RNA exerts self-control

A crystal structure of two bound RNA molecules not only provides insight into how regulatory riboswitch sequences affect messenger RNA expression, but also expands our understanding of RNA structure and architecture. See Letter p.363 Structure of a T-box tRNA binding region Bacterial T-box riboswitches are found in the 5′ UTR of genes encoding aminoacyl-tRNA synthetases, the enzymes that charge tRNAs with amino acids. They differ from other riboswitches in that they bind tRNAs rather than a small molecule or metabolite to regulate expression. Jinwei Zhang and Adrian Ferré-D'Amaré have now solved the crystal structure of the T-box tRNA binding region, Stem I, bound to tRNA. The long-awaited structure shows that this region binds not just the anticodon, but cradles the entire tRNA, forming an extended interface. Binding is facilitated by mutual induced fit in the T-box RNA and tRNA. The T-loop motifs mediate interactions that are similar to those of RNase P and a domain of the large ribosomal subunit, even though the three species do not have a common evolutionary ancestor.