Stepwise activation of the mouse acetylcholine receptor delta- and gamma-subunit genes in clonal cell lines

We used the DNase I-hypersensitive sites around the mouse acetylcholine receptor delta-subunit gene as a guide toward the cloning and sequencing of delta and gamma transcriptional regulatory regions and as a means to assess chromatin structural activation of the delta- and gamma-subunit genes during myogenesis. Genomic cloning of hypersensitive sites downstream of the delta-subunit gene revealed the presence of the gamma-subunit gene approximately 5 kilobases away; the hypersensitive sites mapped to the 5' end of the gamma-subunit gene. Sequence comparison of restriction fragments containing hypersensitive sites in analogous locations at the 5' ends of the delta- and gamma-subunit genes uncovered little overall homology between the two genomic fragments; however, an 11- of 13-base-pair match between the two sequences was found. Homologs to this sequence were also found to be present in the upstream regions of the chicken alpha- and mouse beta-subunit genes. By RNase protection and primer extension analyses, the delta-subunit gene transcription start site was mapped to 56 base pairs upstream of the initiator ATG codon. Clonal cell lines with various potentials to differentiate to the skeletal muscle phenotype were examined for their hypersensitive site pattern within the delta-gamma locus. Only remote hypersensitive sites flanking the locus appear in pluripotential mesodermal cells. A cell line of determined but inducible myoblasts expressed only one more intergenic site, while in permissively differentiating myoblasts hypersensitive sites were already present at the 5' ends of the delta and gamma genes prior to differentiation. Terminal differentiation resulted in an identical pattern of hypersensitive sites in all muscle cell lines examined so far, with an intergenic site near the gamma-subunit gene being the only site specific to the differentiated muscle phenotype.