ESSENTIAL BACTERIAL GENES AND THEIR USE

Abstract: Disclosed are two genes, termed "yphC" and "yqjK", found in Streptococcus pneumoniae, which are essential for survival for a wide range of bacteria. These genes and the essential polypeptides that they encode, as well as homologs and orthologs thereof, can be used to identify antibacterial agents for treating a broad spectrum of bacterial infections.

Description: ESSENTIAL BACTERIAL GENES AND THEIR USE Cross-Reference to Related Application This application claims priority under 35 U.S.C. §119 (e) from USSN 60/099,578, filed September 9, 1998, which is incorporated herein by reference in its entirety. Field of the Invention The invention relates to essential bacterial genes and their use in identifying antibacterial agents. Background of the Invention Bacterial infections may be cutaneous, subcutaneous, or systemic. Opportunistic bacterial infections proliferate, especially in patients afflicted with AIDS or other diseases that compromise the immune system. Most bacteria that are pathogenic to humans are gram positive bacteria. The bacterium Streptococcus pneumoniae, for example, typically infects the respiratory tract and can cause lobar pneumonia, as well as meningitis, sinusitis, and other infections. Summary of the Invention The invention is based on the discovery of two genes in the gram positive bacterium Streptococcus pneumoniae that are essential for the survival of this and other bacteria. For convenience, these genes, yphC and yqjK, are collectively referred to herein as "essential" genes and the polypep ides that these genes encode are referred to as "essential" polypeptides since Streptococcus pneumoniae cells lacking functional yphC or yqjK genes are unable to survive. 0/1420 - 2 - The yphC and yqjK genes are useful molecular tools for identifying similar genes in pathogenic microorganisms. The essential polypeptides that these genes encode are useful targets for identifying compounds that are inhibitors of the pathogens in which the essential polypeptides are expressed. Such compounds diminish bacterial growth by inhibiting the activity of an essential protein, or by inhibiting transcription of an essential gene or translation of the mRNA transcribed from the essential gene. The invention, therefore, features an isolated yphC polypeptide having the amino acid sequence set forth in SEQ ID NO: 2, as depicted in Fig. 1, or conservative variations thereof. An isolated nucleic acid encoding yphC also is included within the invention. In addition, the invention includes (a) an isolated nucleic acid having the sequence of SEQ ID NO:l, as depicted in Fig. 1, or degenerate variants thereof; (b) an isolated nucleic acid having the sequence of SEQ ID NO : 1 , or degenerate variants thereof, wherein T is replaced by U; (c) nucleic acids complementary to (a) and (b) ; and (d) fragments of (a) , (b) , and (c) that are at least 15 base pairs in length and that hybridize under stringent conditions, as described below, to genomic DNA encoding the polypeptide of SEQ ID NO:2. The yphC polypeptide depicted in Fig. 1 is a partial sequence of the full- length polypeptide, which is depicted in Figs. 2A-2B. The invention also features an isolated yphC polypeptide having the amino acid sequence set forth in SEQ ID NO: 5, as depicted in Figs. 2A-2B, or conservative variations thereof. An isolated nucleic acid encoding full-length yphC also is included within the invention. In addition, the invention includes (a) an isolated nucleic acid having the sequence of SEQ ID NO: 4, as depicted in Figs. 2A-2B, or degenerate variants thereof; (b) an isolated nucleic acid having the sequence of SEQ ID NO : 4 , or degenerate variants thereof, wherein T is replace by U; and (c) nucleic acids complementary to (a) and (b) . As described above for yphC, the invention includes an isolated nucleic acid encoding yqjK. In addition, the invention includes (a) an isolated nucleic acid having the sequence of SEQ ID NO: 7, as depicted in Fig. 3, or degenerate variants thereof; (b) an isolated nucleic acid having the sequence of SEQ ID NO: 7, or degenerate variants thereof, wherein T is replaced by U; (c) nucleic acids complementary to (a) and (b) ; and (d) fragments of (a) , (b) , and (c) that are at least 15 base pairs in length and that hybridize under stringent conditions, as described below, to genomic DNA encoding the polypeptide of SEQ ID NO: 8. These sequences are summarized in Table 1. Table 1: Essential Nucleic Acids and Polypeptides Identification of these essential genes allows homologs of the essential genes to be found in other strains within the species, and it allows orthologs of the essential genes to be found in other organisms (e.g., Bacillus sp . , H. influenzae, H. pylori , and E. coli ) . While "homologs" are structurally similar genes contained within the Streptococcus species, "orthologs" are functionally equivalent genes from other species, as determined, for example, in a standard complementation assay. Thus, the essential polypeptides can be used not only as a model for identifying similar genes in other Streptococcus strains, but also to identify homologs and orthologs of essential genes in other species (e.g., other gram positive bacteria, particularly those bacteria that are pathogenic to humans, and other bacteria generally) . Such orthologs can be identified, for example, in a conventional complementation assay. In addition, or alternatively, such orthologs can be expected to exist in bacteria in the same branch of the phylogenetic tree, as set forth, for example, at ftp : //ft . cme . msu . edu/ pub/RDP/SSU_rRNA/SSU/Prok .phylo . For example, B . subtilis is in the B. subtilis subgroup of the B. subtilis group in the Bacillus-Lactobaccillus- Streptococcus Subdivision of the Gram positive phylum. Likewise, S . pneumoniae belong to the Stc. pneumonia subgroup of Streptococci, which also are in the Bacillus- Lactobacillus-Streptococcus subdivision of the Gram positive phylum. E. coli belong to the Escherichia Salmonella group of the Enterics and relatives within the Gamma subdivision of the Purple bacteria. Other bacteria within the same phylum (particularly, bacteria within the same subdivision, group, or subgroup) can be expected to contain an ortholog of the yphC and/or yqjK genes described herein. Examples of orthologs of the Streptococcus yphC and yqjK genes are summarized in Table 2. As shown in Table 2, the Streptococcus gene yphC has an ortholog in B . subtilis, termed "B-yphC, " and an ortholog in E. coli , termed "yfgK, " which is also known as "f503." The Streptococcus gene yqjK also has an ortholog in B . subtilis, termed "B-yqjK, " and an ortholog in E. coli , termed "elaC, " which is also known as "o311." As discussed below, orthologs of essential genes may themselves be essential or non-essential in the organism in which they are found. As determined by the experiments described below, the B-yphC, yfgK, and B-yqjK orthologs are essential for survival of the bacteria in which they are found. Thus, these essential orthologous genes and the polypeptides encoded by these orthologs can be used to identify compounds that inhibit the growth of the host organism (e.g., compounds that inhibit the activity of an essential protein, or inhibit transcription of an essential gene) . Table 2: Orthologs of yphC and yqjK The yphC polypeptides and genes described herein include the polypeptides and genes set forth in Figs. 1 and 2A-2B herein, as well as isozymes, variants, and conservative variations of the sequences set forth in Figs. 1 and 2A-2B. The invention includes various isozymes of yphC and yqjK. For example, the invention includes a gene that encodes an essential polypeptide but which gene includes one or more point mutations, deletions, or promoter variants, provided that the resulting essential polypeptide retains a biological function of an essential polypeptide. The yphC polypeptide has structural characteristics of known GTPases . Using BLAST analysis, the yphC polypeptide has been shown to contain two domains that are predicted to be GTPase domains, and yphC displays GTPase activity in vi tro . This GTPase activity is linked to the essentiality of the yphC polypeptide. When point mutations are made in each GTPase domain of yphC such that the mutants are unable to bind GTP, such mutants no longer are able to complement a bacterial strain that lacks yphC. The yqjK polypeptide has structural characteristics of known sulfatases. Thus, the various isozymes, variants, and conservative variations of the yphC and yqjK sequences set forth in Figs. 1 and 2A-2B retain a biological function of yphC or yqjK as determined, for example, in an assay of GTPase or sulfatase activity, or in a conventional complementation assay. Suitable GTPase and sulfatase activity assays are well known in the art (see, e.g., Bollag, et al . , Meth. Enzymol . 255:161 (1995) and Barbeyron, et al . , Microbiol . 141:2897 (1995), incorporated herein by reference). The GTPase activity of yphC can also be assayed using a conventional Malachite Green phosphorelease assay (see, e.g., Lanzetta et al . , 1979, Analytical Biochemistry 100:95-97) . The inclusion of KC1 in such an assay leads to an approximately 70-fold stimulation of GTPase activity, and thus provides a sensitive assay for detection of GTP activity. Also encompassed by the term yphC gene are degenerate variants of the nucleic acid sequences set forth in Figs. 1 and 2A-2B (SEQ ID NO:l and 4) . Degenerate variants of a nucleic acid sequence exist because of the degeneracy of the amino acid code; thus, those sequences that vary from the sequence represented by SEQ ID NO:l and 4, but which nonetheless encode a yphC polypeptide are included within the invention. Likewise, because of the similarity in the structures of amino acids, conservative variations (as described herein) can be made in the amino acid sequence of the yphC polypeptide while retaining the function of the polypeptide (e.g., as determined in a conventional complementation assay) . Other yphC polypeptides and genes identified in additional bacterial strains may be such conservative variations or degenerate variants of the particular yphC polypeptide and nucleic acid set forth in Figs. 1 and 2A-2B (SEQ ID NOs:l-6) . The yphC polypeptide and gene share at least 80%, e.g., 90%, sequence identity with SEQ ID NOs : 2 and 1, respectively, or SEQ ID NOs : 5 and 4, respectively. Regardless of the percent sequence identity between the yphC sequence and the sequences represented by SEQ ID NOs : 1 , 2 , 4 , and 5 , the yphC genes and polypeptides encompassed by the invention preferably are able to complement for the lack of yphC function (e.g., in a temperature-sensitive mutant) in a standard complementation assay. Additional yphC genes that are identified and cloned from additional bacterial strains, and pathogenic, gram-positive strains in particular, can be used to produce yphC polypeptides for use in the various methods described herein, e.g., for identifying antibacterial agents. Likewise, the term yqjK encompasses isozymes, variants, and conservative variations of the sequences depicted in Fig. 3. In various embodiments, the essential polypeptide used in the assays described herein is derived from a non-pathogenic or pathogenic gram positive bacterium. For example, the polypeptide can be derived from a Streptococcus strain, such as Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus endocardi tis, Streptococcus faecium, Streptococcus sangus , Streptococcus viridans, and Streptococcus hemolyticus . Orthologs of the yphC and yqjK genes can be derived from a wide spectrum of bacteria, such as E. coli and Bacillus subtilis . Having identified the yphC and yqjK genes described herein as being essential for survival, these essential genes and the polypeptides encoded by these essential genes and their essential homologs and orthologs can be used to identify antibacterial agents. Such antibacterial agents can readily be identified with high throughput assays to detect inhibition of the metabolic pathway in which the essential polypeptide participates. This inhibition can be caused by small molecules interacting with (e.g., binding directly or indirectly to) the essential polypeptide or other essential polypeptides in that pathway. An exemplary method for identifying antibacterial compounds involves screening for small molecules that specifically interact with (i.e., bind directly or indirectly to) the essential polypeptide. A variety of suitable interaction and binding assays are known in the art as described, for example, in U.S. Patent Nos. 5,585,277 and 5,679,582, incorporated herein by reference. For example, in various conventional assays, test compounds can be assayed for their ability to interact with an essential polypeptide by measuring the ability of the small molecule to stabilize the essential polypeptide in its folded, rather than unfolded, state. More specifically, the degree of protection from unfolding that is afforded by the test compound can be measured. Test compounds that bind the essential polypeptide with high affinity cause, for example, a large shift in the temperature at which the polypeptide is denatured. Test compounds that stabilize the essential polypeptide in a folded state can be further tested for antibacterial activity in a standard susceptibility assay. Another exemplary method for identifying antibacterial agents involves measuring the ability of a test compound to bind to one of the essential polypeptides described herein. Binding can be assayed in a conventional capillary electrophoresis assay in which binding of the test compound to the essential polypeptide changes the electrophoretic mobility of the essential polypeptide. Another suitable method for identifying inhibitors of the essential polypeptides involves identifying a biochemical activity of the essential polypeptide and then screening for small molecule inhibitors of the activity using, for example, a high throughput screening method. The yphC polypeptide has structural characteristics of known GTPases and displays GTPase activity in vi tro . Therefore, inhibitors of this polypeptide therefore can be identified by their ability to inhibit the GTPase activity of yphC in a conventional assay of GTPase activity. Suitable assays have been described (e.g., Gollag et al . , Meth. Enzymol . 