TRANSPOSABLE ELEMENT INSERTIONS RESPECIFY ALTERNATIVE EXON SPLICING IN 3 DROSOPHILA MYOSIN HEAVY-CHAIN MUTANTS

Insertions of transposable elements into the myosin heavy chain (Mhc) locus disrupt the regulation of alternative pre-mRNA splicing for mult i-alternative exons in the Mhc2, Mhc3 and Mhc4 mutants in Drosophila. Sequence and expression analyses show that each inserted element intro duces a strong polyadenylation signal that defines novel terminal exon s, which are then differentially recognized by the alternative splicin g apparatus, Mhc2 and Mhc4 have insertion elements located within intr on 7c and exon 9a, respectively, and each expresses a single truncated transcript that contains an aberrant terminal exon defined by the pol y(A) signal of the inserted element and the 3' acceptor of the upstrea m common exon. In Mhc3 a poly(A) signal inserted into Mhc intron 7d de fines terminal exons using either the upstream 3' acceptor of common e xon 6 or the 7d acceptor, leading to the expression of 4.1- and 1.7-kb transcripts, respectively. Acceptor selection is regulated in Mhc3 tr anscripts, where the 3' acceptor of common Mhc exon 6 is preferentiall y selected in larvae, whereas the alternative exon 7d acceptor is favo red in adults. These results reflect the adult-specific use of exon 7d and suggest flat the normal exon 7 alternative splicing mechanism con tinues to influence the selection of exon 7d in Mhc3 transcripts. Over all, transposable element-induced disruptions in alternative processin g demonstrate a role for the nonconsensus 3' accepters in Mhc exons 7 and 9 alternative splicing regulation.