Ecdysteroid synthesis ‘Black Box’ illuminated

In insects, the steroid hormone ecdysone controls the timing of moulting and metamorphosis. Some enzymes involved in ecdysone biosynthesis from dietary cholesterol have been identified but not those for the conversion of 7-dehydrocholesterol to 5β-ketodiol, the so-called ‘Black Box’. Now, Ryusuke Niwa, Tetsuro Shinoda and co-workers report that non-molting glossy (nm-g)/shroud (sro) encodes a short-chain dehydrogenase/reductase that functions in this ecdysteroid biosynthesis Black Box (see p. 1991). The researchers first isolated nm-g by positionally cloning the silkworm nm-g mutant gene. This mutant has low ecdysteroid levels and developmentally arrests as a larva. Next, they discovered that the Drosophila mutant sro, which is a Drosophila Halloween-class embryonic lethal mutant, is caused by the loss of function of an nm-g orthologue. Finally, they report that the application of ecdysteroids or 5β-ketodiol, but not that of cholesterol or 7-dehydrocholesterol, reverses nm-g/sro mutant phenotypes. Thus, the researchers conclude, Nm-g/Sro is an essential, but as yet uncharacterised, component of the ecdysteroid biosynthesis Black Box.