COMPOSITIONS AND METHODS FOR PROTEIN SECRETION

Abstract: The present invention relates to compositions and methods for secretion of functional proteins in a soluble form by host cells. In particular, the invention relates to membrane targeting and translocation proteins, MttA, MttB and MttC and to variants and homologs thereof. The membrane targeting and translocation proteins are useful in targeting protein expression to the periplasm of gram negative bacteria and to extracellular media of other host cells. Such expression allows secretion of expressed proteins of interest in a functional and soluble form, thus facilitating purification and increasing the yield of functional proteins of interest.

Description: COMPOSITIONS AND METHODS FOR PROTEIN SECRETION FIELD OF THE INVENTION The present invention relates to compositions and methods for secretion of functional proteins in a soluble form by host cells. In particular, the invention relates to proteins involved in targeting expression of a protein of interest extracellularly and to the periplasm, thus facilitating generation of a functional soluble protein. BACKGROUND OF THE INVENTION Proteins having clinical or industrial value may be obtained using techniques which facilitate their synthesis in bacterial or in eukaryotic cell cultures. However, once synthesized, there are often problems in recovering these recombinant proteins in substantial yields and in a useful form. For example, recombinant proteins expressed in bacteria often accumulate in the bacterial cytoplasm as insoluble aggregates known as inclusion bodies [Marston, (1986) Biochem. J. 240:1-12; Schein (1989) Biotechnology 7:1141-1149], Similarly, recombinant transmembrane proteins which contain both hydrophobic and hydrophilic regions are intractable to solubilization. While transmembrane recombinant proteins and recombinant proteins which are expressed in the cytoplasm may be solubilized by use of strong denaturing solutions (e.g., urea, guanidium salts, detergents, Triton, SDS detergents, etc.), solubilization efficiency is nevertheless variable and there is no general method of solubilization which works for most proteins. Additionally, many proteins which are present at high concentrations precipitate out of solution when the solubilizing agent is removed. Yet a further drawback to solubilization of recombinant proteins is that denaturing chemicals (e.g., guanidium salts and urea) contain reactive primary amines which swamp those of the protein, thus interfering with the protein's reactive amine groups. Thus, what is needed ϊs-ajriethod for producing soluble proteins. SUMMARY OF THE INVENTION The present invention provides a recombinant polypeptide comprising at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NOs:47 and 49. SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO:8 and variants and homologs thereof. - 1 - This invention further provides an isolated nucleic acid sequence encoding at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NOs:47 and 49, SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO:8 and variants and homologs thereof. In one preferred embodiment, the nucleic acid sequence is contained on a recombinant expression vector. In a more preferred embodiment, the expression vector is contained within a host cell. Also provided by the present invention is a nucleic acid sequence that hybridizes under stringent conditions to a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO: 8 and variants and homologs thereof. The invention additionally provides a method for expressing a nucleotide sequence of interest in a host cell to produce a soluble polypeptide sequence, the nucleotide sequence of interest when expressed in the absence of an operably linked nucleic acid sequence encoding a twin-arginine signal amino acid sequence produces an insoluble polypeptide, comprising: a) providing: i) the nucleotide sequence of interest encoding the insoluble polypeptide; ii ) the nucleic acid sequence encoding the twin-arginine signal amino acid sequence; and iii) the host cell, wherein the host cell comprises at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NOs:47 and 49, SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO: 8 and variants and homologs thereof; b) operably linking the nucleotide sequence of interest to the nucleic acid sequence to produce a linked polynucleotide sequence; and c) introducing the linked polynucleotide sequence into the host cell under conditions such that the fused polynucleotide sequence is expressed and the soluble polypeptide is produced. Without intending to limit the location of the insoluble polypeptide, in one preferred embodiment, the insoluble polypeptide is comprised in an inclusion body. In another preferred embodiment, the insoluble polypeptide comprises a cofactor. In a more preferred embodiment, the cofactor is selected from the group consisting of iron-sulfur clusters, molybdopterin, polynuclear copper, tryptophan tryptophylquinone, and flavin adenine dinucleotide. Without limiting the location of the soluble polypetide to any particular location, in one preferred embodiment, the soluble polypeptide is comprised in periplasm of the host cell. In an alternative preferred embodiment, the host cell is cultured in medium, and the soluble polypeptide is contained in the medium. - 2 - The methods of the invention are not intended to be limited to any particular cell. However, in one preferred embodiment, the cell is Escherichia coli. In a more preferred embodiment, the Escherichia coli cell is D-43. It is not intended that the invention be limited to a particular twin-arginine signal amino acid sequence. In a preferred embodiment, the twin-arginine signal amino acid sequence is selected from the group consisting of SEQ ID NO:41 and SEQ ID NO:42. The invention further provides a method for expressing a nucleotide sequence of interest encoding an amino acid sequence of interest in a host cell, comprising: a) providing: i) the host cell; ii) the nucleotide sequence of interest; iii) a first nucleic acid sequence encoding twin-arginine signal amino acid sequence; and iv) a second nucleic acid sequence encoding at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NOs:47 and 49, SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO: 8 and variants and homologs thereof; b) operably fusing the nucleotide sequence of interest to the first nucleic acid sequence to produce a fused polynucleotide sequence; and c) introducing the fused polynucleotide sequence and the second nucleic acid sequence into the host cell under conditions such that the at least portion of the amino acid sequence selected from the group consisting of SEQ ID NOs:47 and 49, SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO: 8 and variants and homologs thereof is expressed, and the fused polynucleotide sequence is expressed to produce a fused polypeptide sequence comprising the twin-arginine signal amino acid sequence and the amino acid sequence of interest. The location of the expressed amino acid sequence of interest is not intended to be limited to any particular location. However, in one preferred embodiment, the expressed amino acid sequence of interest is contained in periplasm of the host cell. In a particularly preferred embodiment, the expressed amino acid sequence of interest is soluble. Also without intending to limit the location of the expressed amino acid sequence of interest, in an alternative preferred embodiment, the host cell is cultured in medium, and the expressed amino acid sequence of interest is contained in the medium. In a particularly preferred embodiment, the expressed amino acid sequence of interest is soluble. - 3 - BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows anaerobic growth of strain a) HB101 and b) D-43 in the presence of various electron acceptors: (Δ) 40 mM nitrate, (D) 35 mM fumarate, (O) 100 mM TMAO or (0) 70 mM DMSO. Figure 2 shows a Western blot analysis of washed membranes and soluble fractions of HBlOl and D-43 harboring pDMS160 expressing DmsABC. Figure 3 shows A) Nitrate-stained polyacrylamide gel containing periplasmic proteins, membrane proteins and cytoplasmic proteins from HBlOl and D-43, B) Nitrite- stained polyacrylamide gel containing periplasmic proteins from HBlOl and D-43, and C) TMAO- stained polyacrylamide gel containing periplasmic proteins from HBlOl and D-43. Figure 4 shows the results of a Western blot analysis of the cellular localization of DmsAB in A) HBlOl expressing either native DmsABC (pDMS160), DmsABΔC (pDMSC59X), or FrdABΔCD, and B) equivalent lanes as in Figure 4A. but with the same plasmids in D-43. Figure 5 shows a gene map of contig AE00459 noting the positions of the ORFs and the clones used in this investigation. Figure 6 shows the amino acid sequence (SEQ ID NO:l) of MttA aligned with the amino acid sequence of YigT of Haemophilus influenzae (SEQ ID NO:2). Figure 7 shows the nucleotide sequence (SEQ ID NO:3) of the mttABC operon which contains the nucleotide sequence of the three open reading frames, ORF RF[3] nucleotides 5640-6439 (SEQ ID NO:4), ORF RF[2] nucleotides 6473-7246 (SEQ ID NO:5). and ORF RF[1] nucleotides 7279-8070 (SEQ ID NO:6) which encode the amino acid sequences of MttA (SEQ ID NO:l), MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8), respectively. Figure 8 shows an alignment of the amino acid sequence of the E. coli MttA sequence (SEQ ID NO:l) with amino acid sequences of Hcfl06-ZEAMA (SEQ ID NO:9), YBEC- ECOLI (SEQ ID NO: 10), SYNEC (SEQ ID NO: 11), ORF13-RHOER (SEQ ID NO: 12), PSEST-ORF57 (SEQ ID NO: 13), YY34-MYCLE (SEQ ID NO: 14), HELPY (SEQ ID NO: 15), HAEIN (SEQ ID NO: 16), BACSU (SEQ ID NO: 17), and ORF4-AZOCH (SEQ ID NO:18). Figure 9 shows an alignment of the amino acid sequence of the E. coli MttB sequence (SEQ ID NO:7) with amino acid sequences of YC43-PROPU (SEQ ID NO: 19), YM16- MARPO (SEQ ID NO:20), ARATH (SEQ ID NO:21), Ymfl6-RECAM (SEQ ID NO:22), Y194-SYNY3 (SEQ ID NO:23), YY33-MYCTU (SEQ ID NO:24), HELPY (SEQ ID NO:25), YigU-HAEIN (SEQ ID NO:26), YcbT-BACSU (SEQ ID NO:27), YH25-AZOCH (SEQ ID NO:28) and ARCFU (SEQ ID NO:29). Figure 10 shows an alignment of the amino acid sequence of the E. coli MttC sequence (SEQ ID NO:8) with amino acid sequences of YCFH-ECOLI (SEQ ID NO:30). Y.IJV-ECOLI (SEQ ID NO:31), METTH (SEQ ID NO:32), Y009-MYCPN (SEQ ID NO:33), YcfH-Myctu (SEQ ID NO:34), HELPY (SEQ ID NO:35), YCFH-HAEIN (SEQ ID NO:36), YABC-BACSU (SEQ ID NO:37), SCHPO (SEQ ID NO:38), CAEEL (SEQ ID NO:39) and Y218-HUMAN (SEQ ID NO:40). Figure 1 1 shows the nucleotide sequence (SEQ ID NO:45) of the mttABC operon which contains the mttAl nucleotide sequence (SEQ ID NO:46) (from nucleic acid number 642 to nucleic acid number 953) encoding the amino acid sequence of MttAl (SEQ ID NO:47), and the mttA2 nucleotide sequence (SEQ ID NO:48) (from nucleic acid number 558 to nucleic acid number 1472) encoding the amino acid sequence of MttA2 (SEQ ID NO:49). DEFINITIONS To facilitate understanding of the invention, a number of terms are defined belo . The term "foreign gene" refers to any nucleic acid (e.g., gene sequence) which is introduced into a cell by experimental manipulations and may include gene sequences found in that cell so long as the introduced gene contains some modification (e.g. , a point mutation. the presence of a selectable marker gene, etc.) relative to the naturally-occurring gene. The term "gene" refers to a DNA sequence that comprises control and coding sequences necessary for the production of RNA or a polypeptide. The polypeptide can be encoded by a full length coding sequence or by any portion of the coding sequence. The terms "gene of interest" and "nucleotide sequence of interest" refer to any gene or nucleotide sequence, respectively, the manipulation of which may be deemed desirable for any reason, by one of ordinary skill in the art. Such nucleotide sequences include, but are not limited to, coding sequences of structural genes (e.g., reporter genes, selection marker genes, oncogenes, drug resistance genes, growth factors, etc.), and of regulatory genes (e.g.. activator protein 1 (API), activator protein 2 (AP2), Spl, etc.). Additionally, such nucleotide sequences include non-coding regulatory elements which do not encode an lnRNA or protein product, such as for example, a promoter sequence, an enhancer sequence, etc. As used herein the term "coding region" when used in reference to a structural gene refers to the nucleotide sequences which encode the amino acids found in the nascent - 5 - polypeptide as a result of translation of an mRNA molecule. The coding region is bounded, in eukaryotes, on the 5' side by the nucleotide triplet "ATG" which encodes the initiator methionine and on the 3' side by one of the three triplets which specify stop codons (i.e. , TAA, TAG, TGA). Transcriptional control signals in eukaryotes comprise "promoter" and "enhancer" elements. Promoters and enhancers consist of short arrays of DNA sequences that interact specifically with cellular proteins involved in transcription [Maniatis, et al., Science 236:1237 (1987)]. Promoter and enhancer elements have been isolated from a variety of eukaryotic sources including genes in yeast, insect and mammalian cells and viruses (analogous control elements, i.e., promoters, are also found in prokaryotes). The selection of a particular promoter and enhancer depends on what cell type is to be used to express the protein of interest. Some eukaryotic promoters and enhancers have a broad host range while others are functional in a limited subset of cell types [for review see Voss, et al.. Trends Biochem. Sci., 11:287 (1986) and Maniatis, et al, Science 236:1237 (1987)]. The term "wild-type" refers to a gene or gene product which has the characteristics of that gene or gene product when isolated from a naturally occurring source. A wild-type gene is that which is most frequently observed in a population and is thus arbitrarily designed the "normal" or "wild-type" form of the gene. In contrast, the term "modified" or "mutant" refers to a gene or gene product which displays modifications in sequence and or functional properties (i.e.. altered characteristics) when compared to the wild-type gene or gene product. It is noted that naturally-occurring mutants can be isolated; these are identified by the fact that they have altered characteristics when compared to the wild-type gene or gene product. The term "expression vector" as used herein refers to a recombinant DNA molecule containing a desired coding sequence and appropriate nucleic acid sequences necessary for the expression of the operably linked coding sequence in a particular host cell. Nucleic acid sequences necessary for expression in prokaryotes include a promoter, optionally an operator sequence, a ribosome binding site and possibly other sequences. Eukaryotic cells are known to utilize promoters, enhancers, and termination and polyadenylation signals. The terms "targeting vector" or "targeting construct" refer to oligonucleotide sequences comprising a gene of interest flanked on either side by a recognition sequence which is capable of homologous recombination of the DNA sequence located between the