Genetic analysis of temperature-sensitive mutants of HSV-1: the combined use of complementation and physical mapping for cistron assignment

Herpesvirus DNA polymerase: Structures, functions, and mechanisms

Herpesviruses comprise a family of DNA viruses that cause a variety of human and veterinary diseases. During productive infection, mammalian, avian, and reptilian herpesviruses replicate their genomes using a set of conserved viral proteins that include a two subunit DNA polymerase. This enzyme is both a model system for family B DNA polymerases and a target for inhibition by antiviral drugs. This chapter reviews the structure, function, and mechanisms of the polymerase of herpes simplex viruses 1 and 2 (HSV), with only occasional mention of polymerases of other herpesviruses such as human cytomegalovirus (HCMV). Antiviral polymerase inhibitors have had the most success against HSV and HCMV. Detailed structural information regarding HSV DNA polymerase is available, as is much functional information regarding the activities of the catalytic subunit (Pol), which include a DNA polymerization activity that can utilize both DNA and RNA primers, a 3'-5' exonuclease activity, and other activities in DNA synthesis and repair and in pathogenesis, including some remaining to be biochemically defined. Similarly, much is known regarding the accessory subunit, which both resembles and differs from sliding clamp processivity factors such as PCNA, and the interactions of this subunit with Pol and DNA. Both subunits contribute to replication fidelity (or lack thereof). The availability of both pharmacologic and genetic tools not only enabled the initial identification of Pol and the pol gene, but has also helped dissect their functions. Nevertheless, important questions remain for this long-studied enzyme, which is still an attractive target for new drug discovery.

Penciclovir-Resistance Mutations in the Herpes Simplex Virus DNA Polymerase Gene

Penciclovir is the active form of the orally available prodrug famciclovir, which is entering clinical use for herpesvirus infections. Like aciclovir, penciclovir is an acyclic guanosine analogue that is phosphorylated by viral thymidine kinase and whose triphosphate can inhibit viral DNA polymerase. We tested several well-characterized herpes simplex virus mutants with aciclovir-resistance mutations in the viral DNA polymerase gene for altered sensitivity to penciclovir. The mutants varied in their susceptibilities to penciclovir with one exhibiting 2-fold hypersensitivity, one marginal resistance and three about 3-fold resistance. Marker rescue and DNA sequencing analyses mapped the penciclovir-resistance mutation of one mutant, AraA r7, to a single base change that alters a glycine to a cysteine at residue 841 within conserved region III of α-like DNA polymerases. The results have implications for the mechanism of selective action of penciclovir, for the potential for development of resistance in the clinic, and for the substrate recognition properties of herpes simplex virus DNA polymerase.

A novel miRNA produced during lytic HSV-1 infection is important for efficient replication in tissue culture

The influence of miRNAs on the host-pathogen environment is largely unknown and under intensive investigation. Whether produced by the pathogen or by the host cell, these miRNAs will sculpt the intracellular landscape, as their activity will ultimately affect levels of target proteins. Using a high-throughput sequencing approach, we identified 19 novel small RNAs produced during the early hours of herpes simplex virus type 1 (HSV-1) infection in epithelial cells. Six of the novel RNAs had predicted folds characteristic of miRNAs. One of the six, miR-92944, which resides in the 5' UTR of the ul42 gene in the sense orientation, was confirmed as a bona fide miRNA by RT-PCR and stem-loop PCR analysis. Northern blot analysis was used to observe the precursor forms of miR-92944. Viral mutants that do not produce miR-92944 exhibited significant reductions in viral titers in both single and multi-step growth analysis and a fourfold reduction in plaque size. The miR-92944 mutants produce wild-type levels of ICP4, UL42, VP5, and gC proteins contain no additional changes in the DNA sequence surrounding the site of mutagenesis. The defective phenotype of miR-92944 mutants was complemented in V42.3 cells, which contain the 5'UTR of ul42. We also found that miR-H1 expression was diminished in cells infected with the miR-92944 mutant virus. This study provides new information on the miRNA landscape during the early stages of HSV-1 infection and reveals novel targets for antagonistic molecules that may curtail the establishment of lytic or latent virus infection.

Modification and reorganization of the cytoprotective cellular chaperone Hsp27 during herpes simplex virus type 1 infection

Chaperone-enriched domains are formed in the nuclei of cells lytically infected with herpes simplex virus type 1 (HSV-1). These domains, called VICE, for virus induced chaperone enriched, contain Hsc70, Hsp70, Hsp40, Hsp90, polyubiquitinated proteins, and components of the proteasome machinery. Accumulating evidence indicates that these sites may be utilized during infection to sequester misfolded, modified, or otherwise unwanted proteins away from viral replication compartments, sites of robust transcription, DNA synthesis, and capsid maturation. To further explore the role of cellular chaperones and VICE domains during HSV-1 infection, we have analyzed the cytoprotective chaperone Hsp27. Here we present evidence that Hsp27, which is known to possess several antioxidant functions, is rapidly reorganized and modified at early stages in response to HSV-1 infection and signaling from the mitogen-activated protein kinase p38. Immunofluorescence analysis and fractionation experiments reveal disparate subcellular localizations of nonphosphorylated and phosphorylated forms of Hsp27 during wild-type HSV-1 infection. Unmodified forms of Hsp27 are localized in nuclear foci that are outside of replication compartments, adjacent to VICE domains, and in the cytoplasm. Conversely, we find that phosphorylated forms of Hsp27 are localized exclusively in the cytoplasm. Last, in cells depleted of all forms of Hsp27, virus replication is significantly reduced.

Involvement of UL24 in herpes-simplex-virus-1-induced dispersal of nucleolin

UL24 of herpes simplex virus 1 is important for efficient viral replication, but its function is unknown. We generated a recombinant virus, vHA-UL24, encoding UL24 with an N-terminal hemagglutinin tag. By indirect immunofluorescence at 9 h post-infection (hpi), we detected HA-UL24 in nuclear foci and in cytoplasmic speckles. HA-UL24 partially co-localized with nucleolin, but not with ICP8 or coilin, markers for nucleoli, viral replication compartments, and Cajal bodies respectively. HA-UL24 staining was often juxtaposed to that of another nucleolar protein, fibrillarin. Analysis of HSV-1-induced nucleolar modifications revealed that by 18 hpi, nucleolin staining had dispersed, and fibrillarin staining went from clusters of small spots to a few separate but prominent spots. Fibrillarin redistribution appeared to be independent of UL24. In contrast, cells infected with a UL24-deficient virus retained foci of nucleolin staining. Our results demonstrate involvement of UL24 in dispersal of nucleolin during infection.

Competition and complementation between thymidine kinase-negative and wild-type herpes simplex virus during co-infection of mouse trigeminal ganglia

Laboratory strains of herpes simplex virus lacking thymidine kinase (TK) cannot replicate acutely to detectable levels in mouse trigeminal ganglia and do not reactivate from latency. However, many pathogenic clinical isolates that are resistant to the antiviral drug acyclovir are heterogeneous populations of TK-negative (TK(-)) and TK-positive (TK(+)) viruses. To recapitulate this in vivo, mice were infected with mixtures of wild-type virus and a recombinant TK(-) mutant in various ratios. Following co-infection, the replication, number of latent viral genomes and reactivation efficiency of TK(+) virus in trigeminal ganglia were reduced in a manner related to the amount of TK(-) virus in the inoculum. TK(+) virus did not always complement the acute replication or increase the number of latent viral genomes of TK(-) mutant in mouse ganglia. Even so, TK(+) virus could still confer the pathogenic phenotype to a TK(-) mutant, somehow providing sufficient TK activity in trans to permit a TK(-) mutant to reactivate from latently infected ganglia.

