Interspecific sequence comparison of the muscle-myosin heavy-chain genes from Drosophila hydei and Drosophila melanogaster

Transcriptional regulation of the Drosophila melanogaster muscle myosin heavy-chain gene

We show that a 2.6kb fragment of the muscle myosin heavy-chain gene (Mhc) of Drosophila melanogaster (containing 458 base pairs of upstream sequence, the first exon, the first intron and the beginning of the second exon) drives expression in all muscles. Comparison of the minimal promoter to Mhc genes of 10 Drosophila species identified putative regulatory elements in the upstream region and in the first intron. The first intron is required for expression in four small cells of the tergal depressor of the trochanter (jump) muscle and in the indirect flight muscle. The 3'-end of this intron is important for Mhc transcription in embryonic body wall muscle and contains AT-rich elements that are protected from DNase I digestion by nuclear proteins of Drosophila embryos. Sequences responsible for expression in embryonic, adult body wall and adult head muscles are present both within and outside the intron. Elements important for expression in leg muscles and in the large cells of the jump muscle flank the intron. We conclude that multiple transcriptional regulatory elements are responsible for Mhc expression in specific sets of Drosophila muscles.

Positive and negative intronic regulatory elements control muscle-specific alternative exon splicing of Drosophila myosin heavy chain transcripts

Alternative splicing of Drosophila muscle myosin heavy chain (MHC) transcripts is precisely regulated to ensure the expression of specific MHC isoforms required for the distinctive contractile activities of physiologically specialized muscles. We have used transgenic expression analysis in combination with mutagenesis to identify cis-regulatory sequences that are required for muscle-specific splicing of exon 11, which is encoded by five alternative exons that produce alternative "converter" domains in the MHC head. Here, we report the identification of three conserved intronic elements (CIE1, -2, and -3) that control splicing of exon 11e in the indirect flight muscle (IFM). Each of these CIE elements has a distinct function: CIE1 acts as a splice repressor, while CIE2 and CIE3 behave as splice enhancers. These CIE elements function in combination with a nonconsensus splice donor to direct IFM-specific splicing of exon 11e. An additional cis-regulatory element that is essential in coordinating the muscle-specific splicing of other alternative exon 11s is identified. Therefore, multiple interacting intronic and splice donor elements establish the muscle-specific splicing of alternative exon 11s.

The role of evolutionarily conserved sequences in alternative splicing at the 3' end of Drosophila melanogaster myosin heavy chain RNA

Exon 18 of the muscle myosin heavy chain gene (Mhc) of Drosophila melanogaster is excluded from larval transcripts but included in most adult transcripts. To identify cis-acting elements regulating this alternative RNA splicing, we sequenced the 3' end of Mhc from the distantly related species D. virilis. Three noncoding regions are conserved: (1) the nonconsensus splice junctions at either end of exon 18; (2) exon 18 itself; and (3) a 30-nucleotide, pyrimidine-rich sequence located about 40 nt upstream of the 3' splice site of exon 18. We generated transgenic flies expressing Mhc mini-genes designed to test the function of these regions. Improvement of both splice sites of adult-specific exon 18 toward the consensus sequence switches the splicing pattern to include exon 18 in all larval transcripts. Thus nonconsensus splice junctions are critical to stage-specific exclusion of this exon. Deletion of nearly all of exon 18 does not affect stage-specific utilization. However, splicing of transcripts lacking the conserved pyrimidine sequence is severely disrupted in adults. Disruption is not rescued by insertion of a different polypyrimidine tract, suggesting that the conserved pyrimidine-rich sequence interacts with tissue-specific splicing factors to activate utilization of the poor splice sites of exon 18 in adult muscle.

