IMPROVED VACCINES

Abstract: Vaccines and bacteria which are virulence attenuated by mutations in a two-component regulatory system.

Description: IMPROVED VACCINES The invention relates to vaccines. Background of the Invention This invention was made in the course of work supported by the United States Government, which has certain rights in the invention. Enteric fevers and diarrheal diseases, e.g., typhoid fever and cholera, are major causes of morbidity and mortality throughout the developing world, Hook et al., 1980, In Harrison's Principles of Internal Medicine, 9th Ed., 641-848, McGraw Hill, New York. Traditional approaches to the development of vaccines for bacterial diseases include the parenteral injection of purified components or killed organisms. These parenterally administered vaccines require technologically advanced preparation, are relatively expensive, and are often, because of dislike for needle-based injections, resisted by patients. Live oral vaccine strains have several advantages over parenteral vaccines: low cost, ease of administration, and simple preparation. The development of live vaccines has often been limited by a lack of understanding of the pathogenesis of the disease of interest on a molecular level. Candidate live vaccine strains require nonrevertible genetic alterations that affect the virulence of the organism, but not its induction of an immune response. Work defining the mechanisms of toxigenesis of vibrio cholerae has made it possible to create live vaccine strains based on deletion of the toxin genes, Mekalanos et al. , 1983, Nature 3_06_:551, Levine et al., 1988, Infect. Immun. 56:161. Recent studies have begun to define the molecular basis of Salmonella typhimurium macrophage survival and virulence. Miller et al., 1989, Proc. Na l. Acad. Sci. USA .86.:5054, hereby incorporated by reference. Salmonella typhimurium strains with mutations in the positive regulatory regulon phoP are markedly attenuated in virulence for BALB/c mice. The phoP regulon is composed of two genes present in an operon, termed phoP and phoQ. The phoP and phoQ gene products are highly similar to other members of bacterial two-component transcriptional regulators that respond to environmental stimuli and control the expression of a large number of other genes. A mutation at one of these phoP regulatory region regulated genes, page, confers a virulence defect. Strains with page, phoP, or phoQ mutations afford partial protection to subsequent challenge by wild-type S. typhimurium. Salmonella species cause a spectrum of clinical disease that includes enteric fevers and acute gastroenteritis, Hook et al., 1980, supra. Infections with Salmonella species are more common in im unosuppressed persons, Celum et al., 1987, J. Infect. Dis. 156:998. S. typhi, the bacterium that causes typhoid fever, can only infect man, Hook et al., 1980, supra. The narrow host specificity of S. typhi has resulted in the extensive use of S. enteriditiε typhimurium infection of mice as a laboratory model of typhoid fever, Carter et al., 1984 J. Exp. Med. 139.1189. S. typhimurium infects a wider range of hosts, causing acute gastroenteritis in man and a disease similar to typhoid fever in the mouse and cow. Salmonella infections are acquired by oral ingestion. The organisms, after traversing the stomach, replicate in the small bowel, Hornik et al., 1970, N. Eng. J. Med. 283;686. Salmonella are capable of invasion of the intestinal mucosal cells, and S. typhi can pass through this mucosal barrier and spread via the Peyer's patches to the lamina propria and regional lymph nodes. Colonization of the reticuloendothelial cells of the host then occurs after bacteremia. The ability of S. typhi to survive and replicate within the cells of the human reticuloendothelial system is essential to its pathogenesis. Hook et al., 1980, supra , Hornick et al., 1970, supra , and Carter et al., 1984, supra . Immunity to Salmonella typhi involves humoral and cell-mediated immunity. Murphy et al., 1987, J. Infect. Dis. 156:1005, and is obtainable by vaccination, Edelman et al., 1986, Rev. Inf. Dis. 8_:324. Recently, human field trials demonstrated significant protective efficacy against S . typhi infection after intramuscular vaccination with partially purified Vi antigen, Lanata et al., 1983, Lancet 2.-441. Antibody-dependent enhancement of S. typhi killing by T cells has been demonstrated in individuals who received a live S. typhi vaccine, indicating that these antibodies may be necessary for the host to generate a cell-mediated immune response, Levine et al., 1987, J. Clin. Invest. 7J9:888. The cell-mediated immune response is important in typhoid immunity since killed vaccines that do not induce this immune response are not protective in man, Collins et al., 1972, Infect. Immun. 41:742. Summary of the Invention In general, the invention features a vaccine, preferably a live vaccine, including a bacterial cell, preferably a Salmonella cell, e.g., a S . typhi , S . enteritidis typhimurium, or S. cholerae-suis cell, the virulence of which is attenuated by the constitutive expression of a gene under the control of a two-component regulatory system. In preferred embodiments the constitutive expression is the result of a mutation at a component of the two-component regulatory system. In preferred embodiments the bacterial cell includes a second mutation which attenuates virulence. In yet other preferred embodiments of the vaccine the two-component regulatory system is the phoP regulatory region, and the gene under the control of the two-component system is a phoP regulatory region regulated gene, e.g., a prg or pag gene, e.g., pag . In preferred embodiments constitutive expression is the result of a change or mutation (preferably a non- revertible mutation) at the promoter of the regulated gene or of the phoP regulatory region, e.g., a mutation in the phoQ or the phoP gene, e.g., the phoP c mutation. In preferred embodiments of the vaccine the Salmonella cell includes a first mutation which attenuates virulence, e.g., a mutation in a phόP regulatory region gene, e.g., a mutation in the phoP or phoQ gene, e.g., phόP c , or a mutation in a phoP regulatory region regulated gene, and a second mutation which attenuates virulence, e.g., a mutation in an aromatic amino acid synthetic gene, e.g., an aro gene, a mutation in a phoP regulatory region regulated gene, e.g., a mutation in a prg or pag locus, e.g., a page mutation. In yet other preferred embodiments the bacterial cell includes a first mutation in a phoP regulatory region gene and a second mutation in an aromatic amino acid synthetic gene, e.g, an aro gene. In another aspect, the invention features a vaccine, preferably a live vaccine, including a bacterial cell, the virulence of which is attenuated by a mutation in a gene under the control of a two-component regulatory system. In preferred embodiments the bacterial cell includes a virulence attenuating mutation in a second gene, e.g., in an aromatic amino acid synthetic gene, e.g., an aro gene. In yet other preferred embodiments of the vaccine the bacterial cell is Salmonella cell, the two-component regulatory system is the phoP regulatory region, and the gene under its control is a prgr or a pag gene, e.g., the pagC gene. In another aspect the invention features a vaccine, preferably a live vaccine, including a Salmonella cell e.g., a S. typhi , S. enteritidis typhimurium, or S. cholerae-suis cell, including a first virulence attenuating mutation in an aromatic amino acid biosynthetic gene, e.g., an aro gene, and a second virulence attenuating mutation in a phoP regulatory region gene, e.g., a phoP " mutation. In another aspect the invention features a bacterial cell, or a substantially purified preparation thereof, preferably a Salmonella cell, e.g., a S. typhi - S. enteritidis typhimurium, or S. cholerae-suis cell, which constitutively expresses a gene under the control of a two-component regulatory system and which includes a virulence attenuating mutation which does not result in constitutive expression of a gene under the control of the two-component regulatory system. In preferred embodiments the bacterial cell includes a mutation in a component of the two-component regulatory system. In preferred embodiments the bacterial cell is a Salmonella cell which expresses a phoP regulatory region regulated gene constitutively (the constitutive expression preferably caused by a mutation, preferably a non-revertible mutation, e.g., a deletion in the phoP regulatory region, e.g., a mutation in the phoQ or phoP gene, e.g., phoP c ) , and which includes a virulence attenuating mutation, preferably a non-revertible mutation, e.g., a deletion, preferably in an aromatic amino acid synthetic gene, e.g., an aro gene, or in a phoP regulatory region regulated gene, e.g., a prg or pag gene, e.g., page which does not result in the constitutive expression of a gene under the control of the phoP regulatory region. In another aspect, the invention features a bacterial cell, or a substantially purified preparation thereof, e.g., a Salmonella cell, e.g., a S. typhi cell, an S. enteritidis typhimurium or a S. cholerae-suis cell, including a virulence attenuating mutation in a gene regulated by a two-component regulatory system. In preferred embodiments the virulence attenuating mutation is in a phoP regulatory region regulated gene, e.g., a prg or pag gene, e.g., page. In preferred embodiments the bacterial cell includes a second mutation, e.g., in an aromatic amino acid synthetic gene, e.g., an aro gene, in a phoP regulatory region gene, e.g., the phoP or phoQ genes, or in a phoP regulating region regulated gene, e.g., a prg or a pag gene, e.g., pag , which attenuates virulence but which does not result in constitutive expression of a phoP regulatory region regulated gene. The invention also features a live Salmonella cell, or a substantially purified preparation thereof, e.g., a S. typhi, S. enter iditis typhimurium, or S. cholerae-suis cell, in which there is inserted into a virulence gene, e.g., a gene in the phoP regulating region, or a phoP regulating region regulated gene, e.g., a prg or a pagr locus, e.g., page, a gene