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Applied and Environmental Microbiology, May 2009, p. 2652-2658, Vol. 75, No. 9
0099-2240/09/$08.00+0     doi:10.1128/AEM.02536-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Eukaryotic nirK Genes Encoding Copper-Containing Nitrite Reductase: Originating from the Protomitochondrion? {triangledown} ,{dagger}

Sang-Wan Kim, Shinya Fushinobu, Shengmin Zhou, Takayoshi Wakagi, and Hirofumi Shoun*

Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

Received 6 November 2008/ Accepted 26 February 2009

Although denitrification or nitrate respiration has been found among a few eukaryotes, its phylogenetic relationship with the bacterial system remains unclear because orthologous genes involved in the bacterial denitrification system were not identified in these eukaryotes. In this study, we isolated a gene from the denitrifying fungus Fusarium oxysporum that is homologous to the bacterial nirK gene responsible for encoding copper-containing nitrite reductase (NirK). Characterization of the gene and its recombinant protein showed that the fungal nirK gene is the first eukaryotic ortholog of the bacterial counterpart involved in denitrification. Additionally, recent genome analyses have revealed the occurrence of nirK homologs in many fungi and protozoa, although the denitrifying activity of these eukaryotes has never been examined. These eukaryotic homolog genes, together with the fungal nirK gene of F. oxysporum, are grouped in the same branch of the phylogenetic tree as the nirK genes of bacteria, archaea, and eukaryotes, implying that eukaryotic nirK and its homologs evolved from a single ancestor (possibly the protomitochondrion). These results show that the fungal denitrifying system has the same origin as its bacterial counterpart.

* Corresponding author. Mailing address: Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan. Phone and fax: 81-3-5841-5148. E-mail: ahshoun{at}mail.ecc.u-tokyo.ac.jp

{triangledown} Published ahead of print on 6 March 2009.

{dagger} Supplemental material for this article may be found at http://aem.asm.org/.

Applied and Environmental Microbiology, May 2009, p. 2652-2658, Vol. 75, No. 9
0099-2240/09/$08.00+0     doi:10.1128/AEM.02536-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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