Molecular characterization of mutations of nit-4, the pathway-specific regulatory gene which controls nitrate assimilation in Neurospora crassa.

The nit-4 genes of three conventional Neurospora crassa mutations and of the closely related species, Neurospora intermedia, have been isolated by amplifying the genomic DNA with the polymerase chain reaction. Nucleotide sequencing has revealed that the three nit-4 mutants, alleles 15, 1214, and 2994, are the result of a missense mutation, a nonsense mutation and a frameshift mutation, respectively. The nucleotide sequence of the NIT4 protein coding region of a nit-4 mutant (allele 2994) and of N. intermedia have been determined and compared with that of wild-type N. crassa. The molecular characteristics confirm that the mutated gene of 2994 originated from N. intermedia and was introgressed into N. crassa. The polyglutamine domains of the N. crassa wild type, the 2994 mutant, or N. intermedia cannot replace an upstream glutamine-rich domain which is essential for nit-4 function.