Shigella flexneri is the predominant bacterial causative agent of human bacillary dysentery, responsible for 125 million endemic cases in Asia annually. The ability of the pathogen to survive in the diverse environmental conditions that it encounters is dependent on its ability to regulate the expression and function of a wide variety of virulence factors. OmpR is the response regulator of the two component system EnvZ/OmpR, a DNA binding transcription factor which regulates Shigella flexneri virulence. It does this by regulating the expression of two membrane proteins, OmpC and OmpF in response to changes in osmolarity. We have recently found that OmpR is tyrosine phosphorylated on Y194, which is present in the DNA binding domain. With tyrosine phosphorylation emerging as a key post-transcriptional regulator of virulence in a range of bacterial pathogens, we hypothesised that this phosphorylation may modulate the activity of OmpR to regulate genes, and thus the virulence of S. flexneri.
To study the effect of tyrosine phosphorylation, we constructed phosphomimetic (OmpRY194E) and phosphoablative substitutions (OmpRY194F)-in the phosphorylated tyrosine. Phosphomimetic substitution resulted in aberrant regulation of OmpC and OmpF similar to that seen in an OmpR mutant. Furthermore, we showed that the OmpR also regulates the virulence factor IcsP. Western immunoblot analysis suggested that OmpR is tyrosine phosphorylated on additional as yet un-identified additional tyrosine residue(s). Thus, this work suggests that tyrosine phosphorylation modulates OmpR regulation, with work investigating whether this is due to an effect on the DNA binding capacity of this important virulence factor currently underway.