Abstract:
Eubacterial cells which produce non-formylated proteins, polypeptides and/or peptides and methods for making such non-formylated products. The eubacterial cells may be produced by inactivating or deleting the def (deformylase) and/or fmt (Met-tRNA transformylase) gene(s) from the def-fmt operon of the corresponding wild-type strain.

Description:
CROSS REFERENCE TO A RELATED APPLICATION 
     This application claims priority to U.S. application Ser. No. 10/189,505, filed Jul. 8, 2002 now U.S. Pat. No. 6,962,807 (now allowed) and to U.S. Provisional Application 60/303,065, which was filed on Jul. 6, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention is useful for production of recombinant proteins in eubacterial hosts. An eubacterial host defective in the genes for Met-tRNAi transformylase and polypeptide deformylase is described which grows in minimal and complex nutrient media at 30° C., 37° C. and 42° C. with near wild-type rate. In this  eubacterium  protein synthesis does not require N-formyl methionine as the initiator methionine, protein synthesis instead is initiated with unmodified methionine. The absence of peptides which retain N-formyl methionine in this  eubacterium  makes it particularly suited for the expression of recombinant proteins for pharmaceutical use. 
     BACKGROUND OF THE INVENTION 
     In eubacteria, peptide synthesis is initiated at methionine start codons which are read by N-formyl methionine tRNA. Prior to translation initiation the methionyl moiety of the charged tRNA is N-formylated by the action of Met-tRNAi transformylase (E.C.2.1.2.9). The N-formyl group is removed from the native protein by polypeptide deformylase (E.C. 3.5.127), and the initiator methionine can then be cleaved off by methionine aminopeptidase, completing the primer methionine cycle. In contrast, archaea and eukaryotes have a primer methionine cycle devoid of N-formylating and deformylating activities (for review see Mazel, D., Pochet, S. and Marliere, P.: Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13 (1994) 914-923, and Mazel et al., 1996). 
     Expression of eukaryotic proteins in eubacterial hosts often results in the production of recombinant proteins that retain an N-terminal forrnylrnethionyl residue (examples include bovine somatotropin (Bogosian, G., et al., (1989) Biosynthesis and incorporation into protein of norleucine by  Escherichia coli  J. Biol. Chem. 264:531-539); eel growth hormone (Sugimoto, S., Yamaguchi, K. and Yokoo, Y.: Isolation and characterization of recombinant eel growth hormone expressed in  Escherichia coli . J. Chromatog. 515 (1990) 483-494); human granulocyte colony-stimulating factor (Clogston, C. L., Hsu, Y. R, Boone, T. C. and Lu, H. S.: Detection and quantitation of recombinant granulocyte colony-stimulating factor charge isoforms: comparative analysis by cationic-exchange chromatography, isoelectric focusing gel electrophoresis, and peptide mapping. Anal. Biochem. 202 (1992) 375-383); bovine fatty acid-binding protein (Specht, B., Oudenampsen-Kruger, E., Ingendoh, A., Hillenkamp, F., Lezius, A. G. and Spener, F.: N-terminal variants of fatty acid-binding protein from bovine heart overexpressed in  Escherichia coli . J. Biotechnol. 33 (1994) 259-269); bovine cytochrome P450 (Dong et al FASEB J. 9 (1995) A1486); Methanothermus fervidus histone A (Sandman, K., Grayling, R. A., and Reeve, J. N. Improved N-terminal Processing of Recombinant Proteins Synthesized in  Escherichia coli . Biotechnology 13 (1995) 504-506); human interleukin-5 (Rose, K., Regarney, P., Anderegg, R, Wells, T., Proudfoot, A., Human interleukin-5 expressed in  Escherichia coli  has N-terminal modifications. Biochem J. 286 (1992) 825-828); human parathyroid hormone (Rabbani, S. A., Yasuda T., Bennett H. P. J., Sung, W. L. Zahab, D. M., Tam, C. S. Goltman, D., and Hendy, G. N. Recombinant Human Parathyroid Hormone Synthesized in  Escherichia coli . Journal of Biological Chemistry 263:3 (1988) 1307-1313; Hogset, A., Blingsmo, O. R., Gaurvk V. T., Saether, O., Jacobsen, P. B., Gordeladzo, J. O., Alestrom, P. and Gautvik, K. M. Expression of Human Parathyroid Hormone In  Escherichia coli  Biochemical and Biophysical Research Communications 166:1 (1990) 50-60); human gamma-interferon (Honda, S., Asano, T., Kajio, T., and Nishimura, O.  Escherichia coli -Derived Human Interferon-.gamma. with Cys-Tyr-Cys at the N-Terminus is Partially Nα-Acylated. Archives of Biochemistry and Biophysics 269 (1989) 612-622)). In addition retention of N-formyl methionine has been found in endogeneous  E. coli  proteins (Hauschild-Rogat, P. N-formylmethionine as a N-terminal group of  E. coli  ribosomal protein. Mol. Geri. Genet. 