Gene structure and multiple mRNA species of Drosophila melanogaster aldolase generating three isozymes with different enzymatic properties.

Genomic clones encoding the Drosophila aldolase gene were isolated and the organization of the gene was determined. The protein-coding region spanning nearly 3.5 kb consists of five coding exons (exon 2, 3, 4 alpha, 4 beta, and 4 gamma). The insect exon 2 corresponds to exons 2 to 7 of vertebrate aldolase genes and thus appears to have been formed by the fusion of these 6 exons into a single exon during evolution. The Drosophila aldolase gene is predicted to generate mRNAs for three isozymes (alpha-, beta-, and gamma-types) from the primary transcripts by alternative usage of the final three exons. The reverse transcriptase-PCR assay revealed the occurrence of mRNAs for the three isozymic forms at different developmental stages, and tissue-specific expression was also found to occur in adult flies. In addition to the usual type mRNA species for the alpha-, beta-, and gamma-isozymes, two novel forms of mRNAs, alpha beta- and beta gamma-type mRNAs, were detected tissue-specifically in adult flies, although their functions are unpredictable. The alpha beta-mRNA is an alpha-type mRNA in which exon 4 beta remains unspliced, while the beta gamma-mRNA is a beta-type mRNA with the exon 4 gamma remaining unspliced. Recombinant enzymes expressed in Escherichia coli were all active and exhibited different enzymatic properties.