| Summary: | The symbiosis island ICEM/SymR7A of Mesorhizobium loti R7A is an integrative and conjugative element (ICE) that carries genes required for a nitrogen-fixing symbiosis with Lotus species. ICEM/SymR7A encodes homologues (TraR, Trad and Tral2) of proteins that regulate plasmid transfer by quorum sensing in rhizobia and agrobacteria. Introduction of traR cloned on a plasmid induced excision of ICEM/SymR7A in all cells, a 1000-fold increase in the production of 3-oxo-C6homoserine lactone (3-oxo-C6-HSL) and a 40-fold increase in conjugative transfer. These effects were dependent on trail but not tral2. Induction of expression from the trail and trail promoters required the presence of plasmid-bome traR and either trail or 10OpM 3-OXO-C6-HSL, suggesting that traR expression or TraR activity is repressed in wild-type cells by a mechanism that can be overcome by additional copies of traR. The tral2 gene formed an operon with hypothetical genes msi172 and msi171 that were essential for ICEAWSymR7A excision and transfer. Our data suggest that derepressed TraR in conjunction with Trail-synthesized 3-oxo-C6-HSL regulates exci-sion and transfer of ICEM/SymR7A through expression of msi172 and msi171. Homologues of msi172 and msi171 were present on putative ICEs in several oc-proteobacteria, indicating a conserved role in ICE excision and transfer. © 2009 Blackwell Publishing Ltd.
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