An intronic splicing enhancer binds U1 snRNPs to enhance splicing and select 5′ splice sites

Intronic G triplets are frequently located adjacent to 5 ' splice sites in vertebrate pre-mRNAs and have been correlated with splicing efficiency and specificity via a mechanism that activates upstream 5 ' splice sites in exons containing duplicated sites (26). Using an intron dependent upon G triplets for maximal activity and 5 ' splice site specificity, we determined that these elements bind U1 snRNPs via base pairing with U1 RNA, This interaction is novel in that it uses nucleotides 8 to 10 of U1 RNA and is independent of nucleotides 1 to 7. In vivo functionality of base pairing was documented by restoring activity and specificity to mutated G triplets through compensating U1 RNA mutations. We suggest that the C-rich region near vertebrate 5 ' splice sites promotes accurate splice site recognition by recruiting the U1 snRNP.