encoding a heterologous protein, or a regulatory element thereof. In preferred embodiments the live Salmonella cell carries a second mutation, e.g., an aro mutation, e.g., an aroA mutation, e.g., aroA " or aroADEL407, that attenuates virulence. In preferred embodiments the DNA encoding a heterologous protein is under the control of an environmentally regulated promoter. In other preferred embodiments the live Salmonella cell further includes a DNA sequence encoding T7 polymerase under the control of an environmentally regulated promoter and a T7 transcriptionally sensitive promoter, the T7 transcriptionally sensitive promoter controlling the expression of the heterologous antigen. The invention also features a vector capable of integrating into the chromosome of Salmonella including: a first DNA sequence encoding a heterologous protein; a second (optional) DNA sequence encoding a marker e.g., a selective marker, e.g., a gene that confers resistance for a heavy metal resistance or a gene that compliments an aurotrophic mutation carried by the strain to be transformed; and a third DNA sequence, e.g., a phoP regulon encoded gene, e.g., a prg or a pag locus, e.g., page, encoding a product necessary for virulence, the third DNA sequence being mutationally inactivated. In other preferred embodiments: the first DNA sequence is disposed on the vector so as to mutationally inactivate the third DNA sequence; the vector cannot replicate in a wild-type Salmonella strain; the heterologous protein is under the control of an environmentally regulated promoter; and the vector further includes a DNA sequence encoding T7 polymerase under the control of an environmentally regulated promoter and a T7 transcriptionally sensitive promoter, the T7 transcriptionally sensitive promoter controlling the expression of the heterologous antigen. In another aspect the invention includes a method of vaccinating an animal, e.g., a mammal, e.g., a human, against a disease caused by a bacterium, e.g.. Salmonella , including administering a vaccine of the invention. The invention also includes a vector including DNA which encodes the pagC gene product; a cell transformed with the vector; a method of producing the page gene product including culturing the transformed cell and purifying the pagC gene product from the cell or culture medium; and a purified preparation of the pagC gene product. In another aspect the invention includes a method of detecting the presence of Salmonella in a sample including contacting the sample with pagC encoding DNA and detecting the hybridization of the page encoding DNA to nucleic acid in the sample. In another aspect the invention features a method of attenuating the virulence of a bacterium, the bacterium including a two-component regulatory system, including causing a gene under the control of the two- component system to be expressed constitutively. In preferred embodiments the bacterium is Salmonella, e.g., S. typhi, S. enteritidis typhimurium, or S. cholerae- suis, and the two-component system is the phoP regulatory region. Two-component regulatory system, as used herein, refers to a bacterial regulatory system that controls the expression of multiple proteins in response to environmental signals. The two-components referred to in the term are a sensor, which may, e.g., sense an environmental parameter and in response thereto promote the activation, e.g. by promoting the phosphorylation, of the second component, the activator. The activator affects the expression of genes under the control of the two-component system. A two-component system can include, e.g., a histidine protein kinase and a phosphorylated response regulator, as is seen in both gram positive and gram negative bacteria. In E. coli. e.g., 10 kinases and 11 response regulators have been identified. They control chemotaxis, nitrogen regulation, phosphate regulation, osmoregulation. sporulation, and many ither cellular functions. Stock et al., 1989 Microbiol. Rev. 51:450-490, hereby incorporated by reference. A two-component system also controls the virulence of Agrobacterium tumefasciens plant tumor formation, Leroux et al. EMBO J jϊ:849-856, hereby incorporated by reference) . Similar virulence regulators are involved in the virulence of Bordetella pertussis Arico et al., 1989, Proc. Natl. Acad. Sci. USA 86:6671- 6675, hereby incorporated by reference, and Shigella flexneri , Bernardini et al., 1990, J. Bact. 172:6274- 6281, hereby incorporated by reference. Environmentally regulated, as used herein refers to a pattern of expression wherein the expression of a gene in a cell depends on the levels of some characteristic or component of the environment in which the cell resides. Examples include promoters in biosynthetic pathways which are turned on or off by the level of a specific component or components, e.g., iron, temperature responsive promoters, or promoters which are expressed more actively in specific cellular compartments, e.g., in macrophages or vacuoles. A vaccine, as used herein, is a preparation including materials that evoke a desired biological response, e.g., an immune response, in combination with a suitable carrier. The vaccine may include live organism, in which case it is usually administered orally, or killed organisms or components thereof, in which case it is usually administered perinterally. The cells used for the vaccine of the invention are preferably alive and thus capable of colonizing the intestines of the inoculated animal. A mutation, as used herein, is any change (in comparison with the appropriate parental strain) in the DNA sequence of an organism. These changes can arise e.g., spontaneously, by chemical, energy e.g.. X-ray, or other forms of mutagenesis, by genetic engineering, or as a result of mating or other forms of exchange of genetic information. Mutations include e.g., base changes, deletions, insertions, inversions, translocations or duplications. A mutation attenuates virulence if, as a result of the mutation, the level of virulence of the mutant cell is decreased in comparison with the level in a cell of the parental strain, as measured by (a) a significant (e.g. , at least 50%) decrease in virulence in the mutant strain compared to the parental strain, or (b) a significant (e.g., at least 50%) decrease in the amount of the polypeptide identified as the virulence factor in the mutant strain compared to the parental strain. A non-revertible mutation, as used herein, is a mutation which cannot revert by a single base pair change, e.g., deletion or insertion mutations and mutations that include more than one lesion, e.g., a mutation composed of two separate point mutations. The phoP regulatory region, as used herein, is a two-component regulatory system that controls the expression of pag and prg genes. It includes the phoP locus and the phoQ locus. phoP regulatory region regulated genes, as used herein, refer to genes such as pag and prg genes. pag, as used herein, refers to a gene which is positively regulated by the phoP regulon. prg, as used herein, refers to a gene which is negatively regulated by the phoP regulon. An aromatic amino acid synthetic gene, as used herein, is a gene which encodes an enzyme which catalyzes a step in the synthesis of an aromatic amino acid. aroA, aroC, and aroD are examples of such genes in Salmonella . Mutations in these genes can attenuate virulence without the total loss of immunogenicity. Abnormal expressions, as used herein, means expression which is higher or lower than that seen in wild type. Heterologous protein, as used herein, is a protein that in wild type, is not expressed or is expressed from a different chromosomal site, e.g., a heterologous protein is one encoded by a gene that has been inserted into a second gene. Virulence gene, as used herein, is a gene the inactivation of which results in a Salmonella cell with less virulence than that of a similar Salmonella cell in which the gene is not inactivated. Examples include the phoP and pagC genes. A marker, as used herein, is gene product the presence of which is easily determined, e.g., a gene product that confers resistance to a heavy metal or a gene product which allows or inhibits growth under a given set of conditions. Purified preparation, as used herein, is a preparation, e.g., of a protein, which is purified from the proteins, lipids, and other material with which it is associated. The preparation is preferably at least 2-10 fold purified. Constitutive expression, as used herein, refers to gene expression which is modulated or regulated to a lesser extent than the expression of the same gene in an appropriate control strain, e.g., a parental or in wild- type strain. For example, if a gene is normally repressed under a first set of conditions and derepressed under a second set of conditions constitutive expression would be expression at the same level, e.g., the repressed level, the derepressed level, or an intermediate level, regardless of conditions. Partial constitutive expression is included within the definition of constitutive expression and occurs when the difference between two levels of expression is reduced in comparison in what is seen in an appropriate control strain, e.g., a wild-type or parental strain. A substantially purified preparation of a bacterial cell is a preparation of cells wherein contaminating cells without the desired mutant genotype constitute less than 10%, preferably less than 1%, and more preferably less than 0.1% of the total number of cells in the preparation. The invention allows for the attenuation of virulence of bacteria and of vaccines that include bacteria, especially vaccines that include live bacteria, by mutations in two-component regulatory systems and/or in genes regulated by these systems. The vaccines of the invention are highly attenuated for virulence but retain immunogenicity, thus they are both safe and effective. The vectors of the invention allow the rapid construction of strains containing DNA encoding heterologous proteins, e.g., antigens. The heterologous protein encoding DNA is chromosomally integrated, and thus stable, unlike plasmid systems which are dependent on antibiotic resistance or other