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Mutations in the seven clustered rpf genes cause downregulated synthesis of extracellular enzymes and reduced virulence of Xanthomonas campestris pathovar campestris (Xcc). The phenotype of mutants in one of the genes, rpfF, can be restored by a diffusible extracellular factor (DSF) produced by all Xcc strains tested, apart from rpfF and rpfB mutants. DSF accumulates in early stationary phase (when synthesis of enzymes is maximal), but levels decline subsequently. Addition of DSF to exponentially‐growing wild‐type bacteria does not cause precocious enzyme synthesis. rpfB and rpfF are expressed throughout growth, but the rate 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 at …