Hartmann, Linda , Barnum, S R .
The Evolutionary History of Nitrogenase as assessed by nifK .
Although 80% of the atmosphere is composed of dinitrogen, living organisms rely on reduced forms to live. The ability to fix nitrogen is widely, but sporadically, dispersed among prokaryotes suggesting either vertical descent with loss of function or gene transfer between organisms in the same ecosystem. Interpreting the evolutionary history of nitrogenase is complicated by instances of gene duplication and forms of the enzyme that utilize different metal cofactors. The nifD and nifK genes encode Mo-dependent nitrogenase. Thirty-three nifK sequences were obtained from GenBank. Additional sequences were generated for heterocystous cyanobacteria. Maximum likelihood analyses were developed from nifDK, nifK, and 16S alignments using PAUP 4.0. Our analyses indicate that the nif phylogenies are largely congruent with the taxonomic placements derived from the 16S rRNA gene supporting vertical descent. However, Wolinella succinogenes, an epsilon-proteobacteria, falls within the cyanobacterial clade suggesting lateral gene transfer. Additionally, the nifDK2 genes from the filamentous cyanobacteria Anabaena variabilis cluster with unicellular cyanobacteria. Further analysis of symbiotic Nostoc strains indicates the presence of a second gene highly similar to nifK2. We conclude that the nitrogenase enzyme is of ancient origin and has been transmitted primarily by vertical descent based on the congruence of the nif and 16S phylogenies and the high level of structural conservation of amino acids. Horizontal gene transfer between a cyanobacterium and a proteobacterium is also supported.
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1 - Miami University, Botany, Pearson Hall, Oxford, OH, 45056, USA
2 - Miami University, Botany
Presentation Type: Plant Biology Abstract
Location: Exhibit Hall (Northeast, Southwest & Southeast)/Hilton
Date: Sunday, July 8th, 2007
Time: 8:00 AM