High level expression of DNA polymerases from herpesviruses

A Comparative Study of the in vitro and in vivo Antiviral Activities of Acyclovir and Penciclovir

The antiviral properties of the compounds acyclovir (ACV) and penciclovir (PCV) have been compared in a number of in vitro and in vivo assays. In vitro, both compounds had good activity against herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV), although ACV showed statistically significant superiority. In addition, ACV had greater activity against herpes simplex virus type 2 (HSV-2), human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV). We examined the effect of time of addition and removal of ACV and PCV under a variety of conditions and found similar results with the two compounds under most conditions. However, at a high multiplicity of infection, when all of the cells would be expected to be synchronously expressing large amounts of the viral thymidine kinase, short exposures to PCV appeared to be superior to similar exposures to ACV. In the HSV-1 zosteriform mouse model there was no significant difference between the activities of ACV and PCV, or its prodrug famciclovir (FCV), in once- or twice-daily treatment. The possible significance of these results and those previously reported on the activity of the compounds in humans is discussed.

Utility of the bacteriophage RB69 polymerase gp43 as a surrogate enzyme for herpesvirus orthologs

Viral polymerases are important targets in drug discovery and development efforts. Most antiviral compounds that are currently approved for treatment of infection with members of the herpesviridae family were shown to inhibit the viral DNA polymerase. However, biochemical studies that shed light on mechanisms of drug action and resistance are hampered primarily due to technical problems associated with enzyme expression and purification. In contrast, the orthologous bacteriophage RB69 polymerase gp43 has been crystallized in various forms and therefore serves as a model system that provides a better understanding of structure–function relationships of polymerases that belong the type B family. This review aims to discuss strengths, limitations, and opportunities of the phage surrogate with emphasis placed on its utility in the discovery and development of anti-herpetic drugs.

Inhibition of p53 transcriptional activity by human cytomegalovirus UL44

Human cytomegalovirus (HCMV) stimulates cellular synthesis of DNA and proteins and induces transition of the cell cycle from G(1) to S and G(2) /M phase, in spite of increased amounts of p53 in the infected cells. The immediate early protein IE2-86  kDa (IE86) tethers a transcriptional repression domain to p53; however, its repression of p53 function is not enough to abrogate the G(1) checkpoint function of p53. Other HCMV proteins that suppress the activity of p53 were investigated in this study. Of the HCMV proteins that bind to p53 when assessed by immunoprecipitation and immunoblot analysis, HCMV UL44 was chosen as a candidate protein. It was found that reporter gene containing p53 consensus sequence was activated by transfection with wild type p53, but when plasmids of p53 with IE86 or UL44 were co-transfected, p53 transcriptional activity was decreased to 3-7% of the p53 control in a dose-dependent manner. When the deletion mutant of UL44 was co-transected with p53, the carboxyl one-third portion of UL44 had little effect on inhibition of p53 transcriptional activity. The amount of mRNA p21 was measured in H1299 by real time PCR after transfection of the combination of p53 and UL44 vectors and it was found that p21 transcription by p53 was inhibited dose-dependently by UL44. Increased G0/G1 and decreased S phases in p53 wild type-transfected H1299 cells were recovered to the level of p53 mutant type-transfected ones by the additional transfection of UL44 in a dose-dependent manner. In conclusion, the transcriptional activity of p53 is suppressed by UL44 as well as by IE86.

Depletion of cellular pre-replication complex factors results in increased human cytomegalovirus DNA replication

Although HCMV encodes many genes required for the replication of its DNA genome, no HCMV-encoded orthologue of the origin binding protein, which has been identified in other herpesviruses, has been identified. This has led to speculation that HCMV may use other viral proteins or possibly cellular factors for the initiation of DNA synthesis. It is also unclear whether cellular replication factors are required for efficient replication of viral DNA during or after viral replication origin recognition. Consequently, we have asked whether cellular pre-replication (pre-RC) factors that are either initially associated with cellular origin of replication (e.g. ORC2), those which recruit other replication factors (e.g. Cdt1 or Cdc6) or those which are subsequently recruited (e.g. MCMs) play any role in the HCMV DNA replication. We show that whilst RNAi-mediated knock-down of these factors in the cell affects cellular DNA replication, as predicted, it results in concomitant increases in viral DNA replication. These data show that cellular factors which initiate cellular DNA synthesis are not required for the initiation of replication of viral DNA and suggest that inhibition of cellular DNA synthesis, in itself, fosters conditions which are conducive to viral DNA replication.

