Changes in parental vaccinia virus DNA after viral penetration into cells

The structure of vaccinia virus DNA, extracted from purified virions or from the cytoplasmic fraction of virus-infected cells, was studied by sensitivity to exonucleases, terminal labeling, and cleavage by restriction endonucleases. The results indicate that virion DNA and parental DNA extracted at 0 time infection do not possess free termini. When virion DNA and parental DNA extracted at 0 time infection were digested with HindIII and the fragments denatured and further treated with N. crassa endonuclease, it was observed that the terminal fragments containing the crosslinks were present. By contrast, parental DNA extracted after 90 min of infection was sensitive to exonucleases and to terminal labeling. Furthermore, when parental DNA extracted after 90 min of infection was digested with HindIII and the denatured fragments treated with N. crassa endonuclease, no terminal fragments were observed. These results support previous findings indicating that crosslinks are removed from parental DNA after viral penetration into cells.