Cooperative interactions between replication origin-bound molecules of herpes simplex virus origin-binding protein are mediated via the amino terminus of the protein

Direct interaction between the N- and C-terminal portions of the herpes simplex virus type 1 origin binding protein UL9 implies the formation of a head-to-tail dimer

UL9, a superfamily II helicase, is a multifunctional protein required for herpes simplex virus type 1 replication in vivo. Although the C-terminal 317-amino-acid DNA binding domain of UL9 exists as a monomer, the full-length protein behaves as a dimer in solution. Thus, it has been assumed that the N-terminal 534 residues contain a region necessary for efficient dimerization and that UL9 dimers are in a head-to-head configuration. We recently showed, however, that residues in the N terminus could modulate the inhibitory properties of UL9 by decreasing the DNA binding ability of the C terminus (S. Chattopadhyay and S. K. Weller, J. Virol. 80:4491-4500, 2006). We suggested that a direct interaction between the N- and C-terminal portions of UL9 might exist and serve to modulate the DNA binding activities of the C terminus. In this study, we used a coimmunoprecipitation assay to show that the N-terminal portion of UL9 can indeed directly interact with the C terminus. A series of truncation mutant proteins were used to show that a region in the N terminus between residues 293 and 321 is necessary for efficient interaction. Similarly, a region in the C terminus between residues 600 and 800 is required for this interaction. The simplest model to explain these data is that UL9 dimers are oriented in a head-to-tail arrangement in which the N terminus is in contact with the C terminus.

Binding of the universal minicircle sequence binding protein at the kinetoplast DNA replication origin

Kinetoplast DNA, the mitochondrial DNA of trypanosomatids, is a remarkable DNA structure that contains, in the species Crithidia fasciculata, 5000 topologically linked duplex DNA minicircles. Their replication initiates at two conserved sequences, a dodecamer, known as the universal minicircle sequence (UMS), and a hexamer, which are located at the replication origins of the minicircle L and H strands, respectively. A UMS-binding protein (UMSBP) binds specifically the 12-mer UMS sequence and a 14-mer sequence that contains the conserved hexamer in their single-stranded DNA conformation. In vivo cross-linking analyses reveal the binding of UMSBP to kinetoplast DNA networks in the cell. Furthermore, UMSBP binds in vitro to native minicircle origin fragments, carrying the UMSBP recognition sequences. UMSBP binding at the replication origin induces conformational changes in the bound DNA through its folding, aggregation and condensation.

DNA binding activity of the herpes simplex virus type 1 origin binding protein, UL9, can be modulated by sequences in the N terminus: correlation between transdominance and DNA binding

UL9, the origin binding protein of herpes simplex virus type 1, is a member of the SF2 family of helicases. Cotransfection of cells with infectious viral DNA and plasmids expressing either full-length UL9 or the C-terminal DNA binding domain alone results in the drastic inhibition of plaque formation which can be partially relieved by an insertion mutant lacking DNA binding activity. In this work, C-terminally truncated mutants which terminate at or near residue 359 were shown to potentiate plaque formation, while other C-terminal truncations were inhibitory. Thus, residues in the N-terminal region appear to regulate the inhibitory properties of UL9. To identify which residues were involved in this regulation, a series of N-terminally truncated mutants were constructed which contain the DNA binding domain and various N-terminal extensions. Mutants whose N terminus is either at residue 494 or 535 were able to bind the origin efficiently and were inhibitory to plaque formation, whereas constructs whose N terminus is at residue 304 or 394 were defective in origin binding activity and were able to relieve inhibition. Since UL9 is required for viral infection at early but not late times and is inhibitory to infection when overexpressed, we propose that the DNA binding activities of UL9 are regulated during infection. For infection to proceed, UL9 may need to switch from a DNA binding to a non-DNA binding mode, and we suggest that sequences residing in the N terminus play a role in this switch.

