Neurospora endo-exonuclease is immunochemically related to the recC gene product of Escherichia coli.

Immunochemical cross-reaction between the endo-exonuclease of Neurospora crassa, an enzyme previously implicated in recombination and recombinational DNA repair, and the recC-encoded polypeptide of Escherichia coli has been detected by immunoblotting extracts of strains of E. coli having a deletion that includes the recBCD genes but carrying multicopy plasmids bearing all three of the recBCD genes or only one or two of these genes. It was predicted that homology would also be found at the amino acid sequence level between the recC polypeptide and both nuclear and mitochondrial endo-exonucleases of Saccharomyces cerevisiae, which cross-react with antibodies raised to the N. crassa endo-exonuclease. Since the gene for the S. cerevisiae mitochondrial enzyme, NUC1, has been cloned and sequenced and the predicted amino acid sequence is known, this sequence was aligned with the predicted amino acid sequence of the recC polypeptide. Extensive homology was found by aligning 306 of the 329 amino acids of the yeast mitochondrial nuclease sequence with the carboxy-terminal one-quarter of the amino acid sequence of the recC polypeptide.     

Site-directed mutagenesis of the ''zinc finger'' DNA-binding domain of the nitrogen-regulatory protein NIT2 of Neurospora

The major nitrogen-regulatory gene nit-2 of Neurospora crassa activates the expression of numerous unlinked structural genes which specify nitrogen-catabolic enzymes during conditions of nitrogen limitation. The nit-2 gene encodes a regulatory protein of 1036 amino acid residues with a single 'zinc finger' and a downstream basic region, which together may constitute a DNA-binding domain. The zinc finger domain of the NIT2 protein was synthesized in vitro and also expressed as a fusion protein in Escherichia coli to examine its DNA-binding activity. The wild-type NIT2 finger domain protein binds to the promoter region of nit-3, the nitrate reductase structural gene. A series of NIT2 mutant proteins obtained by site-directed mutagenesis was expressed and tested for functional activity. The results demonstrate that both the single zinc-finger motif and the downstream basic region of NIT2 are critical for its trans-activating function in vivo and specific DNA-binding in vitro.     

cys-3, the positive-acting sulfur regulatory gene of Neurospora crassa, encodes a sequence-specific DNA-binding protein

cys-3, the positive-acting master sulfur regulatory gene of Neurospora crassa, turns on the expression of an entire set of unlinked structural genes which encode sulfur-catabolic enzymes. cys-3 encodes a protein of 236 amino acid residues and contains a potential bipartite DNA-binding domain which consists of a leucine zipper and an adjacent highly basic region. Gel band mobility shift and DNA footprint experiments were used to demonstrate that the CYS3 protein, expressed in Escherichia coli, binds to three distinct sites in the 5' upstream DNA of cys-14, the structural gene for sulfate permease II. The CYS3 protein also binds to one distinct sequence element upstream of the cys-3 gene itself, which suggests an autoregulatory role for this protein. Two mutant CYS3 proteins, altered in the basic region of the DNA-binding domain, failed to bind to either the cys-14 or the cys-3 upstream recognition elements.