DEREPRESSION OF THE GLYOXYLATE CYCLE IN MUTANTS OF NEUROSPORA-CRASSA ACCELERATED FOR GROWTH ON ACETATE

Two spontaneous allelic mutations have been isolated with the unusual semidominant phenotype of faster-than-wild-type growth on acetate as s ole carbon source. The mutants were designated Aag-1 (accelerated acet ate 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 a nd 6% of the progeny were found to be acu (acetate non-utilizing) muta nts. Ten of these were selected for heterokaryon complementation analy sis with known acu mutants; nine proved to be new alleles of acu-5 (de ficient in acetyl-CoA synthetase), and one was a new acetate non-utili zing class, designated acu-14. Although the Aag-1 mutants clearly have no acetate-growth-related enzyme deficiencies, they did exhibit signi ficant constitutive enzyme levels for acetyl-CoA synthetase and the gl yoxylate cycle enzymes (isocitrate lyase and malate synthase) on the n on-inducing carbon source, sucrose. The derepression was restricted to these enzymes, as representative enzymes from other carbon-assimilato ry pathways remained repressed and subject to carbon catabolite repres sion. Northern blot analysis of the mRNA levels of acetyl-CoA syntheta se and the glyoxylate cycle enzymes from the mutants demonstrated the derepression to occur at the level of transcription. These data sugges t that the physiological explanation for the accelerated acetate growt h phenotype lies in the standing levels of the acetate-assimilatory en zymes, which enable the mutants to forgo some of the normal time requi red for adaption to growth on acetate.