The complete sequence of the 1,683-kb pSymB megaplasmid from the N2-fixing endosymbiont Sinorhizobium meliloti

Abstract

Analysis of the 1,683,333-nt sequence of the pSymB megaplasmid from the symbiotic N(2)-fixing bacterium Sinorhizobium meliloti revealed that the replicon has a high gene density with a total of 1,570 protein-coding regions, with few insertion elements and regions duplicated elsewhere in the genome. The only copies of an essential arg-tRNA gene and the minCDE genes are located on pSymB. Almost 20% of the pSymB sequence carries genes encoding solute uptake systems, most of which were of the ATP-binding cassette family. Many previously unsuspected genes involved in polysaccharide biosynthesis were identified and these, together with the two known distinct exopolysaccharide synthesis gene clusters, show that 14% of the pSymB sequence is dedicated to polysaccharide synthesis. Other recognizable gene clusters include many involved in catabolic activities such as protocatechuate utilization and phosphonate degradation. The functions of these genes are consistent with the notion that pSymB plays a major role in the saprophytic competence of the bacteria in the soil environment.

Figure 1

Genome map of the pSymB megaplasmid of Sinorhizobium meliloti strain 1021. The inner circle displays ORFs on the leading (red) and lagging (green) strands of pSymB. The outer circle shows predicted gene regions encoding transcriptional regulators (pink), ABC transport systems (yellow), and genes involved in polysaccharide biosynthesis (orange). The positions of specific genes or sets of genes (e.g., exp, exs, and exo) are also shown on the outer edge of the map.