Transfer of Thymidine Kinase to Thymidine Kinaseless L Cells by Infection with Ultraviolet-Irradiated Herpes Simplex Virus

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Abstract
L cells lacking thymidine kinase (TK) activity (Ltk− cells) have been stably transformed to a TK-positive phenotype by infection with ultraviolet-irradiated herpes simplex virus (HSV-UV). The highest frequency of the Ltk− to Ltk+ transformation observed in these experiments was approximately 10−3, whereas no measurable transformation was observed (less than 10−8) in the absence of HSV-UV infection. Cell lines of HSV-transformed Ltk+ cell lines contain 7 to 24 times as much TK activity as do the parental Ltk− cells, and they have been maintained in culture for a period exceeding 8 months. The kinetics of thermal inactivation of the TK activity derived from an Ltk+ HSV-transformed cell line and the TK activity from Ltk− cells lytically infected with infectious HSV are similar. Both of these TK activities are much more thermolabile than the TK activity present in wild-type L cells. A mutant strain of HSV which does not induce TK activity during lytic infection does not cause the Ltk− to Ltk+ transformation. These data suggest that either an HSV TK gene has been transferred to Ltk− cells or that an HSV gene product has caused the expression of a previously repressed cellular enzyme.