Induction of DNA synthesis in terminally differentiated myotubes by the activation of the src gene of Rous sarcoma virus.

Mononucleated myogenic cells from 11-day-old chicken embryos were infected with tsLA24 or tsNY68, temperature-sensitive transformation mutants of Rous sarcoma virus. The infected mononucleated myogenic cells were incubated at the nonpermissive temperature (41 degrees C) and allowed to develop into multinucleated myotubes. These myotubes have withdrawn from the cell cycle, and no DNA synthesis was observed as long as the cultures were maintained at the nonpermissive temperature. However, when the incubation temperature of these cultures was lowered to the permissive temperature (36 degrees C) for expression of the src gene, DNA synthesis was induced in multinucleated myotubes. For this induction of DNA synthesis, cells infected with tsLA24 had to be incubated at the permissive temperature for at least 50 hr, while the induction of DNA synthesis in cells infected with tsNY68 required less than 20 hr. Induction of DNA synthesis was observed by autoradiography as well as by measuring incorporation of [3H]thymidine into the macromolecule fraction in these myotube cultures. For the maintenance of capacity to induce DNA synthesis, constant presence of the src gene product is necessary, because when the temperature of these cultures was returned to 41 decrees C, the myotubes lost the capacity to induce DNA synthesis. During the process of DNA induction one biochemical marker of muscle (creatine kinase) remained unchanged.