Constancy of the shift‐up point in two temperature‐sensitive mammalian cell lines that arrest in G1

Abstract

Two cell cycle‐specific temperature sensitive (ts) mutants of mammalian cell lines, AF8 and K12, are known to arrest in G₁ when shifted to the non‐permissive temperature. We have determined the entry into S of both AF8 and K12 cells in five different growth conditions, namely: (1) quiescent sparse cultures stimulated to proliferate by serum; (2) quiescent dense cultures stimulated by serum; (3) quiescent sparse cultures stimulated by trypsinization and replating; (4) quiescent, dense cultures stimulated by trypsinization and replating; and (5) mitotic cells collected by mitotic detachment. In addition, for each cell line and for each different growth condition, we have determined the shift‐up time, i.e., the time at which a shift‐up to the nonpermissive temperature no longer prevents the entry of cells into S. In no case did K12 or AF8 enter S at the nonpermissive temperature. At the permissive temperature, the average time of entry into S varied in different growth conditions, and so did the shift‐up time. However, in both cell lines, the distance of the average shift‐up time from the average time of entry into S was remarkably constant, regardless of the growth conditions, i.e., 1.8 hours in K12 and 8.6 hours in AF8.