Genetic competence in both Bacillus subtilis and Streptococcus pneumoniae,
as well as virulence in Staphylococcus aureus, are regulated by quorum-sens
ing mechanisms that use two-component signal transduction systems to respon
d to extracellular peptide pheromones. Recent data indicate that in all thr
ee systems closely related strains express markedly different pheromones an
d polytopic membrane receptor proteins. This polymorphism may function as a
sexual isolation mechanims, In B. subtilis the downstream segment of the c
ompetence regulatory pathway acts by controlling the stability of a key tra
nscription factor, In S. pneumoniae the downstream segment involves the tra
nscriptional activation of a minor sigma factor that is in turn responsible
for the expression of late competence genes.