Herpes simplex virus as a source of thymidine kinase for thymidine kinase-deficient mouse cells: suppression and reactivation of the viral enzyme.

Thymidine kinase (EC 2.7.1.21)-deficient mouse cells were infected with inactivated herpes simplex virus, after which "transformed" cells that produce viral thymidine kinase were isolated. Shortly after transformation, the expression of the viral enzyme could be suppressed and reactivated with high efficiency. On continued multiplication in nonselective medium, the proportion of cells producing the viral enzyme decayed exponentially. This decay seemed to represent a change in the expression of the viral gene for thymidine kinase rather than the loss of the gene from the cells, since the viral enzyme could be apparently reactivated in every cell, albeit at a very low frequency.