Denitrification by the Fungus Fusarium oxysporum Involves NADH-Nitrate Reductase

Abstract

Fusarium oxysporum JCM11502 expresses a denitrifying (nitrate (NO₃⁻)-respiring) mechanism and can thrive under oxygen (O₂) limitation. The fungus reduces NO₃⁻ to nitrite at the initial step of denitrification. In this study, we cloned the gene coding NADH-NO₃⁻ reductase (NADH-Nar) (niaD) from F. oxysporum JCM11502. The niaD gene complemented the defective NO₃⁻ assimilation by mutant strain M10, indicating that the fungus reduced NO₃⁻ through NADH-Nar activity and assimilated it like other fungi. We found that the transcription of niaD and the production of NADH-Nar activity were enhanced under O₂-limited denitrifying conditions relative to aerobic conditions. Strain M10 produced less NADH-Nar activity and less denitrified product than the wild-type strain. Introducing niaD into the mutant also restored these defects, indicating that niaD is involved in denitrification. These results indicate that the fungus denitrified NO₃⁻ through NADH-Nar activity in addition to the ubiquinol-Nar mechanism.