Nuclear sequestration of cellular chaperone and proteasomal machinery during herpes simplex virus type 1 infection

Herpes simplex virus type 1 (HSV-1) encodes a portal protein that forms a large oligomeric structure believed to provide the conduit for DNA entry and exit from the capsid. Chaperone proteins often facilitate the folding and multimerization of such complex structures. In this report, we show that cellular chaperone proteins, components of the 26S proteasome, and ubiquitin-conjugated proteins are sequestered in discrete foci in the nucleus of the infected cell. The immediate-early viral protein ICP0 was shown to be necessary to establish these foci at early times during infection and sufficient to redistribute chaperone molecules in transfected cells. Furthermore, we found that not only is the portal protein, UL6, localized to these sites during infection, but it is also a substrate for ubiquitin modification. Our results suggest that HSV-1 has evolved an elegant mechanism for facilitating protein quality control at specialized foci within the nucleus.

Characterization of human herpesvirus 8 ORF59 protein (PF-8) and mapping of the processivity and viral DNA polymerase-interacting domains

Human herpesvirus 8 (HHV-8) or Kaposi's sarcoma-associated herpesvirus (KSHV) ORF59 protein (PF-8) is a processivity factor for HHV-8 DNA polymerase (Pol-8) and is homologous to processivity factors expressed by other herpesviruses, such as herpes simplex virus type 1 UL42 and Epstein-Barr virus BMRF1. The interaction of UL42 and BMRF1 with their corresponding DNA polymerases is essential for viral DNA replication and the subsequent production of infectious virus. Using HHV-8-specific monoclonal antibody 11D1, we have previously identified the cDNA encoding PF-8 and showed that it is an early-late gene product localized to HHV-8-infected cell nuclei (S. R. Chan, C. Bloomer, and B. Chandran, Virology 240:118-126, 1998). Here, we have further characterized PF-8. This viral protein was phosphorylated both in vitro and in vivo. PF-8 bound double-stranded DNA (dsDNA) and single-stranded DNA independent of DNA sequence; however, the affinity for dsDNA was approximately fivefold higher. In coimmunoprecipitation reactions, PF-8 also interacted with Pol-8. In in vitro processivity assays with excess poly(dA):oligo(dT) as a template, PF-8 stimulated the production of elongated DNA products by Pol-8 in a dose-dependent manner. Functional domains of PF-8 were determined using PF-8 truncation mutants. The carboxyl-terminal 95 amino acids (aa) of PF-8 were dispensable for all three functions of PF-8: enhancing processivity of Pol-8, binding dsDNA, and binding Pol-8. Residues 10 to 27 and 279 to 301 were identified as regions critical for the processivity function of PF-8. Interestingly, aa 10 to 27 were also essential for binding Pol-8, whereas aa 1 to 62 and aa 279 to 301 were involved in binding dsDNA, suggesting that the processivity function of PF-8 is correlated with both the Pol-8-binding and the dsDNA-binding activities of PF-8.

Novel class of thiourea compounds that inhibit herpes simplex virus type 1 DNA cleavage and encapsidation: resistance maps to the UL6 gene

In our search for novel inhibitors of herpes simplex virus type 1 (HSV-1), a new class of thiourea inhibitors was discovered. N-(4-[3-(5-Chloro-2,4-dimethoxyphenyl)-thioureido]-phenyl)-acetamide and its 2-fluoro-benzamide derivative inhibited HSV-1 replication. HSV-2, human cytomegalovirus, and varicella-zoster virus were inhibited to a lesser extent. The compounds acted late in the replication cycle by impairing both the cleavage of concatameric viral DNA into progeny genome length and the packaging of the DNA into capsids, indicative of a defect in the encapsidation process. To uncover the molecular target of the inhibition, resistant HSV-1 isolates were generated, and the mutation responsible for the resistance was mapped using marker transfer techniques. Each of three independent isolates had point mutations in the UL6 gene which resulted in independent single-amino-acid changes. One mutation was located in the N terminus of the protein (E121D), while two were located close together in the C terminus (A618V and Q621R). Each of these point mutations was sufficient to confer drug resistance when introduced into wild-type virus. The UL6 gene is one of the seven HSV-1 genes known to play a role in DNA packaging. This novel class of inhibitors has provided a new tool for dissection of HSV-1 encapsidation mechanisms and has uncovered a new viable target for the treatment of herpesviral diseases.

A herpes simplex virus DNA polymerase mutation that specifically attenuates neurovirulence in mice

Herpes simplex virus can infect the mammalian brain causing lethal encephalitis (neurovirulence). Previously, herpes simplex virus mutants that are attenuated for neurovirulence have exhibited defects in replication in brain and/or blocks to replication in neuronal cells. We investigated the attenuation of neurovirulence of mutant PAAr5, which exhibits resistance to antiviral drugs due to altered viral DNA polymerase. Following intracerebral inoculation of 7-week-old CD1 mice, PAAr5 was 30-fold attenuated for neurovirulence compared to its wild-type parent. A drug-sensitive virus derived by marker rescue with DNA polymerase gene sequences exhibited neurovirulence that was essentially indistinguishable from that of wild-type virus, demonstrating that attenuation was due to a polymerase mutation. PAAr5 replicated in brain similarly to wild-type virus unlike another polymerase mutant, 615.8, that exhibited a similar degree of attenuation. The attenuation of PAAr5 was not associated with altered particle to PFU ratios nor with any obvious reductions in viral antigen expression in neurons, spread, histopathology, or TUNEL staining suggestive of apoptotic cells. Thus PAAr5 differs from other mutants that are attenuated for neurovirulence. Understanding how a polymerase mutation specifically attenuates neurovirulence may shed light on how herpes simplex virus can cause lethal encephalitis.

Human thymidine kinase can functionally replace herpes simplex virus type 1 thymidine kinase for viral replication in mouse sensory ganglia and reactivation from latency upon explant

Herpes simplex virus type 1 thymidine kinase exhibits a strikingly broad substrate specificity. It is capable of phosphorylating deoxythymidine and deoxyuridine as does human thymidine kinase, deoxycytidine as does human deoxycytidine kinase, the cytosolic kinase whose amino acid sequence it most closely resembles, and thymidylate as does human thymidylate kinase. Following peripheral inoculation of mice, viral thymidine kinase is ordinarily required for viral replication in ganglia and for reactivation from latency following ganglionic explant. To determine which activity of the viral kinase is important for replication and reactivation in mouse ganglia, recombinant viruses lacking viral thymidine kinase but expressing individual human kinases were constructed. Each recombinant virus expressed the appropriate kinase activity with early kinetics following infection of cultured cells. The virus expressing human thymidine kinase exhibited thymidine phosphorylation activity equivalent to approximately 5% of that of wild-type virus in a quantitative plaque autoradiography assay. Nevertheless, it was competent for ganglionic replication and reactivation following corneal inoculation of mice. The virus expressing human thymidylate kinase was partially competent for these activities despite failing to express detectable thymidine kinase activity. The virus expressing human deoxycytidine kinase failed to replicate acutely in neurons or to reactivate from latency. Therefore, it appears that low levels of thymidine phosphorylation suffice to fulfill the role of the viral enzyme in ganglia and that this role can be partially fulfilled by thymidylate kinase activity alone.

