Effects of temperature on frog polyhedral cytoplasmic deoxyribovirus multiplication: Thermosensitivity of initiation, replication, and encapsidation of viral DNA

Abstract

The effects of temperature on frog polyhedral cytoplasmic deoxyribovirus DNA replication and on encapsidation of viral genomes into virions were examined in baby hamster kidney cells. A cell fractionation technique was used to distinguish two classes of viral DNA, replicating DNA and encapsidated DNA. At temperatures permissive for infectious virus production (28°), both classes of DNA were first demonstrable 2–4 hours after infection and they increased concomitantly for about 17 hours. Production of infectious virus paralleled encapsidation of viral genomes. Viral DNA replication was required throughout the virus growth cycle for continuous virion production. At temperatures nonpermissive for infectious virus production (33°–37°), a number of virus-specific thermosensitive functions were detected. At 33° viral DNA replicated but encapsidation was inhibited. At 34° or 37° no viral DNA was made; however, if viral DNA synthesis was initiated at 28°, it continued when infected cultures were shifted to 34° but not if shifted to 37°. At 34° replicated DNA did not become encapsidated. Thermosensitivity of viral DNA synthesis and encapsidation were reversed when cultures were shifted from a nonpermissive temperature of 34° or 37° to permissive temperature (28°). However, if viral DNA was made at 28° and then the infected cells were incubated at 37°, the preformed viral DNA was not encapsidated when cultures were shifted to 28°; de novo synthesis of viral DNA was necessary for subsequent encapsidation. The effect of temperature on virus-induced DNA polymerase activity also was studied at 28°, 34°, or 37°. The “early” temperature block at 34° for initiation of viral DNA synthesis occurred before virus gene expression for DNA polymerase activity. If viral DNA polymerase activity was induced at 28°, enzyme activity was maintained when infected cultures were shifted to 34° but not if shifted to 37°. The thermosensitivity of virus-induced DNA polymerase activity may be responsible for inhibition of viral DNA synthesis at 37°. These experiments indicate that depending on the incubation temperature, several virus-specific functions involved in some event(s) before initiation of viral DNA synthesis and in initiation, replication, and encapsidation of viral DNA are thermosensitive.