MYOD1 PROMOTER AUTOREGULATION IS MEDIATED BY 2 PROXIMAL E-BOXES

We show that in mouse myoblasts the MyoD1 promoter is highly stimulate d by MyoD1 expression, suggesting that it is controlled by a positive feedback loop. Using deletion and mutation analyses, we identified the targets for MyoD1 promoter autoregulation as the two proximal E-boxes located close to the MyoD1 core promoter. Gel mobility shift competit ion assays with MyoD1 antibodies as competitor suggest that the MyoD1 protein is binding directly to these E-boxes. Autoregulation did not o ccur in fibroblasts cotransfected with the expression vector of MyoD1. It is assumed that autoregulation is controlled by the stoichiometry between the MyoD1 protein and negatively regulatory proteins like Id, which is known to be highly expressed in fibroblasts. When the MyoD1 p romoter was methylated, autoregulation only occurred when the density of methylated sites was low. The density of DNA methylation, therefore , can determine the accessibility of the MyoD1 promoter to transcripti on factors and interfere with the auto- and crossregulatory loop. The MyoD1 promoter in vivo was found to be only partially methylated in al l tissues tested except in skeletal muscle where it was demethylated. We propose that high level expression of the MyoD1 gene is a result of release from constraints such as negative regulatory factors and/or D NA methylation interfering with MyoD1 autoregulation.