A NUCLEAR CAP-BINDING COMPLEX FACILITATES ASSOCIATION OF U1-SNRNP WITH THE CAP-PROXIMAL 5' SPLICE-SITE

The mechanism by which intron-containing RNAs are recognized by the sp licing machinery is only partly understood. A nuclear cap-binding comp lex (CBC), which specifically recognizes the monomethyl guanosine cap structure carried by RNA polymerase II transcripts, has previously bee n shown to play a role in pre-mRNA splicing. Using a combination of sp licing complex and psoralen cross-linking analysis we demonstrate that CBC is required for efficient recognition of the 5' splice site by U1 snRNP during formation of E (early) complex on a pre-mRNA containing a single intron. However, in a pre-mRNA containing two introns, CBC is not required for splicing of the cap distal intron. In this case, the presence of an intact polypyrimidine tract in the cap-proximal intron renders splicing of the cap-distal intron independent of CBC. These r esults support models in which the splice sites in a pre-mRNA are orig inally recognized by interactions spanning exons. The defects in splic ing and U1 snRNP binding caused by CBC depletion can be specifically r eversed by recombinant CBC. In smmary, efficient recognition of the ca p-proximal 5' splice site by U1 snRNP is facilitated by CBC in what ma y be one of the earliest steps in pre-mRNA recognition. Data in Colot et al. (this issue) indicate that this function of CBC is conserved in humans and yeast.