Induction of thymidine⁻³h incorporation in multinucleated myotubes by rous sarcoma virus

During the differentiation of skeletal muscle, mononucleated myoblasts fuse to form multinucleated myotubes. After fusion, both DNA synthesis and nuclear division cease. When clones of chick embryonic leg muscle consisting of both myoblasts and myotubes are infected with Rous sarcoma virus, DNA synthesis (as measured by thymidine‐ 3 H incorporation) is induced in the nuclei of myotubes within 24 hours. On the other hand, nuclear division is not reactivated. These findings indicate that the restriction of DNA synthesis which attends the formation of myotubes is not irreversible since RSV, an RNA virus, reactivates DNA synthesis in mature myotubes.