Analysis of the binding sites for the varicella-zoster virus gene 51 product within the viral origin of DNA replication

G-quadruplexes formed by Varicella-Zoster virus reiteration sequences suppress expression of glycoprotein C and regulate viral cell-to-cell spread

G-quadruplex (G4) formed by repetitive guanosine-rich sequences plays important roles in diverse cellular processes; however, its roles in viral infection are not fully understood. In this study, we investigated the genome-wide distribution of G4-forming sequences (G4 motifs) in Varicella-Zoster virus (VZV) and found that G4 motifs are enriched in the internal repeat short and the terminal repeat short regions flanking the unique short region and also in some reiteration (R) sequence regions. A high density of G4 motifs in the R2 region was found on the template strand of ORF14, which encodes glycoprotein C (gC), a virulent factor for viral growth in skin. Analyses such as circular dichroism spectroscopy, thermal difference spectra, and native polyacrylamide gel electrophoresis with oligodeoxynucleotides demonstrated that several G4 motifs in ORF14 form stable G4 structures. In transfection assays, gC expression from the G4-disrupted ORF14 gene was increased at the transcriptional level and became more resistant to suppression by G4-ligand treatment. The recombinant virus containing the G4-disrupted ORF14 gene expressed a higher level of gC mRNA, while it showed a slightly reduced growth. This G4-disrupted ORF14 virus produced smaller plaques than the wild-type virus. Our results demonstrate that G4 formation via reiteration sequences suppresses gC expression during VZV infection and regulates viral cell-to-cell spread.

Varicella-zoster virus (VZV) origin of DNA replication oriS influences origin-dependent DNA replication and flanking gene transcription

The VZV genome has two origins of DNA replication (oriS), each of which consists of an AT-rich sequence and three origin binding protein (OBP) sites called Box A, C and B. In these experiments, the mutation in the core sequence CGC of the Box A and C not only inhibited DNA replication but also inhibited both ORF62 and ORF63 expression in reporter gene assays. In contrast the Box B mutation did not influence DNA replication or flanking gene transcription. These results suggest that efficient DNA replication enhances ORF62 and ORF63 transcription. Recombinant viruses carrying these mutations in both sites and one with a deletion of the whole oriS were constructed. Surprisingly, the recombinant virus lacking both copies of oriS retained the capacity to replicate in melanoma and HELF cells suggesting that VZV has another origin of DNA replication.

An Sp1/Sp3 site in the downstream region of varicella-zoster virus (VZV) oriS influences origin-dependent DNA replication and flanking gene transcription and is important for VZV replication in vitro and in human skin

The distribution and orientation of origin-binding protein (OBP) sites are the main architectural contrasts between varicella-zoster virus (VZV) and herpes simplex virus (HSV) origins of DNA replication (oriS). One important difference is the absence of a downstream OBP site in VZV, raising the possibility that an alternative cis element may replace its function. Our previous work established that Sp1, Sp3, and YY1 bind to specific sites within the downstream region of VZV oriS; we hypothesize that one or both of these sites may be the alternative cis element(s). Here, we show that the mutation of the Sp1/Sp3 site decreases DNA replication and transcription from the adjacent ORF62 and ORF63 promoters following superinfection with VZV. In contrast, in the absence of DNA replication or in transfection experiments with ORF62, only ORF63 transcription is affected. YY1 site mutations had no significant effect on either process. Recombinant viruses containing these mutations were then constructed. The Sp1/Sp3 site mutant exhibited a significant decrease in virus growth in MeWo cells and in human skin xenografts, while the YY1 site mutant virus grew as well as the wild type in MeWo cells, even showing a late increase in VZV replication in skin xenografts following infection. These results suggest that the Sp1/Sp3 site plays an important role in both VZV origin-dependent DNA replication and ORF62 and ORF63 transcription and that, in contrast to HSV, these events are linked during virus replication.

