Herpes Simplex Virus Type 1 TK Research Articles

Human Mitochondrial Thymidine Kinase Is Selectively Inhibited by 3′-Thiourea Derivatives of β-Thymidine: Identification of Residues Crucial for Both Inhibition and Catalytic Activity

Substituted 3'-thiourea derivatives of beta-thymidine (dThd) and 5'-thiourea derivatives of alpha-dThd have been evaluated for their inhibitory activity against recombinant human cytosolic dThd kinase-1 (TK-1), human mitochondrial TK-2, herpes simplex virus type 1 (HSV-1) TK, and varicella-zoster virus TK. Several substituted 3'-thiourea derivatives of beta-dThd proved highly inhibitory to and selective for TK-2 (IC(50) value, 0.15-3.1 microM). The 3'-C-branched p-methylphenyl (compound 1) and 3-CF(3)-4-Cl-phenyl (compound 7) thiourea derivatives of beta-dThd showed competitive inhibition of TK-2 when dThd was used as the variable substrate (K(i) values, 0.40 and 0.05 microM, respectively), but uncompetitive inhibition in the presence of variable concentrations of ATP (K(i) values, 15 and 2.0 microM, respectively). These kinetic properties of compounds 1 and 7 against TK-2 could be accounted for by molecular modeling showing that two hydrogen bonds can be formed between the thiourea nitrogens of compound 7 and the oxygens of the gamma-phosphate of ATP. The importance of several active-site residues was assessed by site-directed mutagenesis experiments on TK-2 and the related HSV-1 TK. The low K(i)/K(m) ratios for compounds 1 and 7 (0.38 and 0.039 against dThd, and 0.75 and 0.12 against ATP, respectively) indicate that these compounds are among the most potent inhibitors of TK-2 described so far. In addition, a striking close correlation was found between the inhibitory activities of the test compounds against TK-2 and Mycobacterium tuberculosis thymidylate kinase that is strongly indicative of close structural and/or functional similarities between both enzymes in relation to their mode of interaction with these nucleoside analog inhibitors.

Functional analysis of transcriptional regulation of herpes simplex virus type 1 tegument protein VP22

The herpes simplex virus type 1 (HSV-1) tegument proteins have important functions in the viral replication process. In order to investigate the role of the HSV-1 tegument protein VP22 in viral replication, its transcriptional regulation of viral promoters was investigated using the chloramphenicol acetyltransferase (CAT) assay. The results indicate that VP22 exerts a dose-dependent transcriptional inhibitory effect on the HSV-1 alpha4, TK, and gC gene promoters. VP22 had the capacity to repress transcriptional activation of promoters via different viral transcription regulatory factors such as VP16 and ICP0, as evidenced by the specific repression of the TK and gC gene promoters by ICP0. In addition, VP22 was capable of inhibiting the promotion of ICP0 transcriptional activation in the presence of HAT PCAF, which is even more remarkable than the VP22 repression of ICP0 transcriptional activation. Finally, the transcriptional inhibitory effect of VP22 on other viral promoters was demonstrated by the analysis of beta-galactosidase activities in internal controls.

Development and validation of a capillary electrophoresis method for the characterization of herpes simplex virus type 1 (HSV-1) thymidine kinase substrates and inhibitors

A fast, convenient capillary electrophoresis (CE) method was developed for monitoring the enzymatic reaction of herpes simplex virus type 1 thymidine kinase (HSV-1 TK). The reaction was performed in a test tube followed by quantitative analysis of the products. The optimized CE conditions were as follows: polyacrylamide-coated capillary (20 cm effective length × 50 μm), electrokinetic injection for 30 s, 50 mM phosphate buffer at pH 6.5, constant current of −60 μA, UV detection at 210 nm, UMP or cAMP were used as internal standards. Phosphorylated products eluted within less than 7 min. The limits of detection were 0.36 μM for dTMP and 0.86 μM for GMP. The method was used to study enzyme kinetics, and to investigate alternative substrates and inhibitors.

