title: Autoregulation of Sinorhizobium meliloti exoR gene expression.
abstract: The successful nitrogen-fixing symbiosis between the gram-negative soil bacterium Sinorhizobium meliloti and its leguminous plant host alfalfa (Medicago sativa) requires the bacterial exopolysaccharide succinoglycan. Succinoglycan and flagellum production, along with the ability to metabolize more than 20 different carbon sources and control the expression of a large number of S. meliloti genes, is regulated by the ExoR-ExoS/ChvI signalling pathway. The ExoR protein interacts with and suppresses the sensing activities of ExoS, the membrane-bound sensor of the ExoS/ChvI two-component regulatory system. Here we show that exoR expression is clearly upregulated in the absence of any functional ExoR protein. This upregulation was suppressed by the presence of the wild-type ExoR protein but not by a mutated ExoR protein lacking signal peptide. The levels of exoR expression could be directly modified in real time by changing the levels of total ExoR protein. The expression of exoR was also upregulated by the constitutively active sensor mutation exoS96, and blocked by two single mutations, exoS* and exoS(supA), in the ExoS sensing domain. Presence of the wild-type ExoS protein further elevated the levels of exoR expression in the absence of functional ExoR protein, and reversed the effects of exoS96, exoS* and exoS(supA) mutations. Altogether, these data suggest that ExoR protein autoregulates exoR expression through the ExoS/ChvI system, allowing S. meliloti cells to maintain the levels of exoR expression based on the amount of total ExoR protein.
authors.affiliation: Graduate School and University Center, The City University of New York, New York, NY 10016-4309, USA.|Biological Sciences Department, Lehman College, The City University of New York, Bronx, NY 10468, USA.
authors.forename: Hai-Yang
authors.initials: HY
authors.lastname: Lu
authors.affiliation: Graduate School and University Center, The City University of New York, New York, NY 10016-4309, USA.|Biological Sciences Department, Lehman College, The City University of New York, Bronx, NY 10468, USA.
authors.forename: Hai-Ping
authors.initials: HP
authors.lastname: Cheng
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D011401:Promoter Regions, Genetic; D015398:Signal Transduction; D016962:Sinorhizobium meliloti / Q000235:genetics* / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins