Specificity and genetic polymorphism of the Bacillus competence quorum-sensing system

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.