Physical and chemical agents which cause DNA strand breakage enhance infection by DNA and RNA oncogenic viruses, presumably by facilitating the incorporation of viral genetic information into the host cell genome. X-irradiation has now been shown to potentiate infectious center formation by radiation leukemia virus (RadLV*). The potentiation was dose-dependent with a peak in the range of 300-450 rads. Doses in this range enhanced infectivity by a factor of about 1.5 to 2. X-irradiation also enhanced the infection of cells phenotypically nonpermissive for murine leukemia virus infection, but did not alter the characteristic 2-hit kinetics observed in such cells. Fractionation of the X-ray exposure into two doses separated by varying time intervals showed that the potentiation persisted up to 30 hr postinfection. The initial dose fraction, given at the time of infection, caused partial synchrony of the host cell population. A sharp peak of enhancement occurred when cells received the second dose fraction at a time when they had just completed one round of DNA synthesis and mitosis.
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