A quorum-sensing mechanism involving the
pheromone ComX and the ComP-
ComA two-component system controls natural competence in Bacillus subtilis. ComX is expressed as a cytoplasmic inactive precursor that is released into the extracellular medium as a cleaved, modified decapeptide. This process requires the product of comQ. In the presence of ComX, the membrane-localized ComP
histidine kinase activates the response regulator
ComA. We compared the sequences of the quorum-sensing genes from four closely related bacilli, and we report extensive genetic polymorphism extending through comQ, comX, and the 5' two-thirds of comP. This part of ComP encodes the membrane-localized and linker domains of the sensor
protein. We also determined the sequences of the comX genes of four additional wild-type bacilli and tested the in vivo activities of all eight
pheromones on isogenic strains containing four different ComP receptor
proteins. A striking pattern of specificity was discovered, providing strong evidence that the
pheromone contacts ComP directly. Furthermore, we show that coexpression of comQ and comX in Escherichia coli leads to the production of active
pheromone in the medium, demonstrating that comQ is the only dedicated
protein required for the processing, modification, and release of active competence
pheromone. Some of the implications of these findings for the evolution and the mechanism of the quorum-sensing system are discussed.