Mapping Functional Domains in the Promoter Region of the Herpes Thymidine Kinase Gene

The cloned herpes simplex virus type 1 (HSV-1) thymidine kinase (TK; ATP:thymidine 5′-phosphotransferase, EC 2.7.1.21) gene can be used to transform TK-cells to a TK+phenotype. Transformants generated in this way express TK at a basal constitutive level that is inducible to a higher level by infection with TK-herpes virus. We have studied the effect of mutations generated in vitro on both the constitutive and virus-induced expression of TK in transformants. Four Xho I linker insertions and two deletions in the 5′untranscribed region of the cloned HSV-1 TK gene were generated in vitro. A deletion that removed all but nine base pairs of the 5′untranscribed region virtually eliminated constitutive expression and completely prevented induction by herpes virus infection. Two of the insertions have particularly interesting properties. One, nine base pairs upstream from the cap site, inactivates constitutive expression without stopping induction. The other, 50 base pairs upstream from the cap site has the opposite effect (i.e., normal constitutive expression but no induction). Analysis of these results leads us to propose that the 5′untranscribed region of the HSV-1 TK gene is quite complex with several functional domains having differential roles in the constitutive and herpes-induced expression of the TK gene.