Functional recognition of the 5 ' splice site by U4/U6.U5 tri-snRNP defines a novel ATP-dependent step in early spliceosome assembly

A sensitive assay based on competition between cis-and trans-splicing sugge sted that factors in addition to U1 snRNP were important for early 5' splic e site recognition. Cross-linking and physical protection experiments revea led a functionally important interaction between U4/U6.U5 tri-snRNP and the 5' splice site, which unexpectedly was not dependent upon prior binding of U2 snRNP to the branch point. The early 5' splice site/tri-snRNP interacti on requires ATP, occurs in both nematode and HeLa cell extracts, and involv es sequence-specific interactions between the highly conserved splicing fac tor Prp8 and the 5' splice site. We propose that U1 and U5 snRNPs functiona lly collaborate to recognize and define the 5' splice site prior to establi shment of communication with the 3' splice site.