The putative herpes simplex virus 1 chaperone protein UL32 modulates disulfide bond formation during infection.

UNLABELLED: During DNA encapsidation, herpes simplex virus 1 (HSV-1) procapsids are converted to DNA-containing capsids by a process involving activation of the viral protease, expulsion of the scaffold proteins, and the uptake of viral DNA. Encapsidation requires six minor capsid proteins (UL6, UL15, UL17, UL25, UL28, and UL33) and one viral protein, UL32, not found to be associated with capsids. Although functions have been assigned to each of the minor capsid proteins, the role of UL32 in encapsidation has remained a mystery. Using an HSV-1 variant containing a functional hemagglutinin-tagged UL32, we demonstrated that UL32 was synthesized with true late kinetics and that it exhibited a previously unrecognized localization pattern. At 6 to 9 h postinfection (hpi), UL32 accumulated in viral replication compartments in the nucleus of the host cell, while at 24 hpi, it was additionally found in the cytoplasm. A newly generated UL32-null mutant was used to confirm that although B capsids containing wild-type levels of capsid proteins were synthesized, these procapsids were unable to initiate the encapsidation process. Furthermore, we showed that UL32 is redox sensitive and identified two highly conserved oxidoreductase-like C-X-X-C motifs that are essential for protein function. In addition, the disulfide bond profiles of the viral proteins UL6, UL25, and VP19C and the viral protease, VP24, were altered in the absence of UL32, suggesting that UL32 may act to modulate disulfide bond formation during procapsid assembly and maturation. IMPORTANCE: Although functions have been assigned to six of the seven required packaging proteins of HSV, the role of UL32 in encapsidation has remained a mystery. UL32 is a cysteine-rich viral protein that contains C-X-X-C motifs reminiscent of those in proteins that participate in the regulation of disulfide bond formation. We have previously demonstrated that disulfide bonds are required for the formation and stability of the viral capsids and are also important for the formation and stability of the UL6 portal ring. In this report, we demonstrate that the disulfide bond profiles of the viral proteins UL6, UL25, and VP19C and the viral protease, VP24, are altered in cells infected with a newly isolated UL32-null mutant virus, suggesting that UL32 acts as a chaperone capable of modulating disulfide bond formation. Furthermore, these results suggest that proper regulation of disulfide bonds is essential for initiating encapsidation.

Mutational analysis of the herpes simplex virus type 1 UL25 DNA packaging protein reveals regions that are important after the viral DNA has been packaged.

The herpes simplex virus type 1 (HSV-1) UL25 gene encodes a minor capsid protein, pUL25, that is essential for packaging the full-length viral genome. Six regions which contain disordered residues have been identified in the high-resolution structure of pUL25. To investigate the significance of these flexible regions, a panel of plasmids was generated encoding mutant proteins, with each member lacking the disordered residues in one of the six regions. In addition, UL25 constructs were produced, which specified proteins that contained missense mutations individually affecting two of the four regions on the surface of pUL25 predicted from evolutionary trace analysis to be important in protein-protein interactions. The impacts of these mutations on viral DNA packaging, virus assembly, and growth were examined. Of the nine mutant proteins analyzed, five failed to complement the growth of a UL25 deletion mutant in Vero cells. These noncomplementing proteins fell into three classes. Proteins in one class did not alter the DNA packaging phenotype of an HSV-1 UL25 deletion mutant, whereas proteins from the other two classes allowed the UL25 null mutant to package full-length viral DNA. Subsequent analysis of the latter classes of mutant proteins demonstrated that one class enabled the null virus to release enveloped virus particles from U2OS cells, whereas the other class prevented egress of mature HSV-1 capsids from the nucleus. These findings reveal a new role for pUL25 in virion assembly, consistent with its flexible structure and location on the capsid.

Analysis of herpes simplex virus type 1 DNA packaging signal mutations in the context of the viral genome.

