The genome of Sinorhizobium meliloti

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

Galilbert et al. reported that S. meliloti strain 1021 has a tripartite genome; a 3.65 megabase (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 predicted that the genome contains 6,204 protein-coding genes, of which the largest proportion (12.5%) are thought to encode transport system proteins. Regulatory genes occupy 9% of the genome. The three nod genes required for the initiation of the rhizhobium (nodM, nodPQ and nodG) are located on pSymA.

Understanding the genome of this bacterium could enable specific genes to be manipulated to increase the yield of alfalfa. It may also enable the bacteria to be altered such that they can be used to infect other non-leguminous species. In addition, as S. meliloti is an a-proteobacterium, understanding its genome could provide important information about plant and animal pathogenic bacterial species such as Agrobacterium and Brucella.