Mapping and molecular characterization of a functional thymidine kinase from Amsacta moorei entomopoxvirus

Identification of a Potential Entry-Fusion Complex Based on Sequence Homology of African Swine Fever and Vaccinia Virus

African swine fever virus (ASFV) belongs to the family of Asfarviridae, part of the group of nucleocytoplasmic large DNA viruses (NCLDV). Little is known about the internalization of ASFV in the host cell and the fusion membrane events that take place at early stages of the infection. Poxviruses, also members of the NCLDV and represented by vaccinia virus (VACV), are large, enveloped, double-stranded DNA viruses. Poxviruses were considered unique in having an elaborate entry-fusion complex (EFC) composed of 11 highly conserved proteins integrated into the membrane of mature virions. Recent advances in methodological techniques have again revealed several connections between VACV EFC proteins. In this study, we explored the possibility of an analogous ASFV EFC by identifying ten candidate proteins exhibiting structural similarities with VACV EFC proteins. This could reveal key functions of these ASFV proteins, drawing attention to shared features between the two virus families, suggesting the potential existence of an ASFV entry-fusion complex.

Transcriptional analysis of the putative glycosyltransferase gene (amv248) of the Amsacta moorei entomopoxvirus

Amsacta moorei entomopoxvirus (AMEV), the most studied member of the genus Betaentomopoxvirus, was initially isolated from Red Hairy caterpillar larvae, Amsacta moorei. According to genome sequence and previous studies it was shown that amv248 encodes a putative glycosyltransferase that is the only conserved attachment protein in betaentomopoxviruses. Transcriptional analysis of the amv248 gene by RT-PCR and qPCR showed that transcription starts at 6h post infection (hpi). Also, transcription was not affected by a DNA replication inhibitor but was severely curtailed by a protein synthesis inhibitor. These results indicate that amv248 belongs to the intermediate class of gene expression. 5' and 3' untranslated regions analysis revealed that transcription initiates at position -126 relative to the translational start site, and ends between 50 and 83 bases after the stop codon. To narrow down the size and location of the gene's promoter, the upstream region as well as several different sized deletions thereof were generated and cloned upstream of a luciferase reporter gene. The constructs were used to measure the Firefly and Renilla luciferase activities in dual assays. The results showed that luciferase activity decreased when bases -198 to -235 of amv248 upstream region were missing. Sequence analysis among the intermediate gene promoters of AMEV showed that TTTAT(T/A)TT(T/A)₂TTA is possibly a common motif, however, further investigations are needed to confirm this conclusion.

Genome-wide analysis of differential mRNA expression of Amsacta moorei entomopoxvirus, mediated by the gene encoding a viral protein kinase (AMV197)

Insect-born entomopoxviruses (Fam: Poxviridae) are potentially important bio-pesticide against insect pests and expression vectors as well as vectors for transient human gene therapies including recombinant viral vaccines. For these reasons, it is necessary to understand the regulatory genes functions to improve its biotechnological potential. Here, we focused on the characterization of serine/threonine (Ser/Thr; ORF AMV197) protein kinase gene from the Amsacta moorei entomopoxvirus (AMEV), the type species of the genus Betaentomopoxvirus. Transcription of the parental and an amv197-null recombinant AMEV was compared by whole-genome gene expression microarray analysis. Blast2GO analysis reflected a broad diversity of upregulated and downregulated genes. Results showed that expression levels of 102 genes (45%) out of 226 tested genes changed significantly in the recombinant AMEV infected cells. Of these transcripts, 72 (70.58%) were upregulated and 30 (29.41%) were downregulated throughout the infection period. Genes involved in DNA repair, replication and nucleotide metabolism, transcription and RNA modification, and protein modification were mostly upregulated at different times in cells infected with the recombinant virus. Furthermore, transcription of all studied cellular genes including metabolism of apoptosis (Nedd2-like caspase, hemolin and elongation factor-1 alpha (ef1a) gene) was downregulated in the absence of amv197. Quantitative real time reverse transcription-PCR confirmed viral transcriptional changes obtained by microarray. The results of this study indicated that the product of amv197 appears to affect the transcriptional regulation of most viral and many cellular genes. Further investigations are, however, needed to narrow down the role of AMV197 throughout the infection process.