102 (1968) 95-101, Marasco, W. A., Phan, S. H., Kruusch, H., Showell, H. J., Feltner, D. E., Nairn, R, Becker, E. L. and Ward, P. A.: Purification and identification of formyl-methionyl-leucyl-phenylalanine as the major peptide neutrophil chemotaetic factor produced by  Escheriehia coli . J. Biol. Chem. 259 (1984) 5430-5439; Milligan, D. L. and Koshland, Jr., D. E.: The amino terminus of the aspartate chemoreceptor is formylmethionine. J. Biol. Chem. 265 (1990) 4455-4460). Since N-formylated peptides are a major indicator of eubacterial infections for the mammalian immune system and are highly immunogenic, incomplete deformylation precludes; for example, the use of N-formylated preparations for therapeutic purposes. Several approaches to circumvent this problem have been proposed, e.g., expression in the presence of trimethoprim and thymidine (Sandman et al., 1995), overexpression of peptide deformylase in the host (Warren, W. C., Bentle, K. A., Schlittler, M. R, Schwane, A. C., O&#39;Neil, J. P. and Bogosian, G. (1996): increased production of peptide deformylase eliminates retention of formylmethionine in bovine somatotropin overproduced in  Escherichia coli . Gene 174, 235-23), expression as a protein fusion either with an N-terminal peptide that can be removed in vitro by a specific protease or with an N-terminal leader peptide which is cleaved during transport of the nascent protein in a non-cytoplasmatic compartment. Finally, the N-formyl group may also be removed by mild acid hydrolysis, or the fraction of the protein retaining N-formyl methionine may be separated from the correctly processed protein by purification procedures. 
     Each of these approaches has significant disadvantages. Addition of trimethoprim and thymidine is costly, requires manipulation of the culture that will express the recombinant protein, and may slow down growth of the host. Overexpression of peptide deformylase requires a stable plasmid construct in the host that has to be selected for; moreover, deformylation may be less than 100% effective. Expression of fusion proteins requires exact molecular constructions; chemical hydrolysis with acid may cause damage to the rest of the protein. Finally, none of these approaches guarantees a final preparation that is absolutely free of N-formylated peptides derived either from the recombinant protein or from contaminations with endogeneous host peptides. 
     SUMMARY OF THE INVENTION 
     The present invention is useful for production of recombinant proteins in eubacterial hosts. An eubacterial host defective in the genes for Met-tRNAi transformylase and polypeptide deformylase is described which grows in minimal and complex nutrient media at 30° C., 37° C. and 42° C. with near wild-type rate. In this  eubacterium  protein synthesis does not require N-formyl methionine as the protein initiator protein synthesis instead being initiated with unmodified methionine. The absence of peptides which retain N-formyl methionine in this  eubacterium  makes it particularly suited for the expression of recombinant proteins for pharmaceutical use. 
     The present invention relates to the use of a bacterial strain lacking of transformylase and deformylase activity for the production of non formylated peptides or proteins and also the selection of a mutant bacteria which is not spontaneously reversible to the wild type for a formylase and deformylase activities but has a doubling generation time similar to the growth rate of the wild type bacteria. 
     More particularly the invention provides the use of a bacterial strain lacking of deformylase and/or transformylase activities characterized in that it is non reversible for the deformylase and/or transformylase activities 
     The invention also provides the use of the bacterial strain according to the invention characterized in that it develops at a temperature of 37° C. 
     The invention further relates the use of a bacterial strain according to the invention characterized in that the bacteria is  E. coli.    
     The invention also provides bacteria strain lacking of the protein deformylation and/or transformylation activity characterized in that: 
     said bacteria is not able to revert spontaneously for these activities, 
     the growth rate of said bacteria is at least equivalent to the wild type growth rate. 
