Distribution of Nitrogen‐Fixation Genes in Prokaryotes Containing Alternative Nitrogenases

Biological nitrogen fixation is an inherent trait exclusive to a select number of prokaryotes. Although molybdenum nitrogenase is the dominant catalyst for dinitrogen reduction, some diazotrophs also contain one or two additional types of nitrogenase that use alternative metal content as the active‐site cofactor. The occurrence of alternative nitrogenases has not been well studied due to the discriminatory expression of the molybdenum nitrogenase and lack of comprehensive genomic data. This study reports on the genomic analysis of 87 unique species containing alternative nitrogenase sequences. The distribution of nitrogen‐fixing genes within these species from distinct taxonomic groups shows the presence of the minimum gene set required for nitrogen fixation, including catalytic and biosynthetic enzymes of the Mo‐dependent system (NifHDKENB) and the varying occurrence of additional Nif‐dedicated components. These include NifS and NifU, found primarily in aerobic species, thus suggesting that these genes are necessary to accommodate the high demand for Fe−S clusters during aerobic nitrogen fixation. All diazotrophs studied to date contain Mo‐dependent nitrogenase; a subset also have V‐dependent and Fe‐only nitrogenases that are expressed under Mo‐deficient conditions. As the concentrations of V in soil are typically much higher than those of Mo, identifying species carrying alternative enzymes is of particular interest for applying diazotrophs as energy‐efficient natural fertilizers.