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^α4^β, and A^γ). 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 daring evolution. The Drosophila aldolase gene is predicted to generate mRNAs for three isozymes (α-, β-, and γ-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 α-, β-, and γ-isozymes, two novel forms of mRNAs, α^beta;- and β^γ-type mRNAs, were detected tissue-specifically in adult files, although their functions are unpredictable. The α^β-mRNA is an α-type mRNA in which exon 4^β remains unspliced, while the β^γ-mRNA is a β-type. mRNA with the exon 4^γ remaining unspliced. Recombinant enzymes expressed in Escherichia coli were all active and exhibited different enzymatic properties.