Analysis of the thymidine kinase of herpes simplex virus type 1 isolate that exhibits resistance to (E)-5-(2-bromovinyl)-2-deoxyuridine

The mechanism responsible for the decreased sensitivity of a clinical herpes implex virus type 1 (HSV-1) isolate, HSV- 145, to (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) was examined. Measurements of 50 % in-hibitory 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 sig-nificantly less than that normally observed for HSV-1. Analysis of the thymidylate kinase (TMP-K) activity of HSV-145 thymidine kinase (TK) demonstrated a de-creased 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-7. When the nucleotide sequence of the HSV-145 TK gene was compared to that of the HSV-1 strains CI(101) and SC16 a single nucleotide sub-stitution (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. Col-lectively, 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 sen-sitivity. The thymidine kinase (TK) of herpes simplex virus (HSV) is a multi-substrate enzyme (Jamieson et al., 1974) which possesses both TK and thymidylate kinase (TMP-K) activities (Chen & Prusoff, 1978). Activation of various anti-HSV nucleoside analogues by HSV T