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

A number of nitrogen-fixing bacteria were screened using PCR for genes (vnf G and anfG) unique to the V-containing nitrogenase (vnf) and the Fe-only ni trogenase (anf) systems. Products with sequences similar to that of vnfG we re obtained from Azotobacter paspali and Azotobacter salinestris genomic DN As, and products with sequences similar to that of anfG were obtained from Azomonas macrocytogenes, Rhodospirillum rebrum, and Azotobacter paspali DNA s. Phylogenetic analysis of the deduced amino acid sequences of anfG and vn fG genes shows that each gene product forms a distinct cluster. Furthermore , amplification of an internal 839-bp region in anfD and vnfD yielded a pro duct similar to anfD from Heliobacterium gestii and a product similar to vn fD from Azotobacter paspali and Azotobacter salinestris. Phylogenetic analy sis of NifD, VnfD, and AnfD amino acid sequences indicates that AnfD and Vn fD sequences are more closely related to each other than either is to NifD. The results of this study suggest that Azotobacter salinestris possesses t he potential to express the vanadium (V)-containing nitrogenase (nitrogenas e 2) and that R. rubrum, Azomonas macrocytogenes, and H. gestii possess the potential to express the Fe-only nitrogenase (nitrogenase 3). Like Azotoba cter vinelandii, Azotobacter paspali appears to have the potential to expre ss both the V-containing nitrogenase and the Fe-only nitrogenase.