Donor site competition is involved in the regulation of alternative splicing of the rat beta-tropomyosin pre-mRNA

The rat beta-tropomyosin (beta-TM) gene encodes both skeletal muscle beta-T M mRNA and nonmuscle TM-1 mRNA via alternative RNA splicing. This gene cont ains eleven exons: exons 1-5, 3, and 9 are common to both mRNAs; exons 6 an d 11 are used in fibroblasts as well as in smooth muscle, whereas exons 7 a nd 10 are used in skeletal muscle. Previously we demonstrated that utilizat ion of the 3' splice site of exon 7 is blocked in nonmuscle cells. In this study, we use both in vitro and in vivo methods to investigate the regulati on of the 5' splice site of exon 7 in nonmuscle cells. The 5' splice site o f exon 7 is used efficiently in the absence of flanking sequences, but its utilization is suppressed almost completely when the upstream exon 6 and in tron 6 are present. The suppression of the 5' splice site of exon 7 does no t result from the sequences at the 3' end of intron 6 that block the use of the 3' splice site of exon 7. However, mutating two conserved nucleotides GU at the 5' splice site of exon 6 results in the efficient use of the 5' s plice site of exon 7. In addition, a mutation that changes the 5' splice si te of exon 7 to the consensus U1 snRNA binding site strongly stimulates the splicing of exon 7 to the downstream common exon 8. Collectively, these st udies demonstrate that 5' splice site competition is responsible, in part, for the suppression of exon 7 usage in nonmuscle cells.