A widely conserved molecular switch controls quorum sensing and symbiosis island transfer in Mesorhizobium loti through expression of a novel antiactivator
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ICEMlSymR7A of Mesorhizobium loti is an integrative and conjugative element (ICE) that confers the ability to form a nitrogen-fixing symbiosis with Lotus species. Horizontal transfer is activated by TraR and N-acyl-homoserine lactone (AHL), which can stimulate ICE excision in 100% of cells. However, in wild-type cultures, the ICE is excised at low frequency. Here we show that QseM, a widely conserved ICE-encoded protein, is an antiactivator of TraR. Mutation of qseM resulted in TraR-dependent activation of AHL production and excision, but did not affect transcription of traR. QseM and TraR directly interacted in a bacterial two-hybrid assay in the presence of AHL. qseM expression was repressed by a DNA-binding protein QseC, which also activated qseC expression from a leaderless transcript. QseC differentially bound two adjacent operator sites, the lower affinity of which overlapped the -35 regions of the divergent qseCqseM promoters. QseC homologues were identified on ICEs, TraR/TraM-regulated plasmids and restriction-modification cassettes, suggesting a conserved mode of regulation. Six QseC variants with distinct operators were identified that showed evidence of reassortment between mobile elements. We propose that QseC and QseM comprise a bimodal switch that restricts quorum sensing and ICEMlSymR7A transfer to a small proportion of cells in the population.
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Ramsay, Joshua; Ronson, C. (2015)The Mesorhizobium loti symbiosis island (ICEMlSymR7A) is a 500-kb mobile integrative and conjugative element (ICE) that converts non-symbiotic mesorhizobia into strains capable of forming a symbiosis with Lotus corniculatus. ...
Ribosomal frameshifting and dual-target antiactivation restrict quorum-sensing-activated transfer of a mobile genetic elementRamsay, Joshua; Tester, L.; Major, A.; Sullivan, J.; Edgar, C.; Kleffmann, T.; Patterson-House, Jackson; Hall, D.; Tate, W.; Hynes, M.; Ronson, C. (2015)Symbiosis islands are integrative and conjugative mobile genetic elements that convert nonsymbiotic rhizobia into nitrogen-fixing symbionts of leguminous plants. Excision of the Mesorhizobium loti symbiosis island ICEMlSymR7A ...
A LuxRI-family regulatory system controls excision and transfer of the Mesorhizobium loti strain R7A symbiosis island by activating expression of two conserved hypothetical genesRamsay, Joshua; Sullivan, J.; Jambari, N.; Ortori, C.; Heeb, S.; Williams, P.; Barrett, D.; Lamont, L.; Ronson, C. (2009)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 ...