The minimal signal required for the cleavage and packaging of replicated concatemeric herpes simplex virus type 1 (HSV-1) DNA corresponds to an approximately 200-bp fragment, Uc-DR1-Ub, spanning the junction of the genomic L and S segments. Uc and Ub occupy positions adjacent to the L and S termini and contain motifs (pac2 and pac1, respectively) that are conserved near the ends of other herpesvirus genomes. We have used homologous Red/ET recombination in Escherichia coli to introduce wild-type and specifically mutated Uc-DR1-Ub fragments into an ectopic site of a cloned HSV-1 genome from which the resident packaging signals had been previously deleted. The resulting constructs were transfected into mammalian cells, and their abilities to replicate and become encapsidated, generate Uc- and Ub-containing terminal fragments, and give rise to progeny virus were assessed. In general, the results obtained agree well with previous observations made using amplicons and confirm roles for the pac2 T element in the initiation of DNA packaging and for the GC-rich motifs flanking the pac1 T element in termination. In contrast to a previous report, the sequence of the DR1 element was also crucial for DNA packaging. Following repair of the resident packaging signals in mammalian cells, recombination occurred at high frequency in progeny virus between the repaired sequences and mutated Uc-DR1-Ub inserts. This restored the ability of mutated Uc-DR1-Ub inserts to generate terminal fragments, although these were frequently larger than expected from simple repair of the original lesion.

Mutational analysis of the herpes simplex virus type 1 DNA packaging protein UL33.

The UL33 protein of herpes simplex virus type 1 (HSV-1) is thought to be a component of the terminase complex that mediates the cleavage and packaging of viral DNA. In this study we describe the generation and characterization of a series of 15 UL33 mutants containing insertions of five amino acids located randomly throughout the 130-residue protein. Of these mutants, seven were unable to complement the growth of the UL33-null virus dlUL33 in transient assays and also failed to support the cleavage and packaging of replicated amplicon DNA into capsids. The insertions in these mutants were clustered between residues 51 and 74 and between 104 and 116, within the most highly conserved regions of the protein. The ability of the mutants to interact with the UL28 component of the terminase was assessed in immunoprecipitation and immunofluorescence assays. All four mutants with insertions between amino acids 51 and 74 were impaired in this interaction, whereas two of the three mutants in the second region (with insertions at positions 111 and 116) were not affected. These data indicate that the ability of UL33 to interact with UL28 is probably necessary, but not sufficient, to support viral growth and DNA packaging.

Upstream-binding factor is sequestered into herpes simplex virus type 1 replication compartments.

Previous reports have shown that adenovirus recruits nucleolar protein upstream-binding factor (UBF) into adenovirus DNA replication centres. Here, we report that despite having a different mode of viral DNA replication, herpes simplex virus type 1 (HSV-1) also recruits UBF into viral DNA replication centres. Moreover, as with adenovirus, enhanced green fluorescent protein-tagged fusion proteins of UBF inhibit viral DNA replication. We propose that UBF is recruited to the replication compartments to aid replication of HSV-1 DNA. In addition, this is a further example of the role of nucleolar components in viral life cycles.

The UL15 protein of herpes simplex virus type 1 is necessary for the localization of the UL28 and UL33 proteins to viral DNA replication centres.

The UL15, UL28 and UL33 proteins of herpes simplex virus type 1 (HSV-1) are thought to comprise a terminase complex responsible for cleavage and packaging of the viral genome into pre-assembled capsids. Immunofluorescence studies confirmed that shortly after infection with wild-type HSV-1 these three proteins localize to viral DNA replication compartments within the nucleus, identified by the presence of the single-stranded DNA-binding protein, ICP8. In cells infected with either UL28- or UL33-null mutants, the other two terminase proteins also co-localized with ICP8. In contrast, neither UL28 nor UL33 was detectable in replication compartments following infection with a UL15-null mutant, although Western blot analysis showed they were present in normal amounts in the infected cells. Provision of UL15 in a complementing cell line restored the ability of all three proteins to localize to replication compartments. These data indicate that UL15 plays a key role in localizing the terminase complex to DNA replication compartments, and that it can interact independently with UL28 and UL33.

The UL25 gene product of herpes simplex virus type 1 is involved in uncoating of the viral genome.