Engineering of a Single Conserved Amino Acid Residue of Herpes Simplex Virus Type 1 Thymidine Kinase Allows a Predominant Shift from Pyrimidine to Purine Nucleoside Phosphorylation

Studies of herpes simplex virus type 1 (HSV-1) thymidine (dThd) kinase (TK) crystal structures show that purine and pyrimidine bases occupy distinct positions in the active site but approximately the same geometric plane. The presence of a bulky side chain, such as tyrosine at position 167, would not be sterically favorable for pyrimidine or pyrimidine nucleoside analogue binding, whereas purine nucleoside analogues would be less affected because they are located further away from the phenylalanine side chain. Site-directed mutagenesis of the conserved Ala-167 and Ala-168 residues in HSV-1 TK resulted in a wide variety of differential affinities and catalytic activities in the presence of the natural substrate dThd and the purine nucleoside analogue drug ganciclovir (GCV), depending on the nature of the amino acid mutation. A168H- and A167F-mutated HSV-1 TK enzymes turned out to have a virtually complete knock-out of dThd kinase activity (at least approximately 4-5 orders of magnitude lower) presumably due to a steric clash between the mutated amino acid and the dThd ring. In contrast, a full preservation of the GCV (and other purine nucleoside analogues) kinase activity was achieved for A168H TK. The enzyme mutants also markedly lost their binding capacity for dThd and showed a substantially diminished feedback inhibition by thymidine 5'-triphosphate. The side chain size at position 168 seems to play a less important role regarding GCV or dThd selectivity than at position 167. Instead, the nitrogen-containing side chains from A168H and A168K seem necessary for efficient ligand discrimination. This explains why A168H-mutated HSV-1 TK fully preserves its GCV kinase activity (Vmax/Km 4-fold higher than wild-type HSV-1 TK), although still showing a severely compromised dThd kinase activity (Vmax/Km 3-4 orders of magnitude lower than wild-type HSV-1 TK).

Negative regulation of herpes simplex virus type 1 ICP4 promoter by IE180 protein of pseudorabies virus.

Recombinant pseudorabies viruses (PRVs) gIS8 and N1aHTK were constructed by the insertion of a chimeric gene (alpha4-TK) from herpes simplex virus type 1 (HSV-1) into wild-type PRV. HSV-1 TK expression by these recombinant viruses resulted in enhanced sensitivity to ganciclovir, compared to that of the wild-type PRV, and was similar to the sensitivity shown by HSV-1. Infection with gIS8 or N1aHTK recombinant viruses led to expression of HSV-1 TK mRNA as an immediate-early (IE) gene, observed by downregulation of the HSV-1 alpha4 promoter. This negative regulation was due to a PRV IE protein, IE180. IE180, however, does not have all the regulatory functions of the infected-cell protein ICP4, as it does not restore the growth of ICP4-deficient HSV-1 mutants.

Non-nucleoside inhibitors of mitochondrial thymidine kinase (TK-2) differentially inhibit the closely related herpes simplex virus type 1 TK and Drosophila melanogaster multifunctional deoxynucleoside kinase.

5'-O-Trityl derivatives of thymidine (dThd), (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU), and their acyclic analogs 1-[(Z)-4-triphenylmethoxy-2-butenyl]thymine (KIN-12) and (E)-5-(2-bromovinyl)-1-[(Z)-4-triphenylmethoxy-2-butenyl]uracil (KIN-52) have been synthesized and evaluated for their inhibitory activity against the amino acid sequence related mitochondrial dThd kinase (TK-2), herpes simplex virus type 1 (HSV-1) TK, and Drosophila melanogaster multifunctional 2'-deoxynucleoside kinase (Dm-dNK). Several compounds proved markedly inhibitory to these enzymes and represent a new generation of nucleoside kinase inhibitors. KIN-52 was the most potent and selective inhibitor of TK-2 (IC(50), 1.3 microM; K(i), 0.50 microM; K(i)/K(m), 0.37) but was not inhibitory against HSV-1 TK and Dm-dNK at 100 microM. As found for the alternative substrate BVDU, the tritylated compounds competitively inhibited the three enzymes with respect to dThd. However, whereas BVDU behaved as a noncompetitive inhibitor (alternative substrate) of TK-2 and HSV-1 TK with respect to ATP as the varying substrate, the novel tritylated enzyme inhibitors emerged as reversible purely uncompetitive inhibitors of these enzymes. Computer-assisted modeling studies are in agreement with these findings. The tritylated compounds do not act as alternative substrates and they showed a type of kinetics against the nucleoside kinases different from that of BVDU. KIN-12, and particularly KIN-52, are the very first non-nucleoside specific inhibitors of TK-2 reported and may be useful for studying the physiological role of the mitochondrial TK-2 enzyme.

