# Patent

## Title
Use of ylqF, yqeG, yybQ, and ysxC, essential bacterial genes and polypeptides

## Abstract

Disclosed are genes found in Streptococcus pneumoniae that are essential for survival for a wide range of bacteria. These genes are termed “S-ylqF,” “S-yqeG,” “S-yybQ,” “S-yerL,” and “S-ysxC.” These genes and the 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.

## Background
RELATED APPLICATION INFORMATION
This application is a divisional of and claims priority under 35 USC §120 to application Ser. No. 10/423,330, filed on Apr. 25, 2003 (now U.S. Pat. No. 6,815,177), which is a divisional of application Ser. No. 10/023,528, filed on Dec. 17, 2001 (now U.S. Pat. No. 6,638,729), which is a divisional of application Ser. No. 09/222,939, filed on Dec. 30, 1998 (now U.S. Pat. No. 6,372,448), all of which are incorporated herein by reference in their entirety.


FIELD OF THE INVENTION
The invention relates to the use of ylqF, yqeG, yybQ, yerL, and ysxC, essential bacterial genes and polypeptides 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 identification of ten genes of the gram positive bacterium Streptococcus pneumoniae and of Bacillus subtilis as being essential for survival. The Streptococcus pneumoniae genes are termed “S-ylqF,” “S-yqeG,” “S-yybQ,” “S-yerL,” and “S-ysxC.” The orthologs of these genes in Bacillus subtilis are termed “B-ylqF,” “B-yqeG,” “B-yybQ,” “B-yerL,” and “B-ysxC,” respectively. The terms “ylqF,” “yqeG,” “yybQ,” “yerL,” and “ysxC” genes and polypeptides are used to refer to the Streptococcus pneumoniae and Bacillus subtilis genes and polypeptides, as well as their homologs and orthologs, collectively. While “homologs” are structurally similar genes contained within a species, “orthologs” are functionally equivalent genes from other species (within or outside of a given genus, e.g., from E. coli). These genes are considered “essential”, genes, and their polypeptides are considered “essential” polypeptides. Each gene and polypeptide can be used in methods for identifying similar genes and polypeptides in pathogenic and non-pathogenic microorganisms. Each polypeptide can be used to identify compounds that are inhibitors of the pathogens in which the polypeptide (ylqF, yqeG, yybQ, yerL, or ysxC) is expressed. Such inhibitors attenuate bacterial growth by inhibiting the activity of ylqF, yqeG, yybQ, yerL, or ysxC polypeptide, or by inhibiting gene transcription or translation.
The amino acid and nucleic acid sequences of the essential polypeptides described herein are set forth in FIGS. 1–10 as shown in Table 1.