selection pressure for stability. Live Salmonella cells of the invention in which the expression of heterologous protein is under the control of an environmentally responsive promoter do not express the heterologous protein at times when such expression would be undesirable e.g., during culture, vaccine preparation, or storage, contributing to the viability of the cells, but when administered to humans or animals, express large amounts of the protein. This is desirable because high expression of many heterologous proteins in Salmonella can be associated with toxicity to the bacterium. The use of only a single integrated copy of the DNA encoding the heterologous protein also contributes to minimal expression of the heterologous protein at times when expression is not desired. In embodiments where a virulence gene, e.g., the page gene, contains the site of integration for the DNA encoding the heterologous protein the virulence of the organism is attenuated. Other features and advantages of the invention will be apparent from the following description of the preferred embodiments and from the claims. Description of the Preferred Embodiments The drawings will first be described. Drawings Fig. 1 is a graph of the survival of Salmonella strains within macrophages. Fig. 2 is a map of the restriction endonuclease sites of the pagC locus. Fig. 3 is a map of the DNA sequence of the pag C region (Sequence ID No. 1) . Strain Deposit PhoP c strain CS022 (described below) has been deposited with the American Type Culture Collection (Rockville, MD) and has received ATCC designation Constitutive Expression of the PhoP Regulon Attenuates Salmonella Virulence and Survival within Macrophages The phoP constitutive allele (PhoP c ) , pho-24, results in derepression of pag loci. Using diethyl sulfate mutagenesis of S. typhimurium LT-2, Ames and co- workers isolated strain TA2367 pho-24 (all strains, materials, and methods referred to in this section are described below) , which contained a phoP locus mutation that resulted in constitutive production of acid phosphatase in rich media, Kier et al. , 1979, J. Bacteriol. 138:155. hereby incorporated by reference. This phoP-regulated acid phosphatase is encoded by the phoN gene, a pag locus, Kier et al., 1979, supra. Miller et al., 1989, supra. To analyze whether the pho-24 allele increased the expression of other pag loci the effect of the pho-24 allele on the expression of other pag loci recently identified as transcriptional (e.g., pagA and pagrB) and translational (e.g., pagC) fusion proteins that required phoP and phoQ for expression, Miller et al., 1989, supra, was determined, pag gene fusion strains, isogenic except for the pho-24 allele, were constructed and assayed for fusion protein activity. PhoP c derivatives of the pagA: :Mu dJ and pagB: :Mu dJ strains produced 480 and 980 U, respectively, of jβ- galactosidase in rich medium, an increase of 9- to 10- fold over values for the fusion strains with a wild-type phoP locus, see Table 1. TABLE 1. Bacterial strains and properties a A. Acid phosphatase; B, ,9-galactosidase; C, alkaline phosphatase. b Gift of Ning Zhu and John Roth. The pagC: :TnphoA gene fusion produced 350 U of alkaline phosphatase, an increase of three- to fourfold over that produced in strain CS119, which is isogenic except for the pho-24 mutation. Miller et al., 1989, supra. These results compare with a ninefold increase in the acid phosphatase activity in strain CS022 on introduction of the pho-24 allele. Therefore, these available assays for pag gene expression document that the pho-24 mutation causes constitutive expression of pag loci other than phoN. Identifications of protein species that are repressed as well as activated in the PhoP c mutant strain Whole-cell proteins of strain CS022 were analyzed to estimate the number of protein species that could be potentially regulated by the PhoP regulon. Remarkably, analysis by one-dimensional polyacrylamide gel electrophoresis of the proteins produced by strains with the PhoP c phenotype indicated that some protein species were decreased in expression when many presumptive pag gene products were fully induced by the pho-24 mutation. The proteins decreased in the PhoP c strain might represent products of genes that are repressed by the PhoP regulator. Genes encoding proteins decreased by the pho-24 allele are designated prg loci, for phoP-repressed genes. Comparison of wild-type, PhoP " , and PhoP c mutant strain proteins shows that growth in LB medium at 37°C represents repressing conditions for pag gene products and derepressing conditions for prg gene products. To estimate the total number of potentially PhoP- regulated gene products, the total cell proteins of wild- type and PhoP c mutant strains grown in LB were analyzed by two-dimensional gel electrophoresis. At least 40 species underwent major fluctuation in expression in response to the pho-24 mutation. Virulence defects of the PhoP c strain Remarkably, strains with the single pho-24 mutation were markedly attenuated for virulence in mice (Table 2) . The number of PhoP c organisms (2 x 10 5 ) that killed 50% of BALB/c mice challenged (LD 50 ) by the intraperitoneal (i.p.) route was near that (6 x 10 5 ) of PhoP~ bacteria. Miller et al., 1989, supra. The PhoP c strains had growth comparable to wild-type organisms in rich and minimal media. The PhoP c mutants were also tested for alterations in lipopolysaccharide, which could explain the virulence defect observed. Strain CS022 had normal sensitivity to phage P22, normal group B reactivity to antibody to O antigen, and a lipopolysaccharide profile identical to that of the parent strain, as determined by polyacrylamide gel electrophoresis and staining. Table 2 Virulence and protective efficacy of PhoP c and PhoP " Salmonella strains No. of No. of survivors total after wild- <_> 1 (*) Organisms were administered by the oral route, In all other experiments. organisms were administered by i.p. challenge. Since the TA2367 pho-24 strain was constructed by chemical mutagenesis and could have another linked mutation responsible for its virulence defect revertants of the PhoP c were isolated to determine whether the pho- 24 allele was responsible for the attenuation of virulence observed. Phenotype PhoP c revertants, identified by the normal levels of acid phosphatase in rich medium, were isolated among the bacteria recovered from the livers of mice infected with strain CS022. Six separate phenotypic revertants, designated CS122 to CS128, were found to be fully virulent (L 50 of less than 20 organisms for BALB/c mice) . The locus responsible for the reversion phenotype was mapped in all six revertants tested for virulence by bacteriophage P22 cotransduction and had linkage characteristics consistent with the phoP locus (greater than 90% linkage to purB) . These data indicate that these reversion mutations are not extragenic suppressors but are intragenic suppressors or true revertants of the pho-24 mutation. Thus, the virulence defect of PhoP c mutants is probably the result of a single revertible mutation in the phoP locus and not the result of a second unrelated mutation acquired during mutagenesis. Reversion frequency of the PhoP c phenotype The reversion frequency of the PhoP c mutation in vivo in mice was investigated to assess whether reversion could reduce the LD 50 of this strain. The presence of the revertants of strain CS022 was tested for by administering 10 , 10 4 , and 10 2 challenge organisms to each of eight animals by i.p. injection. On day 7, three animals died that received 10 6 PhoP c organisms. On that day, the livers and spleens of all animals were harvested and homogenized in saline. After appropriate dilution, 10% of the tissue was plated on LB plates containing the chromogenic phosphatase substrate XP. Revertants were identified by their lighter blue colonies compared with PhoP c bacteria and were confirmed by quantitative acid phosphatase assays. An estimated 10 7 , 10 5 , and 10 3 organisms per organ were recovered from animals at each of the three respective challenge doses. Revertants were identified only at the highest dose and comprised 0.5 to 1%, or 10 5 organisms per organ, at the time of death. It is likely that revertants are able to compete more effectively for growth in these macrophage-containing organs, since strain CS022 is deficient in survival within macrophages (see below) . However, revertants were not identified if fewer than 10 5 organisms were administered in the challenge dose, suggesting that the reversion frequency must be approximately 10~ 5 . The reversion rate of the PhoP c phenotype for CS022 bacteria grown in LB is in fact 6xl0~ 4 when scored by the same colony phenotypes. The percentage of revertants recovered from animals near death suggests that pressure is applied in vivo that selects for revertants of the PhoP c phenotype and implies that the virulence defect observed could be much greater quantitatively for a strain with a nonrevertible PhoP c mutation. The PhoP c strain is deficient in survival within macrophages Because of the importance of survival within macrophages to Salmonella virulence Fields et al., 1986, Proc. Natl. Acad. Sci. USA 83:5189, hereby incorporated by reference, PhoP c bacteria were tested for this property. Strain CS022 was defective in the ability to grow and persist in macrophages as compared with wild- type organisms (Fig. 1) . In Fig. 1 the survival of strain CS022 (PhoP c ) (triangles) in cultured macrophages is compared with that of wild-type S. typhimurium ATCC 10428 (cicles) . The experiment shown is a representative one. The difference between the two strains at 4 and 24 hours is significant (P < 0.05). PhoP " bacteria seemed to have a macrophage survival defect qualitatively similar to that of PhoP c bacteria but survived consistently better by two- to threefold in side-by-side experiments. The increased recovery of organisms that reverted to PhoP c phenotype in mouse organs rich in macrophage content is consistent with the reduced macrophage survival of PhoP c mutants in vitro. Use of the PhoP c strain as a live vaccine It has been previously reported that PhoP " strains are useful as