Ensifer meliloti (formerly Rhizobium meliloti and Sinorhizobium meliloti) are an aerobic, Gram-negative, and diazotrophic species of bacteria. S. meliloti are motile and possess a cluster of peritrichous flagella. S. meliloti fix atmospheric nitrogen into ammonia for their legume symbionts, such as alfalfa. S. meliloti forms a symbiotic relationship with legumes from the genera Medicago, Melilotus and Trigonella, including the model legume Medicago truncatula. This symbiosis promotes the development of a plant organ, termed a root nodule. Because soil often contains a limited amount of nitrogen for plant use, the symbiotic relationship between S. meliloti and their legume hosts has agricultural applications. These techniques reduce the need for inorganic nitrogenous fertilizers. Symbiosis Symbiosis between S. meliloti and its legume hosts begins when the plant secretes an array of betaines and flavonoids into the rhizosphere: 4,4′-dihydroxy-2′-methoxychalcone, chrysoeriol, cynaroside, 4′,7-dihydroxyflavone, 6′′-O-malonylononin, liquiritigenin, luteolin, 3′,5-dimethoxyluteolin, 5-methoxyluteolin, medicarpin, stachydrine, and trigonelline. These compounds attract S. meliloti to the surface of the root hairs of the plant where the bacteria begin secreting nod factors. This initiates root hair curling. The rhizobia then penetrate the root hairs and proliferate to form an infection thread. Through the infection thread, the bacteria move toward the main root. The bacteria develop into bacteroids within newly formed root nodules and perform nitrogen fixation for the plant. A S. meliloti bacterium does not perform nitrogen fixation until it differentiates into a endosymbiotic bacteroid. A bacteroid depends on the plant for survival. Leghemoglobin, produced by leguminous plants after colonization of S. meliloti, interacts with the free oxygen in the root nodule where the rhizobia reside. Rhizobia are contained within symbiosomes in the root nodules of leguminous plants. The leghemoglobin reduces the amount of free oxygen present. Oxygen disrupts the function of the nitrogenase enzyme in the rhizobia, which is responsible for nitrogen fixation. Genome The S. meliloti genome contains four genes coding for flagellin. These include fliC1C2–fliC3C4. The genome contains three replicons: a chromosome (~3.7 megabases), a chromid (pSymB; ~1.7 megabases), and a plasmid (pSymA; ~1.4 megabases). Individual strains may possess additional, accessory plasmids. Five S. meliloti genomes have been sequenced to date: Rm1021, AK83, BL225C, Rm41, and SM11 with 1021 considered to be the wild type. Indeterminate nodule symbiosis by S. meliloti is conferred by genes residing on pSymA. DNA repair The proteins encoded by E. meliloti genes uvrA, uvrB and uvrC are employed in the repair of DNA damages by the