Acid Shock Accumulation of Sigma S in Salmonella Enterica Involves Increased Translation, Not Regulated Degradation

Enteric pathogens such as Salmonella enterica and Escherichia coli face the daunting task of surviving passage through the extremely acid pH of the stomach in order to establish an infection in the host intestinal tract. These organisms have evolved elaborate stress response systems that aid in survival. The alternative sigma factor sigma(S) is a key regulator of many stress responses in S. enterica and is regulated at the levels of transcription, translation, and protein stability. Of these control mechanisms, proteolysis has been considered paramount in determining sigma(S) levels in the cell. Until the current report, acid shock was thought to increase sigma(S) levels by directly regulating degradation. However, mutant strains unable to degrade sigma(S) still exhibited acid shock induction of sigma(S). We demonstrate here that RPOS translation is a major focus of acid stress control and is responsible for the observed increase in sigma(S) levels. A series of deletions of the 566-nucleotide untranslated region of the RPOS mRNA were constructed to examine the importance of this regulatory region in acid shock induction of RPOS. Progressive deletions starting from the 5' end of the RPOS message produced alternating loss and recovery of acid shock control. The results suggest that competing stem-loop structures work in concert to control the acid shock induction of RPOS. Further, the half-life of sigma(S) was unchanged in response to acid shock and over-expression of the MviA recognition protein resulted in constitutive sigma(S) degradation under acid stress conditions. The data indicate that in log phase, nonstressed cells increasing sigma(S) production is sufficient to increase protein half-life. In toto, these results suggest that acid shock stabilization of sigma(S) is the result of increased synthesis via translational control and does not involve changes in the activity of the MviA (RssB/SprE) ClpXP degradation complex. Therefore, constitutive degradation may enable the cell to reset the level of sigma(S) once acid stress is alleviated.