The membrane proteins, Spt23p and Mga2p, play distinct roles in the activation of Saccharomyces cerevisiae OLE1 gene expression - Fatty acid-mediatedregulation of Mga2p activity is independent of its proteolytic processing into a soluble transcription activator

The Saccharomyces OLE1 gene encodes the Delta -9 fatty acid desaturase, an enzyme that converts saturated fatty acyl-CoAs into cis-Delta -9 unsaturate d fatty acids. OLE1 gene expression is regulated by unsaturated fatty acids , which repress transcription and destabilize the OLE1 mRNA. Expression of OLE1 is activated by N-terminal proteolytic fragments of two homologous end oplasmic reticulum membrane proteins, Spt23p and Mga2p. Disruption of eithe r gene does not significantly affect cell growth or fatty acid metabolism; cells that contain null alleles of both genes, however, are unsaturated fat ty acid auxotrophs. An analysis of spt23 Delta and mga2 Delta strains shows that Spt23p and Mga2p differentially activate and regulate OLE1 transcript ion. In glucose-grown cells, both genes activate transcription to similar l evels of activity. Expressed alone, Mga2p induces high levels of OLE1 trans cription in cells exposed to cobalt or grown in glycerol-containing medium. Spt23p expressed alone activates OLE1 transcription to levels similar to t hose in wild type cells. OLE1 expression is strongly repressed by unsaturat ed fatty acids in spt23 Delta or mga2 Delta cells, under all growth conditi ons. To test if OLE1 expression is controlled by fatty acids at the level o f membrane proteolysis, soluble N-terminal fragments of Spt23p and Mga2p th at lack their membrane-spanning regions (Atm) were expressed under the cont rol of their native promoters in spt23 Delta ;mga2 Delta cells. Under those conditions, Mga2p Delta tm acts as a powerful transcription activator that is strongly repressed by unsaturated fatty acids. By comparison, the Spt23 p Delta tm polypeptide weakly activates transcription and shows little regu lation by unsaturated fatty acids. Co-expression of the two soluble fragmen ts results in activation to levels observed with the Mga2p Delta tm protein alone. The fatty acid repression of transcription under those conditions i s attenuated by Spt23 Delta tm, however, suggesting that the two proteins m ay interact to modulate OLE1 gene expression.