255: 161- 170, 1995, which is incorporated herein by reference) . A detailed example of a suitable assay is set forth below. The yqjK polypeptide has structural characteristics of sulfatases and is expected to function as a sulfatase. Accordingly, inhibitors of the yqjK polypeptide can be identified by assaying for the ability of the test compound to inhibit the sulfatase activity of yqjK. An example of a suitable assay is described by Barbeyron et al . , Microbiol . 141:2897-2904, 1995, which is incorporated herein by reference. The invention also includes a method for identifying an antibacterial agent which method entails: (a) contacting an essential polypeptide, or homolog or orthologs thereof, with a test compound; (b) detecting binding of the test compound to the polypeptide or homolog or ortholog; and, optionally, (c) determining whether a test compound that binds to the polypeptide or homolog or ortholog inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of the test compound that binds to the polypeptide or homolog or ortholog, as an indication that the test compound is an antibacterial agent. In another suitable assay, a promoter that responds to depletion of the essential polypeptide by upregulation or downregulation is linked to a reporter gene. To identify a promoter that is up- or down- regulated by the depletion of an essential protein, the gene encoding the essential protein is deleted from the genome and replaced with a version of the gene in which the sequence encoding the essential protein is operably linked to a regulatable promoter. The cells containing this regulatable genetic construct are kept alive by the essential polypeptide produced from the genetic construct containing the regulatable promoter. However, the regulatable promoter allows the expression of the essential polypeptide to be reduced to a level that causes growth inhibition. Total RNA prepared from bacteria under such growth-limiting conditions is compared with RNA from wild-type cells. Standard methods of transcriptional profiling can be used to identify mRNA species that are either more or less abundant (i.e., up- or down-regulated) when expressed under the limiting conditions. Genomic sequence information, e.g., from GenBank, can be used to identify the promoter that drives expression of the identified RNA species. Such promoters are up- or down-regulated by depletion of the essential polypeptide. Having identified a promoter (s) that is up- or down-regulated by depletion of the essential polypeptide, the promoter (s) is operably linked to a reporter gene (e.g., jβ-galactosidase, gus, or green fluorescent protein (GFP) ) . A bacterial strain containing this reporter gene construct is then exposed to test compounds. Compounds that inhibit the essential polypeptide (or other polypeptides in the essential pathway in which the essential polypeptide participates) cause a functional depletion of the essential polypeptide and therefore lead to an upregulation or downregulation of expression the reporter gene. Compounds that inhibit the essential polypeptides in such an assay are expected to be antibacterial and can be further tested, if desired, in standard susceptibility assays. In a related method for identifying antibacterial compounds, the essential polypeptides are used to isolate peptide or nucleic acid ligands that specifically bind the essential polypeptides. These peptide or nucleic acid ligands are then used in a displacement screen to identify small molecules that interact with the essential polypeptide. Such assays can be carried out essentially as described above. In still another method, interaction of a test compound with an essential polypeptide (i.e., direct or indirect binding) can be detected in a conventional two- hybrid system for detecting protein/protein interactions (e.g., in yeast or mammalian cells) . A test compound found to interact with the essential polypeptide can be further tested for antibacterial activity in a conventional susceptibility assay. Generally, in such two-hybrid methods, (a) the essential polypeptide is provided as a fusion protein that includes the polypeptide fused to (i) a transcription activation domain of a transcription factor or (ii) a DNA-binding domain of a transcription factor; (b) the test polypeptide is provided as a fusion protein that includes the test polypeptide fused to (i) a transcription activation domain of a transcription factor or (ii) a DNA-binding domain of a transcription factor; and (c) binding of the test polypeptide to the polypeptide is detected as a reconstitution of a transcription factor. Homologs and orthologs of the essential polypeptides can be used in similar methods. Reconstitution of the transcription factor can be detected, for example, by detecting transcription of a gene that is operably linked to a DNA sequence bound by the DNA-binding domain of the reconstituted transcription factor (See, for example, White, 1996, Proc . Natl. Acad. Sci . 93:10001-10003 and references cited therein and Vidal et al . , 1996, Proc. Natl. Acad. Sci. 93:10315-10320). In an alternative method, an isolated nucleic acid molecule encoding an essential polypeptide is used to identify a compound that decreases the expression of an essential polypeptide in vivo . Such compounds can be used as antibacterial agents. To identify such compounds, cells that express an essential polypeptide are cultured, exposed to a test compound (or a mixture of test compounds) , and the level of expression or activity is compared with the level of essential polypeptide expression or activity in cells that are otherwise identical but that have not been exposed to the test compound (s) . Many standard quantitative assays of gene expression can be utilized in this aspect of the invention. To identify compounds that modulate expression of an essential polypeptide (or homologous or orthologous sequence), the test compound (s) can be added at varying concentrations to the culture medium of cells that express an essential polypeptide (or homolog or ortholog) , as described herein. Such test compounds can include small molecules (typically, non-protein, non- polysaccharide chemical entities), polypeptides, and nucleic acids. The expression of the essential polypeptide is then measured, for example, by Northern blot PCR analysis or RNAse protection analyses using a nucleic acid molecule of the invention as a probe. The level of expression in the presence of the test molecule, compared with the level of expression in its absence, will indicate whether or not the test molecule alters the expression of the essential polypeptide. Because the yphC and yqjK polypeptides are essential for survival, test compounds that inhibit the expression and/or function of the essential polypeptide, or of an essential homolog or ortholog thereof, will inhibit growth of, or kill, the cells that express such polypeptides. The polypeptides encoded by essential genes also can be used, separately or together, in assays to identify test compounds that interact with these polypeptides. Test compounds that interact with these polypeptides then can readily be tested, in conventional assays, for their ability to inhibit bacterial growth. Test compounds that interact with the essential polypeptides are candidate antibacterial agents, in contrast to compounds that do not interact with the essential polypeptides. As described herein, any of a variety of art-known methods can be used to assay for the interaction of test compounds with the essential polypeptides . Typically, the test compound will be a small organic molecule. Alternatively, the test compound can be a test polypeptide (e.g., a polypeptide having a random or predetermined amino acid sequence; or a naturally-occurring or synthetic polypeptide) or a nucleic acid, such as a DNA or RNA molecule. The test compound can be a naturally-occurring compound or it can be synthetically produced, if desired. Synthetic libraries, chemical libraries, and the like can be screened to identify compounds that bind the essential polypeptide. More generally, binding of test a compound to the polypeptide, homolog, or ortholog can be detected either in vi tro or in vivo . If desired, the above- described methods for identifying compounds that modulate the expression of the polypeptides of the invention can be combined with measuring the levels of the essential polypeptides expressed in the cells, e.g., by performing a Western blot analysis using antibodies that bind an essential polypeptide. Regardless of the source of the test compound, the essential polypeptides described herein can be used to identify compounds that inhibit the activity of an essential protein or transcription of an essential gene or translation of the mRNA transcribed from the essential gene. These antibacterial agents can be used to inhibit a wide spectrum of pathogenic or non-pathogenic bacterial strains . In other embodiments, the invention includes pharmaceutical formulations that include a pharmaceutically acceptable excipient and an antibacterial agent identified using the methods described herein. In particular, the invention includes pharmaceutical formulations that contain antibacterial agents that inhibit the growth of, or kill, pathogenic bacterial strains (e.g., pathogenic gram positive bacterial strains such as pathogenic Streptococcus strains) . Such pharmaceutical formulations can be used in a method of treating a bacterial infection in an organism (e.g., a Streptococcus infection). Such a method entails administering to the organism a therapeutically effective amount of the pharmaceutical formulation, i.e., an amount sufficient to ameliorate signs and/or symptoms of the bacterial infection. In particular, such pharmaceutical formulations can be used to treat bacterial infections in mammals such as humans and domesticated mammals (e.g., cows, pigs, dogs, and cats), and in plants. The efficacy of such antibacterial agents in humans can be estimated in an animal model system well known to those of skill in the art (e.g., mouse and rabbit model systems of, for example, streptococcal pneumonia) . Various affinity reagents that are permeable to the microbial membrane (i.e., antibodies and antibody fragments) are useful in practicing the methods of the invention. For example polyclonal and monoclonal antibodies that specifically bind to the yphC polypeptide or yqjK polypeptide can facilitate detection of essential polypeptides in various bacterial strains (or extracts thereof) . These antibodies also are useful for detecting binding of a test compound to essential polypeptides (e.g., using the assays described herein) . In addition, monoclonal antibodies that bind essential polypeptides can themselves be used as antibacterial agents. The invention further features methods of identifying from a large group of mutants those strains that have conditional lethal mutations. In general, the gene and corresponding gene product are subsequently identified, although the strains themselves can be used in screening or diagnostic assays. The mechanism (s) of action for the identified genes and gene products provide a rational basis for the design of antibacterial therapeutic agents. These antibacterial agents reduce the action of the gene product in a wild type strain, and therefore are useful in treating a subject with that type, or a similarly susceptible type, of infection by administering the agent to the subject in a pharmaceutically effective amount. Reduction in the action of the gene product includes competitive inhibition of the gene product for the active site of an enzyme or receptor; non-competitive inhibition; disrupting an intracellular cascade path which requires the gene product; binding to the gene product itself, before or after post-translational processing; and acting as a gene product mimetic, thereby down-regulating the activity. Therapeutic agents include monoclonal antibodies raised against the gene product. Furthermore, the presence of the gene sequence in certain cells (e.g., a pathogenic bacterium of the same genus or similar species) , and the absence or divergence of the sequence in host cells can be determined, if desired. Therapeutic agents directed toward genes or gene products that are not present in the host have several advantages, including fewer side effects, and a lower overall required dosage. Nucleic acids include both RNA and DNA, including genomic DNA and synthetic (e.g., chemically synthesized) DNA. Nucleic acids can be double-stranded or single- stranded. Where single-stranded, the nucleic acid may be a sense strand or an antisense strand. Nucleic acids can be synthesized using oligonucleotide analogs or derivatives (e.g., inosine or phosphorothioate nucleotides) . Such oligonucleotides can be used, for example, to prepare nucleic acids that have altered base- pairing abilities or increased resistance to nucleases . An isolated nucleic acid is a DNA or RNA that is not immediately contiguous with both of the coding sequences with which it is immediately contiguous (one on the 5' end and one on the 3' end) in the naturally occurring genome of the organism from which it is derived. Thus, in one embodiment, an isolated nucleic acid includes some or all of the 5' non-coding (e.g., promoter) sequences that are immediately contiguous to the coding sequence. The term therefore includes, for example, a recombinant DNA that is incorporated into a vector, into an autonomously replicating plasmid or virus, or into the genomic DNA of a prokaryote or eukaryote, or which exists as a separate molecule (e.g., a genomic DNA fragment produced by PCR or restriction endonuclease treatment) independent of other sequences. It also includes a recombinant DNA that is part of a hybrid gene encoding an additional polypeptide sequence. The term "isolated" can refer to a nucleic acid or polypeptide that is substantially free of cellular material, viral material, or culture medium (when produced by recombinant DNA techniques) , or chemical precursors or other chemicals (when chemically synthesized) . Moreover, an isolated nucleic acid fragment is a nucleic acid fragment that is not naturally occurring as a fragment and would not be found in the natural state. A nucleic acid sequence that is substantially identical to an essential nucleotide sequence is at least 80% (e.g., at least 85%) identical to the nucleotide sequence of yphC or yqjK as represented by the SEQ ID NOs listed in Table 1, as depicted in Figs. 1-3. For purposes of comparison of nucleic acids, the length of the reference nucleic acid sequence will generally be at least 40 nucleotides, e.g., at least 60 nucleotides or more nucleotides. To determine the percent identity of two amino acid sequences or of two nucleic acids, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in the sequence of a first amino acid or nucleic acid sequence for optimal alignment with a second amino or nucleic acid sequence) . The amino acid residues or nucleotides at corresponding amino acid positions or nucleotide positions are then compared. When a position in the first sequence is occupied by the same amino acid residue or nucleotide as the corresponding position in the second sequence, the molecules are identical at that position. The percent identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e., % identity = # of identical positions/total # of overlapping positions x 100) . Preferably, the two sequences are the same length. The determination of percent identity or homology between two sequences can be accomplished using a mathematical algorithm. A suitable, mathematical algorithm utilized for the comparison of two sequences is the algorithm of Karlin and Altschul (1990) Proc . Nat ' l Acad . Sci . USA 87:2264-2268, modified as in Karlin and Altschul (1993) Proc . Nat ' l Acad . Sci . USA 90:5873-5877. Such an algorithm is incorporated into the NBLAST and XBLAST programs of Altschul, et al . (1990) J " . Mol . Biol . 