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

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.