LET-767 from C. elegans belongs to a family of short-chain dehydrogenases/ reductases and is homologous to 17ss-hydroxysterol dehydrogenases of type 3 and 3 ketoacyl-CoA reductases. Worms subjected to RNA interference of
let-767 displayed multiple growth and developmental defects in the first generation and arrested in the second generation as L1 larvae. To determine the function of LET-767 in vivo, we exploited a biochemical complementation approach, in which
let-767 (RNAi) arrested larvae were rescued by feeding with compounds isolated from wild type worms. The arrest was only rescued by the addition of triacylglycerides extracted from worms, but not from various natural sources, such as animal fats and plant oils. The mass spectrometric analyses showed alterations in the fatty acid content of triacylglycerides. Essential for the rescue were odd numbered fatty acids with monomethyl branched chains. The rescue was improved when worms were additionally supplemented with long chain even numbered fatty acids. Remarkably,
let-767 completely rescued the yeast 3-ketoacyl-CoA reductase mutant (
ybr159). Since worm ceramides exclusively contain a monomethyl branched chain sphingoid base, we also investigated ceramides in
let-767 (RNAi). Indeed, the amount of ceramides was greatly reduced and unusual sphingoid bases were observed. Taken together, we conclude that LET-767 is a major 3-ketoacyl-CoA reductase in C. elegans required for the bulk production of monomethyl branched and long chain fatty acids and the developmental arrest in
let-767 (RNAi) worms is caused by the deficiency of the former.