215:403-410. BLAST nucleotide searches can be performed with the NBLAST program, score = 100, wordlength = 12 to obtain nucleotide sequences homologous to yphC or yqjK nucleic acid molecules of the invention. BLAST protein searches can be performed with the XBLAST program, score = 50, wordlength = 3 to obtain amino acid sequences homologous to yphC or yqjK protein molecules of the invention. To obtain gapped alignments for comparison purposes, Gapped BLAST can be utilized as described in Altschul et al . , (1997) Nucleic Acids Res . 25:3389-3402. When utilizing BLAST and Gapped BLAST programs, the default parameters of the respective programs (e.g., XBLAST and NBLAST) can be used. See http://www.ncbi.nlm.nih.gov. Another example of a mathematical algorithm utilized for the comparison of sequences is the algorithm of Myers and Miller, CABIOS (1989) . Such an algorithm is incorporated into the ALIGN program (version 2.0) which is part of the GCG sequence alignment software package. When utilizing the ALIGN program for comparing amino acid sequences, a PAM120 weight residue table, a gap length penalty of 12, and a gap penalty of 4 can be used. The percent identity between two sequences can be determined using the techniques described above, with or without allowing gaps. In calculating percent identity, only exact matches are counted. The essential polypeptides useful in practicing the invention include, but are not limited to, recombinant polypeptides and natural polypeptides. Also useful in the invention are nucleic acid sequences that encode forms of essential polypeptides in which naturally occurring amino acid sequences are altered or deleted. Preferred nucleic acids encode polypeptides that are soluble under normal physiological conditions. Also within the invention are nucleic acids encoding fusion proteins in which a portion of an essential polypeptide is fused to an unrelated polypeptide (e.g., a marker polypeptide or a fusion partner) to create a fusion protein. For example, the polypeptide can be fused to a hexa-histidine tag to facilitate purification of bacterially expressed polypeptides, or to a hemagglutinin tag to facilitate purification of polypeptides expressed in eukaryotic cells. The invention also includes, for example, isolated polypeptides (and the nucleic acids that encode these polypeptides) that include a first portion and a second portion; the first portion includes an essential polypeptide, and the second portion includes an immunoglobulin constant (Fc) region or a detectable marker . The fusion partner can be, for example, a polypeptide which facilitates secretion, e.g., a secretory sequence. Such a fused polypeptide is typically referred to as a preprotein. The secretory sequence can be cleaved by the host cell to form the mature protein. Also within the invention are nucleic acids that encode an essential polypeptide fused to a polypeptide sequence to produce an inactive preprotein. Preproteins can be converted into the active form of the protein by removal of the inactivating sequence. The invention also includes nucleic acids that hybridize, e.g., under stringent hybridization conditions (as defined herein) to all or a portion of the nucleotide sequences represented by SEQ ID NO:l or 7, or their complements. The hybridizing portion of the hybridizing nucleic acids is typically at least 15 (e.g., 20, 25, 30, or 50) nucleotides in length. The hybridizing portion of the hybridizing nucleic acid is at least 80%, e.g., at least 95%, or at least 98%, identical to the sequence of a portion or all of a nucleic acid encoding an essential polypeptide or its complement. Hybridizing nucleic acids of the type described herein can be used, for example, as a cloning probe, a primer (e.g., a PCR primer), or a diagnostic probe. Nucleic acids that hybridize to the nucleotide sequences represented by SEQ ID NOs: 1 and 7 are considered "antisense oligonucleotides . " Also part of in the invention are various engineered cells, e.g., transformed host cells, that contain an essential nucleic acid described herein. A transformed cell is a cell into which (or into an ancestor of which) has been introduced, by means of recombinant DNA techniques, a nucleic acid encoding an 5 essential polypeptide. Both prokaryotic and eukaryotic cells are included, e.g., bacteria, such as Streptococcus, Bacillus, and the like. Also within the invention are genetic constructs (e.g., vectors and plasmids) that include a nucleic acid 10 of the invention that is operably linked to a transcription and/or translation sequence to enable expression, e.g., expression vectors. A selected nucleic acid, e.g., a DNA molecule encoding an essential polypeptide, is "operably linked" to a transcription ! 15 and/or translation sequence when it is positioned adjacent to one or more sequence elements, e.g., a promoter, which direct transcription and/or translation of the sequence such that the sequence elements can control transcription and/or translation of the selected ( 20 nucleic acid. The invention also features purified or isolated polypeptides encoded by the essential genes yphC and yqjK. The terms "protein" and "polypeptide" both refer to any chain of amino acids, regardless of length or - 25 post-translational modification (e.g., glycosylation or phosphorylation) . Thus, the terms yphC polypeptide and yqjK polypeptide include full-length, naturally occurring, isolated yphC and yqjK proteins, respectively, as well as recombinantly or synthetically produced 30 polypeptides that correspond to the full-length, naturally occurring proteins, or to a portion of the naturally occurring or synthetic polypeptide (provided that a portion of the yphC polypeptide includes a portion of the sequence depicted in Fig. 1) . 5 A purified or isolated compound is a composition that is at least 60% by weight the compound of interest, e.g., an essential polypeptide or antibody. Preferably the preparation is at least 75% (e.g., at least 90%, 95%, or even 99%) by weight the compound of interest. Purity can be measured by any appropriate standard method, e.g., column chromatography, polyacrylamide gel electrophoresis, or HPLC analysis. Preferred essential polypeptides include a sequence substantially identical to all or a portion of a naturally occurring essential polypeptide, e.g., including all or a portion of the sequences shown in Figs. 1, 2A-2B, and 3 (provided that a portion of the yphC polypeptide includes a portion of the sequence depicted in Fig. 1) . Polypeptides "substantially identical" to the essential polypeptide sequences described herein have an amino acid sequence that is at least 80% identical to the amino acid sequence of the essential polypeptides represented by the SEQ ID NOs listed in Table 1 (measured as described herein) . The new polypeptides can also have a greater percentage identity, e.g., 85%, 90%, 95%, or even higher. For purposes of comparison, the length of the reference essential polypeptide sequence will generally be at least 16 amino acids, e.g., at least 20 or 25 amino acids. In the case of polypeptide sequences that are less than 100% identical to a reference sequence, the non- identical positions are preferably, but not necessarily, conservative substitutions for the reference sequence. Conservative substitutions typically include substitutions within the following groups: glycine and alanine; valine, isoleucine, and leucine; aspartic acid and glutamic acid; asparagine and glutamine; serine and threonine; lysine and arginine; and phenylalanine and tyrosine. Where a particular polypeptide is said to have a specific percent identity to a reference polypeptide of a defined length, the percent identity is relative to the reference polypeptide. Thus, a polypeptide that is 50% identical to a reference polypeptide that is 100 amino acids long can be a 50 amino acid polypeptide that is completely identical to a 50 amino acid long portion of the reference polypeptide. Alternatively, it can be a 100 amino acid long polypeptide that is 50% identical to the reference polypeptide over its entire length. Of course, other polypeptides also will meet the same criteria. The invention also features purified or isolated antibodies that specifically bind to an essential polypeptide. An antibody "specifically binds" to a particular antigen, e.g., an essential polypeptide, when it binds to that antigen, but does not substantially recognize and bind to other molecules in a sample, e.g., a biological sample, that naturally includes an essential polypeptide. In another aspect, the invention features a method for detecting an essential polypeptide in a sample. This method includes : obtaining a sample suspected of containing an essential polypeptide; contacting the sample with an antibody that specifically binds to an essential polypeptide under conditions that allow the formation of complexes of an antibody and the essential polypeptide; and detecting the complexes, if any, as an indication of the presence of an essential polypeptide in the sample. Also encompassed by the invention is a method of obtaining a gene related to an essential gene. Such a method entails obtaining a labeled probe that includes an isolated nucleic acid which encodes all or a portion of an essential nucleic acid, or a homolog thereof; screening a nucleic acid fragment library with the labeled probe under conditions that allow hybridization of the probe to nucleic acid fragments in the library, thereby forming nucleic acid duplexes; isolating labeled duplexes, if any; and preparing a full-length gene sequence from the nucleic acid fragments in any labeled duplex to obtain a gene related to the essential gene. Alternatively, such related genes can be identified by carrying out a BLAST search of various sequenced bacterial genomes, as described above. The invention offers several advantages. For example, the methods for identifying antibacterial agents can be configured for high throughput screening of numerous candidate antibacterial agents . Because the essential genes disclosed herein are thought to be highly conserved, antibacterial drugs targeted to these genes or their gene products are expected to have antibacterial activity against a wide range of bacteria. