Shope fibroma virus. II. Role of the virion-associated nucleases.

The effect of Shope fibroma virus (SFV) infection on host DNA synthesis was investigated. The cytocidal strain, SFV-I, inhibited the incorporation of [3H]thymidine into nuclear DNA very shortly (2 h) after infection, whereas the noncytocidal strain, SFV-W, did so later (10 h postinfection) and to a lesser extent. Furthermore, a two- to threefold stimulation of host DNA synthesis was recorded in SFV-W-infected cells 3 to 4 h after infection. Since virion-associated nucleases have been implicated in the shutoff of host synthesis, these and other enzymatic activities were measured in purified virion preparations. The SFV strains and vaccinia virus contained equivalent amounts of DNA-dependent RNA polymerase, ATPase, and protein kinase activities. However, in SFV-W the pH 4.5 exonuclease activity was lower than in SFV-I and vaccinia virus, and the level of pH 7.8 endonuclease was almost undetectable. To test whether the lack of endonucleolytic activity had some effect on the removal of the cross-links in the parental DNA that occurs after viral penetration, the fate of the virion SFV DNA was followed. The majority (80%) of the SFV-I and SFV-W DNA molecules extracted after viral adsorption sedimented in alkaline sucrose gradients as cross-linked. After 3 h of infection, 75% of the SFV-I DNA molecules lacked cross-links, whereas 78% of the SFV-W DNA still remained cross-linked. The same results were obtained when the presence of cross-links was tested in restriction fragments. Taken together, these results indicate that virion-associated nucleases are involved in the early shutoff of host DNA synthesis and in the elimination of cross-links from the parental viral DNA.