RNAs with multiple personalities

In the last decade, advances in sequencing technology and a renewed focus on the regulatory potential of RNA molecules have combined to stimulate an enormous expansion in the catalog of known eukaryotic RNAs. Beyond the sheer numerical diversity of RNA species, recent studies have begun to uncover hints of even greater functional complexity. An increasing number of RNA molecules, including those from classic, well-studied classes, have been found to act in previously unanticipated regulatory roles, or as substrate for the biogenesis of functionally distinct RNA molecules, or both. Thus, these molecules can fulfill multiple, parallel functions, compounding the already rich landscape of RNA biology, and potentially connecting disparate biological regulatory networks in unexpected ways. In this article, we review recently discovered instances of RNA multifunctionality, with a particular focus on regulatory small RNAs.