cDNA cloning of acyl-CoA desaturase homologs in the silkworm, Bombyx mori

We have isolated two acyl-CoA desaturase clones from a pheromone gland cDNA library by using the EST (expressed sequence tag) database of Bombyx mori. The putative acyl-CoA desaturases encoded by the clones desat 1 (2029 bp) and desat 2 (2341 bp) have 98% identity, and both proteins show 61% identities to Trichoplusia ni acyl-CoA Δ 11 desaturase. The deduced amino acid sequences conserve well the histidine clusters that are catalytically essential for acyl-CoA desaturase activity. Northern blot and RT-PCR analyses revealed that both transcripts of desat 1 and desat 2 were expressed predominantly in the pheromone gland. Both transcripts detected 3 days before adult eclosion dramatically increased a day before adult eclosion, keeping the mRNA levels high even after eclosion. These results, combined with the fact that Δ 11 and Δ 10, 12 desaturation of palmitate is a key step to synthesize pheromone in B. mori, suggest that the desaturases encoded by desat 1 and desat 2 are involved in either or both of the desaturation steps in the pheromone biosynthetic pathway of B. mori. The mRNA levels of desat 1 and desat 2 were not affected by decapitation or injection of the pheromone biosynthesis activating neuropeptide (PBAN) into the adult female moth, suggesting that the transcription of desat 1 and desat 2 is not regulated by PBAN. In addition to the clones in the pheromone gland, eight other clones encoding the same Δ 9 desaturase homolog were found in an embryonic cDNA library by searching from the EST database of B. mori. The deduced amino acid sequence from one of the clones ( desat 3) shows 79% identity to T. ni Δ 9 desaturase but only 52% identity to the desaturases in the pheromone gland of B. mori. Northern blot analysis showed that the mRNA corresponding to the desat 3 was detected in the ovary and fat body, but not in the pheromone gland. Abundance of the Δ 9 desaturase clones (eight out of the 762 randomly sequenced clones) in the library prepared from diapause-destined embryos (40 h after oviposition) suggests that the Δ 9 desaturase encoded by desat 3 plays an important role in embryonic development in B. mori.