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Journal of Bacteriology, September 2004, p. 5753-5761, Vol. 186, No. 17
0021-9193/04/$08.00+0     DOI: 10.1128/JB.186.17.5753-5761.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Identification of Functional mob Regions in Rhizobium etli: Evidence for Self-Transmissibility of the Symbiotic Plasmid pRetCFN42d

Daniel Pérez-Mendoza,¹ Ana Domínguez-Ferreras,¹ Socorro Muñoz,¹ María José Soto,¹ José Olivares,¹ Susana Brom,² Lourdes Girard,² José A. Herrera-Cervera,^1, and Juan Sanjuán^1*

Departamento Microbiología del Suelo y Sistemas Simbióticos. Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Granada, Spain,¹ Programa de Genética Molecular de Plásmidos Bacterianos, Centro de Investigación sobre Fijación de Nitrógeno, UNAM, Cuernavaca, Morelos, México²

Received 29 March 2004/ Accepted 2 June 2004

An approach originally designed to identify functional origins of conjugative transfer (oriT or mob) in a bacterial genome (J. A. Herrera-Cervera, J. M. Sanjuán-Pinilla, J. Olivares, and J. Sanjuán, J. Bacteriol. 180:4583-4590, 1998) was modified to improve its reliability and prevent selection of undesired false mob clones. By following this modified approach, we were able to identify two functional mob regions in the genome of Rhizobium etli CFN42. One corresponds to the recently characterized transfer region of the nonsymbiotic, self-transmissible plasmid pRetCFN42a (C. Tun-Garrido, P. Bustos, V. González, and S. Brom, J. Bacteriol. 185:1681-1692, 2003), whereas the second mob region belongs to the symbiotic plasmid pRetCFN42d. The new transfer region identified contains a putative oriT and a typical conjugative (tra) gene cluster organization. Although pRetCFN42d had not previously been shown to be self-transmissible, mobilization of cosmids containing this tra region required the presence of a wild-type pRetCFN42d in the donor cell; the presence of multiple copies of this mob region in CFN42 also promoted conjugal transfer of the Sym plasmid pRetCFN42d. The overexpression of a small open reading frame, named yp028, located downstream of the putative relaxase gene traA, appeared to be responsible for promoting the conjugal transfer of the R. etli pSym under laboratory conditions. This yp028-dependent conjugal transfer required a wild-type pRetCFN42d traA gene. Our results suggest for the first time that the R. etli symbiotic plasmid is self-transmissible and that its transfer is subject to regulation. In wild-type CFN42, pRetCFN42d tra gene expression appears to be insufficient to promote plasmid transfer under standard laboratory conditions; gene yp028 may play some role in the activation of conjugal transfer in response to as-yet-unknown environmental conditions.

Present address: Departamento Biología Vegetal, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain.

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