Origin-specific unwinding of herpes simplex virus 1 DNA by the viral UL9 and ICP8 proteins: visualization of a specific preunwinding complex

Herpes simplex virus 1 contains three origins of replication; two copies of oriS and one of a similar sequence, oriL. Here, the combined action of multiple factors known or thought to influence the opening of oriS are examined. These include the viral origin-binding protein, UL9, and single-strand binding protein ICP8, host cell topoisomerase I, and superhelicity of the DNA template. By using electron microscopy, it was observed that when ICP8 and UL9 proteins were added together to oriS-containing supertwisted DNA, a discrete preunwinding complex was formed at oriS on 40% of the molecules, which was shown by double immunolabeling electron microscopy to contain both proteins. This complex was relatively stable to extreme dilution. Addition of ATP led to the efficient unwinding of approximately 50% of the DNA templates. Unwinding proceeded until the acquisition of a high level of positive supertwists in the remaining duplex DNA inhibited further unwinding. Addition of topoisomerase I allowed further unwinding, opening >1 kb of DNA around oriS.

ATP-dependent unwinding of a minimal origin of DNA replication by the origin-binding protein and the single-strand DNA-binding protein ICP8 from herpes simplex virus type I

The Herpes simplex virus type I origin-binding protein, OBP, is encoded by the UL9 gene. OBP binds the origin of DNA replication, oriS, in a cooperative and sequence-specific manner. OBP is also an ATP-dependent DNA helicase. We have recently shown that single-stranded oriS folds into a unique and evolutionarily conserved conformation, oriS*, which is stably bound by OBP. OriS* contains a stable hairpin formed by complementary base pairing between box I and box III in oriS. Here we show that OBP, in the presence of the single-stranded DNA-binding protein ICP8, can convert an 80-base pair double-stranded minimal oriS fragment to oriS* and form an OBP-oriS* complex. The formation of an OBP-oriS* complex requires hydrolysable ATP. We also demonstrate that OBP in the presence of ICP8 and ATP promotes slow but specific and complete unwinding of duplex minimal oriS. The possibility that the OBP-oriS* complex may serve as an assembly site for the herpes virus replisome is discussed.

Activation of the herpes simplex virus type-1 origin-binding protein (UL9) by heat shock proteins

Heat shock proteins participate in the initiation of DNA replication of different organisms by facilitating the assembly of initiation complexes. We have examined the effects of human heat shock proteins (Hsp40 and Hsp70) on the interaction of the herpes simplex virus type-1 initiator protein (UL9) with oriS, one of the viral origins of replication. Hsp40 and Hsp70 act substoichiometrically to increase the affinity of UL9 for oriS. The major contributor to this effect is Hsp40. Heat shock proteins also stimulate the ATPase activity of UL9 with oriS and increase opening of the origin. In contrast, heat shock proteins have no effect on the origin-independent activities of UL9 suggesting that their role is not merely in refolding denatured protein. These observations are consistent with a role for heat shock proteins in activating UL9 to efficiently initiate viral origin-dependent DNA replication. The action of heat shock proteins in this capacity is analogous to their role in activating the initiator proteins of other organisms.

Inhibition of herpes simplex virus type 2 (HSV-2) viral replication by the dominant negative mutant polypeptide of HSV-1 origin binding protein

UL9-C535C, the trans-dominant negative mutant polypeptide of herpes simplex virus type 1 (HSV-1) UL9 origin binding protein, is a potent inhibitor of HSV-1 viral DNA replication. This study focused on testing whether HSV-1 UL9-C535C and a genetically engineered UL9-C535C-encoding HSV-1 recombinant virus CJ83193 could inhibit herpes simplex virus type 2 (HSV-2) infection. First, a stable cell line, R-C535C, expressing a high level of UL9-C535C in the presence of tetracycline and little or no UL9-C535C in the absence of tetracycline was established. The single step growth experiment showed that like HSV-1, the de novo synthesis of HSV-2 could be suppressed approximately 1000-fold by UL9-C535C expressed in R-C535C cells in the presence of tetracycline. Secondly, compared with cells singly infected with HSV-2, co-infection of Vero cells with HSV-2 and CJ83193 reduced the replication efficiency of HSV-2 in co-infected cells by 30-40 fold in a single-step growth assay, which coincided with marked reduction in viral late gene expression, but not the expression of viral immediate-early genes. Taken together, in view of our recent demonstration that CJ83193 can serve as an effective vaccine in preventing HSV-1 infection in mice, one can generate a CJ83193-like HSV-2 recombinant virus that could potentially function as a new therapeutic class of recombinant viral vaccine against HSV-2 infection.