The A167Y mutation converts the herpes simplex virus type 1 thymidine kinase into a guanosine analogue kinase.

The thymidine (dThd) kinase (TK) encoded by herpes simplex virus type 1 (HSV-1) is not only endowed with dThd kinase, but also with thymidylate (dTMP) kinase and 2'-deoxycytidine (dCyd) kinase (dCK) activity. HSV-1 TK also recognizes a variety of antiherpetic guanine nucleoside analogues such as acyclovir (ACV), ganciclovir (GCV), lobucavir (LBV), penciclovir (PCV), and others (i.e., A5021). Site-directed mutagenesis of the highly conserved Ala-167 to Tyr in HSV-1 TK completely abolished TK, dTMP-K, and dCK activity, but maintained ACV-, GCV-, LBV-, PCV-, and A5021-phosphorylating capacity. A variety of 5-substituted pyrimidine nucleoside substrates, but also a number of selective HSV-1 TK inhibitors structurally related to thymine lost significant binding affinity for the mutant enzyme and did not markedly compete with GCV phosphorylation by the mutant enzyme. These findings could be explained by computer-assisted modeling data that revealed steric hindrance of the pyrimidine ring in the HSV-1 TK active site by the large 4-hydroxybenzyl ring of 167-Tyr, while the positioning of the purine ring of guanine-based HIV-1 TK substrates in the active site was kept virtually unaltered. Surprisingly, the efficiency of conversion the antiherpetic 2'-deoxyguanosine analogues ACV, GCV, LBV, PCV, and A5021 to their phosphorylated forms by the A167Y mutant HSV-1 TK was far more pronounced than for the wild-type enzyme. Therefore, the single A167Y mutation converts the wild-type HSV-1 TK from a predominantly pyrimidine nucleos(t)ide kinase into a virtually exclusive purine (guanine) nucleoside analogue kinase.

Importance of C-terminus of herpes simplex virus type 1 thymidine kinase for maintaining thymidine kinase and acyclovir-phosphorylation activities.

We previously isolated an acyclovir (ACV)-resistant herpes simplex virus type 1 (HSV-1), strain TAR, from a child with Wiskott-Aldrich syndrome. An acyclovir-sensitive HSV-1, strain TAS, had been isolated from the same patient before the isolation of HSV-1 TAR. The TK protein of ACV-sensitive HSV-1 TAS was composed of 376 amino acids, while that of HSV-1 TAR was composed of 407 amino acids with altered amino acid residue between positions 355-407. The elongation of TK was caused by a single nucleotide deletion of cytosine from a homopolymer stretch of 4 cytosine residues between positions 1061-1064. There was no viral TK activity in HSV-1 TAR-infected Vero cells, indicating the importance of the C-terminal portion of TK protein from positions 355-376. Recombinant TK polypeptides with amino acid deletions at the C-terminus were prepared, and TK and ACV-phosphorylation activities were examined. Deletion of 5 and 6 amino acids from the C-terminus of the TK polypeptide of HSV-1 TAS resulted in a reduction of TK activity by approximately 75% and 100%, respectively. These mutant TK polypeptides did not phosphorylate ACV. These results indicate that amino acid residues from positions 371-376 in the C-terminal portion of HSV-1 TK protein are essential for keeping TK and ACV-phosphorylation activities.