Studies on the herpes simplex virus type 1 UL25-null mutant KUL25NS have shown that the capsid-associated UL25 protein is required at a late stage in the encapsidation of viral DNA. Our previous work on UL25 with the UL25 temperature-sensitive (ts) mutant ts1204 also implicated UL25 in a role at very early times in the virus growth cycle, possibly at the stage of penetration of the host cell. We have reexamined this mutant and discovered that it had an additional ts mutation elsewhere in the genome. The ts1204 UL25 mutation was transferred into wild-type (wt) virus DNA, and the UL25 mutant ts1249 was isolated and characterized to clarify the function of UL25 at the initial stages of virus infection. Indirect immunofluorescence assays and in situ hybridization analysis of virus-infected cells revealed that the mutant ts1249 was not impaired in penetration of the host cell but had an uncoating defect at the nonpermissive temperature. When ts1249-infected cells were incubated initially at the permissive temperature to allow uncoating of the viral genome and subsequently transferred to the restrictive temperature, a DNA-packaging defect was evident. The results suggested that ts1249, like KUL25NS, had a block at a late stage of DNA packaging and that the packaged genome was shorter than the full-length genome. Examination of ts1249 capsids produced at the nonpermissive temperature revealed that, in comparison with wt capsids, they contained reduced amounts of UL25 protein, thereby providing a possible explanation for the failure of ts1249 to package full-length viral DNA.

Blocks to herpes simplex virus type 1 replication in a cell line, tsBN2, encoding a temperature-sensitive RCC1 protein.

Circularization of the herpes simplex virus type 1 (HSV-1) genome is thought to be an important early event during the lytic cycle. Previous studies from another laboratory using a cell line, tsBN2, that carries a temperature-sensitive mutation in the gene encoding the regulator of chromatin condensation 1 (RCC1) indicated that functional RCC1 was required for HSV-1 genome circularization and subsequent viral DNA synthesis. Here, HSV-1 infection of tsBN2 cells has been re-examined by utilizing both wild-type HSV-1 and a derivative that enables a direct demonstration of circularization. At the non-permissive temperature, when RCC1 was absent, both circularization and viral DNA synthesis were reduced, but not abolished. However, no infectious progeny virus was detected under these conditions. An impairment in the cleavage of concatemeric DNA and the failure to express at least one capsid protein indicated that HSV-1 replication is also blocked at a late stage in the absence of RCC1. This conclusion was supported by a temperature-upshift experiment, which demonstrated a role for RCC1 at times later than 6 h post-infection. Finally, a virus constitutively expressing beta-galactosidase produced the protein in a reduced number of cells when RCC1 was inactivated, suggesting that genome delivery to the nucleus or the initial stages of gene expression may also be affected.

Role of the herpes simplex virus helicase-primase complex during adeno-associated virus DNA replication.

A subset of DNA replication proteins of herpes simplex virus (HSV) comprising the single-strand DNA-binding protein, ICP8 (UL29), and the helicase-primase complex (UL5, UL8, and UL52 proteins) has previously been shown to be sufficient for the replication of adeno-associated virus (AAV). We recently demonstrated complex formation between ICP8, AAV Rep78, and the single-stranded DNA AAV genome, both in vitro and in the nuclear HSV replication domains of coinfected cells. In this study the functional role(s) of HSV helicase and primase during AAV DNA replication were analyzed. To differentiate between their necessity as structural components of the HSV replication complex or as active enzymes, point mutations within the helicase and primase catalytic domains were analyzed. In two complementary approaches the remaining HSV helper functions were either provided by infection with HSV mutants or by plasmid transfection. We show here that upon cotransfection of the minimal four HSV proteins (i.e., the four proteins constituting the minimal requirements for basal AAV replication), UL52 primase catalytic activity was not required for AAV DNA replication. In contrast, UL5 helicase activity was necessary for fully efficient replication. Confocal microscopy confirmed that all mutants retained the ability to support formation of ICP8-positive nuclear replication foci, to which AAV Rep78 colocalized in a manner strictly dependent on the presence of AAV single-stranded DNA (ssDNA). The data indicate that recruitment of AAV Rep78 and ssDNA to nuclear replication sites by the four HSV helper proteins is maintained in the absence of catalytic primase or helicase activities and suggest an involvement of the HSV UL5 helicase activity during AAV DNA replication.

Herpes simplex virus type 1 DNA-packaging protein UL17 is required for efficient binding of UL25 to capsids.