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described herein. All publications, patent applications, patents, and other references mentioned herein are incorporated herein by reference in their entirety. In the case of a conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative and are not intended to limit the scope of the invention, which is defined by the claims. Other features and advantages of the invention will be apparent from the following detailed description, and from the claims . Brief Description of the Drawings Fig. 1 is a representation of the amino acid and nucleic acid sequences of the yphC polypeptide and coding and non-coding strands of the yphC gene from a Streptococcus pneumoniae strain (SEQ ID NOs : 2 , 1, and 3, respectively) . Figs. 2A-2B are a representation of the full- length amino acid and nucleic acid sequences of the yphC polypeptide and coding and non-coding strands of the yphC gene from a Streptococcus pneumoniae strain (SEQ ID NOs: 5, 4, and 6, respectively). Fig. 3 is a representation of the amino acid and nucleic acid sequences of the yqjK polypeptide and coding and non-coding strands of the yqjK gene from a Streptococcus pneumoniae strain (SEQ ID NOs : 8 , 7, and 9, respectively) . Figs. 4A-4B are a representation of the amino acid and nucleic acid sequences of the B-yphC polypeptide and coding and non-coding strands of the B-yphC gene from a B . subtilis strain (SEQ ID Nos: 11, 10, and 12, respectively) . Figs. 5A-5B are a representation of the amino acid and nucleic acid sequences of the yfgK polypeptide and coding and non-coding strands of the yfgK gene from an E. coli strain (SEQ ID Nos : 14 , 13, and 15, respectively) . Fig. 6 is a representation of the amino acid and nucleic acid sequences of the B-yqjK polypeptide and coding and non-coding strands of the B-yqjk gene from a B . subtilis strain (SEQ ID Nos : 17, 16, and 18, respectively) . Fig. 7 is a representation of the amino acid and nucleic acid sequences of the elaC polypeptide and gene from an E. coli strain (SEQ ID Nos: 20, 19, and 21, respectively) . Fig. 8 is a schematic representation of the PCR strategy used to produce DNA molecules used for targeted deletions of essential genes in Streptococcus pneumoniae . Fig. 9 is a schematic representation of the strategy used to produce targeted deletions of essential genes in Streptococcus pneumoniae . Fig. 10 is a schematic representation of the strategy used to obtain non-polar gene deletions of yphC and yqjK in B . subtilis . Figs. 11A-11C are schematic representations of the strategy used to construct conditional null mutants of the yphC and yqjK genes . Fig. 12 is a schematic representation of the general strategy used to obtain deletions of essential genes in E. coli and shows the essential phenotype of the E. coli yfgK gene, which is an ortholog of the S . pneumoniae yphC gene. Detailed Description of the Invention At least two genes in the bacterium Streptococcus pneumoniae have been found to be essential for the survival of these bacteria. These so-called essential genes, yphC and yqjK, encode what are referred to herein as essential polypeptides. The yphC and yqjK genes are useful molecular tools for identifying similar genes in pathogenic microorganisms, such as pathogenic strains of Bacillus . The essential polypeptides are useful targets for identifying compounds that are inhibitors of the pathogens in which the essential polypeptides are expressed. Identifying Essential Streptococcus Genes As shown by the experiments described below, both the yphC and yqjK genes are essential for survival of Streptococcus pneumoniae . Streptococcus pneumoniae is available from the ATCC . In general, and for the examples set forth below, essential genes can be identified by creating targeted deletions of genes of interest in a bacterium, e.g., S. pneumoniae . These genes of interest were selected as follows. Using standard molecular biology techniques, a library containing fragments of the Streptococcus pneumoniae genome was made, using M13 phage or plasmid DNA as the vector. Open reading frames (ORFs) contained within this library were randomly sequenced, using primers that hybridized to the vector. The genes of interest selected for targeted deletion satisfied four criteria, as determined by comparing the sequences with the GenBank database of nucleotide sequences: (i) the ORF had no known function; (ii) the ORF had an ortholog in Bacillus subtilis ; (iii) the ORF was conserved in other bacteria, with p < 10 "10 ; and (iv) the ORF had no eukaryotic ortholog, with p > 10 ~3 . The Streptococcus genes yphC and yqjK met each of these criteria, suggesting that a compound that inhibited the yphC or yqjK genes or gene products would have a broad spectrum of antibacterial activity. The yphC and yqjK genes each were replaced with a nucleic acid sequence conferring resistance to the antibiotic erythromycin (an " erm" gene) . Other genetic markers can be used in lieu of this particular antibiotic resistance marker. Polymerase chain reaction (PCR) amplification was used to make a targeted deletion in the Streptococcus genomic DNA, as shown in Fig. 8. Several PCR reactions were used to produce the DNA molecules needed to carry out target deletion of the genes of interest. First, using primers 5 and 6, an erm gene was amplified from pIL252 from B . subtilis (available from the Bacillus Genetic Stock Center, Columbus, OH) . Primer 5 consists of 21 nucleotides that are identical to the promoter region of the erm gene and complementary to Sequence A. Primer 5 has the sequence 5 ' GTG TTC GTG CTG ACT TGC ACC3' (SEQ ID NO: 22) . Primer 6 consists of 21 nucleotides that are complementary to the 3' end of the erm gene. Primer 6 has the sequence 5 'GAA TTA TTT CCT CCC GTT AAA3' (SEQ ID NO: 23) . PCR amplification of the erm gene was carried out under the following conditions: 30 cycles of 94°C for 1 minute, 55°C for 1 minute, and 72 °C for 1.5 minutes, followed by one cycle of 72 °C for 10 minutes. In the second and third PCR reactions, sequences flanking the gene of interest were amplified and produced as hybrid DNA molecules that also contained a portion of the erm gene. The second reaction produced a double- stranded DNA molecule (termed "Left Flanking Molecule") that includes sequences upstream of the 5' end of the gene of interest and the first 21 nucleotides of the erm gene. As shown in Fig. 8, this reaction utilized primer 1, which is 21 nucleotides in length and identical to a sequence that is located approximately 500 bp upstream of the translation start site of the gene of interest. Primers 1 and 2 are gene-specific and have the sequences 5'TGA AGC CTG TCA AGG ACG AGG3 ' (SEQ ID NO: 24) and 5 ' CCT TAC GTG GTC GAA TTG TGG3 ' (SEQ ID NO:25), respectively, for yqjK. For yphC, primers 1 and 2 have the sequences 5'TGT ATG AAT TGG TAC CTC AAG3 ' (SEQ ID NO: 26) and 5'ACA ATG GCA ATA GTT GGT AGG3 ' (SEQ ID NO:27), respectively. Primer 2 is 42 nucleotides in length, with 21 of the nucleotides at the 3' end of the primer being complementary to the 5' end of the sense strand of the gene of interest. The 21 nucleotides at the 5' end of the primer were identical to Sequence A and are therefore complementary to the 5' end of the erm gene. Thus, PCR amplification using primers 1 and 2 produced the left flanking DNA molecule, which is a hybrid DNA molecule containing a sequence located upstream of the gene of interest and 21 base pairs of the erm gene, as shown in Fig. 8. The third PCR reaction was similar to the second reaction, but produced the right flanking DNA molecule, shown in Fig. 8. The right flanking DNA molecule contains 21 base pairs of the 3' end of the erm gene, a 21 base pair portion of the 3' end of the gene of interest , and sequences downstream of the gene of interest. This right flanking DNA molecule was produced with gene-specific primers 3 and 4. For yqjK, primers 3 and 4 have the sequences 5 'GTG GAA ATC TAG CAG TCA CAG3 ' (SEQ ID NO: 28) and 5 'ATC TGG TTC TAG CAG GAA GCG3 ' (SEQ ID NO: 29), respectively. For yphC, primers 3 and 4 have the sequences 5 'CAT TGC CAG TCC TGT TGC TGG3 ' (SEQ ID NO: 30) and 5 'ATG GCA TCC ATG ACA TCG3 ' (SEQ ID NO: 31) , respectively. Primer 3 is 42 nucleotides; the 21 nucleotides at the 5' end of primer 3 are identical to Sequence B and therefore are identical to the 3 ' end of the erm gene. The 21 nucleotides at the 3' end of primer 3 are identical to the 3' end of the gene of interest. Primer 4 is 21 nucleotides in length and is complementary to a sequence located approximately 500 bp downstream of the gene of interest. PCR amplification of the left and right flanking DNA molecules was carried out, separately, in 50 μl reaction mixtures containing: 1 μl Streptococcus pneumoniae (RX1) DNA (0.25 μg) , 2.5 μl primer 1 or primer 4 (10 pmol/μl) , 2.5 μl primer 2 or primer 3 (20 pmol/μl), 1.2 μl a mixture dNTPs (10 mM each), 37 μl H 2 0, 0.7 μl Taq polymerase (5U/μl) , and 5 μl lOx Taq polymerase buffer (10 mM Tris, 50 mM KC1 , 2.5 mM MgCl 2 ) . The left and right flanking DNA molecules were amplified using the following PCR cycling program: 95°C for 2 minutes; 72°C for 1 minute; 94°C for 30 seconds; 49°C for 30 seconds; 72°C for 1 minute; repeating the 94°C, 49°C, and 72 °C incubations 30 times; 72 °C for 10 minutes and then stopping the reactions. A 15 μl aliquot of each reaction mixture then was electrophoresed through a 1.2% low melting point agarose gel in TAE buffer, and then stained with ethidium bromide. Fragments containing the amplified left and right flanking DNA molecules were excised from the gel and purified using a QIAQUICK™ gel extraction kit (Qiagen, Inc.) Other art-known methods for amplifying and isolating DNA can be substituted. The flanking left and right DNA fragments were eluted into 30 μl TE buffer at pH 8.0. The amplified erm gene and left and right flanking DNA molecules were then fused together to produce the fusion product, as shown in Fig. 8. The fusion PCR reaction was carried out in a volume of 50 μl containing: 2 μl of each of the left and right flanking DNA molecules and the erm gene PCR product; 5 μl of lOx buffer; 2.5 μl of primer 1 (10 pmol/μl); 2.5 μl of primer 4 (10 pmol/μl), 1.2 μl dNTP mix (10 mM each) 32 μl H 2 0, and 0.7 μl Taq polymerase. The PCR reaction was carried out using the following cycling program: 95°C for 2 minutes; 72°C for 1 minute; 94°C for 30 seconds, 48°C for 30 seconds; 72 °C for 3 minutes; repeat the 94 °C, 48 °C, and 72 °C incubations 25 times; 72 °C for 10 minutes. After the reaction was stopped, a 12 μl aliquot of the reaction mixture was electrophoresed through an agarose gel to confirm the presence of a final product of approximately 2 kb. A 5 μl aliquot of the fusion product was used to transform S . pneumoniae grown on a medium containing erythromycin in accordance with standard techniques. As shown in Fig. 9, the fusion product and the S . pneumoniae genome undergo a homologous recombination event so that the erm gene replaces the chromosomal copy of the gene of interest, thereby creating a gene knockout. Disruption of an essential gene results in no growth on a medium containing erythromycin. Using this gene knockout method, the yphC and yqjK genes were identified as being essential for survival. The portion of the yphC open reading frame that was sequenced prior to carrying out targeted deletion is depicted in Fig. 1. The full-length yphC sequence (depicted in Figs. 2A-2B) was compiled by searching the TIGR sequence database for a clone from S . pneumoniae having a sequence overlapping the sequence depicted in Fig. 1 and combining the 3 ' end of the gene from the database with the 5' end of the gene depicted in Fig. 1. The sequence contained in the clone from the database was of unknown function. Identification of Orthologs of Essential Genes Having shown that the yphC and yqjK genes are essential for survival of Streptococcus , orthologs of these genes, when identified in other organisms, for example B . subtilis or E. coli , can be tested to determine whether they are essential for survival of those organisms as well. To this end, the coding sequences of yphC and yqjK were used to search the GenBank database of nucleotide sequences, and orthologs of each sequence were identified in B . subtilis and E. coli . Sequence comparisons were performed using the Basic Local Alignment Search Tool (BLAST) (Altschul et al., J. Mol. Biol. 215:403-410, 1990). The percent sequence identity shared by the essential polypeptides and their orthologs was determined using the GAP program from the Genetics Computer Group (GCG) Wisconsin Sequence Analysis Package (Wisconsin Package Version 9.1; Madison, WI) . The default parameters for gap weight (12) and length weight (4) were used. Typically, essential polypeptides and their orthologs share at least 25% (e.g., at least 30% or 40%) sequence identity. Typically, the DNA sequences encoding essential polypeptides and their homologs or orthologs share at least 20% (e.g., at least 30%, 35%, 40% or 45%) sequence identity. Bioinformatics analysis of the yphC and yqjK genes showed that these genes are widely conserved among bacteria. To determine whether the identified orthologs of yphC and yqjK are essential for survival of other bacteria, each of the orthologous genes was separately deleted from the genome of the host organism, as described in detail below. The observation that the B . subtilis and E . coli orthologs of yphC (B-yphC and yfgK, respectively) also are essential for survival of JS. subtilis and E. coli suggests that the yphC gene is essential in all bacteria where it is present. Therefore, an antibacterial agent targeted to this gene or its gene product is expected to have a broad spectrum of antibacterial activity. The observation that the B . subtilis ortholog of yqjK (B-yqjK) , but not the E. coli ortholog (elaC) , is essential for survival suggests that this gene is essential in all gram-positive bacteria in which it is present, and not essential in gram negative bacteria. Therefore, an antibacterial agent targeted to the yqjK gene or its gene product is expected to have antibacterial activity against all gram-positive bacteria . Deletion and Determination of Essentiality of the yphC and vσiK genes in Bacillus subtilis The following examples illustrate that the B . subtilis orthologs of yphC and yqjK (i.e., B-yphC and B-yqjK) are essential for survival of B . subtilis . Two strategies were used to produce knockout mutations of the B-yphC and B-yqjK genes in B . subtilis, and a determination of the essential phenotype of the B-yphC and B-yqjK genes was made, as described below. The first strategy (illustrated in Fig. 10) was similar to the targeted deletion strategy used to knock out genes in Streptococcus, as described above. The significant differences were as follows. (A.) In PCR, a chloramphenicol resistant gene (CmR) of B . subtilis, from plasmid pDG283, was used in lieu of an erythromycin resistance gene. Alternatively, any B . subtilis marker can be used. (B.) The primers used to amplify the CmR gene and primers B and C, which immediately flank the yphC ORF, contain a stretch of 27 nucleotides termed "universal overlapping sequences." These universal overlapping sequences can be used efficiently in PCR amplifications, and facilitate the use of various insertion sequences in fusion PCR reactions. Resistance markers, promoters, regulatory elements, or any nucleic acid sequence can be amplified with such overlapping sequences and be used with the same set of gene deletion primers (primers A, B, C, and D) . The sequence for the 5' overlapping region is 5 ' CACAGGAAACAGC TATGACCATGATTA3 ' (SEQ ID NO: 25) and the sequence for the 3 ' overlapping region is 5 ' GAAATAAATGCATCTGTATTTGAATG3 ' (SEQ ID NO: 26) . (C.) The left and right flanking DNA molecules produced by PCR should be at least 900 (e.g., 1000) nucleotides in length to optimize recombination