A tale of two HSV-1 helicases: roles of phage and animal virus helicases in DNA replication and recombination

Helicases play essential roles in many important biological processes such as DNA replication, repair, recombination, transcription, splicing, and translation. Many bacteriophages and plant and animal viruses encode one or more helicases, and these enzymes have been shown to play many roles in their respective viral life cycles. In this review we concentrate primarily on the roles of helicases in DNA replication and recombination with special emphasis on the bacteriophages T4, T7, and A as model systems. We explore comparisons between these model systems and the herpesviruses--primarily herpes simplex virus. Bacteriophage utilize various pathways of recombination-dependent DNA replication during the replication of their genomes. In fact the study of recombination in the phage systems has greatly enhanced our understanding of the importance of recombination in the replication strategies of bacteria, yeast, and higher eukaryotes. The ability to "restart" the replication process after a replication fork has stalled or has become disrupted for other reasons is a critical feature in the replication of all organisms studied. Phage helicases and other recombination proteins play critical roles in the "restart" process. Parallels between DNA replication and recombination in phage and in the herpesviruses is explored. We and others have proposed that recombination plays an important role in the life cycle of the herpesviruses, and in this review, we discuss models for herpes simplex virus type 1 (HSV-1) DNA replication. HSV-1 encodes two helicases. UL9 binds specifically to the origins of replication and is believed to initiate HSV DNA replication by unwinding at the origin; the heterotrimeric helicase-primase complex, encoded by UL5, UL8, and UL52 genes, is believed to unwind duplex viral DNA at replication forks. Structure-function analyses of UL9 and the helicase-primase are discussed with attention to the roles these proteins might play during HSV replication.

Sequence requirements for interaction of human herpesvirus 7 origin binding protein with the origin of lytic replication

As do human herpesvirus 6 variants A and B (HHV-6A and -6B), HHV-7 encodes a homolog of the alphaherpesvirus origin binding protein (OBP), which binds at sites in the origin of lytic replication (oriLyt) to initiate DNA replication. In this study, we sought to characterize the interaction of the HHV-7 OBP (OBP(H7)) with its cognate sites in the 600-bp HHV-7 oriLyt. We expressed the carboxyl-terminal domain of OBP(H7) and found that amino acids 484 to 787 of OBP(H7) were sufficient for DNA binding activity by electrophoretic mobility shift analysis. OBP(H7) has one high-affinity binding site (OBP-2) located on one flank of an AT-rich spacer element and a low-affinity site (OBP-1) on the other. This is in contrast to the HHV-6B OBP (OBP(H6B)), which binds with similar affinity to its two cognate OBP sites in the HHV-6B oriLyt. The minimal recognition element of the OBP-2 site was mapped to a 14-bp sequence. The OBP(H7) consensus recognition sequence of the 9-bp core, BRTYCWCCT (where B is a T, G, or C; R is a G or A; Y is a T or C; and W is a T or A), overlaps with the OBP(H6B) consensus YGWYCWCCY and establishes YCWCC as the roseolovirus OBP core recognition sequence. Heteroduplex analysis suggests that OBP(H7) interacts along one face of the DNA helix, with the major groove, as do OBP(H6B) and herpes simplex virus type 1 OBP. Together, these results illustrate both conserved and divergent DNA binding properties between OBP(H7) and OBP(H6B).