A sequence within the varicella-zoster virus (VZV) OriS is a negative regulator of DNA replication and is bound by a protein complex containing the VZV ORF29 protein

The architecture of the varicella-zoster virus (VZV) origin of DNA replication (OriS) differs significantly from that of the herpes simplex virus (HSV) DNA replication origin. Novel aspects of the VZV OriS include a GA-rich region, three binding sites for the VZV origin-binding protein (OBP) all on the same strand and oriented in the same direction, and a partial OBP binding site of unknown function. We have designated this partial binding site Box D and have investigated the role it plays in DNA replication and flanking gene expression. This has been done with a model system using a replication-competent plasmid containing OriS and a replication- and transcription-competent dual-luciferase reporter plasmid containing both the OriS and the intergenic region between VZV open reading frames (ORFs) 62 and 63. We have found that (i) Box D is a negative regulator of DNA replication independent of flanking gene expression, (ii) the mutation of Box D results in a decrease in flanking gene expression, thus a sequence within the VZV OriS affects transcription, which is in contrast to results reported for HSV-1, (iii) there is a specific Box D complex formed with infected cell extracts in electrophoretic mobility shift assay experiments, (iv) supershift assays show that this complex contains the VZV ORF29 single-strand DNA-binding protein, and (v) the formation of this complex is dependent on the presence of CGC motifs in Box D and its downstream flanking region. These findings show that the VZV ORF29 protein, while required for DNA replication, also plays a novel role in the suppression of that process.

Roles of cellular transcription factors in VZV replication

Varicella zoster virus (VZV) is the causative agent of chickenpox and shingles. During productive infection the complete VZV proteome consisting of some 68 unique gene products is expressed through interaction of a small number of viral transcriptional activators with the general transcription apparatus of the host cell. Recent work has shown that the major viral transactivator, commonly designated the IE62 protein, interacts with the human Mediator of transcription. This interaction requires direct contact between the MED25 subunit of Mediator and the acidic N-terminal transactivation domain of IE62. A second cellular factor, host cell factor-1, has been shown to be the common element in two mechanisms of activation of the promoter driving expression of the gene encoding IE62. Finally, the ubiquitous cellular transcription factors Sp1, Sp3, and YY1 have been shown to interact with sequences near the VZV origin of DNA replication and in the case of Sp1/Sp3 to influence replication efficiency.

Stepwise evolution of the herpes simplex virus origin binding protein and origin of replication

The herpes simplex virus replicon consists of cis-acting sequences, oriS and oriL, and the origin binding protein (OBP) encoded by the UL9 gene. Here we identify essential structural features in the initiator protein OBP and the replicator sequence oriS, and we relate the appearance of these motifs to the evolutionary history of the alphaherpesvirus replicon. Our results reveal two conserved sequence elements in herpes simplex virus type 1, OBP; the RVKNL motif, common to and specific for all alphaherpesviruses, is required for DNA binding, and the WP XXXGAXXFXX L motif, found in a subset of alphaherpesviruses, is required for specific binding to the single strand DNA-binding protein ICP8. A 121-amino acid minimal DNA binding domain containing conserved residues is not soluble and does not bind DNA. Additional sequences present 220 amino acids upstream from the RVKNL motif are needed for solubility and function. We also examine the binding sites for OBP in origins of DNA replication and how they are arranged. NMR and DNA melting experiments demonstrate that origin sequences derived from many, but not all, alphaherpesviruses can adopt stable boxI/boxIII hairpin conformations. Our results reveal a stepwise evolutionary history of the herpes simplex virus replicon and suggest that replicon divergence contributed to the formation of major branches of the herpesvirus family.

Cellular transcription factors Sp1 and Sp3 suppress varicella-zoster virus origin-dependent DNA replication