into the B . subtilis chromosome. (D.) To produce the fusion product, two simultaneous PCR reactions were used. One reaction used an annealing temperature of 50 °C, and the other used a temperature of 65°C. Longer extension times were used (30-60 more seconds) , and a Long high fidelity polymerase was also used according to the manufacturer's instructions (Boehringer-Mannheim) . (E.) Competent cells of the wild-type strain, PY79, were used according to standard B . subtilis protocols (Molecular Biological Methods for Bacillus, 1990, Harwood and Cutting, Eds . Wiley & Sons, Ltd. England) . (F.) The sequence of the primers shown in Fig. 10 was as follows: primer Ra, (5 ' CACAGGAAACAGCTATGACCA TGATTAAACTAAAGCACCCATTAGTTCA3 ' (SEQ ID NO: 27)) which hybridized to a sequence upstream of the CmR promoter; primer Rb (5 ' CATTCAAATACAGATGCATTTTATTTCCTCATATT ATAAAAGCCAGTCATT3 ' (SEQ ID NO:28)), which hybridized to a sequence located adjacent to the transcription terminator; primer A-YPHC (5 ' GCCATTGCGTTTGAAAG3 ' (SEQ ID NO:29)); primer A-YQJK (5 ' TGCTTCGCCGATTTCTT3 ' (SEQ ID NO:30); primer B-YPHC ( 5 ' TAATCATGGTCATAGCTGTTTCCTGTGTATGAAAAGAAACCCTTCAGAG3 ' (SEQ ID NO:31)), which is located adjacent to the yphC start codon; primer B-YQJK ( 5 ' TAATCATGGTCATAGCTGTTTCCTGTGCATACCG AACGCCTTTCTT3 ' (SEQ ID NO:32)), which is located adjacent to the yqjK start codon; primer C-YPHC (5 ' GAAATAAATGCA TCTGTATTTGAATGTTTTAGAAAACCGAATCAGAGA3' (SEQ ID NO: 33)), which is located adjacent to the yphC stop codon; primer C-YQJK ( 5 ' GAAATAAATGCATCTGTATTTGAATGAATAGCGTGGCGGCATA3 ' (SEQ ID NO:34)), located adjacent to the yqjK stop codon; primer D-YPHC ( 5 ' ATTCAGATCGAATACTCCTG3 ' (SEQ ID NO:35)); and primer D-YQJK (5 'AAAGCGGGCAAAGCAGA3 ' (SEQ ID NO: 36)) . Competent cells that were transformed with the fused left and right flanking DNA molecules were incubated for 18 hours at 37°C on a selective medium (LB, 5 μg/ml Cm) to determine whether the gene in question was essential (as characterized by lack of colony growth) or non-essential (as characterized by the appearance of dozens to hundreds of colonies) . When these deletion experiments were performed with the yphC and yqjK genes, separately, no colonies were detected on the selective medium, indicating that each of these genes is essential for survival of B . subtilis . Several control experiments were performed to ensure that the observed lack of cell growth was due to the essential nature of the B-yphC and B-yqjK genes. Simultaneous PCR reactions and transformations with genes that were known not to be essential produced hundreds of colonies in similar experiments, indicating that experimental conditions were adequate to ensure that the lack of colony growth was indicative of an essential gene. The CmR gene insertion was also shown to have non- polar effects on downstream genes and allowed efficient expression of downstream genes. The second strategy used to obtain deletion mutants of the yphC and yqjK genes of B . subtilis involved the construction of conditional null mutants that can halt the expression of the chosen gene and allow observation of the mutant phenotype (Figs. 11A-11C) . In these mutants, a wild-type copy of the yphC or yqjK gene was placed under the control of a B . subtilis Para promoter, which is a tightly regulatable promoter that can efficiently turn off gene expression. These regulatable genetic constructs were subsequently inserted into the B . subtilis chromosome. As shown in Fig. 11A, the regulatable genetic constructs contained an amylase gene ( amy) sequence and CmR sequence for integration into chromosome at the amy locus. Disruption of the amy gene by double crossover is innocuous to the cell, and recombinants are easily detectable on starch plates because amy + cells produce colonies having transparent halos . After integration of the regulatable yphC or yqjK genes into the chromosome of wild-type cells, the resulting cells were rendered competent and transformed with a fusion PCR fragment containing a replacement resistant marker (Fig. 11B) , as described above. In this case, the fusion PCR fragment contained a Kanamycin resistance gene from B . subtilis (Kan) instead of the CmR gene. Selection for this Kan marker during transformation of the cells containing the ectopically inserted regulated yphC or yqjK genes was performed in the presence of arabinose to allow expression of the yphC or yqjK genes and trans complementation of the deletion (Fig. 11C) . The yphC and yqjK mutants obtained in this manner were able to grow on a selective medium (LB Kan) in the presence of 0.2% arabinose, while selections made in wild type cells did not yield any mutants. Colonies containing the regulated genes and their deletions then were picked and streaked onto a similar medium (LB Kan, 0.2% arabinose), and onto plates containing a selective medium with lower concentrations of arabinose, no arabinose, or in the presence of 0.2% glucose. After 18 hours of incubation to allow for growth, the only plates that contained colonies were those plates that contained 0.2%, 0.02%, or 0.002% arabinose. Lower concentrations of the inducer or repressing conditions (i.e., the lack of arabinose or presence of glucose) did not allow cell growth and formation of colonies. Thus, these experiments indicated that: (i) the deletion created by the fusion PCR did not affect putative essential genes downstream of yphC or yqjK, since expression of only the gene in question was sufficient to obtain efficient complementation, and (ii) expression of the yphC and the yqjK genes is necessary for survival of the B . subtilis cells. The experiments described above confirmed that B- yphC and B-yqjK are essential in B . subtilis . These experiments yielded conditional lethal strains that can be used in a variety of screens and approaches, including underexpression/overexpression assays, transcription profiling, etc. The constructions of knockout mutations of the yphC and yqjK genes can be accomplished using any of various art-known methods. Assay -of the Essential Nature of vphC and yqjK Orthologs in E. coli The discovery that the yphC and yqjK genes are essential in Streptococcus pneumoniae and in B . subtilis suggests that these genes are essential in all Gram- positive bacteria. To further extend these observations to Gram-negative bacteria, and therefore to all bacteria, deletion mutants were produced for the E. coli orthologs of yphC and yqjK. The general strategy used to produce E. coli gene deletions, as conditional null mutants, is schematically represented in Fig. 12. First, a copy of the wild type gene to be deleted was cloned into a runaway, counter- selectable vector containing the Pbad promoter of E. coli . This E. coli promoter is turned on in the presence of arabinose and is tightly controlled like its B . subtilis counterpart, described above. Cells containing this vector were then used to introduce an in-frame deletion of the chosen gene by replacing the gene with a markerless small TAG of approximately 30 nucleotides. The upstream and downstream regions that flank the chosen gene were amplified by PCR using primers that introduced a 27-30 nucleotide overlapping TAG. Fusion PCR reactions were carried out with only these two fragments that are joined with the TAG lying in the middle, thereby replacing the chosen gene. This fragment was then cloned into a temperature- sensitive, counterselectable plasmid, pKO-3, and inserted into the chromosome in accordance with conventional techniques (see, e.g., Church et al . , 1997, J. Bacteriol . ) . The resulting in-frame deletion was complemented by expression of the wild type gene from the Pbad vector in the presence of arabinose (Fig. 11C, P turned on. The deletion mutant can suppress gene expression under conditions lacking arabinose, in the presence of glucose (P turned off) , or in the presence of streptomycin without IPTG, which allows loss of the plasmid (because the origin of replication of the complementing Pbad plasmid is under lac- IPTG control) . As shown in Fig. 12, deletion of the yphC sequence in E. coli and its substitution by a TAG in the presence of a complementing Pbad-yphC ortholog plasmid resulted in mutants that grew well on arabinose plates but which failed to grow on glucose or streptomycin plates. This result indicates that the yfgK gene (i.e., the E. coli yphC ortholog) is essential for the survival of E. coli . In contrast, similar experiments carried out with the E. coli yqjK ortholog, elaC, showed significant cell growth under all conditions, indicating that the yqj K gene is not essential in E. coli . Alternatively, it is possible that other E. coli genes that have arylsulfatase/phosphatase activity that have no sequence similarity are able to complement for the lack of elaC function. The fact that the B . subtilis and E. coli orthologs of the yphC gene are essential for survival indicates that this gene is essential in all bacteria in which it is present. The yqjK gene, which is essential for survival of S . pneumoniae and B . subtilis , is thought to be essential in all Gram-positive bacteria, but not in E . coli . Therefore, an antibacterial agent targeted to the yphC gene or its gene product is expected to have a broad spectrum of antibacterial activity (including Gram- positive and Gram-negative bacteria) , while an antibacterial agent targeted to the yqjK gene or its gene product is expected to have antibacterial activity against Gram-positive bacteria. Identification of Essential Genes and Polypeptides in Additional Bacterial Strains The yphC and yqjK genes and various orthologs, or fragments thereof, can be used to detect homologous or orthologous genes in yet other organisms. In particular, these genes can be used to analyze various pathogenic and non-pathogenic strains of bacteria. Fragments of a nucleic acid (DNA or RNA) encoding an essential polypeptide, homolog or ortholog (or sequences complementary thereto) can be used as probes in conventional nucleic acid hybridization assays of pathogenic bacteria. For example, nucleic acid probes (which typically are 8-30, usually 15-20, nucleotides in length) can be used to detect essential genes or homologs or orthologs thereof in art-known molecular biology methods, such as Southern blotting, Northern blotting, dot or slot blotting, PCR amplification methods, colony hybridization methods, and the like. Typically, an oligonucleotide probe based on the nucleic acid sequences described herein, or fragments thereof, is labeled and used to screen a genomic library constructed from mRNA obtained from a bacterial strain of interest . A suitable method of labeling involves using polynucleotide kinase to add 32 P-labeled ATP to the oligonucleotide used as the probe. This method is well known in the art, as are several other suitable methods (e.g., biotinylation and enzyme labeling) . Hybridization of the oligonucleotide probe to the library, or other nucleic acid sample, typically is performed under moderate to stringent conditions. Nucleic acid duplex or hybrid stability is expressed as the melting temperature or T m , which is the temperature at which a probe dissociates from a target DNA. This melting temperature is used to define the required stringency conditions. If sequences are to be identified that are related and substantially identical to the probe, rather than identical, it is useful to first establish the lowest temperature at which only homologous hybridization occurs with a particular concentration of salt (e.g., SSC or SSPE) . Then, assuming that 1% mismatching results in a 1°C decrease in the T m , the temperature of the final wash in the hybridization reaction is reduced accordingly (for example, if sequences having >, 95% identity with the probe are sought, the final wash temperature is decreased by 5°C) . In practice, the change in T ra can be between 0.5° and 1.5°C per 1% mismatch. Stringent conditions include, for example, hybridizing at 68 °C in 5x SSC/5x Denhardt ' s solution/1.0% SDS, or in 0.5 M NaHP0 4 (pH 7.2) /l mM EDTA/7% SDS, or in 50% formamide/0.25 M NaHP0 4 (pH 7.2) /0.25 M NaCl/l mM EDTA/7% SDS; and washing in 0.2x SSC/0.1% SDS at room temperature or at 42°C, or in 0. lx SSC/0.1% SDS at 68°C, or in 40 mM NaHP0 4 (pH 7.2) /l mM EDTA/5% SDS at 50°C, or in 40 mM NaHP0 4 (pH 7.2) 1 mM EDTA/1% SDS at 50 °C. Moderately stringent conditions include washing in 3x SSC at 42 °C. The parameters of salt concentration and temperature can be varied to achieve the optimal level of identity between the probe and the target nucleic acid. Additional guidance regarding such conditions is readily available in the art, for example, by Sambrook et al . , 1989, Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Press, N.Y.; and Ausubel et al . (eds.), 1995, Current Protocols in Molecular Biology, (John Wiley & Sons, N.Y.) at Unit 2.10. In one approach, libraries constructed from pathogenic or non-pathogenic bacterial strains can be screened. For example, such strains can be screened for expression of essential genes by Northern blot analysis. Upon detection of transcripts of the essential genes or homologs thereof, libraries can be constructed from RNA isolated from the appropriate strain, utilizing standard techniques well known to those of skill in the art. Alternatively, a total genomic DNA library can be screened using an essential gene probe (or a probe directed to a homolog thereof) . New gene sequences can be isolated, for example, by performing PCR using two degenerate oligonucleotide primer pools designed on the basis of nucleotide sequences within the essential genes, or their homologs or orthologs, as depicted herein. The template for the reaction can be DNA obtained from strains known or suspected to express an essential allele or an allele of a homolog or ortholog thereof . The PCR product can be subcloned and sequenced to ensure that the amplified sequences represent the sequences of a new essential nucleic acid sequence, or a sequence of a homolog thereof . Synthesis of the various essential polypeptides or their homologs or orthologs (or an antigenic fragment thereof) for use as antigens, or