Regulation of nitrate reductase in the mycorrhizal ascomycete Wilcoxina mikolae var. mikolae

Abstract

The regulation of nitrate reductase (NR, EC 1.6.6.3) in the mycorrhizal ascomycete Wilcoxina mikolae var. mikolae was studied with the aim of improving understanding of nitrate assimilation in mycorrhiza. It was possible to study induction, de-repression and repression of NR individually and in combination by utilizing the neutral nitrogen source urea. The NR activity required specific induction by nitrate and was not present in de-repressed urea grown cultures. The induction of NR by nitrate required de novo protein synthesis as it was inhibited by cycloheximide and actinomycin D. Both ammonium and glutamine in the medium repressed the NR activity. The repression by ammonium appeared to require its assimilation since treatment of mycelia with l -methionine- dl -sulfoximine, a specific inhibitor of glutamine synthetase, reduced ammonium repression. These studies demonstrated that the regulation of NR in Wilcoxina is unlike the constitutive and non-repressible NR reported in mycorrhizal basidiomycetes.