TABLE 1







Essential Polypeptides Disclosed Herein














SEQ ID NO. of
SEQ ID NO. of





the Coding
the Non-coding


Essential

SEQ ID NO. of
Strand of the
Strand of the


Nucleic Acid
FIG.
Amino Acid
Nucleic Acid
Nucleic Acid


or Polypeptide
NO.
Sequence
Sequence
Sequence














S-ylqF
1A–
2
1
3



1B


S-yqeG
2A–
5
4
6



2B


S-yybQ
3A–
8
7
9



3C


S-yerL
4
11
10
12


S-ysxC
5
14
13
15


B-ylqF
6A–
17
16
18



B


B-yqeG
7
20
19
21


B-yybQ
8A–
23
22
24



8B


B-yerL
9
26
25
27


B-ysxC
10
29
28
30









Since these genes have been identified and shown to be essential for survival, these genes and polypeptides (including homologs and orthologs of the sequences disclosed herein) 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 these essential polypeptides participate. This inhibition can be caused by small molecules interacting with (e.g., binding directly or indirectly to) the ylqF polypeptide, for example, or other essential polypeptides in that pathway.
In an exemplary assay, but not the only assay, a promoter that responds to depletion of the ylqF, yqeG, yybQ, yerL, or ysxC 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 the polypeptide, the gene encoding the polypeptide is deleted from the genome and replaced with a version of the gene in which the sequence encoding the polypeptide (ylqF, yqeG, yybQ, yerL, or ysxC) 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 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 a promoter that drives expression of the identified RNA species. Such promoters are up- or down-regulated by depletion of the ylqF, yqeG, yybQ, yerL, or ysxC polypeptide.
Having identified a promoter(s) that is up- or down-regulated by depletion of the essential polypeptide (ylqF, yqeG, yybQ, yerL, or ysxC), the promoter(s) is operably linked to a reporter gene (e.g., β-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 ylqF, yqeG, yybQ, yerL, or ysxC polypeptide (or other polypeptides in the essential pathway in which the polypeptide participates) will cause a functional depletion of the polypeptide and therefore lead to an upregulation or downregulation of expression of the reporter gene. Because the polypeptides described herein are essential for the survival of bacteria, compounds that inhibit these polypeptides in such an assay are expected to be antibacterial and can be further tested, if desired, in standard susceptibility assays.
Another suitable method for identifying antibacterial compounds involves screening for small molecules that specifically interact with (i.e., bind directly or indirectly to) the ylqF, yqeG, yybQ, yerL, or ysxC polypeptide. A variety of suitable interaction and binding assays are known in the art as described, for example, in U.S. Pat. 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 a ylqF polypeptide by measuring the ability of the small molecule to stabilize the ylqF polypeptide in its folded, rather than unfolded, state. More specifically, one can measure the degree of protection against unfolding that is afforded by the test compound. Test compounds that bind, for example, the ylqF polypeptide with high affinity cause, for example, a large shift in the temperature at which the polypeptide is denatured. Test compounds that stabilize the ylqF polypeptide in a folded state can be further tested for antibacterial activity in a standard susceptibility assay. Similar assays can be performed with the other essential polypeptides described herein.
In a related method for identifying antibacterial compounds, the ylqF, yqeG, yybQ, yerL, or ysxC polypeptide is used to isolate peptide or nucleic acid ligands that specifically bind the polypeptide. These peptide or nucleic acid ligands are then used in a displacement screen to identify small molecules that interact with the polypeptide. Such assays can be carried out essentially as described, above.
Another suitable method for identifying inhibitors of the ylqF, yqeG, yybQ, yerL, or ysxC polypeptides involves identifying a biochemical activity of the polypeptide and then screening for small molecule inhibitors of the activity using, for example, a high throughput screening method.
The ylqF, yqeG, yybQ, yerL, and ysxC polypeptides 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 ylqF, yqeG, yybQ, yerL, or ysxC polypeptides are candidate antibacterial agents, in contrast to compounds that do not interact with the ylqF, yqeG, yybQ, yerL, or ysxC 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 ylqF, yqeG, yybQ, yerL, or ysxC polypeptides.
The invention also includes a method for identifying an antibacterial agent where the method entails:
(a) contacting a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide with a test compound; (b) detecting binding of the test compound to the polypeptide; and, optionally,
(c) determining whether a test compound that binds to the polypeptide inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of the test compound that binds to the polypeptide, as an indication that the test compound is an antibacterial agent.
