UDC 579.852.11:577.25:579.254.22 review ISSN 1330-9862 (FTB-1181) Variability of the Quorum Sensing System in Natural Isolates of Bacillus sp. Ines Mandic-Mulec*, Barbara Kraigher, Ursa Cepon and Ivan Mahne University of Ljubljana, Biotechnical Faculty, Department Food Science and Technology, Chair of Microbiology, Ve~na pot 111, SI-1000 Ljubljana, Slovenia Received: October 1, 2002 Accepted: January 24, 2003 Summary Bacteria communicate with one another by (emitting and/or reacting) to chemical sig- nals. These communications, also known as quorum sensing, enable cells to control gene expression in response to cell density at the intra- and inter-species level. While bacteria use common signaling themes, variations in the design of the extracellular signals, the sig- nal detection apparatus, and the biochemical mechanisms of signal relay have allowed quorum sensing systems to be adapted to diverse uses. The quorum sensing systems that govern natural genetic competence in Bacillus subtilis involve the ComX pheromones and the ComP-ComA, two-component regulator. ComX is synthesized as an inactive precursor and is then cleaved and modified by ComQ before export to the extra-cellular environ- ment. The comQXP' loci of a set of natural Bacillus isolates have been sequenced and a strik- ing polymorphism that correlates with specific patterns of activation of the quorum sens- ing response was shown. The ComX molecules representing different pherotypes were pu- rified and characterized by mass spectroscopy. The analyses revealed that ComX variants also differ at the level of posttranslational modification of a conserved tryptophane resi- due, which was found to be an isoprenoid. The striking variability found in compe- tence quorum sensing systems might be important for the survival of these bacteria in na- ture to escape the inappropriate induction of competence by closely related strains, playing the role of a sexual isolation mechanism. Key words: Bacillus, genetic competence, quorum sensing, polymorphism, sexual isolation Introduction Intercellular communication plays a pivotal role in the physiology and development of living organisms. Many bacterial species, long thought to live the life of single cell existence, coordinate their physiological re- sponses at the population level. Bacteria produce extracellular signaling molecules (also called phero- mones), which accumulate in the environment denoting the presence of relatively dense population of cells and thus appropriateness of coordinated group behavior. The binding of signaling molecule to cognate receptors (membrane or cytoplasmic) triggers a change in tran- scription of target genes, which leads to the change in physiology or behavior of the population. (Bacteria cell-cell communication has been reviewed in two recently published books (1,2) and several reviews (3–6)). Signaling Molecules In Gram-negative bacteria the most commonly iden- tified signaling mechanism for extra-cellular communi- 23 I. MANDIC-MULEC et al.: Quorum Sensing System in Bacillus sp., Food Technol. Biotechnol. 41 (1) 23–28 (2003) * Corresponding author; Phone: ++386 1 42 33 388; Fax: ++386 1 25 73 390; E-mail: ines.mandic@bf.uni-lj.si