Dihydrofolate reductase (FH2 reductase) activity increased following infection of mouse kidney cell cultures with polyoma virus or monkey kidney cell cultures with SV40. The increase was 1st detected about 15–24 hr after infection, and at 30 hr after infection the enzyme activity was about 3-fold greater than that of noninfected cells. Thymidine kinase activity also increased at about 15–24 hr after SV40 and polyoma virus infection, but to a value 10-fold or more greater than noninfected cell cultures. FH2 reductase did not increase after infection of mouse fibroblast, mouse embryo, or monkey kidney cells with vaccinia or herpes simplex viruses, although thymidine kinase activity was induced by the latter viruses.

Neither 5-bromodeoxyuridine nor 1-β-d-arabinofuranosylcytosine inhibited the induction of FH2 reductase activity by SV40 or polyoma virus. However, the addition of actinomycin D at 2 hr after SV40 infection of CV-1 cells inhibited the increase in FH2 reductase and thymidine kinase normally observed at 41 hr after infection. Addition of actinomycin D at 19 hr after SV40 infection had little or no effect on the levels of either enzyme.

Treatment of SV40-infected CV-1 cells with puromycin 17–41 hr after virus infection almost completely stopped the increase in FH2 reductase and thymidine kinase activities normally observed at 41 hr. If puromycin was removed, FH2 reductase and thymidine kinase activities were induced over the next 28 hr.

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Aided by grants-in-aid from the American Cancer Society (E-291), the National Science Foundation (GB-3126), and by USPHS Research Grants (CA-06656, 1-K6-AI-2352, and 1-K3-CA-25,797).

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