Complete sequence of a Rhizobium plasmid carrying genes necessary for symbiotic association with the plant host

The soil bacteria rhizobia have the capacity to establish nitrogen-fixing symbiosis with their leguminous host plants. In most Rhizobium species the genes for nodule development and nitrogen fixation have been localized on large indigenous plasmids that are transmissible, allowing lateral transfer of symbiotic functions. A recent paper reports on the complete sequencing of the symbiotic plasmid pNGR234a from Rhizobium species NGR234⁽¹⁾, revealing not only putative new symbiotic genes but also possible mechanisms for evolution and lateral dispersal of symbiotic nitrogen-fixing abilities among rhizobia.