The herpes simplex virus type 1 cleavage/packaging protein, UL32, is involved in efficient localization of capsids to replication compartments

Six genes, including UL32, have been implicated in the cleavage and packaging of herpesvirus DNA into preassembled capsids. We have isolated a UL32 insertion mutant which is capable of near-wild-type levels of viral DNA synthesis; however, the mutant virus is unable to cleave and package viral DNA, consistent with the phenotype of a previously isolated temperature-sensitive herpes simplex virus type 1 mutant, tsN20 (P. A. Schaffer, G. M. Aron, N. Biswal, and M. Benyesh-Melnick, Virology 52:57-71, 1973). A polyclonal antibody which recognizes UL32 was previously used by Chang et al. (Y. E. Chang, A. P. Poon, and B. Roizman, J. Virol. 70:3938-3946, 1996) to demonstrate that UL32 accumulates predominantly in the cytoplasm of infected cells. In this report, a functional epitope-tagged version of UL32 showed that while UL32 is predominantly cytoplasmic, some nuclear staining which colocalizes with the major DNA binding protein (ICP8, UL29) in replication compartments can be detected. We have also used a monoclonal antibody (5C) specific for the hexon form of major capsid protein VP5 to study the distribution of capsids during infection. In cells infected with wild-type KOS (6 and 8 h postinfection), 5C staining patterns indicate that capsids are present in nuclei within replication compartments. These results suggest that cleavage and packaging occur in replication compartments at least at 6 and 8 h postinfection. Cells infected with the UL32 mutant exhibit a hexon staining pattern which is more diffusely distributed throughout the nucleus and which is not restricted to replication compartments. We propose that UL32 may play a role in "bringing" preassembled capsids to the sites of DNA packaging and that the failure to localize to replication compartments may explain the cleavage/packaging defect exhibited by this mutant. These results suggest that the UL32 protein is required at a step distinct from those at which other cleavage and packaging proteins are required and may be involved in the correct localization of capsids within infected cells.

Importance of the herpes simplex virus UL24 gene for productive ganglionic infection in mice

The UL24 gene of herpes simplex virus overlaps the viral thymidine kinase (tk) gene. Most previous studies of UL24 have examined UL24 mutants that have also contained tk and sometimes other mutations. To address the importance of UL24 for viral replication in cell culture and in infections of a mammalian host, we constructed a mutant virus containing a UL24 nonsense mutation that does not affect TK activity and a second mutant that contains clustered point mutations in UL24 and a mutation in tk that does not by itself affect the ability of the virus to replicate acutely in mouse ganglia or to reactivate from latent infection following corneal inoculation of mice. Both mutant viruses replicated in cells in culture and in the mouse eye, albeit less efficiently than wild type or control viruses. Both mutants were much more severely impaired for acute replication in trigeminal ganglia and for reactivation from latency following explant of these ganglia. Viral DNA and latency-associated transcripts were present, albeit at lower levels in ganglia infected with the nonsense mutant. These results indicate that UL24 is especially important for productive infection of mouse sensory ganglia and may have implications for the behaviors of certain tk mutants in pathogenesis.

Penciclovir and pathogenesis phenotypes of drug-resistant Herpes simplex virus mutants

We compared the penciclovir susceptibilities and pathogenesis phenotypes of mutants of Herpes simplex virus type 1 that are resistant to acyclovir and/or foscarnet. The mutants, which were derived from laboratory strain KOS, included six DNA polymerase mutants, a thymidine kinase negative mutant, a thymidine kinase partial mutant, and a double mutant. Two of four polymerase mutants not previously examined for penciclovir susceptibility exhibited modest resistance to this drug. A thymidine kinase negative mutant exhibited approximately 20-fold resistance while a thymidine kinase partial mutant was penciclovir-sensitive. Following intracerebral inoculation of 7-week old CD1 mice, the mutants ranged from exhibiting near wild-type neurovirulence (thymidine kinase partial) to modest attenuation (e.g. thymidine kinase negative) to more severe attenuation. Following corneal inoculation, three polymerase mutants exhibited modest deficits (relative to those of thymidine kinase negative mutants) in their abilities to replicate acutely in the ganglion and reactivate from latency. For mutant AraA(r)13, the deficit in ganglionic replication was shown to be due to its polymerase mutation by analysis of recombinant viruses derived by marker rescue. These results may have implications for issues of penciclovir action and resistance, for drug resistance in the clinic, and for the interactions of herpes viruses with the peripheral and central nervous systems.

The U(L)15 gene of herpes simplex virus type 1 contains within its second exon a novel open reading frame that is translated in frame with the U(L)15 gene product

The U(L)15 gene of herpes simplex virus type 1 is composed of two exons. A mutation previously shown to preclude viral DNA cleavage and packaging at the nonpermissive temperature was identified as a change from a highly conserved serine to proline at codon 653. Separate viral mutants that contained stop codons inserted into exon I of U(L)15 (designated S648) or an insertion of the Escherichia coli lacZ gene into a truncated U(L)15 exon II [designated HSV-1(delta U(L)15ExII)] were constructed. Recombinant viruses derived from S648 and HSV-1(delta U(L)15ExII) and containing restored U(L)15 genes were constructed and designated S648R and HSV-1(delta U(L)15ExIIR), respectively. Unlike HSV-1(delta U(L)15ExIIR) and S648R, the viruses containing mutant U(L)15 genes failed to cleave and package viral DNA when propagated on noncomplementing cells. As revealed by electron microscopy, large numbers of enveloped capsids lacking viral DNA accumulated within the cytoplasm of cells infected with either S648 or HSV-1(delta U(L)15ExII) but not in cells infected with HSV-1(delta U(L)15ExIIR) or S648R. Thus, one function of the U(L)15 gene is to effectively prevent immature particles lacking DNA from exiting the nucleus by envelopment at the inner lamella of the nuclear membrane. Cells infected with HSV-1(delta U(L)15ExII) did not express the 75,000- or 35,000-apparent-Mr proteins previously shown to be products of the U(L)15 open reading frame, whereas the 35,000-apparent-Mr protein was readily detectable in cells infected with S648. We conclude that at least the 75,000-Mr protein is required for viral DNA cleavage and packaging and hypothesize that the 35,000-Mr protein is derived from translation of a novel mRNA located partially or completely within the second exon of U(L)15.

Herpes simplex virus DNA replication

The Herpesviridae comprise a large class of animal viruses of considerable public health importance. Of the Herpesviridae, replication of herpes simplex virustype-1 (HSV-1) has been the most extensively studied. The linear 152-kbp HSV-1 genome contains three origins of DNA replication and approximately 75 open-reading frames. Of these frames, seven encode proteins that are required for originspecific DNA replication. These proteins include a processive heterodimeric DNA polymerase, a single-strand DNA-binding protein, a heterotrimeric primosome with 5'-3' DNA helicase and primase activities, and an origin-binding protein with 3'-5' DNA helicase activity. HSV-1 also encodes a set of enzymes involved in nucleotide metabolism that are not required for viral replication in cultured cells. These enzymes include a deoxyuridine triphosphatase, a ribonucleotide reductase, a thymidine kinase, an alkaline endo-exonuclease, and a uracil-DNA glycosylase. Host enzymes, notably DNA polymerase alpha-primase, DNA ligase I, and topoisomerase II, are probably also required. Following circularization of the linear viral genome, DNA replication very likely proceeds in two phases: an initial phase of theta replication, initiated at one or more of the origins, followed by a rolling-circle mode of replication. The latter generates concatemers that are cleaved and packaged into infectious viral particles. The rolling-circle phase of HSV-1 DNA replication has been reconstituted in vitro by a complex containing several of the HSV-1 encoded DNA replication enzymes. Reconstitution of the theta phase has thus far eluded workers in the field and remains a challenge for the future.

