The genomes of rhizobia

The genomes of alpha- and beta-rhizobia belonging to the alpha- and beta-proteobacteria respectively contain genes for nitrogen fixation in symbiosis with host plants (encoded by nif and fix genes) and nodulation of their host plants, most often using Nod Factor (synthesized by nod, noe, nol gene products). In alpha-rhizobia symbiotic genes are often organized on plasmids of the repABC family, and symbiotic genes for the beta-rhizobia are also plasmid borne. In some alpha-rhizobia, symbiotic genes are encoded on the chromosome within symbiotic islands that are integrative conjugal elements. In this chapter we give an overview of general features of rhizobial genomes including the symbiotic genes they contain and their organization. We review the current understanding of common features of rhizobial genomes and genomic backgrounds that support nodulation and nitrogen fixation and discuss genome deconstruction as a tool for uncovering new genes involved in symbiosis with legume plants.