Arehiv fiir die gesamte Virusforschung 38, 306--318 (1972) 9 by Springer-Verlag 1972 The Synthesis of Viral- and Cellular-DNA in Mammalian Cells Exposed to Polyoma Virus By JAMES B. HUDSON, LOR•E A. BABIUK, LARRY M. KOHSE, THOMAS S. WONG, a n d CHARLOTTEYOSHIZAWA Department of Microbiology, University of British Columbia, Vancouver, Canada With 3 Figures Received February 11, 1972 Summary Various mammalian cell cultures were examined for their ability to support polyoma virus DNA replication, and for induction of cellular DNA synthesis, as a result of infection by polyoma virus. Viral DNA synthesis was measured by the technique of DNA-DNA hybridization which allowed its detection at a level of 0.2 ~ of that occurring during optimum viral DNA replication in mouse cells. All those cultures which failed to produce significant yields of infectious virus were also negative for viral DNA synthesis, indicating that restriction operates at an early stage in the infection cycle; and also were not induced to synthesize cellular DNA. The only exception was the BHK-21 cell line, in which polyoma virus infection stimulated the incorporation of labelled thymidine into DNA, in the absence of detectable viral DNA synthesis. Polyoma transformed hamster cells, known to harbor viral genetic information, were not induced to synthesize viral DNA following a variety of "rescue" attempts, in contrast to cultures of SV 40 (Simian virus 40) -- transformed cells in which SV 40 DNA replication could be measured after analogous treatments. Thus in all of the non-murine cultures examined, polyoma virus gene expression was restricted at a stage prior to viral DNA replication. 1. Introduction Polyoma virus appears to have a very narrow host range. Significant multipli- cation only occurs in freshly isolated mouse cells and a few continuous line cultures of mouse origin, with occasional reports of multiplication in hamster cells [reviewed by EDDY (10)]. Nevertheless many rodent cell types can be transformed by polyoma, and cells of mouse, hamster, and human origin can take up the viial DNA into their nuclei [BouRGATJX (4)]. We were interested in learning more about this apparent restriction. We wanted to determine to what extent the viral genome is active in different types of cell, or whether there is a common level at which viral gene expression is restricted. We therefore decided to examine a variety of mum-