The complete sequence of the 1,683-kb pSymB megaplasmid from the N-2-fixing endosymbiont Sinorhizobium meliloti

Analysis of the 1,683,333-nt sequence of the pSymB megaplasmid from the sym biotic N-2-fixing bacterium Sinorhizobium meliloti revealed that the replic on has a high gene density with a total of 1,570 protein-coding regions, wi th few insertion elements and regions duplicated elsewhere in the genome. T he only copies of an essential arg-tRNA gene and the minCDE genes are locat ed on pSymB. Almost 20% of the pSymB sequence carries genes encoding solute uptake systems, most of which were of the ATP-binding cassette family. Man y previously unsuspected genes involved in polysaccharide biosynthesis were identified and these, together with the two known distinct exopolysacchari de synthesis gene clusters, show that 14% of the pSymB sequence is dedicate d to polysaccharide synthesis. Other recognizable gene clusters include man y involved in catabolic activities such as protocatechuate utilization and phosphonate degradation. The functions of these genes are consistent with t he notion that pSymB plays a major role in the saprophytic competence of th e bacteria in the soil environment.