A LuxRI-family regulatory system controls excision and transfer of the Mesorhizobium loti strain R7A symbiosis island by activating expression of two conserved hypothetical genes.

The symbiosis island ICEMlSym(R7A) of Mesorhizobium loti R7A is an integrative and conjugative element (ICE) that carries genes required for a nitrogen-fixing symbiosis with Lotus species. ICEMlSym(R7A) encodes homologues (TraR, TraI1 and TraI2) of proteins that regulate plasmid transfer by quorum sensing in rhizobia and agrobacteria. Introduction of traR cloned on a plasmid induced excision of ICEMlSym(R7A) in all cells, a 1000-fold increase in the production of 3-oxo-C6-homoserine lactone (3-oxo-C6-HSL) and a 40-fold increase in conjugative transfer. These effects were dependent on traI1 but not traI2. Induction of expression from the traI1 and traI2 promoters required the presence of plasmid-borne traR and either traI1 or 100 pM 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 traI2 gene formed an operon with hypothetical genes msi172 and msi171 that were essential for ICEMlSym(R7A) excision and transfer. Our data suggest that derepressed TraR in conjunction with TraI1-synthesized 3-oxo-C6-HSL regulates excision and transfer of ICEMlSym(R7A) through expression of msi172 and msi171. Homologues of msi172 and msi171 were present on putative ICEs in several alpha-proteobacteria, indicating a conserved role in ICE excision and transfer.