Replication of Mouse Cytomegalovirus in Thymidine Kinase‐Deficient Mouse Cells

Abstract

AbstractIn an attempt to determine whether mouse cytomegalovirus (MCMV) requires thymidine kinase (TK) for replication and whether it induces TK, TK‐deficient mouse cells were isolated and used as host cells for MCMV. Mutant cells resistant to 200 μg/ml of 5‐Bromodeoxyuridine (BUdR) were selected from SV40‐transformed mouse cells, mks‐A TU‐7, by propagating the cells in the presence of varying concentrations of BUdR graded by serial 2‐fold increments. The mutant cells, designated as TU‐7 BU, showed a very low TK activity (less than 1/20 that of mks‐A TU‐7). Herpes simplex virus type 1 (HSV‐1) replicated in starved as well as in unstarved TU‐7 BU, whereas MCMV could replicate only in growing TU‐7 BU and could not form plaques in monolayers of mks‐A TU‐7 or TU‐7 BU. HSV‐1 infection enhanced TK activity equally in both mks‐A TU‐7 and TU‐7 BU. In contrast, TK activity of MCMV‐infected mks‐A TU‐7 was lower than that of uninfected cells or cells inoculated with UV‐inactivated virus. In addition, TK activity of the MCMV‐infected TU‐7 BU remained minimal without showing any increase. The replication of HSV‐1 was completely inhibited in the presence of BUdR (10 μg/ml), whereas MCMV could replicate even in the presence of 50 μg/ml of BUdR. The results indicate that MCMV neither requires TK nor induces TK activity in the infected cells.