Tiny RNA World Discovered...

Two small temporal RNAs (stRNAs), let-7 and lin-4, play an important role in the development of the nematode Caenorhabditis elegans. Let-7 is also highly conserved throughout bilateral animals, including Drosophila and humans. Are there other small regulatory RNAs? Three reports by Lagos-Quintana et al., Lau et al., and Lee et al. indicate that there are a very large number (>60) of these tiny ~22-nucleotide microRNAs (miRNAs)and that they have molecular characteristics similar to the two known stRNAs. The miRNAs are developmentally and tissue-specifically expressed and are conserved between different organisms. Some of the RNAs are organized in an operon-like fashion and may be processed from a single precursor. In a Perspective, Ruvkun calls the RNAs the "biological equivalent of dark matter" and suggests that they may provide a potent means for regulating gene expression.