An RNA-directed nuclease mediates post-transcriptional gene silencing in Drosophila cells

In a diverse group of organisms that includes Caenorhabditis elegans, Droso phila, planaria, hydra, trypanosomes, fungi and plants, the introduction of double-stranded RNAs inhibits gene expression in a sequence-specific manne r(1-7). These responses, called RNA interference or post-transcriptional ge ne silencing, may provide anti-viral defence, modulate transposition or reg ulate gene expression(1,6,8-10). We have taken a biochemical approach towar ds elucidating the mechanisms underlying this genetic phenomenon. Here we s how that 'loss-of-function' phenotypes can be created in cultured Drosophil a cells by transfection with specific double-stranded RNAs. This coincides with a marked reduction in the level of cognate cellular messenger RNAs. Ex tracts of transfected cells contain a nuclease activity that specifically d egrades exogenous transcripts homologous to transfected double-stranded RNA . This enzyme contains an essential RNA component. After partial purificati on, the sequence-specific nuclease co-fractionates with a discrete, similar to 25-nucleotide RNA species which may confer specificity to the enzyme th rough homology to the substrate mRNAs.