Identification of genes unique to Mo-independent nitrogenase systems in diverse diazotrophs.

A number of nitrogen-fixing bacteria were screened using PCR for genes (vnfG and anfG) unique to the V-containing nitrogenase (vnf) and the Fe-only nitrogenase (anf) systems. Products with sequences similar to that of vnfG were obtained from Azotobacter paspali and Azotobacter salinestris genomic DNAs, and products with sequences similar to that of anfG were obtained from Azomonas macrocytogenes, Rhodospirillum rubrum, and Azotobacter paspali DNAs. Phylogenetic analysis of the deduced amino acid sequences of anfG and vnfG genes shows that each gene product forms a distinct cluster. Furthermore, amplification of an internal 839-bp region in anfD and vnfD yielded a product similar to anfD from Heliobacterium gestii and a product similar to vnfD from Azotobacter paspali and Azotobacter salinestris. Phylogenetic analysis of NifD, VnfD, and AnfD amino acid sequences indicates that AnfD and VnfD sequences are more closely related to each other than either is to NifD. The results of this study suggest that Azotobacter salinestris possesses the potential to express the vanadium (V)-containing nitrogenase (nitrogenase 2) and that R. rubrum, Azomonas macrocytogenes, and H. gestii possess the potential to express the Fe-only nitrogenase (nitrogenase 3). Like Azotobacter vinelandii, Azotobacter paspali appears to have the potential to express both the V-containing nitrogenase and the Fe-only nitrogenase.