FAST-MUSCLE-SPECIFIC EXPRESSION OF HUMAN ALDOLASE-A TRANSGENES

The expression of the human aldolase A gene is controlled by three alt ernative promoters. In transgenic mice, pN and pH are active in all ti ssues whereas pM is activated specifically in adult muscles composed m ainly of fast, glycolytic fibers. To detect potential regulatory regio ns involved in the fast-muscle-specific activation of pM, we analyzed DNase I hypersensitivity in a 4.3-kbp fragment from the 5' end of the human aldolase A gene. Five hypersensitive sites were located near the transcription initiation site of each promoter in those transgenic-mo use tissues in which the corresponding promoter was active. Only one m uscle-specific hypersensitive site was detected, mapping near pM. To f unctionally delimit the elements required for muscle-specific activity of pM, we performed a deletion analysis of the aldolase A 5' region i n transgenic mice. Our results show that a 280-bp fragment containing 235 bp of pM proximal upstream sequences together with the noncoding M exon is sufficient for tissue-specific expression of pM. When a putat ive MEF-2-binding site residing in this proximal pM region is mutated, pM is still active and no change in its tissue specificity is detecte d. Furthermore, we observed a modulation of pM activity by elements ly ing further upstream and downstream from pM. Interestingly, pM was exp ressed in a tissue-specific way in all transgenic mice in which the 28 0-bp region was present (32 lines and six founder animals). This obser vation led us to suggest that the proximal pM region contains elements that are able to override to some extent the effects of the surroundi ng chromatin.