Expression of the mouse alpha-cardiac actin gene in skeletal and cardiac muscle 
is regulated by enhancers lying 5' to the proximal promoter. Here we report the 
characterization of a cardiac-specific enhancer located within -2.354/-1.36 kbp 
of the gene, which is active in cardiocytes but not in C2 skeletal muscle cells. 
In vivo it directs reporter gene expression to the adult heart, where the 
proximal promoter alone is inactive. An 85-bp region within the enhancer is 
highly conserved between human and mouse and contains a central AT-rich site, 
which is essential for enhancer activity. This site binds myocyte enhancer 
factor (MEF)2 factors, principally MEF2D and MEF2A in cardiocyte nuclear 
extracts. These results are discussed in the context of MEF2 activity and of the 
regulation of the alpha-cardiac actin locus.