Evidence that deletion of coding sequences in the 5' end of the thymidine kinase gene of herpes simplex virus type 1 affects the stability of the gene products.

The experiments described in the present work were designed to study the function of the N-terminal end of thymidine kinase (TK) encoded by herpes simplex virus type 1. Specifically we were interested to know whether this end was involved in binding of the enzyme to other molecules, had any influence on its subcellular localization or affected one or more of the activities associated with the enzyme. A parental enzyme and a deletion mutant, lacking the 45 N-terminal amino acids, derived from this strain, were used. Thymidine kinase from the parental virus bound to DNA-Sepharose, but the truncated enzyme did not. This was apparently not due to a specific ability to bind to DNA, since immunofluorescence studies indicated that both the normal and the deleted TK were mainly located in the cytoplasm, preferentially in the perinuclear region. Phosphorylation of thymidine as well as the amounts of TK polypeptides were markedly reduced at late times after infection with the mutant, but not to the same extent after infection with the wild-type. The deleted TK gene was efficiently transcribed as shown by hybridization of RNA to a probe specific for the gene, and this RNA directed the synthesis in vitro of TK polypeptides. Deletion of the 5' end of the gene seems to affect the stability of either the enzyme or TK-specific mRNA, or both. The TMP phosphorylating activity seems to be particularly destabilized relative to the thymidine phosphorylating activity.