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

ABSTRACTThe RpoS sigma factor (σS) is the master regulator of the bacterial response to a variety of stresses. Mutants inrpoSarise 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 naturalrpoSmutant ofSalmonella entericaserovar Typhi (Ty19). We show that therpoSallele of Ty19 (rpoSTy19) led to the synthesis of a σSTy19protein carrying a single glycine-to-valine substitution at position 282 in σSdomain 4, which was much more dependent than the wild-type σSprotein on activation by Crl, a chaperone-like protein that increases the affinity of σSfor 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 σSdomain 2 and that G282V substitution did not directly affect Crl binding. However, this substitution drastically reduced the ability of σSTy19to bind E in a surface plasmon resonance assay, a defect partially rescued by Crl. The modeled structure of the EσSholoenzyme suggested that substitution G282V could directly disrupt a favorable interaction between σSand E. TherpoSTy19allele conferred a competitive fitness when the bacterial population was wild type forcrlbut was outcompeted in Δcrlpopulations. Thus, these results indicate that the competitive advantage of therpoSTy19mutant is dependent on Crl and suggest thatcrlplays a role in the appearance ofrpoSmutants in bacterial populations.