Abstract:
Compositions relating to a combination of two types of separation matrix; and to variant host cells which contain at least one essential host protein that is fused to an affinity binding tag or has been mutated to replace at least two of a plurality of histidines or basic amino acids are provided. Methods are also provided that relate to isolating a recombinant protein from a lysate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Production of proteins of interest is commonly achieved in transformed competent host cells. A problem that arises during purification of such proteins is that contaminant host proteins co-purify with the protein of interest. One approach to tackling this problem is to form a fusion protein between the protein of interest and a protein tag that has an affinity to a matrix. It is intended that the contaminant proteins are washed away and a pure protein is recovered. An example of a protein tag that is widely used is a histidine tag (His-tag). This binds to a metal containing column. The method is called immobilized metal ion affinity chromatography (see for example U.S. Pat. No. 5,310,663). 
         [0002]    Unfortunately, contaminating host cell proteins which do not carry any form of tag may contain non-consecutive histidine residues or other metal binding motifs exposed to the surface of their ternary structure. These contaminating proteins also bind to nickel and/or cobalt containing purification resins to which the His-tagged protein of interest binds (see Bolanos-Garcia and Davies,  BBA  1760: 1304-1313(2006), and Edwards, et al.,  Nature Methods  5: 135-146(2008)), resulting in co-purification of these contaminants and failure to obtain a purified preparation of the protein of interest. 
       SUMMARY OF THE INVENTION 
       [0003]    Embodiments of the invention provide a composition that includes a variant host cell derived from a parent host cell where the host cell may be a prokaryotic cell such as a bacterial cell such as  E. coli  or a eukaryotic cell. The parent host cell is characterized by a genome encoding a plurality of essential host proteins, wherein one or more essential proteins contain a plurality of histidines or basic amino acids residues such that when the cell is lysed, these essential proteins are capable of binding to a metal chelating matrix. Examples of such essential proteins include SlyD, carbonic anhydrase (can), ArnA, ArnD AceE, AceF and GlmS. 
         [0004]    In one embodiment, the variant viable host cell differs from the parent host cell in that in the variant, at least one of the plurality of essential proteins is additionally fused to an affinity binding tag encoded by the genome, the fusion proteins being capable of binding to a non-metal affinity matrix. Examples of affinity binding tags include: an immunoaffinity tag, a peptide tag selected from hemagglutinin, c-myc, T7, Glu-Glu, GST-tag, ZZ, GB1, MCP, and ACP, a streptavidin binding tag or a chitin binding domain tag. Alternatively, at least one of the plurality of essential proteins is mutated such that at least two of the plurality of histidines or basic amino acid residues is replaced with non-histidine residues such as alanine so that the mutated essential protein is no longer able to bind the metal chelating matrix. The variant host cell is capable of being transformed to express a recombinant target protein. 
         [0005]    The host cell variants may further include a non-host DNA encoding a protein of interest. 
         [0006]    In an embodiment of the invention, a method is provided of isolating a recombinant target protein from a cell lysate, that includes (a) lysing variant host cells of the type described above, wherein the variant host cells are transformed with DNA encoding a target protein fused to a histidine-tag, (b) subjecting the lysed host cells to a metal chelating matrix and a non-metal affinity binding matrix where the metal chelating matrix and the non-metal affinity binding matrix may be contained in the same or different reaction vessels; and (c) isolating the recombinant protein from the cell lysate. 
         [0007]    In another embodiment of the invention, a composition is provided which includes an affinity non-metal binding matrix and a metal chelating matrix proximately located in a reaction vessel suitable for receiving a mixture of components and suitable for separating a subset of the components from the mixture. 
         [0008]    In another embodiment of the invention, a method is provided of isolating a recombinant protein from a cell lysate. The method includes: lysing variant host cells that express a recombinant protein fused to a His-tag and purifying the recombinant protein away from essential protein contaminants that have histidine residues or basic amino acids and are capable of binding to a metal chelating matrix. This is achieved either by fusing the DNA encoding one or more essential proteins to DNA encoding an affinity binding tag and substituting the essential protein in the host cell chromosome using homologous recombination; or mutating at least two histidines or basic amino acids in the essential protein contaminant so that the one or more essential proteins no longer bind to a metal chelating matrix. The lysed host cells are then exposed to at least one of a metal chelating matrix and a non-metal binding matrix; and the purified recombinant protein is obtained from the cell lysate. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]    The following description and the accompanying figures further describe and exemplify the features and advantages of the present invention, where: 
           [0010]      FIG. 1  shows pMAK-chitin binding domain (pMAKCBD) with a 3 prime polylinker sites and a 5 prime flank and target gene polylinker used for cloning of genomic DNA fragments for targeted allele exchange. 
           [0011]      FIG. 2  shows the nucleotide sequence of the pMAKCBD allele exchange vector (SEQ ID No: 49). 
           [0012]      FIG. 3A  shows the results of gel electrophoresis of samples obtained by column fractionation of cell lysates of ER3135 over-expressing a target protein; His6-tagged alanine tRNA synthetase (AlaRS(6His)). The column used here is an Ni-NTA column (HisTrap™ column; GE Healthcare, Waukesha, Wis.). 
           [0013]    Lanes 1 and 10 contains a 10-250 kDa protein ladder (New England Biolabs, Inc., Ipswich, Mass.). 
           [0014]    Lane 11 is Ni-NTA column flow through. 
           [0015]    Lanes 2-9 and 12-22 are imidazole elution fractions containing eluted His-tag proteins and metal binding contaminating proteins. 
           [0016]    Arrows identify the target protein AlaRS(6His) (approx 90 kDa). The black box shows that native  E. coli  protein SlyD (approx. 26-28 kDa) is a metal binding contaminating protein and co-elutes with target AlaRS(6His) protein. 
           [0017]      FIG. 3B : shows the results of gel electrophoresis of samples obtained by column fractionation of cell lysates of ER3135(slyD-CBD derivative) over-expressing a target protein AlaRS(6His). The column used here is an Ni-NTA column. In the absence of a chitin purification step, it was shown here that the co-eluting SlyD-CBD migrated at a position that was consistent with the presence of the CBD-tag on the SlyD. 
           [0018]    Lanes 1 and 14 contains the 10-250 kDa protein ladder. 
           [0019]    Lane 15 is HisTrap™ flow through. 
           [0020]    Lanes 2-13 and 16-26 are imidazole elution fractions. 
           [0021]    The arrows identify AlaRS(6His). SlyD-CBD migrates at 35-40 kDa and is highlighted by a dotted black box. 
           [0022]      FIG. 3C  shows results of a Western Blot in which anti-CBD antibody is applied to the gel in  FIG. 3B  confirming that the band on the gel at 35-40 kDa corresponds to SlyD-CBD contamination of the AlaRS(6His) in lanes 9-13 and 16-25 (see dotted black line box). 