Determination and analysis of the complete nucleotide sequence of human herpesvirus

Human herpesvirus 7 (HHV-7) is a recently isolated betaherpesvirus that is prevalent in the human population, with primary infection usually occurring in early childhood. HHV-7 is related to human herpesvirus 6 (HHV-6) in terms of both biological and, from limited prior DNA sequence analysis, genetic criteria. However, extensive analysis of the HHV-7 genome has not been reported, and the precise phylogenetic relationship of HHV-7 to the other human betaherpesviruses HHV-6 and human cytomegalovirus has not been determined. Here I report on the determination and analysis of the complete DNA sequence of HHV-7 strain JI. The data establish that the close biological relationship of HHV-6 and HHV-7 is reflected at the genetic level, where there is a very high degree of conservation of genetic content and encoded amino acid sequences. The data also delineate loci of divergence between the HHV-6 and HHV-7 genomes, which occur at the genome terminal in the region of the terminal direct-repeat elements and within limited regions of the unique component. Of potential significance with respect to biological and evolutionary divergence of HHV-6 and HHV-7 are notable structural differences in putative transcriptional regulatory genes specified by the direct-repeat and immediate-early region A loci of these viruses and the absence of an equivalent of the HHV-6 adeno-associated virus type 2 rep gene homolog in HHV-7.

Prolonged gene expression and cell survival after infection by a herpes simplex virus mutant defective in the immediate-early genes encoding ICP4, ICP27, and ICP22

Very early in infection, herpes simplex virus (HSV) expresses four immediate-early (IE) regulatory proteins, ICP4, ICP0, ICP22, and ICP27. The systematic inactivation of sets of the IE proteins in cis, and the subsequent phenotypic analysis of the resulting mutants, should provide insights into how these proteins function in the HSV life cycle and also into the specific macromolecular events that are altered or perturbed in cells infected with virus strains blocked very early in infection. This approach may also provide a rational basis to assess the efficacy and safety of HSV mutants for use in gene transfer experiments. In this study, we generated and examined the phenotype of an HSV mutant simultaneously mutated in the ICP4, ICP27, and ICP22 genes of HSV. Unlike mutants deficient in ICP4 (d120), ICP4 and ICP27 (d92), and ICP4 and ICP22 (d96), mutants defective in ICP4, ICP27, and ICP22 (d95) were visually much less toxic to Vero and human embryonic lung cells. Cells infected with d95 at a multiplicity of infection of 10 PFU per cell retained a relatively normal morphology and expressed genes from the viral and cellular genomes for at least 3 days postinfection. The other mutant backgrounds were too toxic to allow examination of gene expression past 1 day postinfection. However, when cell survival was measured by the capacity of the infected cells to form colonies, d95 inhibited colony formation similarly to d92. This apparent paradox was reconciled by the observation that host cell DNA synthesis was inhibited in cells infected with d120, d92, d96, and d95. In addition, all of the mutants exhibited pronounced and distinctive alterations in nuclear morphology, as determined by electron microscopy. The appearance of d95-infected cells deviated from that of uninfected cells in that large circular structures formed in the nucleus. d95-infected cells abundantly expressed ICP0, which accumulated in fine punctate structures in the nucleus at early times postinfection and coalesced or grew to the large circular objects that were revealed by electron microscopy. Therefore, while the abundant accumulation of ICPO in the absence of ICP4, ICP22, and ICP27 may allow for prolonged gene expression, cell survival is impaired, in part, as a result of the inhibition of cellular DNA synthesis.

Properties of the protein encoded by the UL32 open reading frame of herpes simplex virus 1

The functions previously assigned to the essential herpes simplex virus 1 UL32 protein were in cleavage and/or packaging of viral DNA and in maturation and/or translocation of viral glycoproteins to the plasma membrane. The amino acid sequence predicts N-linked glycosylation sites and sequences conserved in aspartyl proteases and in zinc-binding proteins. We report the following. (i) The 596-amino-acid UL32 protein accumulated predominantly in the cytoplasm of infected cells but was not metabolically labeled with glucosamine and did not band with membranes containing a known glycoprotein in flotation sucrose density gradients. The UL32 protein does not, therefore, have the properties of an intrinsic membrane protein. (ii) Experiments designed to demonstrate aspartyl protease activity in a phage display system failed to reveal proteolytic activity. Moreover, substitution of Asp-110 with Gly in the sequence Asp-Thr-Gly, the hallmark of aspartyl proteases, had no effect on viral replication in Vero and SK-N-SH cell lines or in human foreskin fibroblasts. Therefore, if the UL32 protein functions as a protease, this function is not required in cells in culture. (iii) Both the native UL32 protein and a histidine-tagged UL32 protein made in recombinant baculovirus-infected insect cells bound zinc. The consensus sequence is conserved in the UL32 homologs from varicella-zoster virus and equine herpesvirus 1. UL32 protein is therefore a cysteine-rich, zinc-binding essential cytoplasmic protein whose function is not yet clear.

The expression of the proteins of equine herpesvirus 1 which share homology with herpes simplex virus 1 glycoproteins H and L

Several expression systems were used in studies aimed at characterizing the equine herpesvirus 1 (EHV-1) glycoprotein H and L homologues of HSV-1 (EHV-1 gH and gL) and the products were compared to the authentic proteins synthesized in virus infected cells. Using an in vitro transcription/translation system two gH species were detected (an unprocessed 89 kDa and a processed 116 kDa product). Three low molecular weight proteins were found in the case of gL (21.8 kDa, 22.9 kDa and 26.9 kDa) and these showed a slight reduction in mobility on the addition of microsomal membranes to the reactions. A gL fusion protein was produced in pGEX-2T, expression being confirmed by Western blotting using a gL-specific antiserum raised against a peptide incorporating the 13 carboxyl terminal amino acids of the protein. A gH specific peptide antiserum precipitated both gH and two smaller proteins from EHV-1 infected cells thought to be two forms of gL. Insect cells infected with gH or gL baculovirus recombinants were used to vaccinate C3H (H-2k) mice. Some protection against EHV-1 infection was conferred to the gH inoculated mice. The results will enable further studies on the importance of the gH and gL interaction in the pathogenesis of EHV-1 to be evaluated and their potential in contributing to a subunit vaccine to be assessed.

Synergistic effects on ganglionic herpes simplex virus infections by mutations or drugs that inhibit the viral polymerase and thymidine kinase

Herpes simplex virus encodes proteins, such as DNA polymerase, that are essential for its replication and proteins, such as thymidine kinase, that are not essential for replication in cell culture, but are important for pathogenesis in animal models. However, certain mutations affecting these proteins exert little or no effect on replication or pathogenesis. We tested the effects of combining two such mutations--one that alters DNA polymerase and one that decreases but does not abolish thymidine kinase activity--on replication in cultured cells and on acute and latent infections in mice. The double mutant replicated similarly to the single mutants and wild-type virus both in cell culture and acutely in the mouse eye. However, it was severely impaired for acute replication in trigeminal ganglia and for reactivatable latent infections. This impairment depended upon the polymerase mutation. Similarly, although Ro 31-5140, a thymidine kinase inhibitor, did not potentiate the antiviral effects of phosphonoacetic acid, a polymerase inhibitor, in cell culture, the two drugs in combination substantially inhibited viral reactivation from latency at concentrations that had little or no effect when used singly. These synergistic effects may have implications for viral functions during pathogenesis and for antiviral chemotherapy.

