Derepression of the glyoxylate cycle in mutants of Neurospora crassa accelerated for growth on acetate

Abstract

Two spontaneous allelic mutations have been isolated with the unusual semi-dominant phenotype of faster-than-wild-type growth on acetate as sole carbon source. The mutants were designated Aag-1 (accelerated acetate growth) and mapped on linkage group II. Upon re-isolation of both the Aag-1 alleles from repeated back-crosses to wild-type, between 1 and 6% of the progeny were found to be acu (acetate non-utilizing) mutants. Ten of these were selected for heterokaryon complementation analysis with known acu mutants; nine proved to be new alleles of acu-5 (deficient in acetyl-CoA synthetase), and one was a new acetate non-utilizing class, designated acu-14. Although the Aag-1 mutants clearly have no acetate-growth-related enzyme deficiencies, they did exhibit significant constitutive enzyme levels for acetyl-CoA synthetase and the glyoxylate cycle enzymes (isocitrate lyase and malate synthase) on the non-inducing carbon source, sucrose. The derepression was restricted to these enzymes, as representative enzymes from other carbon-assimilatory pathways remained repressed and subject to carbon catabolite repression. Northern blot analysis of the mRNA levels of acetyl-CoA synthetase and the glyoxylate cycle enzymes from the mutants demonstrated the derepression to occur at the level of transcription. These data suggest that the physiological explanation for the accelerated acetate growth phenotype lies in the standing levels of the acetate-assimilatory enzymes, which enable the mutants to forgo some of the normal time required for adaption to growth on acetate.