In vitro analysis of bovine growth hormone pre-mRNA alternative splicing. Involvement of exon sequences and trans-acting factor(s).

Bovine growth hormone (bGH) pre-mRNA is alternatively spliced, resulting in retention of the last intron (intron D) in a fraction of the cytosolic bGH mRNA. To study the mechanism of this alternative splicing event, we examined the splicing of bGH pre-mRNA in vitro. The splicing of bGH intron D in vitro required a 115-base pair segment of exon 5, reflecting the positive influence of exon sequences observed in transfected cells. No detectable spliceosome complex formation was observed using bGH pre-mRNA containing the 115-base pair deletion in exon 5. The in vitro splicing of the wild type bGH pre-mRNA was inhibited by the addition of RNA containing the 115-nucleotide exon sequence, but not by nonspecific RNAs. UV irradiation of the in vitro splicing reaction resulted in specific cross-linking of a 35-kDa protein(s) to the 115-nucleotide bGH exon sequence. These results suggest that terminal exon sequences are required at an early step of spliceosome complex formation and are consistent with a mechanism in which saturable, trans-acting factor(s) bind to these exon sequences to activate spliceosome complex formation and splicing of bGH intron D.