Crl Activates Transcription Initiation of RpoS-Regulated Genes Involved in the Multicellular Behavior ofSalmonella entericaSerovar Typhimurium

InSalmonella enterica serovar Typhimurium, the stationary-phase sigma factor σS (RpoS) is required for virulence, stress resistance, biofilm formation, and development of the rdar morphotype. This morphotype is a multicellular behavior characterized by expression of the adhesive extracellular matrix components cellulose and curli fimbriae. The Crl protein ofEscherichia coli interacts with σS and activates expression of σS -regulated genes, such as thecsgBAC operon encoding the subunit of the curli proteins, by an unknown mechanism. Here, we showed using in vivo and in vitro experiments that the Crl protein ofSalmonella serovar Typhimurium is required for development of a typical rdar morphotype and for maximal expression of thecsgD ,csgB ,adrA , andbcsA genes, which are involved in curli and cellulose biosynthesis. In vitro transcription assays and potassium permanganate reactivity experiments with purified His6 -Crl showed that Crl directly activated σS -dependent transcription initiation at thecsgD andadrA promoters. We observed no effect of Crl on σ70 -dependent transcription. Crl protein levels increased during the late exponential and stationary growth phases in Luria-Beratani medium without NaCl at 28°C. We obtained complementation of thecrl mutation by increasing σS levels. This suggests that Crl has a major physiological impact at low concentrations of σS .