     The invention is also concerned with the use of the bacteria according to the invention for production of nonformylated homologous or heterologous peptides or proteins. 
     The invention also relates to a preparation process of peptides or proteins comprising the following steps: 
     a) culture of the bacteria according to the invention, 
     b) transforming the bacteria according to step a) with a plasmid or a vector comprising an insert containing a polynucleotide coding homologous or heterologous peptide or protein, 
     c) production of the peptide or the protein by the bacteria, and 
     d) optionally, separation of the peptide or protein of interest from the bacterial culture. 
     Another object of the invention concerns a process according to the invention in which the bacteria comprises a polynucleotide coding for a mutator allele and a process according the invention in which the bacteria mutator phenotype has been complemented or the mutator allele has been replaced by the wide type allele. 
     The invention also concerned a purified polynucleotide comprising a gene coding for a mutator allele and contained in the bacteria strain according to the invention and a purified polynucleotide according to the invention, in which the mutator phenotype has been complemented or the mutator allele has been replaced by the wild type allele. 
     The invention further relates to an  E. coli  bacterial strain γ 2045 culture deposited at the CNCM (Collection Nationale de Cultures de Microorganismes), Institut Pasteur, 25 rue du Dr Roux, Paris CEDEX 15, France, on Jul. 5, 2001 under accession number I-2694. 
     Another aspect of the invention is directed to a process for the selection of mutants of a bacteria according to the invention characterized by the culture under continuous proliferation conditions of said mutants and separation of the mutant of interest from the static bacteria. 
     The invention is also concerned with a purified protein obtained after the expression of the heterologous or homologous insert of interest and deprived of formyl residue. 
     Other objects and advantages of the present invention will be apparent upon reading the following non restrictive detailed description made with reference to the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 : The primer methionine cycle in eubacteria ( 1   a ) and archaea and eukaryotes ( 1   b ). metG, met-tRNA synthetase; fmt, met-tRNAi transformylase; def, polypeptide deformylase; map, methionine aminopeptidase; aa, amino acid; f, formyl; pp, polypeptide. Modified after (Mazel, D., Pochet, S. and Marliere, P. (1994): Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13, 914-923). 
         FIG. 2 : In vivo evolution of a D(def-fmt) mutant under permanent proliferation. ( FIG. 2   a ) Cells were kept under permanent proliferation in minimal medium at 37° C. A turbidostat regime at 5×10 8  cells/ml was applied. Growth rates are averaged over 24 h periods. Two independent runs are shown. ( FIG. 2   b ) Input ( 1 ) and evolved strains isolated during the process ( 2 ,  3 ,  4 ; c.f., numbers and open circles in a) were grown on minimal agar for 36 h at 37° C. 
         FIG. 3 : In vivo evolution of a D(def-fmt) mutant at increased mutation rates. Two independent runs are shown. 
         FIG. 4 : Emergence and selection of adhesive variants in a conventional turbidostat, and counter-selection of adhesive variants by Evologic&#39;s process. Starting at point  1 , adhesive variants were allowed to compete with cells in suspension, periodic destruction of static variants was re-established at point  3 . ( FIG. 4   a ) Growth rates of the populations as measured in batch culture. ( FIG. 4   b ) Adhesion of cell material to glass surfaces. Isolates (points  1 - 6  in a) were cultivated in glass tubes for 20 h at 37° C. Arrowheads point to material that accumulated on the surface during cultivation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     PEPTIDE: includes any natural or synthetic compounds containing two or more amino acids. Therefore, it comprises proteins, glycoproteins, and protein fragments derived from pathogenic organisms such as viruses, bacteria, parasites and the like, or proteins isolated from normal or pathogenic tissues, such as cancerous cells. It also includes proteins and fragments thereof produced by recombinant means that has been associated or not with other peptides coding for tumoral, viral, bacterial or fungic epitopes for forming a fusion protein. 
     POLYNUCLEOTIDE: any DNA, RNA sequence or molecule having one nucleotide or more, including nucleotide sequences encoding a complete gene. The term is intended to encompass all nucleic acids whether occurring naturally or non naturally in a particular cell, tissue or organism. This includes DNA and fragments thereof, RNA and fragments thereof, cDNAs and fragments thereof, expressed sequence tags, artificial sequences including randomized artificial sequences. 
