Identification of protein binding sites on U3 snoRNA and pre-rRNA by UV cross-linking and high-throughput analysis of cDNAs

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Abstract
The U3 small nucleolar ribonucleoprotein (snoRNP) plays an essential role in ribosome biogenesis but, like many RNA–protein complexes, its architecture is poorly understood. To address this problem, binding sites for the snoRNP proteins Nop1, Nop56, Nop58, and Rrp9 were mapped by UV cross-linking and analysis of cDNAs. Cross-linked protein–RNA complexes were purified under highly-denaturing conditions, ensuring that only direct interactions were detected. Recovered RNA fragments were amplified after linker ligation and cDNA synthesis. Cross-linking was successfully performed either in vitro on purified complexes or in vivo in living cells. Cross-linking sites were precisely mapped either by Sanger sequencing of multiple cloned fragments or direct, high-throughput Solexa sequencing. Analysis of RNAs associated with the snoRNP proteins revealed remarkably high signal-to-noise ratios and identified specific binding sites for each of these proteins on the U3 RNA. The results were consistent with previous data, demonstrating the reliability of the method, but also provided insights into the architecture of the U3 snoRNP. The snoRNP proteins were also cross-linked to pre-rRNA fragments, with preferential association at known sites of box C/D snoRNA function. This finding demonstrates that the snoRNP proteins directly contact the pre-rRNA substrate, suggesting roles in snoRNA recruitment. The techniques reported here should be widely applicable to analyses of RNA–protein interactions.