Autoinduction of the ompR response regulator by acid shock and control of the Salmonella enterica acid tolerance response

Salmonella enterica serovar Typhimurium periodically experiences acid stress in a variety of host and non‐host environments. An encounter with non‐lethal acid stress (pH > 4) induces an assortment of physiological changes, called the acid tolerance response (ATR), that helps the cell to tolerate extreme low pH (pH 3). These physiological changes differ in log phase and stationary phase cells and are controlled by different regulatory proteins. OmpR is an acid‐induced response regulator critical to the stationary phase ATR but not to the log phase ATR. As OmpR also controls the expression of the acid‐induced viru‐lence operon ssrAB, acid shock induction of ompR was examined to gain insight into how Salmonella links virulence with survival at extreme acid pH. The results indicate that acid pH induces ompR from a promoter different from that used for basal expression. Transcription from this promoter is repressed by the histone‐like protein H‐NS and requires OmpR‐P for induction. The classic sensor kinase EnvZ and acetyl phosphate collaborate to produce the optimum level of OmpR‐P needed for autoinduction. Although OmpR‐P is required for acid‐induced expression of ompR in wild‐type cells, OmpR is not needed for ompR transcription in the absence of H‐NS. Thus, the role of OmpR‐P in autoinduction is to help to counteract repression by H‐NS. This evidence, combined with the finding that relaxing DNA supercoiling with novobiocin also increased ompR transcription, suggests that acid stress induces ompR by altering local DNA topology, not by changing the phosphorylation status of OmpR.