     VECTOR: a self replicating RNA or DNA molecule which can be used to transfer an RNA or DNA segment from one organism to another. Vectors are particularly useful for manipulating genetics constructs and different vectors may have properties particularly appropriate to express proteins in a recipient during cloning procedures and may comprise different selectable markers. Bacterial plasmids are commonly used as vectors. 
     The present invention describes an  E. coli  strain with a primer methionine cycle similar to that in eukaryotic cells. This strain no longer harbors the def fmt operon encoding Met-tRNAi transformylase and polypeptide deformylase and thus can not N-formylate Met-tRNAi. Removal of N-formyl groups from expressed proteins by any of the techniques described above is thus no longer required. 
     The fmt and def genes from  E. coli  were previously isolated (Guillon, J. M., Mechulam, Y., Schmitter, J. M., Blanquet, S. and Fayat, G.: Disruption of the gene for Met-tRNA-Net formyltransferase severely impairs growth of  Escherichia coli. J. Bacteriol.  174 (1992) 4294-4301; Mazel, D., Pochet, S. and Marliere, P.: Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13 (1994) 914-923) (def amino acid sequence is shown in SEQ ID NO: 1, def nucleotide sequence is shown in SEQ ID NO: 2; frit amino acid sequence is shown in SEQ ID NO: 3, and fmt nucleotide sequence is shown in SEQ ID NO: 4) and shown to be highly conserved among eubacteria (Mazel, D., Pochet, S. and Marliere, P.: Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13 (1994) 914-923). Deletion mutants for either the fmt gene (Guillon, J. M., Mechulam, Y., Schmitter, J. M., Blanquet, S. and Fayat, G.: Disruption of the gene for Met-tRNA-Net formyltransferase severely impairs growth of  Escherichia coli . J. Bacteriol. 174 (1992) 4294-4301) or the entire def-fmt operon (Mazel, D., Pochet, S. and Marliere, P.: Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13 (1994) 914-923, D[def-fmt]) were created. The resulting mutants were reported to be severely impaired in growth. The fmt mutant has an 8.61-fold decreased growth rate at 37° C. in rich medium and does not grow at 42° C. (Guillon, J. M., Mechulam, Y., Schmitter, J. M., Blanquet, S. and Fayat, G.: Disruption of the gene for Met-tRNA-Net formyltransferase severely impairs growth of  Escherichia coli . J. Bacteriol. 174 (1992) 4294-4301). The def-fmt mutant has a similarly decreased growth rate in minimal medium at 37° C., and growth is completely impaired in this medium at 42° C. (Mazel, D., Pochet, S. and Marliere, P.: Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13 (1994) 914-923). Whereas deletion of the fmt gene alone leaves the mutant bacteria viable, deletion of the def gene alone as well as re-introduction of the fmt gene into a def-fmt background is lethal (Mazel, D., Pochet, S. and Marliere, P.: Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13 (1994) 914-923), demonstrating that essential bacterial proteins either have to be deformylated, and/or that the initiator methionine has to be cleaved off in order to render these proteins functional. 
     For the purpose of the present invention, a def-fmt deletion mutant was selected for enhanced growth rates under permanent proliferation in minimal medium at 37° C. until its growth rate approximated that of the parent wild-type bacterium. 
     For a second application; the def-fmt deletion mutant previously selected for growth at 37° C. was selected for enhanced growth rates under permanent proliferation in minimal medium at 42° C. until its growth rate approximated that of the parent wild-type bacterium.
           eubacterium  with altered translational mechanism such that it contains no fmt and def genes yet grows at wt (wild-type) rate.     E. coli  formyl-free strain growing at temperatures higher than 37° C.   use of such strains for expression of recombinant proteins.   use of such strains for production of any product in  E. coli  which must not be contaminated with N-formylated peptides.       

     EXAMPLES 
     1. Selection of enhanced growth rate in the def-fmt mutant (figure). 
     2. Growth rate of evolved strains at 30° C., 37° C. and 42° C. in minimal medium and rich medium (table). 
     An  E. coli  strain for expression of N-formyl-free polypeptides. In eubacteria peptide synthesis is initiated at methionine start codon which are read by N-formyl methionine tRNA. Prior to translation initiation the methionyl moiety of the charged tRNA is N-formylated by the action of Met-tRNAi transformylase. The N-formyl group is removed from the native protein by polypeptide deformylase, and the initiator methionine can then be cleaved off by methionine aminopeptidase, completing the primer methionine cycle ( FIG. 1   a ). In contrast, archaea and eukaryotes have a primer methionine cycle devoid of N-formylating and deformylating activities ( FIG. 1   b ). 
