Analysis of thein VitroTranslation Product of a Novel-TypeDrosophila melanogasterAldolase mRNA in Which Two Carboxyl-Terminal Exons Remain Unspliced

Drosophila melanogastergenerates three different types of aldolase mRNAs from a single gene by selective usage of the triplicate exons 4 (4α, 4β, and 4γ), which encode three different isozymes having respective carboxyl termini. We have found the presence of a novel-type mRNA (named αβ) in which two final exons, 4α and 4β, were retained unspliced. Herein, a cDNA clone containing the αβsequence was inserted into pINIII and expressed in an Escherichia coli system. The product, which exhibited aldolase activity, was found to be isozyme α from the primary structure and the enzymological properties, with the 4α sequence alone being present as the carboxyl terminus. In tissues of D. melanogaster, the production of mRNA encoding exon 4α is known to be restrained to a low level. This may be understood by the fact that the aldolase gene of this species does not have a typical poly(A) signal at the 3′ end in exon 4α. Instead, the transcript-encoding exons, 4α and 4β, might be produced when AATATA, which resides downstream of the coding frame in exon 4β, is recognized as a poly(A) signal during RNA processing.