Heterologous expression of the Aspergillus nidulans regulatory gene nirA in Fusarium oxysporum

Abstract We have isolated strains of Fusarium oxysporum carrying mutations conferring a phenotype characteristic of a loss of function in the regulatory gene of nitrate assimilation ( nirA in Aspergillus nidulans, nit-4 in Neurospora crassa ). One of these nir − mutants was successfully transformed with a plasmid containing the nirA gene of A. nidulans . The nitrate reductase of the transformants is still inducible, although the maximum activity is lower than in the wild type. Single and multiple integration events were found, as well as a strict correlation between the presence of the nirA gene and the Nir + phenotype of the F. oxysporum transformants. We also investigated how the A. nidulans structural gene ( niaD ) is regulated in F. oxysporum . Enzyme assays and Northern experiments show that the niaD gene is subject to nitrate induction and that it responds to nitrogen metabolite repression in a F. oxysporum genetic background. This indicates that both the mechanisms of specific induction, mediated by a gene product isofunctional to nirA , and nitrogen metabolite repression, presumably mediated by a gene product isofunctional to the homologous gene of A. nidulans , are operative in F. oxysporum .