Abstract
The myogenic determination factors (MDFs) are transcriptional activators that target E boxes in many muscle-specific promoters, including those of the genes coding for the subunits of the acetylcholine receptor. It is not known, however, if in vivo a given E box in a transcriptionally active gene is occupied, either uniquely by one MDF or randomly by all MDFs. We have analyzed expression of MDF and acetylcholine receptor subunits in cultured mouse muscle cells and, using chromatin immunoprecipitation, have determined which individual MDFs reside at promoters of several receptor subunit genes. We find that before fusion, C2C12 cells express myf-5, MyoD, and myogenin, all of which take up residence at promoters of all subunits except epsilon. At this stage, herculin is present in limited amounts and is detected mainly at the gamma and delta subunit genes. On myotube formation, herculin reaches high levels; concomitantly, the epsilon subunit gene becomes a common MDF target and begins to be expressed. In general, any MDF protein that is expressed also is present on transcriptionally active receptor genes; transcriptional activity of target genes correlates with occupancy by MDF, in particular, herculin.
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