Nuts and bolts of human cytomegalovirus lytic DNA replication

HCMV lytic DNA replication is complex and highly regulated. The cis-acting lytic origin of DNA replication (oriLyt) contains multiple repeat motifs that comprise two main functional domains. The first is a bidirectional promoter element that is responsive to UL84 and IE2. The second appears to be an RNA/DNA hybrid region that is a substrate for UL84. UL84 is required for oriLyt-dependent DNA replication along with the six core proteins, UL44 (DNA processivity factor), UL54 (DNA polymerase), UL70 (primase), UL105 (helicase), UL102 (primase-associated factor) and UL57 (single-stranded DNA-binding protein). UL84 is an early protein that shuttles from the nucleus to the cytoplasm, binds RNA, suppresses the transcriptional activation function of IE2, has UTPase activity and is proposed to be a member of the DExH/D box family of proteins. UL84 is a key factor that may act in concert with the other core replication proteins to initiate lytic replication by altering the conformation of an RNA stem loop structure within oriLyt. In addition, new data suggests that UL84 interacts with at least one member of the viral replication proteins and several cellular encoded proteins.

Disruption of protein-protein interactions: towards new targets for chemotherapy

Protein-protein interactions play a key role in various mechanisms of cellular growth and differentiation, and in the replication of pathogen organisms in host cells. Thus, inhibition of these interactions is a promising novel approach for rational drug design against a wide number of cellular and microbial targets. In the past few years, attempts to inhibit protein-protein interactions using antibodies, peptides, and synthetic or natural small molecules have met with varying degrees of success, and these will be the focus of this review.

Residues of human cytomegalovirus DNA polymerase catalytic subunit UL54 that are necessary and sufficient for interaction with the accessory protein UL44

The human cytomegalovirus DNA polymerase contains a catalytic subunit, UL54, and an accessory protein, UL44. Recent studies suggested that UL54 might interact via its extreme C terminus with UL44 (A. Loregian, R. Rigatti, M. Murphy, E. Schievano, G. Palu', and H. S. Marsden, J. Virol. 77:8336-8344, 2003). To address this hypothesis, we quantitatively measured the binding of peptides corresponding to the extreme C terminus of UL54 to UL44 by using isothermal titration calorimetry. A peptide corresponding to the last 22 residues of UL54 was sufficient to bind specifically to UL44 in a 1:1 complex with a dissociation constant of ca. 0.7 microM. To define individual residues in this segment that are crucial for interacting with UL44, we engineered a series of mutations in the C-terminal region of UL54. The UL54 mutants were tested for their ability to interact with UL44 by glutathione S-transferase pulldown assays, for basal DNA polymerase activity, and for long-chain DNA synthesis in the presence of UL44. We observed that deletion of the C-terminal segment or substitution of alanine for Leu1227 or Phe1231 in UL54 greatly impaired both the UL54-UL44 interaction in pulldown assays and long-chain DNA synthesis without affecting basal polymerase activity, identifying these residues as important for subunit interaction. Thus, like the herpes simplex virus UL30-UL42 interaction, a few specific side chains in the C terminus of UL54 are crucial for UL54-UL44 interaction. However, the UL54 residues important for interaction with UL44 are hydrophobic and not basic. This information might aid in the rational design of new drugs for the treatment of human cytomegalovirus infection.

Inhibition of human cytomegalovirus DNA polymerase by C-terminal peptides from the UL54 subunit

