Interspecies homology of nitrogenase genes.

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Abstract
Cloned N2 fixation (nif) genes from Klebsiella pneumoniae hybridize to DNA from 19 out of 19 widely divergent N2-fixing bacterial strains but do not hybridize to DNA from 10 different non-N2-fixing species. K. pneumoniae nif DNA fragments that hybridize to DNA from other species contain part of the 3 structural genes that code for nitrogenase polypeptides. This homology was utilized to clone an EcoRI restriction endonuclease fragment from Rhizobium meliloti that hybridizes to the K. pneumoniae nif structural genes. Some of the species whose DNA hybridizes with K. pneumoniae nif DNA were postulated to have diverged from K. pneumoniae 3 .times. 109 yr ago. Nitrogenase genes are the only known example of such highly conserved prokaryotic translated genes. Nitrogenase genes are either extraordinarily conserved in evolution or were exchanged between different N2 fixing species relatively recently in evolutionary time.