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Journal of Bacteriology, January 2004, p. 15-21, Vol. 186, No. 1
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.1.15-21.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Diversifying Selection at the Bacillus Quorum-Sensing Locus and Determinants of Modification Specificity during Synthesis of the ComX Pheromone

M. Ansaldi and D. Dubnau^*

Public Health Research Institute and Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry of New Jersey, Newark, New Jersey 07103

Received 3 September 2003/ Accepted 8 October 2003

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.

Present address: Laboratoire de Chimie Bactérienne, IBSM-CNRS, 13402 Marseille Cedex 20, France.

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