Lack of protection by an oligonucleotide complementary to exon 10. Panel A: RNase H protection assay was performed using an oligonucleotide complementary to exon 10. The templates were derived from minigenes A, C, K, and L, and lacked the terminal 5' splice site of exon 11. The templates were incubated on ice or at 30°C for 30 min. An oligonucleotide complementary to exon 10 was added followed by 2U RNase H. Reactions were digested for 5 min at 37°C, the RNA extracted and separated on 5% sequencing gels. The time of incubation at 30°C is indicated above each panel along with the template added. Open arrowheads indicate cleavage products from each template. The ratio of cleaved to uncleaved RNA is unaltered with this oligonucleotide. Panel B: the HeLa nuclear extract was pretreated with RNase H and an oligonucleotide complementary to the first 14 nucleotides of the U1 snRNA to deplete functional U1 snRNP. RNaseH protection assays were run as before using the oligonucleotide complementary to the 5' splice site using control (HeLa) or depleted (HeLaΔU1) extracts. Open arrowheads indicate cleavage products.