Differential Processing of RNA Transcribed from the Single-Copy Drosophila Myosin Heavy Chain Gene Produces Four mRNAs That Encode Two Polypeptides

We report the sequence of genomic DNA at the 3' end of the single-copy Drosophila myosin heavy chain (MHC) gene and the structure and sequence at the 3' end of four MHC mRNAs. Two mRNAs, 7.2 kilobases (kb) and 8.0 kb in length, are expressed in all stages of development in which detectable levels of muscle-specific mRNAs accumulate. These mRNAs differ by alternate choice of two poly(A) sites within the same exon. Sequence information predicts that these two mRNAs can encode one MHC polypeptide. Two additional MHC mRNAs, 8.0 kb and 8.6 kb in length, are expressed only in late pupal and adult stages of development. These two stage-specific MHC mRNAs use the same poly(A) sites as the MHC mRNAs described above but have a different splicing pattern and thus include an additional exon. Sequence information predicts that these two stage-specific MHC mRNAs encode a second MHC polypeptide with a different COOH terminus.