SMOOTH MUSCLE-SPECIFIC SWITCHING OF ALPHA-TROPOMYOSIN MUTUALLY EXCLUSIVE EXON SELECTION BY SPECIFIC-INHIBITION OF THE STRONG DEFAULT EXON

Exons 2 and 3 of alpha-tropomyosin are spliced in a strict mutually ex clusive manner. Exon 3 is a default choice, being selected in almost a ll cell types where the gene is expressed. The default selection arise s from a competition between the two exons, in which the stronger bran ch point/pyrimidine tract elements of exon 3 win. Exon 2 is selected p redominantly or exclusively only in smooth muscle cells. We show here that the basis for the smooth muscle-specific switching of exon select ion is inhibition of exon 3. Exon 3 is still skipped with smooth muscl e specificity, even in the absence of exon 2. We have defined two cons erved sequence elements, one in each of the introns flanking exon 3, t hat are essential for this regulation. Mutation of either element seve rely impairs regulated suppression of exon 3. No other exon or intron sequences appear to be necessary for regulation. We have also demonstr ated skipping of exon 3 that is dependent upon both regulatory element s in an in vitro splicing assay. We further show that both splice site s of exon 3 must be inhibited in a concerted fashion to switch to sele ction of exon 2. This may relate to the requirement for negative eleme nts on both sides of the exon.