Modulating alternative splicing by cotranscriptional cleavage of nascent intronic RNA

Cotranscriptional cleavage mediated by a hammerhead ribozyme can affect alternative splicing if interposed between an exon and its intronic regulatory elements. This has been demonstrated using two different alternative splicing systems based on alpha-tropomyosin and fibronectin genes. We suggest that there is a requirement for intronic regulatory elements to be covalently attached to exons that are in turn tethered to the elongating polymerase. In the case of the alternatively spliced EDA exon of the fibronectin gene, we demonstrate that the newly identified intronic downstream regulatory element is associated with the splicing regulatory protein SRp20. Our results suggest that targeted hammerhead ribozyme cleavage within introns can be used as a tool to define splicing regulatory elements.