co-transformed by electroporation into electrocompetent host cells and spotted in the form of a single spot (approx. 10⁶ cells in a spot of a diameter of 1 to 2 mm) onto an LB-soft agar plate supplemented with the appropriate antibiotics (1 .5 % agar, 10 g/L tryptone, 5 g/L yeast extract, 10 g/L NaCI, 15 mg/L tetracycline, 40 mg/mL chloramphenicol). After incubation overnight, the colony had a halo formed clearly due to chemotactic movement of the cells as apparent by the formation of an embossed front and a poorly populated region between the said front and the central colony. Cells from the outer rim were isolated and suspended in LB in a test-tube (3 mL). Then, 10 pL of the LB were plated on an LB plate and the plate was incubated overnight (30°C). At the next day, 20 colonies were picked and send for Sanger sequencing of the inserts. The sequencing results indicated three matches for OK/SW-CL, one for Synaptogyrin 2, one for Granulin precursor, four for Cathepsin D, and one for Rab36 all well know binding partners for CPI-17.

Claims

1. A method for selecting a variant of a polypeptide, wherein said polypeptide catalyses conversion of a substrate to a product,

said method comprising

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a nucleic acid sequence encoding a variant of said polypeptide under control of a first promoter operable in said cell;

said second promotor is activated by the product;

b) exposing said plurality of cells to said stimulus; and

c) selecting a cell capable of exceeding a threshold of tactic behavior from said plurality of cells;

2. The method for selecting a variant of a polypeptide according to claim 1 , wherein the polypeptide is selected from the classes of oxidoreductases (E.C.1 ), transferases (E.C.2.), hydrolases (E.C.3.), lyases (E.C.4.), isomerases (E.C.5.), and ligases (E.C.6.), particularly the polypeptide is selected from glucose oxidase (E.C. 1.1.3.4.), laccase (E.C. 1.10.3.2), catalase (E.C. 1.11.1.6.), lignin peroxidase (E.C. 1.1 1.1.7), manganese peroxidase (E.C. 1.11.1.13), lipoxygenase (E.C. 1.13.11.12.), transglutaminase (E.C. 2.3.2.13), cyclodextrine glucanotransferase (E.C. 2.4.1.19), glucosyltransferase (E.C. 2.4.1.24), esterase (E.C. 3.1.1.1.), triacylglycerol-lipase (E.C. 3.1.1.3), phospholipase A (E.C. 3.1.1.4), phospholipase B (E.C. 3.1.1.5), pectin esterase (E.C. 3.1.1.11 ), tannase (E.C. 3.1.1.20), monoacylglycerol-lipase (E.C. 3.1.1.23), phosphatase (E.C. 3.1.3.2), phytase (E.C. 3.1.3.8.), phosphodiesterase (E.C. 3.1.4.1 ), amylase (E.C. 3.2.1.1.), glucoamylase (E.C. 3.2.1.3.), cellulase (E.C. 3.2.1.4.), beta-glucanase (E.C. 3.2.1.6.), inulase (E.C. 3.2.1.7), xylanase (E.C. 3.2.1.8), dextranase (E.C. 3.2.1.11 ), pectinase (E.C. 3.2.1.15), alpha-glucosidase (E.C. 3.2.1.20), beta-glucosidase (E.C. 3.2.1.21 ), alpha-galactosidase (E.C. 3.2.1.22.), lactase (E.C. 3.2.1.23), invertase (E.C. 3.2.1.26), exo-xylanase (E.C. 3.2.1.37), alpha- rhamnosidase (E.C. 3.2.1.40), pullalanase (E.C. 3.2.1.41 ), arabinofuranosidase (E.C.

3.2.1.55), exo-1 ,3-beta glucosidase (E.C. 3.2.1.58), endo-1 ,4-beta-mannanase (E.C. 3.2.1. 78), cellobiohydrolase (E.C. 3.2.1.91 ), arabinanase (E.C. 3.2.1.99), metalogenic amylase (E.C. 3.2.1.133), cymosin (E.C. 3.4.23.4.), glutaminase (E.C. 3.5.1.2), penicillin amindase (E.C. 3.5.1.11 ), aminoacylase (3.5.1.14), aminopeptidase (E.C. 3.4.11.X), AMP deaminase (E.C. 3.5.4.6.), nitrilases (E.C. 3.5.5.1.), acetolactate decarboxylase (E.C. 4.1.1.5.). pectat lyase (E.C. 4.2.2.2), pectin lyase (E.C. 4.2.2.10), or glucose isomerase (E.C. 5.3.1.5.), or enzymes encoded by metagenomics libraries.