live vaccines in protecting against mouse typhoid. Miller et al., 1989, supra. The immunogenicity of PhoP c when used as live attenuated vaccines in mice was compared with the of PhoP " . This was done by simultaneous determination of survival, after graded challenge doses with the wild-type strain ATCC 10428, in mice previously immunized with graded doses of the two live vaccine strains. CS015 phoP : :Tn20d-Cam and CS022 pho-24 , as well as a saline control. The results obtained (Table 2) suggest the following conclusions: (i) small i.p. doses of the PhoP c strain (e.g., 15 organisms) effectively protect mice from challenge doses as large as 5xl0 5 bacteria (a challenge dose that represents greater than 10* i.p. LD 50 s) , (ii) large doses of PhoP c organisms given orally completely protect mice from an oral challenge consisting of 5x10 wild-type bacteria (over 200 oral wild-type LD 50 s) and (iii) by comparison, a large dose of PhoP " organisms (5xl0 5 ) does not provide similar protection. The reversion of the PhoP c mutation in vivo somewhat complicates the analysis of the use of these strains as vaccines, since revertants of the CS022 strain (i.e., wild-type cells) could increase immunogenicity). However, we were unable to identify revertants by examining 10% of the available spleen and liver tissue from those mice that received 10* or fewer organisms. Strains. Materials and Methods The strains, materials, and methods used in the PhoP regulon work described above are as follows. American Type Culture Collection (ATCC) strain 14028, a smooth virulent strain of S. typhimurium, was the parent strain for all virulence studies. Strain TT13208 was a gift from Nang Zhu and John Roth. Strain TA2367 was a generous gift of Gigi Stortz and Bruce Ames, Kier et al., 1979, supra. Bacteriophage P22HT int was used in transductional crosses to construct strains isogenic except for phoP locus mutations, Davis et al. , 1980, Advanced Bacterial Genetics, p. 78, 87. Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, hereby incorporated by reference. Luria broth was used as rich medium, and minimal medium was M9, Davis et al., 1980, supra. The chromogenic phosphatase substrate 5-bromo-4- chloro-3indolyl phosphate (XP) was used to qualitatively access acid and alkaline phosphatase production in solid media. Derivatives of S. typhimurium ATCC 10428 with the pho-24 mutation were constructed by use of strain TA2367 as a donor of the purB gene in a P22 transductional cross with strain CS003 AphoP ApurB, Miller et al., 1989, supra. Colonies were then selected for the ability to grow on minimal medium. A transductant designated CS022 (phenotype PhoP c ) that synthesized 1,750 U of acid phosphatase in rich medium (a ninefold increase over the wild-type level in rich medium) was used in further studies. Derivatives of strains CS022 and CS023 pho-24 phoN2 zxx: :6251Tnl0d-Cam, and acid phosphatase-negative derivative of CS022, containing pag gene fusions were constructed by bacteriophage P22 transductional crosses. using selection of Tnp oA- or Mu dJ-encoded kanamycin resistance. Strains were checked for the intact pag gene fusion by demonstration of appropriate loss of fusion protein activity on introduction of a phoP105 : :Tn20d or phoP102 : :Tn20d-Cam allele. Assays of acid phosphatase, alkaline phosphatase, and β-galactosidase were performed as previously described. Miller et al., 1989, supra and are reported in units as defined in Miller, 1972, Experiments in molecular genetics, p. 352-355, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY, hereby incorporated by reference. In the mouse virulence and vaccination studies bacteria grown overnight in Luria broth were washed and diluted in normal saline. The wild-type parent strain of CS022 (ATCC 10428) was used for all live vaccine challenge studies. This strain has a 50% lethal dose (LD 50 ) for naive adult BALB/c mice of less than 20 organisms when administered by intraperitoneal (i.p.) injection and 5xl0 4 when administered orally in NaHC0 3 . Mice were purchased from Charles River Breeding Laboratories, Inc. (Wilmington, Mass.) and were 5 to 6 weeks of age at initial challenge. All i.p. inoculations were performed as previously described. Miller et al., 1989, supra. Oral challenge experiments were performed with bacteria grown in LB broth and concentrated by centrifugation. The bacteria were resuspended in 0.1 M NaHC0 3 to neutralize stomach acid, and administered as a 0.5-ml bolus to animals under ether anesthesia. Colony counts were performed to accurately access the number of organisms administered. All challenge experiments were performed 1 month after i.p. inoculation and 6 weeks after oral challenge. Challenge inocula were administered by the same route as vaccinations. The care of all animals was under institutional guidelines as set by the animal are committees at the Massachusetts General Hospital and Harvard Medical School. Protein electrophoresis was performed as follows. One-dimensional protein gel electrophoresis was performed by the method of Laemmli, 1970, Nature 227:680, hereby incorporated by reference, on whole-cell protein extracts of stationary-phase cells grown overnight in Luria broth. The gels were fixed and stained with Coomassie brilliant blue R250 in 10% acetic acid-10% methanol. Two- dimensional protein gel electrophoresis was performed by method of O'Farrell, 1975, J. Biol. Chem. 250:4007, hereby incorporated by reference, on the same whole-cell extracts. Isoelectric focusing using 1.5% pH 3.5 to 10 ampholines (LKB Instruments, Baltimore, Md.) was carried out for 9,600 V h (700 V for 13 h 45 min) . The final tube gel pH gradient extended from pH 4.1 to pH 8.1 as measured by a surface pH electrode (BioRad Laboratories, Richmond, Calif.) and colored acetylated cytochro e pi markers (Calbiochem-Behring, La Jolla, Calif.) run in an adjacent tube. The slab gels were silver stained, Merril et al., 1984, Methods Enzymol. 104:441, hereby incorporated by reference. In the macrophage survival assays experiments were performed as previously described, Miller et al., 1989, supra, by the method of Buchmeier et al., 1989, Infect. Immun. 52:1, hereby incorporated by reference, as modified from the method of Lissner et al, 1983, J. Immunol. 131:3006. hereby incorporated by reference. Stationary-phase cells were opsonized for 30 min in normal mouse serum before exposure to the cultured bone marrow-derived macrophages harvested from BALB/c mice. One hour after infection, gentamicin sulfate (8 μg/ml) was added to kill extracellular bacteria. All time points were done in triplicate and repeated on three separate occasions. PhoP c Mutant Strains Are More Effective as Live Vaccines PhoP c mutant S. typhimurium are very effective when used as a live vaccine against mouse typhoid fever and are superior to PhoP " bacteria. As few a 15 PhoP c bacteria protect mice against 10 5 LD 50 (lethal doses 50%) of wild type organisms by the intraperitoneal route (Table 3) . This suggests that pag gene products are important antigens for protective immunity against mouse typhoid. Preliminary results have documented that antigens recognized by serum of chronic typhoid carriers recognizes some phoP-regulated gene products of S. typhi . If protective antigens are only expressed within the host, then dead vaccines only grown in rich media may not induce an immune response against these proteins. The use of different S. typhimurium dead vaccine preparations containing different mutations in the phoP regulon was evaluated. As can be seen in Table 3 no dead cell preparations (even those containing mixtures of PhoP " and PhoP c bacteria) are as effective vaccines as are live bacteria. This suggests that there are other properties of live vaccines that increase immunogenicity or that important non-PhoP-regulated antigens are not in these preparations. The only protection observed in any animals studied was at the lowest challenge dose for those immunized with PhoP c bacteria. This further suggests that phoP activated genes are important protective antigens. Table 3 Salmonella with phoP regulon mutations used as a dead vaccine Vaccination Challenge dose of wild type organisms Strain phenotype 6 x 10 3 6 x 10 5 None 3) (5) ATCC10428 wild type CS015 PhoP " CS022 PhoP c CS022/CS015 PhoP " /PhoP c (8) (13) CS015 « phoPl02 t i 'Snl0d-Cam CS022 = pho-24 BA B/c mice were immunized twice, 7 days apart, with 5x10 . 8° formalin-killed bacteria. Three weeks after the second vaccination, mice were challenged with wild-type organisms at the two doses indicated. The numbers in parentheses indicate no survivors after challenge and mean number of days until death (*) Ratio of survivors to number challenged. phoP c indicates the constitutive unregulated expression of phoP-activated genes and lack of expression of phoP repressed genes. phoP~ indicates a lack of expression of pAoP-activated genes and expression of pAoP repressed genes. aroA PhoP Regulon Double Mutant Strains Recent efforts by Stocker, Levine, and colleagues have focused on the use of strains with auxotrophic mutations in aromatic amino acid and purine pathways as live vaccines, Hoseith et al., 1981, Nature 291:238. hereby incorporated by reference, Stocker, 1988, Vaccine e>:141, hereby incorporated by reference, and Levine et al., 1987, J. Clin. Invest. 79:888, hereby incorporated by reference. Purine mutations were found to be too attenuating for immunogenicity, likely because purines are not available to the organism within the mammalian host, Sigwart et al., 1989, Infect. Immun. 