flanking recognition sequences into the chromosomes of the target cell or recipient cell. Typically, the targeting vector will contain 10 to 15 kb of DNA homologous to the gene to be recombined; - 6 - this 10 to 15 kb of DNA is generally divided more or less equally on each side of the selectable marker gene. The targeting vector may contain more than one selectable maker gene. When more than one selectable marker gene is employed, the targeting vector preferably contains a positive selectable marker (e.g., the neo gene) and a negative selectable marker (e.g., the Herpes simplex virus tk (HSV-t ) gene). The presence of the positive selectable marker permits the selection of recipient cells containing an integrated copy of the targeting vector whether this integration occurred at the target site or at a random site. The presence of the negative selectable marker permits the identification of recipient cells containing the targeting vector at the targeted site (i.e., which has integrated by virtue of homologous recombination into the target site); cells which survive when grown in medium which selects against the expression of the negative selectable marker do not contain a copy of the negative selectable marker. Integration of a replacement-type vector results in the insertion of a selectable marker into the target gene. Replacement-type targeting vectors may be employed to disrupt a gene resulting in the generation of a null allele (i.e., an allele incapable of expressing a functional protein; null alleles may be generated by deleting a portion of the coding region, deleting the entire gene, introducing an insertion and/or a frameshift mutation, etc.) or may be used to introduce a modification (e.g., one or more point mutations) into a gene. The terms "in operable combination", "in operable order" and "operably linked" as used herein refer to the linkage of nucleic acid sequences in such a manner that a nucleic acid molecule capable of directing the transcription of a given gene and/or the synthesis of a desired protein molecule is produced. The term also refers to the linkage of amino acid sequences in such a manner so that a functional protein is produced. As used herein, the terms "vector" and "vehicle" are used interchangeably in reference to nucleic acid molecules that transfer DNA segment(s) from one cell to another. The term "recombinant DNA molecule" as used herein refers to a DNA molecule which is comprised of segments of DNA joined together by means of molecular biological techniques. The term "recombinant protein" or "recombinant polypeptide" as used herein refers to a protein molecule which is expressed using a recombinant DNA molecule. The term "transfection" as used herein refers to the introduction of a transgene into a cell. The term "transgene" as used herein refers to any nucleic acid sequence which is introduced into the genome of a cell by experimental manipulations. A transgene may be an - 7 - "endogenous DNA sequence," or a "heterologous DNA sequence" (i.e., "foreign DNA"). The term "endogenous DNA sequence" refers to a nucleotide sequence which is naturally found in the cell into which it is introduced so long as it does not contain some modification (e.g.. a point mutation, the presence of a selectable marker gene, etc.) relative to the naturally- occurring sequence. The term "heterologous DNA sequence" refers to a nucleotide sequence which is not endogenous to the cell into which it is introduced. Heterologous DNA includes a nucleotide sequence which is ligated to, or is manipulated to become ligated to, a nucleic acid sequence to which it is not ligated in nature, or to which it is ligated at a different location in nature. Heterologous DNA also includes a nucleotide sequence which is naturally found in the cell into which it is introduced and which contains some modification relative to the naturally-occurring sequence. Generally, although not necessarily, heterologous DNA encodes RNA and proteins that are not normally produced by the cell into which it is introduced. Examples of heterologous DNA include reporter genes, transcriptional and translational regulatory sequences, DNA sequences which encode selectable marker proteins (e.g. , proteins which confer drug resistance), etc. Yet another example of a heterologous DNA includes a nucleotide sequence which encodes a ribozyme which is found in the cell into which it is introduced, and which is ligated to a promoter sequence to which it is not naturally ligated in that cell. Transfection may be accomplished by a variety of means known to the art including calcium phosphate-DNA co-precipitation, DEAE-dextran-mediated transfection, polybrene- mediated transfection, electroporation, microinjection, liposome fusion, lipofection, protoplast fusion, retroviral infection, biolistics (i.e., particle bombardment) and the like. The term "stable transfection" or "stably transfected" refers to the introduction and integration of a transgene into the genome of the transfected cell. The term "stable transfectant" refers to a cell which has stably integrated one or more transgenes into the genomic DNA. As used herein the term "portion" when in reference to a gene refers to fragments of that gene. The fragments may range in size from 5 nucleotide residues to the entire nucleotide sequence minus one nucleic acid residue. Thus, "an oligonucleotide comprising at least a portion of a gene" may comprise small fragments of the gene or nearly the entire gene. The term "portion" when used in reference to a protein (as in a "portion of a given protein") refers to fragments of that protein. The fragments may range in size from four amino acid residues to the entire amino acid sequence minus one amino acid. - 8 - The term "isolated" when used in relation to a nucleic acid, as in "an isolated oligonucleotide" refers to a nucleic acid sequence that is identified and separated from at least one contaminant nucleic acid with which it is ordinarily associated in its natural source. Isolated nucleic acid is nucleic acid present in a form or setting that is different from that in which it is found in nature. In contrast, non-isolated nucleic acids are nucleic acids such as DNA and RNA which are found in the state they exist in nature. For example, a given DNA sequence (e.g., a gene) is found on the host cell chromosome in proximity to neighboring genes; RNA sequences, such as a specific mRNA sequence encoding a specific protein, are found in the cell as a mixture with numerous other mRNAs which encode a multitude of proteins. However, isolated nucleic acid sequences encoding MttAl , MttA2. MttB or MttC polypeptides include, by way of example, such nucleic acid sequences in cells ordinarily expressing MttAl, MttA2, MttB or MttC polypeptides, respectively, where the nucleic acid sequences are in a chromosomal or extrachromosomal location different from that of natural cells, or are otherwise flanked by a different nucleic acid sequence than that found in nature. The isolated nucleic acid or oligonucleotide may be present in single-stranded or double- stranded form. When an isolated nucleic acid or oligonucleotide is to be utilized to express a protein, the oligonucleotide will contain at a minimum the sense or coding strand (i.e., the oligonucleotide may be single-stranded). Alternatively, it may contain both the sense and anti-sense strands (i.e., the oligonucleotide may be double- stranded). As used herein, the term "purified" or "to purify" refers to the removal of undesired components from a sample. For example, where recombinant MttAl, MttA2. MttB or MttC polypeptides are expressed in bacterial host cells, the MttAl, MttA2, MttB or MttC polypeptides are purified by the removal of host cell proteins thereby increasing the percent of recombinant MttAl, MttA2, MttB or MttC polypeptides in the sample. As used herein, the term "substantially purified" refers to molecules, either nucleic or amino acid sequences, that are removed from their natural environment, isolated or separated, and are at least 60% free, preferably 75% free, and more preferably 90% free from other components with which they are naturally associated. An "isolated polynucleotide" is therefore a substantially purified polynucleotide. The term "recombinant DNA molecule" as used herein refers to a DNA molecule which is comprised of segments of DNA joined together by means of molecular biological techniques. The term "recombinant protein" or "recombinant polypeptide" as used herein refers to a protein molecule which is expressed using a recombinant DNA molecule. The term "homology" when used in relation to nucleic acids refers to a degree of complementarity. There may be partial homology or complete homology (i.e., identity). A partially complementary sequence is one that at least partially inhibits a completely complementary sequence from hybridizing to a target nucleic acid is referred to using the functional term "substantially homologous." The inhibition of hybridization of the completely complementary sequence to the target sequence may be examined using a hybridization assay (Southern or Northern blot, solution hybridization and the like) under conditions of low stringency. A substantially homologous sequence or probe (i.e., an oligonucleotide which is capable of hybridizing to another oligonucleotide of interest) will compete for and inhibit the binding (i.e. , the hybridization) of a completely homologous sequence to a target under conditions of low stringency. This is not to say that conditions of low stringency are such that non-specific binding is permitted; low stringency conditions require that the binding of two sequences to one another be a specific (i.e., selective) interaction. The absence of nonspecific binding may be tested by the use of a second target which lacks even a partial degree of complementarity (e.g., less than about 30% identity); in the absence of non-specific binding the probe will not hybridize to the second non-complementary target. Low stringency conditions when used in reference to nucleic acid hybridization comprise conditions equivalent to binding or hybridization at 42°C in a solution consisting of 5X SSPE (43.8 g/1 NaCl, 6.9 g/1 NaH 2 PO 4 »H 2 O and 1.85 g/1 EDTA, pH adjusted to 7.4 with NaOH), 0.1% SDS, 5X Denhardt's reagent [50X Denhardt's contains per 500 ml: 5 g Ficoll (Type 400, Pharmacia), 5 g BSA (Fraction V; Sigma)] and 100 μg/ml denatured salmon sperm DNA followed by washing in a solution comprising 5X SSPE, 0.1 % SDS at 42°C when a probe of about 500 nucleotides in length is employed. High stringency conditions when used in reference to nucleic acid hybridization comprise conditions equivalent to binding or hybridization at 42°C in a solution consisting of 5X SSPE (43.8 g/1 NaCl, 6.9 g/1 NaH 2 PO 4 » H 2 0 and 1.85 g/1 EDTA, pH adjusted to 7.4 with NaOH), 0.5%) SDS, 5X Denhardt's reagent and 100 μg/ml denatured salmon sperm DNA followed by washing in a solution comprising 0.1X SSPE, 1.0% SDS at 42°C when a probe of about 500 nucleotides in length is employed. When used in reference to nucleic acid hybridization the art knows well that numerous equivalent conditions may be employed to comprise either low or high stringency conditions; - 10 - factors such as the length and nature (DNA, RNA, base composition) of the probe and nature of the target (DNA, RNA, base composition, present in solution or immobilized, etc.) and the concentration of the salts and other components (e.g., the presence or absence of formamide, dextran sulfate, polyethylene glycol) are considered and the hybridization solution may be varied to generate conditions of either low or high stringency hybridization different from, but equivalent to, the above listed conditions. As used herein, the terms "nucleic acid molecule encoding," "DNA sequence encoding," and "DNA encoding" refer to the order or sequence of deoxyribonucleotides along a strand of deoxyribonucleic acid. The order of these deoxyribonucleotides determines the order of ribonucleotides along the mRNA chain, and also determines the order of amino acids along the polypeptide (protein) chain. The DNA sequence thus codes for the RNA sequence and for the amino acid sequence. "Nucleic acid sequence" and "nucleotide sequence" as used interchangeably herein refer to an oligonucleotide or polynucleotide, and fragments or portions thereof, and to DNA or RNA of genomic or synthetic origin which may be single- or double-stranded, and represent the sense or antisense strand. "Amino acid sequence" and "polypeptide sequence" are used interchangeably herein to refer to a sequence of amino acids. The term "antisense sequence" as used herein refers to a deoxyribonucleotide sequence whose sequence of deoxyribonucleotide residues is in reverse 5' to 3' orientation in relation to the sequence of deoxyribonucleotide residues in a sense strand of a DNA duplex. A "sense strand" of a DNA duplex refers to a strand in a DNA duplex which is transcribed by a cell in its natural state into a "sense mRNA." Sense mRNA generally is ultimately translated into a polypeptide. Thus an "antisense" sequence is a sequence having the same sequence as the non-coding strand in a DNA duplex. The term "antisense RNA" refers to a ribonucleotide sequence whose sequence is complementary to an "antisense" sequence. Alternatively, the term "antisense RNA" is used in reference to RNA sequences which are complementary to a specific RNA sequence (e.g., mRNA). Antisense RNA may be produced by any method, including synthesis by splicing the gene(s) of interest in a reverse orientation to a viral promoter which permits the synthesis of a coding strand. Once introduced into a cell, this transcribed strand combines with natural mRNA produced by the cell to form duplexes. These duplexes then block either the further transcription of the mRNA or its translation. In this manner, mutant phenotypes may be generated. The term "antisense strand" is used in - 11 - reference to a nucleic acid strand that is complementary to the "sense" strand. The designation (-) (i.e. , "negative") is sometimes used in reference to the antisense strand, with the designation (+) sometimes used in reference to the sense (i.e., "positive") strand. The term "biologically active" when made in reference to MttAl, MttA2, MttB or MttC refers to a MttAl, MttA2, MttB or MttC molecule, respectively, having biochemical functions of a naturally occurring MttAl, MttA2, MttB or MttC. Biological activity of MttAl, MttA2. MttB or MttC is determined, for example, by restoration of wild-type targeting of proteins which contain twin-arginine signal amino acid sequence to cell membranes and/or translocation of such proteins to the periplasm in cells lacking MttA, MttB or MttC activity (i.e., MttAl , MttA2, MttB or MttC null cells). Cells lacking MttAl. MttA2. MttB or MttC activity may be produced using methods well known in the art (e.g.. point mutation and frame-shift mutation) [Sambasivarao et al (1991) J. Bacteriol. 5935-5943; Jasin et al (1984) J. Bacteriol. 