Tef-1 Dependent Expression of the Alpha-tropomyosin Gene in the Three Muscle Cell Types

In vertebrates, the actin binding proteins tropomyosins are encoded by four distinct genes that are expressed in a complex pattern during development and muscle differentiation. In this study, we have characterized the transcriptional machinery of the α-tropomyosin gene in muscle cells. Promoter analysis reveals that a 284-base pair proximal promoter region of the Xenopus laevis α-tropomyosin gene is sufficient for a maximal activity in the three muscle cell types. The transcriptional activity of this promoter in the three muscle cell types depends on both distinct and common cisregulatory sequences. We have identified a 30base pair conserved sequence unique to all vertebrate α-tropomyosin genes and that contains an MCAT site that is critical for the expression of the gene in all muscle cell types. This site can bind TEF-1 present in muscle cells both in vitro and in vivo. In serumdeprived differentiated smooth muscle cells, TEF-1 is redistributed to the nucleus and this correlates with an increased activity of the αtropomyosin promoter. Overexpression of TEF-1 mRNA in Xenopus embryonic cells leads to the activation of both the endogenous α-tropomyosin gene and the exogenous 284base pair promoter. Finally we show that, in transgenic embryos and juveniles, an intact MCAT sequence is required for the correct temporal and spatial expression of the 284base pair gene promoter. These studies represent the first analysis of the trancriptional regulation of the αtropomyosin gene in vivo and highlight a common TEF-1-dependent regulatory mechanism necessary for expression of the gene in the three muscle lineages.