MYOGENIN RESIDES IN THE NUCLEUS AND ACQUIRES HIGH-AFFINITY FOR A CONSERVED ENHANCER ELEMENT ON HETERODIMERIZATION

Myogenin is a member of a family of muscle-specific factors that can activate the muscle differentiation program in nonmyogenic cells. Using antibodies directed against domains of myogenin, we show in the present study that myogenin resides in the nucleus of differentiated muscle cells. Myogenin translated in vitro does not exhibit detectable DNA binding activity; however, when dimerized with the ubiquitous enhancer-binding factor E12, it acquires high affinity for an element in the core of the muscle of the muscle creatine kinase (MCK) enhancer that is conserved among muscle-specific genes. Antibody disruption experiments show that myogenin, synthesized during differentiation of the BC3H1 and C2 muscle cell lines, is part of a complex that binds to the same site in the MCK enhancer as myogenin-E12 translated in vitro. Mutagenesis of the myogenin-E12-binding site in the MCK enhancer abolishes binding of the heterodimer and prevents transactivation of the enhancer by myogenin. The properties of myogenin suggest that it functions as a sequence-specific DNA-binding factor that interacts directly with muscle-specific genes during myogenesis. The dependence of myogenin on E12 for high-affinity DNA binding activity also suggests that the susceptibility of various cell types to the actions of myogenin may be influenced by the cellular factors with which it may interact.