Efficient RNA pseudouridylation by eukaryotic H/ACA ribonucleoproteins requires high affinity binding and correct positioning of guide RNA

H/ACA ribonucleoproteins (H/ACA RNPs) are responsible for introducing many pseudouridines into RNAs, but are also involved in other cellular functions. Utilizing a purified and reconstituted yeast H/ACA RNP system that is active in pseudouridine formation under physiological conditions, we describe here the quantitative characterization of H/ACA RNP formation and function. This analysis reveals a surprisingly tight interaction of H/ACA guide RNA with the Cbf5p-Nop10p-Gar1p trimeric protein complex whereas Nhp2p binds comparably weakly to H/ACA guide RNA. Substrate RNA is bound to H/ACA RNPs with nanomolar affinity which correlates with the GC content in the guide-substrate RNA base pairing. Both Nhp2p and the conserved Box ACA element in guide RNA are required for efficient pseudouridine formation, but not for guide RNA or substrate RNA binding. These results suggest that Nhp2p and the Box ACA motif indirectly facilitate loading of the substrate RNA in the catalytic site of Cbf5p by correctly positioning the upper and lower parts of the H/ACA guide RNA on the H/ACA proteins. In summary, this study provides detailed insight into the molecular mechanism of H/ACA RNPs.