CYTOPLASMIC REGULATION OF 2 G1-SPECIFIC TEMPERATURE-SENSITIVE FUNCTIONS

tsAF8 and ts13 cells are temperature-sensitive (ts) mutants of BHK cells that specifically arrest, at nonpermissive temperature, in the G1 phase of the cell cycle. These two mutants can complement each other. Both cell lines can be made quiescent by serum deprivation (Go). When subsequently stimulated by serum, they can enter S phase at 34°C but not at 39.5°-40.6°C. We have used these mutants to determine whether the nucleus is needed during the Go→S transition for the expression of the G1 ts functions. For this purpose, we fused cytoplasts of Go-tsAF8 with whole ts13 cells in Go, and cytoplasts of Go-ts13 with whole tsAF8 cells in Go. Serum stimulation at the nonpermissive temperature induced DNA synthesis in both types of such fusion products. No DNA synthesis was induced by serum stimulation at the nonpermissive temperature in fusion products constructed between either Go-tsAF8 cytoplasts and whole Go-tsAF8 cells or Go-ts13 cytoplasts and whole Go-ts13 cells. These results demonstrate that the information for these two ts functions, which are required for entry of serum-stimulated cells into the S phase, are already present in the cytoplasm of Go cells-that is, before serum stimulation commits them to the transition from the nonproliferating to the proliferating state. © 1979.