Studies on temperature-sensitive mutants of Chinese hamster ovary cells affected in DNA synthesis

DNA synthesis in two mutants of Chinese hamster overy cells, ts 13A and ts 15C, which were temperature sensitive for growth, was found to be shut off rapidly at the nonpermissive temperature. The mutants did not complement each other and the ts lesion was not located on the X chromosome. Both isolates were found to be considerably more sensitive to the alkylating agents, ethylmethanesulfonate (EMS) and methylmethanesulfonate (MMS), as compared to the parental cells, but showed normal sensitivity to UV irradiation. The mutants also showed interesting differences in their response to EMS-induced mutation frequencies at the ouabain-resistant and thioguanine-resistant loci. At high survival (50%) the frequencies of mutations at these genetic loci were markedly low in the ts mutants as compared to the parental cells. In ts+ revertants isolated from the mutants, the ts phenotype and the increased sensitivity to EMS and MMS were affected simultaneously, indicating that both these characteristics resulted from a single genetic lesion.