     Expression of eukaryotic proteins in eubacterial hosts often results in the production of recombinant proteins that retain an N-terminal formylmethionyl residue. Since N-formylated peptides are highly immunogenic, incomplete deformylation precludes, for example, their use for therapeutic purposes. Several approaches to circumvent this problem have been proposed, e.g., expression in the presence of trimethoprim and thymidine (Sandman, K., Gryling, R. A. and Reeve, J. N. (1995): Improved N-terminal processing of recombinant proteins synthesized in  Escherichia coli . Biotechnology 13, 504-506 or overexpression of peptide deformylase in the host (Warren, W. C., Bentle, K. A., Schlittler, M. R., Schwane, A. C., O&#39;Neil, J. P. and Bogosian, G. (1996): increased production of peptide deformylase eliminates retention of formylmethionine in bovine somatotropin overproduced in  Escherichia coli . Gene 174, 235-238). 
     The inventors have opted for a radical solution, simplifying the primer methionine cycle in  Escherichia coli  by deletion of the def-fmt operon that encodes polypeptide deformylase and met-tRNAi transformylase, and improving the resulting, crippled strain, by selecting for increasing growth rates (and therefore improved rates of protein synthesis) under permanent proliferation in suspension. The inventors have isolated the def and fmt genes from  E. coli  and created a deletion mutant (D[def-fmt]) devoid of both genes (Mazel, D., Pochet, S. and Marliere, P. (1994): Genetic characterization of polypeptide deformylase, a distinctive enzyme of eubacterial translation. EMBO J. 13, 914-923). The resulting strain was found to be viable, however its growth rate was dramatically reduced, from 0.9 per h to 0.25 per h in minimal medium at 37° C. 
     Protein synthesis in living cells is dependent on the concerted action of a complex assembly of the protein and rRNA constituents of ribosomes and a host of factors catalyzing aminoacylation of tRNAs, initiation, elongation and termination of translation as well as maturation of nascent polypeptides. N-terminal formylation is among the most conserved features that distinguish eubacteria from archaea and eukaryotes. Removing the enzymes that catalyze the corresponding reactions is therefore expected to remove the efficiency of protein synthesis far from its wild-type optimum. Evolutionary resurrection from this type of genetic injury will require multiple adaptive mutations to render the bacterial translation machinery more similar to that found in eukaryotes. State-of-the-art technologies for directed evolution ex vivo are unable to predict and select the adaptive mutations that would re-establish wild-type protein synthesis rates in a D(def-fmt) background. 
     In vivo evolution of the D(def-fmt) mutant under permanent proliferation in suspension in a turbidostat regime yields variants with increasing growth rate; approximating wild-type growth rate after about 1 month (ca. 300 generations) of permanent selection ( FIG. 2   a ).  FIG. 2   b  shows drastically increased biomass production of evolved derivatives tested for growth on minimal agar as compared to the input D(def-fmt) mutant. Stepwise increases in the growth rate of the evolving population suggest selection and fixation of successive adaptive mutations. The inventors have evidence that the protein met-tRNA synthetase, certain ribosomal proteins, initiation factor 2, and methionine aminopeptidase are altered in the output strains. 
     The evolutionary process can be accelerated by increasing variation in the population ( FIG. 3 ). When mutation rates in the population under selection were increased by a factor of about 1,000, wild-type growth rates were approximated within about half the time required for the process shown in  FIG. 2   a.    
     Current technology for continuous proliferation of cells in suspension suffers a major drawback, selection of adhesive variants which stick to inner surfaces of the device and escape the selective pressure imposed by continuous or conditional dilution (Chao, L. and Ramsdell, G. (1985): The effects of wall populations on coexistence of bacteria in the liquid phase of chemostat cultures. J. Gen. Microbiol. 131, 1229-1236). In principle, this can be avoided by serial subculture of cells in suspension (Lenski, R. E. and Travisano, M. (1994): Dynamics of adaptation and diversification: A 10,000-generation experiment with bacterial populations. Proc. Natl. Acad. Sci. USA 91, 6808-6814), a technique where cells in suspension are frequently transferred into fresh culture vessels (i.e., surfaces are periodically discarded), creating a selective disadvantage for static variants. At an industrial scale, serial subculture technology has not been systematically exploited because it is laborious and requires absolute sterility during transfers. 