57:1858. hereby incorporated by reference. Because auxotrophic mutations may be complemented by homologous recombination events with wild type copies donated from environmental organisms or by acquiring the needed metabolite within the host, it would seem prudent for live vaccines to contain a second attenuating mutation in a different virulence mechanism, (i.e., not just a second mutation in the same metabolic pathway) . Additionally, in mice the aroA mutants have some residual virulence. Various strains with aroA mutations combined with phoP regulon mutations were investigated for virulence attenuation and immunogenicity. Table 4 demonstrates that a PhoP " or PhoP c mutation further attenuates aroA mutant S . typhimurium by at least 100-fold and that, at least at high levels of vaccinating organisms, immunogenicity is retained. Strains with both a pagC " and phoP c phenotype are also further attenuated than either mutation alone. Therefore, phoP regulon mutations may increase the safety of aroA live vaccine preparations. Table 4A Additional attenuation of aroA mutants by PhoP regulon mutations Survivors of varying numbers of Salmonella mutant organisms (*) Strain Phenotype 10 l 10' 10 10 5 10 10 (**) Table 4B Protective efficacy of Salmonella with aroA/ phoP regulon mutations Survivors of challenge doses of wild type organisms (*) Strain Phenotype Inoculum 5 x 10 5 5 x 10 7 10 4/4 5/5 10 4/4 4/5 10 5/5 10 5/5 10 5/5 10 5/5 10 5/5 10 5/5 5/5 10' 5/5 ( * ) Ratio of survivors to number of mice challenged. ( ** ) Indicates oral inoculation all other experiments were intraperitoneal inoculation. CS004 = aroA554: :rnI0. SL3261 = aroADE 407 hisG46. CS322 = aroA554 : : Tnl0 pho-24. CS323 = aroADE 407 pho-24. CS315 = aroA554 : : Tn20 phoP102 ι :Tnl0d-Cam. CS316 = aroADE 407 hisG46 phoP102 : :TnI Od-Cam. CS026 = pagCl : : TnpήoA pho-24 phoN2 zxx: : 6251TN20d-Cam. Salmonella typhi phoP Regulon Mutations The phoP regulon is at least partially conserved in S. typhi DNA hybridization studies as well as P22 bacteriophage transductional crosses have documented that the phoP, phoQ, and pagC genes appear highly conserved between S. typhi and S. typhimurium mutations in these genes in S. typhi have been made. Salmonella Live Vaccines as Delivery Systems for Heterologous Antigens The vector used in the vaccine delivery system is a derivative of pJM703.1 described in Miller et al., 1988, J. Bact. 170:2575, hereby incorporated by reference. This vector is an R6K derivative with a deletion in the pir gene. R6K derivatives require the protein product of the pir gene to replicate. E. coli that contain the pir gene present as a lambda bacteriophage prophage can support the replication of this vector. Cells that do not contain the pir gene will not support the replication of the vector as a plasmid. This vector also contains the moj region of RP4 which will allow mobilization into other gram negative bacteria by mating from E. coli strains such as SMIOlambda pir, which can provide the mobilization function in trans. The pagC region is shown in Figs. 2 and 3. Fig. 2 shows the restriction endonuclease sites of the pagC locus. The heavy bar indicates page coding sequence. The TnphoA insertion is indicated by a inverted triangle. The direction of transcription is indicated by the arrow and is left to right. The numbers indicate the location of endonuclease sites, in number of base pairs, relative to the start codon of predicted pagC translation with positive numbers indicating location downstream of the start codon and negative numbers indicating location upstream of the start codon. A is AccI , B is Bg l, C is Clal, D is Dral, E is _3σoRI, H is Hpal, N is Nrul , P is PstI, S is Sspl , T is StuI, U is PvuII, V is EσoRV, and II is Bglll. Fig. 3 shows the DNA sequence (Sequence I.D. No. 1) and translation of pagC: :Tnp oA. The heavy underlined sequence indicates a potential riboso al binding site. The single and double light underlines indicate sequences in which primers were constructed complementary to these nucleotides for primer extension of RNA analysis. The asterix indicates the approximate start of transcription. The arrow indicates the direction of transcription. The boxed sequences indicate a region that may function in polymerase binding and recognition. The inverted triangle is the site of the sequenced TnphoA insertion junction. The arrow indicates a potential site for single sequence cleavage. 3 kilobases of DNA containing the pagC gene (from the PstI restriction endonuclease site 1500 nucleotides 5' to the start of pagC translation to the EcoRI restriction endonuclease site 1585 nucleotides downstream of pagC translation termination) were inserted into the pJM703.1 derivative discussed above. The pagC sequence from the Clal restriction endonuclease site was deleted (490 nucleotides) and replaced with a synthetic oligonucleotide polylinker that creates unique restriction endonuclease sites. DNA encoding one or more heterologous proteins, e.g., an antigen, can be inserted into this site. This creates a vector which allows the insertion of multiple foreign genes into the DNA surrounding page. The vector can be mobilized into Salmonella by mating or any other delivery system, e.g., heat shock, bacteriophage transduction or electroporation. .Since it can not replicate, the vector can only insert into Salmonella by site specific recombination with the homologous DNA on both sides of the pagC gene. This will disrupt and inactivate the native pagC locus and replace it with the disrupted pagC DNA carried on the vector. Such recombination events can be identified by marker exchange and selective media if the foreign DNA inserted into the pagC locus confers a growth advantage. The insertion of antibiotic resistance genes for selection is less desirable as this could allow an increase in antibiotic resistance in the natural population of bacteria. Genes which confer resistance to substances other than antibiotics e.g., to heavy metals or arsenic (for mercury resistance, see Nucifora et al., 1989, J. Bact. , 171:4241-4247, hereby incorporated by reference) , can be used to identify transformants. Alternatively, selection can be performed using a Salmonella recipient strain that carries an auxotrophic mutation in a metabolic pathway and a vector that carries DNA that compliments the auxotrophic mutation. Many Salmonella live vaccine prototypes contain mutations in histidine or purine pathways thus complementation of these metabolic auxotrophies can be used to select for integrants. (Purine mutations specifically have been shown to be too attenuated for use in man.) Further proof of marker exchange can be documented by loss of the ampicillin resistance (carried on the plasmid backbone) or by blot hybridization analysis. A gene useful for selection can be cloned by complementation of a vaccine strain with a metabolic auxotrophy. Specific examples include the cloning of the DNA encoding both purB and phoP by complementation of a strain deleted for function of both these genes. Salmonella gene libraries have been constructed in a pLAFR cosmid vector (Frindberg et al., 1984, Anal. Biochem. 137:266-267, hereby incorporated by reference) by methods known to those skilled in the art. pLAFR cosmids are broad host range plasmids which can be mobilized into Salmonella from E. coli . An entire bank of such strains can be mobilized into Salmonella vaccine strains and selected for complementation of an auxotrophic defect (e.g., in the case of purB growth on media without adenine) . The DNA able to complement this defect is then identified and can be cloned into the antigen delivery vector. As discussed above heterologous genes can be inserted into the polylinker that is inserted into the pagC sequence of the vector. The heterologous genes can be under the control of any of numerous environmentally regulated promotor systems which can be expressed in the host and shut off in the laboratory. Because the expression of foreign proteins, especially membrane proteins (as are most important antigens) , is frequently toxic to the bacterium, the use of environmentally regulated promoters that would be expressed in mammalian tissues at high levels but which could be grown in the laboratory without expression of heterologous antigens would be very desirable. Additionally, high expression of antigens in host tissues may result in increased attenuation of the organism by diverting the metabolic fuel of the organism to the synthesis of heterologous proteins. If foreign antigens are specifically expressed in host phagocytic cells this may increase the immune response to these proteins as these are the cells responsible for processing antigens. The promoter systems likely to be useful include those nutritionally regulated promoter systems for which it has been demonstrated that a specific nutrient is not available to bacteria in mammalian hosts. Purines, Sigwart et al., 1989, Infect. Immun. , 52:1858 and iron, Finklestein et al., 1983, Rev. Infect. Dis. 5:S759, e.g., are not available within the host. Promoters that are iron regulated, such as the aerobactin gene promoter, as well as promoters for biosynthetic genes in purine pathways, are thus excellent candidates for testing as promoters that can be shut down by growth in high concentrations of these nutrients. Other useful environmentally regulated Salmonella promoters include promoters for genes which encode proteins which are specifically expressed within macrophages, e.g., the DnaK and GroEL proteins, which are increased by growth at high temperature, as well as some phoP activated gene products, Buchmeier et al., 1990, Science 248:730. hereby incorporated by reference. Therefore, promoters such as the page 5' controlling sequences and the better characterized promoters for heat shock genes, e.g., GroEL and DnaK, will be expected to be activated specifically within the macrophage. The macrophage is the site of antigen processing and the expression of heat shock genes in macrophages and the wide conservation of heat shock genes in nature may explain the immunodominance of these proteins. A consensus heat shock promoter sequence is known and can be used in the vectors (Cowling et al., 1985, Proc. Natl. Acad. Sci. USA 8_2:2679, hereby incorporated by reference) . The vectors can include an environmentally regulated T7 polymerase amplification system to express heterologous proteins. For example, the T7 polymerase gene (cloned by Stan Tabor and Charles Richardson, See Current Protocols in Molecular Biology ed. Ausubel et al., 1989, (page 3.5.1.2) John Wiley and Sons, hereby incorporated by reference) under control of an iron regulated promoter, can be included on the vectors described above. We have inserted the aerobactin gene promoter of _?. coli with the sequence CATTTCTCATTGATAATGAGAATCATTATTGACATAATTGTTATTATTTTACG (Sequence ID No. 2) , Delorenzo et al. J. Bact. 169:2624, hereby incorporated by reference, in front of the T7 polymerase gene and demonstrated iron regulation of the gene product. This version of the vector will also include one or more heterologous antigens under the control of T7 polymerase promoters. It is well known that RNA can be synthesized from synthetic oligonucleotide T7 promoters and purified T7 in vitro. When the organism encounters low iron T7 polymerase will be synthesized and high expression of genes with T7 promoters will be facilitated. The pagC gene and paσC Gene Product Strains, materials, and methods The following strains, materials, and methods were used in the cloning of pagC and in the analysis of the gene and its gene product. Rich media was Luria broth (LB) and minimal media was M9, Davis et al., 1980, supra. The construction of S. typhimurium strain CS119 pagCl: :TnpnoA phoN2 zxx::6251 TnlOd-Cam was previously described. Miller et al., 1989, supra. American Type Culture Collection (ATCC) S. typhimurium strain 10428 included CS018 which is isogenic to CS119 except for phoP105 : :Tnl0d, Miller et al., 1989, supra, CS022 pho-24 , Miller et al., 1990, J. Bacteriol. 