A proximal promoter element in the hamster desmin upstream regulatory region is responsible for activation by myogenic determination factors.

The muscle-specific intermediate filament protein desmin is up-regulated during skeletal muscle differentiation. When myoblasts leave the cell cycle and fusion into multinucleated myotubes starts, genes associated with myogenesis become activated. The activation is believed to be mediated by the muscle-specific determination factors. We present evidence that both MyoD and myogenin are able to activate the transcription of the hamster desmin gene. A proximal promoter fragment of 89 base pairs is sufficient for this transactivation process. The single E-box in this region is essential for desmin promoter activity in mouse C2 skeletal muscle cells and upon co-transfection of a myogenin expression vector also in human primary fibroblasts. Mutation of this MyoD binding site abrogates desmin transcription, and transactivation of the promoter no longer occurs. By using gel electrophoretic mobility shift assays, we were able to demonstrate that nuclear proteins from C2 muscle cells and myogenin/E12 glutathione S-transferase fusion proteins are able to bind to the functional E-box consensus sequence. A second E-box, situated in a more upstream regulatory region, which also binds to purified Helix-Loop-Helix proteins in vitro is only moderately affected by site-directed in vitro mutagenesis.