Replication of herpesvirus DNA II. Sedimentation characteristics of newly synthesized DNA

Abstract

The sedimentation behavior of viral DNA synthesized during short labeling periods at various times after infection was investigated. These experiments indicated that viral DNA synthesis may be divided into the following two phases: (1) During the early phase, newly synthesized DNA is associated with structures which sediment with S values up to approximately twice that of mature viral DNA; (2) during the later phase, newly synthesized DNA is associated with structures that sediment much more rapidly. Both at early and later times after infection, approximately 20% of the newly synthesized DNA sediments in sucrose gradients more slowly than does mature viral DNA. Furthermore, after isopycnic centrifugation in CsCl, most of the newly synthesized viral DNA sediments in sucrose gradients more slowly than does mature viral DNA. The smaller than unit-size, newly synthesized viral DNA molecules are breakage products resulting from the fragility of newly synthesized DNA. These molecules fragment, not only because of their fragility in the regions of the replicative forks but also because of the presence of fragile sites at other positions along the newly synthesized DNA molecule. Experiments dealing with the transfer of parental DNA to progeny virions show that most parental viral DNA strands that are transferred to progeny virions retain their integrity. Breakage and reunion of parental viral DNA strands with progeny DNA is a relatively rare event.