Comparison of nuclear precursors, spliced nuclear mRNA, and cytoplasmic RNA. Single-stranded S1 nuclease protection probes and protected fragments are shown at the bottom. The m13-probes were uniformly labeled. The lack of visible bands at nt 735 (Bcl-2) and nt 183 (β-actin) indicates that nuclease digestion is complete and that only RNA is detected. Cohybridization of the same sample with a β-actin probe controls for equal amounts of RNA. Note that the β-actin probe used here contains only exon sequences (no precursor). (A) Bcl-2 intron II-exon III border. (B) Exon II-intron II border. Precursor protection of the upstream exon is much stronger and probably reflects increased polymerase loading at the 5′ end.