The varicella-zoster virus (VZV) origin of DNA replication (oriS) contains a 46-bp AT-rich palindrome and three consensus binding sites for the VZV origin binding protein (OBP) encoded by VZV ORF51. All three OBP binding sites are upstream of the palindrome in contrast to the sequence of the herpes simplex virus oriS, which has required OBP binding sites upstream and downstream of the AT-rich region. We are investigating the roles that sequences downstream of the palindrome play in VZV oriS-dependent DNA replication. Computer analysis identified two GC boxes, GC box 1 and GC box 2, in the downstream region which were predicted to be binding sites for the cellular transcription factor Sp1. Electrophoretic mobility shift assay and supershift assays showed that two members of the Sp family (Sp1 and Sp3) stably bind to GC box 1, but not to GC box 2. A predicted binding site for the cellular factor Yin Yang 1 (YY1) that overlaps with GC box 2 was also identified. Supershift and mutational analyses confirmed the binding of YY1 to this site. Mutation of GC box 1 resulted in loss of Sp1 and Sp3 binding and an increase in origin-dependent replication efficiency in DpnI replication assays. In contrast, mutation of the YY1 site had a statistically insignificant effect. These results suggest a model where origin-dependent DNA replication and viral transcription are coupled by the binding of Sp1 and Sp3 to the downstream region of the VZV replication origin during lytic infection. They may also have implications regarding establishment or reactivation of viral latency.

Genomic cartography of varicella-zoster virus: a complete genome-based analysis of strain variability with implications for attenuation and phenotypic differences

In order to gain a better perspective on the true variability of varicella-zoster virus (VZV) and to catalogue the location and number of differences, 11 new complete genome sequences were compared with those previously in the public domain (18 complete genomes in total). Three of the newly sequenced genomes were derived from a single strain in order to assess variations that can occur during serial passage in cell culture. The analysis revealed that while VZV is relatively stable genetically it does posses a certain degree of variability. The reiteration regions, origins of replication and intergenic homopolymer regions were all found to be variable between strains as well as within a given strain. In addition, the terminal viral sequences were found to vary within and between strains specifically at the 3' end of the genome. Analysis of single nucleotide polymorphisms (SNPs) identified a total of 557 variable sites, 451 of which were found in coding regions and resulted in 187 different in amino acid substitutions. A comparison of the SNPs present in the two gE mutant strains, VZV-MSP and VZV-BC, suggested that the missense mutation in gE was primarily responsible for the accelerated cell spread phenotype. Some of the variations noted with high passage in cell culture are consistent with variations seen in the IE62 gene of the vaccine strains (S628G, R958G and I1260V) that may help in pinpointing variations essential for attenuation. Although VZV has been considered to be one of the most genetically stable human herpesviruses, this initial assessment of genomic VZV cartography provides insight into ORFs with previously unreported variations.

Molecular studies of Varicella zoster virus

VZV is a highly cell-associated member of the Herpesviridae family and one of the eight herpesviruses to infect humans. The virus is ubiquitous in most populations worldwide, primary infection with which causes varicella, more commonly known as chickenpox. Characteristic of members of the alphaherpesvirus sub-family, VZV is neurotropic and establishes latency in sensory neurones. Reactivation from latency, usually during periods of impaired cellular immunity, causes herpes zoster (shingles). Despite being one of the most genetically stable human herpesviruses, nucleotide alterations in the virus genome have been used to classify VZV strains from different geographical regions into distinct clades. Such studies have also provided evidence that, despite pre-existing immunity to VZV, subclinical reinfection and reactivation of reinfecting strains to cause zoster is also occurring. During both primary infection and reactivation, VZV infects several PBMC and skin cell lineages. Difficulties in studying the pathogenesis of VZV because of its high cell association and narrow host range have been overcome through the development of the VZV severe combined immunodeficient mouse model carrying human tissue implants. This model has provided a valuable tool for studying the importance of individual viral proteins during both the complex intracellular replication and assembly of new virions and for understanding the underlying mechanism of attenuation of the live varicella vaccine. In addition, a rat model has been developed and successfully used to uncover which viral proteins are important for both the establishment and maintenance of latent VZV infection.

