Abstract
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.
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