Abstract
The methanogenic Archaea bring a broadened perspective to the field of nitrogen fixation. Biochemical and genetic studies show that nitrogen fixation in Archaea is evolutionarily related to nitrogen fixation in Bacteria and operates by the same fundamental mechanism. At least six nif genes present in Bacteria (nif H, D, K, E, N and X) are also found in the diazotrophic methanogens. Most nitrogenases in methanogens are probably of the molybdenum type. However, differences exist in gene organization and regulation. All six known nif genes of methanogens, plus two homologues of the bacterial nitrogen sensor-regulator glnB, occur in a single operon in Methanococcus maripaludis. nif gene transcription in methanogens is regulated by what appears to be a classical prokaryotic repression mechanism. At least one aspect of regulation, post-transcriptional ammonia switch-off, involves novel members of the glnB family. Phylogenetic analysis suggests that nitrogen fixation may have originated in a common ancestor of the Bacteria and the Archaea.
Highlights
It has long been recognized that nitrogen fixation occurs in bacteria but not eukaryotes
Daniels independently discovered diazotrophic growth in two different methanogenic Archaea, Methanosarcina barkeri [2] and Methanococcus thermolithotrophicus [3]. 15N incorporation from 15N2 confirmed that nitrogen fixation occurred in M. barkeri, and acetylene reduction confirmed nitrogenase activity in M. thermolithotrophicus
Sequencing revealed nifD and part of nifK downstream from nifH [17]. These findings suggested that nitrogen fixation in methanogens was evolutionarily related to nitrogen fixation in Bacteria and operated with the same basic mechanism
Summary
It has long been recognized that nitrogen fixation occurs in bacteria but not eukaryotes. When Archaea were first discovered to be a prokaryotic domain of life separate from Bacteria [1], it was not known that they too contained nitrogen fixing species. 15N incorporation from 15N2 confirmed that nitrogen fixation occurred in M. barkeri, and acetylene reduction confirmed nitrogenase activity in M. thermolithotrophicus. Daniels independently discovered diazotrophic growth in two different methanogenic Archaea, Methanosarcina barkeri [2] and Methanococcus thermolithotrophicus [3]. This discovery of nitrogen fixation in Archaea raised new questions regarding the diversity of nitrogen fixation at the phylogenetic, biochemical, and genetic levels. A review on nitrogen fixation in methanogens appeared in 1992 [4]
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