Physiological effects of Crl in Salmonella are modulated by ^S level and promoter specificity

Unité des Régulations Transcriptionnelles; URA-CNRS 2172; Institut Pasteur, 25 rue du Docteur Roux, 75724 Paris Cedex 15, France

^* To whom correspondence should be addressed. Email: francoise.norel{at}pasteur.fr.

	Abstract

The small regulatory protein Crl activates ^S (RpoS), the stationary-phase and general stress response sigma factor. Crl has been reported to bind ^S in vitro and to facilitate formation of RNA polymerase holoenzyme. In Salmonella enterica serovar Typhimurium, Crl is required for development of the rdar morphotype and transcription initiation of the ^S -dependent genes csgD and adrA involved in curli and cellulose production. Here, we examined expression of other ^S -dependent phenotypes and genes in a crl mutant of Salmonella. Gene fusions analyses and in vitro transcription assays indicate that the magnitude of Crl activation differs between promoters and is highly dependent on ^S levels. We replaced the wild-type rpoS allele in Salmonella ATCC14028 with the rpoS_LT2 allele that shows reduced expression of ^S; the result was 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 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 ^S, a finding consistent with a role for Crl in competition via facilitation of ^S binding to core RNA polymerase.