Extended base pair complementarity between U1 snRNA and the 5' splice site does not inhibit splicing in higher eukaryotes, but rather increases 5' splice site recognition.

Spliceosome formation is initiated by the recognition of the 5' splice site through formation of an RNA duplex between the 5' splice site and U1 snRNA. We have previously shown that RNA duplex formation between U1 snRNA and the 5' splice site can protect pre-mRNAs from degradation prior to splicing. This initial RNA duplex must be disrupted to expose the 5' splice site sequence for base pairing with U6 snRNA and to form the active spliceosome. Here, we investigated whether hyperstabilization of the U1 snRNA/5' splice site duplex interferes with splicing efficiency in human cell lines or nuclear extracts. Unlike observations in Saccharomyces cerevisiae, we demonstrate that an extended U1 snRNA/5' splice site interaction does not decrease splicing efficiency, but rather increases 5' splice site recognition and exon inclusion. However, low complementarity of the 5' splice site to U1 snRNA significantly increases exon skipping and RNA degradation. Although the splicing mechanisms are conserved between human and S.cerevisiae, these results demonstrate that distinct differences exist in the activation of the spliceosome.