A net +1 frameshift permits synthesis of thymidine kinase from a drug-resistant herpes simplex virus mutant

Clinical resistance to antiviral drugs requires that a virus evade drug therapy yet retain pathogenicity. Thymidine kinase (TK)-negative mutants of herpes simplex virus are resistant to the drug, acyclovir, but are attenuated for pathogenicity in animal models. However, numerous cases of clinical resistance to acyclovir have been associated with viruses that were reported to express no TK activity. We studied an acyclovir-resistant clinical mutant that contains a single-base insertion in its tk gene, predicting the synthesis of a truncated TK polypeptide with no TK activity. Nevertheless, the mutant retained some TK activity and the ability to reactivate from latent infections of mouse trigeminal ganglia. The mutant expressed both the predicted truncated polypeptide and a low level of a polypeptide that comigrated with full-length TK on polyacrylamide gels and reacted with anti-TK antiserum, providing evidence for a frameshifting mechanism. In vitro transcription and translation of mutant tk genes, including constructs in which reporter epitopes could be expressed only if frameshifting occurred, also gave rise to truncated and full-length polypeptides. Reverse transcriptase-polymerase chain reaction analysis coupled with open reading frame cloning failed to detect alterations in tk transcripts that could account for the synthesis of full-length polypeptide. Thus, synthesis of full-length TK was due to an unusual net +1 frameshift during translation, a phenomenon hitherto confined in eukaryotic cells to certain RNA viruses and retrotransposons. Utilization of cellular frameshifting mechanisms may permit an otherwise TK-negative virus to exhibit clinical acyclovir resistance.

Heterogeneity of a herpes simplex virus clinical isolate exhibiting resistance to acyclovir and foscarnet

Resistance of herpes simplex virus to acyclovir is a problem of growing clinical importance. Acyclovir-resistance can be due either to mutations in the viral thymidine kinase gene or in the viral DNA polymerase gene. Although clinical resistance has most frequently been associated with thymidine kinase alterations, heterogeneity in clinical isolates has not been addressed frequently. The potential for such heterogeneity has been emphasized by a report describing a pathogenic clinical isolate containing within its population at least one thymidine kinase-proficient DNA polymerase mutant as well as mutants exhibiting thymidine kinase-deficiency (Sacks, et al., 1989). We provide here additional characterization of this isolate and speculations regarding its significance.

The six conserved helicase motifs of the UL5 gene product, a component of the herpes simplex virus type 1 helicase-primase, are essential for its function

The UL5 protein of herpes simplex virus type 1, one component of the viral helicase-primase complex, contains six sequence motifs found in all members of a superfamily of DNA and RNA helicases. Although this superfamily contains more than 20 members ranging from bacteria to mammalian cells and their viruses, the importance of these motifs has not been addressed experimentally for any one of them. In this study, we have examined the functional significance of these six motifs for the UL5 protein through the introduction of site-specific mutations resulting in single amino acid substitutions of the most highly conserved residues within each motif. A transient replication complementation assay was used to test the effect of each mutation on the function of the UL5 protein in viral DNA replication. In this assay, a mutant UL5 protein expressed from an expression clone is used to complement a replication-deficient null mutant with a mutation in the UL5 gene for the amplification of herpes simplex virus origin-containing plasmids. Eight mutations in conserved regions and three similar mutations in nonconserved regions of the UL5 gene were analyzed, and the results indicate that all six conserved motifs are essential to the function of UL5 protein in viral DNA replication; on the other hand, mutations in nonconserved regions are tolerated. These data provide the first direct evidence for the importance of these conserved regions in any member of the superfamily of DNA and RNA helicases. In addition, three motif mutations were introduced into the viral genome, and the phenotypic analyses of these mutants are consistent with results from the transient replication complementation assay. The ability of these three mutant UL5 proteins to form specific interactions with other members of the helicase-primase complex, UL8 and UL52, indicates that the functional domains required for replication activity of UL5 are separable from domains responsible for protein-protein interactions. It is anticipated that this type of structure-function analysis will lead to the identification of protein domains that contribute not only to the enzymatic activities of helicase or primase but also to protein-protein interactions within members of the complex.

Replacement mutagenesis of the human cytomegalovirus genome: US10 and US11 gene products are nonessential

The US6 gene family, located within the unique short region (US) of the human cytomegalovirus (HCMV) genome, contains six open reading frames (US6 through US11) which may encode glycoproteins, such as gcII (D. Gretch, B. Kari, R. Gehrz, and M. Stinski, J. Virol. 62:1956-1962, 1988). By homologous recombination, several different recombinant HCMV were created which contain a marker gene, beta-glucuronidase, inserted within this gene family. It was demonstrated that beta-glucuronidase has utility as a marker gene for the identification of recombinants in this herpesvirus system, without the occurrence of deletions in other regions of the viral genome. DNA and RNA blot analyses attested to the fidelity of the recombination. Immunoprecipitation experiments using monospecific polyclonal antisera indicated that the US10 and/or US11 gene products were not expressed in the recombinants, as predicted. These results, along with single-cycle growth analyses, indicated that the US10 and US11 gene products are nonessential for virus replication and growth in tissue culture. HCMV recombinants expressing beta-glucuronidase seemed to be genetically stable.

Polymerization activity of an alpha-like DNA polymerase requires a conserved 3'-5' exonuclease active site

Most DNA polymerases are multifunctional proteins that possess both polymerizing and exonucleolytic activities. For Escherichia coli DNA polymerase I and its relatives, polymerase and exonuclease activities reside on distinct, separable domains of the same polypeptide. The catalytic subunits of the alpha-like DNA polymerase family share regions of sequence homology with the 3'-5' exonuclease active site of DNA polymerase I; in certain alpha-like DNA polymerases, these regions of homology have been shown to be important for exonuclease activity. This finding has led to the hypothesis that alpha-like DNA polymerases also contain a distinct 3'-5' exonuclease domain. We have introduced conservative substitutions into a 3'-5' exonuclease active site homology in the gene encoding herpes simplex virus DNA polymerase, an alpha-like polymerase. Two mutants were severely impaired for viral DNA replication and polymerase activity. The mutants were not detectably affected in the ability of the polymerase to interact with its accessory protein, UL42, or to colocalize in infected cell nuclei with the major viral DNA-binding protein, ICP8, suggesting that the mutation did not exert global effects on protein folding. The results raise the possibility that there is a fundamental difference between alpha-like DNA polymerases and E. coli DNA polymerase I, with less distinction between 3'-5' exonuclease and polymerase functions in alpha-like DNA polymerases.

