ABSENCE OF MEF2 BINDING TO THE A T-RICH ELEMENT IN THE MUSCLE CREATINE-KINASE (MCK) ENHANCER CORRELATES WITH LACK OF EARLY EXPRESSION OF THE MCK GENE IN EMBRYONIC MAMMALIAN MUSCLE/

During skeletal muscle development, different types of muscle fibers a re generated, which express different combinations of muscle-specific gene products, For example, the muscle creatine kinase gene (MCK) is h ighly expressed in fetal but not embryonic myotubes, We performed tran sient transfections of CAT reporter constructs, driven by the MCK prom oter with variable lengths of 5'-flanking sequence, into primary cultu res of embryonic and fetal muscle cells, Reporter activity was observe d in fetal but not embryonic muscle cells, We assayed the ability of n uclear extracts prepared from embryonic and fetal muscle and C2C12 myo tubes to bind specific regulatory elements in the MCK enhancer, The pr ofile of DNA/protein complexes resulting from electrophoretic mobility shift assays was qualitatively the same with all extracts used when t he oligonucleotide probes represented the MCK E-box, MHox site, CArG-b ox, and AP2 site, In contrast, no binding activity to the MEF2 site wa s observed with embryonic nuclear extract, Interestingly, MEF2 mRNAs a nd proteins were detected in both fetal and embryonic muscle, with the exception of the MEF2D1b isoform, which is restricted to fetal muscle , Furthermore, we found that protein phosphatase inhibitors included i n the preparation of embryonic nuclear extracts or added to the medium of transfected embryonic myotubes can restore MEF2 DNA binding activi ty, as well as reporter activity driven by the MCK promoter and partia l transcriptional activation of the endogenous MCK gene, We propose th at phosphorylation of MEF2 regulates its activity and represents an im portant aspect of the mechanism controlling stage-specific transcripti on during skeletal myogenesis.