Tissue-specific transcription of the cardiac myosin light-chain 2 gene is regulated by an upstream repressor element.

Physiological expression of the cardiac muscle myosin light-chain 2 (MLC-2) gene in chickens is restricted to cardiac muscle tissue only, at least during the late embryonic to adult stages of development. The mechanism by which cardiac MLC-2 gene expression is repressed in differentiated noncardiac muscle tissues is unknown. Using sequential 5'-deletion mutants of the cardiac MLC-2 promoter introduced into primary skeletal muscle cells in culture, we have demonstrated that a 89-bp region, designated the cardiac-specific sequence (CSS), is essential for repression of cardiac MLC-2 expression in skeletal muscle. Removal of the CSS sequence alone allows transcription in skeletal muscle cells without affecting the transcriptional activity of the promoter in cardiac muscle cells. DNase I footprinting and gel shift assays indicate that protein binding to sequences in the CSS domain occurs readily in nuclear extracts obtained from skeletal muscle but not in extracts isolated under identical conditions from cardiac muscle. Thus, it appears that a negative regulatory mechanism accounts for the lack of expression of the cardiac MLC-2 gene in skeletal muscle and that the CSS element and its binding proteins are important functional components of the regulatory apparatus which ensures the developmental program for cardiac tissue-specific gene expression.