Derivation of TK- clones from revertant TK+ mammalian cells.

In order to obtain a large collection of Chinese hamster cell clones defective in thymidine kinase (TK(-)), BrdU(r) selection experiments have been performed on wild-type and revertant TK(+) cell lines. No clones (< 10(-9)) were obtained from the wild-type TK(+) cell line by single-step selection. In contrast, revertant TK(+) clones readily gave rise to stable TK(-) derivatives (1 - 2 x 10(-4)). Both wild-type and revertant TK(+) clones spontaneously yielded 8-AG(r) colonies with the same frequency (1 - 5 x 10(-6)), suggesting that the differences between wild-type and revertant cell lines specifically affected selection of the TK(-) phenotype. The increased frequency of TK(-) clones reflects perhaps the number (ploidy) or character of the autosomal TK loci in TK(+) revertants, or perhaps the mechanisms which regulate expression of the TK genes. Several mutagens, EMS, MNNG and UV, stimulated the TK(+) revertants' frequency of TK(-) subclones only slightly (< 3-fold). Biochemical and genetic data indicated that the TK(-) clones derived from one revertant are phenotypically different. The phenotypes displayed by these cell lines are stable and do not depend upon the continued presence of the selective agent.