Amsacta moorei entomopoxvirus encodes a functional esterase (amv133) with protease activity

OBJECTIVES

Lipolytic genes have been investigated in several viral genomes, and some of them show enzyme activity which can be used for various functions including the production of DNA replication metabolites, rescue from endosomes, and membrane fusion. Amsacta moorei entomopoxvirus (AMEV) replicates in nearly the entire insect body, especially in the adipose tissue. One of the open reading frames (ORFs) in the AMEV genome, amv133, encodes a putative lipase enzyme. In this study, we therefore investigate the enzyme activity of amv133.

METHODS

amv133 was aligned with known lipase genes and their homologs in entomopoxviruses. Expressed proteins were partially purified and assayed for lipase, esterase and protease.

RESULTS

We found that amv133 contains all the domains required for a functional lipase enzyme and that it shows a significant similarity with homologs in other entomopoxviruses. Since there is a similarity of the catalytic triad between lipases and serine proteases, we also investigated the protease activity of amv133. Lipase, esterase and protease assays showed that amv133 encodes a functional esterase enzyme with protease activity.

CONCLUSION

The current data show that amv133 is a conserved gene in all entomopoxvirus genomes sequenced so far and might contribute greatly to degrading the lipids or proteins and hence improve the virus infection.

The complete genome sequence of the Alphaentomopoxvirus Anomala cuprea entomopoxvirus, including its terminal hairpin loop sequences, suggests a potentially unique mode of apoptosis inhibition and mode of DNA replication

Complete genome sequence of Anomala cuprea entomopoxvirus, which belongs to the genus Alphaentomopoxvirus, including its terminal hairpin loop sequences, is reported. This is the first genome sequence of Alphaentomopoxvirus reported, and hairpin loops in entomopoxviruses have not previously been sequenced. The genome is 245,717 bp, which is smaller than had previously been estimated for Alphaentomopoxvirus. The inverted terminal repeats are quite long, and experimental results suggest that one genome molecule has one type of hairpin at one end and another type at the other end. The genome contains unexpected ORFs, e.g., that for the ubiquitin-conjugating enzyme E2 of eukaryotes. The BIR and RING domains found in a single ORF for an inhibitor of apoptosis in baculoviruses and entomopoxviruses occurred in two different, widely separated ORFs. Furthermore, an ORF in the genome contains a serpin domain that was previously found in vertebrate poxviruses for apoptosis inhibition but not in insect viruses.

Induction of apoptosis by the Amsacta moorei entomopoxvirus

CF-70-B2 cells derived from the spruce budworm (Choristoneura fumiferana) undergo apoptosis when infected with Amsacta moorei entomopoxvirus (AMEV), as characterized by membrane blebbing, formation of apoptotic bodies, TdT-mediated dUTP nick-end labelling (TUNEL) staining, condensed chromatin and induction of caspase-3/7 activity. The apoptotic response was reduced when cells were infected with UV-inactivated AMEV, but not when infected in the presence of the DNA synthesis inhibitor, cytosine β-d-arabinofuranoside. Hence, only pre-DNA replication events were involved in inducing the antiviral response in CF-70-B2 cells. The virus eventually overcame the host’s antiviral response and replicated to high progeny virus titres accompanied by high levels of caspase-3/7 activity. The CF-70-B2 cells were less productive of progeny virus in comparison to LD-652, a Lymantria dispar cell line routinely used for propagation of AMEV. At late stages of infection, LD-652 cells also showed characteristics of apoptosis such as oligosomal DNA fragmentation, TUNEL staining, condensed chromatin and increased caspase-3/7 activity. Induction of apoptosis in LD-652 cells was dependent on viral DNA replication and/or late gene expression. A significantly reduced rate of infection was observed in the presence of general caspase inhibitors Q-VD-OPH and Z-VAD-FMK, indicating caspases may be involved in productive virus infection.

