ALPHA-TROPOMYOSIN GENE-EXPRESSION IN XENOPUS-LAEVIS - DIFFERENTIAL PROMOTER USAGE DURING DEVELOPMENT AND CONTROLLED EXPRESSION BY MYOGENIC FACTORS

Tropomyosins (TMs) constitute a group of contractile proteins encoded by a multigene family showing distinct cell-type-specific and developm ental expression patterns. In mammals and birds, the alpha-TM gene is the most complex and can produce several muscle and non-muscle isoform s. We report here the characterization of the 5' region of the Xenopus laevis alpha-TM gene and its developmental expression. The 5' region of the gene is structurally related to the avian and mammalian cognate s and presents two promoters flanking a pair of alternatively spliced exons, 2a/2b, where exon 2a is a smooth-muscle-specific exon. The inte rnal promoter is used to generate a non-muscle low molecular weight TM whilst muscle TM isoforms originate from the distal promoter. RNase p rotection analysis shows that the two promoters have distinct temporal programs of activation. The internal promoter is activated early in o ogenesis and non-muscle transcripts are found throughout oogenesis, em bryogenesis and in adult tissues. Only low molecular weight non-muscle TM-encoding mRNAs are expressed in oogenesis. The distal promoter is silent during oogenesis, and the skeletal muscle alpha-TM transcripts accumulate from stage 15 in the embryo and are expressed in adult stri ated muscle tissues. In situ hybridization indicates that these transc ripts are expressed in both the somites and heart of the embryo. Ectop ic expression of myogenic factors, but not the MEF2 myocyte-specific e nhancer factor 2 factors SL1 and SL2, can induce the expression of the alpha-TM gene suggesting that the gene is a direct target for myogeni c but not for MEF2 factors. The amphibian alpha-TM gene constitutes a gene marker for studying the developmental control expression of muscl e genes in the different myogenic lineages.