159:783-786]. Complementation is achieved by transfecting cells which lack MttAl, MttA2, MttB or MttC activity with an expression vector which expresses MttAl, MttA2. MttB or MttC, a homolog thereof, or a portion thereof. Details concerning complementation of cells which contain a point mutation in MttAl, MttA2 is provided in Example 6 herein. As used herein "soluble" when in reference to a protein produced by recombinant DNA technology in a host cell is a protein which exists in solution; if the protein contains a twin-arginine signal amino acid sequence the soluble protein is exported to the periplasmic space in gram negative bacterial hosts and is secreted into the culture medium by eukaryotic cells capable of secretion or by bacterial host possessing the appropriate genes (i.e., the kil gene). Thus, a soluble protein is a protein which is not found in an inclusion body inside the host cell. Alternatively, a soluble protein is a protein which is not found integrated in cellular membranes. In contrast, an insoluble protein is one which exists in denatured form inside cytoplasmic granules (called an inclusion body) in the host cell. Alternatively, an insoluble protein is one which is present in cell membranes, including but not limited to, cytoplasmic membranes, mitochondrial membranes, chloroplast membranes, endoplasmic reticulum membranes, etc A distinction is drawn between a soluble protein (i.e., a protein which when expressed in a host cell is produced in a soluble form) and a "solubilized" protein. An insoluble recombinant protein found inside an inclusion body or found integrated in a cell membrane may be solubilized (i.e., rendered into a soluble form) by treating purified inclusion bodies or - 12 - cell membranes with denaturants such as guanidine hydrochloride, urea or sodium dodecyl sulfate (SDS). These denaturants must then be removed from the solubilized protein preparation to allow the recovered protein to renature (refold). Not all proteins will refold into an active conformation after solubilization in a denaturant and removal of the denaturant. Many proteins precipitate upon removal of the denaturant. SDS may be used to solubilize inclusion bodies and cell membranes and will maintain the proteins in solution at low concentration. However, dialysis will not always remove all of the SDS (SDS can form micelles which do not dialyze out); therefore, SDS-solubilized inclusion body protein and SDS-solubilized cell membrane protein is soluble but not refolded. A distinction is also drawn between proteins which are soluble ( i.e., dissolved) in a solution devoid of significant amounts of ionic detergents (e.g., SDS) or denaturants (e.g., urea, guanidine hydrochloride) and proteins which exist as a suspension of insoluble protein molecules dispersed within the solution. A soluble protein will not be removed from a solution containing the protein by centrifugation using conditions sufficient to remove cells present in a liquid medium (e.g., centrifugation at 5,000 x g for 4-5 minutes). DESCRIPTION OF THE INVENTION The present invention exploits the identification of proteins involved in a Sec- independent protein translocation pathway which are necessary for the translocation of proteins which contain twin-arginine signal amino acid sequences to the periplasm of gram negative bacteria, and into the extracellular media of cells which do not contain a periplasm (e.g. , gram positive bacteria, eukaryotic cells, etc.), as well as for targeting such proteins to cell membranes. The proteins of the invention are exemplified by the Membrane Targeting and Translocation proteins MttAl (103 amino acids), MttA2 (161 amino acids). MttB (258 amino acids) and MttC (264 amino acids) of E. coli which are encoded by the mttABC operon. The invention further exploits the presence of a large number of proteins which are widely distributed in organisms extending from archaebacteria to higher eukaryotes. The well characterized Sec-dependent export system translocates an unfolded string of amino acids to the periplasm and folding follows as a subsequent step in the periplasm and mediated by chaperones and disulfide rearrangement. In contrast to the Sec-dependent export pathway, the proteins of the invention translocate fully-folded as well as cofactor-containing proteins from the cytoplasm into the bacterial periplasm and are capable of translocating such proteins into extracellular medium. Such translocation offers a unique advantage over current - 13 - methodologies for protein purification. Because the composition of culture medium can be manipulated, and because the periplasm contains only about 3% of the proteins of gram negative bacteria, expressed proteins which are translocated into the extracellular medium or into the periplasm are more likely to be expressed as functional soluble proteins than if they were translocated to cellular membranes or to the cytoplasm. Furthermore, translocation to the periplasm or to the extracellular medium following protein expression in the cytoplasm allows the expressed protein to be correctly folded by cytoplasmic enzymes prior to its translocation, thus allowing retention of the expressed protein's biological activity. The mttABC operon disclosed herein is also useful in screening compounds for antibiotic activity by identifying those compounds which inhibit translocation of proteins containing twin-arginine signal amino acid sequences in bacteria. For example, DMSO reductase has been found to be essential for the pathogenesis of Salmonella [Bowe and Heffron (1994) Methods in Enzymology 236:509-526]. Thus, compounds which inhibit targeting of DMSO reductase to Salmonella could result in conversion of a virulent bacterial strain to an avirulent nonpathogenic variant. The invention is further described under (A) mttA, mttB, and mttC nucleotide sequences, (B) MttA, MttB, and MttC polypeptides, and (C) Methods for expressing polypeptides to produce soluble proteins. A. mttA, mttB, and mttC nucleotide sequences The present invention discloses the nucleic acid sequence of the mttA l (SEQ ID NO:46), mttAl (SEQ ID NO:48), mttB (SEQ ID NO:5) and mttC (SEQ ID NO:6) genes which form part of the mttABC operon (SEQ ID NO:45) shown in Figure 11. Data presented herein demonstrates that the MttA2 polypeptide encoded by mttAl functions in targeting proteins which contain twin-arginine signal amino acid sequences to cell membranes, and in translocating such proteins to the periplasm of gram negative bacteria and to the extracellular medium of cells which do not contain a periplasm (e.g., gram positive bacteria and eukaryotic cells). Data presented herein further shows that the MttB and MttC polypeptides which are encoded by mttB and mttC, respectively, also serve the same functions as MttA2. This conclusion is based on the inventors' finding that mttAl, mttA2, mttB and mttC form an operon which is expressed as a single polycistronic mRNA. The function of MttB and MttC may be demonstrated by in vivo homologous recombination of chromosomal mttB and mttC by using knockouts in the mttBC operon by - 14 - utilizing insertion of mini-Mudll as previously described [Taylor et al. (1994) J. Bacteriol. 176:2740-2742]. Alternatively, the function of MttB and MttC may also be demonstrated as previously described [Sambasivarao et al (1991) J. Bacteriol. 5935-5943; Jasin et al (1984) J. Bacteriol. 159:783-786]. Briefly, the mttABC operon (Figure 1 1) is cloned into pTZ18R and pBR322 vectors. In pBR322, the Hindlll site in mttB is unique. The pBR322 containing mttB is then modified by insertion of a kanamycin gene cartridge at this unique site, while the unique Nrul fragment contained in mttC are replaced by a kanamycin cartridge. The modified plasmids are then be homologously recombined with chromosomal mttB and mttC in E. coli cells which contain either a recBC mutation or a recD mutation. The resulting recombinant are transferred by PI transduction to suitable genetic backgrounds for investigation of the localization of protein expression. The localization (e.g. , cytoplasm, periplasm, cell membranes, extracellular medium) of expression of proteins which contain twin-arginine signal amino acid sequences is compared using methods disclosed herein (e.g., functional enzyme activity and Western blotting) between homologously recombined cells and control cells which had not been homologously recombined. Localization of expressed proteins which contain twin-arginine signal amino acid sequences in extracellular medium or in the periplasm of homologously recombined cells as compared to localization of expression in other than the extracellular medium and the periplasm (e.g., in the cytoplasm, in the cell membrane, etc. ) of control cells demonstrates that the wild-type MttB or MttC protein whose function had been modified by homologous recombination functions in translocation of the twin argining containing proteins to the extracellular medium or to the periplasm. The present invention contemplates any nucleic acid sequence which encodes one or more of MttAl, MttA2, MttB and MttC polypeptide sequences or variants or homologs thereof. These nucleic acid sequences are used to make recombinant molecules which express the MttAl, MttA2, MttB and MttC polypeptides. For example, one of ordinary skill in the art would recognize that the redundancy of the genetic code permits an enormous number of nucleic acid sequences which encode the MttAl, MttA2, MttB and MttC polypeptides. Thus, codons which are different from those shown in Figure 7 may be used to increase the rate of expression of the nucleotide sequence in a particular prokaryotic or eukaryotic expression host which has a preference for particular codons. Additionally, alternative codons may also be used in eukaryotic expression hosts to generate splice variants of recombinant RNA transcripts which have more desirable properties (e.g., longer or shorter half-life) than transcripts generated using the sequence depicted in Figure 7. In addition, different codons may also be - 15 - desirable for the purpose of altering restriction enzyme sites or, in eukaryotic expression hosts, of altering glycosylation patterns in translated polypeptides. The nucleic acid sequences of the invention may also be used for in vivo homologous recombination with chromosomal nucleic acid sequences. Homologous recombination may be desirable to, for example, delete at least a portion of at least one of chromosomal mttAl. mttA2, mttB and mttC nucleic acid sequences, or to introduce a mutation in these chromosomal nucleic acid sequence as described below. Variants of the nucleotide sequences which encode MttAl , MttA2, MttB and MttC and which are shown in Figure 7 and Figure 11 are also included within the scope of this invention. These variants include, but are not limited to, nucleotide sequences having deletions, insertions or substitutions of different nucleotides or nucleotide analogs. This invention is not limited to the mttAl, mttA2, mttB and mttC sequences (SEQ ID NOs:46. 48. 5 and 6, respectively) but specifically includes nucleic acid homologs which are capable of hybridizing to the nucleotide sequence encoding MttAl, MttA2, MttB and MttC (Figures 1 1 and 7), and to portions, variants and homologs thereof. Those skilled in the art know that different hybridization stringencies may be desirable. For example, whereas higher stringencies may be preferred to reduce or eliminate non-specific binding between the nucleotide sequences of Figure 7 and other nucleic acid sequences, lower stringencies may be preferred to detect a larger number of nucleic acid sequences having different homologies to the nucleotide sequence of Figure 7. Portions of the nucleotide sequence encoding MttAl, mttA2, MttB and MttC of Figures 1 1 and 7 are also specifically contemplated to be within the scope of this invention. It is preferred that the portions have a length equal to or greater than 10 nucleotides and show greater than 50% homology to nucleotide sequences encoding MttAl, mttA2, MttB and MttC of Figures 11 and 7. The present invention further contemplates antisense molecules comprising the nucleic acid sequence complementary to at least a portion of the polynucleotide sequences encoding MttAl, mttA2, MttB and MttC (Figures 11 and 7). The scope of this invention further encompasses nucleotide sequences containing the nucleotide sequence of Figures 11 and 7, portions, variants, and homologs thereof, ligated to one or more heterologous sequences as part of a fusion gene. Such fusion genes may be desirable, for example, to detect expression of sequences which form part of the fusion gene. Examples of a heterologous sequence include the reporter sequence encoding the enzyme - 16 - β-galactosidase or the enzyme luciferase. Fusion genes may also be desirable to facilitate purification of the expressed protein. For example, the heterologous sequence of protein A allows purification of the fusion protein on immobilized immunoglobulin. Other affinity traps are well known in the art and can be utilized to advantage in purifying the expressed fusion protein. For example, pGEX vectors (Promega, Madison WI) may be used to express the MttAl , MttA2, MttB and MttC polypeptides as a fusion protein with glutathione S-transferase (GST). In general, such fusion proteins are soluble and can easily be purified from lysed cells by adsorption to glutathione-agarose beads followed by elution in the presence of free glutathione. Proteins made in such systems are designed to include heparin, thrombin or factor XA protease cleavage sites so that the cloned polypeptide of interest can be released from the GST moiety at will. The nucleotide sequences which encode MttAl, MttA2, MttB and MttC (Figures 1 1 and 7), portions, variants, and homologs thereof can be synthesized by synthetic chemistry techniques which are commercially available and well known in the art. The nucleotide sequence of synthesized sequences may be confirmed using commercially available kits as well as from methods well known in the art which utilize enzymes such as the Klenow fragment of DNA polymerase I, Sequenase ® , Taq DNA polymerase, or thermostable T7 polymerase. Capillary electrophoresis may also be used to analyze the size and confirm the nucleotide sequence of the products of nucleic acid synthesis. Synthesized sequences may also be amplified using the polymerase chain reaction (PCR) as described by Mullis [U.S. Patent No. 4,683,195] and Mullis et al. [U.S. Patent No. 4,683,202], the ligase chain reaction [LCR; sometimes referred to as "Ligase Amplification Reaction" (LAR)] described by Barany, Proc. Natl. Acad. Sci., 88:189 (1991); Barany, PCR Methods and Applic, 1 :5 (1991); and Wu and Wallace, Genomics 4:560 (1989). It is readily appreciated by those in the art that the mttAl, mttA2, mttB and mttC nucleotide sequences of the present invention may be used in a variety of ways. For example, fragments of the sequence of at least about 10 bp, more usually at least about 15 bp. and up to and including the entire (i.e., full-length) sequence can be used as probes for the detection and isolation of complementary genomic DNA sequences from any cell. Genomic sequences are isolated by screening a genomic library with all or a portion of the nucleotide sequences which encode MttAl, MttA2, MttB and MttC (Figures 11 and 7). In