The isolation and characterization of mutants defective in nitrate assimilation in Neurospora crassa

The isolation and characterization of mutants altered for nitrate assimilation in Neurospora crassa is described. The mutants isolated can be subdivided into five classes on the basis of growth test that correspond to the growth patterns of existing mutants at six distinct loci. Mutants with growth characteristics like those of nit-2, nit-3 and nit-6 are assigned to those loci on the basis of noncomplementation and lack of recombination. Mutants that, from their growth patterns, appear to lack the molybdenum-containing cofactor for both nitrate reductase and xanthine dehydrogenase subdivide into three loci (nit-y, nit-8 and nit-9), all of which are gentically distinct from nit-1. nit-9 is a complex locus consisting of three complementation groups and thus appears similar ao the cnxABC locus of Asperillus nidulans. Extensive complementational and recombinational analyses reveal that nit-4 and nit-5 are alleles of the same locus, and two new alleles of that locus have been isolated. The results indicate that, as in A. nidulans, nitrate assimilation in N. crassa requires at least four loci (nit-1, 7, 8 and 9) to produce the molybdenum co-factor for nitrate reductase (and xanthine dehydrogenase), one locus (nit-3) to code for the nitrate reductase apoprotein, one locus (nit-6) to code for the nitrite reductase approtein and only one lous (nit-4/5) for the regulation of induction of the pathway by nitrate and nitrite.