A homolog of the vaccinia virus D13L rifampicin resistance gene is in the entomopoxvirus of the parasitic wasp, Diachasmimorpha longicaudata

The parasitic wasp, Diachasmimorpha longicaudata (Ashmead) (Hymenoptera: Braconidae), introduces an entomopoxvirus (DlEPV) into its Caribbean fruit fly host, Anastrepha suspensa. (Loew) (Diptera: Tephritidae), during oviposition. DlEPV has a 250-300 kb unipartite dsDNA genome, that replicates in the cytoplasm of the host's hemocytes, and inhibits the host's encapsulation response. The putative proteins encoded by several DlEPV genes are highly homologous with those of poxviruses, while others appear to be DlEPV specific. Here, a 2.34 kb sequence containing a 1.64 kb DlEPV open reading frame within a cloned 4.5 kb EcoR1 fragment (designated R1-1) is described from a DlEPV EcoRI genomic library. This open reading frame is a homolog of the vaccinia virus rifampicin resistance (rif) gene, D13L, and encodes a putative 546 amino acid protein. The DlEPV rif contains two EcoRV, two HindIII, one XbaI, and one DraII restriction sites, and upstream of the open reading frame the fragment also contains EcoRV, HindII, SpEI, and BsP106 sites. Early poxvirus transcription termination signals (TTTTTnT) occur 236 and 315 nucleotides upstream of the consensus poxvirus late translational start codon (TAAATG) and at 169 nucleotides downstream of the translational stop codon of the rif open reading frame. Southern blot hybridization of HindIII-, EcoRI-, and BamH1-restricted DlEPV genomic DNA probed with the labeled 4.5 kb insert confirmed the fidelity of the DNA and the expected number of fragments appropriate to the restriction endonucleases used. Pairwise comparisons between DlEPV amino acids and those of the Amsacta moorei, Heliothis armigera, and Melanoplus sanguinipes entomopoxviruses, revealed 46, 46, and 45 % similarity (identity + substitutions), respectively. Similar values (41-45%) were observed in comparisons with the chordopoxviruses. The mid portion of the DlEPV sequence contained two regions of highest conserved residues similar to those reported for H. armigera entomopoxvirus rifampicin resistance protein. Phylogenetic analysis of the amino acid sequences suggested that DlEPV arose from the same ancestral node as other entomopoxviruses but belongs to a separate clade from those of the grasshopper-infecting M. sanguinipes entomopoxvirus and from the Lepidoptera-infecting (Genus B or Betaentomopoxvirus) A. moorei entomopoxvirus and H. armigera entomopoxvirus. Interestingly, the DlEPV putative protein had only 3-26.4% similarity with RIF-like homologs/orthologs found in other large DNA non-poxviruses, demonstrating its closer relationship to the Poxviridae. DlEPV remains an unassigned member of the Entomopoxvirinae (http://www.ncbi.nlm.nih.gov/ICTVdb/Ictv/index.htm) until its relationship to other diptera-infecting (Gammaentomopoxvirus or Genus C) entomopoxviruses can be verified. The GenBank accession number for the nucleotide sequence data reported in this paper is EF541029.

Comparative analysis of selected genes from Diachasmimorpha longicaudata entomopoxvirus and other poxviruses

