Hypersensitive sites in the 5' promoter region of nit-3, a highly regulated structural gene of Neurospora crassa.

The nit-3 gene of the filamentous fungus Neurospora crassa encodes nitrate reductase, the enzyme which catalyzes the first step in nitrate assimilation. The nit-3 gene is subject to a high degree of regulation by metabolic inducers and repressors, and its expression requires two distinct trans-acting regulatory proteins. Hypersensitive sites in the 5' DNA sequence upstream of the nit-3 gene were mapped with the use of three different nucleases as molecular probes. Six hypersensitive sites, three of which are very strong, were detected at essentially identical positions by all three nucleases. The hypersensitive sites appear to develop in a constitutive fashion and are present under conditions in which the nit-3 structural gene is expressed but also when this gene is inactive, although these sites are considerably less prominent in cells subjected to nitrogen catabolite repression. The presence of the hypersensitive sites appears to depend upon both the positively acting NIT2 and the positively acting NIT4 regulatory proteins, which might play a role in positioning of chromatin protein.