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J. Biol. Chem., Vol. 281, Issue 37, 27205-27215, September 15, 2006

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Transcriptional Specificity of RpoN1 and RpoN2 Involves Differential Recognition of the Promoter Sequences and Specific Interaction with the Cognate Activator Proteins^*

Sebastian Poggio, Aurora Osorio, Georges Dreyfus, and Laura Camarena¹

From the Departamento de Biología Molecular y Biotecnología, Instituto de Investigaciones Biomédicas, and Departamento de Genética Molecular, Instituto de Fisiología Celular, Universidad Nacional Autónoma de México, 04510 México D. F., México

The four RpoN factors of Rhodobacter sphaeroides are functionally specialized. In this bacterium, RpoN1 and RpoN2 are specifically required for the transcription of the nitrogen fixation and flagellar genes, respectively. Analysis of the promoter sequences recognized by each of these RpoN proteins revealed some significant differences. To investigate the functional relevance of these differences, the flagellar promoter fliOp was sequentially mutagenized to resemble the nitrogen fixation promoter nifUp. Our results indicate that the promoter sequences recognized by these sigma factors have diverged enough so that particular positions of the promoter sequence are differentially recognized. In this regard, we demonstrate that the identity of the -11-position is critical for promoter discrimination by RpoN1 and RpoN2. Accordingly, purified RpoN proteins with a deletion of Region I, which has been involved in the recognition of the -11-position, did not show differential binding of fliOp and nifUp promoters. Substitution of the flagellar enhancer region located upstream fliOp by the enhancer region of nifUp allowed us to demonstrate that RpoN1 and RpoN2 interact specifically with their respective activator protein. In conclusion, two different molecular mechanisms underlie the transcriptional specialization of these sigma factors.

Received for publication, February 23, 2006 , and in revised form, July 5, 2006.

^* This study was supported by Consejo Nacional de Ciencia y Tecnología Grant 47172/A-1 and Dirección General de Asuntos del Personal Académico, Universidad Nacional Autónoma de México Grant IN222103-3. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Fig. 1.

¹ To whom correspondence should be addressed: Dept. de Biología Molecular y Biotecnología, Instituto de Investigaciones Biomédicas, U.N.A.M. México, D. F., 04510 México. Tel.: 5255-562-23824; Fax: 5255-555-00048; E-mail: rosal{at}servidor.unam.mx.

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