Abstract
Drosophila melanogaster generates three different types of aldolase mRNAs from a single gene by selective usage of the triplicate exons 4 (4 alpha, 4 beta, and 4 gamma), which encode three different isozymes having respective carboxyl termini. We have found the presence of a novel-type mRNA (named alpha beta) in which two final exons, 4 alpha and 4 beta, were retained unspliced. Herein, a cDNA clone containing the alpha beta sequence was inserted into pINIII and expressed in an Escherichia coli system. The product, which exhibited aldolase activity, was found to be isozyme alpha from the primary structure and the enzymological properties, with the 4 alpha sequence alone being present as the carboxyl terminus. In tissues of D. melanogaster, the production of mRNA encoding exon 4 alpha 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 4 alpha. Instead, the transcript-encoding exons, 4 alpha and 4 beta, might be produced when AATATA, which resides downstream of the coding frame in exon 4 beta, is recognized as a poly(A) signal during RNA processing.
