A temperature-sensitive mammalian cell cycle mutant with blocks in G1 and G2 phases of the cell cycle has been isolated in culture. When shifted from the permissive temperature of 33‡ C to the nonpermissive temperature of 39‡ C, the fraction of cells initiating DNA synthesis as well as the fraction of cells entering mitosis decreased rapidly. Combined cytophotometric and autoradiographic analysis on the cells at 39‡ C showed that G1 cells, with the exception of those in late G1, were arrested in that phase. Cells in S phase at the time of temperature shift, together with the late G1 cells which subsequently entered S, continued through S into G2, but were blocked in that phase of the cell cycle and unable to initiate mitosis. Those cells already in mitosis completed cell division at 39‡ C. The G1 block point of ts-550C was found to be located after the serum starvation and isoleucine deprivation arrest points, approximately 3 h before initiation of DNA synthesis.
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