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Nodule bacteria (rhizobia) forming nitrogen-fixing symbiosis with legumes have a multicomponent genome represented by chromosome and a number of plasmids of different sizes. Genes related to symbiotic activity of Sinorhizobium meliloti are located predominantly on two megaplasmids SMa and SMb (1.35 and 1.68 Mb, correspondingly). Genomes at about 80% of native rhizobia nodulating alfalfa contained cryptic plasmids with sizes varied from 7 up to 450 kb. Functional role of this class of plasmids remained unclear up t...