passages.id: split_5_test_1804_passage
passages.type: progene_text
passages.text: The active 17S U2 small nuclear ribonucleoprotein particle ( snRNP ) , which binds to the intron branch site during the formation of the prespliceosome , is assembled in vitro by sequential interactions of the essential splicing factors SF3b and SF3a with the 12S U2 snRNP .