addition to screening genomic libraries, the mttAl, mttA2, mttB and mttC nucleotide sequences can also be used to screen cDNA libraries made using RNA. - 17 - The mttAl, mttA2, mttB and mttC nucleotide sequences of the invention are also useful in directing the synthesis of MttAl, MttA2, MttB, and MttC, respectively. The MttAl , MttA2, MttB, and MttC polypeptides find use in producing antibodies which may be used in, for example, detecting cells which express MttAl, MttA2, MttB and MttC. These cells may additionally find use in directing expression of recombinant proteins to cellular membranes or to the periplasm, extracellular medium. Alternatively, cells containing at least one of MttAl , MttA2, MttB and MttC may be used to direct expression of recombinant proteins which are engineered to contain twin-arginine signal amino acid sequences, or of wild-type proteins which contain twin-arginine signal amino acid sequences, to the periplasm or extracellularly (as described below), thus reducing the likelihood of formation of insoluble proteins. B. MttA, MttB, and MttC polypeptides This invention discloses the amino acid sequence of MttAl (SEQ ID NO:47), and MttA2 (SEQ ID NO:49) which are encoded by the mttAl and mttA2 genes, respectively. Data presented herein demonstrates that the protein MttA2 targets twin arginine containing proteins (i.e., proteins which contain twin-arginine signal amino acid sequences), as exemplified by the proteins dimethylsulfoxide (DMSO) reductase (DmsABC) to the cell membrane (Examples 2 and 5). The function of MttA2 in membrane targeting of twin arginine containing proteins was demonstrated by isolating a pleiotropic-negative mutant in mttA2 which prevents the correct membrane targeting of Escherichia coli dimethylsulfoxide reductase and results in accumulation of DmsA in the cytoplasm. DmsABC is an integral membrane molybdoenzyme which normally faces the cytoplasm and the DmsA subunit has a twin-arginine signal amino acid sequence. The mutation in mttA2 changed proline 25 to leucine in the encoded MttA2, and was complemented by a DNA fragment encoding the mttA 2 gene. Data presented herein further demonstrates that MttA2 also functions in selectively translocating twin arginine containing proteins, as exemplified by nitrate reductase (NapA) and trimethylamine N-oxide reductase (TorA), to the periplasm (Example 4). The mutation in the mttA 2 gene resulted in accumulation of the periplasmic proteins NapA and TorA in the cytoplasm and cell membranes. In contrast, proteins with a sec-dependent leader, as exemplified by nitrite reductase (NrfA), or which contain a twin-arginine signal amino acid sequence and which assemble spontaneously in the membrane, as exemplified by trimethylamine N-oxide (TMAO), were not affected by this mutation (Examples 2 and 4). - 18 - The isolation of mutant D-43 which contained a mutant mttA2 gene was unexpected. The assembly of multisubunit redox membrane proteins in bacteria and eukaryotic organelles has been assumed to be a spontaneous process mediated by protein-protein interactions between the integral anchor subunit(s) and the extrinsic subunit(s) [Latour and Weiner (1987) J. Gen. Microbiol. 133:597-607; Lemire et al. (1983) J. Bacteriol. 155:391-397]. It has previously been shown that the extrinsic subunits of fumarate reductase, FrdAB, can be reconstituted to form the holoenzyme with the anchor subunits, FrdCD, in vitro without any additional proteins [Lemire et al. (1983) J. Bacteriol. 155:391-397]. Because the architecture of DMSO reductase is similar to that of fumarate reductase, it seemed likely that this protein assembled in a similar manner. However, data presented herein demonstrates that this was not the case. Thus, the isolation of mutant D-43 was unexpected and it suggests that the assembly of DmsABC needs auxiliary proteins for optimal efficiency. Alternatively, the assembly of DmsABC may be an evolutionary vestige related to the soluble periplasmic DMSO reductase found in several organisms [McEwan (1994) Antonie van Leeuwenhoek 66:151-164; McEwan et al. (1991) Biochem. J. 274:305-307]. Without limiting the invention to a particular mechanism, MttA2 is predicted to be a membrane protein with two transmembrane segments and a long periplasmic α-helix. Proline 25 is located after the second transmembrane helix and immediately preceding the long periplasmic α-helix suggesting the essential nature of this region of MttA2. Interestingly, the smallest complementing DNA fragment, pGS20, only encoded the amino terminal two thirds of MttA2. This suggests that the carboxy terminal globular domain is not necessary or can be substituted by some other activity. This conclusion is further supported by the observation that the carboxy terminal third of MttA2 is also the least conserved region of MttA2. While the amino terminal of MttA2 is homologous to YigT of Settles et al. (1997) Science 278:1467-1470, the YigT sequence was not correct throughout its length. Data presented herein shows that proteins which were homologous to MttAl and MttA2 were identified by BLAST searches in a wide variety of archaebacteria, eubacteria, cyanobacteria and plants, suggesting that the sec-independent translocation system of which MttAl and MttA2 are members is very widely distributed in nature. The invention further discloses the amino acid sequence of MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8). Without limiting the invention to any particular mechanism, MttB is an integral membrane protein with six transmembrane segments and MttC is a membrane protein with one or two transmembrane segments and a large cytoplasmic domain. Proteins - 19 - homologous to MttB were identified by BLAST searches in a wide variety of archaebacteria, eubacteria, cyanobacteria and plants, suggesting that the protein translocation system of which MttB is a member is very widely distributed in nature. The MttC protein was even more widely dispersed with homologous proteins identified in archaebacteria, mycoplasma, eubacteria, cyanobacteria, yeast, plants, C. elegans and humans. In all cases the related proteins were of previously unknown function. Without limiting the invention to any particular mechanism, the predicted topology of the MttABC proteins suggests that the large cytoplasmic domain of MttC serves a receptor function for twin arginine containing proteins, with the integral MttB protein serving as the pore for protein transport. Based on the observation that the MttA2 can form a long α-helix, this protein is predicted to play a role in gating the pore. The present invention specifically contemplates variants and homologs of the amino acid sequences of MttAl , MttA2, MttB and MttC. A "variant" of MttAl , MttA2. MttB and MttC is defined as an amino acid sequence which differs by one or more amino acids from the amino acid sequence of MttAl (SEQ ID NO:47), MttA2 (SEQ ID NO:49), MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8), respectively. The variant may have "conservative" changes, wherein a substituted amino acid has similar structural or chemical properties, e.g., replacement of leucine with isoleucine. More rarely, a variant may have "nonconservative" changes, e.g., replacement of a glycine with a tryptophan. Similar minor variations may also include amino acid deletions or insertions (i.e., additions), or both. Guidance in determining which and how many amino acid residues may be substituted, inserted or deleted without abolishing biological or immunological activity may be found using computer programs well known in the art, for example, DNAStar software. For example, MttAl, MttA2, MttB and MttC variants included within the scope of this invention include MttAl, MttA2, MttB and MttC polypeptide sequences containing deletions, insertion or substitutions of amino acid residues which result in a polypeptide that is functionally equivalent to the MttAl, MttA2, MttB and MttC polypeptide sequences of Figure 11 and Figure 7. For example, amino acids may be substituted for other amino acids having similar characteristics of polarity, charge, solubility, hydrophobicity, hydrophilicity and/or amphipathic nature. Alternatively, substitution of amino acids with other amino acids having one or more different characteristic may be desirable for the purpose of producing a polypeptide which is secreted from the cell in order to, for example, simplify purification of the polypeptide. - 20 - The present invention also specifically contemplates homologs of the amino acid sequences of MttAl, MttA2, MttB and MttC. An oligonucleotide sequence which is a "homolog" of MttAl (SEQ ID NO:47), MttA2 (SEQ ID NO:49), MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8) is defined herein as an oligonucleotide sequence which exhibits greater than or equal to 50% identity to the sequence of MttAl (SEQ ID NO:47), MttA2 (SEQ ID NO:49), MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8), respectively, when sequences having a length of 20 amino acids or larger are compared. Alternatively, a homolog of MttAl (SEQ ID NO:47), MttA2 (SEQ ID NO:49), MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8) is defined as an oligonucleotide sequence which encodes a biologically active MttAl, MttA2, MttB and MttC amino acid sequence, respectively. The MttAl, MttA2, MttB and MttC polypeptide sequence of Figures 1 1 and 7 and their functional variants and homologs may be made using chemical synthesis. For example, peptide synthesis of the MttAl, MttA2, MttB and MttC polypeptides, in whole or in part, can be performed using solid-phase techniques well known in the art. Synthesized polypeptides can be substantially purified by high performance liquid chromatography (HPLC) techniques, and the composition of the purified polypeptide confirmed by amino acid sequencing. One of skill in the art would recognize that variants and homologs of the MttAl, MttA2. MttB and MttC polypeptide sequences can be produced by manipulating the polypeptide sequence during and/or after its synthesis. MttAl, MttA2, MttB and MttC and their functional variants and homologs can also be produced by an expression system. Expression of MttAl, MttA2, MttB and MttC may be accomplished by inserting the nucleotide sequence encoding MttAl , MttA2. MttB and MttC (Figures 1 1 and 7), its variants, portions, or homologs into appropriate vectors to create expression vectors, and transfecting the expression vectors into host cells. Expression vectors can be constructed using techniques well known in the art [Sambrook et al. (1989) Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Press, Plainview NY; Ausubel et al. (1989) Current Protocols in Molecular Biology, John Wiley & Sons, New York NY]. Briefly, the nucleic acid sequence of interest is placed in operable combination with transcription and translation regulatory sequences. Regulatory sequences include initiation signals such as start (i.e., ATG) and stop codons, promoters which may be constitutive (i.e., continuously active) or inducible, as well as enhancers to increase the efficiency of expression, and transcription termination signals. Transcription termination signals must be provided downstream from the structural gene if the termination - 21 - signals of the structural gene are not included in the expression vector. Expression vectors may become integrated into the genome of the host cell into which they are introduced, or are present as unintegrated vectors. Typically, unintegrated vectors are transiently expressed and regulated for several hours (eg. , 72 hours) after transfection. The choice of promoter is governed by the type of host cell to be transfected with the expression vector. Host cells include bacterial, yeast, plant, insect, and mammalian cells. Transfected cells may be identified by any of a number of marker genes. These include antibiotic (e.g., gentamicin, penicillin, and kanamycin) resistance genes as well as marker or reporter genes (e.g., β-galactosidase and luciferase) which catalyze the synthesis of a visible reaction product. Expression of the gene of interest by transfected cells may be detected either indirectly using reporter genes, or directly by detecting mRNA or protein encoded by the gene of interest. Indirect detection of expression may be achieved by placing a reporter gene in tandem with the sequence encoding one or more of MttAl, MttA2, MttB and MttC under the control of a single promoter. Expression of the reporter gene indicates expression of the tandem one or more MttAl, MttA2, MttB and MttC sequence. It is preferred that the reporter gene have a visible reaction product. For example, cells expressing the reporter gene β-galactosidase produce a blue color when grown in the presence of X-Gal, whereas cells grown in medium containing luciferin will fluoresce when expressing the reporter gene luciferase. Direct detection of MttAl, MttA2, MttB and MttC expression can be achieved using methods well known to those skilled in the art. For example, mRNA isolated from transfected cells can be hybridized to labelled oligonucleotide probes and the hybridization detected. Alternatively, polyclonal or monoclonal antibodies specific for MttAl, MttA2, MttB and MttC can be used to detect expression of the MttAl, MttA2, MttB and MttC polypeptide using enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA) and fluorescent activated cell sorting (FACS). Those skilled in the art recognize that the MttAl, MttA2, MttB and MttC polypeptide sequences of the present invention are useful in generating antibodies which find use in detecting cells that express MttAl, MttA2, MttB and MttC or proteins homologous thereto. Such detection is useful in the choice of host cells which may be used to target recombinant twin arginine containing protein expression to cellular membranes or to the periplasm or to the extracellular medium. Additionally, such detection is particularly useful in selecting host - 22 - cells for cytoplasmic or extracellular expression of recombinant twin arginine containing proteins by disrupting the function of at least one of MttAl, MttA2, MttB and MttC as described below. C. Methods for expressing polypeptides to produce soluble proteins This invention contemplates methods for targeting expression (e.g. , to the periplasm, extracellular medium) of any gene of interest (e.g., to the cytoplasm, extracellular medium) thus reducing the likelihood of expression of insoluble recombinant polypeptides, e.g., in inclusion bodies. The methods of the invention are premised on the discovery of three proteins, MttAl, MttA2, MttB and MttC which function as part of a Sec-independent pathway, and which target expression of twin arginine containing proteins to cell membranes and which direct translocation of such proteins to the periplasm of gram negative bacteria and to the extracellular medium of cells which do not contain a periplasm. This discovery makes possible methods for expression of any gene of interest such that the expressed polypeptide is targeted to the periplasm or extracellular medium thereby allowing its expression in a soluble form and thus facilitating its purification. The methods of the invention contemplate expression of any recombinant polypeptide