Database Searching for Thymidine and Thymidylate Kinase Inhibitors Using Three-dimensional Structure-based Methods

Structure-based drug design methods were used to search for novel inhibitors of herpes simplex virus type 1 (HSV-1) thymidine kinase and Mycobacterium tuberculosis thymidylate kinase. The method involved the use of crystal structure complexes to guide database searching for potential inhibitors. A number of weak inhibitors of HSV-2 were identified, one of which was found to inhibit HSV-1 TK and HSV-1 TK-deficient viral strains. Each compound tested against M. tuberculosis thymidylate kinase was found to have some activity. The best of these compounds was only 4.6-fold less potent than 3′-azido-3′-deoxythymidine-5′-monophosphate (AZTMP). This study demonstrates the utility of structure-based drug design methods in the search for novel enzyme inhibitors.

SYNTHESIS OF UNNATURAL 7-SUBSTITUTED-1-(2-DEOXY-β-D-RIBOFURANOSYL)ISOCARBOSTYRILS1†: “THYMINE REPLACEMENT” ANALOGS OF DEOXYTHYMIDINE FOR EVALUATION AS ANTIVIRAL AND ANTICANCER AGENTS

A group of unnatural 1-(2-deoxy-β-D-ribofuranosyl)isocarbostyrils having a variety of C-7 substituents [H, 4,7-(NO2)2, I, CF3, CN, (E)-CH=CH-I, -C═CH, -C═C-I, -C═C-Br, -C═C-Me], designed as nucleoside mimics, were synthesized for evaluation as anticancer and antiviral agents. This class of compounds exhibited weak cytotoxicity in a MTT assay (CC50=10−3 to 10−5 M range) with the 4,7-dinitro derivative being the most cytotoxic, relative to thymidine (CC50=10−3 to 10−5 M range), against a variety of cancer cell lines. The 4,7-dinitro, 7-I and 7-C═CH compounds exhibited similar cytotoxicity against non-transfected (KBALB, 143B), and HSV-1 TK+ gene transfected (KBALB-STK, 143B-LTK) cancer cell lines possessing the herpes simplex virus type 1 (HSV-1) thymidine kinase gene (TK+). This observation indicates that these compounds are not substrates for HSV type-1 TK, and are therefore unlikely to be useful in gene therapy based on the HSV gene therapy paradigm. †Nucleoside-like numbering is used by analogy to nucleoside nomenclature.

SYNTHESIS OF 1-(2-DEOXY-β-D-RIBOFURANOSYL)-2,4-DIFLUORO-5-SUBSTITUTED-BENZENES1*: “THYMINE REPLACEMENT” ANALOGS OF THYMIDINE FOR EVALUATION AS ANTICANCER AND ANTIVIRAL AGENTS

A group of unnatural 1-(2-deoxy-β-D-ribofuranosyl)-2,4-difluoro-benzenes having a variety of C-5 two-carbon substituents [−C≡C−X, X = I, Br; −C≡CH;(E)−CH=CH−X, X = I, Br; −CH=CH2; −CH2CH3; −CH(N3) CH2Br], designed as nucleoside mimics, were synthesized for evaluation as anticancer and antiviral agents. The 5-substituted (E)−CH=CH−I and −CH2CH3 compounds exhibited negligible cytotoxicity in a MTT assay (CC50 = 10−3 to 10−4 M range), relative to thymidine (CC50 = 10−3 to 10−5 M range), against a variety of cancer cell lines. In contrast, the C-5 substituted −C≡C−I and −CH(N3)CH2Br compounds were more cytotoxic (CC50 = 10−5 to 10−6 M range). The −C≡C−I and −CH2CH3 compounds exhibited similar cytotoxicity against non-transfected (KBALB, 143B) and HSV-1 TK+ gene transfected (KBALB-STK, 143B-LTK) cancer cell lines expressing the herpes simplex virus type 1 (HSV-1) thymidine kinase gene (TK+). This observation indicates that expression of the viral TK enzyme did not provide a gene therapeutic effect. The parent group of 5-substituted compounds, that were evaluated using a wide variety of antiviral assay systems [HSV-1, HSV-2, varicella-zoster virus (VZV), vaccinia virus, vesicular stomatitis, cytomegalovirus (CMV), and human immunodeficiency (HIV-1, HIV-2) viruses], showed that this class of unnatural C-aryl nucleoside mimics are inactive and/or weakly active antiviral agents.

