In vivo splicing of the beta tropomyosin pre-mRNA: a role for branch point and donor site competition.

The chicken beta tropomyosin gene contains two sets of alternatively spliced, mutually exclusive exons whose utilization is developmentally regulated. Exons 6A and 6B are used in nonmuscle cells (or undifferentiated muscle cells) and skeletal muscle cells, respectively. A complex arrangement of cis-acting sequence elements is involved in alternative splicing regulation. We have performed an extensive mutational analysis on the sequence spanning the region from exon 6A to the constitutive exon 7. A large number of mutant minigenes have been tested in transfection assays of cultured myogenic cells, and the splicing products have been analyzed by cDNA polymerase chain reaction. We demonstrate that in undifferentiated myoblasts, exon 6B is skipped as a result of a negative control on its selection, while exon 6A is spliced as a default choice. We provide evidence that the focal point of such a regulation is localized in the intron upstream of exon 6B and probably involves the blockage of its associated branch point. In differentiated myotubes, in contrast, both exons are accessible to the splicing machinery. We show that the preferential choice of exon 6B in this splicing environment depends on the existence of a competition between the two exons for the flanking constitutive splice sites. We demonstrate that both the donors and the branch points of the two exons are involved in this competition.