172:2485-2490. hereby incorporated by reference, and CS015 phoP102 : :Tnl0d-cam, Miller et al., 1989, supra. Other wild type strains used for preparation of chromosomal DNA included S. typhimurium LT2 (ATCC 15277) , S. typhimurium Ql and S. dry pool (Dr. J. Peterson U. Texas Medical Branch, Galveston) , and Salmonella typhi Ty2 (Dr. Caroline Hardegree, Food and Drug Administration) . pLAFR cosmids were mobilized from E. coli to s. typhimurium using the E. coli strain MM294 containing pRK2013, Friedman et al., 1982, Gene 18:289- 296, hereby incorporated by reference. Alkaline phosphatase (AP) activity was screened on solid media using the chromogenic phosphatase substrate 5-bromo-4- chloro-3-indolyl phosphate (XP) . AP assays were performed as previously described, Brickman et al., 1975, J. Mol. Biol. £6:307-316, hereby incorporated by reference, and are reported in units as defined by Miller, Miller, 1972, supra, pp. 352-355. One dimensional protein gel electrophoresis was performed by the method of Laemmli, 1970, Nature, 227:680-685. hereby incorporated by reference, and blot hybridization using antibody to AP was performed as previously described, Peterson et al. , 1988, Infect. Immun. 5_6:2822-2829, hereby incorporated by reference. Whole cell protein extracts were prepared, from saturated cultures grown in LB at 37°C with aeration, by boiling the cells in SDS-pagE sample buffer, Laemmli, 1970, supra. Two dimensional gel electrophoresis was performed by the method of O'Farrell, 1975, J. Biol. Chem. 250:4007, hereby incorporated by reference. Proteins in the 10% polyacrylamide slab gels were visualized by silver staining, Merril et al., 1984, Methods in Enzymology, 104:441. hereby incorporated by reference. Chromosomal DNA was prepared by the method of Mekalanos, 1983, Cell, 3_5_:253-263, hereby incorporated by reference. DNA, size fractionated in agarose gels, was transferred to nitrocellulose (for blot hybridization) by the method of Southern, 1975, J. Mol. Biol. 98.:503-517, hereby incorporated by reference. DNA probes for Southern hybridization analysis were radiolabeled by the random primer method, Frinberg et al., 1984, supra. Plasmid DNA was transformed into E. coli and Salmonella by calcium chloride and heart shock, Mekalanos, 1983, supra, or by electroporation using a Genepulser apparatus (Biorad, Richmond, Ca.) as recommended by the manufacturer. Dower et al., 1988, Nucl. Acids Res. 16:6127-6145, hereby incorporated by reference. DNA sequencing was performed by the dideoxy chain termination method of Sanger et al., 1977, Proc. Natl. Acad. Sci. USA, 24:5463-5467, hereby incorporated by reference, as modified for use with Sequenase (U.S. Biochemical, Cleveland, Ohio) . Oligonucleotides were synthesized on an Applied Biosystems Machine and used as primers for sequencing reactions and primer extension of RNA. Specific primers unique to the two ends of TnphoA, one of which corresponds to the alkaline phosphatase coding sequence and the other to the right IS50 sequence, were used to sequence the junctions of the transposon insertion. Construction of a S. typhimurium cosmid gene bank in pLAFR3 and screening for clones containing the wild type pagC DNA was performed as follows. DNA from S. typhimurium strain ATCC 10428 was partially digested using the restriction endonuclease Sau3A and then size selected on 10-40% sucrose density gradient. T4 DNA ligase was used to ligate chromosomal DNA of size 20-30 kilobases into the cosmid vector pLAFR3, a derivative of pLAFRl, Friedman et al., 1982, Gene .18:289-296, hereby incorporated by reference, that was digested with the restriction endonuclease BamHI. Cosmid DNA was packaged and transfected into E. coli strain DH5-α using extracts purchased from Stratagene, La Jolla, Ca. Colonies were screened by blot hybridization analysis. The analysis of proteins produced from cloned DNA by in vitro transcription/translation assays was analyzed as follows. These assays were performed with cell free extracts, (Amersham, Arlington Heights, Illinois) , and were performed using conditions as described by the manufacturer. The resultant radiolabeled proteins were analyzed by SDS-pagE. RNA was purified from early log and stationary phase Salmonella cultures by the hot phenol method. Case et al., 1988, Gene 22:219-236, hereby incorporated by reference, and run in agarose-formaldehyde gels for blot hybridization analysis, Thomas, 1980, Proc. Natl. Acad. Sci. USA 22*5201, hereby incorporated by reference. Primer extension analysis of RNA was performed as previously described, Miller et al. , 1986, Nuc. Acids. Res. 14.:7341-7360, hereby incorporated by reference, using AMV reverse transcriptase (Promega, Madison, Wisconsin) and synthesized oligonucleotide primers complementary to nucleotides 335-350 and 550-565 of the page locus. Identification of an 18 kDa protein missing in a paσC mutant of S. typhimurium page mutant strain CS119 was analyzed by two dimensional protein electrophoresis to detect protein species that might be absent as a result of the TnphoA insertion. Only a single missing protein species, of approximately 18 kD and pI-8.0, was observed when strains, isogenic except for their transposon insertions, were subjected to this analysis. This 18 kDa species was also missing in similar analysis of Salmonella strains with mutations phoP and phoQ. Though two-dimensional protein gel analysis might not detect subtle changes of protein expression in strain CS119, this suggested that a single major protein species was absent as a result of the page: : TnphoA insertion. Additional examination of the 2-dimensional gel analysis revealed a new protein species of about 45 kDa that is likely the pagC-Ap fusion protein. The pagC-AP fusion protein was also analyzed by Western blot analysis using antisera to AP and found to be similar in size to native AP (45 kDa) and not expressed in PhoP-S. typhimurium . Cloning of the pagrC: :TnphoA insertion Chromosomal DNA was prepared from S. typhimurium strain CS119 and a rough physical map of the restriction endonuclease sites in the region of the page::TnphoA fusion was determined by using a DNA fragment of TnphoA as a probe in blot hybridization analysis. This work indicated that digestion with the restriction endonuclease ecoRV yielded a single DNA fragment that included the pagC::TnphoA insertion in addition to several kilobases of flanking DNA. Chromosomal DNA from strain CS119 was digested with UcoRV (blunt end) and ligated into the bacterial plasmid vector pUC19 (New England Biolabs) that had been digested with the restriction endonuclease Smal (blunt end) . This DNA was electroporated into the E. coli strain DH5-α (BRL) and colonies were plated onto LB agar containing the antibiotics kanamycin (TnphoA encoded and ampicillin (pUC19 encoded) . A single ampicillin and kanamycin resistant clone containing a plasmid designated pSMlOO was selected for further study. A radiolabeled DNA probe from pSMlOO was constructed and used in Southern hybridization analysis of strain CS119 and its wild type parent ATCC 10428 to prove that the pagC: :TnphoA fusion had been cloned. The probe contained sequences immediately adjacent to the transposon at the opposite end of the alkaline phosphatase gene [Hpal endonuclease generated DNA fragment that included 186 bases of the right IS50 of the transposon and 1278 bases of Salmonella DNA (Fig. 2) . As expected, the pSMlOO derived probe hybridized to an 11- 12 kb AccJ endonuclease digested DNA fragment from the strain containing the transposon insertion, CS119. This was approximately 7.7kb (size of TnphoA) larger than the 3.9 kB AccJ fragment present in the wild type strain that hybridizes to the probe. In addition, a derivative of plasmid pSMlOO, pSMlOl (which did not allow expression of the pagC-PhoA gene fusion off the lac promoter) , was transformed into phoP- (strain Cs015) and phoN- (strain CS019) Salmonella strains and the cloned AP activity was found to be dependent on phoP for expression. Therefore we concluded that the cloned DNA contained the page::TnphoA fusion. The presence of the page gene was also demonstrated in other strains of S. typhimurium, as well as in S. typhi, and S. drypool . All Salmonella strains examined demonstrated similar strong hybridization to an 8.0 kb JScoRV and a 3.9 kb Accil restriction endonuclease fragment suggesting that page is a virulence gene common to Salmonella species. The pagC gene probe from nucleotides -46 (with 1 as the first base of the methionine to 802 (PstI site to the Bg ll site) failed to cross hybridize to DNA from Citrobacter freundii, Shigella flexneri, Shigella sonnei, Shigella dysenterial, Escherichia coli. Vibrio cholerae, Vihrio vulnificus, Yersenia entero colitica, and Klibsiella pneumonia. Cloning of the wild type pagC locus DNA and its complementation of the virulence defect of a S. typhimurium pa e mutant The same restriction endonuclease fragment described above was used to screen a cosmid gene