The Diachasmimorpha longicaudata entomopoxvirus (DlEPV) is the first symbiotic EPV described from a parasitic wasp. The DlEPV is introduced into the tephritid fruit fly larval host along with the wasp egg at oviposition. We sequenced a shotgun genomic library of the DlEPV DNA and analyzed and compared the predicted protein sequences of eight ORFs with those of selected poxviruses and other organisms. BlastP searches showed that five of these are homologous to poxvirus putative proteins such as metalloprotease, a putative membrane protein, late transcription factor-3, virion surface protein, and poly (A) polymerase (PAP) regulatory small subunit. Three of these are similar to those of other organisms such as the gamma-glutamyltransferase (GGT) of Arabidopsis thaliana, eukaryotic initiation factor 4A (eIF4A) of Caenorhabditis briggsae and lambda phage integrase (lambda-Int) of Enterococcus faecium. Transcription motifs for early (TGA,A/T,XXXXA) or late (TAAATG, TAAT, or TAAAT) gene expression conserved in poxviruses were identified with those ORFs. Phylogenetic analysis of multiple alignments of five ORFs and 20 poxvirus homologous sequences and of a concatenate of multiple alignments suggested that DlEPV probably diverged from the ancestral node between the fowlpox virus and the genus B, lepidopteran and orthopteran EPVs, to which Amsacta moorei and Melanoplus sanguinipes EPV, respectively, belong. The DlEPV putative GGT, eIF4A, and lambda-Int contained many conserved domains that typified these proteins. These homologues may be involved in either viral pathogenicity or enhancing parasitism via the gamma-glutamyl cycle and compensation of eIF4A levels in the parasitized fly, or via the integration of a portion of the viral genome into the wasp and/or parasitized fly.

Amsacta moorei entomopoxvirus expresses an active superoxide dismutase

The entomopoxvirus from Amsacta moorei serves as the prototype of the group B entomopoxviruses. One of the interesting genes found in Amsacta moorei entomopoxvirus (AmEPV) is a superoxide dismutase (sod) (open reading frame AMV255). Superoxide dismutases (SODs) catalyze the conversion of superoxide radicals to hydrogen peroxide and oxygen. Many vertebrate poxviruses contain a sod gene, but to date, none have been demonstrated to be active. There are three families of SODs, characterized by their metal ion-binding partners, Fe, Mn, or Cu and Zn. Poxvirus enzymes belong to the Cu-Zn SOD family. Unlike inactive vertebrate poxvirus SODs, AMVSOD contains all the amino acids necessary for function. We expressed and purified a 6X-His-tagged version of the AMVSOD in Escherichia coli. The recombinant AMVSOD demonstrates superoxide dismutase activity both in an in situ gel assay and by stopped flow spectrophotometry. The k(cat)/K(m) for AMVSOD is 4 x 10(7) M(-1)s(-1). In infected cells, the AMVSOD protein behaves as a dimer and is catalytically active; however, disruption of the gene in AMEPV has little or no effect on growth of the virus in cell culture. An analysis of mRNA expression indicates that AMVsod is expressed late during infection of Lymantria dispar (Ld652) cells and produces a discrete nonpolydisperse transcript. Characterization of protein expression with a monoclonal antibody generated against AMVSOD confirms that the AMVSOD protein can be classified as a late, postreplicative gene. Therefore, AMVSOD is the first example of an active poxvirus SOD.

Complete genomic sequence of the Amsacta moorei entomopoxvirus: analysis and comparison with other poxviruses

The genome of the genus B entomopoxvirus from Amsacta moorei (AmEPV) was sequenced and found to contain 232,392 bases with 279 unique open reading frames (ORFs) of greater than 60 amino acids. The central core of the viral chromosome is flanked by 9.4-kb inverted terminal repeats (ITRs), each of which contains 13 ORFs, raising the total number of ORFs within the viral chromosome to 292. ORFs with no known homology to other poxvirus genes were shown to constitute 33.6% of the viral genome. Approximately 28.6% of the AmEPV genome encodes homologs of the mammalian poxvirus colinear core genes, which are found dispersed throughout the AmEPV chromosome. There is also no significant gene order conservation between AmEPV and the orthopteran genus B poxvirus of Melanoplus sanguinipes (MsEPV). Novel AmEPV genes include those encoding a putative ABC transporter and a Kunitz-motif protease inhibitor. The most unusual feature of the AmEPV genome relates to the viral encoded poly(A) polymerase. In all other poxviruses this heterodimeric enzyme consists of a single large and a single small subunit. However, AmEPV appears to encode one large and two distinct small poly(A) polymerase subunits. AmEPV is one of the few entomopoxviruses which can be grown and manipulated in cell culture. The complete genomic sequence of AmEPV paves the way for an understanding and comparison of the molecular properties and pathogenesis between the entomopoxviruses of insects and the more intensively studied vertebrate poxviruses.

