Identification of nonsense mutations in Neurospora: Application to the complex arg-6 locus

Processing and function of a polyprotein precursor of two mitochondrial proteins in neurospora crassa

In Neurospora crassa, the mitochondrial arginine biosynthetic enzymes, N-acetylglutamate kinase (AGK) and N-acetyl-gamma-glutamyl-phosphate reductase (AGPR), are generated by processing of a 96-kDa cytosolic polyprotein precursor (pAGK-AGPR). The proximal kinase and distal reductase domains are separated by a short connector region. Substitutions of arginines at positions -2 and -3 upstream of the N terminus of the AGPR domain or replacement of threonine at position +3 in the mature AGPR domain revealed a second processing site at position -20. Substitution of arginine at position -22, in combination with changes at -2 and -3, prevented cleavage of the precursor and identified two proteolytic cleavage sites, Arg-Gly downward arrow Tyr-Leu-Thr at the N terminus of the AGPR domain and Arg-Gly-Tyr downward arrow Ser-Thr located 20 residues upstream. Inhibitors of metal-dependent peptidases blocked proteolytic cleavage at both sites. Amino acid residues required for proteolytic cleavage in the connector were identified, and processing was abolished by mutations changing these residues. The unprocessed AGK-AGPR fusion had both catalytic activities, including feedback inhibition of AGK, and complemented AGK-AGPR- mutants. These results indicate that cleavage of pAGK-AGPR is not required for functioning of these enzymes in the mitochondria.

Amber nonsense mutations in regulatory and structural genes of the nitrogen control circuit of Neurospora crassa

Neurospora crassa possesses a set of nitrogen-regulated enzymes whose expression requires a lifting of nitrogen catabolite repression and specific induction. The nit-2 gene is a major regulatory locus which appears to act in a positive way to turn on the expression of these nitrogen-related enzymes whereas the nit-4 gene appears to mediate nitrate induction of nitrate and nitrite reductase. The nit-3 gene specifies nitrate reductase and is subject to control by both nit-2 and nit-4. Many new nit-2, nit-3, and nit-4 mutants were isolated in order to obtain amber nonsense mutations in these loci which were suppressible by the suppressor gene, Ssu-1. A nit-2 nonsense mutant was isolated which has altered regulatory properties for control of nitrate reductase. L-amino acid oxidase, and uricase, and which may encode a truncated regulatory protein. Four nit-3 nonsense mutations were isolated, each of which completely lacks nitrate reductase activity, which is restored to markedly different levels by suppression with Ssu-1. Studies of heat activation and thermal lability of nitrate reductase suggest a qualitative alteration of the enzyme occurs in two of the Ssu-1 nit-3 strains.