Sinorhizobium meliloti Megaplasmids and Symbiosis in S. meliloti

Sinorhizobium meliloti is a Gram-negative soil bacterium forming a symbiotic nitrogen-fixing relationship with legumes such as Medicago sativa. All strains analyzed so far contain three replicons: one chromosome and two inherently stable megaplasmids, the maintenance of which is not only due to their contribution to the cell viability but also to enhancement of the competitiveness of the cells in their natural environment. pSymB contains both plasmid and chromosomal features and is designed as a second chromosome, whereas pSymA is considered as an symbiotic accessory megaplasmid, as it can be cured without affecting S. meliloti viability. S. meliloti genome architecture was shown to be highly dynamic, as the three replicons continuously cointegrate and excise. Many of the genes identified on pSymA and pSymB in S. meliloti are involved in the formation and functioning of nitrogen-fixing root nodules. Genes located on pSymA are necessary for nodulation and nitrogen fixation, while those located on pSymB are involved in exopolysaccharide synthesis and uptake of various nutrients. Functional analyses of the genome have contributed to our understanding of the influence of the megaplasmids on S. meliloti's metabolic and symbiotic abilities as well as of its successful occupation of natural niches, soil survival, plant colonization, and atmospheric dinitrogen fixation.