           [0023]      FIG. 4  shows removal of the CBD-tagged contaminants from the AlaRS(6His) (protein of interest) using a chitin column following a Ni-NTA chromatography step (QIA express manual, (Qiagen, Germantown, Md.)). Ni-NTA and chitin column fractions were analyzed by Western blotting using an anti-CBD antibody (New England Biolabs, Ipswich, Mass.) which reacts with any CBD in the fraction. The positions of bands corresponding to AceE-CBD; ArnA-CBD; SlyD-CBD; and Can-CBD are marked. The host strain used in this experiment is ER3203. The Ni-NTA fractions correspond to lanes identified as Ly, S, ft, W1, W2 and P The fractions obtained after affinity binding to a chitin column overnight are shown in lanes F, W, B, F, W B. 
           [0024]    Lane L contains a biotinylated protein ladder (Cell Signaling Technology, Beverly, Mass.). 
           [0025]    Lane Ly is a sonication lysate. 
           [0026]    Lane S is a supernatant from the sonicated sample. 
           [0027]    Lane ft is a flow-through from the Ni-NTA column. 
           [0028]    Lane w1 contains the first wash of the Ni-NTA column. 
           [0029]    Lane w2 contains the second wash of the Ni-NTA column. 
           [0030]    Lane P contains the pooled fractions of Ni-NTA eluate which contain tagged protein without chitin column purification 
           [0031]    Lanes F contains a flow-through from chitin columns after 1 hr or 18 hrs of incubation. 
           [0032]    Lanes W contains a wash of chitin columns after 1 hr or 18 hrs of incubation. 
           [0033]    Lanes B contains the eluate obtained from boiling the chitin resin which had been incubated for 1 hr or 18 hrs. 
           [0034]    Lane WC contains whole cell lysate from  E. coli  ER3203 encoding AceE-CBD, ArnA-CBD, SlyD-CBD, Can-CBD and over-expressing AlaRS(6His). 
           [0035]      FIG. 5  shows positions of histidine (His) residues within the GlmS amino acid sequence which were mutated to alanine (Ala). Three mutants are shown: the GlmS(2Ala) mutant containing the mutations His62Ala and His65Ala, the GlmS(4Ala) mutant containing the mutations His432Ala, His436Ala, His466Ala and His467Ala, and the GlmS(6Ala) mutant containing all the mutations of the 2Ala and 4Ala mutants. 
           [0036]      FIG. 6  shows the results of SDS-PAGE of the Ni-NTA column elution of cell extract proteins of  E. coli  ER3135 expressing WT GlmS, Glms(2Ala) or GlmS(6Ala) from the pMAK705 vector. 
           [0037]    Lane (M) contains 7-175 kDa protein marker. 
           [0038]    Lane (K) shows Ni-NTA binding proteins from ER3135 expressing WT GlmS. 
           [0039]    Lane (2) shows Ni-NTA binding proteins from ER3135 expressing the GlmS(2Ala). 
           [0040]    Lane (6) shows Ni-NTA binding proteins from ER3135 expressing the GlmS(6Ala). An arrow indicates the expected position of GlmS (67 kDa) which is absent owing to the altered properties of GlmS(6Ala) that prevent it from binding. 
           [0041]      FIG. 7  shows SDS-PAGE results of over-expressing AlaRS(6His) in 3 strains—ER3135 which are wild type cells with no modification; Nico21(DE3) which contains three CBD-tagged non-target nickel binding proteins and GlmS(6Ala), and Nico22(DE3) which contains four CBD-tagged non-target nickel binding proteins and the GlmS(6Ala). 
           [0042]    M=marker 
           [0043]    P=pooled samples from Ni-NTA eluate enriched in AlaRS(6His) target protein 
           [0044]    FT=flow through after exposing Ni-NTA eluate to chitin resin 
           [0045]    B=eluate obtained from boiling the chitin resin to release bound CBD-tagged proteins 
           [0046]    Arrows identify AceE-CBD, AlaRS(6His), ArnA-CBD and SlyD-CBD. GlmS(6Ala) is not detected in Nico21(DE3) and Nico22(DE3) samples because the mutations eliminate binding to the Ni-NTA column. The position of WT GlmS is identified by a black triangle and WT SlyD is identified by black square in ER3135 pool (P) and flow through (FT). 
           [0047]      FIG. 8A-D  are DNA sequences cloned into pMAKCBD in order to perform chromosomal allele exchange. 
           [0048]      FIG. 8A  is a sequence of the aceE-CBD-aceF allele (SEQ ID NO: 50). 
           [0049]      FIG. 8B  is a sequence of the arnA-CBD-arnD allele (SEQ ID NO: 51). 
           [0050]      FIG. 8C  is a sequence of the can-CBD allele (SEQ ID NO: 52). 
           [0051]      FIG. 8D  is a sequence of the slyD-CBD allele (SEQ ID NO: 53). 
           [0052]      FIG. 9  shows a schematic of the process of using a metal chelating matrix and an affinity binding matrix to purify a protein of interest expressed in Nico cells. 
           [0053]    1. Cell lysate in solution is applied to the metal chelate chromatography column. 
           [0054]    2. The lysate is allowed to flow through the column. The His-tagged protein of interest (*) and the affinity-tagged contaminating proteins (white squares and circles) remain bound to the column after rinsing. 
           [0055]    3. The bound proteins are eluted with buffer which weakens the binding of the proteins to the metal ion. Fractions are collected from the column and tested for the presence of the protein of interest. Fractions containing the protein of interest are pooled. 
           [0056]    4. The pooled fractions are applied to a second chromatography column containing a matrix which specifically binds the affinity-tagged contaminating proteins. The flow-through containing the isolated protein of interest is retained. 
           [0057]    5. The affinity-tagged contaminating proteins bound to the matrix may be eluted with a buffer which weakens the binding of the affinity-tag to the matrix. A particular example is illustrated here, in which a column containing a chitin matrix is treated with boiling water to release bound CBD-tagged proteins. 
           [0058]      FIG. 10  shows a schematic of the process of using an affinity layer and a metal chelate layered column to purify a protein of interest expressed in Nico cells. 
           [0059]    1. Cell lysate in solution is applied to a column containing distinct matrix layers, wherein the solution runs through a matrix layer which specifically binds affinity-tagged contaminating proteins (“affinity layer”) before the solution runs through a metal chelating matrix (“metal chelate layer”). 
           [0060]    Note: the position of the layers may be reversed so that the cell lysate is exposed to the metal chelate layer before the affinity layer. 
           [0061]    2. The lysate is allowed to flow through the column and then the column is rinsed. The affinity-tagged contaminating proteins (white squares and circles) bind to the affinity layer while the His-tagged proteins of interest flow through the affinity layer and are bound by the metal chelate layer. 
           [0062]    3. The bound proteins of interest are eluted with buffer which weakens the binding of the metal-binding peptide tag to the metal chelate layer but does affect the binding of the affinity-tagged proteins for the affinity layer. Fractions are collected from the column and tested for the presence of the protein of interest. Fractions containing the isolated protein of interest are retained. 
           [0063]    4. The affinity-tagged contaminating proteins bound to the matrix may be eluted with a buffer which weakens the binding of the affinity-tag to the matrix. A particular example is illustrated here, in which a column containing a chitin matrix is treated with boiling water to release bound CBD-tagged proteins. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0064]    Embodiments of the invention provide solutions to the problem of co-purification of contaminating essential host cell proteins containing histidines with His-tagged proteins of interest so that His tagged proteins of interest may be readily isolated from essential host cell proteins containing histidine residues. (“His-tag” as used here refers to more than 2 consecutive His residues in a protein fusion tag (EP0282042; U.S. Pat. No. 5,284,933, U.S. Pat. No. 5,310,663) and includes protein fusion tags capable of metal binding where histidine residues are non-consecutive (see US 2006/0030007 and U.S. Pat. No. 7,176,298)). The methods described herein can be readily applied to non-essential host proteins also although it is a relatively easy matter to delete or mutate the host genes encoding the non-essential proteins. 