3. A method for selecting a variant of a polypeptide, wherein said polypeptide is capable of binding to a transcriptional regulatory protein, thereby modulating the activity of an inducible promotor,

said method comprising

a) providing a plurality of cells, wherein each of said plurality of cells:

comprises a nucleic acid sequence encoding a variant of said polypeptide under a promoter operable in said cell;

said second promotor is activated by the polypeptide;

b) exposing said plurality of cells to said stimulus; and

4. A method for selecting a variant of a polypeptide, wherein said polypeptide catalyses conversion of a substrate to a product, wherein said product is capable of binding to a translational regulator, thereby modulating the translation efficiency of

i. an mRNA encoded by a taxis-enabling gene or

ii. an mRNA encoded by a taxis-enabling gene repressor, wherein the taxis-enabling gene repressor inhibits gene expression of the taxis-enabling gene;

said method comprising

a) providing a plurality of cells, wherein each of said plurality of cells: comprises a nucleic acid sequence encoding a variant of said polypeptide under a promoter operable in said cell;

i. translation of an mRNA encoded by said taxis-enabling gene is under control of a translational regulator, and said translational regulator is activated by the product of the polypeptide, or

ii. translation of an mRNA encoded by said taxis-enabling gene repressor is under control of a translational regulator, and said translational regulator is deactivated by the product of the polypeptide, and

b) exposing said plurality of cells to said stimulus; and

5. The method for selecting a variant of a polypeptide according to any one of the previous claims, wherein said taxis-enabling gene is a taxis-enabling gene selected from a prokaryote organism selected from any one of the genera Bacillus, Pseudomonas, Eschericha, and Salmonella, particularly a taxis-enabling gene selected from any one of the species Escherichia coli, Pseudomonas aeroguinosa, Pseudomonas fluorescence, Pseudomonas stutzeri, Pseudomonas pseudoalcaligenes, Pseudomonas spiringae, Pseudomonas putida, Pseudomonas oleovorans, Salmonella thyphimurium, Bacillus subtilis, or Bacillus licheniformis, and more particularly a taxis-enabling gene selected from

any one gene having a function in a prokaryote’s detection of an exogenous stimulus, particularly to an exogenous small chemical compound in the environment of the bacterium, or a gene having a function in intracellular signalling in response to the exogenous stimulus, particularly the E. coli and Salmonella thyphimurium chemoreceptors Tar, Tap, Trg, Tsr, the B. subtilis and B. licheniformis chemoreceptors HemAT, McpA, McpB, TlpA, TlpB, McpC, TlpC, YvaQ, YfmS, YoaH, the P. oleovorans, P. putida, P. fluorescence, P. aeroguinosa chemoreceptors McpA, McpB, PilJ, WspA, Pet A, PctB, PctC, CtpH, CtpL, McpS, McpK, PA1251 , PA1423, Pa1561 , PA1608, PA1646, PA2573, PA2652, PA2654, PA2788, PA2867, PA2920, PA4290, AP4530, PA4633, PA4915, and conserved bacterial signalling proteins CheA, CheB, CheC, CheD, CheR, CheV, CheW, CheZ, or FliY; a protein involved in the formations of the flagellar apparatus, particularly for assembly of the flagella filaments extending into the extracellular space, the base of the flagella-stand anchoring the flagella in the bacterial wall and extending into the cytosol, or the assembly of the machinery employed for generation of the torque enabling rotation of the flagella, more particularly a protein selected from FliD, FliC, FlgL, FlgK, FliK, FlgD, FlgG, FlgF, MotY, MotX, FlgH, FlgL, MotB, MotA, FliE, FlgB, FlgC, FliF, FliG, FliM, FlhB, FlhA, FiH, FliL, FliO, FluQ, FliT, FliS, FliJ, FliN, FliA, FliP, FliR, FlhC, FlhD, FlgM, and FliA.