Phosphorylation of the herpes simplex virus type 1 origin binding protein

The herpes simplex virus type 1 (HSV-1) origin binding protein (OBP), the product of the UL9 gene, is one of seven HSV-encoded proteins required for viral DNA replication. OBP performs multiple functions characteristic of a DNA replication initiator protein, including origin-specific DNA binding and ATPase and helicase activities, as well as the ability to interact with viral and cellular proteins involved in DNA replication. Replication initiator proteins in other systems, including those of other DNA viruses, are known to be regulated by phosphorylation; however, the role of phosphorylation in OBP function has been difficult to assess due to the low level of OBP expression in HSV-infected cells. Using a metabolic labeling and immunoprecipitation approach, we obtained evidence that OBP is phosphorylated during HSV-1 infection. Kinetic analysis of metabolically labeled cells indicated that the levels of OBP expression and phosphorylation increased at approximately 4 h postinfection. Notably, when expressed from a transfected plasmid, a recombinant baculovirus, or a recombinant adenovirus (AdOBP), OBP was phosphorylated minimally, if at all. In contrast, superinfection of AdOBP-infected cells with an OBP-null mutant virus increased the level of OBP phosphorylation approximately threefold, suggesting that HSV-encoded viral or HSV-induced cellular factors enhance the level of OBP phosphorylation. Using HSV mutants inhibited at sequential stages of the viral life cycle, we demonstrated that this increase in OBP phosphorylation is dependent on early protein synthesis and is independent of viral DNA replication. Based on gel mobility shift assays, phosphorylation does not appear to affect the ability of OBP to bind to the HSV origins.

Cysteine 111 affects coupling of single-stranded DNA binding to ATP hydrolysis in the herpes simplex virus type-1 origin-binding protein

Herpes simplex virus type-1 origin-binding protein (UL9 protein) initiates viral replication by unwinding the origins. It possesses sequence-specific DNA-binding activity, single-stranded DNA-binding activity, DNA helicase activity, and ATPase activity that is strongly stimulated by single-stranded DNA. We have examined the role of cysteines in its action as a DNA helicase. The DNA helicase and DNA-dependent ATPase activities of UL9 protein were stimulated by reducing agent and specifically inactivated by the sulfhydryl-specific reagent N-ethylmaleimide. To identify the cysteine responsible for this phenomenon, a conserved cysteine in the vicinity of the ATP-binding site (cysteine 111) was mutagenized to alanine. UL9C111A protein exhibits defects in its DNA helicase and DNA-dependent ATPase activities and was unable to support origin-specific DNA replication in vivo. A kinetic analysis indicates that these defects are due to the inability of single-stranded DNA to induce high affinity ATP binding in UL9C111A protein. The DNA-dependent ATPase activity of UL9C111A protein is resistant to N-ethylmaleimide, while its DNA helicase activity remains sensitive. Accordingly, sensitivity of UL9 protein to N-ethylmaleimide is due to at least two cysteines. Cysteine 111 is involved in coupling single-stranded DNA binding to ATP-binding and subsequent hydrolysis, while a second cysteine is involved in coupling ATP hydrolysis to DNA unwinding.

The herpes simplex virus type 1 origin-binding protein. sequence-specific activation of adenosine triphosphatase activity by a double-stranded DNA containing box I

Origin-dependent replication of the herpes simplex virus type 1 genome requires the virally encoded origin-binding protein, UL9. UL9 binds specifically to the herpes simplex virus type 1 replication origin at two high affinity binding sites on the DNA, Boxes I and II. UL9 also has ATP-dependent DNA helicase and DNA-stimulated ATPase activities that are used to unwind the origin DNA. Origin-specific binding is mediated by the C-terminal domain (C-domain) of the enzyme. ATPase and helicase activities are mediated by the N-terminal domain (N-domain). Previous studies have shown that single-stranded DNA is a good coeffector for ATPase activity. We have analyzed several DNAs for their ability to stimulate the ATPase activity of UL9 and of a truncated UL9 protein (UL9/N) consisting only of the N-domain. We report here that duplex Box I DNA specifically and potently stimulates the ATPase activity of UL9 but not of UL9/N. We also find that removal of the C-domain significantly increases the ATPase activity of UL9. We have incorporated these results into a model for initiation in which the C-domain of UL9 serves to regulate the enzymatic activity of the N-domain.

