Biosynthesis of polyunsaturated fatty acids in C. elegans is initiated by t
he introduction of a double bond at the Delta 9 position of a saturated fat
ty acid. We identified three C. elegans fatty acid desaturase genes related
to the yeast Delta 9 desaturase OLE1 and the rat stearoyl-CoA desaturase S
CD1. Heterologous expression of all three genes rescues the fatty acid auxo
trophy of the yeast Delta 9 desaturase mutant ole1. Examination of the fatt
y acid composition of the transgenic yeast reveals striking differences in
the substrate specificities of these desaturases. Two desaturases, FAT-6 an
d FAT-7, readily desaturate stearic acid (18:0) and show less activity on p
almitic acid (16:0). In contrast, the other desaturase, FAT-B, readily desa
turates palmitic acid (16:0), but shows nearly undetectable activity on the
common Delta 9 substrate stearic acid. This is the first report of a palmi
toyl-CoA-specific membrane fatty acid desaturase. (C) 2000 Academic Press.