The Saccharomyces cerevisiae SCS7 and SUR2 genes are members of a gene family that encodes enzymes that desaturate or hydroxylate lipids. Sur2p is required for the hydroxylation of C-4 of the sphingoid moiety of ceramide, and Scs7p is required for the hydroxylation of the very long chain fatty acid. Neither SCS7 nor SUR2 are essential for growth, and lack of the Scs7p-or Sur2p-dependent hydroxylation does not prevent the synthesis of mannosyldiinositolphosphorylceramide, the mature sphingolipid found in yeast. Deletion of either gene suppresses the Ca 2؉ -sensitive phenotype of csg2⌬ mutants, which arises from overaccumulation of inositolphosphorylceramide due to a defect in sphingolipid mannosylation. Characterization of scs7 and sur2 mutants is expected to provide insight into the function of ceramide hydroxylation.

Saccharomyces cerevisiae mutants lacking Scs7p fail to accumulate the inositolphosphorylceramide (IPC) species, IPC‐C, which is the predominant form found in wild‐type cells. Instead scs7 mutants accumulate an IPC‐B species believed to be unhydroxylated on the amide‐linked C26‐fatty acid. Elimination of the SCS7 gene suppresses the Ca2+‐sensitive phenotype of csg1 and csg2 mutants. The CSG1 and CSG2 genes are required for mannosylation of IPC‐C and accumulation of IPC‐C by the csg mutants renders them Ca2+‐sensitive. The SCS7 gene encodes a protein that contains both a cytochrome b5‐like domain and a domain that resembles the family of cytochrome b5‐dependent enzymes that use iron and oxygen to catalyse desaturation or hydroxylation of fatty acids and sterols. Scs7p is therefore likely to be the enzyme that hydroxylates the C26‐fatty acid of IPC‐C. © 1998 John Wiley & Sons, Ltd.