IDENTIFICATION OF AN ELEMENT CRUCIAL FOR THE SUB-SYNAPTIC EXPRESSION OF THE ACETYLCHOLINE-RECEPTOR EPSILON-SUBUNIT GENE

The adult neuromuscular junction displays an accumulation of both the acetylcholine receptor (AChR) protein in the subneural domain of the p ost-synaptic membrane and the mRNAs coding for all its subunits at the level of the subjunctional ''fundamental nuclei,'' In the course of e nd plate development, the epsilon-subunit, at variance with other subu nits, becomes exclusively expressed at the level of the fundamental nu clei, yet at a rather late stage (around birth). To analyze the promot er region of the epsilon-subunit gene which directs its specific expre ssion at the synapse, we used a quantitative transient in vivo express ion assay in intact muscle tissue using constructs of the epsilon-subu nit promoter placed upstream of the beta-galactosidase reporter gene. One crucial element for synapse-specific expression was detected betwe en the -11 and -6 positions, Disruption of this element, either by a s canning mutation or single base mutations, greatly diminishes, or even completely inhibits, preferential expression of the transgene at the end plate. Gel shift experiments reveal the presence of a complex in n uclear muscle extracts that bind the core sequence of this element, Th e identification of such a site opens the possibility to identify regu latory factors responsible for compartmentalized expression at the neu romuscular junction.