Cell cycle dependence of the reactivation of chick erythrocyte nuclei after transplantation into mouse L929 cell cytoplasts

Monolayer cultures of cytoplasts were formed from synchronized mouse L929 cells in the G1, S and G2 phases of the cell cycle by cytochalasin‐induced enucleation. Chick embryo erythrocytes were then fused to the cytoplasts using Sendai virus. Extensive reactivation of the dormant erythrocyte nuclei‐determined by nuclear swelling, nucleoli formation and incorporation of 3H‐uridine into RNA‐was observed upon fusion to the three types of cytoplasts. By all of these criteria, reactivation was greatest when nuclei were introduced into G1 cytoplasts. Similar experiments in which DNA synthesis was measured showed that both G1 and G2 cytoplasts almost completely lacked the ability to induce DNA synthesis in the chick nuclei. But at least 26% of the erythrocyte nuclei within S cytoplasts exhibited significant levels of DNA synthesis. Cytoplasts prepared from cells three hours after entry into the S phase, which is six to seven hours in length in L929 cells, possessed the maximum capacity to induce DNA replication in chick nuclei. As many as 60% of the nuclei introduced into 3‐hour S phase cytoplasts synthesized significant quantities of DNA. The effect of cycloheximide, an inhibitor of protein synthesis, on erythrocyte nuclear reactivation within G1 phase cytoplasts was also investigated. Protein synthesis was necessary to maximize the reactivation of erythrocyte nuclei. However, it appeared that protein synthesis occurring prior to enucleation was more important in the reactivation process than synthesis occurring following enucleation.