Expression of brain-specific ID sequence is not restricted to the brain, and a novel complementary cID sequence is found in L-type pyruvate kinase mRNA (a liver-specific messenger)

The differences between the aldolase A mRNA forms seem therefore to occur through their 5‘ non-coding stretches. Several possible mechanisms leading to the expression of such different messengers could then be proposed. Two promoters, one ubiquitous and the other specific to adult fast twitch muscle, could exist. Otherwise, the same premRNA transcript might be differentially spliced, or even both mechanisms might occur, as has been demonstrated for myosin light chains (Periasmy et al., 1984). Nevertheless, one cannot totally exclude that the different aldolase A mRNAs could be specified by different genes. It has been shown that in man, at least two genes carried by different chromosomes (Hagenauer et a[., 1985), and in rat at least five genes (P. Maire, unpublished work) are present. There is no information until now on how many of these genes are transcribed. In fast twitch fibres, the rise of the aldolase A mRNA lighter species level is parallel to that of other specific muscular protein mRNAs such as glycogen phosphorylase M, creatine kinase M (F. Schweighhoffer, unpublished work) cr-actin (Minty et al., 1982) and adult myosin heavy chain (Wild et al., 1984). In conclusion, the expression of aldolase A mRNAs is qualitatively and quantitatively related to muscle development. The lighter mRNA species is absolutely specific to the differentiated fast twitch muscle fibres, while the heavier species accounts for foetal, ubiquitous aldolase A expression.