In common with other herpesviruses, the human cytomegalovirus (HCMV) DNA polymerase contains a catalytic subunit (Pol or UL54) and an accessory protein (UL44) that is thought to increase the processivity of the enzyme. The observation that antisense inhibition of UL44 synthesis in HCMV-infected cells strongly inhibits viral DNA replication, together with the structural similarity predicted for the herpesvirus processivity subunits, highlights the importance of the accessory protein for virus growth and raises the possibility that the UL54/UL44 interaction might be a valid target for antiviral drugs. To investigate this possibility, overlapping peptides spanning residues 1161 to 1242 of UL54 were synthesized and tested for inhibition of the interaction between purified UL54 and UL44 proteins. A peptide, LPRRLHLEPAFLPYSVKAHECC, corresponding to residues 1221 to 1242 at the very C terminus of UL54, disrupted both the physical interaction between the two proteins and specifically inhibited the stimulation of UL54 by UL44. A mutant peptide lacking the two carboxy-terminal cysteines was markedly less inhibitory, suggesting a role for these residues in the UL54/UL44 interaction. Circular dichroism spectroscopy indicated that the UL54 C-terminal peptide can adopt a partially alpha-helical structure. Taken together, these results indicate that the two subunits of HCMV DNA polymerase most likely interact in a way which is analogous to that of the two subunits of herpes simplex virus DNA polymerase, even though there is no sequence homology in the binding site, and suggest that the UL54 peptide, or derivatives thereof, could form the basis for developing a new class of anti-HCMV inhibitors that act by disrupting the UL54/UL44 interaction.

Characteristics of DNA-binding activity of human cytomegalovirus ppUL44

Human cytomegalovirus (HCMV)-specific monoclonal antibody, SCMVM34, recognizes the early antigen encoded by UL44 of HCMV. This antigen is confined to the nucleus of HCMV-infected cells. This study was performed to characterize the DNA-binding activity of the protein encoded by UL44 of HCMV. The nuclear and cytoskeletal fraction of HCMV-infected cells was subjected to 0.4 M NaCl extraction, DEAE-Sephacel ion exchange chromatography, DNA-cellulose chromatography and SDS-PAGE analysis with monitoring of the reactive protein using SCMVM34 monoclonal antibody. The molecular weights of the resultant proteins were found to be 34, 40 and 52 kDa. The internal peptide fragments were isolated by tryptic digestion and reverse-phase HPLC. The internal amino acid sequence analysis of the peptides from the HPLC profile revealed that the antigen recognized by SCMVM34 monoclonal antibody was ppUL44. The reactive antigen began to be eluted from 250 mM NaCl (Tris-HCl pH 7.4) in DNA cellulose. The 34 kDa protein seems to bind to DEAE more tightly than the 52 kDa protein. The surface charge of 34 kDa might be more basic. Conclusively, the antigen recognized by SCMVM34 was the protein encoded by HCMV UL44, which was localized in the nuclei after HCMV infection, and was the DNA-binding protein with the characteristic that the surface charge of the molecule was more basic, as the molecular weights of the protein were decreased.

Soluble expression and complex formation of proteins required for HCMV DNA replication using the SFV expression system

Several of the viral proteins required for human cytomegalovirus (HCMV) DNA replication have been difficult to study due to their low abundance in infected cells and low solubility in bacterial or insect-cell expression systems. Therefore we used the Semliki Forest virus expression system to express these proteins in mammalian cells. All of the recombinant proteins were soluble, on the basis of ultracentrifugation properties and their ability to be immunoprecipitated from solution with specific antibodies. Pulse-chase analysis of the 86-kDa major immediate-early protein (IE86) revealed two expressed forms-a precursor and a product-indicating that this recombinant protein, like the native HCMV protein, is posttranslationally processed. The recombinant proteins (polymerase core and accessory as well as the IE86 and pUL84) formed stable complexes similar to those known to form in HCMV-infected cells. The recombinant DNA polymerase holoenzyme also exhibited enzyme activity that was phosphonoformic acid sensitive, as is the infected-cell DNA polymerase activity. This expression system offers many advantages for the expression and study of the HCMV replication proteins, including the expression of soluble, active proteins that are able to interact to form complexes. Additionally, the relative ease with which SFV recombinants can be made lends itself to the construction and evaluation of mutants.

Synthesis and evaluation of some masked phosphate esters of the anti-herpesvirus drug 882C (netivudine) as potential antiviral agents

A number of symmetric and asymmetric 5'-phosphate esters of the potent anti-varicella-zoster virus (VZV) agent 1-(beta-D-arabinofuranosyl)-5-prop-1-ynyluracil (882C; netivudine) were prepared as potential lipophilic, membrane-soluble prodrugs of the bio-active phosphate forms. The compounds were prepared by the base-catalysed coupling of various phosphorochloridates with the free nucleoside analogue. Compounds were fully characterized by a range of spectroscopic and analytical methods and were studied for their inhibition of several viruses in tissue culture. All of the phosphate esters were inactive against human cytomegalovirus, herpes simplex virus type 2, VZV, human immunodeficiency virus type 1 and influenza A virus (EC50 > 100 microM) except the 5'-(4-nitrophenyl phenyl) phosphate, which inhibited influenza A virus. The relative rate of esterase-mediated hydrolysis of one of the lead target structures was measured in order to rationalize the poor antiviral action, and data were collected on possible metabolites in support of this analysis. Cell-specific esterases are implicated as key determinants of the antiviral potency of prodrugs of this type.