for other purposes, can readily be accomplished using any of the various art- known techniques. For example, an essential polypeptide or homolog or ortholog thereof, or an antigenic fragment (s), can be synthesized chemically in vi tro, or enzymatically (e.g., by in vi tro transcription and translation) . Alternatively, the gene can be expressed in, and the polypeptide purified from, a cell (e.g., a cultured cell) by using any of the numerous, available gene expression systems. For example, the polypeptide antigen can be produced in a prokaryotic host (e.g., E. coli or B . subtilis) or in eukaryotic cells, such as yeast cells or in insect cells (e.g., by using a baculovirus-based expression vector) . Proteins and polypeptides can also be produced in plant cells, if desired. For plant cells, viral expression vectors (e.g., cauliflower mosaic virus and tobacco mosaic virus) and plasmid expression vectors (e.g., Ti plasmid) are suitable. Such cells are available from a wide range of sources (e.g., the American Type Culture Collection, Rockland, MD; also, see, e.g., Ausubel et al . , Current Protocols in Molecular Biology, John Wiley & Sons, New York, 1994) . The optimal methods of transformation or transfection and the choice of expression vehicle will depend on the host system selected. Transformation and transfection methods are described, e.g., in Ausubel et al . , supra ; expression vehicles may be chosen from those provided, e.g., in Cloning Vectors : A Laboratory Manual (P.H. Pouwels et al., 1985, Supp. 1987). The host cells harboring the expression vehicle can be cultured in conventional nutrient media, adapted as needed for activation of a chosen gene, repression of a chosen gene, selection of transformants, or amplification of a chosen gene. If desired, the yphC or yqjK polypeptides or their homologs or orthologs can be produced as fusion proteins. For example, the expression vector pUR278 (Ruther et al . , EMBO J. , 2:1791, 1983) can be used to create lacZ fusion proteins. The art -known pGEX vectors can be used to express foreign polypeptides as fusion proteins with glutathione S-transferase (GST) . In general, such fusion proteins are soluble and can be easily purified from lysed cells by adsorption to glutathione-agarose beads followed by elution in the presence of free glutathione. The pGEX vectors are designed to include thrombin or factor Xa protease cleavage sites so that the cloned target gene product can be released from the GST moiety. In an exemplary expression system, a baculovirus such as Autographa californica nuclear polyhedrosis virus (AcNPV) , which grows in Spodoptera frugiperda cells, can be used as a vector to express foreign genes . A coding sequence encoding an essential polypeptide or homolog or ortholog thereof can be cloned into a non-essential region (for example the polyhedrin gene) of the viral genome and placed under control of a promoter, e.g., the polyhedrin promoter or an exogenous promoter. Successful insertion of a gene encoding an essential polypeptide or homolog can result in inactivation of the polyhedrin gene and production of non-occluded recombinant virus (i.e., virus lacking the proteinaceous coat encoded by the polyhedrin gene) . These recombinant viruses are then typically used to infect insect cells (e.g., Spodoptera frugiperda cells) in which the inserted gene is expressed (see, e.g., Smith et al . , J " . Virol . , 46:584, 1983; Smith, U.S. Patent No. 4,215,051). If desired, mammalian cells can be used in lieu of insect cells, provided that the virus is engineered such that the gene encoding the desired polypeptide is placed under the control of a promoter that is active in mammalian cells. In mammalian host cells, a number of viral -based expression systems can be utilized. When an adenovirus is used as an expression vector, the nucleic acid sequence encoding the essential polypeptide or homolog can be ligated to an adenovirus transcription/ translation control complex, e.g., the late promoter and tripartite leader sequence. This chimeric gene can then be inserted into the adenovirus genome by in vi tro or in vivo recombination. Insertion into a non-essential region of the viral genome (e.g., region El or E3) will result in a recombinant virus that is viable and capable of expressing an essential gene product in infected hosts (see, e.g., Logan, Proc. Natl. Acad. Sci. USA, 81:3655, 1984) . Specific initiation signals may be required for efficient translation of inserted nucleic acid sequences. These signals include the ATG initiation codon and adjacent sequences. In general, exogenous translational control signals, including, perhaps, the ATG initiation codon, should be provided. Furthermore, the initiation codon must be in phase with the reading frame of the desired coding sequence to ensure translation of the entire sequence. These exogenous translational control signals and initiation codons can be of a variety of origins, both natural and synthetic. The efficiency of expression may be enhanced by the inclusion of appropriate transcription enhancer elements, or transcription terminators (Bittner et al . , Methods in Enzymol . , 153:516, 1987). The essential polypeptides and their homologs and orthologs can be expressed individually or as fusions with a heterologous polypeptide, such as a signal sequence or other polypeptide having a specific cleavage site at the N-and/or C-terminus of the protein or polypeptide. The heterologous signal sequence selected should be one that is recognized and processed, i.e., cleaved by a signal peptidase, by the host cell in which the fusion protein is expressed. A host cell can be chosen that modulates the expression of the inserted sequences, or modifies and processes the gene product in a specific, desired fashion. Such modifications and processing (e.g., cleavage) of protein products may facilitate optimal functioning of the protein. Various host cells have characteristic and specific mechanisms for post- translational processing and modification of proteins and gene products. Appropriate cell lines or host systems familiar to those of skill in the art of molecular biology can be chosen to ensure the correct modification and processing of the foreign protein expressed. To this end, eukaryotic host cells that possess the cellular machinery for proper processing of the primary transcript, and phosphorylation of the gene product can be used. Such mammalian host cells include, but are not limited to, CHO, VERO, BHK, HeLa, COS, MDCK, 293, 3T3 , WI38, and choroid plexus cell lines. If desired, the essential polypeptide or homolog or ortholog thereof can be produced by a stably- transfected mammalian cell line. A number of vectors suitable for stable transection of mammalian cells are available to the public, see, e.g., Pouwels et al . (supra) ; methods for constructing such cell lines are also publicly known, e.g., in Ausubel et al . (supra) . In one example, DNA encoding the protein is cloned into an expression vector that includes the dihydrofolate reductase (DHFR) gene. Integration of the plasmid and, therefore, the essential polypeptide-encoding gene into the host cell chromosome is selected for by including 0.01-300 μM methotrexate in the cell culture medium (as described in Ausubel et al . , supra) . This dominant selection can be accomplished in most cell types. Recombinant protein expression can be increased by DHFR-mediated amplification of the transfected gene. Methods for selecting cell lines bearing gene amplifications are described in Ausubel et al . (supra) ; such methods generally involve extended culture in medium containing gradually increasing levels of methotrexate. DHFR-containing expression vectors commonly used for this purpose include pCVSEII-DHFR and pAdD26SV(A) (described in Ausubel et al . , supra) . A number of other selection systems can be used, including but not limited to, herpes simplex virus thymidine kinase genes, hypoxanthine-guanine phosphoribosyl-transferase genes, and adenine phosphoribosyltransferase genes, which can be employed in tk, hgprt , or aprt cells, respectively. In addition, grpt, which confers resistance to mycophenolic acid (Mulligan et al . , Proc . Natl . Acad . Sci . USA , 78:2072, 1981) ; neo, which confers resistance to the aminoglycoside G-418 (Colberre-Garapin et al . , J. Mol . Biol . , 150:1, 1981); and hygro, which confers resistance to hygromycin (Santerre et al . , Gene, 30:147, 1981), can be used. Alternatively, any fusion protein can be readily purified by utilizing an antibody or other molecule that specifically binds the fusion protein being expressed. For example, a system described in Janknecht et al . , Proc . Na tl . Acad . Sci . USA, 88:8972 (1981), allows for the ready purification of non-denatured fusion proteins expressed in human cell lines. In this system, the gene of interest is subcloned into a vaccinia recombination plasmid such that the gene's open reading frame is translationally fused to an amino-terminal tag consisting of six histidine residues. Extracts from cells infected with recombinant vaccinia virus are loaded onto Ni + nitriloacetic acid-agarose columns, and histidine-tagged proteins are selectively eluted with imidazole-containing buffers . Alternatively, yphC or yqjK, or a homolog or ortholog thereof, or a portion thereof, can be fused to an immunoglobulin Fc domain. Such a fusion protein can be readily purified using a protein A column, for example. Moreover, such fusion proteins permit the production of a chimeric form of an essential polypeptide or homolog or ortholog having increased stability in vivo . Once the recombinant essential polypeptide (or homolog) is expressed, it can be isolated (i.e., purified) . Secreted forms of the polypeptides can be isolated from cell culture media, while non-secreted forms must be isolated from the host cells. Polypeptides can be isolated by affinity chromatography . For example, an anti-yphC antibody (e.g., produced as described herein) can be attached to a column and used to isolate the protein. Lysis and fractionation of cells harboring the protein prior to affinity chromatography can be performed by standard methods (see, e.g., Ausubel et al . , supra) . Alternatively, a fusion protein can be constructed and used to isolate an essential polypeptide (e.g., a yphC-maltose binding fusion protein, a yphC-/3- galactosidase fusion protein, or a yphC-trpE fusion protein; see, e.g., Ausubel et al . , supra ; New England Biolabs Catalog, Beverly, MA) . The recombinant protein can, if desired, be further purified, e.g., by high performance liquid chromatography using standard techniques (see, e.g., Fisher, Laboratory Techniques In Biochemistry And Molecular Biology, eds . , Work and Burdon, Elsevier, 1980) . Given the amino acid sequences described herein, polypeptides useful in practicing the invention, particularly fragments of essential polypeptides, can be produced by standard chemical synthesis (e.g., by the methods described in Solid Phase Peptide Synthesis, 2nd ed. , The Pierce Chemical Co., Rockford, IL, 1984) and used as antigens, for example. Antibodies The yphC and yqjK polypeptides (or antigenic fragments or analogs of such polypeptides) can be used to raise antibodies useful in the invention, and such polypeptides can be produced by recombinant or peptide synthetic techniques (see, e.g., Solid Phase Peptide Synthesis , supra ; Ausubel et al . , supra) . Likewise, antibodies can be raised against homologs or orthologs of yphC and yqjK (or antigenic fragments and analogs of such homologs and orthologs) . In general, the polypeptides can be coupled to a carrier protein, such as KLH, as described in Ausubel et al . , supra , mixed with an adjuvant, and injected into a host mammal. A "carrier" is a substance that confers stability on, and/or aids or enhances the transport or immunogenicity of, an associated molecule. Antibodies can be purified, for example, by affinity chromatography methods in which the polypeptide antigen is immobilized on a resin. In particular, various host animals can be immunized by injection of a polypeptide of interest. Examples of suitable host animals include rabbits, mice, guinea pigs, and rats. Various adjuvants can be used to increase the immunological response, depending on the host species, including but not limited to Freund' s (complete and incomplete adjuvant) , adjuvant mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpet hemocyanin, dinitrophenol, BCG (bacille Calmette-Guerin) and Corynebacterium parvum. Polyclonal antibodies are heterogeneous populations of antibody molecules derived from the sera of the immunized animals. Antibodies useful in the invention include monoclonal antibodies, polyclonal antibodies, humanized or chimeric antibodies, single chain antibodies, Fab fragments, F(ab') 2 fragments, and molecules produced using a Fab expression library. Monoclonal antibodies (mAbs) , which are homogeneous populations of antibodies to a particular antigen, can be prepared using yphC, yqjK, or homologs or orthologs thereof and standard hybridoma technology (see, e.g., Kohler et al . , Nature, 256:495, 1975; Kohler et al., Eur. J. Immunol . , 6:511, 1976; Kohler et al . , Eur . J. Immunol . , 6:292, 1976; Hammerling et al . , In Monoclonal Antibodies and T Cell Hybridomas , Elsevier, ΝY, 1981; Ausubel et al . , supra) . In particular, monoclonal antibodies can be obtained by any technique that provides for the production of antibody molecules by continuous cell lines in culture, such as those described in Kohler et al . , Nature, 256:495, 1975, and U.S. Patent No. 4,376,110; the human B-cell hybridoma technique (Kosbor et al . , Immunology Today, 4:72, 1983; Cole et al . , Proc . Natl . Acad . Sci . USA, 80:2026, 1983); and the EBV-hybridoma technique (Cole et al . , Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96, 1983). Such antibodies can be of any immunoglobulin class including IgG, IgM, IgE, IgA, IgD, and any subclass thereof. The hybridomas producing the mAbs of this invention can be cultivated in vi tro or in vivo . Once produced, polyclonal or monoclonal antibodies are tested