passages.id: split_4_test_225_passage
passages.type: progene_text
passages.text: Using an RNase H protection assay and specific blocking oligonucleotides , we find that recognition of the 5' splice - site ( 5'ss) and branchpoint sequence ( BPS ) elements by U11 and U12 snRNPs , respectively , displays strong cooperativity , requiring both sites in the pre - mRNA substrate for efficient complex formation .