In still another method, interaction of a test compound with a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide (e.g., binding) can be detected in a conventional two-hybrid system for detecting protein/protein interactions (e.g., in yeast or mammalian cells). A test polypeptide found to interact with the ylqF, yqeG, yybQ, yerL, or ysxC polypeptide can be further tested for antibacterial activity in a conventional susceptibility assay. Generally, in such two-hybrid methods, (a) the ylqF, yqeG, yybQ, yerL, or ysxC polypeptide is provided as a fusion protein that includes the essential 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 compound is provided as a fusion protein that includes a 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 ylqF, yqeG, yybQ, yerL, or ysxC polypeptide is detected as a reconstitution of a transcription factor. Homologs and orthologs of the polypeptides described herein can be used in such 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 ylqF, yqeG, yybQ, yerL, or ysxC is used to identify a compound that decreases the expression of the polypeptide in vivo (i.e., in a cell). Such compounds can be used as antibacterial agents. To discover such compounds, cells that express a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide are cultured, exposed to a test compound (or a mixture of test compounds), and the level of ylqF, yqeG, yybQ, yerL, and ysxC expression or activity is compared with the level of 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 a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide, the test compound(s) can be added at varying concentrations to the culture medium of cells that express the polypeptide, 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 polypeptide is then measured, for example, by Northern blot PCR analysis or RNAse protection analyses using a nucleic acid molecule described herein 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 polypeptide. Because the ylqF, yqeG, yybQ, yerL, and ysxC polypeptides (including homologs and orthologs of the sequences disclosed herein) are essential for survival, test compounds that inhibit the expression and/or function of any of these polypeptides will inhibit growth of, or kill, the cells that express such 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 ylqF, yqeG, yybQ, yerL, or ysxC polypeptide. More generally, binding of test a compound to the polypeptide can be detected either in vitro or in vivo. If desired, the above-described methods for identifying compounds that modulate the expression of the essential polypeptides of the invention can be combined with measuring the levels of the essential polypeptides of the invention expressed in the cells, e.g., by performing a Western blot analysis using antibodies that bind one of the polypeptides.
Regardless of the source of the test compound, the essential polypeptides of the invention can be used to identify compounds that inhibit the activity of a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide, or inhibit transcription of a gene encoding such a polypeptide, or inhibit translation of the mRNA transcribed from such a gene. These antibacterial agents can be used to inhibit a wide spectrum of pathogenic or non-pathogenic bacterial strains, particularly gram-positive bacteria.
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 a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide can facilitate detection of such polypeptides in various bacterial strains (or extracts thereof). These antibodies also are useful for detecting binding of a test compound to such polypeptides (e.g., using the assays described herein). In addition, monoclonal antibodies that bind these essential polypeptides can themselves be used as antibacterial agents.
The invention further features methods of identifying from a large group of mutant strains 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 lower overall dosage.
In various embodiments, the ylqF, yqeG, yybQ, yerL, and ysxC polypeptides used in the assays described herein are derived from a non-pathogenic or pathogenic gram positive bacterium. For example, these polypeptides can be derived from a Streptococcus strain, such as Streptococcus pneumoniae, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus endocarditis, Streptococcus faecium, Streptococcus sangus, Streptococcus viridans, and Streptococcus hemolyticus. Suitable orthologs of the S-ylqF, S-yqeG, S-yybQ, S-yerL, and S-ysxC genes can be derived from a wide spectrum of bacteria, such as Bacillus subtilis and E. coli. The Bacillus subtilis orthologs are B-ylqF, B-yqeG, B-yybQ, B-yerL, B-and B-ysxC, respectively.
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 a broad spectrum of antibacterial activity.
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
FIGS. 1A–1B are a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a S-ylqF polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:2, 1, and 3, respectively).
FIGS. 2A–2B are a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a S-yqeG polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:5, 4, and 6, respectively).
FIGS. 3A–3C are a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a S-yybQ polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:8, 7, and 9, respectively).
FIG. 4 is a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a S-yerL polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:11, 10 and 12, respectively).
FIG. 5 is a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a S-ysxC polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:14, 13, and 15, respectively).
FIGS. 6A–6B are a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a B-ylqF polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs: 17, 16, and 18, respectively).
FIG. 7 is a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a B-yqeG polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:20, 19, and 21, respectively).
FIGS. 8A–8B are a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a B-yybQ polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:23, 22, and 24, respectively).
FIG. 9 is a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a B-yerL polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:26, 25, and 27, respectively).
FIG. 10 is a listing of the amino acid and coding strand and non-coding strand nucleic acid sequences of a B-ysxC polypeptide and gene from a Streptococcus pneumoniae strain (SEQ ID NOs:29, 28, and 30, respectively).
FIG. 11 is a schematic representation of the PCR strategy used to produce DNA molecules used for targeted deletions of essential genes in Streptococcus pneumoniae. 
FIG. 12 is a schematic representation of the strategy used to produce targeted deletions of essential genes in Streptococcus pneumoniae. 



