Nitrogenase structural genes are unlinked in the nonlegume symbiont Parasponia rhizobium.

Several Rhizobium strains are capable of biological nitrogen fixation in symbiotic association with nonleguminous plants. The gene encoding the iron-protein component of nitrogenase (nifH) from one such strain, Parasponia Rhizobium sp. ANU289, has been isolated and completely sequenced. Unlike previously studied nitrogen-fixing prokaryotes, the Fe-protein subunit is encoded on a separate operon from other components of the nitrogenase enzyme complex. Comparative analysis of Fe-protein amino acid sequences indicates that the symbiotic nitrogen fixation phenotype in Rhizobium may have arisen on at least two separate occasions during its evolution.