Polymerization activity of an alpha-like DNA polymerase requires a conserved 3'-5' exonuclease active site

Most DNA polymerases are multifunctional proteins that possess both polymerizing and exonucleolytic activities. For Escherichia coli DNA polymerase I and its relatives, polymerase and exonuclease activities reside on distinct, separable domains of the same polypeptide. The catalytic subunits of the alpha-like DNA polymerase family share regions of sequence homology with the 3'-5' exonuclease active site of DNA polymerase I; in certain alpha-like DNA polymerases, these regions of homology have been shown to be important for exonuclease activity. This finding has led to the hypothesis that alpha-like DNA polymerases also contain a distinct 3'-5' exonuclease domain. We have introduced conservative substitutions into a 3'-5' exonuclease active site homology in the gene encoding herpes simplex virus DNA polymerase, an alpha-like polymerase. Two mutants were severely impaired for viral DNA replication and polymerase activity. The mutants were not detectably affected in the ability of the polymerase to interact with its accessory protein, UL42, or to colocalize in infected cell nuclei with the major viral DNA-binding protein, ICP8, suggesting that the mutation did not exert global effects on protein folding. The results raise the possibility that there is a fundamental difference between alpha-like DNA polymerases and E. coli DNA polymerase I, with less distinction between 3'-5' exonuclease and polymerase functions in alpha-like DNA polymerases.

Herpes simplex virus (HSV) glycoprotein H is partially processed in a cell line that expresses the glycoprotein and fully processed in cells infected with deletion or ts mutants in the known HSV glycoproteins

Cell lines that constitutively express herpes simplex virus 1 (HSV-1) glycoprotein H (gH-1) failed to synthesize the mature form of gH and accumulated a precursor-like form of the glycoprotein, which was retained intracellularly, most likely in RER. Fine-structure analysis of the oligosaccharides present in recombinant gH revealed oligosaccharides processed by RER enzymes; sialylated complex-type and biantennary oligosaccharides, which are assembled in the trans-Golgi, were absent. A small fraction had the characteristics of oligosaccharides processed by the early mannosidases of the Golgi. These findings suggest that a defect in the transport out of RER to the Golgi may account for the intracellular retention of the immature form of gH in cells that express the glycoprotein constitutively. Upon superinfection of cells expressing gH-1 with HSV-2, recombinant gH-1 underwent maturation, indicating that a viral function is required to attain full processing of gH. The known HSV glycoproteins do not appear to carry out this function, since in cells infected with deletion mutants in gD, gG, gE, and gE-gI, with a spontaneous gC- mutant, or with a temperature-sensitive mutant in gB, maturation of gH occurred independently of the presence or of the maturation of the single glycoproteins tested. The present findings together with previous observations on HSV, human CMV, and the EBV homologue of gH suggest that inability of gH to undergo full processing in the absence of viral protein(s) is a property of gH.

Sequence characteristics of a gene in equine herpesvirus 1 homologous to glycoprotein H of herpes simplex virus

A gene in equine herpesvirus 1 (EHV-1, equine abortion virus) homologous to the glycoprotein H gene of herpes simplex virus (HSV) was identified and characterised by its nucleotide and derived amino acid sequence. The EHV-1 gH gene is located at 0.47-0.49 map units and contains an open reading frame capable of specifying a polypeptide of 848 amino acids, including N- and C-terminal hydrophobic domains consistent with signal and membrane anchor regions respectively, and 11 potential sites for N-glycosylation. Alignment of the amino acid sequence with those published for HSV gH, varicella zoster virus gpIII, Epstein Barr virus gp85 and human cytomegalovirus p86 shows similarity of the EHV gene with the 2 other alpha-herpesviruses over most of the polypeptide, but only the C-terminal half could be aligned for all 5 viruses. The identical positioning of 6 cysteine residues and a number of highly conserved amino acid motifs supports a common evolutionary origin of this gene and is consistent with its role as an essential glycoprotein of the herpesvirus family. An origin of replication is predicted to occur at approximately 300 nucleotides downstream of the EHV-1 gH coding region, on the basis of similarity to other herpesvirus origins.

Engineered herpes simplex virus DNA polymerase point mutants: the most highly conserved region shared among alpha-like DNA polymerases is involved in substrate recognition

Eucaryotic, viral, and bacteriophage DNA polymerases of the alpha-like family share blocks of sequence similarity, the most conserved of which has been designated region I. Region I includes a YGDTDS motif that is almost invariant within the alpha-like family and that is similar to a motif conserved among RNA-directed polymerases and also includes adjacent amino acids that are more moderately conserved. To study the function of these conserved amino acids in vivo, site-specific mutagenesis was used to generate herpes simplex virus region I mutants. A recombinant virus constructed to contain a mutation within the nearly invariant YGDTDS motif was severely impaired for growth on Vero cells which do not contain a viral polymerase gene. However, three recombinants constructed to contain mutations altering more moderately conserved residues grew on Vero cells and exhibited altered sensitivities to nucleoside and PPi analogs and to aphidicolin. Marker rescue and DNA sequencing of one such recombinant demonstrated that the region I alteration confers the altered drug sensitivity phenotype. These results indicate that this region has an essential role in polymerase function in vivo and is involved directly or indirectly in drug and substrate recognition.

Quantitative polymerase chain reaction analysis of herpes simplex virus DNA in ganglia of mice infected with replication-incompetent mutants

To study the roles of viral genes in the establishment and maintenance of herpes simplex virus (HSV) latency, we have developed a polymerase chain reaction assay that is both quantitative and sensitive. Using this assay, we analyzed the levels of viral DNA in trigeminal ganglia of mice inoculated corneally with HSV mutants that are defective for virus replication at one or more sites in mice and for reactivation upon ganglionic explant. Ganglia from mice infected with thymidine kinase-negative mutants, which replicate at the site of inoculation and establish latency but do not replicate acutely in ganglia or reactivate upon explant, contained a range of levels of HSV DNA that overlapped with the range found in ganglia latently infected with wild-type virus. On average, these mutant-infected ganglia contained one copy of HSV DNA per 100 cell equivalents (ca. 10(4) molecules), which was 50-fold less than the average for wild-type virus. Ganglia from mice infected with a ribonucleotide reductase deletion mutant, which is defective for acute replication and reactivation upon ganglionic explant, also contained on average one copy of HSV DNA per 100 cell equivalents. We also detected substantial numbers of HSV DNA molecules (up to ca. 10(3] in ganglia of mice infected with an ICP4 deletion mutant and other replication-negative mutants that are severely impaired for viral DNA replication and gene expression. These results raise the possibility that such mutants can establish latency, which could have important implications for mechanisms of latency and for vaccine and antiviral drug development.

Identification of three genes nonessential for growth in cell culture near the right terminus of the unique sequences of long component of herpes simplex virus 1

We report the construction of herpes simplex virus 1 recombinants from which all or part of the coding sequences of four open reading frames have been deleted. In recombinants R7101 and R7108, the BamHI D' fragment containing the coding sequences of the dUTPase gene and the promoter-regulatory domain of a late gene was either replaced with a fragment containing a thymidine kinase gene or deleted, respectively. The recombinant R7107 lacks, in addition to BamHI D', 202 of a total of 244 predicted amino acids from the 3' end of the adjacent open reading frame UL51. The deletion in recombinant R7105 encompasses two genes, i.e., the entire open reading frame of UL47 and the amino terminus of UL46. These genes map next to that specifying the alpha trans-inducing factor. R7105, R7101, and R7108 do not exhibit demonstrable defects in viral replication. The recombinant R7107 forms minute plaques and replicates best in multiplying cells in subconfluent cultures. The results indicate that the multiplying cells supply at least some of the functions expressed by the protein encoded in UL51.

