Pleiotropic deficiencies of the laccase-derepressed mutant lah-1 are caused by constitutively increased expression of the cross-pathway control gene cpc-1 in Neurospora crassa

Abstract

In Neurospora crassa, expression of the laccase gene is induced by treatment with the protein synthesis inhibitor cycloheximide (CHX). This expression is mediated by CPC1, which acts as a general transcriptional activator when mycelia are treated with CHX or starved for any one of the amino acids. A laccase-derepressed mutant, lah-1, shows pleiotropic deficiencies in growth, hyphal morphology, CHX sensitivity, and production of protoperithecia. Moreover, in the lah-1 mutant, transcript levels of CHX-inducible genes, including lacc, tub-2, tef-1, and amino acid biosynthetic genes such as cpc-1, trp-3, and arg-12, are increased without exposure to CHX. All of the defects exhibited in the lah-1 mutant are suppressed by a mutation in the cpc-1 locus. These findings suggest that the cpc-1 mutation is epistatic to the lah-1 mutation and that the pleiotropic defects in the lah-1 mutant are attributable to constitutive expression of CPC1. These conclusions are supported by a developmental Northern blot analysis of the CHX-inducible genes. Based on these results, the lah-1 gene product appears to regulate expression of the cpc-1 gene negatively. Expression of the CHX-inducible genes was induced by CHX treatment in the lah-1 cpc-1 mutant, as well as in the cpc-1 mutant. This observation indicates that LAH1 is not a component of CHX-responsive pathway itself.