Additional layers of gene regulatory complexity from recently discovered microRNA mechanisms.

In recent years microRNAs have become recognized as pervasive, versatile agents of gene regulation. Some widely embraced rules involving Watson-Crick hybridization of microRNAs with mRNAs have generated great interest as scientists envision potential RNA cargoes for gene therapy and other experimental systems. However, while researchers ardently seek simplifying principles, nature seems very uncooperative. This article reviews some small RNA mechanisms that potentially regulate genes and which are not covered by previous microRNAs characterizations. In addition, we report here results of fluorescence microscopy experiments to directly demonstrate nuclear import of small RNAs equal in length to typical mature microRNAs, implying that gene regulation at the locus of transcription might be possible. Copyright 2009 Elsevier Ltd. All rights reserved.