Strains of varicella-zoster virus resistant to 1-p-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil (BV-araU) were isolated from varicella-zoster virus-infected Vero cells which were pulse-treated with 5-iododeoxyuridine or 5-bromodeoxyuridine or both and then treated with BV-araU. These BV-araU-resistant strains (BVaraUr) could not be isolated from varicella-zoster virus-infected cells treated with BV-araU alone and had reduced viral thymidine kinase activity. Two offive BV-araUr strains were also resistant to 5-icdodeoxyuridine and 5-bromodeoxyuridine, whereas other BV-araUr strains were relatively susceptible to these drugs.All clones from the BV-araUr strain were susceptible to 1-p-D-arabinofuranosylcytosine and phosphonoacetic acid, but 7 of 10 clones from the BV-araUr strain were resistant to 1-p-D-arabinofuranosyladenine. The possible mechanisms of induction of BV-araU resistance are discussed.Molecular analysis of the viral genome and its relation to viral replication has been extensively studied in herpes simplex virus (HSV), Epstein-Barr virus, and cytomegalovirus of the family Herpetoviridae (8,10,12,20,22,25,30). Dubbs ahd Kit (9) have reported a mutant strain of HSV that is deficient in viral thymidine kinase-inducing activity, and recently, many kinds of mutant strains of HSV have been reported (4,5,15,29).Until recently, the study of the molecular properties of varicella-zoster virus (VZV) has been greatly hampered due to problems in the isolation offree virions with high infectivity and in the purification of viral DNA. A few results on the susceptibility and resistance of VZV to anti-herpesvirus agents have been reported (7,26,32). Yokota et al. (32) have isolated a mutant strain of VZV, which was deficient in viral thymidine kinase-inducing activity, from cells persistently infected with VZV in the presence of 5-bromo-2'-deoxyuridine. Recently, Machida et al. (18,19) University, Osaka, Japan. The YS strain was isolated in our laboratory on HEL cells from vesicular fluid of a patient with varicella (H. Kawai, T. Sakuma, K. Sasaki, M. Sato, H. Watanaloe, M. Honda, S. Ozaki, and M. Azuma, manuscript in preparation). BV-araU-resistant (BV-araUr) strains YSR, 03, 07, 018, and 026 were obtained as follows. Vero cell sheets (30-by 50-mm bottle) were infected with the stocked YS-infected cells (cell-to-cell infection) at a multiplicity of 0.1. After adsorption for 1 h at room temperature, the cell sheets were refed with maintenance medium containing 10 to 20 ,ug of IUdR or BUdR or both per ml and incubated for 24 h at 37°C. The cell sheets were washed with phosphate-buffered saline solution (pH 7.4), replenished with fresh maintenance medium, and incubated for 48 h at 370C. Then, 0.1 to 1 jig of BV-araU (200 to 2,000 times the MIC for parent YS strain) per ml was added and cultivated until a cytopathogenic effect (CPE) was apparent. One or a few small plaque-like CPE was observed on the Vero cell sheets about 7 days after infection, and after an additional 2 or 3 days, the CPE was dominant. The virus-in...
