Analysis of the 5' End of the Drosophila Muscle Myosin Heavy Chain Gene. Alternatively Spliced Transcripts Initiate at a Single Site and Intron Locations Are Conserved Compared to Myosin Genes of Other Organisms

We have localized the transcription start site of the Drosophila melanogaster muscle myosin heavy chain (MHC) gene and find that all forms of the alternatively spliced MHC mRNA initiate at the same location. Therefore the alternative inclusion/exclusion of the 3' penultimate exon in transcripts from this gene (Bernstein, S.I., Hansen, C.J., Becker, K.D., Wassenberg, D.R., II, Roche, E.S., Donady, J.J., and Emerson, C. P., Jr. (1986) Mol. Cell. Biol. 6, 2511-2519; Rozek, C.E., and Davidson, N. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 2128-2134) does not result from the use of different 5' transcription initiation sites. This gene is the first invertebrate MHC gene shown to have TATA and CAAT box consensus sequences and a noncoding 5' exon, properties that are shared with some vertebrate and invertebrate contractile protein genes. The intron that splits the 5' noncoding region of the Drosophila MHC gene contains no major conserved elements relative to other Drosophila contractile protein genes. The introns within the coding region near the 5' end of the Drosophila MHC gene are located at the same sites as nematode and vertebrate MHC gene introns, indicating that these MHC genes are derived from a common ancestral sequence. The putative ATP binding domain encoded in the fourth exon of the Drosophila MHC gene is highly conserved relative to vertebrate, invertebrate, and non-muscle MHC genes suggesting that each of these myosins bind ATP by the same mechanism. Two divergent copies of the third exon are present within the 5' region of the Drosophila MHC gene, suggesting that alternative splicing produces MHC isoforms with different globular head regions.