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Summary

Dickeya dadantii is a plant pathogen that secretes cell wall-degrading enzymes (CWDE) that are responsible for soft-rot symptoms. Virulence genes are expressed in a concerted manner and culminate when bacterial multiplication slows. We identify a 25 kb vfm cluster required for D. dadantii CWDE production and pathogenesis. The vfm cluster encodes proteins displaying similarities both with enzymes involved in amino acid activation and with enzymes involved in fatty acid biosynthesis. These similarities suggest that the vfm genes direct the production of a metabolite. Cell-free supernatant from the D. dadantii wild-type strain restores CWDE production in vfm mutants. Collectively, our results indicate that vfm genes direct the synthesis of an extracellular signal and constitute a new quorum sensing system. Perception of the signal is achieved by the two-component system VfmH–VfmI, which activates the expression of the vfmE gene encoding an AraC regulator. VfmE then activates both the transcription of the CWDE genes and the expression of the vfm operons. The vfm gene cluster does not seem to be widespread among bacterial species but is conserved in other Dickeya species and could have been laterally transferred to Rahnella. This work highlights that entirely new families of bacterial languages remain to be discovered.