REGULATION OF ALTERNATIVE POLYADENYLATION BY U1 SNRNPS AND SRP20

Although considerable information is currently available about the fac tors involved in constitutive vertebrate polyadenylation, the factors and mechanisms involved in facilitating communication between polyaden ylation and splicing are largely unknown. Even less is known about the regulation of polyadenylation in genes in which 3'-terminal exons are alternatively recognized. Here we demonstrate that an SR protein, SRp 20, affects recognition of an alternative 3'-terminal exon via an effe ct on the efficiency of binding of a polyadenylation factor to an alte rnative polyadenylation site. The gene under study codes for the pepti des calcitonin and calcitonin gene-related peptide. Its pre-mRNA is al ternatively processed by the tissue-specific inclusion or exclusion of an embedded 3'-terminal exon, exon 4, via factors binding to an intro nic enhancer element that contains both 3' and 5' splice site consensu s sequence elements. In cell types that preferentially exclude exon 4, addition of wild-type SRp20 enhances exon 4 inclusion via recognition of the intronic enhancer. In contrast, in cell types that preferentia lly include exon 4, addition of a mutant form of SRp20 containing the RNA-binding domain but missing the SR domain inhibits exon 4 inclusion . Inhibition is likely at the level of polyadenylation, because the mu tant SRp20 inhibits binding of CstF to the exon 4 poly(A) site. This i s the first demonstration that an SR protein can influence alternative polyadenylation and suggests that this family of proteins may play a role in recognition of 3'-terminal exons and perhaps in the communicat ion between polyadenylation and splicing.