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Journal of Bacteriology, November 2005, p. 7341-7350, Vol. 187, No. 21
0021-9193/05/$08.00+0     doi:10.1128/JB.187.21.7341-7350.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Identification of the rctA Gene, Which Is Required for Repression of Conjugative Transfer of Rhizobial Symbiotic Megaplasmids{dagger}

Daniel Pérez-Mendoza,1 Edgardo Sepúlveda,2 Victoria Pando,2 Socorro Muñoz,1 Joaquina Nogales,1 José Olivares,1 Maria J. Soto,1 José A. Herrera-Cervera,1,{ddagger} David Romero,2 Susana Brom,2 and Juan Sanjuán1*

Departamento Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas (CSIC), Granada, Spain,1 Programa de Ingeniería Genómica, Centro de Ciencias Genómicas, UNAM, Cuernavaca, Morelos, México2

Received 26 May 2005/ Accepted 5 August 2005

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.

* Corresponding author. Mailing address: Dpto. Microbiología del Suelo y Sistemas Simbióticos, Estación Experimental del Zaidín, Profesor Albareda 1, 18008 Granada, Spain. Phone: 34-958181600, ext. 219. Fax: 34-958129600. E-mail: juan.sanjuan{at}eez.csic.es.

{dagger} Supplemental material for this article may be found at http://jb.asm.org/.

{ddagger} Present address: Dpto. Fisiología Vegetal, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada, Spain.

Journal of Bacteriology, November 2005, p. 7341-7350, Vol. 187, No. 21
0021-9193/05/$08.00+0     doi:10.1128/JB.187.21.7341-7350.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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