bank of wild type strain ATCC 10428. A single clone, designated pWP061, contained 18 kilobases of S. typhimurium DNA and hybridized strongly to the pagC DNA probe. pWPOβl was found to contain Salmonella DNA identical to that of pSMlOO when analyzed by restriction endonuclease analysis and DNA blot hybridization studies. Probes derived from pWP061 were also used in blot hybridization analysis with DNA from wild type and CS119 S. typhimurium. Identical hybridization patterns were observed to those seen with pSMlOO. pWP061 was also mobilized into strain CS119, a page mutant strain. The resulting strain had wild type virulence for BALB/c mice (a LD 50 less than 20 organisms when administered by IP injection) . Therefore the cloned DNA complements the virulence defect of a page mutant strain. Since, a wild type cosmid containing pagC locus DNA was found to complement the virulence defect of a pagC mutant S. typhimurium strain, it was concluded that the pagC protein is an 188 amino acid (18 kDa) membrane (see below) protein essential for survival within microphages and virulence of S. typhimurium. Physical mapping of restriction endonuclease sites f DNA sequencing, and determination of the pagC gene product Restriction endonuclease analysis of plasmid pSMlOO and pWP061 was performed to obtain a physical map of the page locus, and, in the case of PSMlOO, to determine the direction of transcription (Fig. 2) . DNA subclones were generated and the TnphoA fusion junctions were sequenced, as well as the Salmonella DNA extending from the Hpal site, 828 nucleotides 5' to the phoA fusion junction, to the .EcoRI site 1032 nucleotides 3' to the TnphoA insertion (Fig. 2 and 3). The correct reading frame of the DNA sequence was deduced from that required to synthesize an active AP gene fusion. The deduced amino acid sequence of this open reading frame was predicted to encode a 188 amino acid protein with a predicted pI+8.2. This data were consistent with the 2- D polyacrylamide gel analysis of strain CS119 in which an 18 kDa protein of approximate pI+8.0 was absent. No other open reading frames, predicted to encode peptides larger than 30 amino acids, were found. The deduced amino acid sequence of the 188 amino acid open reading frame contains a methionine start codon 33 amino acids from the fusion of pagC and AP (Fig. 3) . This 33 amino acid pagC contribution to the fusion protein was consistent with the size observed in Western blot analysis and contains a hydrophobic N-terminal region, identified by the method of Kyle et al., 1982, J. Mol. Biol. 157:105-132. hereby incorporated by reference, that is a typical bacterial signal sequence. Von Heinje, 1985, J. Mol. Biol. 184:99-105, hereby incorporated by reference. Specifically, amino acid 2 is a positively charged lysine, followed by a hydrophobic domain and amino acid 24 is a negatively charged aspartate residue. A consensus cleavage site for this leader peptide is predicted to be at an alanine residue at amino acid 23, Von Heinje, 1984, J. Mol. Biol. 121:243-251, hereby incorporated by reference. The DNA sequence also revealed a typical ribosomal binding site, Shine et al., 1974, Proc. Natl. Acad. Sci. USA 21:1342-1346, hereby incorporated by reference, at 6-2 nucleotides 5' to the predicted start of translation (Fig. 3) nucleotides 717- 723) . This suggested that the open reading frame was, in fact, translated and further supported the assumption that this was the deduced amino acid sequence of the pagC protein interrupted by the TnphoA insertion (Fig. 3) . In vitro synthesis of proteins by the cloned page locus To detect if other proteins were encoded by pagC and to determine the approximate size of the pagC gene product, an in vitro coupled transcription/translation analysis was performed. A 5.3 kilobase JJcoRI fragment of pWP06l was inserted into pUC19 so that the page gene would not be expressed off the lac promotor. This plasmid was used in an in vitro coupled transcription- translation assay. A single protein of approximately 22 kilodaltons was synthesized by the cell free system. The size was compatible with this being the precursor of the pagC protein containing its leader peptide. These data further support the conclusion the single and the single page gene product had been identified. Identification of the pagC encoded RNA An approximately 1100 nucleotide RNA is encoded by page. The page gene is highly expressed by cells with a phoP constitutive phenotype of pag activation, as compared to wild type and phoP constitutive phenotype of pag activation, as compared to wild type and phoP- bacteria. In these blot hybridization experiments page is only detected in wild type cells grown in rich media during stationary growth. This result, coupled with previous work, Miller et al., 1989, supra. Miller et al., 1990, supra, demonstrates that page is transcriptionally regulated by the phoP gene products and is only expressed during early logarithmic phase growth in rich media by cells with a phoP constitutive phenotype. The size of the page transcript is approximately 500 nucleotides greater than that necessary to encode the 188 amino acid protein. Primer extension analysis of Salmonella RNA using oligonucleotide primers specific for page sequence was performed to determine the approximate start site of transcription and to determine whether these nucleotides might be transcribed 5' or 3' to the 188 amino acid pagC gene product. Primer extension analysis with an oligonucleotide predicted to be complementary to nucleotides 550-565 of page, 150 nucleotides 5' to the predicted start codon, resulted in an approximately 300 nucleotide primer extension product. Therefore a primer further upstream was constructed complementary to nucleotides 335-350 of pagC and used in a similar analysis. A primer extension product of 180 nucleotides was observed to be primer specific. This is consistent with transcription starting at nucleotide 170 (Fig. 3) . Upstream of the predicted transcriptional start, at nucleotides 153-160, a classic RNA polymerase binding site was observed with the sequence TATAAT at - 12 nucleotides as well as the sequence TAATAT at -10 nucleotides. No complete matches were observed for the consensus RNA polymerase recognition site (TTGACA) 15-21 nucleotides upstream from the -10 region. AT -39 (126- 131) nucleotides (TTGGAA) , -38 (127-132) nucleotides (TTGTGG) , and -25 (135-140) nucleotides (TTGATT) are sequences that have matches with the most frequently conserved nucleotides of this sequence. Based on the above results transcription was predicted to terminate near the translational stop codon of the 188 amino acid protein (nucleotide 1295, Fig. 3) . Indeed, a stem loop configuration was found at nucleotides 1309-1330 that may function as a transcription terminator. This was consistent with the lack of evidence of open reading frames downstream of the 188 amino acid protein and the lack of synthesis of other transcription/translation using the cloned pagC DNA. This further suggests that the pagC::TnphoA insertion inactivated the synthesis of only a single protein. Similarity of pa C to Ail and Lom A computer analysis of protein similarity using the National Biomedical Research Foundation/Protein Identification Resource, George et al., 1986, Nucleic Acids Res. 14:11- 15, hereby incorporated by reference, protein sequence base was conducted to identify other proteins that had similarity to pagC in an attempt to find clues to the molecular function of this protein. Remarkably, pagC was found to be similar to a bacteriophage lambda protein, Lorn, that has been localized to the outer membrane in minicell analysis. Court et al., 1983, Lambda II, Hendrix, R.W. et al. ed. Cold Spring Harbor Laboratory (Cold Spring Harbor NY), pp. 251-277, hereby incorporated by reference, and demonstrated to be expressed by lambda lysogens of E. coli , Barondess, et al., 1990, Nature 346:871-874, hereby incorporated by reference. Recently, the deduced amino acid sequence of the cloned ail gene product of Y. enter ocol it ica was determined and found to also be similar to Lorn, Miller et al. , 1990b, J. Bacteriol. 122:1062-1069. Therefore, a protein family sequence alignment was performed using a computer algorithm that establishes protein sequence families and consensus sequences. Smith et al., 1990, Proc. Natl. Acad. Sci. 82:118-122, hereby incorporated by reference. The formation of this family is indicated by the internal data base values of similarity between these proteins : pagC and Lom (107.8), pagC and Ail (104.7), and Ail and Lom (89.8). These same proteins were searched against 314 control sequences in the data base and mean values and ranges were 39.3 (7.3-52.9) pagC, 37.4 (7.3-52.9) Ail, and 42.1 (7.0-61.9) Lom. The similarity values for this protein family are all greater than 3.5 standard deviations above the highest score obtained for similarity to the 314 random sequences. No other similarities or other family members were found in the database. Regions of similarity are located not only in the leader peptide transmembrane domains but throughout the protein. paσC Mutant Strains Are Attenuated For Virulence Salmonella typhimurium strains with a pagC mutation are most likely inactivated for the phoP- regulated gene product, as these strains are attenuated for virulence by at least 1,000-fold. Attenuation of Bacterial Virulence by Constitutive Expression of Two-component Regulatory Systems. The virulence of a bacterium can be attenuated by inducing a mutation or which results in the constitutive expression of genes under the control of a two-component regulatory system or by inducing a mutation that inactivates a gene under the control of the two-component systems. A balance between the expression of the genes under the control of the two-component system, e.g., between pag and prg gene expression, and possibly beteen two-component system regulated genes and other genes, is necessary for full virulence. Mutations that disrupt this balance, e.g., mutations that cause the constitutive expression of a gene under the control of the two- component system, or a mutation that inactivates a gene under the control of the two-component system, e.g., the