Physiological Effects of Crl in Salmonella Are Modulated by σ S Level and Promoter Specificity

The small regulatory protein Crl activates σS (RpoS), the stationary-phase and general stress response sigma factor. Crl has been reported to bind σS in vitro and to facilitate the formation of RNA polymerase holoenzyme. InSalmonella enterica serovar Typhimurium, Crl is required for the development of the rdar morphotype and transcription initiation of the σS -dependent genescsgD andadrA , involved in curli and cellulose production. Here, we examined the expression of other σS -dependent phenotypes and genes in a Δcrl mutant ofSalmonella . Gene fusion analyses and in vitro transcription assays indicate that the magnitude of Crl activation differs between promoters and is highly dependent on σS levels. We replaced the wild-typerpoS allele inS. enterica serovar Typhimurium strain ATCC 14028 with therpoS LT2 allele that shows reduced expression of σS ; the result was an increased Crl activation ratio and larger physiological effects of Crl on oxidative, thermal, and acid stress resistance levels during stationary phase. We also found thatcrl ,rpoS , andcrl rpoS strains grew better on succinate than did the wild type and expressed the succinate dehydrogenasesdhCDBA operon more strongly. Thecrl andrpoS LT2 mutations also increased the competitive fitness ofSalmonella in stationary phase. These results show that Crl contributes to negative regulation by σS , a finding consistent with a role for Crl in sigma factor competition via the facilitation of σS binding to core RNA polymerase.