as a fusion polypeptide with a twin-arginine signal amino acid sequence as the fusion partner. Such expression may be accomplished by introducing a nucleic acid sequence which encodes the fusion polypeptide into a host cell which expresses wild-type MttAl, MttA2, MttB or MttC, or variants or homologs thereof, or which is engineered to express MttAl, MttA2, MttB or MttC, or variants or homologs thereof. While expressly contemplating the use of the methods of the invention for the expression of any polypeptide of interest, the methods disclosed herein are particularly useful for the expression of cofactor-containing proteins. The methods of the invention are further described under (i) Cofactor-containing proteins, (ii) Expression of fusion proteins containing twin-arginine signal amino acid sequences, and (iii) Construction of host cells containing deletions or mutations in at least a portion of the genes mttAl, MttA2, mttB and mttC. i. Cofactor-containing proteins A strong correlation has been reported between possession of a twin-arginine signal amino acid sequence in the preprotein and the presence of a redox cofactor in the mature protein; approximately 40 out of 135 preprotein amino acid sequences which contain a twin- arginine signal amino acid sequence have been found by Berks [Berks (1996) Molecular - 23 - Microbiology 22 393-104; http://www.blackwell-science.com/products/journals/ contents/berks. htm] to result in a mature protein which binds, or can be inferred to bind, a redox cofactor. The entire contents of Berks are hereby expressly incorporated by reference. The cofactors associated with a twin-arginine signal amino acid sequence include, but are not limited to, iron-sulfur clusters, at least two variants of the molybdopterin cofactor, certain polynuclear copper sites, the tryptophan tryptophylquinone (TTQ) cofactor, and flavin adenine dinucleotide (FAD). A representative selection of bacterial twin-arginine signal amino acid sequences is shown in Table 1. TABLE 1 Evidence Length I. PERIPLASMIC PROTEINS BINDING IRON-SULFUR CLUSTERS A. MauM family ferredoxins P. denitriflcans MauM MEA MTGRRKVTRRDAMADAARAVGVACLG VH 46 GFSLAALVRTASPVDA E. coli NapG MSRSAKPQNGRRRFLRDVVRTAGGLAAVGVA VH 41 LGLQQQTARA B. '16Fe' ferredoxin superfamily E. coli NrfC MTWSRRQFLTGVGVLAAVSGTAGRVVA VH 27 D. vitlgaris Hmc2 MDRRRFLTLLGSAGLTATVATAGTAKA VH 27 C. High potential iron protein (HiPIP) T. ferrooxidans Iro MSEKDKMITRRDALRNIAVVVGSVATTTMMG EX 37 VGVADA D. Periplasmically-located (Fe| hydrogenase small subunits D. vulgat s HydB MQIVNLTRRGFLKAACVVTGGALISIRMTGKA VH 34 VA E. Periplasmically-located [NiFe] hydrogenase small subunits E. coli HyaA M NEETFYQAMRRQGVTRRSFLKYCSLAATS EX 45 LGLGAGMAPKIAWA +M. ma∑ei VhoG MSTGTTNLVRTLDSMDFLKMDRRTFMKAVSA EX 48 LGATAFLGTYQTEIV A D. gigas HynB MKCYIGRGKNQVEERLERRGVSRRDFMKFCT EX 50 AVAVAMGMGPAFAPKVAEA E. coli HybA MNRRNFIKAASCGALLTGALPSVSHA VH 26 F. Membrane-anchored Rieske proteins P. denitrific ns FbcF MSHADEHAGDHGATRRDFLYYATAGAGTVA AGAAAWTLVNQM P 24 Evidence Length +S nechocystis PetC MTQISGSPDVPDLGRRQFMNLLTFGTITGVAA GALYPAVKYLIP +S. ucidocaldarius SoxF MDRRTFLRLYLLVGAAIAVAPVI PALDYVGY II. PERIPLASMIC PROTEINS BINDING THE MOLYBDOPTERIN COFACTOR A. Molybdopterin guanine dinucleotide-binding proteins, some of which also bind an iron-sulfur cluster R. sphaeroides DmsA MTKLSGQELHAELSRRAFLSYTAAVGALGLCG EX 42 TSLLAQGARA E. coli BisZ MTLTRREFIKHSGIAAGALVVTSAAPLPAWA VH 3 1 T. pantotropha NapA MTISRRDLLKAQAAG1AAMAAN1PLSSQAPA VH 3 1 W. succinogenes FdhA MSEALSGRGNDRRKFLKMSALAGVAGVSQAV EX 32 G E. coli DmsA MKTKIPDAVLAAEVSRRGLVKTTAIGGLAMAS EX 45 SALTLPFSRIAHA H. inflnenzae DmsA MSNFNQISRRDFVKASSAGAALAVSNLTLPFN VH 35 VMA S. typhimurium PhsA MSISRRSFLQGVGIGCSACALGAFPPGALA VH 30 B. Molybdopterin cytosine dinucleotide-binding proteins P. dimimita lorB MKTVLPSVPETVRLSRRGFLVQAGTITCSVAFG VH 37 SVPA A. polyoxogenes Aid MGRLNRFRLGKDGRREQASLSRRGFLVTSLGA EX 44 GVMFGFARPSSA III. PERIPLASMIC ENZYMES WITH POLYNUCLEAR COPPER SITES A. Nitrous oxide reductases P. stut∑eri NosZ MSDKDSKNTPQVPEKLGLSRRGFLGASAVTGA EX 50 AVAATALGGAVMTRESWA B. Multicopper oxidase superfamily P. syringae CopA MESRTSRRTFVKGLAAAGVLGGLGLWRSPSW VH 32 A E. coli Sufi MSLSRRQFIQASGIALCAGAVPLKASA VH 27 IV. METHYLAMINE DEHYDROGENASE SMALL SUBUNITS (TRYPTOPHAN TRYPTOPHYLQUINONE COFACTOR) M. extorquens MauA MLGKSQFDDLFEKMSRKVAGHTSRRGFIGRVG EX 57 TAVAGVALVPLLPVDRRGRVSRANA V. PERIPLASMIC PROTEINS BINDING FLAVIN ADENINE DINUCLEOTIDE C. vinosum FccB MTLNRRDFIKTSGAAVAAVGILGFPHLAFG EX 30 +B. sterolicum ChoB MTDSRANRADATRGVASVSRRRFLAGAGLTA EX 45 GAIALSSMSTSASA 25 - A more complete listing of bacterial twin-arginine signal amino acid sequences is available at http://www.blackwell-science.com/products/journals/mole.htm, the entire contents of which are incorporated by reference. Amino acids with identity to the most preferred (S/T)-RR-x-F-L-K consensus motif are indicated in bold. Signal sequences are from Proteobacterial preproteins except where indicated (+). 'Evidence' indicates the method used to determine the site of protease processing: EX, experimentally determined; VH. inferred using the algorithm of von Heijne (1987). [1] van der Palen et al. ( 1995); [2] Richterich et al. (1993); [3] Hussain et al. (1994); [4] Rossi et al. (1993); [5] Kusano et al. (1992); [6] Voordouw et al. (1989); [7] Menon et al. (1990); [8] Deppenmeier et al. (1995); [9] Li et al. (1987); [10] Menon et al. (1994); [11] Kurowski and Ludwig (1987); [12] Mayes and Barber (1991); [13] Castresana et al. (1995); [14] Hilton and Rajagopalan (1996); [15] Campbell and Campbell (1996); [16] Berks et al. (1995a); [17] Bokranz et al. (1991); [18] Bilous et al. ( 1988); [19] Fleischmann et al. (1995); [20] Heinzinger et al. (1995); [21] Lehmann et al. (1995); [22] Tamaki et al. (1989); [23] Viebrock and Zumft (1988): [24] Mellano and Cooksey (1988); [25] Plunkett (1995); [26] Chistoserdov and Lidstrom (1991); [27] Dolata et al. (1993); [28] Ohta et al. (1991). In contrast to twin-arginine signal amino acid sequences, Sec signal sequences are associated with periplasmic proteins binding other redox cofactors, /. e. , iron porphyrins (including the cytochromes c), mononuclear type I or II copper centers, the dinuclear Cu Λ center, and the pyrrolo-quinoline quinone (PQQ) cofactor. Currently the assembly of cofactor-containing proteins is limited to the cytoplasm because the machinery to insert the cofactor is located in this compartment. The present invention offers the advantage of providing methods for periplasmic and extracellular expression of cofactor-containing proteins which contain a twin-arginine signal amino acid sequence, thus facilitating their purification in a functional and soluble form. ii. Expression of fusion proteins containing twin-arginine signal amino acid sequences The methods of the invention exploit the inventors' discovery of proteins MttAl, MttA2, MttB and MttC which are involved in targeting expression of proteins which contain a twin-arginine amino acid signal sequence to cell membranes and in translocation of such proteins to the periplasm of gram negative bacteria and the extracellular medium of cell that - 26 - do not contain a periplasm. The term "twin-arginine signal amino acid sequence" as used herein means an amino acid sequence of between 2 and about 200 amino acids, more preferably between about 10 and about 100 amino acids, and most preferably between about 25 and about 60 amino acids, and which comprises the amino acid sequence, from the N- terminal to the C-terminal, A-B-C-D-E-F-G, wherein the amino acid at position B is Arg, and the amino acid at position C is Arg. The amino acid at positions A, D, E, F, and G can be any amino acid. However, the amino acid at position A preferably is Gly, more preferably is Glu, yet more preferably is Tin:, and most preferably is Ser. The amino acid at position D preferably is Gin, more preferably is Gly, yet more preferably is Asp. and most preferably is Ser. The amino acid at position E preferably is Leu and more preferably is Phe. The amino acid at position F preferably is Val, more preferably is Met, yet more preferably is He, and most preferably is Leu. The amino acid at position G preferably is Gin, more preferably is Gly and most preferably is Lys. In one preferred embodiment, the twin-arginine amino acid signal sequence is Ser-Arg-Arg-Ser-Phe-Leu-Lys (SEQ ID NO:41). In yet another preferred embodiment, the twin-arginine amino acid signal sequence is Thr-Arg-Arg-Ser-Phe-Leu-Lys (SEQ ID NO:42). The invention contemplates expression of wild-type polypeptide sequences which contain a twin-arginine amino acid signal sequence as part of a preprotein. To date, 135 polypeptide sequences have been reported to contain a twin-arginine amino acid signal sequence motif [Berks (1996) Molecular Microbiology 22 393-104; http://www.blackwell- science.com/products/journals/contents/berks.htm the entire contents of which are incorporated by reference]. The invention further contemplates expression of recombinant polypeptide sequences which are engineered to contain a twin-arginine amino acid signal sequence as part of a fusion protein. Fusion protein containing one or more twin-arginine amino acid signal sequences may be made using methods well known in the art. For example, one of skill in the art knows that nucleic acid sequences which encode a twin-arginine amino acid signal sequence may be operably ligated in frame (directly, or indirectly in the presence of intervening nucleic acid sequences) to a nucleotide sequence which encodes a polypeptide of interest. The ligated nucleotide sequence may then be inserted in an expression vector which is introduced into a host cell for expression of a fusion protein containing the polypeptide of interest and the twin-arginine amino acid signal sequence. 27 - Fusion proteins containing twin-arginine amino acid signal sequences are expected to be targeted to the periplasm or extracellular medium by the MttAl . MttA2, MttB and MttC proteins of the invention and by variants and homologs thereof; Keon and Voordouw [Keon and Voordouw (1996) Anaerobe 2:231-238] have reported that a fusion protein containing E. coli alkaline phosphatase (phoA) linked to a signal amino acid sequence from the Hmc complex of Desulfovibrio vulgaris subsp. vulgaris was exported to E. coli periplasm. Similarly, a fusion protein containing a hydrogenase signal peptide to β-lactamase from which the signal peptide had been removed led to export in E. coli under both aerobic and anaerobic conditions [Niviere et al. (1992) J. Gen. Microbiol. 138:2173-2183]. Fusion proteins which contain twin-arginine amino acid signal sequences are also expected to be cleaved to generate a mature protein from which the twin-arginine amino acid signal sequences has been cleaved. Two signal peptidases have so far been identified in E. coli: Signal peptidase I and signal peptidase II. The signal peptidase II which has a unique cleavage site involving a cystine residue at the cleavage site [Bishop et al. (1995) J. Biol. Chem. 270:23097-23103] is believed not to participate in cleavage of twin-arginine amino acid signal sequences. Rather, signal peptidase I, which cleaves Sec signal sequences has been Suggested by Berks to cleave twin-arginine amino acid signal sequences. Berks also suggested that signal peptidase I has the same recognition site in Sec signal sequences as in twin-arginine amino acid signal sequences [Berks (1996)]. This suggestion was based on (a) the "-1/-3" rule for Sec signal peptidase in which the major determinant of signal peptidase processing is the presence of amino acids with small neutral side-chains at positions -1 and -3 relative to the site of cleavage, and (b) the good agreement between the cleavage site of twin- arginine amino acid signal sequences as determined using the "-1/-3" rule (with the invariant arginine at the N-terminus of the signal sequence, i. e. , position B in the A-B-C-D-Ε-F-G sequence, designated as position zero) and the experimentally determined amino terminus of the mature protein [Berks (1996)]. Evidence presented herein (Example 9) further confirms cleavage of twin-arginine amino acid signal sequences to release a mature protein which lacks the twin-arginine amino acid signal sequence. iii. Construction of host cells containing deletions or mutations in at least a portion of the genes mttA, mttB and mttC The function of any portion of E. coli MttAl, MttA2, MttB and MttC polypeptides and variants and homologs thereof, as well as the function of any polypeptide - 28 - which is encoded by a nucleotide sequence that is a variant or homolog of the mttAl, MttA2. mttB and mttC sequences disclosed herein may be demonstrated in any host cell by in vivo homologous recombination of chromosomal sequences which are variants or homologs of mttAl, MttA2, mttB and mttC using previously described methods [Sambasivarao et al (1991 ) J. Bacteriol. 5935-5943; Jasin et al (1984) J. Bacteriol. 159:783-786]. Briefly, the nucleotide sequence whose function is to be determined is cloned into vectors, and the gene is mutated. e.g., by insertion of a nucleotide sequence within the coding region of the gene. The plasmids are then homologously recombined with chromosomal variants or homologs of mttAl, MttA 2, mttB or mttC sequences in order to replace the chromosomal variants or homologs of mttAl, MttA2, mttB or mttC genes with the mutated genes of the vectors. The effect of the mutations on the localization of proteins containing twin-arginine amino acid signal sequences is compared between the wild-type host cells and the cells containing the mutated mttAl, MltA2, mttB or mttC genes. The localization (e.g. , cytoplasm, periplasm. cell membranes, extracellular medium) of expressed twin arginine containing proteins is compared using methods disclosed herein (e.g., functional enzyme activity and Western blotting) between homologously recombined cells and control cells which had not been homologously recombined. Localization of expressed twin arginine containing proteins extracellularly, in the periplasm. or in the cytoplasm of homologously recombined cells as compared to localization of expression in cell membranes of control cells demonstrates that the wild-type MttAl , MttA2. MttB or MttC protein whose function had been modified by homologous recombination functions in targeting expression of the twin arginine containing protein to the cell membrane. Similarly, accumulation of expressed twin arginine containing proteins in extracellular medium, in the cytoplasm, or in cell membranes of homologously recombined cells as compared to periplasmic localization of the expressed twin arginine containing protein in control cells which had not been homologously recombined indicates that the protein (i.e.. MttAl , MttA2, MttB or MttC) whose function had been modified by homologous recombination functions in translocation of the twin arginine containing protein to the periplasm. EXPERIMENTAL The following examples serve to illustrate certain preferred embodiments and aspects of the present invention and are not to be construed as limiting the scope thereof. The strains and plasmids used in this investigation are listed in Table 2. - 29 - TABLE 2 Bacteria and Plasmids used in this Investigation Genotype or Strain/Plasmid Gene Combinations Present Reference/Source HB l Ol F-, hsdS20(r- B m-g), leu, supE44, aral4,galK2, Boyer and Roulland- lacYl, proA2, rpsL20, xyl-5, mtl-1, recA13, mcrB Dussoix, 1969 TG I K 12A(lac-pro) sup EF ' traD36 proAB lacP Amersham Corp. MacZMIS D43 HB l Ol ; mttA Bilous and Weiner, 1985 pBR322 cloning vector Tet', Amp" Pharmacia pTZ18R cloning vector Amp r , lacZ Pharmacia pJBS633 blaM fusion vector Broome-Smith and Spratt, 1986 pFRD84 frdABCD cloned into pBR322 Lemire et al., 1982 pFRD1 17 AfrdCD version of pFRD84 Lemire el al. , 1982 pDMS160 dmsABC cloned into pBR322 Rothery and Weiner, 1991 pDMS223 dmsABC operon in pTZ 18R Rothery and Weiner, 1991 pDMSL71 dmsABC: : blaM ' in pJBS633 fusion after residue 12 Weiner et al., 1993 pD SL5 dmsABC ' :: blaM in pJBS633 fusion after residue 216 Weiner et al., 1993 pDMSL29 dmsABC:: blaM in pJBS633 fusion after residue 229 Weiner et al.. 1993 pD SL4 dmsABC:: blaM in pJBS633 fusion after residue 267 Weiner el al.. 1993 pDMSC59X dmsC truncate after residue 59 Sambasivarao and Weiner, 1991 pDSR31 1 yigO,P, R, T and U in pBR322 This investigation pGS20 b3835', b3836, b3837, and b3838' in pBR322 This investigation pTZmttABC region of ORF's b3836, b3838, yigU, yigW, cloned This investigation into pTZ18R pBRmttABC region of ORF's b3836, b3838, yigU, yigW, cloned This investigation into pBR322 pTZb3836 ORF b3836 cloned into pTZ18R This investigation pBRb3836 ORF b3836 cloned into pBR322 This investigation EXAMPLE 1 Isolation And Properties of D-43 Mutants Defective In DmsABC Targeting DMSO reductase is a "twin arginine" trimeric enzyme composed of an extrinsic membrane dimer with catalytic, DmsA, and electron transfer, DmsB, subunits bound to an intrinsic anchor subunit, DmsC. The DmsA subunit has a "twin arginine" leader but it has been exhaustively shown that the DmsA and DmsB subunits face the cytoplasm [Rothery and Weiner (1996) Biochem. 