         [0065]    In one embodiment, a desired expression strain was generated using an allele exchange vector containing a gene encoding a contaminating histidine-containing essential protein fused to an open reading frame (ORF) encoding a protein affinity binding tag. The term “affinity binding tag” refers to a peptide or protein that is not a metal binding protein or peptide. Once transformed with the plasmid, the host cell could express a fusion protein that included an affinity binding protein fused to the contaminating essential host protein wherein the essential protein was active and the host cell viable. The fusion gene was inserted into the host cell chromosome at the native gene locus to replace the native gene by homologous recombination (For example, see Hamilton, et al.,  Journal of Bacteriology:  4617-4622 (1989)). In this way viability of the host cell was preserved by expression of an active affinity-tagged protein from the native gene locus and subsequent expression of the His-tagged protein of interest was possible. 
         [0066]    The protein of interest could then be purified away from contaminating host metal binding proteins. In one embodiment, the crude cell lysate was first added to a metal chelating matrix which separates metal binding proteins from non-metal binding proteins resulting in a purified mixture of the protein of interest and contaminating histidine-containing fusion proteins. The fusion protein (affinity binding protein-contaminating essential host protein) was then removed by means of an affinity matrix from the target proteins. 
         [0067]    In other embodiments of the method, the metal binding chelating matrix could be used after binding of the cell lysate to an affinity matrix or both matrices could be used at the same time for purifying the protein of interest. The purification may be performed in separate or the same reaction vessels. 
         [0068]    Examples of  E. coli  host proteins that were reported to be co-eluted from IMAC resin in significant amounts were DnaK, GlmS, AceE, EF-Tu, ArnA, RNase E, AtpF, CRP, Can, the Rho transcription terminator and SlyD. The most consistent and significant contaminants are SlyD, GlmS, Can, ArnA and AceE. The examples show how genes expressing these proteins were targeted by modifying the host chromosome using homologous recombination. Using the same or similar approach, any desired protein additional to those in the examples can be similarly targeted and then removed from the preparation containing the protein of interest (target protein) by affinity-binding to a selected affinity matrix. 
         [0069]    There are a wide range of affinity binding proteins or peptide-tags known in the art that are characterized by being capable of binding to an affinity matrix. Any of these may be utilized in embodiments of the invention. These include: immuno-affinity tags such as FLAG tag (DYKDDDDK)(SEQ ID NO: 1) that binds to ANTI-FLAG® M2 Affinity Gel (Sigma, St. Louis, Mo.); hemagglutinin (HA); c-myc, T7; Glu-Glu (which mediates protein binding to the respective immobilized antibody or ligand (Table 9.9.1 in  Current Protocols in Protein Science , authors Michelle E. Kimple and John Sondek (2004)); StrepII Tag (WSHPQFEK) (SEQ ID NO: 2) (that binds to streptavidin and StrepTactin™ resin (GE Healthcare, Waukesha, Wis.)); and Biotin Carboxyl Carrier Protein (BCCP) (a natural substrate for BirA biotin ligase (Cronan, J. E.,  J. Biol. Chem.  265:10327-10333 (1990)). BCCP-tagged proteins are biotinylated in vivo in birA+ expression hosts. The biotin group mediates protein binding to streptavidin and StrepTactin™ resin. AviTag™ (GeneCopoeia, Rockville, Md.) (GLNDIFEAQKIEWH) (SEQ ID NO: 3) may also be biotinylated by the BirA protein in vivo or in vitro (Beckett D., et al.,  Protein Science  8: 921-929 (1999)). This biotinylated peptide is capable of high affinity binding to streptavidin and StrepTactin™ resin. The S-Tag™ (EMD Biosciences, Darmstadt, Germany) binds to S-protein agarose. The GST-tag, ZZ-tag, GB1-tag are also suitable for contaminant protein tagging (F. Freuler, et al.,  Protein Expression and Purification  59: 232-241(2008)). YbbR-tags (J. Yin, et al.,  PNAS  102: 15815-15820 (2005)) may be specifically labeled with biotin by Sfp phosphopantetheinyl transferase for subsequent binding to streptavidin and StrepTactin™ resin. S6 and A1 peptides were identified from a phage-display library as efficient substrates for site-specific protein labeling catalyzed by Sfp and AcpS phosphopantetheinyl transferases (Zhou, Z., et al.,  ACS Chem. Biol.  2: 337-346 (2007)). Labeling with biotin-CoA allows for subsequent binding of the tagged protein to streptavidin and StrepTactin™ resin. The MCP-tag and ACP-tag (New England Biolabs, Inc., Ipswich, Mass.) may be labeled with derivatives of coenzyme A (e.g. biotin-CoA). In the labeling reaction, the substituted phosphopantetheine group of CoA is covalently attached to a conserved serine residue by SFP-Synthase or ACP-Synthase, respectively. CBD tag is small and binds chitin very tightly. In the examples described herein, the chitin binding domain from  Bacillus circulans  is used as the fusion affinity-tag for  E. coli  contaminant proteins. 
         [0070]    Where “matrix” is used, this is intended to refer to any of a porous or non-porous two dimensional surface coating of a surface such as a coating of surface of a reaction vessel or chip or three dimensional porous or non-porous structure such as a bead, column, or paper. 
         [0071]    The host cell can be any bacterial cell such as  E. coli  or a eukaryotic cell that is capable of being transformed or transfected with a vector suitable for making a protein of interest. 
         [0072]    Various vectors can be designed for use in homologous recombination such as for example, allele exchange vectors. A desirable feature of allele exchange vectors which recombine with the chromosome is the ability to select for those cells in which recombination has occurred. The examples of vectors provided here are not intended to be limiting and any person of ordinary skill in the art will appreciate that any selectable marker commonly in use will be effective. In the examples below, the pMAKCBD vector which contains a temperature-sensitive origin of replication, has a chloramphenicol selectable marker. These features allow for selection of cells where the vector is integrated in the chromosome when agar plates are incubated at the non-permissive temperature for plasmid replication. Alternatively, direct allele exchange may be performed by introduction of linear DNA into cells. Transformation of linear DNA is preferably linked to direct selection of cells with the desired phenotype. (Swingle B., et al.,  Mol. Microbiol.  75: 138-148 (2010)). 
         [0073]    The examples provided demonstrate the proof of principle described here using a CBD-tag fused to the essential contaminating proteins. Any of the other affinity-tags described may be utilized for this purpose. The contaminating proteins naturally contain a plurality of histidines or other basic amino acids that are capable of binding to a nickel column along with His-tagged proteins of interest. For essential contaminating proteins, the activity of the protein must be preserved to maintain viability of the host cell in order to express the protein of interest. 
         [0074]    Embodiments of the modified host cells described herein contain multiple contaminating essential proteins where some or all of the chromosomal genes encoding these proteins have been individually modified so as to be fused to a non-His-tag when expressed in the cell. In addition certain essential contaminating proteins may also be mutated such that the plurality of histidines (or basic amino acids) are replaced by a different amino acid such as alanine (GlmS)(see also Example 5). 
         [0075]    All references cited herein are incorporated by reference including Robichon et al. Applied and Environmental Microbiology, 77, p 4634-4646 (2011) and provisional application 61/381,736 which is the priority document for the present application. 
       EXAMPLES 
       [0076]    To assist in understanding the present embodiments of the invention strain genotypes and descriptions are given below: 
         [0000]    Parent strain: ER3135=BL21(DE3) fhuA2
 