for specific recognition of an essential polypeptide or homolog or ortholog thereof in an immunoassay, such as a Western blot or immunoprecipitation analysis using standard techniques, e.g., as described in Ausubel et al . , supra . Antibodies that specifically bind to the essential polypeptides, or conservative variants and homologs and orthologs thereof, are useful in the invention. For example, such antibodies can be used in an immunoassay to detect an essential polypeptide in pathogenic or non-pathogenic strains of bacteria. Preferably, antibodies of the invention are produced using fragments of the essential polypeptides that appear likely to be antigenic, by criteria such as high frequency of charged residues. In one specific example, such fragments are generated by standard techniques of PCR, and are then cloned into the pGEX expression vector (Ausubel et al . , supra) . Fusion proteins are expressed in E. coli and purified using a glutathione agarose affinity matrix as described in Ausubel, et al . , supra . If desired, several (e.g., two or three) fusions can be generated for each protein, and each fusion can be injected into at least two rabbits. Antisera can be raised by injections in a series, typically including at least three booster injections. Typically, the antisera is checked for its ability to immunoprecipitate a recombinant essential polypeptide or homolog, or unrelated control proteins, such as glucocorticoid receptor, chloramphenicol acetyltransferase, or luciferase . Techniques developed for the production of "chimeric antibodies" (Morrison et al . , Proc . Natl . Acad . Sci . , 81:6851, 1984; Neuberger et al . , Nature, 312:604, 1984; Takeda et al .. Nature, 314:452, 1984) can be used to splice the genes from a mouse antibody molecule of appropriate antigen specificity together with genes from a human antibody molecule of appropriate biological activity. A chimeric antibody is a molecule in which different portions are derived from different animal species, such as those having a variable region derived from a murine mAb and a human immunoglobulin constant region. Alternatively, techniques described for the production of single chain antibodies (U.S. Patents 4,946,778; 4,946,778; and 4,704,692) can be adapted to produce single chain antibodies against an essential polypeptide or homolog or ortholog thereof. Single chain antibodies are formed by linking the heavy and light chain fragments of the Fv region via an amino acid bridge, resulting in a single chain polypeptide. Antibody fragments that recognize and bind to specific epitopes can be generated by known techniques. For example, such fragments can include but are not limited to F(ab') 2 fragments, which can be produced by pepsin digestion of the antibody molecule, and Fab fragments, which can be generated by reducing the disulfide bridges of F(ab') 2 fragments. Alternatively, Fab expression libraries can be constructed (Huse et al . , Science, 246:1275, 1989) to allow rapid and easy identification of monoclonal Fab fragments with the desired specificity. Polyclonal and monoclonal antibodies that specifically bind to essential polypeptides, homologs, or orthologs can be used, for example, to detect expression of an essential gene, homolog, or ortholog in another bacteria. For example, an essential polypeptide can be readily detected in conventional immunoassays of bacteria cells or extracts. Examples of suitable assays include, without limitation, Western blotting, ELISAs, radioimmune assays, and the like. Assay for Antibacterial Agents The invention provides methods for identifying antibacterial agents. Without being bound by any particular theory as to the biological mechanism involved, the new antibacterial agents are thought to inhibit specifically (1) the function of the yphC or yqjK polypeptide (s) , or homologs or orthologs thereof, or (2) expression of the yphC or yqjK genes, or homologs or orthologs thereof . Alignment of the yphC protein sequence with similar sequences from the GenBank database suggests that the yphC protein has GTPase activity. Similarly, the alignment of the yqjK protein with sequences from GenBank suggests that the yqjK protein has arylsulfatase activity. In experiments designed to test whether the yphC and yqjK proteins have the proposed biochemical activities, the yphC protein was shown to have GTPase activity, and the yqjK protein was shown to have arylsulfatase activity. The enzymatic activity of each protein suggests novel features of each enzyme. For example, the yphC protein contains two GTP-binding sites, but only one of the sites appears to be active. The yqjK protein has phosphatase activity in addition to arylsulfatase activity, and the protein carries out an activation step that is catalyzed by manganese ions. In the genetic experiments described herein, the observed biochemical activities of yphC and yqjK were shown to be linked to the essential nature of the proteins. Mutants of the GTP-binding sites of the yphC gene result in proteins that lack GTPase activity and that are unable to complement null yphC mutants. Similarly, yqjK mutants that lack arylsulfatase activity are unable to complement null yqjK mutants. Additionally, mutants that lacked arylsulfatase activity also lacked phosphatase activity and vice versa . These experiments indicate that inhibition of the enzymatic activities of yphC or yqjK in cell cultures impairs the viability of the cells and results in cell death. Thus, these biochemical activities can be used in vivo or in vi tro, alone or in combination with any art-known methods to detect these activities, to identify antibacterial agents. In various suitable methods, screening for antibacterial agents is accomplished by identifying those compounds (e.g., small organic molecules) that inhibit the activity of an essential polypeptide or the expression of an essential gene. Screening for antibacterial agents can be accomplished by (i) identifying those compounds that interact with or bind to an essential polypeptide and (ii) further testing such compounds for their ability to inhibit bacterial growth in vi tro or in vivo. Examples of suitable screening methods are set forth in U.S. Patent Nos. 5,679,582 and 5,585,277, which are incorporated herein by reference. Briefly, in these methods, a target protein is incubated in the presence of a test compound (i.e., test ligand) to produce a "test combination, " and the target protein is incubated in the absence of a test compound to produce a "control combination." The test and control combinations are then treated to cause a detectable fraction of the target protein to exist in a partially or totally unfolded state. The extent to which the target protein occurs in a folded state, an unfolded state, or both, in the test and control combinations is then determined. When the target protein is present in the folded state to a greater or lesser extent in the test combination than in the control combination, the test compound is a compound that binds the target protein. In an alternative method, binding of a test compound to a target protein is detected using capillary electrophoresis . Briefly, test compounds (e.g., small molecules) that bind to the target protein cause a change in the electrophoretic mobility of the target protein during capillary electrophoresis. Suitable capillary electrophoresis methods are known in the art (see, e.g., Freitag, J. Chromatography B, Biomedical Sciences & Applications: 722 (1-2) : 279-301 , Feb. 5, 1999; Chu and Cheng, Cellular & Molecular Life Sciences: 54 (7) : 663 -83 , July 1998; Thormann et al . , Forensic Science International: 92(2-3): 157-83, April 5, 1998; Rippel et al., Electrophoresis: 18(12-13): 2175-83, Nov. 1997; Hage and Tweed, J. Chromatography. B, Biomedical Sciences & Applications: 699 (1-2) :499-525, October 10, 1997; Mitchelson et al . , Trends In Biotechnology: 15 (11) :448-58, Nov. 1997; Jenkins and Guerin J. Chromatography B. Biomedical Applications: 682 (1) :23-34 , June 28, 1996; and Chen and Gallo, Electrophoresis: 19 (16-17) .2861-9, Nov. 1998. Inhibitors of yphC can also be identified in the following biochemical assay for detection of GTPase inhibitors. This assay uses a colorimetric detection system for the detection of nanomolar amounts of inorganic phosphate . The assay can be carried out in a clear bottom 96-well microplate (e.g., Corning-COSTAR Catalog #9710) . A 20 μl aliquot of each test compound in 10% DMSO is placed into each well of the plate, except those wells that are used as controls. A 20 μl aliquot of 420 μM GTP then is added to each well of the plate, except those wells that are used for control reactions. 20 μl of IC 50 controls, containing 225 μM GDP, is dispensed into two of the control wells; 20 μl of IC 100 (2.25 ml of 0.5M EDTA in 12.75 ml of 420 μM GTP) is dispensed into two other control wells; and 20 μl of no inhibition controls containing 420 μM GTP is dispensed into four other control wells. 20 μl of 2X-buffer (100 mM Tris HCL, 500 mM KCl , 10 mM MGC1 2 , 0.2 mg/ml Acetylated-BSA, H 2 0) plus yphC enzyme solution (to provide 1 μg/well) then is dispensed into each well, and the plate is incubated at room temperature for 3.5 hours. To stop the enzyme reaction, 150 μl of 0.045% Malachite Green/35 mM EDTA solution is added to each well. After 25 minutes, 50 μl of 15% citrate is added to each well to prevent further color development. The samples then are mixed vigorously (e.g., with a TOMTEC-Quadra-96, Model 320) until a homogenous solution results. The plates subsequently are read using a plate reader (e.g., a Wallac-Victor 2 plate reader) set at a wavelength of 650 nm. Generally, the plates should be read within 24 hours of adding the Malachite Green and citrate. The percent inhibition for each sample well can be calculated as follows. The average of the two wells that contained IC 100 controls can be used as the background counts. The percent inhibition can calculated according the following formula: % inhibition = [1- (sample counts - background counts) / (average counts - background counts)] x 100. Test compounds that produce greater than 40% inhibition may be retested with a dose response at a higher concentration, if desired. Other methods for identifying antibacterial agents include various cell-based methods for identifying polypeptides that bind yphC, yqjK, or homologs or orthologs thereof, such as the conventional two-hybrid assays of protein/protein interactions (see e.g., Chien et al., Proc . Natl . Acad . Sci . USA, 88:9578, 1991; Fields et al., U.S. Pat. No. 5,283,173; Fields and Song, Nature, 340:245, 1989; Le Douarin et al . , Nucleic Acids Research, 23:876, 1995; Vidal et al . , Proc . Natl . Acad . Sci . USA, 93:10315-10320, 1996; and White, Proc . Natl . Acad . Sci . USA, 93:10001-10003, 1996). Generally, the two-hybrid methods involve reconstitution of two separable domains of a transcription factor in a cell . One fusion protein contains the essential polypeptide (or homolog or ortholog thereof) fused to either a transactivator domain or DΝA binding domain of a transcription factor (e.g., of Gal4) . The other fusion protein contains a test polypeptide fused to either the DΝA binding domain or a transactivator domain of a transcription factor. Once brought together in a single cell (e.g., a yeast cell or mammalian cell) , one of the fusion proteins contains the transactivator domain and the other fusion protein contains the DΝA binding domain. Therefore, binding of the essential polypeptide to the test polypeptide (i.e., candidate antibacterial agent) reconstitutes the transcription factor. Reconstitution of the transcription factor can be detected by detecting expression of a gene (i.e., a reporter gene) that is operably linked to a DΝA sequence that is bound by the DΝA binding domain of the transcription factor. Kits for practicing various two-hybrid methods are commercially available (e.g., from Clontech; Palo Alto, CA) . In another exemplary assay, but not the only assay, a promoter that responds to depletion of the essential polypeptide by upregulation or downregulation is linked to a reporter gene (e.g., 3-galactosidase, gus, or GFP) , as described above. A bacterial strain containing this reporter gene construct is then exposed to test compounds. Compounds that inhibit the essential polypeptide (or other polypeptides in the essential pathway in which the essential polypeptide participates) will cause a functional depletion of the essential polypeptide and therefore lead to an upregulation or downregulation of expression the reporter gene. Because the polypeptides described herein are essential for the survival of bacteria, compounds that inhibit the essential polypeptides in such an assay are expected to be antibacterial agents and can be further tested, if desired, in conventional susceptibility assays. The methods described above can be used for high throughput screening of numerous test compounds to identify candidate antibacterial (or anti-bacterial) agents. Having identified a test compound as a candidate antibacterial agent, the candidate antibacterial agent can be further tested for inhibition of bacterial growth in vi tro or in vivo (e.g., using an animal, e.g., rodent, model system) if desired. Using other, art-known variations of such methods, one can test the ability of a nucleic acid (e.g., DNA or RNA) used as the test compound to bind yphC, yqjK, or a homolog or ortholog thereof. In vi tro, further testing can be accomplished by means known to those in the art such as an enzyme inhibition assay or a whole-cell bacterial growth inhibition assay. For example, an agar dilution assay identifies a substance that inhibits bacterial growth. Microtiter plates are prepared with serial dilutions of the test compound, adding to the preparation a given amount of growth substrate, and providing a preparation of bacteria. Inhibition of bacterial growth is determined, for example, by observing changes in optical densities of the bacterial cultures. Inhibition of bacterial growth is demonstrated, for example, by comparing (in the presence and absence of a test