Transactivation of cardiac MLC-2 promoter by MyoD in 10T1/2 fibroblast cells is independent of E-box requirement but depends upon new proteins that recognize MEF-2 site.

MyoD-mediated activation of skeletal muscle genes, which is dependent upon the consensus E-box sequence, involves, at least in one group of muscle genes, another transcription factor, the myocyte enhancer factor-2 (MEF-2). Since the cardiac myosin light chain-2 (MLC-2) gene promoter lacks the functional E-box but contains the activator MEF-2 site, we tested the effect of ectopic expression of MyoD on cardiac MLC-2 promoter function. Here, we demonstrate that either transient or stable expression of MyoD in otherwise nonpermissive C3H10T1/2 fibroblast cells can promote the expression of MLC-2/CAT. Deletion and site-specific mutation analysis demonstrate that the MEF-2 site (Element B) in the MLC-2 promoter is the target of activation by MyoD. Gel mobility shift assay using nuclear extracts from the normal and MyoD-transfected fibroblast cells did not show a difference in the major MEF-2 binding complexes, except for one complex of fast-moving mobility, which suggested that new MEF-2-like regulatory proteins are induced by MyoD.