When HeLa cells were pre-labelled with tritiated thymidine and infected with the or strain of vaccinia virus, the host-cell DNA was broken down and reutilized for virus particle DNA biosynthesis. This phenomenon did not occur in LM cells, and was not explained by pseudovirus particle formation. Utilization of host-cell DNA, pre-labelled with [C]-thymidine did not occur. Therefore, breakdown and re-utilization of host-cell DNA in HeLa cells resulted from intranuclear irradiation of the cells. Under these circumstances there was a markedly increased activity in the cells of an exonuclease with a pH optimum of 9.2, a requirement for magnesium ion, and which used denatured DNA as a substrate. The activity of this enzyme in such cells was directly proportional to the amount of irradiation, measured as the specific activity of the pre-labelled host-cell DNA. Associated with the vaccinia virus particles obtained from intranuclearly irradiated HeLa cells was an alkaline DNase inhibited by magnesium ion.


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