Mouse lymphoma 388 cells, growing in vitro, are inhibited in their growth rate by glucocorticoids. Studies with whole cells incorporating thymidine into DNA, uridine into RNA and leucine into protein suggested an early (12 hr) depression of thymidine incorporation, with less effect on uridine or leucine. DNA and RNA polymerase activities in these cells were studied more directly by examining a broken cell system. Washed nuclei isolated from ML-388 cells incorporate nucleoside and deoxynucleoside triphosphates into RNA and DNA. Using a washed nuclear system, we have shown that glucocorticoids have no direct effect on either RNA or DNA polymerase activity. Nuclei taken from cells exposed to glucocorticoids for 12 or 24 hr show no consistent differences when compared to control preparations with respect to their ability to synthesize DNA. RNA synthesis is depressed about 30 per cent in nuclei taken from steroid-treated cells.
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