35:3247-3257; Rothery and Weiner (1993) Biochem. 32:5855-5861; Sambasivarao et al. (1990) J. Bacteriol. 172:5938-5948; Weiner et al. ( 1992) Biochem. Biophys. Acta 1 102:1-18; Weiner et al. (1993) J. Biol. Chem. 268:3238-3244]. In order to isolate a E. coli mutant defective in membrane targeting of DmsABC, plieotropic mutants which were unable to grow on DMSO were produced by nitrosoguanidine mutagenesis of HBlOl and the growth rates on DMSO of both the mutants and HBl Ol were determined. Mutant D-43, which grew anaerobically on fumarate and nitrate, nevertheless failed to grow on DMSO or TMAO. These results are further described in the following sections. A. Isolation of mutant Nitrosoguanidine mutagenesis and ampicillin enrichment were as described by Miller (1992) in A Short Course in Bacterial Genetics, Cold Spring Harbor Laboratory Press. Sixteen mutants were isolated that were defective for anaerobic growth on DMSO but grew with nitrate or fumarate as the alternate electron acceptor. Each of the mutants was transformed with pDMS160 [Rothery and Weiner (1991) Biochem. 30:8296-8305] carrying the entire dms operon and again tested for growth on DMSO. All of the transformants failed to grow on DMSO. When tested for DMSO reductase activity 14 of the 16 transformants lacked measurable enzyme activity. Two of the mutants expressed high levels of DMSO reductase activity but the activity was localized in the cytoplasm rather than the membrane fraction. One of these mutants, D-43, was chosen for further study. B. Anaerobic growth rates of HBlOl and D-43 For growth experiments, bacteria were initially grown aerobically overnight at 37°C in LB plus 10 μg/ml " ' vitamin Bl . A 1% inoculum was added to 150 ml of minimal salts medium containing 0.8% (w/v) glycerol, 10 μg/ml " ' each of proline, leucine, vitamin Bl and - 31 - 0.15% peptone and supplemented with either DMSO 70 mM, fumarate 35 mM, nitrate 40 mM, or trimethylamine N-oxide (TMAO) lOOmM. Cultures were grown anaerobically at 37°C in Klett flasks and the turbidity monitored in a Klett spectrophotometer with a No. 66 filter. The rates of anaerobic growth of strains HBlOl and D-43 with a range of electron acceptors and a nonfermentable carbon source, glycerol, were compared. The results are shown in Figure 1. All the terminal electron acceptors tested supported the growth of the parent HBlOl (Figure la). In contrast, only nitrate and fumarate stimulated the growth rate of the mutant (Figure lb). However, even in the presence of nitrate and fumarate the growth yield was half that of strain HBlOl . The reduced growth rate may reflect the pleiotropic effects of the mutation of various metabolic reactions needed for optimal growth in addition to the terminal electron transfer reaction. Only DmsABC supports growth on DMSO whereas both DmsABC and the periplasmic TMAO reductase support growth on TMAO [Sambasivarao and Weiner (1991) J. Bacteriol. 173:5935-5943]. The observation that D-43 is unable to grow on either DMSO or TMAO indicates that both of these enzymes were non- functional. EXAMPLE 2 DmsA Is Not Anchored To the Membrane In D-43 Previous studies have exhaustively shown that DmsABC is localized on the cytoplasmic membrane of wild-type E. coli strains with the DmsAB subunits anchored to the cytoplasmic surface [Rothery and Weiner (1996) Biochem. 35:3247-3257; Rothery and Weiner (1993) Biochem. 32:5855-5861 ; Sambasivarao et al. (1990) J. Bacteriol. 172:5938- 5948; Weiner et al. (1992) Biochem. Biophys. Acta 1102:1-18; Weiner et al. (1993) J. Biol. Chem. 268:3238-3244]. In order to determine he localization of DmsABC in D-43 mutants, cell fractions were assayed for the presence of DmsA and DmsB by immunoblot analysis, and for DMSO reductase activity as follows. A. Functional enzyme activity assays Cell fractions were assayed for DMSO reductase activity by measuring the DMSO- dependent oxidation of reduced benzyl viologen at 23°C [Bilous and Weiner (1985) J. Bacteriol. 162:1151-1 155]. This assay is dependent only on the presence of DmsAB. - 32 - To test the localization of DmsABC in D-43, enzyme activity in the soluble fraction and membrane band fraction of HB101/pDMS160 and of D-43/pDMS160 was determined. 250 ml anaerobic cultures of HB101/pDMS160 and D-43/pDMS160 were grown on Gly/Fum medium. HB101/pDMS160 yielded 1 14 mg total protein, 3240 units of membrane-bound TMAO reductase activity, and 2900 units of soluble activity. D-43/pDMS160 yielded 99 mg total protein, 320 units were membrane-bound and 4000 units were soluble. Thus, although the total DmsABC activity was lower in D-43, (4300 total units compared to 6200 for HB101/pDMS160) the vast majority was not targeted to the membrane. This suggested that D-43 was defective in targeting to the membrane rather than in a biosynthetic step. B. Western blot analysis of DmsA and DmsB To determine the cellular locations of DmsA and DmsB by Western blots. D- 43/pDMS160 and HB101/pDMS160 were grown anaerobically on Gly/fumerate medium at 37°C in 19 I batches [Bilous and Weiner (1985) J. Bacteriol. 162: 1 151-1 155], Cultures were grown for 24hr, at 37°C and the cells harvested and membranes prepared by French pressure cell lysis at 16,000 psi followed by differential centrifugation as previously described [Rothery and Weiner (1991) Biochem. 30:8296-8305]. The crude membranes were washed twice with lysis buffer (50 mM MOPS, 5 mM EDTA pH 7.0). DmsABC was purified as described by Simala-Grant and Weiner (1996) Microbiology 142:3231-3229. For the determination of subunit anchoring to the membrane, membrane preparations were first washed with lysis buffer and then with lysis buffer containing 1 M NaCI. The osmotic shock procedure of Weiner and Heppel (1971) J. Biol. Chem. 246:6933-6941 ) was used to isolate the periplasmic fraction tested for fumarate and DMSO reductase polypeptides. For Western blot analysis, antibodies to purified DmsA and DmsB were used [Sambasivarao et al. (1990) J. Bacteriol. 172:5938-5948]. Typically, samples were separated on 10% (w/v) SDS-PAGE and then blotted onto nitrocellulose. The protein bands were detected using the enhanced chemiluminescence detection system from Amersham and goat anti-rabbit lgG (H+L) horseradish peroxidase conjugate. The results are shown in Figure 2. Figure 2 shows a Western blot of washed membranes and soluble fractions of HBlOl and D-43 harboring pDMS160 expressing DmsABC. The blot was probed with either purified anti-DmsA or anti-DmsB. S; soluble fraction, M; Washed membranes, sM; salt washed membranes, sS; soluble fraction from the salt washed membranes, P; purified DmsABC. Figure 2 clearly shows that DmsA is not targeted to the membrane in D-43. The - 33 - DmsA polypeptide was expressed and was present in the cytoplasm at levels equivalent to the wild-type. Equivalent samples probed with anti-DmsB demonstrated that significant amounts of DmsB were targeted to the membrane. Membrane incorporation of DmsC in the absence of DmsAB is lethal [Turner et al. (1997) Prof. Engineering 10:285-290] and the presence of DmsB on the membrane may overcome the lethality normally associated with incorporation of DmsC in the absence of the catalytic subunits. EXAMPLE 3 DmsC Is Anchored To the Membrane In D-43 Because polyclonal antibodies against DmsC could not successfully be raised [Sambasivarao et al. (1990) J. Bacteriol. 172:5938-5948; Turner et al. (1997) Prof. Engineering 10:285-290], three BlaM (β-lactamase) fusions were used to determine whether the anchor subunit is translated and correctly inserted into the membranes of D-43 [Weiner et al. (1993) J. Biol. Chem. 268:3238-3244]. These fusions were located after amino acid positions 216, 229 and 267 of DmsC. Fusion 216 was localized to the periplasm and mediated very high resistance. Fusions 229 and 267 were localized to the seventh and eighth transmembrane helices and mediated intermediate levels of resistance [Weiner et al. (1993) J. Biol. Chem. 268:3238-3244]. The minimal inhibitory concentrations of ampicillin. for each of these fusions expressed in D-43 under anaerobic growth conditions, were the same or within one plate dilution of the wild-type values. Additionally, Western blots, using antibody directed against BlaM, of cell fractions of membrane, cytoplasmic and osmotic shock fluids of D-43/pDMSL29 (fusion at amino acid 229) showed DmsC-BlaM in the membrane fractions (results not shown). These data suggest that the DmsC protein is translated and inserted into the membrane and has the same topology as that found in wild-type E. coli cells. EXAMPLE 4 Enzyme Activity Of Nitrate Reductase and Trimethylamine N-Oxide Reductase With A Twin Arginine Signal Sequence Is Not Targeted To the Periplasm Of D-43 While Enzyme Activity of Nitrite Reductase With A Sec-Signal Sequence Is Present In the Periplasm Of D-43 In order to determine whether the mutation in D-43 (which resulted in failure to anchor DmsA and DmsB to the cell membrane as described above) selectively prevented - 34 - membrane targeting of proteins with a twin-arginine signal amino acid sequence, the enzyme activity of periplasmic enzymes having a twin-arginine signal amino acid sequence (i.e., nitrate reductase (NapA) and trimethylamine N-oxide reductase (TorA)) and of a periplasmic enzyme having a Sec-leader sequence (i.e., nitrite reductase (NrfA)) was determined in the periplasm of D-43 and HB 101. E. coli can reduce nitrate to ammonia using two periplasmic electron transfer chains, the Nap and Nrf pathways [Grove et al. (1996) Mol. Microbiol. 19:467-481 ; Cole (1996) FEMS Microbiol. Letts. 136: 1-1 1]. The catalytic subunit of the periplasmic nitrate reductase, NapA. is a large molybdoprotein with similarity to DmsA and is synthesized with a twin- arginine signal amino acid sequence. NrfA, the periplasmic nitrite reductase. is not a molybdoprotein but a c-type cytochrome and contains a Sec-leader peptide. Accumulation of both of these redox enzymes in the periplasm of strain D-43 was assayed by staining the periplasmic proteins separated by PAGE with reduced methyl viologen in the presence of nitrate and nitrite as follows. Periplasmic proteins were released from washed bacterial suspensions as described by McEwan et al. (1984) Arch. Microbiol. 137:344-349 except that the EDTA concentration was 5 mM. The periplasmic fraction was dialyzed against two changes of a 20-fold excess of 10 mM Na+/K+ phosphate, pH 7.4 to remove sucrose and excess salt, freeze dried and dissolved in 10 mM phosphate pH 7.4 to a protein concentration of about 15 mg/ml " '. Protein concentrations were determined by the Folin phenol method described previously [Newman and Cole (1978) J. Gen. Microbiol. 106:1-12]. The periplasmic proteins were separated on a 7.5% non-denaturing polyacrylamide gel. After electrophoresis. the 18 cm square gel was immersed in 5 μg ml " ' methyl viologen containing 5 mM nitrate. Dithionite was added to keep the viologen reduced; bands of activity were detected as transparent areas against a dark purple background. The same protocol was used to detect periplasmic nitrite and TMAO reductase activity but 5 mM nitrate was replaced by 2.5 mM nitrite or 5 mM TMAO, respectively. The results are shown in Figure 3. Figure 3a shows A nitrate-stained polyacrylamide gel containing periplasmic proteins, membrane proteins and cytoplasmic proteins from HBlOl and D-43. Lanes 1) and 2) contain periplasmic proteins from HBlOl and D-43, respectively. Lanes 3) and 4) contain membrane proteins from HBlOl and D-43, respectively and lanes 5) and 6) contain soluble cytoplasmic proteins from HBlOl and D-43, respectively. Figure 3b shows nitrite-stained polyacrylamide gel containing periplasmic proteins from 1) HBlOl and 2) D-43. Approximately 30 μg of - 35 - protein was loaded into each lane. Figure 3c shows TMAO-stained polyacrylamide gel containing periplasmic proteins from 1) HBlOl and 2) D-43. The results in Figure 3 show that nitrate reductase activity due to NapA was present in the periplasmic proteins extracted from the parental strain HBlOl but was not observed in periplasmic proteins prepared from strain D-43 (Figure 3a). In contrast, activity of NrfA, the otype cytochrome nitrite reductase, was similar in periplasmic proteins prepared from both HBlOl and D-43 (Figure 3b). Significantly, the nitrate reductase activity was higher in membranes prepared from strain D-43 than in membranes prepared from the parental strain HBlOl , suggesting that NapA protein was "stuck" in the membrane fraction. No nitrate reductase activity was detected in soluble cytoplasmic proteins prepared from either strain (data not shown). Additionally, the rate of electron transfer from physiologic electron donors to NrfA was measured by assaying the rate of nitrite reduction by a suspension of whole cells in the presence of formate or glycerol. The effects of the mutation on periplasmic nitrite reductase activity provided a key control to test whether MttA2 plays a major role in protein targeting. Nrf activity can be assessed in two ways: by detecting the activity of the terminal nitrite reductase which is a c-type cytochrome secreted by the Sec pathway and assembled in the periplasm (Figure 3b) [Thony-Meyer and Kunzler (1997) Eur. J. Biochem. 