Thus ER3135=fhuA2 [Ion] ompT gal (α DE3) [dcm] ΔhsdS
 
λ DE3 is defined as λ sBamHIo ΔEcoRI-B int::(lacI::PlacUV5::T7 gene1) i21 Δnin5
 
Derivative 1: ER3200=ER3135 carrying the slyD-CBD allele
 
Derivative 2: ER3201=ER3135 carrying the slyD-CBD and can-CBD alleles
 
Derivative 3: ER3202=ER3135 carrying the slyD-CBD, can-CBD, and arnA-CBD alleles
 
Derivative 4: ER3203=ER3135 carrying the slyD-CBD, can-CBD, arnA-CBD and aceE-CBD alleles
 
Derivative 5: ER3204=ER3135 carrying the slyD-CBD, can-CBD, arnA-CBD and glmS(6Ala) alleles aka (Nico21(DE3))
 
derivative 6: ER3205=ER3135 carrying the slyD-CBD, can-CBD, arnA-CBD, aceE-CBD and glmS(6Ala) aka (Nico22(DE3))
 
note: can is the carbonic anhydrase gene, also known as the yadF gene arnA is also known as yfbG.
 
       Example 1 
     Construction of the pMAKslyD-CBD and Allele Exchange to Replace the Chromosomal slyD Gene with slyD-CBD 
       [0077]    The pMAK705 vector (Hamilton, et al.,  Journal of Bacteriology:  4617-4622 (1989)) was modified to create a vector for introducing the CBD affinity tag open reading frame (ORF) at the 3′ end of chromosomal genes encoding contaminant proteins. CBD-ORF from vector pTYB1 (New England Biolabs, Inc., Ipswich, Mass.) was inserted into the polylinker region of pMAK705. A protein coding linker region was inserted upstream of the CBD-ORF. The linker region codes for the following nineteen amino acid sequence: LQASSS(N) 10 LQS (SEQ ID NO: 4), where the first LQ codons correspond to a PstI restriction site and the last LQS codons contain a SalI restriction site. (See  FIG. 1  for a polylinker map of the C-terminal CBD-tagging vector named pMAKCBD). 
         [0078]    The allele exchange method described by Hamilton, et al. (1989) relies on homologous recombination. Efficient allele exchange occurs when the allele exchange vector contains at least 300 bp of homology to both the 5′ and 3′ regions flanking the target site on the bacterial host chromosome. These DNA segments of at least 300 bp are most easily isolated by PCR amplification from the target host chromosome and subsequently cloned into the allele exchange vector by restriction site ligation, ligase independent cloning (LIC), or uracil-specific excision reagent (USER) cloning (Sambrook, J., et al.,  Molecular Cloning: A Laboratory Manual,  2nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1989); Aslanidis, C., et al.,  Nucleic Acids Research  18: 6069-6074 (1990); Haun, R. S., et al.,  Biotechniques  13: 515-518 (1992); and Bitinaite, J., et al.,  Nucleic Acids Research  35(6): 1992-2002 (207)). Alternatively, DNA fragments corresponding to target sites on the chromosome may be created by in vitro DNA synthesis techniques. We introduced useful restriction sites into the allele exchange vector to facilitate target gene cloning and 3′ flanking DNA cloning (see  FIG. 1 ). Genes encoding  E. coli  essential proteins that contaminate samples on a nickel column may be cloned into the unique HindIII, SphI and/or PstI(SbfI) sites in the pMAKCBD allele exchange vector. DNA corresponding to 3′ gene flanking sequence may be cloned into the AsiSI, Acc65I and/or the EagI unique restriction sites to provide a 3′ region of homologous sequence.  FIG. 2  shows the nucleotide sequence of the pMAKCBD allele exchange vector. 
         [0079]    pMAKslyD-CBD was constructed from pMAKCBD to replace the slyD allele in ER3135 with the slyD-CBD allele. The slyD gene of ER3135 was PCR-amplified with primers 4853 and 4854 and cloned into the HindIII and SbfI sites of pMAKCBD. In a second step, the 3′ flanking DNA downstream from the slyD gene was PCR-amplified from the ER3135 using primers 4855 and 4856 and cloned into the AsiSI and EagI sites to create clone pMAKslyD-CBD. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Primers and Primer Sequences for 
               
