Non-coding RNA: a bridge between small RNA and DNA.

Small RNAs are not only involved in post-transcriptional gene silencing in the cytoplasm, but also modulate activities of the nuclear genome. Although silencing of mRNAs involves RNA-RNA base-pairing, it remains unclear whether the RNAi machinery responsible for chromatin or DNA modification interacts with DNA or RNA. The identification of an RNA helicase required for chromatin-Argonaute protein interaction as well as the presence of Argonaute-associated long non-coding transcripts in Tetrahymena suggest the presence of RNA-RNA interactions prior to small RNA-induced heterochromatin formation in ciliates.