Complete DNA sequence analyses of the first two varicella-zoster virus glycoprotein E (D150N) mutant viruses found in North America: evolution of genotypes with an accelerated cell spread phenotype

Varicella-zoster virus (VZV) is considered to be one of the most genetically stable of all the herpesviruses. Yet two VZV strains with a D150N missense mutation within the gE glycoprotein were isolated in North America in 1998 and 2002. The mutant strains have an accelerated cell spread phenotype, which distinguishes them from all wild-type and laboratory viruses. Since the VZV genome contains 70 additional open reading frames (ORFs), the possibility existed that the phenotypic change was actually due to an as-yet-undiscovered mutation or deletion elsewhere in the genome. To exclude this hypothesis, the entire genomes of the two mutant viruses were sequenced and found to contain 124,883 (VZV-MSP) and 125,459 (VZV-BC) nucleotides. Coding single-nucleotide polymorphisms (SNPs) were identified in 14 ORFs. One missense mutation was discovered in gH, but none was found in gB, gI, gL, or gK. There were no coding SNPs in the major regulatory protein ORF 62. One polymorphism was discovered which could never have been anticipated based on current knowledge of herpesvirus genomics, namely, the origins of replication differed from those in the prototype strain but not in a manner expected to affect cell spread. When the two complete mutant VZV sequences were surveyed in their entirety, the most reasonable conclusion was that the increased cell spread phenotype was dependent substantially or solely on the single D150N polymorphism in glycoprotein gE. The genomic results also expanded the evolutionary database by identifying which VZV ORFs were more likely to mutate over time.

Differences in DNA binding specificity among Roseolovirus origin binding proteins

The Roseolovirus genus of the Betaherpesvirinae consists of the very closely related viruses, human herpesvirus 6 variants A and B (HHV-6A and HHV-6B) plus the somewhat more distantly related human herpesvirus 7 (HHV-7). The roseoloviruses each encode a homolog of the alphaherpesvirus origin binding protein (OBP) which is required for lytic DNA replication. In contrast, members of the other betaherpesvirus genera, the cytomegaloviruses, initiate DNA replication by a different mechanism. To better understand the basis of roseolovirus OBP sequence specificity, we investigated their ability to recognize each other's binding sites. HHV-6A OBP (OBP(H6A)) and HHV-6B OBP (OBP(H6B)) each bind to both of the HHV-7 OBP sites (OBP-1 and OBP-2) with similar strengths, which are also similar to their nearly equivalent interactions with their own sites. In contrast, HHV-7 OBP (OBP(H7)) had a gradient of binding preferences: HHV-7 OBP-2 > HHV-6 OBP-2 > HHV-7 OBP-1 > HHV-6 OBP-1. Thus, the roseolovirus OBPs are not equally reciprocal in their recognition of each other's OBP sites, suggesting that the sequence requirements for the interaction of OBPH7 at the OBP sites in its cognate oriLyt differ from those of OBPH6A and OBPH6B.

Complementary intrastrand base pairing during initiation of Herpes simplex virus type 1 DNA replication

The herpes simplex virus type 1 origin of DNA replication, oriS, contains three copies of the recognition sequence for the viral initiator protein, origin binding protein (OBP), arranged in two palindromes. The central box I forms a short palindrome with box III and a long palindrome with box II. Single-stranded oriS adopts a conformation, oriS*, that is tightly bound by OBP. Here we demonstrate that OBP binds to a box III-box I hairpin with a 3' single-stranded tail in oriS*. Mutations designed to destabilize the hairpin abolish the binding of OBP to oriS*. The same mutations also inhibit DNA replication. Second site complementary mutations restore binding of OBP to oriS* as well as the ability of mutated oriS to support DNA replication. OriS* is also an efficient activator of the hydrolysis of ATP by OBP. Sequence analyses show that a box III-box I palindrome is an evolutionarily conserved feature of origins of DNA replication from human, equine, bovine, and gallid alpha herpes viruses. We propose that oriS facilitates initiation of DNA synthesis in two steps and that OBP exhibits exquisite specificity for the different conformations oriS adopts at these stages. Our model suggests that distance-dependent cooperative binding of OBP to boxes I and II in duplex DNA is succeeded by specific recognition of a box III-box I hairpin in partially unwound DNA.

