MGA2 or SPT23 Is Required for Transcription of the Δ9 Fatty Acid Desaturase Gene, OLE1, and Nuclear Membrane Integrity in Saccharomyces cerevisiae

MGA2 and SPT23 are functionally and genetically redundant homologs in Saccharomyces cerevisiae. Both genes are implicated in the transcription of a subset of genes, including Ty retrotransposons and Tyinduced mutations. Neither gene is essential for growth, but mga2 spt23 double mutants are inviable. We have isolated a gene-specific activator, SWI5, and the Δ9 fatty acid desaturase of yeast, OLE1, as multicopy suppressors of an mga2Δ spt23 temperature-sensitive mutation (spt23-ts). The level of unsaturated fatty acids decreases 35–40% when the mga2Δ spt23-ts mutant is incubated at 37°. Electron microscopy of these cells reveals a separation of inner and outer nuclear membranes that is sometimes accompanied by vesicle-like projections in the intermembrane space. The products of Ole1p catalysis, oleic acid and palmitoleic acid, suppress mga2Δ spt23-ts and mga2Δ spt23Δ lethality and restore normal nuclear membrane morphology. Furthermore, the level of the OLE1 transcript decreases more than 15-fold in the absence of wild-type Mga2p and Spt23p. Our results suggest that Mga2p/Spt23p control cell viability by stimulating OLE1 transcription.