Formation of cytomegalovirus DNA replication compartments defined by localization of viral proteins and DNA synthesis

To characterize the formation of replication compartments in human cytomegalovirus-infected cells, and to determine the fate of newly synthesized DNA, we localized viral replication proteins and DNA synthesis at early and late times during infection. As expected, ppUL57 (single-stranded DNA binding protein) and ppUL44 (DNA polymerase processivity factor) both localized to replication compartments beginning at 48 hpi. BrdU was incorporated into viral DNA in these compartments that was found to mature into progeny virus based on our ability to chase the label into the cytoplasm and out of the cell over the ensuing 72-h period. Although the pattern of BrdU incorporation at early times (20 or 24 hpi) was punctate, and distinct from the replication compartment that formed later during infection, viral DNA synthesized at this time also matured into progeny virus during a chase. Interestingly, sites of ppUL57 localization did not overlap completely with sites of BrdU incorporation at early times. Products from the UL112-113 gene localized to subnuclear regions by 6 hpi, earlier than ppUL57. Between 12 and 24 hpi, both ppUL57 and ppUL44 joined UL112-113 gene products at sites that subsequently developed into replication compartments. When infection was carried out in the presence of phosphonoformate or ganciclovir, replication compartment formation was blocked. A viral mutant deficient in uracil DNA glycosidase, previously shown to exhibit a delay in the initial phase of DNA replication, also exhibited delayed formation of replication compartments. These results raise the possibility that subnuclear sites defined by UL112-113 localization orchestrate the assembly of the CMV replication compartment and implicate punctate sites of BrdU incorporation as sites of early viral DNA replication that precedes the formation of the replication compartment.

Expression of the catalytic subunit (UL54) and the accessory protein (UL44) of human cytomegalovirus DNA polymerase in a coupled in vitro transcription/translation system

The catalytic subunit (UL54) and accessory protein (UL44) of human cytomegalovirus (HCMV) DNA polymerase have been cloned and expressed in an in vitro-coupled transcription/translation reticulocyte lysate system. The influence of the 5'-untranslated region (5'-UTR) on the efficiency of expression from the circular plasmids has been investigated. For expression of both UL54 and UL44, a truncated form of the alfalfa mosaic virus (AMV) RNA4 5'-UTR was found to be superior to the full-length AMV 5'-UTR or the original HCMV 5'-UTRs of different lengths. Protein products with Mr approximately 140 and 55 kDa were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis in the UL54 and UL44 in vitro expression reactions, respectively. The properties of the expressed enzyme were compared with those of native HCMV DNA polymerase purified from HCMV-infected cells. DNA polymerase and 3'-5' exonuclease activities of the expressed UL54/UL44 complex were found to be dependent on salt concentration in the same manner as the activities of the native enzyme. The in vitro-expressed enzyme resembles the purified HCMV DNA polymerase in its affinity for deoxynucleoside triphosphates as well as in its sensitivity to known inhibitors (cidofovir diphosphate, ganciclovir triphosphate, and foscarnet). This straightforward method for protein expression may also be applicable to other enzymes where reproducible generation of fully functional products is desirable.

Varicella-zoster virus. The virus

Varicella-zoster virus (VZV) causes chickenpox and herpes zoster. After acute infection the virus becomes latent in dorsal root and trigeminal ganglia for the lifetime of the individual. The viral genome encodes about 70 proteins, at least three of which are thought to be expressed during latency in humans. VZV grows in cell culture, but is very cell-associated; it is relatively difficult to obtain high titers of cell-free virus.

