Mapping of the transcriptional initiation site of the herpes simplex virus type 1 ICP8 gene in infected and transfected cells

Sp1 facilitates continued HSV-1 gene expression in the absence of key viral transactivators

Productive replication of herpes simplex virus (HSV) relies upon a well-ordered transcriptional cascade flowing from immediate-early (IE) to early (E) to late (L) gene products. While several virus-encoded transcriptional activators are involved in this process, IE and E gene promoters also contain multiple binding sites for the ubiquitously expressed cellular transcription factor Sp1. Sp1 has been previously implicated in activating HSV-1 gene transcription downstream of these sites, but why Sp1-binding sites are maintained in the promoters of genes activated by virus-encoded activators remains unclear. We hypothesized that Sp1 enables continued HSV-1 transcription and replication when viral transactivators are limited. We used a depletion-based approach in human foreskin fibroblasts to investigate the specific contribution of Sp1 to the initiation and progression of the HSV-1 lytic gene cascade. We found that Sp1 increased viral transcript levels, protein expression, and replication following infection with VP16- or ICP0-deficient viruses but had little to no effect on rescued viruses or during wild-type (WT) HSV-1 infection. Moreover, Sp1 promoted WT virus transcription and replication following interferon treatment of fibroblasts and thus may contribute to viral immune evasion. Interestingly, we observed reduced expression of Sp1 and Sp1-family transcription factors in differentiated sensory neurons compared to undifferentiated cells, suggesting that reduced Sp1 levels may also contribute to HSV-1 latent infection. Overall, these findings indicate that Sp1 can promote HSV-1 gene expression in the absence of key viral transactivators; thus, HSV-1 may use Sp1 to maintain its gene expression and replication under adverse conditions.IMPORTANCEHerpes simplex virus (HSV) is a common human pathogen that actively replicates in the epithelia but can persist for the lifetime of the infected host via a stable, latent infection in neurons. A key feature of the HSV replication cycle is a complex transcriptional program in which virus and host-cell factors coordinate to regulate expression of the viral gene products necessary for continued viral replication. Multiple binding sites for the cellular transcription factor Sp1 are located in the promoters of HSV-1 genes, but how Sp1 binding contributes to transcription and replication of wild-type virus is not fully understood. In this study, we identified a specific role for Sp1 in maintaining HSV-1 gene transcription under adverse conditions, as when virus-encoded transcriptional activators were absent or limited. Preservation of Sp1-binding sites in HSV-1 gene promoters may thus benefit the virus as it navigates diverse cell types and host-cell conditions during infection.

Herpesviral ICP0 Protein Promotes Two Waves of Heterochromatin Removal on an Early Viral Promoter during Lytic Infection

Herpesviruses must contend with host cell epigenetic silencing responses acting on their genomes upon entry into the host cell nucleus. In this study, we confirmed that unchromatinized herpes simplex virus 1 (HSV-1) genomes enter primary human foreskin fibroblasts and are rapidly subjected to assembly of nucleosomes and association with repressive heterochromatin modifications such as histone 3 (H3) lysine 9-trimethylation (H3K9me3) and lysine 27-trimethylation (H3K27me3) during the first 1 to 2 h postinfection. Kinetic analysis of the modulation of nucleosomes and heterochromatin modifications over the course of lytic infection demonstrates a progressive removal that coincided with initiation of viral gene expression. We obtained evidence for three phases of heterochromatin removal from an early gene promoter: an initial removal of histones and heterochromatin not dependent on ICP0, a second ICP0-dependent round of removal of H3K9me3 that is independent of viral DNA synthesis, and a third phase of H3K27me3 removal that is dependent on ICP0 and viral DNA synthesis. The presence of ICP0 in transfected cells is also sufficient to promote removal of histones and H3K9me3 modifications of cotransfected genes. Overall, these results show that ICP0 promotes histone removal, a reduction of H3K9me3 modifications, and a later indirect reduction of H3K27me3 modifications following viral early gene expression and DNA synthesis. Therefore, HSV ICP0 promotes the reversal of host epigenetic silencing mechanisms by several mechanisms.

