Two distinct protein-protein interactions between the NIT2 and NMR regulatory proteins are required to establish nitrogen metabolite repression in Neurospora crassa.

Nitrogen metabolism is a highly regulated process in Neurospora crassa. The structural genes that encode nitrogen catabolic enzymes are subject to nitrogen metabolite repression, mediated by the positive-acting NIT2 protein and by the negative-acting NMR protein. NIT2, a globally acting factor, is a member of the GATA family of regulatory proteins and has a single Cys2/Cys2 zinc finger DNA-binding domain. The negative-acting NMR protein interacts via specific protein-protein binding with two distinct regions of the NIT2 protein, a short alpha-helical motif within the NIT2 DNA-binding domain and a second motif at its carboxy terminus. Deletions of segments of NIT2 throughout most of its length result in truncated proteins, which are still functional for activating gene expression; most of these mutant NIT2 proteins still allow proper nitrogen repression of nitrate reductase synthesis. In contrast, deletions or certain amino acid substitutions within the zinc finger and the carboxy-terminal tail result in a loss of nitrogen metabolite repression. Those mutated forms of NIT2 that are insensitive to nitrogen repression have also lost one of the NIT2-NMR protein-protein interactions. These results provide compelling evidence that the specific NIT2-NMR interactions have a regulatory function and play a central role in establishing nitrogen metabolite repression.