nit-2, the major nitrogen regulatory gene of Neurospora crassa, encodes a protein with a putative zinc finger DNA-binding domain

Molecular organisation of the malate synthase genes of Aspergillus nidulans and Neurospora crassa

The sequencing and comparison of the genes encoding the glyoxylate bypass enzyme malate synthase of Aspergillus nidulans (acuE) and Neurospora crassa (acu-9) are presented. The predicted amino acid sequences of the A. nidulans and N. crassa enzymes are 538 and 542 residues respectively and the proteins are 87% homologous. In fungi, the malate synthase proteins are located in glyoxysomes and the deduced acuE and acu-9 proteins both contain a C-terminal S-K-L sequence, which has been implicated in transport into peroxisomes. The acuE coding region is interrupted by four introns and the acu-9 coding region is interrupted by one intron which occurs at the same position as the C-terminal acuE intron. The 5' non-coding regions of the two genes were examined for short homologous sequences that may represent the binding sites for regulatory proteins. Pyrimidine-rich sequences with weak homology to the amdI9 sequence, which has been implicated in facB-mediated acetate regulation of the amdS gene, were found but their functional significance remains to be determined.

Nit-3, the structural gene of nitrate reductase in Neurospora crassa: nucleotide sequence and regulation of mRNA synthesis and turnover

The nit-3 gene of the filamentous fungus Neurospora crassa encodes the enzyme nitrate reductase, which catalyzes the first reductive step in the highly regulated nitrate assimilatory pathway. The nucleotide sequence of nit-3 was determined and translates to a protein of 982 amino acid residues with a molecular weight of approximately 108 kDa. Comparison of the deduced nit-3 protein sequence with the nitrate reductase protein sequences of other fungi and higher plants revealed that a significant amount of homology exists, particularly within the three cofactor-binding domains for molybdenum, heme and FAD. The synthesis and turnover of the nit-3 mRNA were also examined and found to occur rapidly and efficiently under changing metabolic conditions.

Nitrogen regulation in Aspergillus: are two fingers better than one?

The areA gene, mediating nitrogen metabolite repression in Aspergillus nidulans, encodes a positive-acting regulatory protein with a single putative DNA-binding 'zinc finger' which is remarkably similar to the two 'zinc fingers' of the major regulatory protein of vertebrate erythroid cells (GF-1/Eryf1/NF-E1). The areA-300 mutation alters the specificity of gene activation in that it elevates expression of certain structural genes whilst reducing expression of certain others. It is an 'in-frame' tandem duplication of 417 bp including the entire DNA-binding region. The consequences of areA didactyly are further explored by construction of a double mutant having an altered loop residue in the N-terminal 'finger'.

Nucleotide sequence and DNA recognition elements of alc, the structural gene which encodes allantoicase, a purine catabolic enzyme of Neurospora crassa

The nitrogen regulatory circuit of Neurospora crassa contains structural genes that encode nitrogen catabolic enzymes which are subject to complex genetic and metabolic regulation. This set of genes is controlled by nitrogen limitation, by specific induction, and by the action of nit-2, a major positive-acting regulatory gene, and nmr, a negative-acting control gene. The complete nucleotide sequence of alc, the gene that encodes allantoicase, a purine catabolic enzyme, is presented. The alc gene contains a single intron, is transcribed from two initiation sites situated approximately 50 nb upstream of the translation start site, and encodes a protein comprised of 354 amino acids. Mobility shift and DNA footprint experiments identified a single binding site for the NIT2 regulatory protein in the alc promoter region. The binding site contains a 10 nucleotide base pair symmetrical sequence which is flanked by two possible core binding sequences, TATCT and TATCG. Mutant NIT2/beta-gal fusion proteins with amino acid substitutions in a putative zinc-finger motif were shown to be completely deficient in the ability to bind to the alc promoter DNA fragment.

Analysis of conventional and in vitro generated mutants of nmr, the negatively acting nitrogen regulatory gene of Neurospora crassa

The nmr gene is the major negative regulatory gene in the nitrogen control circuit of Neurospora crassa, which, together with positive regulatory genes, governs the expression of multiple unlinked structural genes of the circuit. Possible functional domains of the NMR protein were investigated by mutational analyses using three different approaches. First, the polymerase chain reaction was used to clone the nmr locus from two conventional mutants, V2M304 and MS5, and the mutant amino acid codons were identified. A single point mutation was shown to be responsible for the mutant phenotype in each of these strains. The V2M304 allele contains a nonsense codon, and in the MS5 allele an aspartate has been substituted for glycine at residue 386. Our second approach studied possible functionally important regions in the nmr gene by the use of site-directed mutagenesis. The region containing the naturally occurring substitution in MS5 appears to be essential for function whereas a region in the N-terminal part of the protein does not seem important for NMR function. Finally, over 50% of the protein coding region was randomly mutagenized and amino acid residues that are essential for function and others that are functionally unimportant were identified.

Nucleotide sequence and analysis of NMR, a negative-acting regulatory gene in the nitrogen circuit of Neurospora crassa

In Neurospora the expression of a set of unlinked structural genes, which allows utilization of various nitrogen-containing compounds, is controlled by the positive-acting nit-2 gene and the negative-acting nmr gene. The nucleotide sequence of the nmr gene has been determined and a long open reading frame which encodes a putative protein of 54854 daltons has been identified. A full-length cDNA clone was obtained and its the sequence revealed that the nmr gene contains no introns. The transcriptional start and stop sites have been mapped by S1 nuclease and primer extension. Site-directed mutagenesis was used to introduce stop codons at various locations in the nmr coding region. Transformation assays showed that the proteins lacking up to 16% of the carboxyl-terminus were still functional. Homology searches showed that the nmr protein is homologous to the yeast arginine regulatory gene AR-GRII.