REGULATION OF NITRATE REDUCTASE IN THE MYCORRHIZAL ASCOMYCETE WILCOXINA-MIKOLAE VAR MIKOLAE

The regulation of nitrate reductase (NR, EC 1.6.6.3) in the mycorrhiza l ascomycete Wilcoxina mikolae var. mikolae was studied with the aim o f improving understanding of nitrate assimilation in mycorrhiza. It wa s possible to study induction, de-repression and repression of NR indi vidually and in combination by utilizing the neutral nitrogen source u rea. The NR activity required specific induction by nitrate and was no t present in de-repressed urea grown cultures. The induction of NR by nitrate required de novo protein synthesis as it was inhibited by cycl oheximide and actinomycin D. Both ammonium and glutamine in the medium repressed the NR activity. The repression by ammonium appeared to req uire its assimilation since treatment of mycelia with L-methionine-DL- sulfoximine, a specific inhibitor of glutamine synthetase, reduced amm onium repression. These studies demonstrated that the regulation of NR in Wilcoxina is unlike the constitutive and non-repressible NR report ed in mycorrhizal basidiomycetes.