Since Saccharomyces cerevisiae contains Delta 9 fatty acid desaturase (OLE1) as a sole fatty acid desaturase, it produces saturated and monounsaturated fatty acids of 16- and 18-carbon compounds. We showed earlier that Kluyveromyces lactis Delta 12 (KlFAD2) and omega 3 (KlFAD3) fatty acid desaturase genes enabled S. cerevisiae to make also polyunsaturated fatty acids (PUFAs), linoleic (18:2n-6), and alpha-linolenic (18:3n-3) acids. Unlike Delta 9 fatty acid desaturase Ole1p, the two added fatty acid desaturases (KlFAD2and KlFAD3) do not contain a cytochrome b5 domain, and we now report on effects of the overexpression of K. lactis and S. cerevisiae cytochrome b5 (CYB5) genes as well as temperature effects on PUFA synthesis. Without extra cytochrome b5, while PUFA synthesis is significant at low temperature (20 A degrees C), it was marginal at 30 A degrees C. Overexpression of cytochrome b5 at 20 A degrees C did not affect the fatty acid synthesis so much, but it significantly enhanced the synthesis of PUFA at 30 A degrees C.