EXPRESSION OF THE ACETYLCHOLINE-RECEPTOR DELTA-SUBUNIT GENE IN DIFFERENTIATING CHICK MUSCLE-CELLS IS ACTIVATED BY AN ELEMENT THAT CONTAINS 2 16 BP COPIES OF A SEGMENT OF THE ALPHA-SUBUNIT ENHANCER

The acetylcholine receptor is a multimeric membrane protein whose expression is activated during muscle differentiation and upon denervation of adult muscle. To gain insight into the coordinate expression of receptor subunits during myogenesis we have analyzed the chick muscle receptor δ-subunit gene upstream region. The δ-subunit gene lacks canonical promoter elements (CCAAT and TATA boxes). Nuclease protection and primer extension analysis revealed that transcription starts at six major and several minor sites between -110 and -30 upstream of the translational initiation site; two sites, at positions -77 and -66, give rise to ~ 50% of all transcripts. Using nested deletions of the proximal 960 bp of the 5' flanking region of this gene we have identified a 62 bp sequence (-207 to -146) that activates transcription in a position independent manner. This enhancer-like element is activated during myotube formation; it contains two distinct functional moieties, each resembling the same 16 bp portion of the stage and tissue specific α-subunit gene enhancer which we have characterized previously [Wang et al. (1988) Neuron, 1, 527-534]. This common element, which also comprises several previously proposed skeletal muscle specific motifs [Buskin, J.N; and Hauschka, S.D. (1989) Mol. Cell Biol., 9, 2627-2640; Mar, J.H. and Ordahl, C.P. (1988) Proc. Natl. Acad. Sci. USA, 85, 6404-6408], may account for the coordinate expression of the two subunits. The cell specificity of the δ-subunit gene 5' flanking region is partly due to the enhancer, partly to the inhibitory element upstream of -207.