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OmpR/EnvZ is a two-component system that senses osmotic signals and controls downstream gene expression in many species of  Enterobacteriaceae . However, the role of OmpR/EnvZ in  Klebsiella pneumoniae  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  ompR  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  K. pneumoniae  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  mrkA   sequence,  mrkA  mRNA levels and P  mrkA   activity were decreased in the Δ ompR  and  ompR  D55A  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,  mrkHIJ , regulates the expression of type 3 fimbriae. We found that both the  ompR  and  ompR  D55A  mutants exhibited decreased  mrkHIJ  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  K. pneumoniae  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  K. pneumoniae  in response to osmotic stresses.

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