CIS-ELEMENTS INVOLVED IN DIFFERENTIAL EXPRESSION OF NA-K+-ATPASE ALPHA-2 SUBUNIT GENE IN MUSCLE DIFFERENTIATION()

The mRNA of the Na+-K+-ATPase alpha 2 subunit gene (Atpla2) increases during myogenesis in rat following birth. To investigate the mechanism of the transcriptional regulation underlying this process, we examine d the promoter activity of Atpla2 by transient transfection assays in myocyte differentiation, using C2C12 murine skeletal myoblast cell lin e. The promoter activity was increased about 5-fold in the differentia ted cells, comparing with that in the growing cells. By analyses of 5' -deletion mutations of the gene, the responsible elements were localiz ed in the region between -108 and +60. We identified a binding factor that exists only in the nuclear extract from the growing cells but not in the extract from the differentiated cells with a probe DNA fragmen t from +20 to +50 by gel retardation assays. The element to which the factor binds can mediate the inhibition of alpha 2 gene expression in the growing C2C12 cells but not in the differentiated cells. Finally w e revealed protein occupancy in this region in the growing cells by in vivo footprinting. We propose a model that the increased expression o f Atpla2 during muscle differentiation is mediated by the disappearanc e of a negative factor.