compound) the rate of growth or the absolute growth of bacterial cells. Inhibition includes a reduction of one of the above measurements by at least 20%. Particularly potent test compounds may further reduce the growth rate (e.g., by at least 25%, 30%, 40%, 50%, 75%, 80%, or 90%) . Rodent (e.g., murine) and rabbit animal models of bacterial infections are known to those of skill in the art, and such animal model systems are accepted for screening antibacterial agents as an indication of their therapeutic efficacy in human patients. In a typical in vivo assay, an animal is infected with a pathogenic strain of bacteria, e.g., by inhalation of bacteria such as Streptococcus pneumoniae, and conventional methods and criteria are used to diagnose the mammal as being afflicted with a bacterial infection. The candidate antibacterial agent then is administered to the mammal at a dosage of 1-100 mg/kg of body weight, and the mammal is monitored for signs of amelioration of disease. Alternatively, the test compound can be administered to the mammal prior to infecting the mammal with the bacteria, and the ability of the treated mammal to resist infection is measured. Of course, the results obtained in the presence of the test compound should be compared with results in control animals, which are not treated with the test compound. Administration of candidate antibacterial agents to the mammal can be carried out as described below, for example. Pharmaceutical Formulations Treatment includes administering a pharmaceutically effective amount of a composition containing an antibacterial agent to a subject in need of such treatment, thereby inhibiting bacterial growth in the subject. Such a composition typically contains from about 0.1 to 90% by weight (such as 1 to 20% or 1 to 10%) of an antibacterial agent of the invention in a pharmaceutically acceptable carrier. Solid formulations of the compositions for oral administration may contain suitable carriers or excipients, such as corn starch, gelatin, lactose, acacia, sucrose, microcrystalline cellulose, kaolin, mannitol, dicalcium phosphate, calcium carbonate, sodium chloride, or alginic acid. Disintegrators that can be used include, without limitation, micro-crystalline cellulose, corn starch, sodium starch glycolate and alginic acid. Tablet binders that may be used include acacia, methylcellulose, sodium carboxymethylcellulose, polyvinylpyrrolidone (Povidone) , hydroxypropyl methylcellulose, sucrose, starch, and ethylcellulose . Lubricants that may be used include magnesium stearates, stearic acid, silicone fluid, talc, waxes, oils, and colloidal silica. Liquid formulations of the compositions for oral administration prepared in water or other aqueous vehicles may contain various suspending agents such as methylcellulose, alginates, tragacanth, pectin, kelgin, carrageenan, acacia, polyvinylpyrrolidone, and polyvinyl alcohol. The liquid formulations may also include solutions, emulsions, syrups and elixirs containing, together with the active compound(s), wetting agents, sweeteners, and coloring and flavoring agents. Various liquid and powder formulations can be prepared by conventional methods for inhalation into the lungs of the mammal to be treated. Injectable formulations of the compositions may contain various carriers such as vegetable oils, dimethylacetamide, dimethylformamide, ethyl lactate, ethyl carbonate, isopropyl myristate, ethanol, polyols (glycerol, propylene glycol, liquid polyethylene glycol, and the like) . For intravenous injections, water soluble versions of the compounds may be administered by the drip method, whereby a pharmaceutical formulation containing the antibacterial agent and a physiologically acceptable excipient is infused. Physiologically acceptable excipients may include, for example, 5% dextrose, 0.9% saline, Ringer's solution or other suitable excipients. Intramuscular preparations, a sterile formulation of a suitable soluble salt form of the compounds can be dissolved and administered in a pharmaceutical excipient such as Water-for-Injection, 0.9% saline, or 5% glucose solution. A suitable insoluble form of the compound may be prepared and administered as a suspension in an aqueous base or a pharmaceutically acceptable oil base, such as an ester of a long chain fatty acid, (e.g., ethyl oleate) . A topical semi-solid ointment formulation typically contains a concentration of the active ingredient from about 1 to 20%, e.g., 5 to 10% in a carrier such as a pharmaceutical cream base. Various formulations for topical use include drops, tinctures, lotions, creams, solutions, and ointments containing the active ingredient and various supports and vehicles. The optimal percentage of the antibacterial agent in each pharmaceutical formulation varies according to the formulation itself and the therapeutic effect desired in the specific pathologies and correlated therapeutic regimens. Appropriate dosages of the antibacterial agents can be readily determined by those of ordinary skill in the art of medicine by monitoring the mammal for signs of disease amelioration or inhibition, and increasing or decreasing the dosage and/or frequency of treatment as desired. The optimal amount of the antibacterial compound used for treatment of conditions caused by or contributed to by bacterial infection may depend upon the manner of administration, the age and the body weight of the subject, and the condition of the subject to be treated. Generally, the antibacterial compound is administered at a dosage of 1 to 100 mg/kg of body weight, and typically at a dosage of 1 to 10 mg/kg of body weight . Other Embodiments The invention also features fragments, variants, analogs, and derivatives of the polypeptides described above that retain one or more of the biological activities of the yphC and yqjK polypeptides, e.g., GTPase or sulfatase activities. Included within the invention are naturally-occurring and non-naturally- occurring variants. Compared with the naturally- occurring essential gene sequences depicted in Figs. 1 and 3 , the nucleic acid sequences encoding variants may have a substitution, deletion, or addition of one or more nucleotides. The preferred variants retain a function of an essential polypeptide, e.g., as determined in a complementation assay. It is to be understood that, while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims. For example, other art -known assays to detect interactions of test compounds with proteins, or to detect inhibition of bacterial growth also can be used with the essential genes, gene products, and homologs and orthologs thereof.

Claims: What is claimed is: 1. An isolated nucleic acid molecule that is at least 85% identical to SEQ ID NO:l; SEQ ID NO:4; or SEQ ID NO: 7. 2. An isolated nucleic acid molecule of claim 1, wherein the nucleic acid molecule encodes: a yphC polypeptide comprising the amino acid sequence of SEQ ID NO:2, as depicted in Fig. 1 ; a yphC polypeptide comprising the amino acid sequence of SEQ ID NO: 5, as depicted in Figs. 2A-2B; or a yqjK polypeptide comprising the amino acid sequence of SEQ ID NO: 8, as depicted in Fig. 3. 3. An isolated nucleic acid molecule comprising a nucleic acid sequence selected from the group consisting of: (1) the sequence of SEQ ID NO:l, as depicted in Fig. 1, or degenerate variants thereof; (2) the sequence of SEQ ID NO:l, as depicted in Fig. 1, or degenerate variants thereof, wherein T is replaced by U; (3) nucleic acid sequences complementary to sequences of (1) and (2) ; (4) nucleic acid fragments of sequences of (1) , (2) , and (3) that are at least 15 base pairs in length and which hybridize under stringent conditions to genomic DNA encoding the polypeptide of SEQ ID NO: 2; (5) the sequence of SEQ ID NO : 4 , as depicted in Figs. 2A-2B, or degenerate variants thereof; (6) the sequence of SEQ ID NO: 4, as depicted in Figs. 2A-2B, or degenerate variants thereof, wherein T is replaced by U; (7) nucleic acids complementary to (5) and (6) ; (8) the sequence of SEQ ID NO : 7 , as depicted in Fig. 3, or degenerate variants thereof; (9) the sequence of SEQ ID NO : 7 , or degenerate variants thereof, wherein T is replaced by U; (10) nucleic acid sequences complementary to sequences of (8) and (9) ; and (11) nucleic acid fragments of sequences of (8) , (9) , and (10) that are at least 15 base pairs in length and that hybridize under stringent conditions to genomic DNA encoding the polypeptide of SEQ ID NO: 8. 4. An isolated nucleic acid molecule that is at least 15 base pairs in length and hybridizes under stringent conditions to SEQ ID NO:l or SEQ ID NO:4. 5. A vector comprising a nucleic acid molecule of claim 1. 6. A host cell comprising an exogenously introduced nucleic acid molecule of claim 1, 2, or 3. 7. An isolated polypeptide encoded by a nucleic acid molecule of claim 1. 8. A method for identifying an antibacterial agent, the method comprising: (a) contacting a yphC or yqjK polypeptide with a test compound; and (b) detecting interaction of the test compound with the yphC or yqjK polypeptide, wherein interaction indicates that the test compound is an antibacterial agent . 9. A method of claim 8, further comprising: (c) determining whether a test compound that interacts with the polypeptide inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of a test compound that interacts with the polypeptide, wherein inhibition of growth indicates that the test compound is an antibacterial agent. 10. A method of claim 8, wherein the polypeptide is derived from a non-pathogenic bacterial strain. 11. A method of claim 8, wherein the polypeptide is derived from a pathogenic bacterial strain. 12. A method of claim 8, wherein the test compound is selected from the group consisting of polypeptides, ribonucleic acids, small molecules, and deoxyribonucleic acids. 13. A pharmaceutical formulation comprising an antibacterial agent identified by the method of claim 8, and a pharmaceutically acceptable excipient . 14. A method for treating a bacterial infection in an organism, the method comprising administering to the organism a therapeutically effective amount of the pharmaceutical formulation of claim 13. 15. The method of claim 14, wherein the organism is a human. 16. An antibody that specifically binds to a polypeptide of claim 7. 17. A method of claim 8, wherein said interaction is detected by detecting a decrease in function of the polypeptide contacted with the test compound; and further comprising : determining whether a test compound that decreases function of a contacted polypeptide inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of a test compound that decreases function of a contacted polypeptide, wherein inhibition of growth indicates that the test compound is an antibacterial agent . 18. A method for identifying an antibacterial agent, the method comprising: (a) contacting a nucleic acid encoding yphC or yqjK with a test compound; and (b) detecting interaction of the test compound with the nucleic acid, wherein interaction indicates that the test compound is an antibacterial agent. 19. A method of claim 18, further comprising: (c) determining whether a test compound that interacts with the nucleic acid inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of the test compound that interacts the nucleic acid, wherein inhibition of growth indicates that the test compound is an antibacterial agent. 20. A method for identifying an antibacterial agent, the method comprising: (a) contacting an ortholog of a yphC or yqjK polypeptide with a test compound; and (b) detecting interaction of the test compound with the ortholog, wherein interaction indicates that the test compound is an antibacterial agent. 21. A method of claim 20, further comprising: (c) determining whether a test compound that interacts with the ortholog inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of the test compound that interacts with the ortholog, wherein inhibition of growth indicates that the test compound is an antibacterial agent. 22. A method of claim 21, wherein the ortholog is selected from the group consisting of B-yphC, B-yqjK, yfgK, and elaC. 23. A method of claim 21, wherein the ortholog is derived from a pathogenic bacterium. 24. A method for identifying an antibacterial agent, the method comprising: (a) contacting an ortholog of a yphC or yqjK polypeptide with a test compound; (b) detecting a decrease in function of the ortholog contacted by the test compound; and (c) determining whether a test compound that decreases function of a contacted ortholog inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of a test compound that decreases function of a contacted ortholog, wherein inhibition of growth indicates that the test compound is an antibacterial agent. 25. A method of claim 24, wherein the ortholog is selected from the group consisting of B-yphC, B-yqjK, yfgK, and elaC. 26. A method for identifying an antibacterial agent, the method comprising: (a) contacting a nucleic acid encoding an ortholog of yphC or yqjK with a test compound; and (b) detecting interaction of the test compound with the nucleic acid, wherein interaction indicates that the test compound is an antibacterial agent. 27. A method of claim 26, further comprising: (c) determining whether a test compound that interacts with the nucleic acid inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of a test compound that interacts with the nucleic acid, wherein inhibition of growth indicates that the test compound is an antibacterial agent. 28. A method of claim 27, wherein the ortholog is selected from the group consisting of B-yphC, B-yqjK, yfgK, and elaC. 29. A method for treating a mammal having a Streptococcus pneumonia infection, the method comprising inhibiting the function of a yphC or yqjK polypeptide in Streptococcus pneumonia infecting the mammal .

Inventors: FRITZ CHRISTIAN, GUZMAN LUZ-MARIA, YOUNGMAN PHILIP, FRITZ, CHRISTIAN, GUZMAN, LUZ-MARIA, YOUNGMAN, PHILIP
Applicants: MILLENNIUM PHARM INC, MILLENNIUM PHARMACEUTICALS, INC.
Application year: 1999
Main classification: C12N
IPC codes: A61K45, A61P31, C07K14, C07K16, C12N1, C12N5, C12N15, C12Q1, G01N33