Several genes encoding putative myogenic regulatory factors have been isolated on the basis of their ability to convert nonmuscle cells into myoblasts. Four of these genes code for nuclear proteins that belong to a larger family characterized by a conserved helix-loop-helix motif required for DNA-binding and dimerization. At least one protein, MyoD1, can function as a transcription factor and activate muscle-specific genes during differentiation. But the promoter of the delta-subunit gene of the mouse acetylcholine receptor (AChR) was recently reported to be functional in the absence of MyoD1 binding sites and it has been suggested that the genes coding for the AChR could be regulated independently of MyoD1 protein. Here, we identify two functional MyoD1-binding sites in the muscle-specific enhancer of the chicken AChR alpha-subunit gene that are essential for full activity in transfected myotubes.

PACS. 87.30. -Biophysics of neurophysiological processes. PACS. 75.10H. -Ising and other classical spin models. PACS. 64.60. -General studies of phase transitions.

Acetylcholine receptor (AChR) genes are repressed in extrajunctional domains of adult muscle fiber by neurally evoked electrical activity. Denervation to give rise to KS-842-and KS-110-ALA. The Spe I-Kpn I fragments of KS-842-and KS-110-ALA have been subsequently introduced in the Xba I-Kpn I AdK8 sites. Targeted mutagenesis of promoter fragments was performed as described (9).The -256 to -62 promoter fragment ofthe chicken skeletal a-actin gene (25) was amplified from genomic DNA using

The 5' end and promoter region of the a-subunit gene of chicken muscle acetylcholine receptor was mapped and sequenced. It includes a TATA and a CAAT box and a potential Spl-binding site. When inserted in front of the chloramphenicol acetyltransferase gene, this promoter (including 850 base pairs of upstream sequence) directed high transient chloramphenicol acetyltransferase expression in transfected mouse C2.7 myotubes but not in C2.7 myoblasts or nonmyogenic 3T6 cells.The genes encoding the nicotinic acetylcholine receptor (AChR) of vertebrate muscle (and the closely related electric organ of some fishes) have recently been cloned from several species (reviewed in reference 41) and constitute a system of choice for studying the control of gene expression in higher eucaryotes. Indeed, muscle AChR is made up of four subunits assembled in an a243Y5 pentamer (9, 41), and one