Pyruvate carboxylase deficiency in yeast: a mutant affecting the interaction between the glyoxylate and Krebs cycles

A single-gene nuclear mutant has been isolated in Saccharomyces cerevisiae which cannot grow on minimal medium supplemented with ethanol, acetate, pyruvate, aspartate, or oxaloacetate as sole carbon sources. It will grow on complete medium with these carbon sources, and on minimal medium with dextrose as carbon source. The only supplement which will permit growth on minimal medium with ethanol or pyruvate is aspartate, so the mutant is an aspartate auxotroph when grown on these nonfermentable substrates. It exhibits enhanced levels of phosphoenolpyruvate carboxykinase (EC 4.1.1.49) when grown on dextrose. The mutant can survive as an alcohol dehydrogenase-negative, indicating that the defect is not in the Krebs Cycle or in electron transport. When grown on pyruvate, it produces two to three times as much free alanine and half as much aspartate plus asparagine as the wild type. Two different assays show that the mutant phenotype is due to a deficiency of pyruvate carboxylase (EC 6.4.1.1), an important anaplerotic enzyme. Inferences that can be drawn from the characteristics of this mutant include (a) the glyoxylate cycle is probably located entirely outside the mitochondria, (b) the inner mitochondrial membrane appears to be impermeable to oxaloacetate, and (c) a succinate-malate exchange across the inner mitochondrial membrane connects the glyoxylate and Krebs cycles when yeast is grown on minimal medium with ethanol as a sole carbon source.