Human microRNA Polyadenylation Signals

MicroRNAs (miRNAs) are small non-coding RNAs that are involved in posttranscriptional gene regulation (cf., Bartel, 2004). First identified in C. elegans just over a decade ago (Lee et al., 1993), miRNAs have been identified in virtually every metazoan and plant species examined. Experimental evidence is rapidly accumulating that shows miRNAs to play key roles in process such as cellular differentiation, cell death, and fat storage. Some miRNAs appear to be ancient (Pasquinelli et al., 2000; Floyd and Bowman, 2003) while others, particularly those found in only a few plant or animal taxa, may be quite young in an evolutionary sense. Allen et al. (2004) offered a model of miRNA evolution based upon an analysis of sequences in A. thaliana but the origin and subsequent development of miRNAs remains an open issue.