Heat-inactivated murine cytomegalovirus (MCMV) stimulates cellular DNA synthesis in WME, NMG, 3T3, Wg1A, chick and NIK-8 cells, but active or u.v.-irradiated MCMV does not. The stimulation of DNA synthesis in NIL-8 and chick cells was studied in detail. We found that both the nuclear and the mitochondrial DNA synthesis were stimulated in these cells. There was no virus DNA synthesis during the period we studied (48 h). The stimulation of nuclear DNA synthesis was about threefold in NIL-8 and 2.5-fold in chick cells as measured by the rate of incorporation of H-thymidine (H-dThd) in the CsCl fractions which banded at the density of cell DNA. The stimulation was about 9.5-fold in NIL-8 and 1.7-fold in chick cells as detected by autoradiography. There was a 3-fold and a 2.2-fold increase in the degree of incorporation of H-dThd into mitochondrial DNA of NIL-8 and chick cells, respectively. The amount of mitochondrial DNA obtained in infected cells of both kinds was about twice that in control cells. The synthesis of mitochondrial DNA was also stimulated by a factor of 2 in the thymidine kinaseless 3T3 cells which incorporate exogenous thymidine exclusively into mitochondrial DNA. There were no MCMV specific antigens detectable by immunofluorescence 5 h after infection, but diffuse nuclear fluorescence could be demonstrated 24 h after infection. Our results indicate that the heat-inactivated virus penetrates the cells, stimulates host DNA synthesis and induces synthesis of early MCMV antigens.


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