Tissue-specific expression of the alternately processed Drosophila myosin heavy-chain messenger RNAs.

The myosin heavy-chain (Mhc) gene of Drosophila is a single-copy gene from which four messenger RNAs are transcribed. Two of these mRNAs, CA-1 and CA-2, are expressed in all stages of development when Mhc mRNA is detected. The 3' ends of these mRNAs differ by alternate choice of poly(A) addition sites. Two additional Mhc mRNAs, CBA-1 and CBA-2, are detected only in midpupal to adult stages of development. The 3' ends of these mRNAs are alternately polyadenylated as the above mRNAs; however, these mRNAs contain an additional alternately spliced exon. We have used in situ hybridization to tissue sections to determine the tissue-specific expression of the alternately processed Mhc mRNAs. Four probes were used in the in situ hybridization experiments: one that detects all Mhc mRNAs, one that is specific for mRNA molecules polyadenylated at the downstream site 2, one that is specific for alternately spliced mRNAs containing the B exon, and one that is specific for Mhc mRNAs Ca-1 and CA-2. This last probe is an oligodeoxynucleotide, while the others are single-stranded RNA molecules synthesized in vitro. Our results demonstrate that the alternate splicing of Mhc mRNAs is muscle-cell-type-specific during pupal development, while the polydenylation site usage at the downstream site 2 is not muscle-cell-type-specific during either embryonic or pupal development.