FIGURE 2 Cross-linking experiment to determine the protein bound to the radiolabeled stem-loop of histone mRNA. The cross-linked species were separated by denaturing gel electrophoresis, based on their slower migration relative to non-cross-linked molecules, revealing a prominent species that was found to contain CPSF-73. The cross-linked species corresponding to CPSF-73– pre-mRNA is present in the gel with the wild-type RNA, but is greatly reduced in a reaction that used a mutant RNA defective for 3′-end processing.