Ultraviolet-Irradiated Herpes Simplex Virus

L cells lacking thymidine kinase (TK) activity (Ltkcells) have been stably transformed to a TK-positive phenotype by infection with ultraviolet-irradiated herpes simplex virus (HSV-UV). The highest frequency of the Ltkto 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 Ltkcells, 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 Ltkc 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 Ltkto Ltk+ transformation. These data suggest that either an HSV TK gene has been transferred to Ltkcells or that an HSV gene product has caused the expression of a previously repressed cellular enzyme.