A PURINE-RICH EXON SEQUENCE ENHANCES ALTERNATIVE SPLICING OF BOVINE GROWTH-HORMONE PRE-MESSENGER-RNA

A previous study has demonstrated that deletion of a region within the last exon of bovine growth hormone (bGH) pre-mRNA results in almost c omplete retention of the upstream intron (Hampson, R. K., LaFollette, L., and Rottman, F. M. (1989) Mol. Cell Biol. 9, 1604-1610), We now de monstrate that insertion of a simple purine-rich element (GGAAG), whic h is present within the deleted region, activates intron splicing upon expression in transfected cells. Moreover, several repeats of the GGA A(G) sequence restore splicing to near wild-type levels and direct the binding of a factor present in HeLa cell nuclear extracts. Mutation o f the 5'-splice site toward U1 small nuclear RNA complementarity elimi nates dependence on the downstream exon sequence for splicing. These r esults support a model for alternative intron retention in which purin e-rich sequences function as part of an ''exonic splicing enhancer'' t o complement a weak 5'-splice site and thereby facilitate intron remov al. As a result, the majority of bGH mRNA is processed to remove intro n D while still allowing a fraction of bGH mRNA containing the intact intron to reach the cytoplasm.