Conjugative Transfer of the Integrative and Conjugative Element ICE Bs1 from B acillus subtilis Likely Initiates at the Donor Cell Pole

Abstract

Mobile genetic elements (MGEs) are ubiquitous in bacteria. They contain genes for antibiotic resistance, symbiosis, and virulence, and their dissemination plays an important role in bacterial evolution by conferring new genes and phenotypes to their recipients. The most common MGEs are phages, plasmids, and integrative and conjugative elements (ICEs), also known as conjugative transposons. Conjugative plasmids and ICEs generally encode their own conjugation systems and are transferred from cell to cell via direct cell-cell contact (16). ICEBs1 is an ICE found integrated into the leucine tRNA gene of Bacillus subtilis strains (3, 7). When induced, ICEBs1 excises from the chromosome and can transfer to recipient cells. Gene expression and excision are induced by the SOS response or when cells are at high density surrounded by cells lacking a copy of ICEBs1. Regulation by population density is mediated by the regulator RapI and the pentapeptide PhrI (3). In this issue of the Journal of Bacteriology, Berkmen et al. (4) report that a fully functional ConE, a protein of the HerA/FtsK superfamily of ATPases, is required for ICEBs1 conjugation. A ConE-GFP (green fluorescent protein) fusion associated with the membrane predominantly at the cell poles in ICEBs1 donor cells. At least one additional ICEBs1 gene product was needed to target ConE to the membrane and cell poles. When integrated in the chromosome, ICEBs1 was located near midcell along the length of the cell, whereas following excision, the ICE was more often detected near a cell pole. Excision of ICEBs1 also resulted in altered subcellular positioning of the replisome. Similarities with other ICEs. Tn916, found in several different gram-positive cocci, is the smallest, least complex, and best-characterized ICE. The closest relative of Tn916 (similar gene organization with approximately 30% sequence identity between common genes) is ICEBs1 (8). A comparison of the