Many plants have evolved symbiotic relationships with bacteria that result in the development of nodules called rhizhobia, which enable the plants to obtain nitrogen compounds by bacterial reduction of nitrogen to ammonium. In 27 July Science Galilbert and colleagues from the Laboratoire de Génétique et Développement, CNRS, Rennes, France revealed the sequence of the α-proteobacterium Sinorhizobium meliloti, the bacteria involved in a rhizhobial association with the crop plant alfalfa (Medicago sativa).

Galilbert et al. reported that the bacterium has a tripartite genome; a 3.65 Mb chromosome, and 1.35 Mb pSymA and 1.68 Mb pSymB megaplasmids. Further analysis of the genome suggests that all three genetic elements are involved, to a greater or lesser degree, in the establishment and maintenance of the plant/bacterium symbiosis (Science 2001, 293:668-672).

They analysed S. meliloti strain 1021 and predicted the genome to contain 6204 protein-coding genes. Of these they estimated...

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