Human cytomegalovirus UL102 gene

We have identified and characterized the transcript for the human cytomegalovirus (HCMV) UL102 gene. The UL102 gene product is proposed to encode the primase-associated factor. The primase-associated factor is one of the three components of the helicase-primase complex, along with UL105 (helicase) and UL70 (primase). In order to characterize the UL102 transcription unit we used single-stranded antisense RNA probes to identify an abundant 2.7-kb transcript originating from the UL102 region. This transcript can be initially detected at 24 h postinfection and in the presence of phosphonoformic acid but not in the presence of cycloheximide. A 2.7-kb cDNA clone containing this transcript was isolated from a 72-h HCMV (strain Towne) cDNA library. Sequence analysis of this clone revealed a continuous unspliced transcript between the region of UL101X and UL102; the only in-frame translational stop codon is 2,619 bp downstream from the first ATG in the message. Genome sequencing of the UL102 region from strains AD169 and Towne revealed that the UL101X stop codon TAA was actually TAC and that the cDNA and genomic sequences were in agreement. The cDNA clone starts 5 nucleotides (nt) upstream of the UL101X ATG, continues through the putative ATG of UL102, and ends 97 nt downstream of the putative termination codon of the UL102 open reading frame. Primer extension analysis indicated a transcriptional start site 23 nt upstream of the UL101X open reading frame.

Functional characterization of partially purified Epstein-Barr virus DNA polymerase expressed in the baculovirus system

The DNA polymerase gene of Epstein-Barr virus (EBV) was cloned into baculovirus transfer vector (pBlueBac). The recombinant baculovirus (AcEBP-15) was obtained by cotransfection of Spodoptera frugiperda (Sf9) cells with infectious DNA from Autographa californica multiple nuclear polyhedrin virus (AcMNPV) and pBlueBac plasmid carrying EBV polymerase gene. Infection of Sf9 cells with the recombinant virus produced substantial quantities of the EBV DNA polymerase protein of the expected size (110 kD). The identity of the EBV polymerase 110-kD polypeptide was determined by (a) immunoprecipitation and Western blot analyses with rabbit polyclonal antiserum specific for a synthetic peptide derived from the coding sequence of the polymerase gene; (b) identification of a polypeptide of identical size (110 kD) from EBV-infected cells; (c) measurement of DNA polymerase activity similar to that of the enzyme induced in EBV-infected cells; and (d) neutralization of the enzymatic activity by the rabbit antiserum and inhibition by phosphonoacetic acid. Our results indicate that the baculovirus expression system provides large quantities of functional polymerase suitable for biochemical and structural analyses, thereby furthering our understanding of the mechanism of viral DNA replication and its inhibition by antiviral drugs.

Serological Responses of Patients with Nasopharyngeal Carcinoma to an N-Terminal Epstein-Barr Virus DNA Polymerase Protein Expressed in Prokaryotic Cells

A 1.7-kb cDNA clone, pGEM-cDP, was isolated from a cDNA library of IUdR-induced p3HR1 cells. It contains the upstream nucleotide sequence of the Epstein-Barr virus (EBV) DNA polymerase gene from 156,859 to 155,088, and was subcloned into expression vector pET3cp* by the polymerase chain reaction, giving the plasmid pDP1. Using a T7 RNA polymerase expression system, a 77-kD polypeptide was produced from pDP1 in Escherichia coli and specific hyperimmune serum was generated in mice. The truncated EBV DNA polymerase was shown to possess the authentic antigenicity by an indirect immunofluorescence assay and by immunoblotting using EBV-containing cells as antigens. Serum from nasopharyngeal carcinoma (NPC) patients and healthy donors was examined for antibodies against the 77-kD polypeptide by Western blot analyses and ELISAs. About 70% NPC patients were positive, while less than 15% of healthy persons showed weak reactivities in ELISAs. Copyright 1994 S. Karger AG, Basel

Eleven loci encoding trans-acting factors are required for transient complementation of human cytomegalovirus oriLyt-dependent DNA replication

