FUNCTIONAL CROSSTALK BETWEEN EXON ENHANCERS, POLYPYRIMIDINE TRACTS AND BRANCHPOINT SEQUENCES

We recently identified enhancer elements that activate the weak 3' spl ice site of a-tropomyosin exon 2 as well as a variety of heterologous weak 3' splice sites. To understand their mechanism of action, we devi sed an iterative selection strategy to identify functional pyrimidine tracts and branchpoint sequences in the presence or absence of enhance r elements. Surprisingly, we found that strong pyrimidine tracts were selected regardless of the presence of enhancer elements, However, the presence of enhancer elements resulted in the selection of multiple, non-consensus branchpoint sequences, Thus, enhancer elements apparentl y activate weak 3' splice sites primarily by increasing the efficiency of splicing of introns containing branchpoint sequences with less tha n optimal U2-branchpoint pairing arrangements, Comparison of consensus sequences from both our selection strategy and compilations of publis hed intron sequences suggests that exon enhancer elements could be wid espread and play an important role in the selection of 3' splice sites .