DETAILED DESCRIPTION OF THE INVENTION
Various genes in the bacterium Streptococcus pneumoniae have each been found to be essential for the survival of this bacterium. These essential genes have been termed S-ylqF, S-yqeG, S-yybQ, S-yerL, and S-ysxC. These genes can be used to identify homologs and orthologs in pathogenic microorganisms, such as pathogenic gram-positive bacteria (e.g., Bacillus). The Bacillus subtilis orthologs of these essential Streptococcus pneumoniae genes have been termed B-ylqF, B-yqeG, B-yybQ, B-yerL, and B-ysxC and are essential for survival. The essential polypeptides described herein can be used in methods for identifying compounds that are inhibitors of the pathogens in which these polypeptides are expressed (e.g., pathogenic and non-pathogenic bacteria, find gram-positive bacteria in particular).
Nucleic acids used in practicing the methods of the invention 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 S-ylqF or B-ylqF nucleotide sequence is at least 80% identical to the nucleotide sequence represented by SEQ ID NO:1, as depicted in FIGS. 1A–1B. Likewise, nucleic acid sequences that are substantially identical to each of the other essential genes disclosed herein are at least 80% identical to each of the nucleotide sequences presented by SEQ ID NOs:1, 4, 7, 10, 13, 18, 21, 22, 25, and 30, as depicted in FIGS. 2A–10 respectively. 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 acid 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×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 the nucleic acid molecules described herein. BLAST protein searches can be performed with the XBLAST program, score=50, wordlength=3 to obtain amino acid sequences homologous to yneS protein molecules. 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 techniques similar to those 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 the ylqF, yqeG, yybQ, yerL, or ysxC 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 useful are nucleic acids encoding fusion proteins in which a portion of one of the essential polypeptides described herein 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. Also useful in practicing the methods of the invention are isolated polypeptides (and the nucleic acids that encode these polypeptides) that include a first portion and a second portion; the first portion includes, e.g., a ylqF 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. Additional useful nucleic acids include nucleic acids that encode, for example, a ylqF 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.
Nucleic acids that hybridize, e.g., under stringent or highly stringent hybridization conditions (as defined herein) to all or a portion of a nucleotide sequence represented by SEQ ID NOs:1, 4, 7, 10, 13, 18, 21, 22, 25, or 30, or their complements, can be used to identify additional homologs or orthologs of the essential genes described herein. 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 one of the essential polypeptides described herein or its complement. Hybridizing nucleic acids of the type described herein can be used 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, 4, 7, 10, 13, 18, 21, 22, 25, or 30, are considered “antisense oligonucleotides.”
Cells that may be used in practicing the methods of the invention include various engineered cells, e.g., transformed host cells, that contain one of the essential nucleic acids 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 one of the essential polypeptides described herein. Both prokaryotic and eukaryotic cells are included, e.g., bacteria, such as Streptococcus, Bacillus, and the like.
Various genetic constructs (e.g., vectors and plasmids) that include one of the essential nucleic acids described herein that is operably linked to a transcription and/or translation sequence to enable expression (e.g., expression vectors) can be used in practicing the methods of the invention. A selected nucleic acid, e.g., a DNA molecule encoding a ylqF polypeptide, is “operably linked” to a transcription 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 nucleic acid.
The terms “protein” and “polypeptide” both refer to any chain of amino acids, regardless of length or post-translational modification (e.g., glycosylation or phosphorylation). Thus, the term ylqF polypeptide, for example, includes full-length, naturally occurring, isolated ylqF proteins, as well as recombinantly or synthetically produced polypeptides that correspond to the full-length, naturally occurring protein, or to a portion of the naturally occurring or synthetic polypeptide.
A purified or isolated compound is a composition that is at least 60% by weight the compound of interest, e.g., a ylqF 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 polypeptides include a sequence substantially identical to all or a portion of a naturally occurring ylqF, yqeG, yybQ, yerL, or ysxC polypeptide, e.g., including all or a portion of a sequence shown in any of FIGS. 1A–10. Polypeptides “substantially identical” to the essential polypeptides described herein have an amino acid sequence that is at least 80% identical to the amino acid sequence of a polypeptide, represented by SEQ ID NO:2, 5, 8, 11, 14, 17, 20, 23, 26, or 29 (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 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 methods of the invention also make use of purified or isolated antibodies that specifically bind to one of the essential polypeptides described herein. An antibody “specifically binds” to a particular antigen, e.g., a ylqF 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 a ylqF polypeptide.
The various nucleic acids described herein can be used in a method of obtaining a gene related to an essential gene described herein. Such a method can entail obtaining a labeled probe that includes an isolated nucleic acid which encodes all or a portion of a ylqF nucleic acid, for example, 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 ylqF gene (e.g., homologs and orthologs of ylqF).
Identification the S-ylqF, S-yqeG, S-yybQ, S-yerL, and S-ysxC Genes
As shown by the experiments described below, the S-ylqF, S-yqeG, S-yybQ, S-yerL, and S-ysxC genes each are essential for survival of Streptococcus pneumoniae. Streptococcus pneumoniae is available from the ATCC. In general, and for the examples set forth below, each of ylqF, yqeG, yybQ, yerL, and ysxC genes can be identified by creating targeted deletions of genes of interest in a bacterium, e.g., S. pneumoniae. The gene of interest was 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 gene of interest selected for targeted deletion satisfied four criteria, as determined by comparing the sequences with the GenBank database of nucleotide sequence (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 S-ylqF, S-yqeG, S-yybQ, S-yerL, and S-ysxC met each of these criteria, suggesting that a compound that inhibited each of these genes or their gene products would have a broad spectrum of antibacterial activity.
Each of these essential genes was, separately, 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. 11. 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 (including coding sequences, promoter sequences, and 5′ and 3′ untranslated regions) was amplified from the plasmid pIL252 (available from the Bacillus Genetic Stock Center, Columbus, Ohio). 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:31). 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:32). 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. 11, 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 include the sequences 5′ GAA AAG TCA TCA TTT GAT GGC3′ (SEQ ID NO:33) and 5′ GGT GCA AGT CAG CAC GAA CAC CTA GAT AGA GGC AAG CGT GCA3′ (SEQ ID NO:34), respectively, for S-ylqF. 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. 11.
The third PCR reaction was similar to the second reaction, but produced the right flanking DNA molecule, shown in FIG. 11. 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 S-ylqF, primers 3 and 4 included the sequences 5′ TTT AAC GGG AGG AAA TAA TTC GTC TCT TCG TGA AGG AAG TCC 3′ (SEQ ID NO:35) and 5′ CTG TGC CAT ATC CTG CAT TAG3′ (SEQ ID NO:36), 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 (including coding sequences, promoter sequences, and 5′ and 3′ untranslated regions). 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. As discussed above, primers 1–4 are gene-specific, and the sequences disclosed above were used for S-ylqF. Gene-specific primers were used to identify the other essential genes described herein, as shown in Table 2.







