title: Transcriptional interference and repression modulate the conjugative ability of the symbiotic plasmid of Rhizobium etli.
abstract: Bacteria of the order Rhizobiales are able to establish nitrogen-fixing symbioses with legumes. Commonly, genes for symbiosis are harbored on large symbiotic plasmids. Although the transfer of symbiotic plasmids is commonly detected in nature, there are few experimentally characterized examples. In Rhizobium etli, the product of rctA inhibits the conjugation of the symbiotic plasmid by reducing the transcription of the virB operon. rctA is transcribed divergently from this operon, and its product is predicted to have a DNA binding domain. In the present study, using DNase I footprinting and binding assays, we demonstrated the specific binding of RctA to the virB operon promoter. A 9-bp motif in the spacer region of this promoter (the rctA binding motif box) and the presence of a functional -10 region were critical elements for RctA binding. Transcriptional fusion analyses revealed that the elimination of either element provoked a relief of RctA-mediated repression. These data support a model in which RctA inhibits the access of the RNA polymerase to the virB promoter. Interestingly, rctA expression levels were modulated by transcriptional interference from transcripts emanating from the virB promoter. This phenomenon adds another level of regulation for this system, thus revealing a novel mechanism of plasmid transfer regulation in the Rhizobiales.
authors.affiliation: Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Apartado Postal 565-A, Cuernavaca, Morelos, México.
authors.forename: Edgardo
authors.initials: E
authors.lastname: Sepúlveda
authors.forename: Daniel
authors.initials: D
authors.lastname: Pérez-Mendoza
authors.forename: Miguel A
authors.initials: MA
authors.lastname: Ramírez-Romero
authors.forename: María J
authors.initials: MJ
authors.lastname: Soto
authors.forename: Isabel M
authors.initials: IM
authors.lastname: López-Lara
authors.forename: Otto
authors.initials: O
authors.lastname: Geiger
authors.forename: Juan
authors.initials: J
authors.lastname: Sanjuán
authors.forename: Susana
authors.initials: S
authors.lastname: Brom
authors.forename: David
authors.initials: D
authors.lastname: Romero
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D003227:Conjugation, Genetic; D018983:DNA Footprinting; D024202:Electrophoretic Mobility Shift Assay; D008954:Models, Biological; D009876:Operon / Q000235:genetics*; D010957:Plasmids / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics; D011485:Protein Binding; D042163:Rhizobium etli / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: D001426:Bacterial Proteins