Infection of a chicken cell with avian sarcoma virus requires division of the infected cell before synthesis of infectious progeny is initiated. This requirement for a cell division for the complete expression of avian sarcoma virus has been examined further with chicken embryo fibroblasts infected with two distinct viruses. Chicken cells infected with and producing a mutant of Rous sarcoma virus temperature sensitive for transfornation (tsLA24PR-A) were arrested in Go by depletion of serum

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... ors from growth medium. These stationary cells continued to produce infectious progeny in the absence of further cell division. Superinfection of the stationary cells with the wild-type Prague strain of Rous sarcoma virus (PR-RSV-C) produced a stable double infection in these cells. Progeny of the superinfecting PR-RSV-C, however, were not detected until these cells underwent division after stimulation with fresh serum-containing medium. The addition of colchicine to these serum-stimulated cells, although not affecting production of the tsLA24PR-A, inhibited the appearance of progeny of the superinfecting PR-RSV-C. These experiments indicate that each avian sarcoma virus infection of a chicken embryo fibroblast requires division of the infected cell for production of that virus regardless of whether or not the cell is already producing a similar virus. The results suggest, therefore, that the requirement for a cell division represents a requirement for an event that controls virus expression in a "cis-acting" fashion specific for the provirus. The replication cycle of viruses belonging to the avian leukosis-sarcoma virus complex involves a DNA intermediate, the provirus (1, 12). Full expression of these viruses requires the complete synthesis of this DNA intermediate and its transcription into viral RNA. It has previously been demonstrated, using chicken embryo fibroblasts (CEF) synchronized by the depletion of specific serum factors from growth medium, that both the initiation of viral RNA synthesis and the production of viral progeny require division of the newly infected cells (8, 9) . The addition of colchicine to infected cells after the addition of serum-containing medium inhibits the appearance of new progeny virus. The requirement for division of the infected cell for production of avian leukosis-sarcoma virus progeny has been called activation (13). Recent studies have demonstrated that, whereas proviral DNA synthesis is initiated in stationary cells (2), the newly synthesized DNA is incomplete (6, 15). Analysis of minus-and plus-strand viral DNA shows that both the amount of DNA synthesis and the length of the DNA strand synthesized are reduced in stationary cells when compared with dividing cells. It has also been demonstrated that synthesis and integration ofthe provirus require host cell func-tions associated with serum stimulation of the infected cell (15). Recent experiments using synchronized cells have shown that both synthesis and integration of proviral DNA occur during Sphase cellular DNA synthesis (E. H. Humphries, C. Glover, and M. E. Reichmann, Proc. Natl. Acad. Sci. U.S.A., in press). This study further demonstrated that, whereas colchicine inhibits the synthesis of progeny virus, synthesis and integration of the provirus are unaffected. It appears, therefore, that avian leukosis-sarcoma virus infection can be inhibited in two ways. First, the proviral DNA is not completely synthesized in stationary cells. Second, although serum stimulation is required for the complete synthesis and integration of the provirus, the integrated provirus is not expressed until after the first cell division. The present experiments were carried out to determine whether the control on the expression of the provirus that requires cellular division, that is, the requirement for an event that is blocked by treatment of cells with colchicine, is specific for each provirus. The results demonstrate that, although a cell is producing one virus, the requirement for a subsequent cell division to activate the provirus formed by a superinfecting virus is not eliminated. These ex-721 on May 8, 2020 by guest