Phosphorylated OmpR Is Required for Type 3 Fimbriae Expression in Under Hypertonic Conditions.

OmpR/EnvZ is a two-component system that senses osmotic signals and controls downstream gene expression in many species of . However, the role of OmpR/EnvZ in remains unknown. In this study, we found that production of MrkA, the major subunit of type 3 fimbriae, was decreased under hypertonic conditions. A deletion mutant of and a site-directed mutant with a single amino acid substitution of aspartate 55 to alanine (D55A), which mimics the unphosphorylated form of OmpR, markedly reduced MrkA production under hypertonic conditions. These results indicate that type 3 fimbriae expression is activated by the phosphorylated form of OmpR (OmpR∼P). Although no typical OmpR∼P binding site was found in the P sequence, mRNA levels and P activity were decreased in the Δ and strains compared with the wild type (WT) strain, indicating that OmpR∼P mediates type 3 fimbriae expression at the transcriptional level. Previous reports have demonstrated that a cyclic-di-GMP (c-di-GMP) related gene cluster, , regulates the expression of type 3 fimbriae. We found that both the and mutants exhibited decreased mRNA levels, intracellular c-di-GMP concentration, and bacterial biofilm amount, but increased total intracellular phosphodiesterase activity in response to hypertonic conditions. These results indicate that OmpR∼P regulates type 3 fimbriae expression to influence biofilm formation via MrkHIJ and modulation of intracellular c-di-GMP levels. Taken together, we herein provide evidence that OmpR∼P acts as a critical factor in the regulation of the c-di-GMP signaling pathway, type 3 fimbriae expression, and biofilm amount in in response to osmotic stresses.