We have isolated a mutant clone from mouse FM3A cells with temperature-sensitive defects both in cytokinesis and in thymidine kinase enzyme activity. The clone, designated tsCl.B59, was isolated after mutagenesis at 33°C followed by exposure to cytosine arabinoside at 39°C. It was derived from a thymidine kinase deficient, 5-bromodeoxyuridine-resistant clone (S-BUCl.42) which was originally derived from wild-type clone H-5 of FM3A cells. The temperature-sensitive mutant clone grows normally at 33°C, but not at 39°C, where it exhibits an increased frequency of multinucleate cells due to defective cytokinesis. Unlike the parental S-BUCl.42 cells, which have negligible thymidine kinase activity and are unable to incorporate3H-thymidine, the mutant incorporates substantial amounts of3H-thymidine at 33°C, although its thymidine kinase activity remains lower than that of wild-type H-5 cells. When cultures of tsCl.B59 cells are transferred to 39°C, incorporation of3H-thymidine decreases markedly. The decrease has been shown to be due to thermolability of the thymidine kinase in tsCl.B59 cells.
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