SPLICE-JUNCTION ELEMENTS AND INTRONIC SEQUENCES REGULATE ALTERNATIVE SPLICING OF THE DROSOPHILA MYOSIN HEAVY-CHAIN GENE TRANSCRIPT

The Drosophila muscle myosin heavy chain (Mhc) gene primary transcript contains five alternatively spliced exon groups (exons 3, 7, 9, 11 an d 15), each of which contains two to five mutually exclusive members. Individual muscles typically select a specific alternative exon from e ach group for incorporation into the processed message. We report here on the cis-regulatory mechanisms that direct the processing of altern ative exons in Mhc exon 11 in individual muscles using transgenic repo rter constructs, RT-PCR and directed mutagenesis. The 6.0-kilobase exo n 11 domain is sufficient to direct the correct processing of exon 11 alternatives, demonstrating that the alternative splicing cis-regulato ry elements are local to Mhc exon 11. Mutational analysis of Mhc exon 11 reveals that the alternative exon nonconsensus 5'-splice donors are essential for alternative splicing regulation in general, but do not specify alternative exons for inclusion in individual muscles. Rather, we show, through exon substitutions and deletion analyses, that a 360 -nucleotide intronic domain precisely directs the normal processing of one exon, Mhc exon 11e, in the indirect flight muscle. These and othe r data indicate that alternative exons are regulated in appropriate mu scles through interactions between intronic alternative splice-specifi city elements, nonconsensus exon 11 splice donors and, likely, novel e xon-specific alternative splicing factors.