The composite genome of the legume symbiont Sinorhizobium meliloti

The scarcity of usable nitrogen frequently limits plant growth. A tight met abolic association with rhizobial bacteria allows legumes to obtain nitroge n compounds by bacterial reduction of dinitrogen (N-2) to ammonium (NH4+). We present here the annotated DNA sequence of the alpha -proteobacterium Si norhizobium 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 p SymB megaplasmids. Genome sequence analysis indicates that all three elemen ts contribute, in varying degrees, to symbiosis and reveals how this genome may have emerged during evolution. The genome sequence will be useful in u nderstanding the dynamics of interkingdom associations and of life in soil environments.