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The scarcity of usable nitrogen frequently limits plant growth. A tight metabolic association with rhizobial bacteria allows legumes to obtain nitrogen compounds by bacterial reduction of dinitrogen (N2) to ammonium (NH4 +). We present here the annotated DNA sequence of the α-proteobacteriumSinorhizobium meliloti, the symbiont of alfalfa. The tripartite 6.7-megabase (Mb) genome comprises a 3.65-Mb chromosome, and 1.35-Mb pSymA and 1.68-Mb pSymB megaplasmids. Genome sequence analysis indicates that all three elements contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in understanding the dynamics of interkingdom associations and of life in soil environments.
↵† Present address: Institut Curie, 26 rue d'Ulm, 75005 Paris, France.
↵‡ Present address: Exelixis, Inc., 170 Harbor Way, Post Office Box 511, South San Francisco, CA 94083–0511, USA.
↵§ Present address: Incyte Genomics, 3160 Porter Drive, Palo Alto, CA 94304, USA.
↵|| Present address: Département de Biologie Moléculaire Sciences 2, Université de Genève, Geneva, Switzerland 1211.
↵⁋ Present address: Institute of BioAgricultural Sciences, Academia Sinica, Nankang, Taipei, Taiwan 11529.