The formation of the glyoxylate cycle enzymes isocitrate lyase and malate synthase is strongly regulated in and the enzymes are induced at high levels in mycelium grown on acetate and found at low levels in mycelium grown on hexose. A search was made for constitutive mutants forming the enzymes when grown on hexose by the use of a pyruvate carboxylaseless () strain. This strain does not grow on hexose since it requires a source of C tricarboxylic acid cycle intermediates and it was hoped that among revertants selected for growth on hexose some may effect C synthesis by the constitutive formation of the glyoxylate cycle enzymes.

Four constitutive strains were isolated. Three had no pyruvate carboxylase activity and each was found to contain two new mutations: a suppressor mutation of the pyruvate carboxylase lesion, and a mutation which caused low constitutive isocitrate lyase activity. The greater activities in the constitutive strains were due to interactions between the and the and mutations.

The four low-level constitutive mutations defined two genes affecting isocitrate lyase formation. Recessive mutations in (linkage group IV; three alleles) caused 10-fold increased activities in sucrose-grown mycelium, as did a semi-dominant mutation in (linkage group I). The genes are not linked to (structural gene for isocitrate lyase) and did not detectably affect the formation of either malate synthase or acetyl-CoA synthase, the structural gene () for which is tightly linked to There was a synergistic interaction in the double mutant ; Two alternative interpretations of the genes remain open: one is a model for genetic regulation with negative ( gene product) and positive ( gene product) elements; the other is endogenous induction due to the accumulation of metabolites resulting from the unidentified metabolic lesions. We did not find any mutations uninducible for isocitrate lyase amongst acetate non-utilizing mutants.

A fifth isocitrate lyase constitutive mutant was isolated in the wild-type strain. The mutant contained another recessive mutation () in the gene.

The high constitutive isocitrate lyase activities in ; double mutants did not provide replacement of the C requirement in a pyruvate carboxylaseless strain since the triple mutant did not grow on sucrose. Revertants of this strain selected for growth on sucrose again contained suppressor mutations of the lesion and the high constitutive activities for malate synthase decreased as known mutant genes were replaced. The nature of the suppressor gene function(s) providing an alternative source of C intermediates for growth is not known.


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