THE STATIONARY-PHASE SIGMA-FACTOR SIGMA(S) (RPOS) IS REQUIRED FOR A SUSTAINED ACID TOLERANCE RESPONSE IN VIRULENT SALMONELLA-TYPHIMURIUM

The acid tolerance response (ATR) of log-phase Salmonella typhimurium is induced by acid exposures below pH4.5 and will protect cells agains t more extreme acid. Two systems are evident: a transiently induced sy stem dependent on the iron regulator Fur that provides a moderate degr ee of acid tolerance and a more effective sustained ATR that-requires the alternate sigma factor sigma(S) encoded by rpoS. Differences betwe en the acid responses of virulent S. typhimurium and the attenuated la boratory strain LT2 were attributed to disparate levels of RpoS caused by different translational starts. The sustained ATR includes seven n ewly identified acid shock proteins (ASPs) that are dependent upon sig ma(S) for their synthesis. It is predicted that one or more of these A SPs is essential for the sustained system. The sustained ATR also prov ided cross-protection to a variety of other environmental stresses (he at, H2O2 and osmolarity); however, adaptation to the other stresses di d not provide significant acid tolerance. Therefore, in addition to st arvation, acid shock serves as an important signal for inducing genera l stress resistance. Consistent with this model, sigma(S) proved to be induced by acid shock. Our results also revealed a connection between the transient and sustained ATR systems. Mutations in the regulator a tbR are known to cause the overproduction of ten proteins, of which on e or more can suppress the acid tolerance defect of an rpoS mutant. On e member of the AtbR regulon, designated atrB, was found to be co-regu lated by sigma(S) and AtbR. Both regulators had a negative effect on a trB expression. The results suggest AtrB serves as a link between the sustained and transient ATR systems.