TABLE 2







Primers Used in Identifying Essential Genes


 











Gene
Primer 1
Primer 2
Primer 3
Primer 4















S-ylqF
5′GAAAAGTCATCA
5′GGTGCAAGTCAG
5′TTTAACGGGAGGAA
5′CTGTGCCATAT



 



TTTGATGGC3′
CACGAACACCTAGA
ATAATTCGTCTCTTCG
CCTGCATTAG3′


 



(SEQ ID NO:33)
TAGAGGCAAGCGTGCA3′
TGAAGGAAGTCC3′
(SEQ ID NO:36)


 




(SEQ ID NO:34)
(SEQ ID NO:35)


 


S-yqeG
5′CTTGCTCTATAT
5′GGTGCAAGTCAG
5′TTTAACGGGAGGAA
5′TTAAGCTGATA


 



CAGTTTGGC3′
CACGAACACGGTTG
ATAATTCATCACTGAA
GCTCTTAGGC3′


 



(SEQ ID NO:37)
TTCCAAGCAATGAGGG3′
AAGTACGGACCG3′
(SEQ ID NO:40)


 




(SEQ ID NO:38)
(SEQ ID NO:39)


 


S-yybQ
5′AGCGCCTGCAGT
5′GGTGCAAGTCAG
5′TTTAACGGGAGGAA
5′CTGGGAGCAGT


 



TTGTTTATC3′
CACGAACACCAGAT
ATAATTCGACAAGGTC
AGAAACCAGC3′


 



(SEQ ID NO:41)
GACCCGATGGCATCTG3′
GAAGCGGCTTTC3′
(SEQ ID NO:44)


 




(SEQ ID NO:42)
(SEQ ID NO:43)


 


S-yerL
5′GTCCTCGTGCCC
5′GGTGCAAGTCAG
5′TTTAACGGGAGGAA
5′AGGTTGGCGGA


 



TGGCGTGTC3′
CACGAACACGGCAA
ATAATTCCTAGACAAT
TGGAACCACC3′


 



(SEQ ID NO:45)
CGTGTGTTACCTCTTC3′
GGAGGAGATGCC3′
(SEQ ID NO:48)


 




(SEQ ID NO:46)
(SEQ ID NO:47)


 


S-ysxC
5′CTGTTGCTAGGT
5′GGTGCAAGTCAG
5′TTTAACGGGAGGAA
5′CCAGCTGCAAA


 



ACCGCCTC3′
CACGAACACCCTGC
ATAATTCTGACCCAAG
AACTTAGGC3′


 



