Mapping U2 snRNP‐‐pre‐mRNA interactions using biotinylated oligonucleotides made of 2′‐OMe RNA

Biotinylated 2′‐OMe RNA oligonucleotides complementary to two separate regions of human U2 snRNA have been used as affinity probes to study U2 snRNP‐‐pre‐mRNA interactions. Both oligonucleotides bind specifically and allow highly selective removal of U2 snRNP from HeLa cell nuclear extracts. Pre‐mRNA substrates can also be specifically affinity selected through oligonucleotides binding to U2 snRNP particles in splicing complexes. Stable binding of U2 snRNP to pre‐mRNA is blocked by the pre‐binding of an oligonucleotide to the branch site complementary region of U2 snRNA, but not by an oligonucleotide binding to the 5′ terminus of U2. Both oligonucleotides affinity select the intron product, but not the intron intermediate, when added after spliceosome assembly has taken place. The effect of 2′‐OMe RNA oligonucleotides on splicing complex formation has been used to demonstrate that complexes containing U2 snRNP and unspliced pre‐mRNA are precursors to functional spliceosomes.