Myocyte-specific enhancer factor 2 acts cooperatively with a muscle activator region to regulate Drosophila tropomyosin gene muscle-expression

MEF2 (myocyte-specific enhancer factor 2) is a MADS box transcription factor that is thought to be a key regulator of myogenesis in vertebrates. Mutations in the Drosophila homologue of the mef2 gene indicate that it plays a key role in regulating myogenesis in Drosophila, We show here that the Drosophila tropomyosin I (TmI) gene is a target gene for mef2 regulation. The TmI gene contains a proximal and a distal muscle enhancer within the first intron of the gene, We show that both enhancers contain a MEF2 binding site and that a mutation in the MEF2 binding site of either enhancer significantly reduces reporter gene expression in embryonic, larval, and adult somatic body wall muscles of transgenic flies. We also show that a high level of proximal enhancer-directed reporter gene expression in somatic muscles requires the cooperative activity of MEF2 and a cis-acting muscle activator region located within the enhancer. Thus, mef2 null mutant embryos show a significant reduction but not an elimination of TmI expression in the body wall myoblasts and muscle fibers that are present. Surprisingly, there is little effect in these mutants on TmI expression in developing visceral muscles and dorsal vessel (heart), despite the fact that MEF2 is expressed in these muscles in wild-type embryos, indicating that TmI expression is regulated differently in these muscles. Taken together, our results show that mef2 is a positive regulator of tropomyosin gene transcription that is necessary but not sufficient for high level expression in somatic muscle of the embryo, larva, and adult.