Herpes simplex virus type 1 packages its DNA genome into a precursor capsid, referred to as the procapsid. Of the three capsid-associated DNA-packaging proteins, UL17, UL25, and UL6, only UL17 and UL6 appear to be components of the procapsid, with UL25 being added subsequently. To determine whether the association of UL17 or UL25 with capsids was dependent on the other two packaging proteins, B capsids, which lack viral DNA but retain the cleaved internal scaffold, were purified from nonpermissive cells infected with UL17, UL25, or UL6 null mutants and compared with wild-type (wt) B capsids. In the absence of UL17, the levels of UL25 in the mutant capsids were much lower than those in wt B capsids. These results suggest that UL17 is required for efficient incorporation of UL25 into B capsids. B capsids lacking UL25 contained about twofold-less UL17 than wt capsids, raising the possibilities that UL25 is important for stabilizing UL17 in capsids and that the two proteins interact in the capsid. The distribution of UL17 and UL25 on B capsids was examined using immunogold labeling. Both proteins appeared to bind to multiple sites on the capsid. The properties of the UL17 and UL25 proteins are consistent with the idea that the two proteins are important in stabilizing capsid-DNA structures rather than having a direct role in DNA packaging.

Structural characterization of the UL25 DNA-packaging protein from herpes simplex virus type 1.

Herpesviruses replicate their double stranded DNA genomes as high-molecular-weight concatemers which are subsequently cleaved into unit-length genomes by a complex mechanism that is tightly coupled to DNA insertion into a preformed capsid structure, the procapsid. The herpes simplex virus type 1 UL25 protein is incorporated into the capsid during DNA packaging, and previous studies of a null mutant have demonstrated that its function is essential at the late stages of the head-filling process, either to allow packaging to proceed to completion or for retention of the viral genome within the capsid. We have expressed and purified an N-terminally truncated form of the 580-residue UL25 protein and have determined the crystallographic structure of the region corresponding to amino acids 134 to 580 at 2.1-Angstroms resolution. This structure, the first for any herpesvirus protein involved in processing and packaging of viral DNA, reveals a novel fold, a distinctive electrostatic distribution, and a unique "flexible" architecture in which numerous flexible loops emanate from a stable core. Evolutionary trace analysis of UL25 and its homologues in other herpesviruses was used to locate potentially important amino acids on the surface of the protein, leading to the identification of four putative docking regions for protein partners.

New genes from old: redeployment of dUTPase by herpesviruses.

Published work (D. J. McGeoch, Nucleic Acids Res. 18:4105-4110, 1990; J. E. McGeehan, N. W. Depledge, and D. J. McGeoch, Curr. Protein Peptide Sci. 2:325-333, 2001) has indicated that evolution of dUTPase in the class of herpesviruses that infect mammals and birds involved capture of a host gene followed by a duplication event that resulted in a coding region comprising two fused dUTPase domains. Some of the conserved residues required for enzyme activity were then lost, resulting in a dUTPase containing a single active site with different elements contributed by each half of the protein. Further conserved residues were lost in one subfamily (the Betaherpesvirinae), yielding a protein that is related to herpesvirus dUTPases but has a different and as yet unrecognized function. Evidence from sequence similarities and structural predictions now indicates that several additional genes were derived from the herpesvirus dUTPase gene, probably by duplication. These are UL31, UL82, UL83, and UL84 in human cytomegalovirus (and counterparts in other members of the Betaherpesvirinae) and ORF10 and ORF11 in human herpesvirus 8 (and counterparts in other members of the Gammaherpesvirinae). The findings clarify the evolutionary history of these genes and provide novel insights for structural and functional studies.

Circularization of the herpes simplex virus type 1 genome upon lytic infection.

For many years, the generally accepted model for the replication of the double-stranded DNA genome of herpes simplex virus type 1 (HSV-1) incorporated initial circularization of linear molecules in the cell nucleus. Ensuing DNA synthesis resulted in the generation of head-to-tail concatemers which were subsequently cleaved into monomeric units and packaged into the nascent viral capsid. Recently, however, it has been proposed that circularization of HSV-1 genomes does not occur at the onset of lytic infection and moreover that this event is specifically inhibited by the HSV-1 transcriptional transactivator, ICP0 (S.A. Jackson and N.A. DeLuca, Proc. Natl. Acad. Sci. USA 100:7871-7876, 2003). To further investigate genome circularization, we have generated HSV-1 derivatives in which the viral a sequences, which contain the cleavage-packaging signals, have been replaced by a minimal packaging element located in the thymidine kinase gene. In contrast to wild-type HSV-1, fusion of the genomic termini of these viruses produces a novel fragment in circular or concatemeric DNA which can be detected by Southern blot hybridization. Utilizing these viruses, we demonstrate that fusion of the genomic termini occurred rapidly upon infection and in the presence of inhibitors of viral DNA or protein synthesis. We provide evidence indicating that the end joining represented circularization rather than concatemerization of input molecules and that circularized molecules functioned as templates for replication. Since the termini of these viruses lack direct repeats, our findings indicate that circularization can be mediated by direct end-to-end ligation of linear input genomes.

