Cumulative Mutations Affecting Sterol Biosynthesis In The Yeast Saccharomyces Cerevisiae Result In Synthetic Lethality That Is Suppressed By Alterations In Sphingolipid Profiles

UPC2 and ECM22 belong to a Zn(2)-Cys(6) family of fungal transcription factors and have been implicated in the regulation of sterol synthesis in Saccharomyces cerevisiae and Candida albicans. Previous reports suggest that double deletion of these genes in S. cerevisiae is lethal depending on the genetic background of the strain. In this investigation we demonstrate that lethality of upc2 Delta ecm22 Delta in the S288c genetic background is attributable to a mutation in the HAP1 transcription factor. In addition we demonstrate that strains containing upc2 Delta ecm22 Delta are also inviable when carrying deletions of ERG6 and ERG28 but not when carrying deletions of ERG3, ERG4, or ERG5. It has previously been demonstrated that UPC2 and ECM22 regulate S. cerevisiae ERG2 and ERG3 and that the erg2 Delta upc2 Delta ecm22 Delta triple mutant is also synthetically lethal. We used transposon mutagenesis to isolate viable suppressors of hap1 Delta, erg2 Delta, erg6 Delta, and erg28 Delta in the upc2 Delta ecm22 Delta genetic background. Mutations in two genes (YND1 and GDA1) encoding apyrases were found to suppress the synthetic lethality of three of these triple mutants but not erg2 Delta upc2 Delta ecm22 Delta. We show that deletion of YND1, like deletion of GDA1, alters the sphingolipid profiles, suggesting that changes in sphingolipids compensate for lethality produced by changes in sterol composition and abundance.