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FixJ-regulated genes evolved through promoter duplication in Sinorhizobium meliloti

Lionel Ferrières

Anne Francez-Charlot

Laboratoire des Interactions Plantes-Microorganismes, UMR 2594 INRA-CNRS, Chemin de Borde-Rouge, BP 27, 31326 Castanet-Tolosan Cedex, France

Correspondence
Daniel Kahn
dkahn{at}toulouse.inra.fr

The FixLJ two-component system of Sinorhizobium meliloti is a global regulator, turning on nitrogen-fixation genes in microaerobiosis. Up to now, nifA and fixK were the only genes known to be directly regulated by FixJ. We used a genomic SELEX approach in order to isolate new FixJ targets in the genome. This led to the identification of 22 FixJ binding sites, including the known sites in the fixK1 and fixK2 promoters. FixJ binding sites are unevenly distributed among the three replicons constituting the S. meliloti genome: a majority are carried either by pSymA or by a short chromosomal region of non-chromosomal origin. Thus FixJ binding sites appear to be preferentially associated with the pSymA replicon, which carries the fixJ gene. Functional analysis of FixJ targets led to the discovery of two new FixJ-regulated genes, smc03253 and proB2. This FixJ-dependent regulation appears to be mediated by a duplication of the whole fixK promoter region, including the beginning of the fixK gene. Similar duplications were previously reported for the nifH promoter. By systematic comparison of all promoter regions we found 17 such duplications throughout the genome, indicating that promoter duplication is a common mechanism for the evolution of regulatory pathways in S. meliloti.

Present address: Molecular Microbiology Laboratory, Department of Biology and Biochemistry, University of Bath, Bath BA2 7AY, UK.

Present address: Laboratoire de Microbiologie et de Génétique Moléculaires, UMR5100 CNRS-UPS, 118 route de Narbonne, 31062 Toulouse Cedex, France.

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				C. Bobik, E. Meilhoc, and J. Batut FixJ: a Major Regulator of the Oxygen Limitation Response and Late Symbiotic Functions of Sinorhizobium meliloti J. Bacteriol., July 1, 2006; 188(13): 4890 - 4902. [Abstract] [Full Text] [PDF]