title: Identification of the rctA gene, which is required for repression of conjugative transfer of rhizobial symbiotic megaplasmids.
abstract: An analysis of the conjugative transfer of pRetCFN42d, the symbiotic plasmid (pSym) of Rhizobium etli, has revealed a novel gene, rctA, as an essential element of a regulatory system for silencing the conjugative transfer of R. etli pSym by repressing the transcription of conjugal transfer genes in standard laboratory media. The rctA gene product lacks sequence conservation with other proteins of known function but may belong to the winged-helix DNA-binding subfamily of transcriptional regulators. Similar to that of many transcriptional repressors, rctA transcription seems to be positively autoregulated. rctA expression is greatly reduced upon overexpression of another gene, rctB, previously identified as a putative activator of R. etli pSym conjugal transfer. Thus, rctB seems to counteract the repressive action of rctA. rctA homologs are present in at least three other bacterial genomes within the order Rhizobiales, where they are invariably located adjacent to and divergently transcribed from putative virB-like operons. We show that similar to that of R. etli pSym, conjugative transfer of the 1.35-Mb symbiotic megaplasmid A of Sinorhizobium meliloti is also subjected to the inhibitory action of rctA. Our data provide strong evidence that the R. etli and S. meliloti pSym plasmids are indeed self-conjugative plasmids and that this property would only be expressed under optimal, as yet unknown conditions that entail inactivation of the rctA function. The rctA gene seems to represent novel but probably widespread regulatory systems controlling the transfer of conjugative elements within the order Rhizobiales.
authors.affiliation: Dpto. Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín, Profesor Albareda 1, 18008 Granada, Spain.
authors.forename: Daniel
authors.initials: D
authors.lastname: Pérez-Mendoza
authors.forename: Edgardo
authors.initials: E
authors.lastname: Sepúlveda
authors.forename: Victoria
authors.initials: V
authors.lastname: Pando
authors.forename: Socorro
authors.initials: S
authors.lastname: Muñoz
authors.forename: Joaquina
authors.initials: J
authors.lastname: Nogales
authors.forename: José
authors.initials: J
authors.lastname: Olivares
authors.forename: Maria J
authors.initials: MJ
authors.lastname: Soto
authors.forename: José A
authors.initials: JA
authors.lastname: Herrera-Cervera
authors.forename: David
authors.initials: D
authors.lastname: Romero
authors.forename: Susana
authors.initials: S
authors.lastname: Brom
authors.forename: Juan
authors.initials: J
authors.lastname: Sanjuán
mesh_terms: D003227:Conjugation, Genetic; D004251:DNA Transposable Elements; D015964:Gene Expression Regulation, Bacterial; D020868:Gene Silencing; D005798:Genes, Bacterial; D005809:Genes, Regulator; D016254:Mutagenesis, Insertional; D009876:Operon; D010957:Plasmids / Q000235:genetics*; D042163:Rhizobium etli / Q000235:genetics*; D017386:Sequence Homology, Amino Acid; D016962:Sinorhizobium meliloti / Q000235:genetics*; D013559:Symbiosis / Q000235:genetics*; D026801:Synteny
chemical_list: D004251:DNA Transposable Elements