               
                 Construction of pMAKslyD-CBD 
               
             
          
           
               
                   
                   
                 Restriction 
               
               
                 Primer 
                 Sequence† 
                 Enzyme 
               
               
                   
               
               
                 4853For 
                 CCACCA AAGCTT GTTAAGTGCGGACATCAG 
                 HindIII 
               
               
                   
                 (SEQ ID NO: 5) 
                   
               
               
                   
               
               
                 4854Rev  
                 GGTGGT CCTGCAGG TGGCAACCGCAACCGC 
                 SbfI 
               
               
                   
                 CG (SEQ ID NO: 6) 
                   
               
               
                   
               
               
                 4855For 
                 CCACCA GCGATCGC ATACCGAAAAAGTGAC 
                 AsiSI 
               
               
                   
                 AAAAAAGCG (SEQ ID NO: 7) 
                   
               
               
                   
               
               
                 4856Rev  
                 GGTGGT CGGCCG GCAGCTTCAGCAGCAAAA 
                 EagI 
               
               
                   
                 GTGA (SEQ ID NO: 8) 
               
               
                   
               
               
                 †Underlined nucleotides indicate the recognition sequence of the restriction enzyme named in the rightmost column. 
               
             
          
         
       
     
         [0080]    SlyD was selected as the first contaminating essential protein to be tagged by the CBD-tag. pMAKslyD-CBD was transformed into ER3135 which is a T1-phage resistant version of BL21(DE3). We determined that selection of pMAK705-derived constructs in ER3135 was preferably undertaken using rich agar plates with chloramphenicol dosed at 4 ug/mL (“Rich-Cam4 plates”). The pMAKslyD-CBD construct was transformed into ER3135, and individual clones were grown in Rich-Cam4 liquid media until OD=0.5, and then approximately 2×10 6  colony forming units were plated on Rich-Cam4 agar. Plates were incubated at either 30° C. to allow plasmid replication or 42.5° C. to prevent plasmid replication. The ratio of colonies resulting from the two different plating temperatures was approximately 10,000/1. Thus, the chromosomal integration frequency was about 1 in 10 4  cells. PCR analysis was carried out on individual colonies to confirm slyD locus integration by the pMAKslyD-CBD construct. slyD locus integration was confirmed by positive PCR amplification using a forward primer specific for the plasmid (s1233) and reverse primer 4060, annealing to the chromosomal DNA downstream of the slyD locus (and outside of the sequence cloned into pMAK-slyD-CBD). Positive integrants were inoculated into rich media with Cam concentration dosed at 10 ug/mL (“Rich-Cam10 media”) and grown at 30° C. to enable re-activation of the plasmid origin of replication. The higher level of Cam (10 ug/mL) encourages growth of the strains where the pMAK construct becomes episomal. Thus, after three continuous outgrowths to saturation, the respective culture was populated with cells containing replicating plasmid. The plasmid allele was analyzed by PCR amplification using primers s1233 and s1224. Strains where allele exchange occurred were identified by the size of the PCR amplicon and additionally by resistance to MfeI digestion. The CBD-ORF contains a unique MfeI site that is useful for allele exchange analysis. The strains positive for allele exchange were cured of the pMAK vector carrying the WT slyD allele derived from the chromosome. pMAK vector curing was accomplished by coumermycin treatment (Chen, et al.,  J. Biol. Chem.  278: 23295-23300(2003)). The cured strain (ER3200) was genetically characterized by sequencing the slyD-CBD allele amplified by primers 4059 and 4060. PCR amplification with these two primers confirmed that the slyD-CBD allele was present at the correct position within the chromosome. The amplified ER3200 genomic DNA was sequenced to confirm the presence of an in-frame genetic fusion between the slyD gene and the CBD-ORF. Strain ER3200 exhibited the same growth rate in rich media when compared to parent strain ER3135. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Primers and Primer Sequences  
               
               
                 for Allele Exchange to Create the  
               
               
                 slyD-CBD Derivative of ER3135 
               
             
          
           
               
                 Primer 
                 Sequence 
               
               
                   
               
               
                 S1233For 
                 AGCGGATAACAATTTCACACAGGA (SEQ ID NO: 9) 
               
               
                   
               
               
                 4060Rev 
                 GCACCCAGTGCATAAGCTGATTTCT (SEQ ID NO: 10) 
               
               
                   
               
               
                 S1224 
                 CGCCAGGGCCCAGTCACGAC (SEQ ID NO: 11) 
               
               
                   
               
               
                 4059Forv 
                 GCCTGTCAGGCGCAGGATTCA (SEQ ID NO: 12) 
               
               
                   
               
             
          
         
       
     
         [0081]      FIG. 3A  demonstrates the problem of wild type SlyD co-eluting from Ni-NTA resin with the his-tagged protein of interest (also referred to as a target protein) over-expressed in parent strain ER3135.  FIG. 3B  shows co-elution of SlyD-CBD from Ni-NTA resin with the target protein after overexpression in the slyD-CBD derivative strain ER3200. Note that SlyD-CBD protein exhibits a much slower migration rate in SDS-PAGE when tagged with the 7 kDa CBD-tag.  FIG. 3C  confirms that the most of the Ni-NTA elution fractions shown in  FIG. 3B  are contaminated with the SlyD-CBD fusion protein (prior to chitin affinity chromatography). 
       Example 2 
     Construction of pMAKcan-CBD and Allele Exchange to Tag the Chromosomal can Carbonic Anhydrase Gene 
       [0082]    A second contaminating essential protein chosen for tagging was the can gene product carbonic anhydrase. The 3′ flanking DNA downstream of the can gene (formerly yadF) was PCR-amplified from ER3135 genomic DNA using primers 4839 and 4840 and cloned into the AsiSI and EagI sites of pMAKCBD. The can gene was PCR-amplified from ER3135 genomic DNA using primers 4841 and 4842 and cloned into the HindIII and SacI sites. The resulting construct pMAKcan-CBD was confirmed by DNA sequencing and then transformed into ER3135. The allele exchange procedure was carried out in the same manner described in Example 1, except that chromosomal integration analysis and can locus amplification were accomplished using primers 4841 and 2187. 
         [0083]    The can-CBD derivative of ER3135 exhibited the same growth rate in rich media as the parent strain. Thus, we proceeded to add the can-CBD allele to strain ER3200 (slyD-CBD strain) by allele exchange to create the double CBD-tagged derivative ER3201, which also exhibited the same growth rate in rich media when compared to parent strain ER3135. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Primers and Primer Sequences for  
               
               
                 Construction of pMAKcan-CBD  
               
               
                 and Allele Exchange 
               
             
          
           
               
                   
                   
                 Restriction 
               
               
                 Primer 
                 Sequence 
                 Enzyme 
               
               
                   
               
             
          
           
               
                 4839For 
                 ACCACC GCGATCGC AAATGCCATGCCGGAT 
                 AsiSI 
               
               
                   
                 GCAACACATCC (SEQ ID NO: 13) 
                   
               
               
                   
               
               
                 4840Rev 
                 ACCACC CGGCCG CATATGGTTAGAGATATG 
                 EagI 
               
               
                   
                 AAACATAC (SEQ ID NO: 14) 
                   
               
               
                   
               
               
                 4841For 
                 ACCACC AAGCTT CGAGATCGTAACCAAATA 
                 HindIII 
               
               
                   
                 CGCTG (SEQ ID NO: 15) 
                   
               
               
                   
               
               
                 4842Rev 
                 ACCACC GAGCTC GATTTGTGGTTGGCGTGT 
                 SacI 
               
               
                   
                 TTCAGCTTGAG (SEQ ID NO: 16) 
                   
               
               
                   
               
               
                 2187Rev 
                 CGAGTAATCGTCGCGAGCCTGTATTG  
                   
               
               
                   
                 (SEQ ID NO: 17) 
               
               
                   
               
               
                 †Underlined nucleotides indicate the recognition sequence of the restriction enzyme named in the rightmost column. 
               
             
          
         
       
     
       Example 3 
     Construction of pMAKarnA-CBD-arnD and Allele Exchange to Tag the Chromosomal arnA Gene at the 3′End 
       [0084]    The ArnA protein was selected as the third contaminating essential protein for CBD-tagging. The arnA gene resides within an operon where the arnA stop codon overlaps the downstream arnD gene start codon (ATGA). To maintain this native genetic context at the arnA/arnD junction, we designed the arnA-CBD-arnD allele to encode the last 4 codons of arnA after the CBD-ORF so that the native arnA/arnD junction would be maintained. This engineered allele expresses an ArnA-CBD fusion protein with the DKPS amino acid sequence repeated before and after the C-terminal CBD-tag. The pMAKarnA-CBD-arnD construct was created as follows. First, the arnD gene was PCR-amplified from ER3135 genomic DNA using primers 9990 and 0001 (see Table 4). Next, the pMAKCBD vector was PCR-amplified using primers 0003 and 9991 to create a blunt ended DNA that ends with the last codon of the CBD-ORF. Ligation of these two fragments creates a genetic fusion coding for CBD-DKPSArnD. Finally, the arnA gene was PCR-amplified from ER3135 genomic DNA using primers 0000 and 0002 and cloned into the HindIII and PstI sites to create the final allele exchange construct pMAKarnA-CBD-arnD. 
         [0085]    The arnA-CBD-arnD allele was inserted at the arnA-arnD locus of ER3135 using the allele exchange method as described in Examples 1 and 2. Chromosomal integration analysis and arnA-arnD locus amplification was accomplished using primers 5032 and 5031. The arnA-CBD-arnD derivative of ER3135 exhibited the same growth rate in rich media when compared to parent strain ER3135. Thus, the arnA-CBD-arnD allele was also added to strain ER3201 to create the triple CBD-tagged strain ER3202. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Primers and Primer Sequences for  
               
               
                 Construction of pMAKarnA-CBD-arnD  
               
               
                 and Allele Exchange 
               
             
          
           
               
                   
                   
                 Restriction 
               
               
                 Primer 
                 Sequence†‡ 
                 Enzyme 
               
               
                   
               
               
                 9990For 
                 P-GATAAACCATCATGACCAAAGTAGG  
                   
               
               
                   
                 (SEQ ID NO: 18) 
                   
               
               
                   
               
               
                 0001Rev 
                 CAGGTG GGTACC GTCACCGGAATTTGC 
                 Acc65I 
               
               
                   
                 G (SEQ ID NO: 19) 
                   
               
               
                   
               
               
                 0003 
                 GATATTGCTG GGTACC GAGCTCGAA  
                 Acc65I 
               
               
                   
                 (SEQ ID NO: 20) 
                   
               
               
                   
               
               
                 9991 
                 P-TTGAAGCTGCCACAAGGCAGGAACG  
                   
               
               
                   
                 (SEQ ID NO: 21) 
                   
               
               
                   
               
               
                 0000For 
                 CGGCAT AAGCTT ACTCGGTGAATATAT  
                 HindIll 
               
               
                   
                 CGG (SEQ ID NO: 22) 
                   
               
               
                   
               
               
                 0002Rev 
                 P-AGCCTGCAGGGAAGGTTTATCCGTA 
                   
               
               
                   
                 AGATCAACGGTGCG  
                   
               
               
                   
                 (SEQ ID NO: 23) 
                   
               
               
                   
               
               
                 5032For 
                 GATGTACGACCTGGTGACCTGC  
                   
               
               
                   
                 (SEQ ID NO: 24) 
                   
               
               
                   
               
               
                 5031Rev 
                 GGATGCGGTTGAGTAACCAACC 
                   
               
               
                   
                 (SEQ ID NO: 25) 
               
               
                   
               
               
                 †Underlined nucleotides indicate the recognition sequence of the restriction enzyme named in the rightmost column. 
               
               
                 ‡“P” indicates the position of phosphorylation. 
               
             
          
         
       
     
       Example 4 
     Construction of pMAKaceE-CBD and Allele Exchange to Tag the Chromosomal aceE Gene at the 3′End 
       [0086]    The AceE protein was selected as the fourth contaminating essential protein for CBD-tagging. The aceE gene codes for the E1 subunit of the pyruvate dehydrogenase mufti-subunit complex. The downstream chromosomal gene is aceF, which codes for the E2 subunit of the pyruvate dehydrogenase complex. Together with the dihydrolipoyl dehydrogenase subunit (E3), this key metabolic enzyme is composed of E1:E2:E3 at a ratio of 24:24:12 subunits per complex in  E. coli  (Lehninger et al. “ Principles of Biochemistry”  2 nd  edition, 1993 by Worth Publishers). Thus, the expression level of each subunit is important to the viability of the cell. 
         [0087]    The pMAKaceE′-CBD-aceF allele exchange clone maintained the native 14 nucleotide spacing between the aceE and aceF genes: TAA GAGGTAAAAGAATA ATG (SEQ ID NO: 26). The aceE′ designation indicates that the aceE gene is truncated at the 5′ end. Thus, integrants were not isolated in the first step of the allele exchange method if the aceE-CBD allele was not tolerated. pMAKaceE′-CBD-aceF was constructed as follows: First, the full-length aceE gene was cloned using primers 4845 and 4846 (see Table 5). Then, the 5′ end of aceE was deleted by SphI-BsiWI digestion, followed by a blunting reaction with Klenow fragment and ligation to reclose the plasmid. Next, the full-length aceF gene was PCR-amplified from ER3135 using primers 0077 and 0076 and cloned into the deltaSphI-BsiWI clone. The vector fragment was prepared by PCR-amplification using 0079 and 0078 and subsequent digestion with EagI. 
         [0088]    pMAKaceE′-CBD-aceF allele exchange construct was transformed into the triple CBD-tagged strain ER3202 to create the quadruple CBD-tagged strain ER3203. Primers 4845 (upstream forward) and 0078 (CBD-tag reverse) were used to confirm proper integration at the aceE locus. Primers 4845 and 0076 were used to PCR-amplify the aceE locus for sequence characterization. The amplicon was digested with MfeI to rapidly identify the strains encoding aceE-CBD at the aceE locus as the WT aceE gene lacks this site and the CBD-ORF contains a single MfeI site. 
         [0089]    Strain ER3203 exhibited a reduced growth rate in rich media when compared to parent strain ER3135. However, the same cell density was obtained after overnight shaking (225 rpm at 37° C. in 2 L flasks). Under high-density cultivation conditions, ER3203 achieved a saturation density of OD600=31.6, whereas ER3135 achieved a saturation density of OD600=91.3 when both strains were induced with 1 mM IPTG to over-express Alanyl tRNA-synthetase(6His) from a plasmid. In the same experiment, the triple CBD-tagged strain ER3202 achieved a saturation density of OD=100.1 and the 6His expression level was comparable to the expression level observed in ER3135. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Primers and Primer Sequences for  
               
               
                 Tagging the 3′ End of the aceE  
               
               
                 Gene with the CBD-ORF 
               
             
          
           
               
                   
                   
                   
                 Restriction 
               
               
                   
                 Primer 
                 Sequence†‡ 
                 Enzyme 
               
               
                   
                   
               
               
                   
                 4845For 
                 ACCACC GCATGC GAATTGCTCTAT 
                 SphI 
               
               
                   
                   
                 TCGCGTCGCGAGATG 
                   
               
               
                   
                   
                 (SEQ ID NO: 27) 
                   
               
               
                   
                   
               
               
                   
                 4846Rev 
                 ACCACC GAGCTC GACGCCAGACGC 
                 SacI 
               
               
                   
                   
                 GGGTTAACTTTATCTGC 
                   
               
               
                   
                   
                 (SEQ ID NO: 28) 
                   
               
               
                   
                   
               
               
                   
                 0077For 
                 P-GAGGTAAAAGAATAATGGCTAT 
                   
               
               
                   
                   
                 CG (SEQ ID NO: 29) 
                   
               
               
                   
                   
               
               
                   
                 0076Rev 
                 CAAACG GCGGCCGC TTTGTCTATT 
                 NotI 
               
               
                   
                   
                 CGCTA (SEQ ID NO: 30) 
                   
               
               
                   
                   
               
               
                   
                 0079For 
                 CAGCTC CGGCCG ACGCGCTGGGCT  
                 EagI 
               
               
                   
                   
                 (SEQ ID NO: 31) 
                   
               
               
                   
                   
               
               
                   
                 0078Rev 
                 P-TTATTGAAGCTGCCACAAGGCA 
                   
               
               
                   
                   
                 GG (SEQ ID NO: 32) 
               
               
                   
                   
               
               
                   
                 †Underlined nucleotides indicate the recognition sequence of the restriction enzyme named in the rightmost column. 
               
               
                   
                 ‡“P” indicates the position of phosphorylation. 
               
             
          
         
       
     
       Example 5 
     Construction and Evaluation of GlmS Mutants with Either Two or Six Surface Histidines Replaced by Alanines 
       [0090]    The chromosomal glmS gene was mutated to replace histidine codons with alanine codons so that the respective strain would express a GlmS protein with reduced affinity for IMAC resins.  FIG. 5  shows positions of histidine codons within the glmS gene, which were mutated to alanine in the mutants described below. 
         [0091]    The pMAK-glmS clone was generated by PCR amplification of the glmS gene from ER3135 genomic DNA with the primers HindIII-glmS For and glmS-SacI Rev (see Table 6). This PCR product corresponding to the glmS gene with 200 bp 5′ flanking sequence and no 3′ flanking sequence was cloned into the HindIII and SacI sites of pMAK705. 
         [0092]    pMAK-glmS(2Ala) was generated by PCR amplification of the plasmid pMAK-g/mS with the reverse primer 3 (His62Ala) and forward primer 4 (His65Ala) and followed by ligation to circularize the linear PCR product (Phusion® Site-Directed Mutagenesis Kit, New England Biolabs, Inc., Ipswich, Mass.). The glmS(2Ala) gene has the sequence  GC TCCTCTG GC T (SEQ ID NO: 33) modified from the WT sequence (CATCCTCTGCAT) (SEQ ID NO: 34) so that the four mutated nucleic acids resulted in alanine codons at positions 62 and 65 of the glmS ORF. 
         [0093]    pMAK-glmS(4Ala) was generated by PCR amplification of the plasmid pMAK-glmS(2Ala) with the reverse primer 5 (His432Ala) and forward primer 6 (His436Ala) followed by the ligation to circularize the linear PCR product. The glmS(4Ala) has the sequence  GC TGACATTGTG GC  (SEQ ID NO: 35) modified from the glmS(2Ala) sequence (CATGACATTGTGCAT) (SEQ ID NO: 36) so that the four mutated nucleic acids resulted in additional alanine codons at positions 432 and 436 of the glmS ORF. 
         [0094]    pMAK-glmS(6Ala) was generated by 2 PCR amplifications. First, the pMAK-glmS(4Ala) template was amplified with the reverse primer 7 containing two mutated bases resulting in a DNA encoding (His466Ala) in the GlmS and the forward primer 8 (His467Ala) and followed by the ligation of the linear PCR product to generate the plasmid pMAK-glmS(5Ala) (His62Ala, His65Ala, His432Ala, His436Ala, His466Ala). Second, the pMAK-glmS(5Ala) template was amplified with the reverse primer 9 (His466Ala) and the forward primer 10 (His467Ala) followed by ligation of the linear PCR product to generate the plasmid pMAK-glmS(6Ala). The glmS(6Ala) has the sequence AAA GC T GC CGCG (SEQ ID NO: 37) modified from the glmS(4Ala) sequence (AAACATCACGCG) (SEQ ID NO: 38) so that the four mutated nucleic acids resulted in additional alanine codons at positions 466 and 467 of the glmS ORF. 
         [0095]      FIG. 6  shows results from over-expressing the WT GlmS protein, the GlmS(2Ala) protein and the GlmS(6Ala) protein from the pMAK vector (lac promoter) in ER3135. In each case, cell lysates were prepared and subjected to ÄKTA™ HisTrap™ chromatography (GE Healthcare, Waukesha, Wis.). The results show that the GlmS(6Ala) does not bind to the HisTrap™ resin whereas the GlmS wild type protein and the GlmS(2Ala) do bind to the resin in the presence of 20 mM imidazole. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Primers and Primer Sequences 
               