A 189-bp repeat region within the human cytomegalovirus replication origin contains a sequence dispensable but irreplaceable with other sequences

The human cytomegalovirus (HCMV) replication origin exhibits a strain-dependent difference in the number of copies of a 189-bp region: the AD169 and Towne strains contain one and three copies of the region, respectively. A nearly complete deletion of the 189-bp repeat region of the Towne strain does not eliminate the origin's ability to initiate DNA synthesis. Here we report that the replication ability of the HCMV replication origin in infected cells disappeared after replacements of an internal sequence (152 bp) of the 189-bp repeat region with lambda DNA of identical and different lengths as well as after introduction of multiple nucleotide substitutions within the 152-bp internal sequence of the 189-bp repeat. In contrast, a variation in the copy number of 189-bp region (either one or two copies) or an inversion of the 152-bp internal sequence of the 189-bp repeat maintained replication abilities similar to those of the wild-type origin of the Towne strain. These results indicate that the 189-bp repeat region within the HCMV replication origin is not just a dispensable spacer sequence but instead contains an irreplaceable sequence that may play a supporting role in HCMV DNA replication.

Identification of residues within the herpes simplex virus type 1 origin-binding protein that contribute to sequence-specific DNA binding

Gene UL9 of herpes simplex virus type 1 encodes an 851-amino-acid protein which is essential for viral DNA synthesis and functions as a sequence-specific origin-binding protein and DNA helicase. We generated monoclonal antibodies against purified UL9 protein and identified one such antibody (MAb 13924) that can block the interaction of the UL9 C-terminal DNA-binding domain (amino acids 534-851) with its recognition sequence. MAb 13924 interacted with immobilized peptides containing residues 780-786 of UL9. Although the corresponding region of the homologous protein encoded by varicell-azoster virus differs at only a single position it was not recognized by MAb 13924. Site-directed mutagenesis experiments confirmed that residues within this region contribute to the epitope recognized by MAb 13924 and may be involved in sequence-specific DNA binding. In addition, all eight lysine residues within the DNA-binding domain were separately changed to alanine and the DNA-binding properties of the mutated proteins were examined. The results showed that lysine residues that are located close to the peptide recognized by MAb 13924 or lie within the region of the DNA-binding domain most highly conserved among homologous alphaherpesvirus proteins play a role in sequence-specific DNA binding. Moreover, alteration of a lysine residue 18 amino acids from the recognized peptide prevented the interaction of MAb 13924 with the UL9 C-terminal DNA-binding domain. Three helical segments are predicted to occur within the region containing mutations that affect sequence-specific binding and interaction with MAb 13924.

Initiation of DNA replication at palindromic telomeres is mediated by a duplex-to-hairpin transition induced by the minute virus of mice nonstructural protein NS1

The linear single-stranded DNA genome of the minute virus of mice (MVM) is replicated via a double-stranded replicative form (RF) intermediate. Amplification of this RF is initiated by the folding-back of palindromic sequences serving as primers for strand-displacement synthesis and formation of dimeric RF DNA. Using an in vitro replication assay and a cloned MVM DNA template, we observed hairpin-primed DNA replication at both MVM DNA termini, with a bias toward right-end initiation. Initiation of DNA replication is favored by nuclear components of A9 cell extract and highly stimulated by the MVM nonstructural protein NS1. Hairpin-primed DNA replication is also observed in the presence of NS1 and the Klenow fragment of the Escherichia coli DNA polymerase I. Addition of ATPgammaS (adenosine 5'-O-(thiotriphosphate)) blocks the initiation of DNA replication but not the extension of pre-existing hairpin primers formed in the presence of NS1 only. The NS1-mediated unwinding of the right-end palindrome may account for the recently reported capacity of NS1 for driving dimer RF synthesis in vitro.

Human herpesvirus 6

Human herpesvirus 6 variant A (HHV-6A) and human herpesvirus 6 variant B (HHV-6B) are two closely related yet distinct viruses. These visuses belong to the Roseolovirus genus of the betaherpesvirus subfamily; they are most closely related to human herpesvirus 7 and then to human cytomegalovirus. Over 95% of people older than 2 years of age are seropositive for either or both HHV-6 variants, and current serologic methods are incapable of discriminating infection with one variant from infection with the other. HHV-6A has not been etiologically linked to any human disease, but such an association will probably be found soon. HHV-6B is the etiologic agent of the common childhood illness exanthem subitum (roseola infantum or sixth disease) and related febrile illnesses. These viruses are frequently active and associated with illness in immunocompromised patients and may play a role in the etiology of Hodgkin's disease and other malignancies. HHV-6 is a commensal inhabitant of brains; various neurologic manifestations, including convulsions and encephalitis, can occur during primary HHV-6 infection or in immunocompromised patients. HHV-6 and distribution in the central nervous system are altered in patients with multiple sclerosis; the significance of this is under investigation.