Superior cytostatic activity of the ganciclovir elaidic acid ester due to the prolonged intracellular retention of ganciclovir anabolites in herpes simplex virus type 1 thymidine kinase gene-transfected tumor cells.

Ganciclovir (GCV) and its lipophilic elaidic acid ester prodrug E-GCV were evaluated for their antiherpetic, cytostatic and metabolic properties, E-GCV proved exquisitely inhibitory to the replication of herpes simplex virus type 1 (HSV-1) and HSV-2 in cell cultures (50% effective concentration (EC50): 0.002 microM). It was five- to 10-fold more effective than its parent drug GCV. E-GCV was at least 2000-fold more cytostatic to HSV-1 or HSV-2 thymidine kinase (tk) gene-transfected mammary carcinoma FM3A tk-/HSVtk+ tumor cells than to the corresponding nontransfected tumor cells. The cytostatic activity of E-GCV to the HSVtk gene-transfected tumor cells was far superior to that of GCV. Metabolic studies revealed that both GCV and E-GCV were converted to the mono-, di- and tri-phosphate derivatives of GCV to a markedly higher extent in FM3Atk-/HSV-1 tk+ cells than in wild-type FM3A/0 cells. Strikingly, mono-, di- and tri-phosphate metabolites of GCV were retained for a substantially longer time in E-GCV-treated cells (half-life approximately 50 h) than in GCV-treated cells (half-life approximately 20 h). The longer retention time of the GCV metabolites most likely explains why E-GCV is superior to GCV against herpes simplex virus replication and HSVtk gene-transfected tumor cell proliferation. Taking into account the marked stability of E-GCV in human plasma and its much higher lipophilicity than GCV, E-GCV should be considered as an effective lipophilic prodrug of GCV with a markedly enhanced cytostatic activity in HSVtk gene-transfected tumor cells compared with parental ganciclovir. Its usefulness in the combined gene/chemotherapy of HSVtk gene-transfected tumors should be further pursued.

Fine-resolution analysis of products of intrachromosomal homeologous recombination in mammalian cells.

Mouse Ltk- cell lines that contained a herpes simplex virus type 1 (HSV-1) thymidine kinase (tk) gene with a 16-bp insertion mutation linked to either a defective HSV-2 tk gene or a hybrid tk sequence comprised of HSV-1 and HSV-2 tk sequences were constructed. HSV-1 and HSV-2 tk genes have 81% nucleotide identity and hence are homeologous. Correction of the insertion mutant HSV-1 tk gene via recombination with the hybrid tk sequence required an exchange between homeologous tk sequences, although recombination could initiate within a region of significant sequence identity. Seven cell lines containing linked HSV-1 and HSV-1-HSV-2 hybrid tk sequences gave rise to tk+ segregants at an average rate of 10(-8) events per cell division. DNA sequencing revealed that each recombinant from these lines displayed an apparent gene conversion which involved an accurate transfer of an uninterrupted block of information between homeologous tk sequences. Conversion tract lengths ranged from 35 to >330 bp. In contrast, cell lines containing linked HSV-1 and HSV-2 tk sequences with no significant stretches of sequence identity had an overall rate of homeologous recombination of <10(-9). One such cell line produced homeologous recombinants at a rate of 10(-8). Strikingly, all homeologous recombinants from this latter cell line were due to crossovers between the HSV-1 and HSV-2 tk genes. Our results, which provide the first detailed analysis of homeologous recombination within a mammalian genome, suggest that rearrangements in mammalian genomes are regulated by the degree of sequence divergence located at the site of recombination initiation.

