Rhizobia are gram-negative bacteria which perform symbiotic nitrogen fixation within nodules formed on the roots of plants of the Leguminosae family(legumes). This ability to convert atmospheric nitrogen to ammonia is strictly limited to prokaryotic organisms and is a crucial function for commercially important crops such as alfalfa and soybean. The independence from nitrogenous fertilizers is an economic saving and minimizes environmental pollution.

The bacterium Rhizobium meliloti (c.f. Sinorhizobium), Bradyrhizobium and Azorhizobium initiate a complex series of interactions with the plant host, they enter the root tissue via infection threads and form nodules in which they fix nitrogen which is then transferred to the plant host.

The genome of Rhizobium meliloti consists of a circular chromosome of 3.5 million nucleotides, two megaplasmids - pEXO of 1.7 million nucleotides and pnod-nif of 1.3 million nucleotides. In addition there is a 536,000 nucleotide pNGR234a plasmid that carries symbiotic determinants which has been completely sequenced (Freiberg et al. 1997). Projects are currently underway to sequence the remainder of this agriculturally important bacterium. These pages are to aid those searching out information regarding the genomes of Rhizobium.