Recently we described the use of human cytomegalovirus (HCMV) cosmid clones in a cotransfection assay of HCMV oriLyt replication (G. S. Pari, M. A. Kacica, and D. G. Anders, J. Virol. 67:2575-2582, 1993). We have now used this assay to identify 11 distinct required loci encoding trans-acting factors sufficient for transient complementation of oriLyt-dependent DNA replication. This set includes all of the virus genes essential to initiate and perform DNA synthesis together with the virus genes required to express these replication functions from their native promoters. Six of the identified loci span open reading frames (ORFs) that encode homologs or probable homologs of herpes simplex virus type 1 replication genes, consistent with predictions based on sequence similarities and biochemical properties. These include the DNA polymerase UL54 and polymerase-associated protein UL44, the single-stranded-DNA-binding protein UL57, and proposed subunits of a helicase-primase complex, UL70, UL105, and UL101-102. Frameshift mutations in any one of these essential ORFs abrogated complementation of DNA replication. Three required loci, UL36-38, IRS1 (or TRS1), and IE1/IE2, encode known regulatory proteins. The remaining two loci span ORFs UL84 and UL112-113 and encode early temporal class nucleus-associated proteins of unknown function. Neither of these genes have been implicated previously in DNA replication or in regulating gene expression, nor have counterparts in herpes simplex virus type 1 or Epstein-Barr virus been described. The results presented here will facilitate investigation of the mechanisms and regulation of HCMV lytic-phase DNA replication.

In vitro expression of the human cytomegalovirus DNA polymerase gene: effects of sequence alterations on enzyme activity

Genomic DNA of the Towne strain human cytomegalovirus polymerase (pol) gene (4.4-kb RsrII-NcoI segment of the EcoRI J fragment) was cloned into plasmids containing either the T3 or the T7 promoter for in vitro transcription-translation studies. The translation efficiency of unmodified pol cRNA was poor in this system and could not be improved by capping. However, the efficiency could be enhanced by replacing the leader sequence with a 40-bp AT-rich sequence derived from an alfalfa mosaic virus, R4. pol cRNA directed the synthesis of a 140-kDa polypeptide in a rabbit reticulocyte translation system. The in vitro-translated wild-type enzyme possessed significant polymerization activity which could be stimulated by salt as could that of the authentic enzyme purified from virus-infected cells. To study the critical domains of this enzyme, nine mutations were introduced into the pol gene around the conserved domains of eukaryotic polymerase by oligonucleotide-directed mutagenesis. Two constructs with mutations at amino acid residues 323 to 325 (M32QS) and 725 to 726 (M72II) remained active, with partial loss of enzyme activity, while the enzyme activities of other mutants with alterations at four domains located around amino acid residues 729 to 730 (M73HN), 804 to 807 (M80 and DE80), 910 to 913 (M91 and DE91), and 962 to 964 (M96 and DE96) were abolished. DNA template and triphosphate binding assays indicated that the mutation at 804 to 807 (conserved region III) lost the ability to bind DNA template, and four mutants, M73HN (within conserved region II), M80 (in region III), M91 (in region I), and M96 (around region V [962 to 964; amino acid sequence KKR]), failed to bind deoxyribonucleoside triphosphate. These data suggest that conserved region III is essential for DNA template binding, while residues between conserved region II and V (725 to 964) are involved in triphosphate binding.

Functional expression and characterization of the Epstein-Barr virus DNA polymerase catalytic subunit

A recombinant baculovirus containing the complete sequence for the Epstein-Barr virus (EBV) DNA polymerase catalytic subunit, BALF5 gene product, under the control of the baculovirus polyhedrin promoter was constructed. Insect cells infected with the recombinant virus produced a protein of 110 kDa, recognized by anti-BALF5 protein-specific polyclonal antibody. The expressed EBV DNA polymerase catalytic polypeptide was purified from the cytosolic fraction of the recombinant virus-infected insect cells. The purified protein exhibited both DNA polymerase and 3'-to-5' exonuclease activities, which were neutralized by the anti-BALF5 protein-specific antibody. These results indicate that the 3'-to-5' exonuclease activity associated with the EBV DNA polymerase (T. Tsurumi, Virology 182:376-381, 1991) is an inherent feature of the polymerase catalytic polypeptide. The DNA polymerase and the exonuclease activities of the EBV DNA polymerase catalytic subunit were sensitive to ammonium sulfate in contrast to those of the polymerase complex purified from EBV-producing lymphoblastoid cells, which were stimulated by salt. Furthermore, the template-primer preference for the polymerase catalytic subunit was different from that for the polymerase complex. These observations strongly suggest that the presence of EBV DNA polymerase accessory protein, BMRF1 gene product, does influence the enzymatic properties of EBV DNA polymerase catalytic subunit.

