Characterization of mutants of the yeast Yarrowia lipolytica defective in acetyl-coenzyme A synthetase.

The expression of the glyoxylate cycle enzymes is required for growth of the yeast Yarrowia lipolytica on acetate or fatty acids as sole carbon source. Acetyl-coenzyme A, which is produced by acetyl-coenzyme A synthetase (ACS) from acetate, is needed for induction of this expression. Acetate-non-utilizing mutants of this yeast were investigated in order to identify mutants which express no or strongly reduced activity of this enzyme. Mutations in gene ICL2 exhibited the strongest effects on the activity. In icl2 mutants, lack of ACS activity resulted in a non-induced glyoxylate cycle on acetate; however, induction on fatty acids was not affected. Gene ICL2 was identified as the structural gene encoding the monomer of ACS. It is shown that a high level of ACS activity is necessary for full expression of the glyoxylate cycle enzymes. Mutations in gene ICL1, which encodes isocitrate lyase, resulted in overproduction of ACS without any growth on acetate. A new gene (GPR1 = glyoxylate pathway regulation) was detected in which trans-dominant mutations inhibit expression of ACS and the glyoxylate cycle on acetate as carbon source.