A Temperature-Sensitive Mutant of CHO-Ki Cell Arrested in the G2 or Late S Phase

A new type of temperature-sensitive mutant, 11C3 has been isolated from CHO-K₁cell and partially characterized. At a nonpermissive temperature the number of mutant cells hardly increases. Incubating mutant cells first at a nonpermissive temperature (39°C) for 24 h reduces the plating efficiency to one third that found without prior incubation. DNA synthesis slows down gradually for 30 h at 39°C, but protein synthesis does not decrease until 48 h after the shift up. Analysis of the DNA content of the individual cells shows that the proportion of cells containing the 4C amount of DNA markedly increases in 28 h after the temperature is shifted up. Light microscopy shows that mitotic figures decrease during this period and that the cells become large and flat with a single large nucleus. Experiments on synchronized cell populations suggest that although the temperature sensitive period exists in the S-phase, cells are arrested near the end of the S-phase or in the G₂-phase. The colchicine binding assay revealed that the colchicine binding protein in mutant cells increased during incubation at a nonpermissive temperature. This supports the results described above.