Retroviral transfer of herpes simplex thymidine kinase gene into glioma cells causes targeting of gancyclovir cytotoxic effect.

The thymidine kinase (tk) gene of herpes simplex virus type 1 (HSV-1) was transduced into three glioma cell lines (T98, U251MG, T9) using a retrovirus vector. The supernatants of viral producer cell line PA317/LTRNL was used for infection and three transduced cell lines (T98/LTRNL, U251MG/LTRNL, T9/LTRNL) were established. The toxicities of the anti-herpetic drugs, acyclovir and gancyclovir, were evaluated in vitro. The cytotoxicities of acyclovir and gancyclovir to the HSV-1 tk gene-transduced cells increased 100-1000 fold compared to the non-transduced parental cell lines. The cytotoxic effect of gancyclovir was higher than that of acyclovir, requiring a concentration of less than 0.31 microM to obtain 50% inhibition. Deoxyribonucleic acid analysis of the gancyclovir-treated cells demonstrated fragmentation, suggesting that apoptosis is involved in the mechanism of cell death. The HSV-1 tk gene-transduced cells were co-cultured with parental cells and treated with gancyclovir. More than 95% of cells were killed in a mixture ratio of 1:1, suggesting that the "bystander effect" operated in this system. Selective transduction of HSV-1 tk gene into glioma cells using a retroviral vector and treatment with gancyclovir is a promising therapy for patients with malignant glioma.

Herpes simplex virus type 1 genes in human mononuclear cells and affecting cell‐mediated immunity

To evaluate whether herpes simplex virus type 1 (HSV-1) replicates in freshly isolated human mononuclear cells (HMNC) and results in cell-mediated immunity, HMNC purified from the peripheral blood of a healthy HSV-1-seronegative individual were infected with HSV-1 (KOS strain). The kinetic data indicated that HSV-1 particles were detected in HMNC at 9 hr postinfection, and the highest HSV-1 titer was obtained at 24 hr postinfection. It was found that the newly synthesized HSV-1 DNA increased in HMNC corresponding to the duration of time postinfection. The HSV-1 tk mRNA was detected in HSV-1-infected HMNC by Northern blot analysis. The Western blot analysis indicated that the HSV-1-infected HMNC could express the viral proteins. These results suggest that HSV-1 replicates in freshly isolated HMNC and expresses its DNA, RNA, and proteins. Interleukin-2 (IL-2) production and cell-mediated cytotoxicity (CMC) were also determined in HSV-1-infected HMNC but were lower than those found in uninfected HMNC. On the other hand, HSV-1 DNA was detectable in HMNC of patients suffering from recurrent HSV-1 infection. The results show that, in the recrudescent phase, the patients' HMNC contain more HSV-1 DNA than in the convalescent phase. We conclude that the impairments of immune responses are related to the infection of HMNC by HSV-1.

Selection for spontaneous null mutations in a chromosomally-integrated HSV-1 thymidine kinase gene yields deletions and a mutation caused by intragenic illegitimate recombination

Spontaneous null mutations represent low frequency events that irreversibly and completely inactivate a gene, and can often consist of major gene alterations. To study the molecular mechanisms leading to recessive spontaneous null mutations in the human genome, we designed and tested a selection procedure in cell culture to enrich for this rare class of spontaneous mutations. The KT cell line contains the herpes simplex virus type 1 (HSV-1)_thymidine kinase ( tk) gene and the neomycin-resistance gene ( neo), from plasmid pSV2 neoKT, integrated as a single-copy in the human tk − cell line 143B. The HSV-1 tk gene was the target for spontaneous gene inactivation, and antiviral drugs (acyclovir, trifluorothymidine and ganciclovir) were used, in combination, to provide a selective enrichment for null mutations over the background of more frequent and revertible point mutations. The tk − mutations obtained with this multiple drug selection assay appeared at a very low frequency, rarely reverted to wild-type ( tk +), and the TK protein was observed only in 4.8% of these null mutants. Deletions of the entire tk gene, or its 3′ region, constituted the major class of DNA rearrangements seen in the null mutations. Additionally, one of the null mutants contained an intragenic 106-bp duplication within a 43-bp deleted region of the tk gene. We propose this mutation to be the outcome of an intragenci gene conversion event which may have been facilitated by short regions of junctional homology.

