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

Proc Natl Acad Sci U S A. 2001 Aug 14;98(17):9889-94. doi: 10.1073/pnas.161294698. Epub 2001 Jul 31.


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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / physiology
  • Carbohydrate Metabolism
  • Carrier Proteins / genetics
  • Chaperonins / genetics
  • DNA, Bacterial / genetics
  • Drug Resistance, Microbial / genetics
  • Enzymes / genetics
  • Gene Expression Regulation, Bacterial
  • Gene Library
  • Genes, Bacterial
  • Lipopolysaccharides / biosynthesis
  • Molecular Sequence Data
  • Nitrogen Fixation / genetics
  • Plasmids / genetics*
  • RNA, Transfer, Arg / genetics
  • Replication Origin / genetics
  • Replicon / genetics
  • Sequence Analysis, DNA
  • Sinorhizobium meliloti / genetics*
  • Species Specificity
  • Transcription, Genetic / genetics


  • Bacterial Proteins
  • Carrier Proteins
  • DNA, Bacterial
  • Enzymes
  • Lipopolysaccharides
  • RNA, Transfer, Arg
  • Chaperonins

Associated data

  • GENBANK/AL591985