Sequence and analysis of a swinepox virus homologue of the vaccinia virus major envelope protein P37 (F13L)

P37 (F13L gene product), the most abundant protein in the envelope of the extracellular virus form of the prototype poxvirus, vaccinia virus (VV), is a crucial player in the process leading to acquisition of the envelope, virus egress and transmission. We have cloned and sequenced a swinepox virus (SPV) gene homologous to VV F13L. The SPV gene product, termed P42, was 54% identical to P37, the VV F13L gene product, and, among the poxviruses, was most similar (73% identity) to the myxoma virus homologue. The SPV P42 gene contained late transcription signals and was expressed only at late times during infection. The protein was palmitylated, and showed an intracellular distribution similar to that of VV P37, both by immunofluorescence and by subcellular fractionation. As with VV P37, SPV P42 was incorporated in extracellular enveloped SPV particles, but was absent from the intracellular mature virus form. To check the ability of SPV P42 to function in the context of VV infection, we inserted the SPV gene into a VV deficient in P37, which is severely blocked in virus envelopment and cell-to-cell transmission. Despite correct expression of SPV P42, the resulting recombinant VV showed no rescue of extracellular virus formation or cell-to-cell virus spread. The lack of function of SPV P42 in the VV genetic background suggests that specific interactions between SPV P42 or VV P37 and other viral proteins is required to drive the envelopment process.

The genome of Melanoplus sanguinipes entomopoxvirus

The family Poxviridae contains two subfamilies: the Entomopoxvirinae (poxviruses of insects) and the Chordopoxvirinae (poxviruses of vertebrates). Here we present the first characterization of the genome of an entomopoxvirus (EPV) which infects the North American migratory grasshopper Melanoplus sanguinipes and other important orthopteran pests. The 236-kbp M. sanguinipes EPV (MsEPV) genome consists of a central coding region bounded by 7-kbp inverted terminal repeats and contains 267 open reading frames (ORFs), of which 107 exhibit similarity to previously described genes. The presence of genes not previously described in poxviruses, and in some cases in any other known virus, suggests significant viral adaptation to the arthropod host and the external environment. Genes predicting interactions with host cellular mechanisms include homologues of the inhibitor of apoptosis protein, stress response protein phosphatase 2C, extracellular matrixin metalloproteases, ubiquitin, calcium binding EF-hand protein, glycosyltransferase, and a triacylglyceride lipase. MsEPV genes with putative functions in prevention and repair of DNA damage include a complete base excision repair pathway (uracil DNA glycosylase, AP endonuclease, DNA polymerase beta, and an NAD+-dependent DNA ligase), a photoreactivation repair pathway (cyclobutane pyrimidine dimer photolyase), a LINE-type reverse transcriptase, and a mutT homologue. The presence of these specific repair pathways may represent viral adaptation for repair of environmentally induced DNA damage. The absence of previously described poxvirus enzymes involved in nucleotide metabolism and the presence of a novel thymidylate synthase homologue suggest that MsEPV is heavily reliant on host cell nucleotide pools and the de novo nucleotide biosynthesis pathway. MsEPV and lepidopteran genus B EPVs lack genome colinearity and exhibit a low level of amino acid identity among homologous genes (20 to 59%), perhaps reflecting a significant evolutionary distance between lepidopteran and orthopteran viruses. Divergence between MsEPV and the Chordopoxvirinae is indicated by the presence of only 49 identifiable chordopoxvirus homologues, low-level amino acid identity among these genes (20 to 48%), and the presence in MsEPV of 43 novel ORFs in five gene families. Genes common to both poxvirus subfamilies, which include those encoding enzymes involved in RNA transcription and modification, DNA replication, protein processing, virion assembly, and virion structural proteins, define the genetic core of the Poxviridae.

