title: Sinorhizobium meliloti, a bacterium lacking the autoinducer-2 (AI-2) synthase, responds to AI-2 supplied by other bacteria.
abstract: Many bacterial species respond to the quorum-sensing signal autoinducer-2 (AI-2) by regulating different niche-specific genes. Here, we show that Sinorhizobium meliloti, a plant symbiont lacking the gene for the AI-2 synthase, while not capable of producing AI-2 can nonetheless respond to AI-2 produced by other species. We demonstrate that S. meliloti has a periplasmic binding protein that binds AI-2. The crystal structure of this protein (here named SmlsrB) with its ligand reveals that it binds (2R,4S)-2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran (R-THMF), the identical AI-2 isomer recognized by LsrB of Salmonella typhimurium. The gene encoding SmlsrB is in an operon with orthologues of the lsr genes required for AI-2 internalization in enteric bacteria. Accordingly, S. meliloti internalizes exogenous AI-2, and mutants in this operon are defective in AI-2 internalization. S. meliloti does not gain a metabolic benefit from internalizing AI-2, suggesting that AI-2 functions as a signal in S. meliloti. Furthermore, S. meliloti can completely eliminate the AI-2 secreted by Erwinia carotovora, a plant pathogen shown to use AI-2 to regulate virulence. Our findings suggest that S. meliloti is capable of 'eavesdropping' on the AI-2 signalling of other species and interfering with AI-2-regulated behaviours such as virulence.
authors.affiliation: Bacterial Signaling Laboratory, Instituto Gulbenkian de Ciência, Oeiras, Portugal.
authors.forename: Catarina S
authors.initials: CS
authors.lastname: Pereira
authors.forename: J Randall
authors.initials: JR
authors.lastname: McAuley
authors.forename: Michiko E
authors.initials: ME
authors.lastname: Taga
authors.forename: Karina B
authors.initials: KB
authors.lastname: Xavier
authors.forename: Stephen T
authors.initials: ST
authors.lastname: Miller
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D006714:Homoserine / Q000031:analogs & derivatives* / Q000378:metabolism; D007783:Lactones / Q000378:metabolism*; D008958:Models, Molecular; D009876:Operon; D010429:Pentoses / Q000378:metabolism; D017434:Protein Structure, Tertiary; D053038:Quorum Sensing; D012329:RNA, Bacterial / Q000235:genetics; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012486:Salmonella typhimurium / Q000235:genetics / Q000378:metabolism; D016962:Sinorhizobium meliloti / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: C492713:2-methyl-2,3,3,4-tetrahydroxytetrahydrofuran; D001426:Bacterial Proteins; D007783:Lactones; C091088:N-octanoylhomoserine lactone; D010429:Pentoses; D012329:RNA, Bacterial; D011956:Receptors, Cell Surface; D006714:Homoserine