A novel anti-herpes simplex virus type 1-specific herpes simplex virus type 1 recombinant

A recombinant herpes simplex virus (HSV) capable of inhibiting its own replication as well as the replication of wild-type virus would have greatly increased safety as a general purpose vector for in vivo gene transfer, antitumor therapy, and viral vaccine against HSV infection. By using a tetracycline repressor (tetR)-mediated HSV-1 viral replication switch [Yao and Eriksson (1999). Hum. Gene Ther. 10, 419-427], we have generated a novel anti-HSV-1-specific HSV-1 recombinant (CJ83193) that expresses a trans-dominant negative HSV-1 UL9 origin-binding protein, UL9-C535C. The de novo synthesis of CJ83193 can be suppressed by UL9-C535C by at least 1 x 10(6)-fold in non-tetR-expressing cells, and is subject to tetracycline regulation over a range of four to five orders of magnitude in a tetR-expressing osteosarcoma line. In particular, the UL9-C535C peptides expressed from the CJ83193 genome can inhibit the replication of wild-type HSV-1 by 100- to 200-fold in single-step growth assays. The construction of CJ83193 creates a new general strategy for developing recombinant viral vectors able to function as an intracellular therapy against wild-type viral infections.

A novel tetracycline-inducible viral replication switch

With the cloning of DNA encoding the trans-dominant negative mutant form of the HSV-1 origin-binding protein UL9, UL9-C535C, under the control of the tet operator-bearing hCMV major immediate-early promoter (pcmvtetO), this article demonstrates that the tetR-mediated mammalian transcription repression switch (Yao et al., Hum. Gene Ther. 9:1939-1950, 1998) can be converted to a novel HSV-1-specific viral replication switch. Using this viral replication switch, the plaque-forming efficiency of infectious HSV-1 DNA can be reversibly regulated by tetR over 100-fold in transient viral infection assays. Moreover, while less than 0 PFU/ml of HSV-1 was detected from tetR-expressing cells transfected with infectious HSV-1 DNA and plasmid pcmvtetOUL9-C535C in the presence of tetracycline, close to 1000 PFU/ml of HSV-1 was produced when similar experiments were carried out in the absence of tetracycline. The tetracycline treatment led no reduction in HSV-1 synthesis in cells transfected with infectious HSV-1 DNA alone. Taken together, given that the UL9-C535C-associated antiviral activity can be silenced in the context of this HSV-1 replication switch, the establishment of this reversible switch would allow construction of a new generation of HSV-1 recombinants able to inhibit its own replication as well as replication of wild-type virus.

Interaction between the herpes simplex virus type 1 origin-binding and DNA polymerase accessory proteins

Interactions between the herpes simplex virus type 1 (HSV-1) origin (ori)-binding protein (UL9) and two other components of the functional DNA replication complex have been observed. However, to date, no interaction between UL9 and a component of the DNA polymerase holoenzyme has been demonstrated. In this report, we demonstrate that UL9 and the DNA polymerase accessory protein (UL42) can form a stable complex in vitro as determined by coimmunoprecipitation with specific antibodies to each protein and by affinity chromatography using glutathione S-transferase (GST) fusion proteins. Complex formation does not require the presence of other viral proteins and occurs in the presence of ethidium bromide, indicating that UL9-UL42 interaction is DNA independent. Affinity beads charged with increasing concentrations of GST-42 fusion protein up to 5 microM bound increasing amounts of UL9 expressed by in vitro transcription/translation in rabbit reticulocyte lysates. Binding of N- and C-terminal portions of UL9 to GST affinity matrices revealed that the N-terminal 533 amino acids were sufficient for binding to GST-42, albeit at approximately a four- to six-fold reduced affinity compared to the full-length protein. No binding of a polypeptide containing the remainder of the UL9 C-terminal residues was observed. Thus the ori-binding protein, UL9, can physically associate with at least one member of each of the complexes (helicase/primase, DNA polymerase holoenzyme, single-stranded DNA-binding protein) required for origin-dependent DNA replication. These specific interactions provide a means by which the ordered assembly of HSV-1 DNA replication proteins at origins of replication can occur in the infected cell for initiation of viral DNA synthesis.

