Zinc finger proteins: getting a grip on RNA.

C2H2 (Cys-Cys-His-His motif) zinc finger proteins are members of a large superfamily of nucleic-acid-binding proteins in eukaryotes. On the basis of NMR and X-ray structures, we know that DNA sequence recognition involves a short alpha helix bound to the major groove. Exactly how some zinc finger proteins bind to double-stranded RNA has been a complete mystery for over two decades. This has been resolved by the long-awaited crystal structure of part of the TFIIIA-5S RNA complex. A comparison can be made with identical fingers in a TFIIIA-DNA structure. Additionally, the NMR structure of TIS11d bound to an AU-rich element reveals the molecular details of the interaction between CCCH fingers and single-stranded RNA. Together, these results contrast the different ways that zinc finger proteins bind with high specificity to their RNA targets.