The human pathogen herpes simplex virus (HSV) has evolved multiple strategies to counteract host-mediated epigenetic silencing during productive infection. However, the mechanisms by which viral and cellular effectors contribute to these processes are not well defined. The results from this study demonstrate that HSV counteracts host epigenetic repression in a dynamic stepwise process to remove histone 3 (H3) and subsequently target specific heterochromatin modifications in two distinct waves. This provides the first evidence of a stepwise reversal of host epigenetic silencing by viral proteins. This work also suggests that targets capable of disrupting the kinetics of epigenetic regulation could serve as potential antiviral therapeutic agents.

Establishment of a novel foreign gene delivery system combining an HSV amplicon with an attenuated replication-competent virus, HSV-1 HF10

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have been used widely in genetic engineering with many advantages for gene delivery, being easily constructed. An attenuated and replication-competent HSV-1 HF10 clone demonstrating an oncolytic effect on cancer cells in vitro and in vivo has been applied recently for clinical virotherapy of breast cancers and the present studies were conducted to test its efficacy in combination with an HSV-1 amplicon. For this purpose, a new system was developed to produce high titers of the HSV-1 amplicon vector and the results showed that its package efficiency and the titer ratio to HF10 were improved by passage through two cell lines. A high ratio of amplicon/helper virus HF10 (A/H) (>1) was required to express the foreign gene efficiently. Furthermore, in order to express the foreign gene conditionally, an HSV-1 ICP8 promoter was introduced in place of the human cytomegalovirus MIE promoter, this driving expression of the transgene when replication of HF10 progressed. The methodology for simple preparation of mixtures of viruses containing the amplicon with the oncolytic virus is documented. This system should find application for studies of cancer therapy.

Differential cellular requirements for activation of herpes simplex virus type 1 early (tk) and late (gC) promoters by ICP4

The herpes simplex virus type 1 immediate-early protein, ICP4, activates the transcription of viral early and late genes and is essential for viral growth. It has been shown to bind DNA and interact with components of the general transcription machinery to activate or repress viral transcription, depending upon promoter context. Since early and late gene promoters have different architectures and cellular metabolism may be very different at early and late times after infection, the cellular requirements for ICP4-mediated activation of early and late genes may differ. This hypothesis was tested using tk and gC as representative early and late promoters, respectively. Nuclear extracts and phosphocellulose column fractions derived from nuclear extracts were able to reconstitute basal and ICP4-activated transcription of both promoters in vitro. When examining the contribution of the general transcription factors on the ability of ICP4 to activate transcription, the fraction containing the general transcription factor TFIIA was not essential for ICP4 activation of the gC promoter, but it was required for efficient activation of the tk promoter. The addition of recombinant TFIIA restored the ability of ICP4 to efficiently activate the tk promoter, but it had no net effect on activation of the gC promoter. The dispensability of TFIIA for ICP4 activation of the gC promoter required an intact INR element. In addition, microarray and Northern blot analysis indicated that TFIIA abundance may be reduced at late times of infection. This decrease in TFIIA expression during infection and its dispensability for activation of late but not early genes suggest one of possibly many mechanisms for the transition from viral early to late gene expression.

Herpes simplex virus type 1 origins of DNA replication play no role in the regulation of flanking promoters

Herpes simplex virus (HSV) exhibits altered gene regulation in neuronal compared to nonneuronal tissues. It has been hypothesized that initiation of DNA synthesis at the viral origins of replication (oriS and oriL) is a critical step in the upregulation of transcriptional activity of flanking divergent promoters, thereby increasing productive gene expression in neurons. Notably, oriS is flanked by the immediate-early (IE) ICP4 and ICP22/47 promoters, and oriL is flanked by the early (E) UL29 and UL30 promoters. To test this hypothesis further, a series of constructs were generated in which these promoters were placed upstream of luciferase genes. In addition, DNA replication origins were deleted in the context of these promoter constructs. All cassettes were recombined into the viral genome of HSV type 1 strain KOS at a site distal to its native origins. Recombinant reporter expression was monitored in vitro and in vivo to determine the role of viral origins of DNA replication in the regulation of their flanking promoters. Reporter gene expression was unaffected by the presence or absence of oriS or oriL, with the exception of a twofold increase in ICP22/47 promoter activity in the absence of oriS. DNA synthesis inhibitors resulted in a decrease of both IE- and E-promoter activity in primary cells but not continuous cell cultures. Reporter activity was readily assayed in vivo during acute infection and reactivation from latency and was also sensitive to DNA synthesis inhibitors. In all assays, reporter gene expression was unaffected by the presence or absence of either oriS or oriL. These data support the requirement of DNA synthesis for full viral gene expression in vivo but suggest that the origin elements play no role in the regulation of their flanking promoters.

