Transcriptional specificity of RpoN1 and RpoN2 involves differential recognition of the promoter sequences and specific interaction with the cognate activator proteins.
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.
Poggio
Sebastian
S
Departamento de Biología Molecular y Biotecnología, Instituto de Investigaciones Biomédicas, Universidad Nacional Autónoma de México, 04510 México D. F., México.
Osorio
Aurora
A
Dreyfus
Georges
G
Camarena
Laura
L
D001483:Base Sequence; D005407:Flagella / Q000378:metabolism; D016680:Genome, Bacterial; D005966:Glucuronidase / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009584:Nitrogen / Q000737:chemistry; D010957:Plasmids / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D051877:RNA Polymerase Sigma 54 / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000378:metabolism; D014158:Transcription, Genetic
D051877:RNA Polymerase Sigma 54; D005966:Glucuronidase; D009584:Nitrogen