(SEQ ID NO:49)
GGATAGTGGGACTTA3′
TGACGATTTCA3′
(SEQ ID NO:52)


 




(SEQ ID NO:50)
(SEQ ID NO:51)









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 H2O, 0.7 μl Taq polymerase (5 U/μl), and 5 μl 10× Taq polymerase buffer (10 mM Tris, 50 mM KCl, 2.5 mM MgCl2). 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 the QIAQUICKTM 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. 11. 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 10× 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 H2O, 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 and plated on a medium containing erythromycin in accordance with standard techniques. As shown in FIG. 12, 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 ylqF, yqeG, yybQ, yerL, and ysxC genes were each identified as being essential for survival.
Identification of Homologs and Orthologs
Having shown that the ylqF, yqeG, yybQ, yerL, and ysxC genes each are essential for survival of Streptococcus, it can be expected that homologs and orthologs of these genes, when present in other organisms, For example B. subtilis, are essential for survival of those organisms as well. The coding sequences of each gene was used to search the GenBank database of nucleotide sequences, and an ortholog of each sequence was identified in B. subtilis. 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 Streptococcus pneumoniae polypeptides and the B. subtilis orthologs was determined using the GAP program from the Genetics Computer Group (GCG) Wisconsin Sequence Analysis Package (Wisconsin Package Version 9.1; Madison, Wis.). The default parameters for gap weight (12) and length weight (4) were used.
Typically, each of the ylqF, yqeG, yybQ, yerL, and ysxC polypeptides and their homologs or orthologs share at least 25% (e.g., at least 30%, 35%, or 40%) sequence identity. Typically, the DNA sequences encoding these 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 each of the ylqF, yqeG, yybQ, yerL, and ysxC genes showed that each of these genes is widely conserved among bacteria. To confirm that the identified orthologs each are essential for survival of B. subtilis, the orthologous genes each were individually deleted from the B. subtilis genome. Such deletion strains have been constructed and do not survive, confirming the essential nature of the polypeptides. The fact that the B-ylqF, B-yqeG, B-yybQ, B-yerL, and B-ysxC genes also are essential for survival suggests that these genes are essential in all bacteria in which they are present. Therefore, an antibacterial drug targeted to any one of these essential genes or gene products is expected to have a broad spectrum of antibacterial activity.
Identification of Essential Genes and Polypeptides in Additional Bacterial Strains and Species
Since the Streptococcus pneumoniae and Bacillus subtilis ylqF, yqeG, yybQ, yerL, and ysxC genes have been identified as being essential for survival, these genes, or fragments thereof, can be used to detect homologous or orthologous genes in yet other organisms (e.g., E. coli) In particular, these genes can be used to analyze various pathogenic and non-pathogenic strains of bacteria, particularly gram-positive bacteria. Fragments of a nucleic acid (DNA or RNA) encoding a ylqF polypeptide, for example, (including homologs and orthologs of the sequences listed herein, or sequences complementary thereto) can be used as probes in conventional nucleic acid hybridization assays of nucleic acids from bacteria. For example, nucleic acid probes (which typically are 8–30, or usually 15–20, nucleotides in length) can be used to detect ylqF genes 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 fragment 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 32P-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 stringent to highly stringent conditions. Nucleic acid duplex or hybrid stability is expressed as the melting temperature or Tm, 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, then 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 Tm, 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 Tm can be between 0.5° C. and 1.5° C. per 1% mismatch.
High stringency hybridization conditions refer to hybridizing at 68° C. in 5×SSC/5× Denhardt's solution/1.0% SDS, and washing in 0.2×SSC/0.1% SDS at 42° C. Stringent conditions refer to washing in 3×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 ylqF genes by Northern blot analysis. Upon detection of transcripts of a ylqF gene, 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 a ylqF gene probe. Similar methods can be used to screen for the other essential genes described herein.
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 ylqF gene, for example, (including homologs and orthologs of the sequences disclosed herein), as described herein. The template for the reaction can be DNA obtained from strains known to express, or suspected of expressing, a ylqF allele (including alleles of homologs or orthologs of the sequences disclosed herein). The PCR product can be subcloned and sequenced to ensure that the amplified sequences represent the sequences of a new ylqF nucleic acid sequence. Such methods also are suitable for isolating new gene sequences of all of the essential genes disclosed herein.