               
                 for Construction and 
               
               
                 Evaluation of GlmS Mutants 
               
             
          
           
               
                   
                 Primer 
                 Sequence* 
               
               
                   
                   
               
               
                   
                 HindIII- 
                 GGAGGA AAGCTT GACTCAGAA 
               
               
                   
                 glmS For 
                 AGAAGGCTGG (SEQ ID NO: 39) 
               
               
                   
                   
               
               
                   
                 glmS- 
                 CCACCA GAGCTCTTATTA CTC 
               
               
                   
                 SacI Rev 
                 AACCGTAACCGATTTTGCC  
               
               
                   
                   
                 (SEQ ID NO: 40) 
               
               
                   
                   
               
               
                   
                 His62- 
                 GGA GC TTCTTCCGCTGCCTGA 
               
               
                   
                 Ala Rev 
                 GCC (SEQ ID NO: 41) 
               
               
                   
                   
               
               
                   
                 His65Ala  
                 TCTG GC TGGCGGCACCGGTAT 
               
               
                   
                 For 
                 TGCTCAT (SEQ ID NO: 42) 
               
               
                   
                   
               
               
                   
                 His432Ala 
                 ATGTCA GC TTCAATGGAGGCA 
               
               
                   
                 Rev 
                 TCCAGACCTT (SEQ ID NO: 43) 
               
               
                   
                   
               
               
                   
                 His436Ala  
                 TGTG GC TGGTCTGCAGGCGTT 
               
               
                   
                 For 
                 GCCGAGCCGTAT (SEQ ID NO: 44) 
               
               
                   
                   
               
               
                   
                 His466Ala 
                 A GC TTTGTCAGAGAAATCTTC  
               
               
                   
                 Rev 
                 (SEQ ID NO: 45) 
               
               
                   
                   
               
               
                   
                 glmS467  
                   C ACGCGCTGTTCCTGGGCCGT  
               
               
                   
                 For 
                 (SEQ ID NO: 46) 
               
               
                   
                   
               
               
                   
                 glmS466  
                   A TGTTTGTCAGAGAAATCTTC  
               
               
                   
                 REV 
                 (SEQ ID NO: 47) 
               
               
                   
                   
               
               
                   
                 His467Ala  
                   GC CGCGCTGTTCCTGGGCCGT 
               
               
                   
                 For 
                 GGCGATCAG (SEQ ID NO: 48) 
               
               
                   
                   
               
               
                   
                 *Bold and underlined nucleotides indicate mutations relative to WT glmS sequence. 
               
             
          
         
       
     
       Example 6 
     Allele Exchange to Introduce the glmS(6Ala) Mutant Gene 
       [0096]    pMAK-glmS(6Ala) construct was transformed into strains ER3202 and ER3203 and the allele exchange method was used to replace the chromosomal WT glmS gene with the glmS(6Ala) gene. The protein expression host derived from ER3202 was named NiCo21(DE3) and the host derived from ER3203 was named NiCo22(DE3). 
       Example 7 
     Mass Spectrometry Analysis of  E. coli  Ni-NTA Binding Proteins from: ER3135, Nico21(DE3) and Nico22(DE3) 
       [0097]    Mass spectrometry analysis of  E. coli  proteins which bound to a Ni-NTA column (Qiagen, Germantown, Md.) from: ER3135, Nico21(DE3) and Nico22(DE3). Each strain was grown to saturation at 37° C. in luria broth (LB) plus 0.1% glucose. Cell pellets were resuspended in buffer A [20 mM sodium phosphate (pH 7.4), 0.5M NaCl, 20 mM imidazole] and sonicated to prepare a cell lysate. The clarified lysate was loaded onto a 1 mL HisTrap™ column. The column was washed with 90 column volumes of buffer A (20 mM imidazole). Then the high affinity Ni-NTA binding proteins were eluted with buffer A containing 400 mM imidazole. The eluted proteins were analyzed by mass spectrometry. Zero GlmS peptides were detected in the samples originating from Nico21(DE3) and Nico22(DE3) in contrast to the parent strain where GlmS was the primary Ni-NTA binding protein. 
       Example 8 
     Overexpression of Target Protein Alanyl tRNA-Synthetase in ER3135, Nico21(DE3) and Nico22(DE3) 
       [0098]    The  E. coli  protein contaminant profile was evaluated when His-tagged Alanyl tRNA synthetase was over-expressed from pQE30 (Qiagen, Germantown, Md.) in three strains of interest: ER3135=BL21(DE3) fhuA versus Nico21(DE3) versus Nico22(DE3). Note that each strain also carried the miniF-lacIq plasmid isolated from T7 Express I q  (New England Biolabs, Inc., Ipswich, Mass.). Cell lysates resulting from 500 mL of IPTG-induced cells were loaded, washed and eluted from a 5 mL HisTrap™ column according to manufacturer&#39;s recommendations. The imidazole elution fractions enriched in 6His were pooled (see lanes P in  FIG. 7 ). The Ni-NTA pools (P) were passed through a chitin column. The proteins flowing through the chitin column are shown in lanes FT in  FIG. 7 . Finally, lanes labeled B were the result of boiling the chitin resin in the presence of SDS to strip the resin of CBD-tagged proteins. As expected, very little protein is present in  FIG. 7  lane B of ER3135 since this is the parent strain lacking CBD-tagged proteins. In contrast, lanes B corresponding to Nico21(DE3) and Nico22(DE3) are enriched with CBD-tagged proteins that have been removed from the target protein (as indicated in  FIG. 7 ). A black triangle indicates the 67 kDa GlmS protein in lanes P and FT of ER3135, whereas the GlmS protein is absent in all lanes corresponding to Nico21(DE3) and Nico22(DE3).