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Journal of Bacteriology, June 2008, p. 4189-4197, Vol. 190, No. 12
0021-9193/08/$08.00+0     doi:10.1128/JB.00041-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Transcriptional Interference and Repression Modulate the Conjugative Ability of the Symbiotic Plasmid of Rhizobium etli{triangledown} ,{dagger}

Edgardo Sepúlveda,1 Daniel Pérez-Mendoza,2 Miguel A. Ramírez-Romero,1 María J. Soto,2 Isabel M. López-Lara,1 Otto Geiger,1 Juan Sanjuán,2 Susana Brom,1 and David Romero1*

Centro de Ciencias Genómicas, Universidad Nacional Autónoma de México, Apartado Postal 565-A, Cuernavaca, Morelos, México,1 Estación Experimental del Zaidín, CSIC, Granada, España2

Received 9 January 2008/ Accepted 7 April 2008

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.

* Corresponding author. Mailing address: Programa de Ingeniería Genómica, Centro de Ciencias Genómicas-UNAM, Apartado Postal 565-A, 62210 Cuernavaca, Morelos, México. Phone: 52 (777) 3175867 or 52 (777) 3291691. Fax: 52 (777) 3175581. E-mail: dromero{at}ccg.unam.mx

{triangledown} Published ahead of print on 18 April 2008.

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

Journal of Bacteriology, June 2008, p. 4189-4197, Vol. 190, No. 12
0021-9193/08/$08.00+0     doi:10.1128/JB.00041-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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