Identification of important residues within the putative nucleoside binding site of HSV-1 thymidine kinase by random sequence selection: analysis of selected mutants in vitro.

Random sequence mutagenesis in conjunction with genetic complementation was used to map the function of amino acid residues within the putative nucleoside binding site of the herpes simplex virus type 1 (HSV-1) thymidine kinase (TK). Six codons of the putative nucleoside binding site of the HSV-1 tk were substituted by a duplex of extended oligonucleotides containing 20% random sequences. Approximately 260 mutants were screened for the ability to genetically complement a TK-deficient Escherichia coli. Of those screened, 32% conferred TK activity. Approximately 60% of the TK positive clones contained single amino acid changes, 23% contained double changes, and 13.4% encoded the wild-type TK amino acid sequence. A small percentage of clones, 2.4% and 1.2%, contained triple or quadruple alterations, respectively. Three residues (D162, H163, and R164) appeared to be highly conserved especially with regard to the type of residues able to substitute. Secondary screening results indicated that several of the mutants had higher affinities for acyclovir and/or 3'-azido-3'-deoxythymidine than thymidine in complementation assays. In addition, a number of clones were unable to form colonies on selection medium at elevated temperatures (42 degrees C). Eight selected mutants were subcloned into an in vitro transcription vector and the derived transcripts used to program a rabbit reticulocyte lysate cell-free translation system. Biologically active translation products were then analyzed in vitro for thymidine kinase activity, for thermal stability, and for the ability to phosphorylate selected nucleoside analogues. Two of the eight mutants had an elevated thymidine kinase activity, two were significantly thermolabile, and three exhibited enhanced efficiency in phosphorylation of nucleoside analogues.

Effects of acyclovir, oxetanocin-G, and carbocyclic oxetanocin-G in combinations on the replications of herpes simplex virus type 1 and type 2 in Vero cells.

9-(2-hydroxyethoxymethyl)guanine (acyclovir, ACV) and novel nucleosides, 9-(2-deoxy-2-hydroxymethyl-beta-D-erythro-oxetanocyl)guanine (oxetanocin-G, OXT-G) and (+)-9-[(1R, 2R, 3S)-2, 3-bis(hydroxymethyl)cyclobutyl]guanine (carbocyclic oxetanocin-G, carbocyclic OXT-G) possessed substantial antiviral activities against herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2). ACV inhibited only viral thymidine kinase positive (TK+) herpes viruses, although the latter two compounds inhibited the replications of the TK deficient (TK-) mutants of HSV-1 and HSV-2 as well as the TK+ parent strains in vitro. The TK- mutants of HSV-1 and HSV-2 (HSV-1 TK- and HSV-2 TK-) were as susceptible to OXT-G as the TK parent strains. However, the TK- mutants were less susceptible to carbocyclic OXT-G than the TK+ parent strains. We demonstrated synergistic inhibition of the replications of HSV-1 and HSV-2 by ACV and OXT-G in combination, additive inhibition of HSV-1 and HSV-2 by ACV and carbocyclic OXT-G in combination, synergistic inhibition of HSV-1 by OXT-G and carbocyclic OXT-G in combination, and additive inhibition of HSV-2 by these two compounds. We investigated the metabolism of ACV and OXT-G in HSV-1 TK(+)-, HSV-1 TK(-)- and mock-infected Vero cells by thin layer chromatography. ACV-triphosphate increased more in HSV-1 TK(+)-infected Vero cells than in HSV-1 TK(-)- and mock-infected Vero cells. The metabolism of OXT-G had almost the same pattern in HSV-1 TK(+)-, HSV-1 TK(-)- and mock-infected Vero cells. These results suggest that ACV is phosphorylated by virus-induced TK, and OXT-G is phosphorylated by cellular nucleoside and nucleotide kinases.