Herpes simplex virus activates expression of a cellular gene by specific binding to the cell surface

The herpes simplex virus (HSV) type 1 mutant ts1204 attaches to the cell surface at 38.5 degrees but fails to penetrate the plasma membrane. A striking feature of human fetal lung cells infected with ts1204 at 38.5 degrees was the presence of enhanced amounts of a 56,000 molecular weight host protein, p56. Studies with protein and RNA synthesis inhibitors suggested that binding of the mutant virus to cells activated expression of the cellular gene encoding p56 and not an intermediary protein. Evidence presented in this paper supports the idea that p56 is induced by a specific interaction between ts1204 virions and the cell surface.

Isolation and characterization of herpes simplex virus mutants containing engineered mutations at the DNA polymerase locus

We have derived Vero cell lines containing the herpes simplex virus DNA polymerase (pol) gene that complement temperature-sensitive pol mutants. These cell lines were used to recover viruses containing new mutations at the pol locus. Two spontaneously arising host-range mutants, 6C4 and 7E4, were isolated. These mutants did not grow efficiently on Vero cells or synthesize late polypeptides but formed plaques on a cell line containing the pol gene (DP6 cells). Whereas mutant 6C4 specified a wild-type-size Pol protein, we detected no full-length Pol protein in 7E4-infected cell extracts. Complementation studies demonstrated that 6C4 and 7E4 contain different mutations and indicated that 6C4 is in a complementation group different from that of pol temperature-sensitive mutant tsC7 or tsD9. A mutant in which 2.2 kilobases of pol sequences were replaced with the Escherichia coli lacZ gene under the control of the herpes simplex virus thymidine kinase promoter was constructed. This mutant formed blue plaques on DP6 cells in the presence of 5-bromo-4-chloro-3-indolyl-beta-D-galactoside. Using this virus in marker rescue experiments, we engineered three mutants containing deletions in the pol coding region which grew efficiently on DP6 cells but not on Vero cells and which differed in their synthesis of Pol polypeptides. The lacZ insertion virus was also used to introduce a deletion in the region upstream of the pol long open reading frame, which removes a short open reading frame that could encode a 10-amino-acid peptide. This mutant grew to similar titers on Vero and DP6 cells, indicating that these sequences are not essential for growth of the virus in tissue culture.

Enzymatic activities of overexpressed herpes simplex virus DNA polymerase purified from recombinant baculovirus-infected insect cells

Biochemical characterization of the herpes simplex virus (HSV) DNA polymerase, a model DNA polymerase and an important target for antiviral drugs, has been limited by a lack of pure enzyme in sufficient quantity. To overcome this limitation, the HSV DNA polymerase gene was introduced into the baculovirus, Autographa californica nuclear polyhedrosis virus, under the control of the polyhedrin promoter to give rise to a recombinant baculovirus, BP58. BP58-infected Spodoptera frugiperda insect cells expressed a polypeptide that was indistinguishable from authentic polymerase by several immunological and biochemical properties, at levels approximately ten-fold higher per infected cell than found in HSV-infected Vero cells. The DNA polymerase was purified to apparent homogeneity from BP58-infected insect cells. Using activated DNA as primer-template, the purified enzyme exhibited specific activity similar to that of enzyme isolated from HSV-infected Vero cells, indicating that additional polymerase-associated proteins from HSV-infected cells are not critical for activity with this primer-template. 3'-5' exonuclease activity co-purified with the BP58-expressed HSV DNA polymerase, demonstrating that this activity is intrinsic to the polymerase polypeptide. The purified enzyme also exhibited RNAse H activity. The recombinant baculovirus should permit detailed biochemical and biophysical studies of this enzyme.

Localization on the herpes simplex virus type 1 genome of a region encoding proteins involved in adsorption to the cellular receptor

We have previously shown that aminoglycosides such as neomycin and the polyamino acids polylysine and polyarginine selectively inhibit the binding of herpes simplex virus type 1 (HSV-1) to the cellular receptor, whereas HSV-2 infection is unaffected. In the present study we took advantage of this difference between HSV-1 and HSV-2 by using HSV(-1)-HSV(-2) intertypic recombinants to locate a region on the HSV-1 genome encoding proteins affecting the binding of the virion to the cellular receptor. The results were consistent with those obtained by marker rescue experiments. The identified region, which mapped between coordinates 0.580 and 0.687, contains two partial and eight complete genes, including the glycoprotein C (gC) gene and two others with potential transmembrane sequences. Various gC monoclonal antibody-resistant mutants of HSV-1 as well as a mutant completely lacking gC were found to be fully sensitive to neomycin, suggesting that gC is not the site of drug sensitivity and is not essential for adsorption of virus to the cellular receptor. However, the rate of adsorption was reduced in the absence of gC, indicating a facilitating function of the glycoprotein. The universal nature of this HSV-1 receptor binding was revealed by the similarity in drug sensitivity of infectivity in four different cell lines from various tissues and species.

Antigenic properties and cellular localization of herpes simplex virus glycoprotein H synthesized in a mammalian cell expression system

Herpes simplex virus type 1 glycoprotein H (HSV-1 gH) was synthesized in an inducible mammalian cell expression system, and its properties were examined. The gH coding sequence, together with the stable 5' untranslated leader sequence from xenopus beta-globin, was placed under control of the strong promoter from the human cytomegalovirus major immediate-early gene in an amplifiable plasmid which contains the simian virus 40 (SV40) virus origin for replication (ori). This expression vector was transfected into ts COS cells constitutively expressing a temperature-sensitive SV40 T antigen which allows utilization of the SV40 ori at permissive temperatures. The results of transient expression assays at the permissive temperature showed that HSV-1 gH could be synthesized in greater amounts than those produced by a high-multiplicity virus infection. The proteins produced were detected in Western blots (immunoblots) with a HSV-1 gH-specific polyclonal serum raised against a TrpE-gH fusion protein. The transfected gH had an apparent molecular weight of approximately 105,000, intermediate in size to those of the precursor (100,000) and fully processed forms (110,000) of HSV-1 gH from infections. Antigenicity was investigated by reactions with three virus-neutralizing monoclonal antibodies specific for conformational epitopes on gH. Only one of these monoclonal antibodies could immunoprecipitate the synthesized gH. However, equal recognition of the transfected gH was achieved by superinfection with virus. In addition, detectable amounts of gH were not expressed on the cell surface unless the cells were superinfected with virus. Studies with a temperature-sensitive mutant, ts1201, defective in encapsidation showed that the changes in antigenic structure and cell surface expression caused by superinfection with virus were not due simply to incorporation of gH into virions. These results suggest that gH requires additional virus gene products for cell surface localization and formation of an antigenic structure important for its function in mediating infectivity.

A herpes simplex virus ribonucleotide reductase deletion mutant is defective for productive acute and reactivatable latent infections of mice and for replication in mouse cells

Herpes simplex virus encodes a ribonucleotide reductase that is not essential for virus growth in dividing cells at 37 degrees. This enzyme has been proposed as a target for antiviral drugs; its utility in this regard could depend upon its importance in vivo. To test the requirement of viral ribonucleotide reductase in a mammalian host, we tested a mutant virus, lacking most of the gene encoding the ribonucleotide reductase large subunit, in a mouse eye model of pathogenesis and latency where the wild-type virus establishes reactivatable latent infections in trigeminal ganglia following corneal inoculation. The deletion mutant was severely impaired in its ability to replicate acutely in the eye and in the trigeminal ganglion and failed to establish reactivatable latent infections. In contrast, a recombinant virus in which the deleted sequences were restored was competent for both acute and latent infections. The defects of the deletion mutant in the mouse may be related to its severely impaired growth at 38 degrees in mouse cells relative to its growth in Vero cells. These results indicate that ribonucleotide reductase is critical for productive acute and reactivatable latent infections in mice and replication in mouse cells at 38 degrees and suggest that caution be exercised in extrapolating from studies conducted in mice to human infections when judging the utility of this enzyme as a target for antiviral chemotherapy.