Varicella-zoster virus (VZV) transcription during latency in human ganglia: detection of transcripts mapping to genes 21, 29, 62, and 63 in a cDNA library enriched for VZV RNA

Information on the extent of virus DNA transcription and translation in infected tissue is crucial to an understanding of herpesvirus latency. To detect low-abundance latent varicella-zoster virus (VZV) transcripts, poly(A)+ RNA extracted from latently infected human trigeminal ganglia was enriched for VZV transcripts by hybridization to biotinylated VZV DNA. After hybridization, the RNA-DNA hybrid was isolated by binding to avidin-coated beads and extensively washed, and the RNA was released by heat denaturation. A lambda-based cDNA library was then constructed from the enriched RNA. PCR and DNA sequencing of DNA extracted from the cDNA library revealed the presence of VZV genes 21, 29, 62, and 63, but not VZV genes 4, 10, 40, 51, and 61, in the enriched cDNA library. These findings confirm the detection of VZV gene 29 and 62 transcripts on Northern (RNA) blots prepared from latently infected human ganglia (J.L. Meier, R.P. Holman, K.D. Croen, J.E. Smialek, and S.E. Straus, Virology 193:193-200, 1993) and the presence of VZV gene 21 transcripts in a cDNA library from mRNA of latently infected ganglia (R.J. Cohrs, K. Srock, M.B. Barbour, G. Owens, R. Mahalingam, M.E. Devlin, M. Wellish and D.H. Gilden, J. Virol. 68:7900-7908,1994) and also reveal, for the first time, the presence of VZV gene 63 RNA in latently infected human ganglia.

Strain-dependent differences in the human cytomegalovirus replication origin

The nucleotide sequence of the human cytomegalovirus replication origin of strain Towne (an AatII-SacI fragment corresponding to nt 90372-94637 of strain AD169) was determined and compared with AD169. Two differences were found in the nucleotide sequence level. One was the alteration of structural organization (a major difference): a 189-bp region of AD169 (nt 93337-93525) was directly repeated three times in Towne. The other was a change in the nucleotide residue level including substitution, insertion, or deletion (a minor difference). The divergent residues were predominantly localized within the nt 92591-92855 region of AD169. A replication assay revealed that replication ability remained after deletion of the 189-bp repeat but disappeared after either a 1.5-kb deletion from the AatII end or a 0.9-kb deletion from the SacI end. The 1.5- and 0.9-kb regions were relatively conserved. These results indicate that at least two regions essential for replication ability lie outside of both the relatively variable region and the 189-bp repeat and suggest that these essential regions support replication even with a spatial separation of either one (AD169) or three repeats (Towne) of the 189-bp region.

The inverted repeat regions of the simian varicella virus and varicella-zoster virus genomes have a similar genetic organization

Simian varicella virus (SVV) causes a varicella-like disease in nonhuman primates. The DNA sequence and genetic organization of the inverted repeat region (RS) of the SVV genome was determined. The SVV RS is 7559 bp in size with 56% guanine+cytosine (G+C) content and includes 3 open reading frames (ORFs). The SVV RS1 ORF encodes a 1279 amino acid (aa) protein with 58 and 39% identity to the varicella-zoster virus (VZV) gene 62 and herpes simplex virus type 1 (HSV-1) ICP4 homologs, respectively. The predicted 261 aa SVV RS2 polypeptide possesses 52% identity with the VZV gene 63 homolog and 23% identity with the HSV-1 ICP22. The SVV RS3 encodes a 187 aa polypeptide with 56% and 28% identity to the VZV gene 64 and the HSV-1 US10 homologs, respectively, and includes an atypical zinc finger motif. A G+C-rich 16 base-pair (bp) sequence which is repeated 7 times and a putative SVV origin of replication were identified between the RS1 and RS2 ORFs. Comparison with the VZV RS indicates the SVV and VZV RS regions are similar in size and genetic organization.

