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Nitrogenase phylogeny
Nitrogenase is likely to be an ancient enzyme. Nitrogenase genes are distributed throughout the prokaryotic kingdom, including representatives of the Archaea as well as the Eubacteria and Cyanobacteria.
Although the phylogeny of nifH reflects the phylogeny of organisms based on ribosomal RNA genes, there are some differences.
One deeply branching cluster is anomalous and is likely to represent an independent line of evolution, and includes some sequences from gram positive organisms, such as Clostridium.
Since nitrogenase gene sequences do reflect phylogenetic affiliation, the sequence of nitrogenase genes can be used to identify the types of nitrogen-fixing microorganisms in different habitats.
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