MEF2B IS A COMPONENT OF A SMOOTH MUSCLE-SPECIFIC COMPLEX THAT BINDS AN A T-RICH ELEMENT IMPORTANT FOR SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN GENE-EXPRESSION/

To understand smooth muscle-specific gene expression, we have focused our studies on the smooth muscle myosin heavy chain (SMHC) gene, a smo oth muscle-specific marker, In this study, we demonstrate that the SMH C promoter region (-1594 to -1462 base pairs) containing the A/T-rich element can activate the heterologous thymidine kinase promoter in smo oth muscle cells, but not in fibroblasts, Mutations of this APT-rich e lement decreased SMHC promoter activity significantly, Both gel mobili ty shift assays and DNase I footprinting revealed that this region bin ds to specific protein complexes from smooth muscle nuclear extracts, whereas nuclear extracts from skeletal muscle and fibroblasts produced a different binding pattern, We also demonstrate that the protein com plex obtained from smooth muscle nuclear extract reacts with MEF2B-spe cific antibody, but not with antibodies specific to MEF2A, MEF2C, or M EF2D, suggesting that only MEF2B protein binds to the A/T-rich element , Furthermore, MEF2B overexpression in smooth muscle cells up-regulate d the SMHC promoter, suggesting that MEF2B is important for SMHC gene regulation, This is the first report demonstrating a role for MEF2 fac tors in smooth muscle-specific gene expression.