Replication, recombination and packaging of amplicon DNA in cells infected with the herpes simplex virus type 1 alkaline nuclease null mutant ambUL12.

The alkaline nuclease (AN) encoded by gene UL12 of herpes simplex virus type 1 (HSV-1) is essential for efficient virus replication but its role during the lytic cycle remains incompletely understood. Inactivation of the UL12 gene results in reductions in viral DNA synthesis, DNA packaging, egress of DNA-containing capsids from the nucleus and ability of progeny virions to initiate new cycles of infection. Mechanistically, AN has been implicated in resolving branched structures in HSV-1 replicative intermediates prior to encapsidation, and promoting DNA strand-exchange. In this study, amplicons (bacterial plasmids containing functional copies of a virus replication origin and packaging signal) were used to analyse further the defects of the UL12 null mutant ambUL12. When ambUL12 was used as a helper virus both replication and packaging of the transfected amplicon were reduced in comparison with cells infected with wild-type (wt) HSV-1, and to extents similar to those previously observed for genomic ambUL12 DNA. By using amplicons differing at a specific restriction endonuclease site it was demonstrated that replicating molecules exhibit high frequency intermolecular recombination in both wt- and mutant-infected cells. Surprisingly, in the absence of the UL12 product, amplicons lacking a functional encapsidation signal were packaged. Moreover, these packaged molecules could be serially propagated indicating that they had been incorporated into functional virions. This difference in packaging specificity between wt HSV-1 and ambUL12 might indicate that replicative intermediates accumulating in the absence of AN contain an increased incidence of structures that can serve for the initiation of DNA packaging.

Virus particles produced by the herpes simplex virus type 1 alkaline nuclease null mutant ambUL12 contain abnormal genomes.

Open reading frame UL12 of herpes simplex virus type 1 (HSV-1) encodes an alkaline nuclease that has previously been implicated in processing the complex, branched, viral DNA replication intermediates and allowing egress of DNA-containing capsids from the nucleus. This report describes experiments using the HSV-1 UL12 null mutant ambUL12, which aim to explain the approximately 200- to 1000-fold decrease in the yield of infectious virus, compared with wild-type (wt) HSV-1, from non-complementing cells. A detailed examination revealed that both DNA replication and encapsidation were affected in ambUL12-infected cells, resulting in an approximately 15- to 20-fold reduction in the amount of packaged DNA. In contrast to previous reports, the absence of UL12 function did not greatly impair capsid release into the cytoplasm, and virus particles were readily detected in the supernatant medium from ambUL12-infected cells. The released virus, however, exhibited much higher particle/p.f.u. ratios than wt HSV-1, and this made a further important contribution to the overall reduction in yield. Gel analyses of packaged ambUL12 and wt DNAs revealed the presence of structural abnormalities. The DNA obtained from extracellular ambUL12 virions was non-infectious in transfection assays, and both ambUL12 DNA and virus particles exerted a dominant inhibitory effect on the growth of wt virus. These results suggest that ambUL12 virions produced in non-complementing cells have a greatly reduced ability to initiate new cycles of infection, and that this defect results from the encapsidation of abnormal genomes.

Herpes simplex virus type 1 portal protein UL6 interacts with the putative terminase subunits UL15 and UL28.

The herpes simplex virus type 1 (HSV-1) UL6, UL15, and UL28 proteins are essential for cleavage of replicated concatemeric viral DNA into unit length genomes and their packaging into a preformed icosahedral capsid known as the procapsid. The capsid-associated UL6 DNA-packaging protein is located at a single vertex and is thought to form the portal through which the genome enters the procapsid. The UL15 protein interacts with the UL28 protein, and both are strong candidates for subunits of the viral terminase, a key component of the molecular motor that drives the DNA into the capsid. To investigate the association of the UL6 protein with the UL15 and UL28 proteins, the three proteins were produced in large amounts in insect cells with the baculovirus expression system. Interactions between UL6 and UL28 and between UL6 and UL15 were identified by an immunoprecipitation assay. These results were confirmed by transiently expressing wild-type and mutant proteins in mammalian cells and monitoring their distribution by immunofluorescence. In cells expressing the single proteins, UL6 and UL15 were concentrated in the nuclei whereas UL28 was found in the cytoplasm. When the UL6 and UL28 proteins were coexpressed, UL28 was redistributed to the nuclei, where it colocalized with UL6. In cells producing either of two cytoplasmic UL6 mutant proteins and a functional epitope-tagged form of UL15, the UL15 protein was concentrated with the mutant UL6 protein in the cytoplasm. These observed interactions of UL6 with UL15 and UL28 are likely to be of major importance in establishing a functional DNA-packaging complex at the portal vertex of the HSV-1 capsid.