246:794-799], and by measuring the rate of nitrite reduction by washed bacteria in the presence of the physiologic substrate, formate. Only the latter activity requires the membrane-bound iron- sulfur protein, NrfC, which is synthesized with an N-terminal twin-arginine signal amino acid sequence. The rate of nitrite reduction in suspensions of strain HBlOl was 34 μmol nitrite reduced/min ' Vmr 1 while that measured with suspensions of D-43 was 1 1 μmol nitrite reduced/min ' Vml " '. These results show that cytochrome c 55 , was correctly targeted in the mutant and able to catalyse nitrite reduction with dithionite-reduced methyl viologen as the artificial electron donor, but strain D-43 was deficient in formate-dependent nitrite reductase activity. Loss of electron transport to NrfA from physiologic electron donors, but not from reduced methyl viologen was probably due to the presence of a twin-arginine signal amino acid sequence motif in either NrfC, which is a protein essential for the transfer of electrons from quinones to NrfA [Hussain et al. (1996) Mol. Microbiol. 12:153-163] or in FdnG which - 36 - contributes to the transfer of electrons from formate to nitrite [Darwin et al. (1993) J. Gen. Microbiol. 139: 1829-1840]. Trimethylamine N-oxide reductase (TorA) is another periplasmic terminal reductase related to DmsA [Mejean et al. (1994) Mol. Microbiol. 1 1 : 1 169-1179] which contains a twin- arginine signal amino acid sequence. In strain D-43 this enzyme activity was not observed in the periplasmic protein fraction (Figure 3c). EXAMPLE 5 MttA2 Protein Targets DmsAB To The Membrane And Does Not Translocate DmsAB To The Periplasm In order to determine whether MttA2 is involved in targeting DmsAB to the membrane rather than in the translocation of DmsAB to the periplasm, and whether the role of DmsC is to prevent translocation of DmsAB to the periplasm, the intracellular location was examined in HBlOl and D-43 for the DmsA and DmsB subunits expressed from a plasmid encoding the wild-type DmsABC operon as well as a truncated form lacking the anchor subunit DmsC. The results are shown in Figure 4. Figure 4 shows a Western blot of DmsAB. Figure 4A shows HBlOl expressing either native DmsABC (pDMS160), DmsABΔC (pDMSC59X), or FrdABΔCD. Figure 4B shows equivalent lanes as in Figure 4A, with the same plasmids in D-43. P; purified or enriched sample protein of either DmsABC or FrdAB, M; washed membranes. S; soluble fraction. O; osmotic shock fraction, 20; 2 fold osmotic shock fraction. Purified FrdAB was obtained from HB101/pFRD84 expressing high levels of the wild-type enzyme and purified by the method of [Dickie and Weiner (1979) Can. J. Biochem. 57:813-821 ; Lemire and Weiner (1986) Meth. Enzymol. 126:377-386]. All lanes had the equivalent concentration of protein loaded. As shown in Figure 4A, (compare lanes 8 and 9 to lanes 4 and 5) significant amounts of DmsA and DmsB accumulated in the periplasm only when the DmsC subunit was absent. As a control for this experiment, plasmids carrying the intact frdABCD (pFRD84) (not shown) and truncated frdAB (pFRD 117) [Lemire et al. ( 1982) J. Bacteriol. 152: 1 126-1 131 ] lacking the anchor subunits of fumarate reductase were also expressed. As fumarate reductase does not have a twin-arginine signal amino acid sequence and assembles spontaneously in the membrane [Latour and Weiner (1987) J. Gen. Microbiol. 133:597-607] neither a Mtt - 37 - mutation, nor loss of the anchor subunits, FrdC and FrdD, should result in secretion of FrdAB into the periplasm. This was confirmed (lanes 13 and 14). In Figure 4B the same experiment is shown for strain D-43. As expected neither DmsA nor DmsB accumulated in the periplasm. These results demonstrate that MttA is not involved in the translocation of DmsAB to the periplasm but in targeting them to the membrane. These results also suggest that the role of DmsC is to prevent translocation of DmsAB to the periplasm. EXAMPLE 6 Plasmid Complementation Of D-43 And Sequencing Of The mttA Region Complementation of the D-43 mutant with plasmid pDMS 160 (which carries the wild- type DmsABC operon) was carried out to determine whether the mutation was located within or outside the DmsABC structural gene. A. Plasmid complementation of mutant D-43 For initial complementation experiments, an E. coli DNA library was prepared by Hindlll digestion of an E. coli HBlOl chromosomal DNA preparation and ligated into the Hindll site of pBR322. The ligation mixture was transformed directly into D-43. The transformants were grown anaerobically on glycerol/DMSO (Gly/DMSO) plates and incubated anaerobically at 37°C for 72 hr. The complementing clone identified form this library, pDSR31 1 , was isolated and restriction mapped. The map was compared with the integrated E. coli restriction map version 6 [Berlyn et al. (1996) Edition 9 in Escherichia coli and Salmonella 2:1715-1902, ASM Press, Washington DC]. A second gene bank was prepared using random 5-7 kb Sau3a fragments of E. coli W1485 ligated into the BamHI site of pBR322. This E. coli gene bank was a gift from Dr. P. Miller, Parke-Davis Pharmaceuticals, Ann Arbor, MI. D-43 was transformed with 2 μg of this library and transformants were plated onto Luria-Bertani (LB) broth plates containing 100 μg/mf' ampicillin. After overnight growth at 37°C the cells were washed off the plates into 5 ml of LB broth and 20 μl of this suspension was diluted with 10 ml of Minimal A medium [Miller ( 1992) in A Short Course in Bacterial Genetics, Cold Spring Harbor Laboratory Press] containing 100 μg/ml " ' ampicillin and 10 μg/ml "1 vitamin Bl, proline and leucine and grown aerobically at 37°C for 16 hr. The cells were washed twice in phosphate buffered - 38 - saline (PBS) and samples were serially diluted into PBS buffer. Each dilution (100 μl) was plated on Gly/DMSO plates and incubated anaerobically at 37°C for 72 hr. Colonies were further tested for anaerobic growth in 9 ml screw-top test tubes containing Gly/DMSO broth medium. The location of the complementing clones in the E. coli chromosome obtained from both libraries was confirmed by DNA sequencing the ends of the clones using primers which flanked the Hindlll and BamHI sites of pBR322. Subclones of the complementing clones from each of the libraries were constructed utilizing standard cloning methods [Sambrook et al. (1989)] and ligated into the cloning vector pTZ18R. DNA from subclones was restriction mapped to verify the insert. Positive subclones were tested for anaerobic growth in Gly/DMSO and Gly/Fumarate broth medium. A single clone. pDSR31 1, which allowed growth on Gly/DMSO was identified. Through restriction map analysis and sequencing the ends of the insert, the clone was mapped to the 88 min region of the chromosome, within contig AE00459 covering the 4,013,851 - 4.022,41 1 bp region of the sequence of Blattner et al. [Blattner et al. ( 1997) Science 277: 1453-1462]. The clone contained the previously undefined open reading frames yigO, P, R, T, and U (based on the original yig nomenclature for unidentified ORFs) (Figure 5). All attempts to use available restriction sites to subclone this region into ORF groups yigOP. yigR, yigRTU, and yigTU were unsuccessful. Therefore, a second library consisting of E. coli chromosomal DNA which had been partially-digested with Sau3a was ligated into BamHI- digested pBR322. This library generated a number of complementing clones. The smallest was pGS20 which encoded the 3' end of yigR and approximately three quarters of yigT as shown in Figure 5. This suggested that the products of the putative genes yigTUW were responsible for DmsA targeting to the membrane and Nap translocation to the periplasm and these genes were renamed mttABC (membrane targeting and translocation). This region was cloned from wild-type HBlOl utilizing PCR as follows. For PCR cloning of the mttABC region, the chromosomal DNA template for PCR was prepared from HBlOl . Bacteria from 1.5 ml of an overnight culture were pelleted in an Eppendorf tube and resuspended in 100 μl of water. The cells were frozen and thawed three times, pelleted by centrifugation and 5 μl of the supernatant was used as the PCR template. The region of the putative mttABC operon was cloned utilizing PCR. The 5" primer was located at the end of the coding sequence for yigR(b3S35) (position 5559-5573 of contig AE00459) and included the intervening sequence between yigR and mttA. The 3' primer - 39 - hybridized immediately after the stop codon of mttC (position 8090-81 10). The primers contained the restriction sites EcoRI and Sail to facilitate cloning into the phagemid pTZ18R and recombinants were screened in e. coli strain TGI. The ends of the clones were sequenced to verify the region cloned. Clones of the ORF region mttABC were subcloned utilizing standard cloning methods [Sambrook et al. (1989)] and ligated into the vector pBR322. Positive clones and subclones were transformed into D-43 and tested for anaerobic growth in Gly/DMSO and Gly/Fumarate broth medium. The clone of mttABC was able to complement the D-43 mutation only when cloned into the lower copy number plasmid pBR322 (pBRmttABC) and no complementation (or growth) was observed when mttABC was cloned into the high copy number plasmid pTZ18R (pTZmttABC). The D-43 mutant could not be complemented with plasmid pDMS160 carrying the wild-type DmsABC operon suggesting that the mutation mapped outside the structural genes. Interestingly, the mutant expressed nearly normal levels of DMSO reductase activity but the activity was soluble rather than membrane-bound. This was surprising given that the membrane anchor, DmsC, was expressed in these cells (see below) and this suggested that the mutant was defective in membrane targeting or assembly. B. Sequencing the mttA region We compared the sequence of clone pGS20 with the identical region of strain D-43 b\ PCR sequencing of both strands as follows. Chromosomal DNA from strains HBl Ol and D- 43 was prepared as above. The 976 bp region which complements the D-43 mutation was amplified, the PCR products were sequenced directly and the DNA sequences of both strains were compared to the published sequence of E. coli [Blattner et al. (1997)]. As Taq DNA polymerase was used for PCR, two different reaction products, resulting from separately prepared templates, were sequenced to identify any mutations which may have resulted from the PCR reaction. Both strands were sequenced in the region of any identified mutations. We identified only one nucleotide change altering a C to a T at position 743 of pGS20. When this region was compared to the sequence of contig AE00459 in the E. coli genome sequence [Blattner et al. (1997) Science 277:1453-1462], it appeared that the mutation mapped within the proposed ORF termed b3837. This ORF did not have a normal E. coli codon usage and so we determined the DNA sequence of this region of AE00459. - 40 - Several differences were identified and a revised ORF map of this contig is shown in Figure 5. This revision resulted in several changes: ORF b3836, b3837 and b3838 are no longer observed and are replaced by a polypeptide which is very similar throughout its length to the YigT protein of H. influenzae [Fleischmann et al. (1995) Science 269:496-512] (Figure 6). Figure 6 shows the sequence (SEQ ID NO:l) of E. coli wild-type MttA aligned with YigT of Haemophilus influenzae (Fleischmann et al., 1995) (SEQ ID NO:2). The two potential transmembrane segments are denoted as TMSl and TMS2, respectively, a) denotes the position of the mutation in MttA which changes proline 25 to leucine. b) denotes the termination of MttA in clone pGS20. The potential α-helical region is indicated. The mutation in D-43 resulted in the mutation of proline 25 of MttA2 to leucine. Interestingly, clone pGS20 did not encode the entire MttA polypeptide but terminated at amino acid 205. The MttA protein is composed of 277 amino acids and has a mass of -30.6 kDa. Without limiting the invention to any particular mechanism, the MttA protein has two potential transmembrane helices between residues 15-34 and 107-126. The most likely orientation is with the amino and carboxyl termini exposed to the periplasm. Residues 150 to 200 are predicted to form a very long α-helix. The mutation in D-43 altered the proline immediately after the second transmembrane helix and could disrupt this structure of the protein. C. Proteins homologous to the MttA protein A database search of sequences which are related to mttA (i.e., mttAl and mttA 2) identified a large family of related proteins whose function was previously unknown. In addition to the Zea mays protein of Settles et al. (1997) Science 278:1467-1470. related sequences were identified by BLAST searches in Azotobacter chroococcum, Bacillus subtilis, Heamophilus influenzae, Helicobacter pylori, Mycobacterium leprae, Mycobacterium tuberculosis, Pseudomonas stutzerii, Rhodococcus erythropolis, and Synechocystis PCC6803 as well as the Ybec sequence of E. coli (Figure 8). 41 EXAMPLE 7 E. coli mttB And mttC Form An Operon With mttA A. The mttABC operon Examination of the DNA sequence adjacent to mttA suggested that the upstream gene, yigR. encodes an aminoglycosyl transferase (BLAST search of the non-redundant data base). A potential transcription terminator at position 5590-5610 of contig AE00459 [Blattner et al. (1997) Science 277:1453-1462] separates yigR from mttA. To test whether the adjacent genes mttB and mttC form an operon with mttA, mRNA was isolated from aerobically grown HBlOl and RT-PCR was used with a primer within mttC to make a cDNA product. This cDNA was then amplified by PCR with primers within mttA and mttB giving the expected product of 270 bp., and mttA and mttC giving a product of 1091 bp. confirming a single polycistronic mRNA for the mttA, mttB. and mttC genes. To ensure that the PCR products were not the result of contaminating chromosomal DNA, the mRNA preparation was extensively digested with DNase prior to PCR and a control omitting the RT-PCR step did not give any products after PCR amplification. The nucleotide sequence (SEQ ID NO:45) of the mttABC operon is shown in Figure 1 1. Figure 7 also shows the nucleotide sequence of the three open reading frames, ORF RF[3], ORF RF[2] and ORF RF[1], and the encoded amino acid sequences of MttA (SEQ ID NOT). MttB (SEQ ID NO:7) and MttC (SEQ ID NO:8), respectively. B. Proteins homologous to the MttB and MttC proteins A database search of sequences which are related to mttB and mttC identified a large family of related proteins which are organized contiguously in several organisms. In all cases the function of these proteins was previously unknown. The nucleotide sequence of mttB (SEQ ID NO:)5 is shown in Figure 7. mttB encodes an integral membrane protein of 258 amino acids with six predicted transmembrane segments. A large number of related sequences was identified in a BLAST search extending from the archaebacteria (Archeoglobus fulgidus), through the eubacteria (Azotobacter chroococcum, Bacillus subtilis, Heamophilus influenzae, Helicobacter pylori, Mycobacterium laprae, Mycobacterium tuberculosis), cyanobacteria (Synechocystis PCC6803) to mitochondria of algae (Reclimonas americana, Chondrus crispus) and plants (Arabidopsis thalania, 42 - Marchantia polymorpha) as well as chloroplasts of Porphyra purpurea and Odentella sinensis (Figure 9). The nucleotide sequence of the neighboring gene mttC (SEQ ID NO:6) is shown in Figure 7. mttC encodes a polypeptide of 264 amino acids which is predicted to have at least one potential transmembrane segment (residues 24-41). The most likely orientation of this protein results in a large cytoplasmic domain extending from residue 41 to 264. Without limiting the invention to any particular mechanism, there is the possibility of a second transmembrane domain at residues 165-182. This possibility may be confirmed by a blaM gene fusion analysis. Like MttA and MttB, the MttC protein also is a member of a very large family of homologous proteins which includes two homologous sequences in E. coli (Ycfh and Yjjv) as well as homologous sequences in archaebacteria (Methanobacterium thermoautotrophicum), Mycoplasma (Mycoplasma pneumoniae and Mycoplasma gentitaluium), eubacteria (Bacillus subtillis, Heamophilus influenzae, Helicobacter pylori, Mycobacterium tuberculosis), cyanobacteria (Synechocytis PCC6803), yeast (Schizosaccharomyces pombe and Saccharomyces cerevisae), C. elegans and humans (Figure 10). The human protein is notable in having a 440 amino acid extension at the amino terminus which is not found in the other proteins. This extension is not related to MttA or MttB. EXAMPLE 8 Construction of host cells containing a deletion of at least a portion of the genes mttA, mttB and mttC The function of MttA, MttB and MttC proteins in a host cell is demonstrated by in vivo homologous recombination of chromosomal mttA, mttB and mttC as previously described [Sambasivarao et al (1991) J. Bacteriol. 