title: A widely conserved molecular switch controls quorum sensing and symbiosis island transfer in Mesorhizobium loti through expression of a novel antiactivator.
abstract: ICEMlSym(R7A)  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 qseC-qseM 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 ICEMlSym(R7A)  transfer to a small proportion of cells in the population.
authors.affiliation: Department of Microbiology and Immunology, University of Otago, Dunedin, New Zealand. joshramsay@gmail.com
authors.forename: Joshua P
authors.initials: JP
authors.lastname: Ramsay
authors.forename: Anthony S
authors.initials: AS
authors.lastname: Major
authors.forename: Victor M
authors.initials: VM
authors.lastname: Komarovsky
authors.forename: John T
authors.initials: JT
authors.lastname: Sullivan
authors.forename: Ron L
authors.initials: RL
authors.lastname: Dy
authors.forename: Michael F
authors.initials: MF
authors.lastname: Hynes
authors.forename: George P C
authors.initials: GP
authors.lastname: Salmond
authors.forename: Clive W
authors.initials: CW
authors.lastname: Ronson
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives / Q000494:pharmacology; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D003227:Conjugation, Genetic; D015964:Gene Expression Regulation, Bacterial; D022761:Gene Transfer, Horizontal; D044404:Genomic Islands; D006714:Homoserine / Q000031:analogs & derivatives / Q000494:pharmacology; D000070116:Lotus / Q000378:metabolism / Q000382:microbiology; D059128:Mesorhizobium / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D009586:Nitrogen Fixation / Q000235:genetics; D010957:Plasmids / Q000235:genetics; D011401:Promoter Regions, Genetic; D053038:Quorum Sensing / Q000235:genetics*; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D013559:Symbiosis / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D015533:Transcriptional Activation / Q000187:drug effects
chemical_list: D001426:Bacterial Proteins; C109860:N-(3-oxododecanoyl)homoserine lactone; D012333:RNA, Messenger; D014157:Transcription Factors; D006714:Homoserine; D015107:4-Butyrolactone