Engineering Structurally Interacting RNA (sxRNA).

RNA-based three-way junctions (3WJs) are naturally occurring structures found in many functional RNA molecules including rRNA， tRNA， snRNA and ribozymes. 3WJs are typically characterized as resulting from an RNA molecule folding back on itself in cis but could also form in trans when one RNA， for instance a microRNA binds to a second structured RNA， such as a mRNA. Trans-3WJs can influence the final shape of one or both of the RNA molecules and can thus provide a means for modulating the availability of regulatory motifs including potential protein or microRNA binding sites. Regulatory 3WJs generated in trans represent a newly identified regulatory category that we call structurally interacting RNA or sxRNA for convenience. Here we show that they can be rationally designed using familiar cis-3WJ examples as a guide. We demonstrate that an sxRNA "bait" sequence can be designed to interact with a specific microRNA "trigger" sequence， creating a regulatable RNA-binding protein motif that retains its functional activity. Further， we show that when placed downstream of a coding sequence， sxRNA can be used to switch "ON" translation of that sequence in the presence of the trigger microRNA and the amount of translation corresponded with the amount of microRNA present.