Human herpesvirus 6B origin-binding protein: DNA-binding domain and consensus binding sequence

We previously demonstrated by a DNA-binding assay that the human herpesvirus 6B (HHV-6B) replication origin has a structure similar to those of alphaherpesviruses, although the HHV-6B and herpes simplex virus type 1 (HSV-1) origin-binding proteins (OBPs) and origins are not interchangeable. Here we describe additional properties of the interaction between HHV-6B OBP and the HHV-6B origin. Competitive electrophoretic mobility shift assays (EMSAs) with DNA duplexes containing single-base alterations allowed deduction of a consensus DNA sequence for HHV-6B-specific OBP binding, YGWYCWCCY, where Y is T or C and W is T or A, while that for HSV-1-specific binding was reported to be YGYTCGCACT. By EMSA, the HHV-6B OBP DNA-binding domain was mapped to a segment containing amino acids 482 to 770. However, in Southwestern (protein-DNA) blotting, the region sufficient for the DNA binding encompassed only amino acids 657 to 770. Similarly, Southwestern blotting showed that amino acids 689 to 851 of HSV-1 OBP had HSV-1 origin-binding activity, although this region was insufficient for origin binding in the EMSA. Although the longer DNA-binding domains identified by EMSA have marginal overall homology among HHV-6B and alphaherpesvirus OBP homologs, the smaller regions sufficient for the binding observed by Southwestern blotting have significant similarity. From these results, we propose a hypothesis that the DNA-binding domain of herpesvirus OBPs consists of two subdomains, one containing a conserved motif that contacts DNA directly, and another, less well conserved, that may modulate either the conformation or accessibility of the binding domain.

Varicella-zoster virus gene 51 complements a herpes simplex virus type 1 UL9 null mutant

Varicella-zoster virus (VZV) gene 51 encodes a protein which is homologous to UL9, the origin of DNA replication-binding protein of herpes simplex virus type 1. No genetic information is available on VZV gene 51, but its product has been shown to bind to virtually the same recognition sequence as does UL9 (D. Chen and P. D. Olivo, J. Virol. 68:3841-3849, 1994; N. D. Stow, H. M. Weir, and E. C. Stow, Virology 177:570-577, 1990). We report here that gene 51 can complement a UL9 null mutant (hr94) (A. K. Malik, R. Martinez, L. Muncy, E. P. Carmichael, and S. K. Weller, Virology 190:702-715, 1992), but at a level which is only 20% of that of UL9. Quantitation of viral DNA synthesis suggests that this phenotype is due to a defect in viral DNA synthesis. Regardless, the ability of VZV gene 51 to complement UL9 suggests that alphaherpesviruses have a highly conserved mechanism of initiation of viral DNA synthesis.

The varicella-zoster virus origin-binding protein can substitute for the herpes simplex virus origin-binding protein in a transient origin-dependent DNA replication assay in insect cells

We isolated two recombinant baculoviruses each of which expresses a varicella-zoster virus (VZV) homolog of one of the seven herpes simplex virus type 1 (HSV-1) genes required for DNA replication. We performed transient origin-dependent DNA replication assays in insect cells in which we substituted a baculovirus which expresses a VZV protein for a baculovirus which expresses its HSV homolog. VZV gene 51 protein was found to be able to support origin-dependent DNA synthesis when it was substituted for UL9, the HSV-1 origin-binding protein (OBP). This occurred whether an HSV-1 or a VZV origin-containing plasmid was used in the assay. These results suggest that VZV gene 51 protein is able to interact with the HSV replication machinery, and in light of the extensive structural divergence of these proteins, it suggests that initiation of VZV and HSV-1 DNA synthesis may involve a limited number of interactions between the OBP and other replication factors. Substitution of infected-cell protein 8 (ICP8), the major single-stranded DNA-binding protein of HSV-1, with VZV gene 29 protein, however, did not result in amplification of plasmids containing either an HSV-1 or a VZV origin. In the absence of ICP8, addition of both VZV gene 51 protein and gene 29 protein was also negative for origin-dependent replication whether or not UL9 was present. Although demonstration that our baculovirus-expressed VZV gene 29 protein is functional for DNA replication will await development of a VZV replication system, our results suggest that VZV gene 29 protein is unable to interact functionally with one or more of the HSV replication proteins. This approach should contribute to efforts to define the interactions among the alphaherpesvirus DNA replication proteins.