Unwinding of the box I element of a herpes simplex virus type 1 origin by a complex of the viral origin binding protein, single-strand DNA binding protein, and single-stranded DNA

The herpes simplex virus type 1 (HSV-1) genome contains three origins of replication: oriL and two copies of oriS. These origins contain specific sequences, box I and box II, linked by an AT-rich segment, that are recognized by an HSV-1-encoded origin binding protein (UL9 protein) which also possesses DNA helicase activity. Despite its intrinsic helicase activity, the UL9 protein is unable to unwind oriS or the box I element of oriS, either in the presence or absence of the HSV-1-encoded single-strand DNA binding protein, ICP8. However, a complex of the UL9 protein and ICP8 can unwind box I if it contains a 3' single-stranded tail at least 18 nt in length positioned downstream of box I. These findings suggest a model for the initiation of HSV-1 DNA replication in which a complex consisting of the UL9 protein bound to box I, and ICP8 bound to single-stranded DNA generated at the A+T rich linker, perhaps as a consequence of transcription, unwinds an HSV-1 origin of replication to provide access to the replication machinery with the consequent initiation of viral DNA replication. This mode of unwinding is distinct from that observed for other animal viruses--e.g., simian virus 40 or bovine papilloma virus--in which the initiator protein, T antigen, or E1 protein alone, unwinds elements of the origin sequence, and the single-strand DNA binding protein serves only to keep the separated strands apart.

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.

Cloning and sequence analysis of a Marek's disease virus origin binding protein (OBP) reveals strict conservation of structural motifs among OBPs of divergent alphaherpesviruses

Marek's disease virus (MDV) is a highly cell-associated avian herpesvirus. In its natural host, MDV induces Marek's disease (MD), a lethal condition characterized by malignant lymphoma of T cells. Although symptoms of MD may be prevented by vaccination, no practical pharmacological method of control has been widely accepted. Viral replication represents a point at which pharmacological control of herpesvirus infection may be most successful. However, this requires detailed knowledge of viral replication proteins. Studies in HSV-1 DNA replication implicate the UL9 protein as a key initiator of replication. For example, binding of UL9 to HSV-1 origins is a prerequisite for assembly of additional replication proteins. In this study, a protein, whose apparent molecular size is similar to that of HSV-1 UL9, was identified in extracts of MDV infected cells by western blot analysis with anti-HSV-1 UL9 antibody. A putative MDV UL9 gene was subsequently identified through sequencing of MDV genome fragments (BamHI G and C). Extended DNA sequence analysis revealed an open reading frame (ORF) which could encode a protein homologous to HSV-1 UL9. The MDV UL9 ORF encodes 841 amino acids, producing a sequence 49% identical to HSV-1 UL9 and 46% identical to VZV gene 51 product (VZV UL9). MDV UL9 shares numerous structural motifs with HSV-1 and VZV UL9 proteins, including six conserved N-terminal helicase motifs, an N-terminal leucine zipper motif, a C-terminal pseudo-leucine zipper sequence, and a putative helix-turn-helix structure.

