Originally published as Genetics Published Articles Ahead of Print on May 15, 2006.

Genetics, Vol. 173, 1893-1908, August 2006, Copyright © 2006
doi:10.1534/genetics.105.053025

Cumulative Mutations Affecting Sterol Biosynthesis in the Yeast Saccharomyces cerevisiae Result in Synthetic Lethality That Is Suppressed by Alterations in Sphingolipid Profiles

Martin Valachovic^*,1,2, Bart M. Bareither^*,1, M. Shah Alam Bhuiyan^*,1, James Eckstein, Robert Barbuch, Dina Balderes, Lisa Wilcox, Stephen L. Sturley, Robert C. Dickson and Martin Bard^*,3

^* Department of Biology, Indiana University–Purdue University, Indianapolis, Indiana 46202, Department of Drug Disposition, Eli Lilly, Lilly Corporate Center, Indianapolis, Indiana 46285, Institute of Human Nutrition, Columbia University Medical Center, New York, New York 10032 and Department of Molecular and Cellular Biochemistry, University of Kentucky, Lexington, Kentucky, 40536

³ Corresponding author: Department of Biology, Indiana University–Purdue University, 723 W. Michigan St., Indianapolis, IN 46202.
E-mail: mbard{at}iupui.edu

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 ecm22 in the S288c genetic background is attributable to a mutation in the HAP1 transcription factor. In addition we demonstrate that strains containing upc2 ecm22 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 upc2 ecm22 triple mutant is also synthetically lethal. We used transposon mutagenesis to isolate viable suppressors of hap1, erg2, erg6, and erg28 in the upc2 ecm22 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 upc2 ecm22. 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.

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