Reinitiation of DNA synthesis in postmitotic nuclei of myotubes by virus-mediated fusion with embryonic fibroblasts.

Myotubes, whose nuclei have stopped DNA synthesis were fused with replicative embryonic fibroblasts. In heterokaryons the postmitotic muscle nuclei resumed DNA synthesis. Incorporation of radioactive thymidine into muscle, and also into fibroblast nuclei was dependent upon the time elapsed between virus-mediated fusion and administration of radioactive thymidine. Whereas incorporation into fibroblast nuclei diminished with time, there was an early increase of labelling into muscle nuclei followed by a decrease of incorporation of 3H thymidine. DNA synthesis was also dependent upon the ratio of noncycling (muscle) to cycling (fibroblast) nuclei. There was a greater incorporation of 3H thymidine into muscle and fibroblast nuclei in myotubes containing larger numbers of fibroblast nuclei. A model is discussed for the control of DNA synthesis in polykaryocytes derived from fusion of cycling and noncycling cells.