NIT2, THE NITROGEN REGULATORY PROTEIN OF NEUROSPORA-CRASSA, BINDS UPSTREAM OF NIA, THE TOMATO NITRATE REDUCTASE GENE, INVITRO

The nit-2 gene of Neurospora crassa encodes a trans-acting regulatory protein that activates the expression of a number of structural genes which code for nitrogen catabolic enzymes, including nitrate reductase. The NIT2 protein contains a Cys2/Cys2-type zinc-finger DNA-binding domain that recognizes promoter regions of the Neurospora nitrogen-related genes. The NIT2 zinc-finger domain/beta-Gal fusion protein was shown to recognize and bind in a specific manner to two upstream fragments of the nia gene of Lycopersicon esculentum (tomato) in vitro, whereas two mutant NIT2 proteins failed to bind to the same fragments. The dissociation kinetics of the complexes formed between the NIT2 protein and the Neurospora nit-3 and the tomato nia gene promoters were examined; NIT2 binds more strongly to the nit-3 promoter DNA fragment than it does to fragments derived from the plant nitrate reductase gene itself. The observed specificity of the binding suggests the existence of a NIT2-like homolog which regulates the expression of the nitrate assimilation pathway of higher plants.