Synthesis of a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide can readily be accomplished using any of the various art-known techniques. For example, a polypeptide can be synthesized chemically in vitro, or enzymatically (e.g., by in vitro 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, a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide (including homologs and orthologs of the sequences disclosed herein) can be produced as a fusion protein. 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 a ylqF polypeptide, for example, 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 a ylqF polypeptide, for example, 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. Pat. 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 an essential polypeptide (e.g., ylqF) 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 vitro or in vivo recombination. Insertion into a non-essential region of the viral genome (e.g., region E1 or E3) will result in a recombinant virus that is viable and capable of expressing the desired 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 ylqF, yqeG, yybQ, yerL, and ysxC polypeptides each 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 ylqF, yqeG, yybQ, yerL, or ysxC polypeptide (including homologs and orthologs of the sequences described herein) can be produced by a stably-transfected mammalian cell line. A number of vectors suitable for stable transfection 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 vector (including DNA encoding the protein) 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, gpt, 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. Natl. 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 Ni2+ nitriloacetic acid-agarose columns, and histidine-tagged proteins are selectively eluted with imidazole-containing buffers.
Alternatively, the ylqF, yqeG, yybQ, yerL, or ysxC polypeptide, or a portion thereof, can be fused to an innunoglobulin 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 the desired essential polypeptide having increased stability in vivo.
Once the recombinant polypeptide 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, a ylqF 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 the desired essential polypeptide (e.g., a ylqF-maltose binding fusion protein, a ylqF-β-galactosidase fusion protein, or a ylqF-trpE fusion protein; see, e.g., Ausubel et al., supra; New England Biolabs Catalog, Beverly, Mass.). 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). The ylqF, yqeG, yybQ, yerL, or ysxC polypeptides can be used in similar methods.
Given the amino acid sequences described herein, polypeptides useful in practicing the invention, particularly fragments of ylqF, yqeG, yybQ, yerL, and ysxC 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, Ill., 1984) and used as antigens, for example.
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 a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide or (2) expression of a ylqF, yqeG, yybQ, yerL, or ysxC gene. In preferred methods, screening for antibacterial agents is accomplished by identifying those compounds (e.g., small organic molecules) that inhibit the activity of one of the essential polypeptides described herein or the expression of one of the essential genes described herein.
In an exemplary assay, but not the only assay, a promoter that responds to depletion of an essential polypeptide described herein, e.g., ylqF, by upregulation or downregulation is linked to a reporter gene (e.g., β-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 of the reporter gene. Because the polypeptides described herein are essential for the survival of bacteria, compounds that inhibit the ylqF, yqeG, yybQ, yerL, or ysxC polypeptides in such an assay are expected to be antibacterial agents and can be further tested, if desired, in conventional susceptibility assays.
In other suitable methods, screening for antibacterial agents is accomplished by (i) identifying those compounds that interact with or bind to a ylqF, yqeG, yybQ, yerL, or ysxC polypeptide and (ii) further testing such compounds for their ability to inhibit bacterial growth in vitro or in vivo.
Specific binding of a test compound to a polypeptide can be detected, for example, in vitro by reversibly or irreversibly immobilizing the test compound(s) on a substrate, e.g., the surface of a well of a 96-well polystyrene microtitre plate. Methods for immobilizing polypeptides and other small molecules are well known in the art. For example, the microtitre plates can be coated with a test polypeptide(s), for example, by adding the polypeptide(s) in a solution (typically, at a concentration of 0.05 to 1 mg/ml in a volume of 1–100 μl) to each well, and incubating the plates at room temperature to 37° C. for 0.1 to 36 hours. Polypeptides that are not bound to the plate can be removed by shaking the excess solution from the plate, and then washing the plate (once or repeatedly) with water or a buffer. Typically, the polypeptide is in water or a buffer. The plate is then washed with a buffer that lacks the bound polypeptide. To block the free protein-binding sites on the plates, the plates are blocked with a protein that is unrelated to the bound polypeptide. For example, 300 μl of bovine serum albumin (BSA) at a concentration of 2 mg/ml in Tris-HCl is suitable. Suitable substrates include those substrates that contain a defined cross-linking chemistry (e.g., plastic substrates, such as polystyrene, styrene, or polypropylene substrates from Corning Costar Corp. (Cambridge, Mass.), for example). If desired, a beaded particle, e.g., beaded agarose or beaded sepharose, can be used as the substrate.
