Studies with recombinant U7 snRNP demonstrate that CPSF73 is both an endonuclease and a 5′–3′ exonuclease

Cleavage of mH2a* histone pre-mRNA by recombinant U7 snRNP depends on the sequence of U7 snRNA. (A) Base-pairing between the HDE of mH2a* pre-mRNA (top sequence) and the 5′ end region of three different U7 snRNAs (bottom sequences). The mutated region of Sup U7 snRNA is indicated with lower case letters. Vertical lines and dots indicate Watson–Crick and GU base pairs, respectively. (B–F) Recombinant U7 snRNP containing indicated components was used to cleave the following histone pre-mRNAs: mH2a* (panel B), Drosophila-specific dH3* (panel C), H2a+4 (panel D), and H2a WT/Ys (panels E,F). The arrow in panel D indicates a shift in the length of the upstream cleavage product resulting from inserting 4 nt into mouse U7 snRNA. The arrow in panels E and F indicates a new upstream cleavage product generated as a result of substituting the U7-specific Sm site in Sup U7 snRNA with that present in the spliceosomal snRNAs (Sup U7^Spl). Processing of H2a WT/Ys pre-mRNA by semirecombinant Sup U7^Sp snRNP is shown in lanes 1,8 of panel F.