A SMALL DIFFUSIBLE SIGNAL MOLECULE IS RESPONSIBLE FOR THE GLOBAL CONTROL OF VIRULENCE AND EXOENZYME PRODUCTION IN THE PLANT PATHOGEN ERWINIA-CAROTOVORA

Virulence of the plant pathogen Erwinia carotovora subsp. carotovora i s dependent on the production and secretion of a complex arsenal of pl ant cell wall-degrading enzymes. Production of these exoenzymes is con trolled by a global regulatory mechanism. Avirulent mutants in one of the regulatory loci, expI, show a pleiotropic defect in the growth pha se-dependent transcriptional activation of exoenzyme gene expression. The expI gene encodes a 26 kDa polypeptide that is structurally and fu nctionally related to the luxI gene product of Vibrio fischeri. Functi onal similarity of expI and luxI has been demonstrated by reciprocal g enetic complementation experiments. LuxI controls bioluminescence in V .fischeri in a growth phase-dependent manner by directing the synthesi s of the diffusible autoinducer, N-(3-oxohexcanoyl) homoserine lactone . E.c. subsp. carotovora expI+ strains or Escherichia coli harboring t he cloned expI gene excrete a small diffusible signal molecule that co mplements the expI mutation of Erwinia as well as a luxI mutation of V .fischeri. This extracellular complementation can also be achieved by E.coli harboring the luxI gene from V.fischeri or by adding the synthe tic V.fischeri autoinducer. Both the production of the plant tissue-ma cerating exoenzymes and the ability of the bacteria to propagate in pl anta are restored in expI mutants by autoinducer addition. These data suggest that the same signal molecule is employed in control of such d iverse processes as virulence in a plant pathogen and bioluminescence in a marine bacterium, and may represent a general mechanism by which bacteria modulate gene expression in response to changing environmenta l conditions.