A NOVEL REGULATORY SYSTEM REQUIRED FOR PATHOGENICITY OF XANTHOMONAS-CAMPESTRIS IS MEDIATED BY A SMALL DIFFUSIBLE SIGNAL MOLECULE

Mutations in the seven clustered rpf genes cause downregulated synthes is of extracellular enzymes and reduced virulence of Xanthomonas campe stris pathovar campestris (Xcc). The phenotype of mutants in one of th e genes, rpfF, can be restored by a diffusible extracellular factor (D SF) produced by all Xcc strains tested, apart from rpfF and rpfB mutan ts. DSF accumulates in early stationary phase (when synthesis of enzym es is maximal), but levels decline subsequently. Addition of DSF to ex ponentially-growing wild-type bacteria does not cause precocious enzym e synthesis. rpfB and rpfF are expressed throughout growth, but the ra te increases in early stationary phase. RpfB is predicted to be a long -chain fatty acyl CoA ligase, and RpfF shows some relatedness to enoyl CoA hydratases. The properties of DSF suggest that it may be a fatty- acid derivative, and certain lipid preparations possess DSF activity a t higher concentrations. These include lipid extracts and acid-hydroly sed lipopolysaccharide and lipid A from Xcc, and purified dodecanoic a nd hydroxydodecanoic acid. DSF production is confined to certain xanth omonads. We propose a model for the DSF system, which represents a nov el mechanism for regulating virulence factor synthesis in response to physiological or environmental changes.