       FIG. 4 : automated technology for the permanent proliferation of populations of cells exclusively in suspension. During the course of an experiment similar to that shown in  FIG. 3 , operation of the device was manipulated such that static, adhesive variants were no longer destroyed and could freely compete with cells in suspension. Highly adhesive variants accumulated rapidly ( FIG. 4   b ). In parallel, the growth rate of the population decreased, demonstrating that these static variants are not subject to the selective pressure imposed on the cells in suspension. When proper operation of the device was re-established, these variants were rapidly and effectively eliminated from the evolving population. 
     The automated device describe in PCT WO 00/34433 is the first operational apparatus which allows permanent proliferation of living cells under defined, selective conditions. 
     The automated process frequently and effectively destroys static variants in any part of the device, overcoming the primary obstacle to continuous proliferation of cells in suspension for indefinite periods of time. 
     The inventors have created derivatives of  Escherichia coli  with a primer methionine cycle similar to that in eukaryotic cells. The strains will allow for expression of N-formyl-free polypeptides in  E. coli.    
     Evolved microbial strains with unique genetic and metabolic imprints will serve as ancestors for the diversification of lines of industrially fit microorganisms. 
     Our reference strain is γ2045:  Escherichia coli  MG1655 Δ(def-fmt):: cat, dnaQ::miniTn10 [CMR R , Tc R ]. 
     MG 1655 is a wild-type K12 strain of  E. coli  (see EMBO J. (1994) 13:914-923). Δ(def-fmt) means deletion of the def-fmt operon, which encodes the polypeptide deformylase and the Met-tRNAi transformylase activities. This allele has been described (EMBO J. (1994) 13:914-923). 
     The writing::cat means the insertion of a cat (chloramphenicol acetyl-transferase) germ in the Δ(def-fmt) locus dnaQ::miniTn10 means that the dnaQ gene (epsilon subunit of the DNA polymerase, the proof-reading subunit) is interrupted by the insertion of a minitransposon Tn10 which confers the tetracycline resistance [CmR, TcR] means that the strain is resistant to chloramphenicol (25 micg/ml) and to Tetracycline (15 micg/ml). 
     This bacterial strain carries a deletion of the def-fmt operon and is consequently defective in the Met-tRNAi transformylase and polypeptide deformylase activities. This strain also carries dnaQ mutation and consequently shows a mutator phenotype. This strain is a derivative of β2124 that has been selected to grow in minimal and complex nutrient media at 30° C., 37° C. and 42° C. with near wild-type rate (approximately 25 min in LB and approximately 80 min in MS minimal medium (Richard (1993), J. Biol. Chem. 268:26827-26835) with mannitol as carbon source at final concentration 0.2% at 37° C.). The original β2124 strain shows a growth rate of approximately 200 min in MS minimal medium+mannitol at 37° C. γ2045, the 2124 derivative, was selected for enhanced growth rates under permanent proliferation in minimal medium at 37° C. until its growth rate reached that of MG 1655, the parent wild-type bacterium. 
     The mutator phenotype can be rescued by complementation with a dnaQ wild type allele expressed either from a plasmid or from the chromosome, through an allele replacement in γ2045. 
     Subject: β2137. Our reference strain is 2137:  Escherichia coli  MG1655, Δfmt::cat [44°CS, CmR] MG1655 is a wild-type K12 strain of  E. coli  (see EMBO J. (1994) 13:914-923). Δfmt means deletion of the fmt gene which encodes the Met-tRNAi transformylase activity. This allele has never been described, it is a Pst I deletion, internal to fmt (nucleotides 247 to 484). 
     The writing::cat means the insertion of a cat (chloramphenicol acetyl-transferase) gene at the PstI site of the deletion. 
     The cat gene is identical to the one used for the construction of the (def-fmt)::cat allele (see EMBO J. (1994) 13:914-923). 
     [44 CS, CmR] means that the strain is thermosensitive and resistant to chloramphenicol (25 micg/ml). 
     This bacterial strain carries a deletion of the fmt gene and is consequently defective in the Met-tRNAi transformylase activity. 
     This strain is a derivative of MG1655 that has a growth rate of approximately 200 min in MS minimal medium+mannitol at 37° C.