| Abstract: | Three different temperature sensitive mutants derived from the Syrian hamster cell line BHK 21 were found to have greatly reduced DNA synthesis at the non-permissive temperature. These mutants are distinct by complementation analysis and behave at the non-permissive temperature as cell cycle traverse defective mutants. Microfluorometric analysis of mutant populations arrested at the non-permissive temperature shows an accumulation of cells with G1 DNA content. Mutants ts 13 and ts HJ4 synchronized in G1 by serum or isoleucine deprivation and shifted to the non-permissive temperature at the time of release do not enter the S phase, while in the case of mutant ts 11 preincubation at the non-permissive temperature before release is required to completely prevent its entry into S. Ts 13 and ts 11 are able to traverse the S phase at the non-permissive temperature when synchronized at the boundary G1/S; in this case, preincubation of ts 11 at the non-permissive temperature before release does not affect the ability of these cells to perform DNA synthesis. On the other hand, ts HJ4 appears to traverse S only partially when tested under similar conditions. Temperature shift experiments of mutant populations at different times after isoleucine synchronization suggest that ts 13 and ts 11 are blocked at the non-permissive temperature in early G1, whereas ts HJ4 is probably affected near the initiation of DNA synthesis, or in some early S function. |
