Virus-specific RNA and DNA in nuclei of cells infected with fowlpox virus

Poxviruses appear to be assembled exclusively in the cytoplasm of infected cells, and it has been generally held that all replicative events are restricted to the cytoplasm. Previous work in this laboratory has shown that fowlpox virus (FPV) stimulates DNA synthesis both in vivo and in vitro, which, together with the ability of FPV to cause hyperplasia in vivo, suggests that the host cell nucleus is affected during FPV infection. To investigate this possibility, FPV-infected cells were pulse labeled with either [ 14C]thymidine or [ 14C]uridine at various times after infection, harvested, and separated into nuclear and cytoplasmic fractions. By hybridization techniques, both viral RNA and DNA were detected in the nucleus of infected cells from 24 to 72 hr after infection. Less than 10% of the total labeled nuclear DNA is viral specific at 24 hr after infection, but by 48–72 hr, more than 50% of the newly synthesized nuclear DNA is viral specific. The newly synthesized nuclear viral-specific DNA represents more than half of the total labeled viral-specific DNA found in the infected cell at all times studied. In contrast, the viral-specific RNA located in the nucleus always comprises the minor fraction of the total FPV-specific RNA. At 24 hr after infection, approximately 10% of the newly synthesized viral-specific RNA was located in the nucleus, while at 72 hr, about one-third of the total labeled viral-specific RNA could be found in the nuclear fraction. FPV infection does not result in the rapid cessation of host cell nucleic acid synthesis. The amount of ribosomal RNA synthesized in FPV-infected cells from 30 to 48 hr p.i. was found to be only slightly less than in mock-infected cells. Significant host cell DNA synthesis could also be demonstrated until approximately 48 hr after infection.