Thymidine kinase-negative herpes simplex virus mutants establish latency in mouse trigeminal ganglia but do not reactivate

Herpes simplex virus infection of mammalian hosts involves lytic replication at a primary site, such as the cornea, translocation by axonal transport to sensory ganglia and replication, and latent infection at a secondary site, ganglionic neurons. The virus-encoded thymidine kinase, which is a target for antiviral drugs such as acyclovir, is not essential for lytic replication yet evidently is required at the secondary site for replication and some phase of latent infection. To determine the specific stage in viral pathogenesis at which this enzyme is required, we constructed virus deletion mutants that were acyclovir resistant and exhibited no detectable thymidine kinase activity. After corneal inoculation of mice, the mutants replicated to high titers in the eye but were severely impaired for acute replication in trigeminal ganglia and failed to reactivate from ganglia upon cocultivation with permissive cells. Nevertheless, latency-associated transcripts were expressed in neuronal nuclei of ganglia from mutant-infected mice and superinfection of the ganglia with a second virus rescued the latent mutant virus. Thus, contrary to a widely accepted hypothesis, the thymidine kinase-negative mutants established latent infections, implying that neither thymidine kinase activity nor ganglionic replication is necessary for establishment of latency. Rather, thymidine kinase appears to be necessary for reactivation from latency. These results suggest that acyclovir-resistant viruses could establish latent infections in clinical settings and have implications for the use of genetically engineered herpesviruses to deliver foreign genes to neurons.

Herpes simplex virus 1 helicase-primase: a complex of three herpes-encoded gene products

In an earlier report, we described a DNA helicase that is specifically induced upon infection of Vero cells with herpes simplex virus 1. We have purified this enzyme to near homogeneity and found it to consist of three polypeptides with molecular weights of 120,000, 97,000, and 70,000. Immunochemical analysis has shown these polypeptides to be the products of three of the genes UL52, UL5, and UL8 that are required for replication of a plasmid containing a herpes simplex 1 origin (oriS). In addition to helicase activity, the enzyme contains a tightly associated DNA primase. Thus, the three-subunit enzyme is a helicase-primase complex that may prime lagging-strand synthesis as it unwinds DNA at the viral replication fork.

Herpes simplex virus type 1 DNA synthesis requires the product of the UL8 gene: isolation and characterization of an ICP6::lacZ insertion mutation

We investigated the role of the herpes simplex virus type 1 UL8 gene product in viral DNA replication. First, we unambiguously fine mapped the mutation in tsS38 (complementation group 1-26) to an open reading frame, designated UL8, predicted to encode an 80-kilodalton protein. Previous studies indicated that tsS38 was capable of synthesizing low to moderate levels of viral DNA at the nonpermissive temperature (C. T. Chu, D. S. Parris, R. A. F. Dixon, F. E. Farber, and P. A. Schaffer, Virology 98:168-181, 1979); thus, it was not clear whether the UL8 gene product is essential for viral DNA synthesis. Therefore, a deletion-insertion mutation was constructed in the UL8 gene by removing most of its coding sequences and replacing them with the Escherichia coli lacZ gene under control of the viral ICP6 regulatory signals. The resulting recombinant, hr80, was propagated in helper cells (S22) which express the wild-type version of the UL8 gene, but was incapable of forming plaques in Vero cells. Furthermore, hr80 was totally defective in the synthesis of viral DNA and late proteins under nonpermissive growth conditions. These results demonstrated that the UL8 gene product is essential for viral DNA synthesis.

Effect of an amber mutation in the herpes simplex virus thymidine kinase gene on polypeptide synthesis and stability

KG111 is a mutant of herpes simplex virus (HSV), strain KOS, that exhibits temperature-dependent drug resistance. For example, it is almost as resistant as a thymidine kinase (tk)-deficient virus at 39 degrees, but is relatively sensitive to acyclovir at 34 degrees, Using marker transfer techniques, we have mapped the mutation conferring temperature-dependent drug resistance in KG111 to the 5' portion of the tk gene. Sequencing of this region revealed an amber mutation at codon 44, which lies between the first and second methionine codons of the tk polypeptide. This mutation is identical to that found in TK4, an HSV mutant derived from Cl 101 (L. Haarr et al., 1985, J. Virol. 56, 512-519). Analyses of immunoprecipitated tk proteins from KG111- and TK4-infected cells showed that KG111 and TK4 do not synthesize full-length tk polypeptides, but instead produce a truncated form of the protein. Small amounts of a similar truncated tk polypeptide are also produced in wild-type-infected cells and are thought to arise from initiation at a downstream AUG. The relative amounts and size of the mutant tk proteins compared with those of the wild-type are consistent with the amber mutation eliminating translation of full-length polypeptide and causing a four- to fivefold increase in the utilization of downstream AUG codons for initiation. The truncated polypeptides specified by KG111 and TK4 are less stable than the full-length polypeptide at 39 degrees, which may contribute to the conditional drug-resistant phenotype. On the other hand, the truncated polypeptides normally expressed by wild-type virus at low levels and the more highly expressed truncated tk polypeptides from a deletion mutant are relatively stable at 39 degrees. These results suggest that stability of the truncated tk polypeptide is influenced by the amount of tk present.

Low levels of herpes simplex virus thymidine- thymidylate kinase are not limiting for sensitivity to certain antiviral drugs or for latency in a mouse model

Herpes simplex virus mutant KG111 contains a nonsense mutation at codon 44 of the viral thymidine kinase (tk) gene and produces low amounts of a truncated tk polypeptide. We tested mutant KG111 and related viruses that specify varying amounts of similar truncated tk polypeptides for their sensitivities to antiviral nucleoside analogs at different temperatures using plaque reduction assays. The results of these assays showed that the nonsense mutation confers high resistance to bromovinyldeoxyuridine (BVdU) at any temperature and temperature-dependent resistance to acyclovir (ACV), buciclovir (BCV), ganciclovir (DHPG), and fluoroiodoarabinouracil (FIAU). Above relatively low threshold levels of tk that varied depending on the drug tested, viruses exhibited full sensitivity to ACV, BCV, DHPG, and FIAU at 34 degrees. Below these threshold levels, however, decreases in drug sensitivity were linear with decreases in tk levels, forming the basis of a pharmacological assay for tk gene expression. Studies of thymidine (TdR) anabolism in infected 143 tk-cells showed that when high TdR concentrations were added to the medium, KG111 directed thymidine monophosphate (TMP) formation at rates consonant with the amount of tk polypeptide produced by the mutant. When low concentrations to TdR were added to the medium, however, KG111 directed TMP formation at a rate similar to that directed by wild-type virus, indicating that the truncation of the tk polypeptide had little or no effect on tk activity at 34 degrees. Subsequent anabolism to thymidine diphosphate and thymidine triphosphate was reduced in KG111-infected cells, indicating a defect in TMP kinase activity that explains this mutant's resistance to BVdU. Despite the low levels of tk and TMP kinase activity expressed by KG111, this mutant established reactivatable latent infections as efficiently as wild-type virus in a mouse model.