Effects of excess succinate and retrograde control of metabolite accumulation in yeast tricarboxylic cycle mutants

Cellular and mitochondrial metabolite levels were measured in yeast TCA cycle mutants (sdh2Δ or fum1Δ) lacking succinate dehydrogenase or fumarase activities. Cellular levels of succinate relative to parental strain levels were found to be elevated ~8-fold in the sdh2Δ mutant and ~4-fold in the fum1Δ mutant, and there was a preferential increase in mitochondrial levels in these mutant strains. The sdh2Δ and fum1Δ strains also exhibited 3-4-fold increases in expression of Cit2, the cytosolic form of citrate synthase that functions in the glyoxylate pathway. Codisruption of the SFC1 gene encoding the mitochondrial succinate/fumarate transporter resulted in higher relative mitochondrial levels of succinate and in substantial reductions of Cit2 expression in sdh2Δsfc1Δ and fum1Δsfc1Δ strains as compared with sdh2Δ and fum1Δ strains, suggesting that aberrant transport of succinate out of mitochondria mediated by Sfc1 is related to the increased expression of Cit2 in sdh2Δ and fum1Δstrains. A defect (rtg1Δ) in the yeast retrograde response pathway, which controls expression of several mitochondrial proteins and Cit2, eliminated expression of Cit2 and reduced expression of NAD-specific isocitrate dehydrogenase (Idh) and aconitase (Aco1) in parental, sdh2Δ, and fum1Δ strains. Concomitantly, co-disruption of the RTG1 gene reduced the cellular levels of succinate in the sdh2Δ and fum1Δ strains, of fumarate in the fum1Δ strain, and citrate in an idhΔ strain. Thus, the retrograde response is necessary for maintenance of normal flux through theTCA and glyoxylate cycles in the parental strain and for metabolite accumulation in TCA cycle mutants.