Open reading frames UL44, IRS1/TRS1, and UL36-38 are required for transient complementation of human cytomegalovirus oriLyt-dependent DNA synthesis

Previous results showed that plasmids containing human cytomegalovirus (HCMV) oriLyt are replicated after transfection into permissive cells if essential trans-acting factors are supplied by HCMV infection (D. G. Anders, M. A. Kacica, G. S. Pari, and S. M. Punturieri, J. Virol. 66:3373-3384, 1992). We have now used oriLyt as a reporter of HCMV DNA replication in a transient complementation assay in which cotransfected cosmid clones, instead of HCMV infection, provided essential trans-acting factors. Complemented replication was oriLyt dependent and phosphonoformic acid sensitive and produced tandem arrays typical of HCMV lytic-phase DNA synthesis. Thus, this assay provides a valid genetic test to find previously unidentified genes that are essential for DNA synthesis and to corroborate functional predictions made by nucleotide sequence comparisons and biochemical analyses. Five cosmids were necessary and sufficient to produce origin-dependent DNA synthesis; all but one of these required cosmids contain at least one candidate homolog of herpes simplex virus type 1 replication genes. We further used the assay to define essential regions in two of the required cosmids, pCM1017 and pCM1052. Results presented show that UL44, proposed on the basis of biochemical evidence to be the HCMV DNA polymerase accessory protein, was required for complementation. In addition, three genomic regions encoding regulatory proteins also were needed to produce origin-dependent DNA synthesis in this assay: (i) IRS1/TRS1, which cooperates with the major immediate-early proteins to activate UL44 expression; (ii) UL36-38; and (iii) the major immediate-early region comprising IE1 and IE2. Combined, these results unequivocally establish the utility of this approach for mapping HCMV replication genes. Thus, it will now be possible to define the set of HCMV genes necessary and sufficient for initiating and performing lytic-phase DNA synthesis as well as to identify those virus genes needed for their expression in human fibroblasts.

Humoral immune response to human cytomegalovirus DNA polymerase

In this work, we show that antibodies to baculovirus recombinant DNA polymerase of human cytomegalovirus are present in human sera during natural infection. Specific antibody was found to be of the immunoglobulin G class, produced at low titers only for a short period of time at the beginning of acute infection and not detectable in sera from patients with a convalescent or a latent phase of infection. These results were compared with those obtained with procaryotically expressed p52, the polymerase accessory protein.

Human cytomegalovirus UL97 open reading frame encodes a protein that phosphorylates the antiviral nucleoside analogue ganciclovir

Human cytomegalovirus (HCMV, a betaherpes virus) is the cause of serious disease in immunologically compromised individuals, including those with acquired immunodeficiency syndrome. One of the compounds used in the chemotherapy of HCMV infections is the nucleoside analogue 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (ganciclovir). The mechanism of action of this drug is dependent on the formation of the nucleoside triphosphate, which is a strong inhibitor of the viral DNA polymerase. Thymidine kinase, which is encoded by many of the herpesviruses, catalyses the initial phosphorylation of ganciclovir. But there is no evidence for the coding of this enzyme by HCMV, and DNA sequence analysis of the HCMV genome has shown that there is no open reading frame characteristic of a herpesvirus thymidine kinase. Here we present biochemical and immunological evidence that the HCMV UL97 open reading frame codes for a protein capable of phosphorylating ganciclovir. This protein seems to be responsible for the selectivity of ganciclovir and will be useful tool in the understanding and refinement of the antiviral activity of new selective anti-HCMV compounds.

Physical and functional interaction of human cytomegalovirus DNA polymerase and its accessory protein (ICP36) expressed in insect cells

Expression of the human cytomegalovirus (HCMV) (AD169) DNA polymerase gene under the control of the polyhedrin promoter of Autographa californica nuclear polyhedrosis virus in Spodoptera frugiperda (Sf9) cells has provided a source of highly active CMV DNA polymerase. In extracts from CMV-infected cells, the CMV DNA polymerase is found strongly associated with an additional polypeptide, ICP36. This protein has been identified as the CMV homolog of the herpes simplex virus type 1 UL42 gene product and may have a similar function. We have expressed HCMV DNA polymerase and ICP36 in the same system and demonstrated that they interact to form a stable complex. Moreover, ICP36 functions to stimulate the DNA polymerase activity in a template-dependent manner. We have compared the activity of the recombinant DNA polymerase in the presence and absence of ICP36 on a number of DNA templates and measured the effect of the polymerase inhibitors phosphonoformic acid and acyclovir triphosphate.