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Diversifying Selection at the Bacillus Quorum-Sensing Locus and Determinants of Modification Specificity during Synthesis of the ComX Pheromone. M. Ansaldi, 2004.The competence quorum-sensing system of Bacillus subtilis consists of two-component regulatory proteins, ComP (histidine kinase) and the response regulator, ComA, an extracellular pheromone (ComX), and a protein that is needed for the proteolytic cleavage and modification of pre-ComX (ComQ) . ComQ and pre-ComX are both necessary and sufficient for the production of active pheromone, which is released as an isoprenylated peptide . Laboratory strain 168 and a number of natural isolates of bacilli differ in the primary sequences of their pheromones as well as in the masses of their isoprenyl adducts . We have shown that ComX, ComQ, and the membrane-localized sensor domain of ComP are highly polymorphic in natural isolates of bacilli all closely related to the laboratory strain of B . subtilis . In this study, we used two statistical tests (the ratio of synonymous and nonsynonymous substitution rates and the Tajima D test) to demonstrate that these polymorphic sequences evolved by diversifying selection rather than by neutral drift . We show that the choice of isoprenyl derivative is determined by the C-terminal (mature) sequence of pre-ComX rather than by the ComQ protein . The implications of these findings for the evolution of the quorum-sensing system and for the protein-protein interactions involved in determining specificity are discussed . Mutations Affecting DNA-Binding Activity of the MexR Repressor of mexR-mexA-mexB-oprM Operon Expression. Kohjiro Saito, 2003.
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