GENETIC EVALUATION OF PHYSIOLOGICAL FUNCTIONS OF THIOLASE ISOZYMES INTHE N-ALKANE-ASSIMILATING YEAST CANDIDA-TROPICALIS

The n-alkane-assimilating diploid yeast Candida tropicalis possesses t hree thiolase isozymes encoded by trio pairs of alleles: cytosolic and peroxisomal acetoacetyl-coenzyme A (CoA) thiolases, encoded by CT-T1A and CT-TIE, and peroxisomal 3-ketoacyl-CoA thiolase, encoded by CT-T3 A and CT-T3B. The physiological functions of these thiolases have been examined by gene disruption, The homozygous ct-t1a Delta/t1b Delta nu ll mutation abolished the activity of acetoacctyl-CoA thiolase and res ulted in mevalonate auxotrophy. The homozygous ct-t3a Delta/t3b Delta null mutation abolished the activity of 3-ketoacyl-CoA thiolase and re sulted in growth deficiency on fz-alkanes (C-10 to C-13), All thiolase activities in this yeast disappeared with the ct-t1a Delta/t1b Delta and ct-t3a Delta/t3b Delta null mutations. To further clarify the func tion of peroxisomal acetoacetyl-CoA thiolases, the site-directed mutat ion leading acetoacetyl-CoA thiolase without a putative C-terminal per oxisomal targeting signal was introduced on the CT-TIA locus in the ct -t1b Delta null mutant. The truncated acetoacetyl-CoA thiolase vies so lely present in cytoplasm, and the absence of acetoacetyl-CoA thiolase in peroxisomes had no effect on growth on all carbon sources employed . Growth on butyrate was not affected by a lack of peroxisomal acetoac etyl-CoA thiolase, while a retardation of growth hy a lack of peroxiso mal 3-ketoacyl-CoA thiolase was observed. A defect of both peroxisomal isozymes completely inhibited growth on butyrate. These results demon strated that cytosolic acetoacetyl-CoA thiolase was indispensable for the mevalonate pathway and that both peroxisomal acetoacetyl-CoA thiol ase and 3-ketoacyl-CoA thiolase could participate in peroxisomal beta- oxidation. In addition to its essential contribution to the beta-oxida tion of longer-chain fatty acids, 3-ketoacyl-CoA thiolase contributed greatly el en to the beta-oxidation of a C-4 substrate butyrate.