We studied a protein antigen, designated pH 6 Ag, that has the same regulation of expression as the previously described Yersinia pestis pH 6 Ag. Monospecific antiserum to this antigen recognized several proteins, ranging from 15 to over 75 kilodaltons (kDa), which were strongly expressed when Y. pestis was cultivated at 37 degrees C and pH 6 but were expressed weakly, if at all, at 37 degrees C and pH 8 and at 26 degrees C. The antigen appeared to be composed of aggregates of a 15-kDa subunit. Escherichia coli minicell analysis and Western blotting (immunoblotting) of minicell extracts containing the cloned pH 6 Ag locus revealed that a 1.7-kilobase-pair (kb) region of Y. pestis chromosomal DNA produced 16- and 15-kDa immunoreactive proteins. We used transposon mutagenesis of the pH 6 Ag-coding region to demonstrate that the 16- and 15-kDa polypeptides were produced by the same cistron. The pH 6 Ag structural gene, psaA, was located within a 0.5-kb region of DNA. A Tn10lacZ transposon insertion 1.2 kb upstream of the psaA locus but outside the psaA transcriptional unit caused decreased expression of pH 6 Ag in both E. coli and Y. pestis and defined the psaE locus necessary for maximum pH 6 Ag expression. This locus itself was not regulated by temperature or pH. However, psaA remained responsive to both of these environmental signals in a Y. pestis psaE mutant. Mutation of either psaE or psaA resulted in at least a 100-fold reduction in the intravenous 50% lethal dose of Y. pestis in mice. Accordingly, pH 6 Ag is involved in the pathogenesis of bubonic plague.