AN RPOS (SIGMA(S)) HOMOLOG REGULATES ACYLHOMOSERINE LACTONE-DEPENDENTAUTOINDUCTION IN RALSTONIA-SOLANACEARUM

Many bacteria sense an appropriate growth condition or a critical popu lation density for gene expression by producing acylhomoserine lactone s (acyl-HSLs) that act as intercellular autoinduction signals. We rece ntly showed that, in Ralstonia (Pseudomonas) solanacearum, a phytopath ogenic bacterium, acyl-HSL production requires sell, which encodes a p utative acyl-HSL synthase, and that its expression is positively regul ated by the acyl-HSL-responsive SolR transcriptional regulator. This a cyl-HSL-dependent autoinduction system is noteworthy because (i) it is regulated by a 'higher level' autoinducer system (responsive to 3-hyd roxypalmitic acid methyl ester) via PhcA, a LysR-type transcriptional regulator and (ii) acyl-HSL production requires two additional unlinke d loci. As reported here, cloning and sequencing of one of these other loci revealed that it encodes a homologue of RpoS, an alternative sig ma factor (sigma(S)) that in other bacteria activates gene expression during stationary phase or in response to stress conditions, R. solana cearum RpoS (RpoS(RSO)) was demonstrated to function as a a factor bec ause when introduced in trans into an Escherichia coli rpoS mutant it largely restored expression of the RpoS-dependent bolAp1 gene. Mutatio n of rpoS(RSO) in R. solanacearum reduced survival during starvation a nd low pH conditions, but did not affect survival during exposure to h ydrogen peroxide, high osmolarity or high temperature. This mutant was also altered in its production of several virulence factors and wilte d tomato plants several days more slowly than the wild-type parent. Tr anscription of solR and solI were decreased in an rpoS(RSO) background (thereby reducing acyl-HSL production), but neither mutations in solR , solI or phcA nor addition of acyl-HSLs affected rpoS(RSO) expression . Therefore, in R. solanacearum the acyl-HSL-dependent autoinduction s ystem is controlled both by a second autoinduction system and by the R poS(RSO), sigma factor.