Deletions in the cloned thymidine kinase (TK) gene of herpes simplex virus type 1 (HSV-1), strain 17 , were produced by two methods. Removal of a 506 base pair fragment from between the unique I and II restriction endonuclease sites of pTK1 (HSV-1 HI cloned in pAT153) and subsequent transformation of resulted in the isolation of 50 deleted plasmids. Sequential digestion of pTK1 with II and nuclease BAL 31 followed by ligation and recleavage with II resulted in the isolation of 31 deleted plasmids. Three clones, pTK2, pTK3 and pTK4, obtained following II and I treatment of pTK1 were recombined with wild-type (wt) HSV-1 (17) DNA in baby hamster kidney (BHK) cells to produce TK deletion mutants HSV-1 (17) TK 1301, HSV-1 (17) TK 1302 and HSV-1 (17) TK 1303 respectively. 5-Bromo-2′-deoxyuridine, 5-bromo-2′-deoxycytidine and 9-(2-hydroxyethoxymethyl)guanine were used to reduce the background of TK virus in heterogeneous recombinant stocks analysed for the presence of TK recombinants. All recombinant clones isolated produced a small syncytial plaque morphology in BHK cells. The mutants HSV-1 (17) TK 1301 and HSV-1 (17) TK 1302 were TK, failed to produce polypeptides of molecular weights 43000 and 19000 found in wt-infected cells and demonstrated one-step growth curves different from wt virus and the TK mutant HSV-1 (17) dPyk. Superinfection studies with HSV-1 (17) TK 1301, HSV-1 (17) TK 1302, HSV-1 (MDK) and HSV-1 (17) dPyk indicated that all TK mutants except dPyK produce a -acting gene product which can switch on the transforming HSV-1 TK gene.


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