ENZYMATIC AND METABOLIC STUDIES ON RETROGRADE REGULATION MUTANTS OF YEAST

Two nuclear genes, RTG1 and RTG2, which sense the functional state of yeast mitochondria, have been described recently. Yeast strains with n ull alleles of either of these two genes (Delta rtg1, Delta rtg2) cann ot grow on acetate as the sole carbon source and are auxotrophic for g lutamate and aspartate. We report here a series of metabolic experimen ts and enzyme activity measurements that were made in an attempt to de termine the reason for the acetate(-) phenotype and the glutamate/aspa rtate auxotrophy. Decreases in the activities (similar to 50%) in mito chondrial citrate synthase (CS1), acetyl-CoA synthetase, NAD isocitrat e dehydrogenase, and pyruvate carboxylase were noted. When CS1 was ove rexpressed in the Delta rtg1 and Delta rtg2 mutants, these strains cou ld grow on acetate but were still auxotrophic for glutamate/aspartate. We propose that, in the mutant strain, CS1 activity becomes limiting for efficient acetate utilization, but that other complex metabolic in teractions are affected, limiting production of intermediates that wou ld allow synthesis of glutamic and aspartic acids.