title: nifH sequences and nitrogen fixation in type I and type II methanotrophs.
abstract: Some methane-oxidizing bacteria (methanotrophs) are known to be capable of expressing nitrogenase and utilizing N2 as a nitrogen source. However, no sequences are available for nif genes in these strains, and the known nitrogen-fixing methanotrophs are confined mainly to a few genera. The purpose of this work was to assess the nitrogen-fixing capabilities of a variety of methanotroph strains. nifH gene fragments from four type I methanotrophs and seven type II methanotrophs were PCR amplified and sequenced. Nitrogenase activity was confirmed in selected type I and type II strains by acetylene reduction. Activities ranged from 0.4 to 3.3 nmol/min/mg of protein. Sequence analysis shows that the nifH sequences from the type I and type II strains cluster with nifH sequences from other gamma proteobacteria and alpha proteobacteria, respectively. The translated nifH sequences from three Methylomonas strains show high identity (95 to 99%) to several published translated environmental nifH sequences PCR amplified from rice roots and a freshwater lake. The translated nifH sequences from the type II strains show high identity (94 to 99%) to published translated nifH sequences from a variety of environments, including rice roots, a freshwater lake, an oligotrophic ocean, and forest soil. These results provide evidence for nitrogen fixation in a broad range of methanotrophs and suggest that nitrogen-fixing methanotrophs may be widespread and important in the nitrogen cycling of many environments.
authors.affiliation: Department of Microbiology, University of Washington, Seattle, Washington 98195, USA. aauman@u.washington.edu
authors.forename: A J
authors.initials: AJ
authors.lastname: Auman
authors.forename: C C
authors.initials: CC
authors.lastname: Speake
authors.forename: M E
authors.initials: ME
authors.lastname: Lidstrom
mesh_terms: D000595:Amino Acid Sequence; D008697:Methane / Q000378:metabolism; D008772:Methylococcaceae / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation; D010088:Oxidoreductases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D012230:Rhizobiaceae / Q000235:genetics / Q000378:metabolism*; D017422:Sequence Analysis, DNA
chemical_list: D010088:Oxidoreductases; C029130:nitrogenase reductase; D008697:Methane