The Herpes Simplex Virus Type 1 Thymidine Kinase Is Expressed in the Testes of Transgenic Mice under the Control of a Cryptic Promoter

We reported previously that the herpes simplex virus type 1 (HSV-1) thymidine kinase reporter gene (tk) was expressed in the testes of transgenic mice when coupled to the promoter of a liver-specific mouse major urinary protein (MUP) gene. Here we show that HSV-1 tk is also expressed in the testis when coupled to a MUP pseudogene promoter, to a truncated MUP promoter that is not active in the liver, and to the promoter of the bovine thyroglobulin gene. Furthermore, HSV-1 tk itself was expressed in the testis, although its normal expression had been disabled by removing an upstream regulator of transcription. In every case, the same multiple transcripts were observed, with their 5' ends located downstream of the normal HSV-1 tk translation initiation codon. We conclude that the transcription of HSV-1 tk in the testis is directed by a cryptic TATA box-independent promoter located in the coding region of the gene. The longest HSV-1 thymidine kinase (TK) polypeptides synthesized in the testis were shorter than full-length TK and probably result from translational initiation at Met46 and Met60, the second and third ATG codons of the tk reading frame. Male mice of most transgenic lines were sterile, and the severity of the lesion in spermatogenesis was directly related to the level of TK expression. In the most highly expressing lines, sperm counts were low and morphologically defective sperm were common. In other sterile lines, TK was expressed at a lower level and sperm counts were normal but sperm motility was greatly reduced. Lines with the lowest levels of HSV-1 TK expression were fertile. HSV-1 TK was expressed in germ line cells, mainly in the haploid spermatids. However, low-level HSV-1 TK activity was found in the testis before the first germ cells entered meiosis, showing that if expression is confined to the germ cells, it also occurs in spermatogonia.

The herpes simplex virus type 1 thymidine kinase is expressed in the testes of transgenic mice under the control of a cryptic promoter.

We reported previously that the herpes simplex virus type 1 (HSV-1) thymidine kinase reporter gene (tk) was expressed in the testes of transgenic mice when coupled to the promoter of a liver-specific mouse major urinary protein (MUP) gene. Here we show that HSV-1 tk is also expressed in the testis when coupled to a MUP pseudogene promoter, to a truncated MUP promoter that is not active in the liver, and to the promoter of the bovine thyroglobulin gene. Furthermore, HSV-1 tk itself was expressed in the testis, although its normal expression had been disabled by removing an upstream regulator of transcription. In every case, the same multiple transcripts were observed, with their 5' ends located downstream of the normal HSV-1 tk translation initiation codon. We conclude that the transcription of HSV-1 tk in the testis is directed by a cryptic TATA box-independent promoter located in the coding region of the gene. The longest HSV-1 thymidine kinase (TK) polypeptides synthesized in the testis were shorter than full-length TK and probably result from translational initiation at Met46 and Met60, the second and third ATG codons of the tk reading frame. Male mice of most transgenic lines were sterile, and the severity of the lesion in spermatogenesis was directly related to the level of TK expression. In the most highly expressing lines, sperm counts were low and morphologically defective sperm were common. In other sterile lines, TK was expressed at a lower level and sperm counts were normal but sperm motility was greatly reduced. Lines with the lowest levels of HSV-1 TK expression were fertile. HSV-1 TK was expressed in germ line cells, mainly in the haploid spermatids. However, low-level HSV-1 TK activity was found in the testis before the first germ cells entered meiosis, showing that if expression is confined to the germ cells, it also occurs in spermatogonia.