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A widely conserved molecular switch controls quorum sensing and symbiosis island transfer in M esorhizobium loti through expression of a novel antiactivator
Author(s) -
Ramsay Joshua P.,
Major Anthony S.,
Komarovsky Victor M.,
Sullivan John T.,
Dy Ron L.,
Hynes Michael F.,
Salmond George P. C.,
Ronson Clive W.
Publication year - 2013
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/mmi.12079
Subject(s) - biology , quorum sensing , symbiosis , microbiology and biotechnology , genetics , bacteria , biofilm
Summary ICE Ml Sym R7A of M esorhizobium loti is an integrative and conjugative element ( ICE ) that confers the ability to form a nitrogen‐fixing symbiosis with L otus 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 qseC ‐ qseM promoters. QseC homologues were identified on ICE s, 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 ICE Ml Sym R7A transfer to a small proportion of cells in the population.