Modulation of exon skipping by high-affinity hnRNP A1-binding sites and byintron elements that repress splice site utilization

The RNA-binding protein hnRNP A1 is a splicing regulator produced by exclus ion of alternative exon 7B from the A1 pre-mRNA. Each intron flanking exon 7B contains a high-affinity A1-binding site. The A1-binding elements promot e exon skipping in vivo, activate distal 5' splice site selection in vitro and improve the responsiveness of pre-mRNAs to increases in the concentrati on of A1. Whereas the glycine-rich C-terminal domain of A1 is not required for binding, it is essential to activate the distal 5' splice site. Because A1 complexes can interact simultaneously with two A1-binding sites, we pro pose that an interaction between bound A1 proteins facilitates the pairing of distant splice sites. Based on the distribution of putative A1-binding s ites in various pre-mRNAs, an A1-mediated change in pre-mRNA conformation m ay help define the borders of mammalian introns. We also identify an intron element which represses the 3' splice site of exon 7B, The activity of thi s element is mediated by a factor distinct from A1. Our results suggest tha t exon 7B skipping results from the concerted action of several intron elem ents that modulate splice site recognition and pairing.