Analysis of the chromosome sequence of the legume symbiont Sinorhizobium meliloti strain 1021

Sinorhizobium meliloti is an alpha -proteobacterium that forms agronomicall y important N-2-fixing root nodules in legumes. We report here the complete sequence of the largest constituent of its genome, a 62.7% GC-rich 3,654,1 35-bp circular chromosome. Annotation allowed assignment of a function to 5 9% of the 3,341 predicted protein-coding ORFs, the rest exhibiting partial, weak, or no similarity with any known sequence. Unexpectedly, the level of reiteration within this replicon is low, with only two genes duplicated wi th more than 90% nucleotide sequence identity, transposon elements accounti ng or 2.2% of the sequence, and a few hundred short repeated palindromic mo tifs (RIME1, RIME2, and C) widespread over the chromosome. Three regions wi th a significantly lower GC content are most likely of external origin. Det ailed annotation revealed that this replicon contains all housekeeping gene s except two essential genes that are located on pSymB. Amino acid/peptide transport and degradation and sugar metabolism appear as two major features of the S. meliloti chromosome. The presence in this replicon of a large nu mber of nucleotide cyclases with a peculiar structure, as well as of genes homologous to virulence determinants of animal and plant pathogens, opens p erspectives in the study of this bacterium both as a free-living soil micro organism and as a plant symbiont.