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

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Abstract
The small regulatory protein Crl activates σ S (RpoS), the stationary-phase and general stress response sigma factor. Crl has been reported to bind σ S in vitro and to facilitate the formation of RNA polymerase holoenzyme. In Salmonella enterica serovar Typhimurium, Crl is required for the development of the rdar morphotype and transcription initiation of the σ S -dependent genes csgD and adrA , involved in curli and cellulose production. Here, we examined the expression of other σ S -dependent phenotypes and genes in a Δ crl mutant of Salmonella . Gene fusion analyses and in vitro transcription assays indicate that the magnitude of Crl activation differs between promoters and is highly dependent on σ S levels. We replaced the wild-type rpoS allele in S. enterica serovar Typhimurium strain ATCC 14028 with the rpoS LT2 allele that shows reduced expression of σ S ; 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 that crl , rpoS , and crl rpoS strains grew better on succinate than did the wild type and expressed the succinate dehydrogenase sdhCDBA operon more strongly. The crl and rpoS LT2 mutations also increased the competitive fitness of Salmonella in stationary phase. These results show that Crl contributes to negative regulation by σ S , a finding consistent with a role for Crl in sigma factor competition via the facilitation of σ S binding to core RNA polymerase.