The AT-rich element MEF-2 plays an important role in the maintenance of the 
muscle-specific expression of a number of cardiac and skeletal muscle genes. In 
the MLC-2 gene, an AT-rich element (HF-1b) which contains a consensus MEF-2 site 
is required for cardiac tissue-specific expression. The present study reports 
the isolation and characterization of a cDNA which encodes a novel C2H2 zinc 
finger (HF-1b) that binds in a sequence-specific manner to the HF-1b/MEF-2 site 
in the MLC-2 promoter. A number of independent criteria suggest that this HF-1b 
zinc finger protein is a component of the endogenous HF-1b/MEF-2 binding 
activity in cardiac muscle cells and that it can serve as a transcriptional 
activator of the MLC-2 promoter in transient assays. These studies suggest that, 
in addition to the previously reported RSRF proteins, structurally divergent 
transcriptional factors can bind to MEF-2-like sites in muscle promoters. These 
results underscore the complexity of the regulation of the muscle gene program 
via these AT-rich elements in cardiac and skeletal muscle.