The frequency of sister chromatid exchange (SCE) was determined in rat fibroblasts transformed by wild-type polyoma virus or by a mutant temperature sensitive for viral large tumor antigen function (ts-a). Elevated SCE frequencies were observed in two wild-type transformed cell lines growing at 37° and in four ts-a-transformed lines upon growth at the permissive temperature for large viral tumor antigen (33°). The increase in SCE frequency in ts-a-transformed cells at 33° was reversed by growth at 39° (nonpermissive for T-antigen function). An increase in SCE at 33° was not observed in untransformed cells or in a ts-a-transformed cell line which makes a defective large viral tumor antigen. These results suggest that large viral tumor antigen can induce SCEs. Since large viral tumor antigen is also responsible for amplification of integrated viral DNA sequences (4), we tried to correlate this phenomenon with the increased SCE frequency. However, increasing SCE artificially by growing cells in the presence of 12-O-tetradecanoylphorbol-13-acetate did not result in amplification of integrated viral DNA in the absence of large viral tumor antigen function. Thus, there is no simple causal relationship between increased SCE and amplification.

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Supported by USPHS Grant CA 26070 from the National Cancer Institute.

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