Specificity and Genetic Polymorphism of the Bacillus Competence Quorum-Sensing System

Abstract

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.Under certain growth conditions, Bacillus subtilis expresses a set of proteins involved in the uptake and integration of extracellular DNA (reviewed in reference 3). The expression of these proteins in the competent state is tightly controlled and initially regulated by a quorum-sensing (QS) system involving the ComX pheromone (14-17, 30, 31) and the ComP-ComA two-component system (38, 40). The membrane-localized histidine kinase ComP, in the presence of ComX, activates the response regulator ComA, permitting the expression of downstream regulatory genes (4). This signal transduction cascade culminates with the expression of the competence transcription factor ComK (35,36), resulting in the synthesis of the DNA uptake and recombination proteins. A convergent pathway utilizes a second pheromone, the competence-stimulating factor (CSF) peptide, to modulate this response (16,31). This second pathway, which exerts a relatively minor effect, probably acts by inhibiting a phosphatase specific for ComA-ComP (22, 23). The two pathways thus appear to converge, both controlling the level of ComA phosphorylation (14,15,30).The ComX pheromone is translated as an inactive propeptide, which is then cleaved and released in the extracellular medium as a modified decapeptide, with the addition of an uncharacterized lipid moiety to a tryptophan residue (17). At least one other gene, comQ (39), is required for the production of modified, active ComX (17), but the precise role of ComQ is not understood.In the laboratory strains of B. subtilis, derived from strain 168, the comQ, comX, comP, and comA genes are...

Private link between signal and response in Bacillus subtilis quorum sensing

“…Whereas signal-deficient mutants are common among certain bacterial species such as P. aeruginosa (28-30), they have not been detected among more than 60 strains of B. subtilis isolated from various environments (9,22,31,32). Our data offer an explanation for this observation.…”

Facultative cheating supports the coexistence of diverse quorum-sensing alleles

“…We found two exceptions to this trend when either pherotype RO-FF-1 or NAF-4 were cocultured as a minority with pherotype 168. The former case most likely arises from the asymmetric signaling interactions between strains 168 and RO-FF-1, where the former activates the latter, but not vice versa (20).…”

“…To test this key prediction in B. subtilis, we studied the interaction between several different pherotypes under swarming conditions. We deleted the endogenous comQXP allele and introduced a comQXP locus of one of four different strains with different pherotypes (168, RO-H-1, NAF4, and RO-FF-1) into an ectopic location (20). We cocultured all possible pairs of strains.…”

“…The ComX autoinducer production genes (comQ, comX) and the region of comP encoding for the extracellular part of the ComP receptor are highly variable and encode for multiple different pherotypes, which coexist in the soil and undergo rapid horizontal gene transfer (10,15,16,20). The interaction between different comQXP pherotypes also includes cases of asymmetric cross-activation or cross-inhibition, where a ComX autoinducer of one pherotype would activate the receptor of another pherotype, but not vice versa, or when the autoinducer inhibits the receptor of another pherotype (10,20). The ComQXP quorum-sensing system activates the ComA transcription factor, which regulates a large array of genes, including the srfA operon (19).…”

Exploring Quorum Sensing Loci and Biofilm Formation in Bacillus Isolates from Pigeonpea Rhizosphere

“…The PCR profile of the comQXP locus amplification consisted of 30 cycles of denaturation at 94°C for 30 s, annealing at 55°C for 45 s, extension at 72°C for 3 min, and final extension at 72°C for 5 min (Tortosa et al, 2001). …”