Role of the Pst system in plaque formation by the intracellular pathogen Shigella flexneri

In response to the host cell environment, the intracellular pathogen Shigella flexneri induces the expression of numerous genes, including those in the pst operon which is predicted to encode a high-affinity phosphate acquisition system that is expressed under reduced phosphate conditions. An S. flexneri pst mutant forms smaller plaques in Henle cell monolayers than does the parental strain. This mutant exhibited normal production and localization of the S. flexneri IcsA protein. The pst mutant had the same growth rate as the parental strain in both phosphate-reduced and phosphate-replete media in vitro and during the first 3 h of growth in Henle cells in vivo. During growth in phosphate-replete media, the PhoB regulon was constitutively expressed in the pst mutant but not the parental strain. This suggested that the inability of the S. flexneri pst mutant to form wild-type plaques in Henle cell monolayers may be due to aberrant expression of the PhoB regulon. A mutation in phoB was constructed in the S. flexneri pst mutant, and the phoB mutation suppressed the small plaque phenotype of the pst mutant. Additionally, a specific mutation (R220Q) was constructed in the pstA gene of the pst operon that was predicted to eliminate Pst-mediated phosphate transport but allow normal PhoB-regulated gene expression, based on the phenotype of an Escherichia coli strain harboring the same mutation. Addition of this pstA(R220Q) mutation to a S. flexneri pst mutant, as part of the pst operon, restored normal plaque formation and regulation of phoA expression.