RNA interference (RNAi) is an ancient mechanism for executing the epigenetic phenomenon called gene silencing. It has quickly become a standard way to figure out what a gene is doing, even though not much is known about how it works.
When Thomas Tuschl and Phillip Zamore reported last year that, in vitro, RNAi involves cleavage into short pieces of both a double-stranded RNA and subsequently the rest of the target mRNA, Brenda Bass immediately had a hunch that the responsible enzyme was one she had encountered a decade ago. She and her colleague Scott Knight (University of Utah, Salt Lake City, UT) have now verified that hunch. They have shown not only that the enzyme (known appropriately as Dicer) is required for RNAi, but that it is essential for normal development.
With the help of the C. elegans Gene Knockout Consortium, Knight and Bass isolated worm knockouts that contained a null mutation in the Dicer gene. The knockouts had germline defects that rendered them sterile. So Knight mated knockout heterozygotes that also contained transgenes encoding both GFP and a heat-shock inducible RNA hairpin that matched the GFP sequence. Heat shock caused GFP to disappear in wild-type and heterozygote progeny because GFP RNA was knocked out by RNAi. In the mutant homozygote progeny, however, GFP stayed on, suggesting that RNAi needs Dicer in order to work.
Under some conditions, however, RNAi worked just fine in the mutant homozygotes. Bass says one possibility is that Dicer from the heterozygote mother sometimes sticks around, but she's doubtful that this is the whole story. Her latest hunch: “I think this means there are other pathways by which RNA interference can occur.” ▪