An analysis of the requirements for human cytomegalovirus oriLyt-dependent DNA synthesis in the presence of the herpes simplex virus type 1 replication fork proteins.

Activation of the human cytomegalovirus (HCMV) origin of replication (oriLyt) was previously demonstrated in transient transfection assays in permissive human fetal fibroblasts and nonpermissive Vero cells, and shown to require six viral proteins that function at the replication fork plus a number of HCMV products that perform auxiliary roles. The six replication fork proteins could be substituted by their Epstein-Barr virus homologues. In this paper we demonstrate that the corresponding herpes simplex virus type 1 replication fork proteins can similarly replace those of HCMV in Vero cells. Under these conditions the essential auxiliary functions were mapped to two plasmids: pSVH (containing the major immediate-early locus) and pZP8 (spanning genes UL32-UL38). Mutants of pSVH and pZP8 and cloned cDNAs encoding the IE1-p72 and IE2-p86 proteins were tested for their ability to support DNA synthesis. The results showed that IE2-p86 was necessary for activation of the origin, and that the UL37x1 and IE1-p72 products exerted strong stimulatory effects. In contrast to the previous work, omission of the UL84 protein had no effect upon oriLyt-dependent DNA synthesis.

Detection of human cytomegalovirus DNA replication in non-permissive Vero and 293 cells.

Human cytomegalovirus (HCMV) displays an exceptionally restricted host range in tissue culture with human fibroblasts being the principal fully permissive system. Nevertheless, immediate early (IE) proteins are expressed following infection of many non-permissive cell types of human, simian and murine origin, and viral origin-dependent DNA synthesis has been reconstituted by transfection of plasmids into Vero cells, a non-permissive line from African green monkey. We have examined the accumulation of HCMV strain AD169 DNA, and the replication of transfected HCMV origin-containing plasmids, in infected Vero and human embryonic kidney 293 cells, which were previously reported to express the major IE protein in a small proportion of infected cells but to be non-permissive for viral DNA synthesis. In Vero cells accumulation of origin-containing plasmid but not viral DNA occurred, whilst in 293 cells both DNAs accumulated. Immunofluorescence experiments indicated that following infection with 3 p.f.u. per cell, a small fraction of both cell types expressed the UL44 DNA replication protein. Neither cell line, however, supported the generation of infectious progeny virus. These results suggest that IE proteins expressed in Vero and 293 cells can induce the synthesis of early proteins capable of functioning in viral DNA replication, but there is a failure in later events on the pathway to infectious virus production. This provides further support for transfected Vero cells being a valid system in which to study HCMV DNA synthesis, and suggests that 293 cells may also prove useful in similar experiments.

Fundamental and accessory systems in herpesviruses.

Evolutionary studies have a large theoretical component and will not directly provide therapies for herpesvirus infections. However, they do provide a conceptual framework within which we can evaluate the origins of the various systems that contribute to viral lifestyle. An evolutionary context allows ancient systems that are fundamental to the replication of all herpesviruses to be distinguished from those that have developed relatively recently in order to tailor viruses to particular biological niches. Both categories are in principle accessible to intervention, either to prevent basic replicative capabilities or to reduce the advantages that the virus has in its interactions with the host. Phylogenetic data provide estimates of evolutionary rate for herpesviruses that are only between one and two orders of magnitude greater than those of their hosts. However, it is becoming apparent that certain genes have evolved much faster under selection pressures and by mechanisms that are not well understood. Nonetheless, the mutation rates of even the most highly conserved genes are sufficient to permit herpesviruses to escape from antiviral therapy. Greater understanding of the origins and functions of herpesvirus genes may lead to new insights into the determinants of pathogenesis and hence to new diagnostic and therapeutic targets.