Hydroxyl radical "footprinting" of RNA: application to pre-mRNA splicing complexes.

We present an adaptation of the hydroxyl radical DNA "footprinting" technique that permits high-resolution mapping of protected regions of RNA. Hydroxyl radical cleaves RNA independently of base sequence and secondary structure of the RNAs examined and allows resolution of protected regions at the single nucleotide level. By using this technique, we show that several regions of the 3' splice site of mRNA precursors are protected during the formation of splicing-specific ribonucleoprotein complexes in an in vitro splicing system. These regions include the 3' intron/exon junction and a portion of the adjacent exon, the polypyrimidine tract, and the site of branch formation. These protections appear to be due to splicing specific complexes since their formation is sensitive to point mutations at crucial residues and requires ATP and incubation. The formation of these protected regions is independent of the presence of a 5' splice site.