Organization of Muscle-Specific Genes in the Rodent Genome

In vitro myogenesis offers and experimental model for the study of the molecular mechanisms involved in gene expression during cell differentiation. Terminal differentiation of muscle cells is characterized by the fusion of mononucleated myoblasts into multinucleated fibers. The morphological changes are accompanied by biochemical modifications, including the onset or a great increase in the synthesis of the major muscle contractile proteins, their regulatory polypeptides and enzymes needed to produce the energy for muscle contraction (reviewed in ref. 1). Several of these proteins (actin, myosin heavy chain, tropomyosin, creatine kinase) have been shown to be members of families of closely related isoforms, some of which are muscle-specific and are synthesized during terminal differentiation, others are present in many cell types. The capacity of myogenic cells (as well as other precursor cells) to proliferate during extended periods without expressing the genes involved in terminal differentiation (2, 3), indicate the existence of mechanisms which retain the latent program of gene expression, and mechanisms of gene activation which recognize these genes.