Transposable element insertions respecify alternative exon splicing in three 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 multi-alternative exons in the Mhc2, Mhc3, and Mhc4 mutants in Drosophila. Sequence and expression analyses show that each inserted element introduces a strong polyadenylation signal that defines novel terminal exons, which are then differentially recognized by the alternative splicing apparatus. Mhc2 and Mhc4 have insertion elements located within intron 7c and exon 9a, respectively, and each expresses a single truncated transcript that contains an aberrant terminal exon defined by the poly(A) signal of the inserted element and the 3' acceptor of the upstream common exon. In Mhc3, a poly(A) signal inserted into Mhc intron 7d defines terminal exons using either the upstream 3' acceptor of common exon 6 or the 7d acceptor, leading to the expression of 4.1- and 1.7-kb transcripts, respectively. Acceptor selection is regulated in Mhc3 transcripts, where the 3' acceptor of common Mhc exon 6 is preferentially selected in larvae, whereas the alternative exon 7d acceptor is favored in adults. These results reflect the adult-specific use of exon 7d and suggest that the normal exon 7 alternative splicing mechanism continues to influence the selection of exon 7d in Mhc3 transcripts. Overall, transposable element-induced disruptions in alternative processing demonstrate a role for the nonconsensus 3' acceptors in Mhc exons 7 and 9 alternative splicing regulation.

Myosin rod protein: a novel thick filament component of Drosophila muscle

Myosin rod protein (MRP), a 155 kDa protein encoded by a gene internal to the Drosophila muscle myosin heavy chain (Mhc) gene, contains the MHC rod domain, but has 77 unique N-terminal residues that exactly replace the MHC motor and light chain binding domains. Originally described as an abundant testis protein, we now demonstrate the MRP also is a major component of myofilaments in Drosophila. Specifically, the Mrp promoter directs the expression of a LacZ reporter transgene in somatic, cardiac and visceral muscles. MRP-specific antibodies detect the protein in detergent-insoluble fractions of muscle extracts and co-localize the protein with MHC to the sarcomeric A-band in immunostained muscles. Immunoblot analysis shows that in a set of adult direct flight muscles (DFM), the ratio of MRP to MHC is 1:3. Chemical cross-link and co-immunoprecipitation experiments using 0.5 M KCl-extracted thick filament proteins indicate that native MRP is a homodimer. Electron microscopy of DFM49, which has a high MRP content, shows in cross section, disordered myofilament packing and a variable thin to thick filament ratio and, in longitudinal section, severely bent thin filaments that are not well associated with thick filaments. In rigor, thick filaments from DFM49 consist of segments with cross bridges that are interspersed with smooth domains lacking cross bridges. These data indicate that MRP is a novel contractile protein that co-integrates with myosin into the thick filament, thereby changing structure and function of the sarcomere.

Structure and expression of histone H3.3 genes in Drosophila melanogaster and Drosophila hydei

We demonstrate that in Drosophila melanogaster the histone H3.3 replacement variant is encoded by two genes, H3.3A and H3.3B. We have isolated cDNA clones for H3.3A and cDNA and genomic clones for H3.3B. The genes encode exactly the same protein but are widely divergent in their untranslated regions (UTR). Both genes are expressed in embryos and adults; they are expressed in the gonads as well as in somatic tissues of the flies. However, only one of them, H3.3A, shows strong testes expression. The 3' UTR of the H3.3A gene is relatively short (approximately 250 nucleotides (nt)). H3.3B transcripts can be processed at several polyadenylation sites, the longest with a 3' UTR of more than 1500 nt. The 3' processing sites, preferentially used in the gonads and somatic tissues, are different. We have also isolated the Drosophila hydei homologues of the two H3.3 genes. They are quite similar to the D. melanogaster genes in their expression patterns. However, in contrast to their vertebrate counterparts, which are highly conserved in their noncoding regions, the Drosophila genes display only limited sequence similarity in these regions.

Minor-myosin, a novel myosin isoform synthesized preferentially in Drosophila testis is encoded by the muscle myosin heavy chain gene

Searching for structural proteins involved in spermatogenesis of Drosophila, we found a novel myosin isoform in the testis of Drosophila hydei and D. melanogaster. The transcript encoding this isoform, which we called 'minor-myosin', initiates within the intron between exons 12 and 13 of the muscle myosin heavy chain (mMHC) gene. Minor-myosin contains a common myosin tail but no ordinary myosin head domain. Instead, it has a short N-terminal domain which displays similarity with the N-termini of certain myosin light chain proteins. Western blots with male germ line mutants showed that the novel mMHC isoform is synthesized in the male germ cells, mainly postmeiotically. However, minor-myosin is not testis-specific, as it is expressed at a low level in the fly carcasses. The possible functions of the myosin isoform in the male germ line are discussed.