The mechanism responsible for the decreased sensitivity of a clinical herpes simplex virus type 1 (HSV-1) isolate, HSV-145, to (E)-5-(2-bromovinyl)-2′-deoxyuridine (BVDU) was examined. Measurements of 50% inhibitory doses of several drugs demonstrated that although HSV-145 was sensitive to phosphonoacetic acid, adenine arabinoside and acyclovir, its sensitivity to BVDU and 5-(2-chloroethyl)-2′-deoxyuridine was significantly less than that normally observed for HSV-1. Analysis of the thymidylate kinase (TMP-K) activity of HSV-145 thymidine kinase (TK) demonstrated a decreased level of TMP-K activity when compared to HSV-1 TK. The TMP-K activity of HSV-145 resembled that observed for HSV-2 and the TK-deficient strain HSV-1 TK. When the nucleotide sequence of the HSV-145 TK gene was compared to that of the HSV-1 strains C1(101) and SC16 a single nucleotide substitution (G changed to A at base position 502) was detected which would result in the substitution of threonine at amino acid position 168 for alanine. The substitution is the same as that for the laboratory-derived BVDU-resistant virus HSV-1 SC16B3. Collectively, these studies highlight the importance of amino acid conservation at position 168 of the HSV-1 TK in conferring efficient TMP-K activity and BVDU sensitivity.


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