MyoD belongs to a small family of basic helix-loop-helix transcription factors implicated in skeletal muscle lineage determination and differentiation. Previously, we identified a transcriptional enhancer that regulates the embryonic expression of the human myoD gene. This enhancer had been localized to a 4 kb fragment located 18 to 22 kb upstream of the myoD transcriptional start site. We now present a molecular characterization of this enhancer. Transgenic and transfection analyses localize the myoD enhancer to a core sequence of 258 bp. In transgenic mice, this enhancer directs expression of a lacZ reporter gene to skeletal muscle compartments in a spatiotemporal pattern indistinguishable from the normal myoD expression domain, and distinct from expression patterns reported for the other myogenic factors. In contrast to the myoD promoter, the myoD enhancer shows striking conservation between humans and mice both in its sequence and its distal position. Furthermore, a myoD enhancer/heterologous promoter construct exhibits muscle-specific expression in transgenic mice, demonstrating that the myoD promoter is dispensable for myoD activation. With the exception of E-boxes, the myoD enhancer has no apparent sequence similarity with regulatory regions of other characterized muscle-specific structural or regulatory genes. Mutation of these E-boxes, however, does not affect the pattern of lacZ transgene expression, suggesting that myoD activation in the embryo is E-box-independent. DNase I protection assays reveal multiple nuclear protein binding sites in the core enhancer, although none are strictly muscle-specific. Interestingly, extracts from myoblasts and 10T1/2 fibroblasts yield identical protection profiles, indicating a similar complement of enhancer-binding factors in muscle and this non-muscle cell type. However, a clear difference exists between myoblasts and 10T1/2 cells (and other non-muscle cell types) in the chromatin structure of the chromosomal myoD core enhancer, suggesting that the myoD enhancer is repressed by epigenetic mechanisms in 10T1/2 cells. These data indicate that myoD activation is regulated at multiple levels by mechanisms that are distinct from those controlling other characterized muscle-specific genes.

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