pag gene, reduce virulence. Constitutive mutations in two-component regulators can be identified by the use of a strain containing a recorder gene fusion to a gene regulated by the two-component system. Such gene fusions would most typically include DNA encoding the aσZ gene or alkaline phosphatase fused to a gene under the control of the two- component system. Strains containing fusions that are (as compared to wild type or parental strains) highly expressed in an unregulated fashion, i.e., constitutive, can be detected by increased color on chromogenic substrates for the enzymes. To detect constitutive mutations a cloned virulence regulator could be mutagenized e.g., by passage through an E. coli strain defective in DNA repair or by chemical mutagenesis. The mutated DNA for the regulator would then be transferred to the strain containing the gene fusion and constitutive mutations identified by the high gene fusion expression (blue color in the case of a acZ fusion grown on media containing X-gal) . Constitutive mutations in a component of a two-component regulatory system could also be made by in vitro mutagenesis after other constitutive mutations have been sequenced and a specific amino acid change responsible for constitutivity identified. Putting several amino acid changes that all result in a PhoP constitutive phenotype would result in a decreased frequency of reversion by spontaneous base changes. A constitutive mutation could also be constructed by deletion of the portion of the amino terminus of the phospho-accepting regulator which contains the phosphoacceptor domain e.g., deletion of sequences encoding amino acids amino terminal to amino acid 119 in the phoP gene or deletion of analogous phospho accepting sequences in genes of other two-component regulatory systems. This could result in a conformational change similar to that induced by phosphorylation and result in increased DNA binding and transcriptional activation. Use The Salmonella cells of the invention are useful as sources of immunological protection against diseases, e.g., typhoid fever and related diseases, in an animal, e.g., a mammal, e.g., a human, in particular as the basis of a live-cell vaccine capable of colonizing the inoculated animal's intestine and provoking a strong immune reaction. Appropriate dosages and conditions of administration of such a live, attenuated vaccine are as described in Holem et al.. Acute Enteric Infections in Children. New Prospects for Treatment and Prevention (1981) Elsevier/North-Holland biomedical Press, Ch. 26, pp. 443 et seq. (Levine et al.), hereby incorporated by reference. Other Embodiments Other embodiments, e.g., strains which in addition to a phoP related mutation or genetic alteration also contain an attenuating mutation in another gene, e.g., an aromatic amino acid synthetic gene, e.g., aroA or aroD, or in cya gene (adenylate cyclase) or crp gene (adenylate cyclase receptor) are also within the claims. What is claimed is: COMPUTER SUBMISSION OF DNA AND AMINO ACID SEQUENCES (1) GENERAL INFORMATION: (i) APPLICANT: Miller, Samuel I. Mekalanos, John J. (ii) TITLE OF INVENTION: Improved Vaccines (iii) NUMBER OF SEQUENCES: 2 (iv) CORRESPONDENCE ADDRESS: (A) ADDRESSEE: Fish & Richardson (B) STREET: 225 Franklin Street (C) CITY: Boston (D) STATE: Massachusetts (E) COUNTRY: U.S.A. (F) ZIP CODE: 02110-2804 (v) COMPUTER READABLE FORM: (A) MEDIUM TYPE: 3.5" Diskette, 1.44 Mb storage (B) COMPUTER: IBM PS/2 Model 50Z or 55SX (C) OPERATING SYSTEM: IBM P.C. DOS (Version 3.30) (D) SOFTWARE: WordPerfect (Version 5.0) (vi) CURRENT APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (C) CLASSIFICATION: (vii) PRIOR APPLICATION DATA: (A) APPLICATION NUMBER: (B) FILING DATE: (viii) ATTORNEY/AGENT INFORMATION: (A) NAME: Clark, Paul T. (B) REGISTRATION NUMBER: 30,162 (C) REFERENCE/DOCKET NUMBER: 00786/065001 (ix) TELECOMMUNICATION INFORMATION: (A) TELEPHONE: (617) 542-5070 (B) TELEFAX: (617) 542-8906 (C) TELEX: 200154 (2 ) INFORMATION FOR SEQUENCE IDENTIFICATION NUMBER: ( i) SEQUENCE CHARACTERISTICS : (A) LENGTH: 2320 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (xi) SEQUENCE DESCRIPTION: SEQUENCE ID NO: 1: GTTAACCACT CTTAATAATA ATGGGTTTTA TAGCGAAATA CACTTTTTTA TCGCGTGTTC 60 AATATTTGCG TTAGTTATTA TTTTTTTGGA ATGTAAATTC TCTCTAAACA CAGGTGATAT 120 TTATGTTGGA ATTGTGGTGT TGATTCTATT CTTATAATAT AACAAGAAAT GTTGTAACTG 180 ATAGATATAT TAAAAGATTA AATCGGAGGG GGAATAAAGC GTGCTAAGCA TCATCGTGAA 240 TATGATTACA GCGCCTGCGA TGGCATATAA CCGTATTGCG GATGGAGCGT CACGTGAGGA 300 CTGTGAAGCA CAATGOGATA TGTTCTGATT ATATGGCGAG TTTGCTTAAT GACATGTTTT 360 TAGCCGAACG GTGTCAAGTT TCTTAATGTG GTTGTGAGAT TTTCTCTTTA AATATCAAAA 420 TGTTGCATGG GTGATTTGTT GTTCTATAGT GGCTAAAGAC TTTATGGTTT CTGTTAAATA 480 TATATGCGTG AGAAAAATTA GCATTCAAAT CTATAAAAGT TAGATGACAT TGTAGAACCG 540 GTTACCTAAA TGAGCGATAG AGTGCTTCGG TAGTAAAAAT ATCTTTCAGG AAGTAAACAC 600 ATCAGGAGCG ATAGCGGTGA ATTATTCGTG GTTTTGTCGA TTCGGCATAG TGGCGATAAC 660 TGAATGCCGG ATCGGTACTG CAGGTGTTTA AACACACCGT AAATAATAAG TAGTATTAAG 720 GAGTTGTT 728 ATG AAA AAT ATT ATT TTA TCC ACT TTA GTT ATT ACT ACA AGC GTT TTG 776 Met Lys Asn lie lie Leu Ser Thr Leu Val lie Thr Thr Ser Val Leu 5 10 15 GTT GTA AAT GTT GCA CAG GCC GAT ACT AAC GCC TTT TCC GTG GGG TAT 824 Val Val Asn Val Ala Gin Ala Asp Thr Asn Ala Phe Ser Val Gly Tyr 20 25 30 GCA CGG TAT GCA CAA AGT AAA GTT CAG GAT TTC AAA AAT ATC CGA GGG 872 Ala Arg Tyr Ala Gin Ser Lys Val Gin Asp Phe Lys Asn He Arg Gly 35 40 45 GTA AAT GTG AAA TAC CGT TAT GAG GAT GAC TCT CCG GTA AGT TTT ATT 920 Val Asn Val Lys Tyr Arg Tyr Glu Asp Asp Ser Pro Val Ser Phe He 50 55 60 TCC TCG CTA AGT TAC TTA TAT GGA GAC AGA CAG GCT TCC GGG TCT GTT 968 Ser Ser Leu Ser Tyr Leu Tyr Gly Asp Arg Gin Ala Ser Gly Ser Val 65 70 75 80 GAG CCT GAA GGT ATT CAT TAC CAT GAC AAG TTT GAG GTG AAG TAC GGT 1016 Glu Pro Glu Gly He His Tyr His Asp Lys Phe Glu Val Lys Try Gly 85 90 95 TCT TTA ATG GTT GGG CCA GCC TAT CGA TTG TCT GAC AAT TTT TCG TTA 1064 Ser Leu Met Val Gly Pro Ala Tyr Arg Leu Ser Asp Asn Phe Ser Leu 100 105 110 TAC GCG CTG GCG GGT GTC GGC ACG GTA AAG GCG ACA TTT AAA GAA CAT 1112 Tyr Ala Leu Ala Gly Val Gly Thr Val Lys Ala Thr Phe Lys Glu His 115 120 125 TCC ACT CAG GAT GGC GAT TCT TTT TCT AAC AAA ATT TCC TCA AGG AAA 1160 Ser Thr Gin Asp Gly Asp Ser Phe Ser Asn Lys He Ser Ser Arg Lys 130 135 140 ACG GGA TTT GCC TGG GGC GCG GGT GTA CAG ATG AAT CCG CTG GAG AAT 1208 Thr Gly Phe Ala Trp Gly Ala Gly Val Gin Met Asn Pro Leu Glu Asn 145 150 155 160 ATC GTC GTC GAT GTT GGG TAT GAA GGA AGC AAC ATC TCC TCT ACA AAA 1256 He Val Val Asp Val Gly Tyr Glu Gly Ser Asn He Ser Ser Thr Lys 165 170 175 ATA AAC GGC TTC AAC GTC GGG GTT GGA TAC CGT TTC TGA AAAGC 1300 He Asn Gly Phe Asn Val Gly Val Gly Tyr Arg Phe 180 185 ATAAGCTATG CGGAAGGTTC GCCTTCCGCA CCGCCAGTCA ATAAAACAGG GCTTCTTTAC 1360 CAGTGACACG TACCTGCCTG TCTTTTCTCT CTTCGTCATA CTCTCTTCGT CATAGTGACG 1420 CTGTACATAA CATCTCACTA GCATAAGCAC AGATAAAGGA TTGTGGTAAG CAATCAAGGT 1480 TGCTCAGGTA GGTGATAAGC AGGAAGGAAA ATCTGGTGTA AATAACGCCA GATCTCACAA 1540 GATTCACTCT GAAAAATTTT CCTGGAATTA ATCACAATGT CATCAAGATT TTGTGACCGC 1600 CTTCGCATAT TGTACCTGCC GCTGAACGAC TACTGAAAAG TAGCAAGGTA TGTATTTTAT 1660 CCAGGAGAGC ACCTTTTTTG CGCCTGGCAG AAGTCCCCAG CCGCCACTAG CTCAGCTGGA 1720 TAGAGCATCA ACCTCCTAAG TTGATGGTGC GAGGTTCGAG GCCTCGGTGG CGGTCCAATG 1780 TGGTTATCGT ATAATGTTAT TACCTCAGTG TCAGGCTGAT GATGTGGGTT CGACTCCCAC 1840 TGACCACTTC AGTTTTGAAT AAGTATTGTC TCGCAACCCT GTTACAGAAT AATTTCATTT 1900 ATTACGTGAC AAGATAGTCA TTTATAAAAA ATGCACAAAA ATGTTATTGT CTTTTATTAC 1960 TTGTGAGTTG TAGATTTTTC TTATGCGGTG AATCCCCCTT TGCGGCGGGG CGTCCAGTCA 2020 AATAGTTAAT GTTCCTCGCG AACCATATTG ACTGTGGTAT GGTTCACCGG GAGGCACCCG 2080 GCACCGCAAT TTTTTATAAA ATGAAATTCA CACCCTATGG TTCAGAGCGG TGTCTTTTTA 2140 CATCAGGTGG GCAAGCATAA TGCAGGTTAA CTTGAAAGAT ACGATCAATA GCAGAAACCA 2200 GTGATTTCGT TTATGGCCTG GGGATTTAAC CGCGCCAGAG CGTATGCAAG ACCCTGGCGC 2260 GGTTGGCCGG TGATCGTTCA ATAGTGCGAA TATGAATGGT TACCAGCCGC CTGCGAATTC 2320 (2) INFORMATION FOR SEQUENCE IDENTIFICATION NUMBER: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 53 (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) SEQUENCE DESCRIPTION: SEQUENCE ID NO: 2: CATTTCTCAT TGATAATGAG AATCATTATT GACATAATTG TTATTATTTT ACG 53

Claims: Claims 1. A vaccine comprising a Salmonella cell the virulence of which is attenuated by a first mutation in the phoP regulatory region causing constitutive expression of a gene under the control of said region and by a second mutation at an aro, pag, or prg gene. 2. A vaccine comprising a Salmonella cell the virulence of which is attenuated by a mutation in a pag or a prg gene and by a mutation in an aro gene. 3. A Salmonella cell which constitutively expresses a phoP regulatory region regulated gene and which comprises a virulence attenuating mutation in an aro, a prg, or a pag gene. 4. A Salmonella cell which comprises a first virulence attenuating mutation in a pag or a prg gene and a second virulence attenuating mutation in an aro gene. 5. A live Salmonella cell in which there is inserted into a pag or a prg gene a gene encoding a heterologous protein, or a regulatory element, of said heterologous protein gene. 6. The live Salmonella cell of claim 5, wherein said DNA encoding a heterologous protein is under the control of an environmentally regulated promoter. 7. A vector capable of integrating into the chromosome of Salmonella comprising a first DNA sequence encoding a heterologous protein, a second DNA sequence encoding a marker, and a third DNA sequence encoding a product necessary for virulence, said third DNA sequence being mutationally inactivated. 8. A vector comprising DNA which encodes the page gene product. 9. A purified preparation of the page gene product. 10. A method of detecting the presence of Salmonella in a sample comprising contacting said sample with pagC encoding DNA and detecting the hybridization of said pagC encoding DNA to nucleic acid in said sample.

Inventors: MEKALANOS JOHN J, MILLER SAMUEL I III, MEKALANOS, JOHN, J., MILLER, SAMUEL, I., III
Applicants: GEN HOSPITAL CORP, HARVARD COLLEGE, PRESIDENT AND FELLOWS OF HARVARD COLLEGE, THE GENERAL HOSPITAL CORPORATION
Application year: 1991
Main classification: C12Q1
IPC codes: A61K39, A61P31, C07K14, C12N1, C12N15, C12Q1, C12R1