A purine-rich exon sequence enhances alternative splicing of bovine growth hormone pre-mRNA

A previous study has demonstrated that deletion of a region within the last exon of bovine growth hormone (bGH) pre-mRNA results in almost complete retention of the upstream intron (Hampson, R. K., LaFollette, L., and Rottman, F. M. (1989) Mol. Cell. Biol. 9, 1604-1610). We now demonstrate that insertion of a simple purine-rich element (GGAAG), which is present within the deleted region, activates intron splicing upon expression in transfected cells. Moreover, several repeats of the GGAA(G) sequence restore splicing to near wild-type levels and direct the binding of a factor present in HeLa cell nuclear extracts. Mutation of the 5'-splice site toward U1 small nuclear RNA complementarity eliminates dependence on the downstream exon sequence for splicing. These results support a model for alternative intron retention in which purine-rich sequences function as part of an "exonic splicing enhancer" to complement a weak 5'-splice site and thereby facilitate intron removal. As a result, the majority of bGH mRNA is processed to remove intron D while still allowing a fraction of bGH mRNA containing the intact intron to reach the cytoplasm.