MULTIPLE SPLICING SIGNALS CONTROL ALTERNATIVE INTRON RETENTION OF BOVINE GROWTH-HORMONE PRE-MESSENGER-RNA

A fraction of bovine growth hormone (bGH) pre-mRNA undergoes alternati ve splicing in which the last intron is retained and transported to th e cytoplasm. Our goal was to characterize the cis-acting signals in bG H pre-mRNA that collectively determine the distribution between intron splicing and intron retention. We now demonstrate that the balance be tween splicing and intron retention in cytoplasmic mRNA is primarily d etermined by the interaction of three splicing signals and the degree to which these signals deviate from consensus splicing signals. Intron retention requires the presence of both suboptimal 5'- and 3'-splice sites. Mutation of either splice site toward consensus leads to comple te splicing of the intron. In the presence of both wild-type, suboptim al splice sites, efficient splicing of this intron is ensured by the p resence of a third splicing element, a purine rich exonic splicing enh ancer (ESE). Although strong ESEs can be contained within very small s equences, the bGH ESE activity appears to be composed of multiple sequ ences spread throughout a 115-nucleotide region of exon 5. Consequentl y, the final ratio of splicing to intron retention depends on the bala nce between the relative strengths of each of these three splicing sig nals, which still allow intron containing coding sequences to be trans ported to the cytoplasm.