The ylqF, yqeG, yybQ, yerL, or ysxC polypeptide (typically 0.05 to 1 mg/ml in 1–100 μl then is allowed to contact the bound substrate at room temperature 37° C. for 0.1 to 36 hours. Excess polypeptide can be removed as described above. Interaction of the test compound with polypeptide can be detected by any of a variety of art-known methods. For example, an antibody that specifically binds a ylqF polypeptide can be used in an immunoassay to detect the ylqF polypeptide. If desired, the antibody can be labeled (e.g., fluorescently or with a radioisotope) and detected directly (see, e.g., West and McMahon, J. Cell Biol. 74:264, 1977). Alternatively, a second antibody can be used for detection (e.g., a labeled antibody that binds the Fc portion of an anti-ylqF antibody). In an alternative detection method, the ylqF polypeptide is labeled, and the label is detected (e.g., by labeling a ylqF polypeptide with a radioisotope, fluorophore, chromophore, or the like). In still another method, the ylqF polypeptide is produced as a fusion protein with a protein that can be detected optically, e.g., green fluorescent protein (which can be detected under UV light). In an alternative method, the polypeptide (e.g., ylqF) can be produced as a fusion protein with an enzyme having a detectable enzymatic activity, such as horse radish peroxidase, alkaline phosphatase, β-galactosidase, or glucose oxidase. Genes encoding all of these enzymes have been cloned and are readily available for use by those of skill in the art. If desired, the fusion protein can include an antigen, and such an antigen can be detected and measured with a polyclonal or monoclonal antibody using conventional methods. Suitable antigens include enzymes (e.g., hose radish peroxidase, alkaline phosphatase, and β-galactosidase) and non-enzymatic polypeptides (e.g., serum proteins, such as BSA and globulins, and milk proteins, such as caseins).
In various in vivo methods for identifying polypeptides that bind ylqF polypeptides, the conventional two-hybrid assays of protein/protein interactions can be used (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 in vivo reconstitution of two separable domains of a transcription factor. One fusion protein contains, for example, a ylqF polypeptide fused to either a transactivator domain or DNA binding domain of a transcription factor (e.g., of Gal4). The other fusion protein contains a test polypeptide fused to either the DNA 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 DNA binding domain. Therefore, binding of the essential polypeptide (e.g., ylqF) 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 DNA sequence that is bound by the DNA binding domain of the transcription factor. Kits for practicing various two-hybrid methods are commercially available (e.g., from Clontech; Palo-Alto, Calif.).
The methods described above can be user; 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 vitro 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 ylqF, yqeG, yybQ, yerL, or ysxC.
In vitro, 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 methods of the invention also can be practiced with various fragments, variants, analogs, and derivatives of the polypeptides described above that retain one or more of the biological activities of such polypeptides. Naturally-occurring and non-naturally-occurring variants are useful in the invention. Compared with the naturally-occurring gene sequences depicted in FIGS. 1A–10, 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 the ylqF, yqeG, yybQ, yerL, or ysxC 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 ylqF, yqeG, yybQ, yerL, and ysxC genes and gene products (including homologs and orthologs of the sequences disclosed herein). Furthermore, while certain of the methods disclosed herein have been described by using ylqF as an example, it is to be understood that ylqF, yqeG, yybQ, yerL, and ysxC can readily be substituted for ylqF.

## Claims

1. A method for identifying an antibacterial agent, the method comprising:
(a) contacting a test compound with a cell that expresses a polypeptide selected from the group consisting of S-ylqF (SEQ ID NO:2), S-yqeG (SEQ ID NO:5), S-yybQ (SEQ ID NO:8), and S-ysxC (SEQ ID NO:14);
(b) detecting a decrease in expression of the polypeptide; and
(c) determining whether a test compound that decreases expression of the polypeptide inhibits growth of bacteria, relative to growth of bacteria cultured in the absence of the test compound, wherein inhibition of growth indicates that the test compound is an antibacterial agent.

2. A method of claim 1, wherein the test compound is selected from the group consisting of nucleic acids, polypeptides and small organic molecules.

3. The method of claim 1, wherein decreased expression is detected using Northern blot analysis, Western blot analysis, polymerase chain reaction (PCR) analysis, or RNAse protection analysis.