HETEROLOGOUS EXPRESSION AND REGULATION OF THE NEUROSPORA-CRASSA NIT-4 PATHWAY-SPECIFIC REGULATORY GENE FOR NITRATE ASSIMILATION IN ASPERGILLUS-NIDULANS

The nirA gene of Aspergillus nidulans and the nit-4 gene of Neurospora crassa appear to be equivalent pathway-specific regulatory genes which mediate nitrate induction of nitrate reductase and nitrite reductase (NR and NiR) activities. We have transformed the nit-4 wild-type (wt) gene into the A. nidulans loss-of-function (pleiotropic negative) nirA 1 mutant strain. The nit-4 gene was found to complement the nirA 1 mutation, thus permitting the nirA 1 mutant strain to grow on nitrate or nitrite as the sole source of nitrogen. Integration of the nit-4 gene in transformants appears to have occurred at a number of 'ectopic', i.e., non-nirA, sites. Nitrate is required for the induction of NR activity in nit-4-transformed strains whilst NR production remains markedly subject to nitrogen-metabolite repression. However, NR levels are modestly higher than wt under all growth conditions.