The regulatory protein NIT4 that mediates nitrate induction in Neurospora crassa contains a complex tripartite activation domain with a novel leucine-rich, acidic motif.

Expression of nit-3 and nit-6, the structural genes which encode nitrate reductase and nitrite reductase in Neurospora crassa, requires the global-acting NIT2 and the pathway specific NIT4 regulatory proteins. NIT4, which consists of 1090 amino-acid residues, possesses a Cys6/Zn2 zinc cluster DNA-binding-domain. NIT4 was dissected to localize transactivation domains by fusion of various segments of NIT4 to the DNA-binding domain of GAL4 for in vivo analysis in yeast. Three separate activation subdomains, and one negative-acting region, which function in yeast were located in the carboxyl-terminal region of NIT4. The C-terminal tail of 28 amino-acid residues was identified as a minimal activation domain and consists of a novel leucine-rich, acidic region. Most deletions which removed even small segments of the NIT4 protein were found to lead to the loss of NIT4 function in vivo in N. crassa, implying that the central region of the protein which lies between the DNA-binding and activation domains is essential for function. The yeast two-hybrid system was employed to identify regions of NIT4 responsible for dimer formation. A short isoleucine-rich segment downstream from the zinc cluster, predicted to form a coiled coil, allowed dimerization in vivo; this same isoleucine-rich region also showed dimerization in vitro when examined via chemical cross linking. The enzyme nitrate reductase has been postulated to exert autogenous regulation by directly interacting with the NIT4 protein. This possible nitrate reductase-NIT4 interaction was investigated with the yeast two-hybrid system and by direct in vitro binding assays; both assays failed to identify such a protein-protein interaction.