Expression of the varicella-zoster virus origin-binding protein and analysis of its site-specific DNA-binding properties

The varicella-zoster virus (VZV) genome contains homologs to each of the seven herpes simplex virus (HSV) genes that are required for viral DNA synthesis. VZV gene 51 is homologous to HSV UL9, which encodes an origin of DNA replication binding protein (OBP). It was previously shown, by using a protein A fusion protein, that the product of gene 51 is a site-specific DNA-binding protein which binds to sequences within the VZV origin (Stow et al., Virology 177:570-577, 1990). In this report, gene 51 was expressed in an in vitro translation system. Rabbit antiserum raised against the carboxyl-terminal 20 amino acids was used to confirm expression of the full-length gene 51 protein, and site-specific DNA-binding activity was demonstrated in a gel retardation assay. The origin-binding domain was located within a 263-amino-acid region of the carboxyl terminus by using a series of deletion mutants. The affinity of binding of the VZV OBP to the three binding sites in the VZV origin was found to be similar. In addition, as with UL9, a CGC triplet within a 10-bp consensus sequence is critical to the interaction between the OBP and the origin. The HSV and VZV OBPs, therefore, appear to have virtually identical recognition sequences despite only 33% identity and 44% similarity in the primary structure of their site-specific DNA-binding domains.

Cellular protein interactions with herpes simplex virus type 1 oriS

The herpes simplex virus type 1 (HSV-1) origin of DNA replication, oriS, contains an AT-rich region and three highly homologous sequences, sites I, II, and III, identified as binding sites for the HSV-1 origin-binding protein (OBP). In the present study, interactions between specific oriS DNA sequences and proteins in uninfected cell extracts were characterized. The formation of one predominant protein-DNA complex, M, was demonstrated in gel shift assays following incubation of uninfected cell extracts with site I DNA. The cellular protein(s) that comprises complex M has been designated origin factor I (OF-I). The OF-I binding site was shown to partially overlap the OBP binding site within site I. Complexes with mobilities indistinguishable from that of complex M also formed with site II and III DNAs in gel shift assays. oriS-containing plasmid DNA mutated in the OF-I binding site exhibited reduced replication efficiency in transient assays, demonstrating a role for this site in oriS function. The OF-I binding site is highly homologous to binding sites for the cellular CCAAT DNA-binding proteins. The binding site for the CCAAT protein CP2 was found to compete for OF-I binding to site I DNA. These studies support a model involving the participation of cellular proteins in the initiation of HSV-1 DNA synthesis at oriS.

Cellular protein interactions with herpes simplex virus type 1 oriS

The herpes simplex virus type 1 (HSV-1) origin of DNA replication, oriS, contains an AT-rich region and three highly homologous sequences, sites I, II, and III, identified as binding sites for the HSV-1 origin-binding protein (OBP). In the present study, interactions between specific oriS DNA sequences and proteins in uninfected cell extracts were characterized. The formation of one predominant protein-DNA complex, M, was demonstrated in gel shift assays following incubation of uninfected cell extracts with site I DNA. The cellular protein(s) that comprises complex M has been designated origin factor I (OF-I). The OF-I binding site was shown to partially overlap the OBP binding site within site I. Complexes with mobilities indistinguishable from that of complex M also formed with site II and III DNAs in gel shift assays. oriS-containing plasmid DNA mutated in the OF-I binding site exhibited reduced replication efficiency in transient assays, demonstrating a role for this site in oriS function. The OF-I binding site is highly homologous to binding sites for the cellular CCAAT DNA-binding proteins. The binding site for the CCAAT protein CP2 was found to compete for OF-I binding to site I DNA. These studies support a model involving the participation of cellular proteins in the initiation of HSV-1 DNA synthesis at oriS.