title: The Sinorhizobium meliloti EmrR regulator is required for efficient colonization of Medicago sativa root nodules.
abstract: The nitrogen-fixing bacterium Sinorhizobium meliloti must adapt to diverse conditions encountered during its symbiosis with leguminous plants. We characterized a new symbiotically relevant gene, emrR (SMc03169), whose product belongs to the TetR family of repressors and is divergently transcribed from emrAB genes encoding a putative major facilitator superfamily-type efflux pump. An emrR deletion mutant produced more succinoglycan, displayed increased cell-wall permeability, and exhibited higher tolerance to heat shock. It also showed lower tolerance to acidic conditions, a reduced production of siderophores, and lower motility and biofilm formation. The simultaneous deletion of emrA and emrR genes restored the mentioned traits to the wild-type phenotype, except for survival under heat shock, which was lower than that displayed by the wild-type strain. Furthermore, the ΔemrR mutant as well as the double ΔemrAR mutant was impaired in symbiosis with Medicago sativa; it formed fewer nodules and competed poorly with the wild-type strain for nodule colonization. Expression profiling of the ΔemrR mutant showed decreased expression of genes involved in Nod-factor and rhizobactin biosynthesis and in stress responses. Expression of genes directing the biosynthesis of succinoglycan and other polysaccharides were increased. EmrR may therefore be involved in a regulatory network targeting membrane and cell wall modifications in preparation for colonization of root hairs during symbiosis.
authors.forename: Mário R
authors.initials: MR
authors.lastname: Santos
authors.forename: Andreia T
authors.initials: AT
authors.lastname: Marques
authors.forename: Jörg D
authors.initials: JD
authors.lastname: Becker
authors.forename: Leonilde M
authors.initials: LM
authors.lastname: Moreira
mesh_terms: D000595:Amino Acid Sequence; D001422:Bacterial Adhesion / Q000502:physiology; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D018441:Biofilms; D017353:Gene Deletion; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology; D000455:Medicago sativa / Q000382:microbiology*; D009068:Movement; D055170:Plant Root Nodulation / Q000502:physiology*; D016962:Sinorhizobium meliloti / Q000235:genetics / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins