Pt. Chaure et If. Connerton, DEREPRESSION OF THE GLYOXYLATE CYCLE IN MUTANTS OF NEUROSPORA-CRASSA ACCELERATED FOR GROWTH ON ACETATE, Microbiology, 141, 1995, pp. 1315-1320
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.