The DNA ligands influence the interactions between the herpes simplex virus 1 origin binding protein and the single strand DNA-binding protein, ICP-8

The herpes simplex virus type 1 (HSV-1) origin binding protein, OBP, is a DNA helicase specifically stimulated by the viral single strand DNA-binding protein, ICP-8. The stimulation is dependent on direct protein-protein interactions between the C-terminal domain of OBP, delta OBP, and ICP 8 (Boehmer, P.E., Craigie, M.C., Stow, N.D., and Lehman, I.R. (1994) J. Biol. Chem. 269, 29329-29334). We have now observed that this interaction is dramatically influenced by the nature of the DNA ligand. Stable complexes between delta OBP, ICP 8, and double-stranded DNA, presented either as a specific duplex oligonucleotide or a restriction fragment containing the HSV-1 origin of replication, oriS, can be detected by gel chromatography and gel electrophoresis. In contrast, a single-stranded oligonucleotide, oligo(dT)65, will completely disrupt the complex between delta OBP and ICP 8. We therefore suggest that the interaction between delta OBP and ICP 8 serves to position the single strand DNA-binding protein with high precision onto single-stranded DNA at a replication fork or at an origin of DNA replication.

Sequence-dependent primer synthesis by the herpes simplex virus helicase-primase complex

The herpes simplex virus helicase-primase complex, a heterotrimer of the UL5, UL8, and UL52 proteins, displays a single predominant site of primer synthesis on phi X174 virion DNA (Tenney, D. J., Hurlburt, W. W., Micheletti, P. M., Bifano, M., and Hamatake, R. K. (1994) J. Biol. Chem. 269, 5030-5035). This site was mapped and found to be deoxycytosine-rich, directing the synthesis of a primer initiating with several guanine residues. The size and sequence requirements for primer synthesis were determined using oligonucleotides containing variations of the predominant template. Although the efficiency of primer synthesis on oligonucleotides was influenced by template size, it was absolutely dependent on nucleotide sequence. Conversely, the ATPase activity on oligonucleotide templates was dependent on template size rather than nucleotide sequence. Furthermore, only oligonucleotides containing primase templates were inhibitory in a coupled primase-polymerase assay using phi X174 DNA as template, suggesting that primer synthesis or primase turnover is rate-limiting. Additionally, stimulation of helicase-primase by the UL8 component and that by the ICP8 protein were shown to differ mechanistically using different templates: the UL8 component stimulated the rate of primer synthesis on phi X174 virion DNA and oligonucleotide templates, while ICP8 stimulation occurred only on phi X174 virion DNA.

The stoichiometry of binding of the herpes simplex virus type 1 origin binding protein, UL9, to OriS

A number of studies have demonstrated that the herpes simplex virus type 1 (HSV-1) UL9 protein, which is a homodimer in solution, binds to two high affinity binding sites in each origin of replication. Interaction between the proteins bound at the two sites leads to the formation of a complex nucleoprotein structure. The simplest models for this binding interaction predict two possible binding stoichiometries: 1) one UL9 dimer is bound at each site; or 2) one UL9 monomer is bound at each site so that one UL9 dimer occupies both sites. Two recent papers have addressed this issue by using indirect methods to measure the binding stoichiometry. Martin et al. (Martin, D. W., Muñoz, R. M., Oliver, D., Subler, M. A., and Deb, S. (1994) Virology 198, 71-80) reported that a monomer of UL9 binds to a single high affinity site, and Stabell and Olivo (Stabell, E. C., and Olivo, P.D. (1993) Nucleic Acids Res. 21, 5203-5211) concluded that a dimer of UL9 binds to a single high affinity site. We have directly measured the stoichiometry of binding of the carboxyl-terminal DNA binding domain of UL9 (t-UL9) to the origin of replication using a double-label gel shift assay. Using a short synthetic double-stranded oligonucleotide containing a single UL9 binding site, one protein-DNA complex was detected in the gel shift assay, and the molar ratio of UL9 DNA binding domains to DNA binding sites in this complex was determined to be 2.0 +/- 0.1 (n = 13). Using the minimal origin sequence excised from plasmid DNA, two protein-DNA complexes were detected. The binding stoichiometry of the faster migrating complex was 1.8 +/- 0.1 (n = 15), and the stoichiometry of the more slowly migrating band was 3.7 +/- 0.4 (n = 15). The simplest explanation for these data is that UL9 binds to the origin of replication as a homodimer with one dimer bound at both high affinity sites.

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.