Molecular comparison of the negative-acting nitrogen control gene,nmr, inNeurospora crassa and otherNeurospora and fungal species

Abstract

InNeurospora crassa, the expression of unlinked structural genes which encode nitrogen catabolic enzymes is subject to genetic and metabolic regulation. The negative-actingnmr regulatory gene appears to play a role in nitrogen catabolite repression. Using theN. crassa nmr gene as a probe, homologous sequences were identified in a variety of other filamentous fungi. The polymerase chain reaction was used to isolate thenmr-like gene from the exotic Mauriceville strain ofN. crassa and from the two related species,N. intermedia andN. sitophila. Sequence comparisons were carried out with a 1.7-kb DNA segment which includes the entire coding region ofnmr plus 5′ and 3′ noncoding sequences. The size of thenmr coding region was identical in all threeNeurospora species. Approximately 30 nucleotide base substitutions were found in the coding region of thenmr gene of each of the sister species when compared to the standardN. crassa sequence. However, most of the base changes occurred in third codon positions and were silent. The NMR proteins ofN. sitophila and ofN. intermedia display only three and four amino acid substitutions, respectively, from theN. crassa protein. Two regions of high variability, which include deletions and insertions of bases, were found in the 5′ and 3′ noncoding regions of the gene.