Transient, nonlethal expression of genes in vertebrate cells by recombinant entomopoxviruses

The group B entomopoxvirus (EPV) from Amsacta moorei (AmEPV) productively infects only insect cells. A series of AmEPV-lacZ recombinants was constructed in which the lacZ gene was regulated by either late (the AmEPV spheroidin or the cowpox virus A-type inclusion [ATI]) or early (the AmEPV esp [early strong promoter; derived from a 42-kDa AmEPV protein] or the Melolontha melolontha EPV fusolin, fus) virus promoters. When the AmEPV recombinants were used to infect vertebrate cells, beta-galactosidase expression occurred (in >30% of the cells) when lacZ was regulated by either the fus or esp early promoters but not when lacZ was regulated by the late promoters (spheroidin or ATI). Therefore, AmEPV enters vertebrate cells and undergoes at least a partial uncoating and early, but not late, viral genes are expressed. Neither viral DNA synthesis nor cytopathic effects were observed under any infection conditions. When an AmEPV recombinant virus containing the Aequorea victoria green fluorescent protein gene (gfp) under the control of the esp promoter was used to infect vertebrate cells at a low multiplicity of infection, single fluorescent cells resulted, which continued to divide over a period of several days, ultimately forming fluorescent cell clusters, suggesting that vertebrate cells survive the infection and continue to grow. Therefore, AmEPV may prove to be a highly efficient, nontoxic method of gene delivery into vertebrate cells for transient gene expression.

Characterization of a DNA topoisomerase encoded by Amsacta moore entomopoxvirus

We have identified an Amsacta moorei entomopoxvirus (AmEPV) gene encoding a DNA topoisomerase. The 333-amino acid AmEPV topoisomerase displays instructive sequence similarities to the previously identified topoisomerases encoded by five genera of vertebrate poxviruses. One hundred nine amino acids are identical or conserved among the six proteins. The gene encoding AmEPV topoisomerase was expressed in bacteria and the recombinant enzyme was partially purified. AmEPV topoisomerase is a monomeric enzyme that catalyzes the relaxation of supercoiled DNA. Like the vaccinia, Shope fibroma virus, and Orf virus enzymes, the AmEPV topoisomerase forms a covalent adduct with duplex DNA at the target sequence CCCTT decreases. The kinetic and equilibrium parameters of the DNA cleavage reaction of AmEPV topoisomerase (Kobs = 0.08 sec-1; Kcl = 0.22) are similar to those of the vaccinia virus enzyme.

Identification and characterization of a filament-associated protein encoded by Amsacta moorei entomopoxvirus

A novel protein which is expressed at high levels in insect cells infected with Amsacta moorei entomopoxvirus was identified by our laboratory. This viral gene product migrates as a 25/27-kDa doublet when subjected to electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. It is expressed at late times of infection and is present in infected cells but is absent in purified extracellular virions and occlusion bodies. The gene encoding this polypeptide was mapped on the viral genome, and cDNA clones were generated and sequenced. The predicted protein was shown to be phosphorylated and contained an unusual 10-unit proline-glutamic acid repeat element. A polyclonal antiserum was produced against a recombinant form of the protein expressed in Escherichia coli, and a monoclonal antibody which reacted with the proline-glutamic acid motif was also identified. Immunofluorescence and immunoelectron microscopy techniques revealed that this protein is associated with large cytoplasmic fibrils which accumulate in the cytoplasm between 96 and 120 h postinfection. We subsequently called this viral polypeptide filament-associated late protein of entomopoxvirus. The fibrils containing this polypeptide are closely associated with occlusion bodies and may play a role in their morphogenesis and maturation.

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.

Viral thymidine kinases and their relatives

Thymidine kinases were described for cellular life long before it was shown that they could also be encoded by viruses, but the viral thymidine kinase genes were the first to be sequenced. These enzymes have been extraordinarily useful to the researcher, serving first to help label DNA, then to get thymidine analogs incorporated into DNA for therapeutic and other purposes and more recently to move genes from one genome to another. Knowledge of the nucleotide and amino acid sequences of these enzymes has allowed some deductions about their possible three-dimensional structure, as well as the location on the polypeptide of various functions; it has also allowed their classification into two main groups: the herpesviral thymidine/eukaryotic deoxycytidine kinases and the poxviral and cellular thymidine kinases; the relationships of the mitochondrial enzyme are still not clear.