Crl Binds to Domain 2 of σ S and Confers a Competitive Advantage on a Natural rpoS Mutant of Salmonella enterica Serovar Typhi

Abstract

ABSTRACT The RpoS sigma factor (σ S ) is the master regulator of the bacterial response to a variety of stresses. Mutants in rpoS arise in bacterial populations in the absence of stress, probably as a consequence of a subtle balance between self-preservation and nutritional competence. We characterized here one natural rpoS mutant of Salmonella enterica serovar Typhi (Ty19). We show that the rpoS allele of Ty19 ( rpoS Ty19 ) led to the synthesis of a σ S Ty19 protein carrying a single glycine-to-valine substitution at position 282 in σ S domain 4, which was much more dependent than the wild-type σ S protein on activation by Crl, a chaperone-like protein that increases the affinity of σ S for the RNA polymerase core enzyme (E). We used the bacterial adenylate cyclase two-hybrid system to demonstrate that Crl bound to residues 72 to 167 of σ S domain 2 and that G282V substitution did not directly affect Crl binding. However, this substitution drastically reduced the ability of σ S Ty19 to bind E in a surface plasmon resonance assay, a defect partially rescued by Crl. The modeled structure of the Eσ S holoenzyme suggested that substitution G282V could directly disrupt a favorable interaction between σ S and E. The rpoS Ty19 allele conferred a competitive fitness when the bacterial population was wild type for crl but was outcompeted in Δ crl populations. Thus, these results indicate that the competitive advantage of the rpoS Ty19 mutant is dependent on Crl and suggest that crl plays a role in the appearance of rpoS mutants in bacterial populations.