6. The method for selecting a variant of a polypeptide according to any one of the preceding claims, wherein said stimulus is a concentration gradient of a chemical compound, particularly a chemical compound selected from:

a list of sugars, aminoacids, acids, di- or oligopeptides, sugars, di- or oligosaccharides, and environmental chemicals, more particularly of acetate, butyrate, citrate, glutaric acid, indole acetic acid, malate, succinate, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryoptophane, tyrosine, valine, 2-aminobenzoate, 3- aminobenzoate, 4-aminobenzoate, arogenate, benzene, benzoate, benzonitrile, benzoylformate, m-bromotoluene, p-bromotoluene, butylbenzole, catechol, 4-chloroaniline, 2-chlorobenzoate, 3-chlorobenzoate, 4-chlorobenzoate, o-chlorotoluene, m-chlorotoluene, p-chlorotoluene, 2,4- dichlorophenoxyacetate, 2,3-dimethylphenol, 3,4-dimethylphenol, ethylbenzene, p-ethyltoluene, fluorobenzene, o-fluoroptoluene, m- fluoroptoluene, p-fluoroptoluene, hydroquinone, p-hydroxymandelate, o- iodotoluene, m-iodorotoluene, p-iodorotoluene, isopropylbenzene, mandelate, 3-methylcatechol, 4-methylcatechol, 2-methylphenol, 3- methylphenol, nitrobenzene, 2-nitrobenzoate, 3-nitrobenzoate, 4- nitrobenzoate, 4-nitrocatechol, p-nitrophenol, o-nitrotoluene, m-nitrotoluene, phenol, phenylactic acid, phenoxyacetate, beta-phenylbutyrate, propylbenzene, salicylate, o-toluate, m-toluate, p-toluate, toluene, o-toluidine, m-toluidine, p-toluidine, 2,4,5-trichlorophenoxyacetate, 1 ,2,4- trimethylbenzene, 1 ,3,5-trimethylbenzene, o-xylene, m-xylene, biphenyl, 2- chlorobiphenyl, 3-chlorobiphenyl, 4-chlorobiphenyl, 2,3-dichlorobiphenyl, cis- 1 ,2-dichloroethane, trans-1 ,2-dichloroethane, 1 ,1-dichloroethane, ethylene, trichloroethylene, tetrachloroethylene, vinyl chloride, furfural, furfural alcohol, furoic acid, 5-hydroxymethylfurfural, dichloromethane, chloroform, trichloroethane, cytidine, cytosine, thymidine, thymine, uracil, uridine, ametryn, atrazine, cyanuric acid, N-iospropylammelidine, gamma- aminobutyric acid, alpha-aminoisobutyrate, ammonium, ammonium chloride, fumarate, galactose, glucose, Maleate, maltose, methylthioisocyanate, methylparathion, morphine, naphthalene, oxygen, phosphate, ribose, 1 ,2,3,4- tetrahydronaphthalene, thiocyanic esters, isothiocyanic esters, dilaurosylphosphatidylethanolamine, dioleoylphosphatidylethanolamine, phosphatidylethanolamine, palmitoleic acid, oleic acid, arachidonic acid, N- acetyl-D-glucosamine, D-fructose, 1-D-glycerol- beta,-D-galactoside, 6- deoxy-D-glucose, methyl-beta-D-glucoside, D-fucose, methyl-beta-D- galactoside, methyl-beta-D-glucoside, L-arabinose, D-xylose, L-sorbose, 2- deoxy-D-glucose, D-glucosamine, D-mannose, D-glucosamine, 2-deoxy-D- glucose, methyl-beta-D-glucoside, methyl-alpha-D-glucoside, D-mannitol, D- sorbitol, D-ribose, D-ribulose, trehalose or their halogenated derivatives or Ni²⁺, Cu²⁺, Cd²⁺, Zn²⁺.

7. The method for selecting a variant of a polypeptide according to any one of the preceding claims, wherein the taxis-enabling gene is activated in response to a binding event of

a polypeptide or polynucleotide based transcriptional or translational regulator, particularly a transcriptional regulatory protein;

a regulated promoter sequence;

an auxiliary regulatory protein component;

a riboswitch, or an siRNA.

8. The method for selecting a variant of a polypeptide according to any one of the preceding claims, wherein said cells capable of exceeding a threshold of directional tactic behavior are selected via

a disc diffusion assay,

a Boyden chamber,

a Zigmond chamber,

a Dunn chamber, a thermotaxis or a magnetotaxis assay.

9. The method for selecting a variant of a polypeptide according to claim 8, wherein said threshold of directional, tactic behavior exceeds the average tactic behavior level by at least 10 %, particularly by at least 30 % and more particularly by at least 80 % when being subjected to any of the assay conditions outlined in claim 8.