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5 The Lytic Cycle of Polyoma Virus and SV40


Polyoma virus and SV40 can give rise to either lytic (productive) infections or incomplete (abortive) infections, depending on the type of cell which is infected. During both types of response the virus adsorbs to and penetrates the cell and is uncoated. During productive infections the viral genome, once liberated from its capsid, is transcribed and translated in the permissive environment; T antigen and the synthesis of cellular enzymes involved in DNA metabolism are induced. The viral DNA is replicated and the synthesis of cellular DNA is initiated about 12 hours after infection. Progeny virus particles are assembled from the newly made genomes, viral capsid protein and the other structural proteins of the virion. The infected cells do not divide and by 36 hours after infection, they begin to die, liberating virus. Figure 5.1 shows the time course of these events during the productive infection of primary cultures of monkey kidney cells by SV40 (Hatanaka and Dulbecco, 1966); productive polyoma virus infections of mouse cells (Dulbecco et al., 1965; Sabin and Koch, 1964) are very similar, but progeny polyoma virus particles are usually matured and released sooner than progeny SV40.

By contrast during incomplete infections of nonpermissive cells only part of the viral genome appears to be expressed; cellular enzymes involved in DNA synthesis, T antigen and cellular DNA synthesis are all induced, but little or no capsid antigen, viral DNA or progeny virus are made. The cells transiently exhibit some of the characteristics of transformed cells, but many revert to...

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