The recognition of a noncanonical RNA base pair by a zinc finger protein

Background: The zinc finger (ZF) is the most abundant nucleic-acid-interacting protein motif. Although the interaction of ZFs with DNA is reasonably well understood, little is known about the RNA-binding mechanism. We investigated RNA binding to ZFs using the Zif268-DNA complex as a model system. Zif268 contains three DNA-binding ZFs; each independently binds a 3 base pair (bp) subsite within a 9 bp recognition sequence. Results: We constructed a library of phage-displayed ZFs by randomizing the a helix of the Zif268 central finger. Successful selection of an RNA binder required a noncanonical base pair in the middle of the RNA triplet. Binding of the Zif268 variant to an RNA duplex containing a G . A mismatch (rG . A) is specific for RNA and is dependent on the conformation of the mismatched middle base pair. Modeling and NMR analyses revealed that the rG . A pair adopts a head-to-head configuration that counterbalances the effect of S-puckered riboses in the backbone. We propose that the structure of the rG A duplex is similar to the DNA in the original Zif268-DNA complex. Conclusions: it is possible to change the specificity of a ZF from DNA to RNA. The ZF motif can use similar mechanisms in binding both types of nucleic acids. Our strategy allowed us to rationalize the interactions that are possible between a ZF and its RNA substrate. This same strategy can be used to assess the binding specificity of ZFs or other protein motifs for noncanconical RNA base pairs, and should permit the design of proteins that bind specific RNA structures.