5935-5943; Jasin et al (1984) J. Bacteriol. 159:783- 786]. Briefly, the mttABC operon is cloned into vectors, and the gene whose function is to be determined (i.e., mttA, mttB or mttC) is mutated, e.g., by insertion of a nucleotide sequence within the coding region of the gene. The plasmids are then homologously recombined with chromosomal mttA, mttB or mttC sequences in order to replace the chromosomal mttA, mttB or mttC genes with the mutated genes of the vectors. The effect of the mutations on the localization of proteins containing twin-arginine amino acid signal - 43 sequences is compared between the wild-type host cells and the cells containing the mutated mttA, mttB or mttC genes. These steps are further described as follows. A. Construction of plasmids carrying deletions or insertions in mttA, mttB and mttC genes The mttABC operon (Figure 11) is cloned into pTZ18R and pBR322 vectors. In pBR322, the Hindlll site in mttB is unique. The pBR322 containing mttB is then modified by insertion of a kanamycin gene cartridge at this unique site, while the unique Nrul fragment contained in mttC is replaced by a kanamycin cartridge. B. Homologous recombination and PI transduction The modified plasmids are homologously recombined with chromosomal mttA, mttB and mttC in E. coli cells which contain either a recBC mutation or a recD mutation. The resulting recombinant is transferred by PI transduction to suitable genetic backgrounds for investigation of the localization of protein expression. The localization (e.g., cytoplasm. periplasm, cell membranes, extracellular medium) of expression of twin arginine containing proteins is compared using methods disclosed herein (e.g., functional enzyme activity and Western blotting) between homologously recombined cells and control cells which had not been homologously recombined. Localization of expressed twin arginine containing proteins extracellularly, in the periplasm, or in the cytoplasm of homologously recombined cells as compared to localization of expression in cell membranes of control cells demonstrates that the wild-type MttA, MttB or MttC protein whose function had been modified by homologous recombination functions in targeting expression of the twin arginine containing protein to the cell membrane. Similarly, accumulation of expressed twin arginine containing proteins in extracellular medium, in the cytoplasm, or in cell membranes of homologously recombined cells as compared to periplasmic localization of the expressed twin arginine containing protein in control cells which had not been homologously recombined indicates that the protein (i.e., MttA, MttB or MttC) whose function had been modified by homologous recombination functions in translocation of the twin arginine containing protein to the periplasm. 44 EXAMPLE 9 Wild-type and mutant twin-arginine amino acid signal sequences of preDmsA are cleaved to release mature DmsA In this Example, the following numbering system for DmsA has been used: the mature protein starts at Val 46; the leader extends from Metl to Ala 45 and the double Arg signal is at residues 15-21. In order to determine whether preproteins which contain twin-arginine amino acid signal sequences are cleaved to release a mature polypeptide as suggested by Berks [Berks (1996)], the two alanine amino acids at the -1 and -3 positions of the twin- arginine amino acid signal sequences of wild-type DmsA preprotein were replaced with asparagine, and cleavage of both the wild-type and the mutated twin-arginine amino acid signal sequences was investigated. A. Cell culture conditions Cells were grown anaerobically in Luria Broth [Sambrook (1989)] and these cultures were used for a 1% inoculum into glycerol minimal medium with 0.167%) peptone and vitamin Bl, proline, leucine at final concentrations of 0.005%. All manipulations of plasmids and strains were carried out as described by Sambrook et al. (1989)]. The upstream untranslated region of DmsA was examined using software from the Center for Biological Analysis (http://www.cbs.dtu.dk/) to identify potential leader peptidase I cleavage sites. This analysis indicated that mutation of both Ala43 and Ala45 was needed to inhibit cleavage. An additional secondary cleavage site with low probability was identified between Thr36 and Leu37. The two Ala mutated in this study were Ala43 and Ala45 which are underlined in the following DmsA leader sequence (SEQ ID NO:43) that contains the twin-arginine amino acid signal sequence: 1 15 30 43 45 MKTKIPDAVLAAEVSRRGLVKTTIAFFLAMASSALTLPFSRIAHAVDSAI Mutants were generated by site-directed mutagenesis of single stranded DNA of plasmid pDMS223 [Rothery and Weiner (1991) Biochemistry 30:8296-8305] using the Sculptor kit (Amersham) and mutagenic primers to generate the mutants A43N and A43N,A45N. The mutagenic primer (SEQ ID NO:44) 5'-TTAGTCGGATTAATCACAATGTCGATAGCG-3 " 45 - was used. Mutant DNA was subcloned into pDMSlόO [Rothery and Weiner (1991)] using Bglll and EcoRI restriction sites, and resequenced to confirm the mutation. B. Expression studies Samples were removed from the cultures after 30-48 hours of anaerobic growth, the cells pelleted by centrifugation at 9500g for 10 min., resuspended and everted envelopes prepared by French Press lysis. The cytoplasm and membrane fractions were separated by differential centrifugation. Membranes were washed twice with 50mM MOPS pH7.0 prior to use. Membrane proteins were solubilized with 1% SDS and polyacrylamide gel electrophoresis was performed using the Bio-Rad minigel system with a discontinuous SDS buffer system [Laemmli (1970) Nature 227:680-685]. Western blotting was performed using affinity purified DmsA antibody with the ECL Western blotting detection reagents from Amersham Life Sciences. The results (data not shown) demonstrated cleavage of both the preDmsA proteins which contained alanine and which contained asparagine in the twin-arginine amino acid signal sequence to release mature DmsA. These results suggest that twin-arginine amino acid signal sequences are cleaved by signal peptidase I which also cleaves Sec signal sequences. Alternatively, a signal peptidase which is different from signal peptidase I and signal peptidase II, and which has different specificity may be operative. This possibility is investigated by N-terminal amino acid sequencing. C. N-terminal amino acid sequencing N-terminal amino acid sequencing is carried out as previously described [Bilous et al (1988) Molec. Microbiol. 2:785-795] in order to determine the cleavage site in preDmsA and other preproteins which contain twin-arginine amino acid signal sequences, e.g., preTorA, and preNapA. A signal peptidase I temperature sensitive mutant is used to determine if preDmsA. preTorA and preNapA are cleaved at the restrictive temperature. Amino terminal sequences are determined by automated Edman degradation on an Applied Biosystems Model 470A gas phase sequenator. Subunits are separated by SDS PAGE and electroblotted onto polyvinylidene fluoride membranes and electroeluted as described by Cole et al. [J. Bacteriol. 170:2448-2456 (1988)]. - 46 The above-presented data shows that mttAl, MttA2, mttB and mttC encode proteins MttAl , MttA2, MttB and MttC which are essential in a Sec-independent pathway, and which function in targeting twin arginine containing proteins to cell membranes and in translocating twin arginine containing proteins to the periplasm and extracellular medium. The above- disclosed data further demonstrates that disruption of the function of any one or more of MttAl , MttA2, MttB and MttC results in translocation of twin arginine containing proteins to the periplasm, to extracellular medium, or to cellular compartments other than those compartments in which the twin arginine containing proteins are translocated in cells containing wild-type MttAl, MttA2, MttB and MttC. These results demonstrate that mttAl, MttA2, MttB and mttC are useful in translocating twin arginine containing proteins to the periplasm and extracellular medium. Such translocation is particularly useful in generating soluble proteins in a functional form, thus facilitating purification of such proteins and increasing their recovery. All publications and patents mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described method and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art and related fields are intended to be within the scope of the following claims. 47 -

Claims: CLAIMS 1. A recombinant polypeptide comprising at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NO:47, of SEQ ID NO:49. of SEQ ID NO: 7 and variants and homologs thereof, and of SEQ ID NO: 8 and variants and homologs thereof. 2. An isolated nucleic acid sequence encoding at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NO:47. of SEQ ID NO:49. of SEQ ID NO:7 and variants and homologs thereof, and of SEQ ID NO:8 and variants and homologs thereof. 3. The nucleic acid sequence of Claim 2, wherein said nucleic acid sequence is contained on a recombinant expression vector. 4. The nucleic acid sequence of Claim 3. wherein said expression vector is contained within a host cell. 5. A nucleic acid sequence that hybridizes under stringent conditions to a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of SEQ ID NO:7 and variants and homologs thereof, and SEQ ID NO:8 and variants and homologs thereof. 6. A method for expressing a nucleotide sequence of interest in a host cell to produce a soluble polypeptide sequence, said nucleotide sequence of interest when expressed in the absence of an operably linked nucleic acid sequence encoding a twin-arginine signal amino acid sequence produces an insoluble polypeptide. comprising: a) providing: i) said nucleotide sequence of interest encoding said insoluble polypeptide; ii) said nucleic acid sequence encoding said twin-arginine signal amino acid sequence; and 48 iii) said host cell, wherein said host cell comprises at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NO:47, of SEQ ID NO:49, of SEQ ID NO:7 and variants and homologs thereof, and of SEQ ID NO:8 and variants and homologs thereof; b) operably linking said nucleotide sequence of interest to said nucleic acid sequence to produce a linked polynucleotide sequence; and c) introducing said linked polynucleotide sequence into said host cell under conditions such that said fused polynucleotide sequence is expressed and said soluble polypeptide is produced. 7. The method of Claim 6, wherein said insoluble polypeptide is comprised in an inclusion body. 8. The method of Claim 6, wherein said insoluble polypeptide comprises a cofactor. 9. The method of Claim 8, wherein said cofactor is selected from the group consisting of iron-sulfur clusters, molybdopterin, polynuclear copper, tryptophan tryptophylquinone. and flavin adenine dinucleotide. 10. The method of Claim 6, wherein said soluble polypeptide is comprised in periplasm of said host cell. 1 1. The method of Claim 6, wherein said host cell is cultured in medium, and wherein said soluble polypeptide is contained in said medium. 12. The method of Claim 6, wherein said cell is Escherichia coli. 13. The method of Claim 12, wherein said Escherichia coli cell is D-43. 14. The method of Claim 6, wherein said twin-arginine signal amino acid sequence is selected from the group consisting of SEQ ID NO:41 and SEQ ID NO:42. 49 - 15. A method for expressing a nucleotide sequence of interest encoding an amino acid sequence of interest in a host cell, comprising: a) providing: i) said host cell; 5 ii) said nucleotide sequence of interest; iii) a first nucleic acid sequence encoding twin-arginine signal amino acid sequence; and iv) a second nucleic acid sequence encoding at least a portion of an amino acid sequence selected from the group consisting of SEQ ID NO:47, of 10 SEQ ID NO:49, of SEQ ID NO:7 and variants and homologs thereof, and of SEQ ID NO:8 and variants and homologs thereof; b) operably fusing said nucleotide sequence of interest to said first nucleic acid sequence to produce a fused polynucleotide sequence; and c) introducing said fused polynucleotide sequence and said second nucleic 15 acid sequence into said host cell under conditions such that said at least portion of said amino acid sequence selected from the group consisting of SEQ ID NO:47, of SEQ ID NO:49, of SEQ ID NO:7 and variants and homologs thereof, and of SEQ ID NO:8 and variants and homologs thereof is expressed, and said fused polynucleotide sequence is expressed to produce a fused polypeptide sequence comprising said twin- 20 arginine signal amino acid sequence and said amino acid sequence of interest. 16. The method of Claim 15. wherein said expressed amino acid sequence of interest is contained in periplasm of said host cell. 25 17. The method of Claim 16, wherein said expressed amino acid sequence of interest is soluble. 18. The method of Claim 15, wherein said host cell is cultured in medium, and wherein said expressed amino acid sequence of interest is contained in said medium. J 0 19. The method of Claim 18, wherein said expressed amino acid sequence of interest is soluble. - 50

Inventors: TURNER RAYMOND JOSEPH, WEINER JOEL HIRSCH, TURNER, RAYMOND, JOSEPH, WEINER, JOEL, HIRSCH
Applicants: TURNER RAYMOND JOSEPH, UNIV ALBERTA, WEINER JOEL HIRSCH, THE GOVERNORS OF THE UNIVERSITY OF ALBERTA, TURNER, RAYMOND, JOSEPH, WEINER, JOEL, HIRSCH
Application year: 1999
Main classification: C12N15
IPC codes: C07K14, C12N15