Reactivation of G0 nuclei by S-phase cells.

tsAF8 cells are a temperature sensitive (ts) mutant of BHK that arrest in G1 at the nonpermissive temperature or after serum deprivation. G0 tsAF8 were fused by polyethylene glycol with other G0 tsAF8 cells, and the fusion products were incubated at the nonpermissive temperature. The homokaryons were incapable of entering S phase under these conditions. However, when S phase tsAF8 cells were fused with G0 tsAF8 cells, both nuclei in the homokaryons entered S phase, even when the fusion products were incubated at the nonpermissive temperature. In addition S phase tsAF8 cells, if fused with chick erythrocytes, can reactivate the chick nucleus, even if the heterokaryons are placed at nonpermissive temperature. Therefore S phase information of tsAF8 can induce DNA synthesis, after fusion, in mammalian G0 AF8 cells and chick erythrocytes even when the homokaryons or heterokaryons are incubated at the nonpermissive temperature.