Regulated expression of a Sindbis virus replicon by herpesvirus promoters

We describe the use of herpesvirus promoters to regulate the expression of a Sindbis virus replicon (SINrep/LacZ). We isolated cell lines that contain the cDNA of SINrep/LacZ under the control of a promoter from a herpesvirus early gene which requires regulatory proteins encoded by immediate-early genes for expression. Wild-type Sindbis virus and replicons derived from this virus cause death of most vertebrate cells, but the cells discussed here grew normally and expressed the replicon and beta-galactosidase only after infection with a herpesvirus. Vero cell lines in which the expression of SINrep/LacZ was regulated by the herpes simplex virus type 1 (HSV-1) infected-cell protein 8 promoter were generated. One Vero cell line (V3-45N) contained, in addition to the SINrep/LacZ cDNA, a Sindbis virus-defective helper cDNA which provides the structural proteins for packaging the replicon. Infection of V3-45N cells with HSV-1 resulted in the production of packaged SINrep/LacZ replicons. HSV-1 induction of the Sindbis virus replicon and packaging and spread of the replicon led to enhanced expression of the reporter gene, suggesting that this type of cell could be used to develop sensitive assays to detect herpesviruses. We also isolated a mink lung cell line that was transformed with SINrep/LacZ cDNA under the control of the promoter from the human cytomegalovirus (HCMV) early gene UL45. HCMV carries out an abortive infection in mink lung cells, but it was able to induce the SINrep/LacZ replicon. These results, and those obtained with an HSV-1 mutant, demonstrate that this type of signal amplification system could be valuable for detecting herpesviruses for which a permissive cell culture system is not available.

Activation of gene expression by herpes simplex virus type 1 ICP0 occurs at the level of mRNA synthesis

ICP0 is a nuclear phosphoprotein involved in the activation of herpes simplex virus type 1 (HSV-1) gene expression during lytic infection and reactivation from viral latency. Although available evidence suggests that ICP0 acts at the level of transcription, definitive studies specifically addressing this issue have not been reported. In the present study we measured the ability of ICP0 to activate gene expression (i) from promoters representing the major kinetic classes of viral genes in transient expression assays and (ii) from the same promoters during viral infection at multiplicities of infection ranging from 0.1 to 5.0 PFU/cell. The levels of synthesis and steady-state accumulation of mRNA, mRNA stability, and levels of protein synthesis were compared in cells transfected with a reporter plasmid in the presence and absence of ICP0 and in cells infected with wild-type HSV-1 or an ICP0 null mutant, n212. In transient expression assays and during viral infection at all multiplicities tested, the levels of steady-state mRNA and protein were significantly lower in the absence of ICP0, indicating that ICP0 activates gene expression at the level of mRNA accumulation. In transient expression assays and during infection at low multiplicities (< 1 PFU/cell) in the presence or absence of ICP0, marked increases in the levels of viral mRNAs accompanied by proportional increases in the levels of protein synthesis were observed with increasing multiplicity. At a high multiplicity (5 PFU/cell) in the presence or absence of ICP0, mRNA levels did not increase as a function of multiplicity and changes in the levels of protein were no longer related to changes in the levels of mRNA. Collectively, these tests indicate that transcription of viral genes is rate limiting at low multiplicities and that translation is rate limiting at high multiplicities, independent of ICP0. Consistent with the lower levels of mRNA detected in the absence of ICP0, the rates of transcription initiation measured by nuclear run-on assays were uniformly lower in cells infected with the ICP0 null mutant at all multiplicities tested, implying that ICP0 enhances transcription at or before initiation or both. No evidence was found of posttranscriptional effects of ICP0 (i.e., effects on the stability of mRNA, nuclear-cytoplasmic distribution, polyribosomal mRNA distribution, or rates of protein synthesis). Taken together, these results suggest that ICP0 activates gene expression prior to or at the level of initiation of mRNA synthesis in transient expression assays and during viral infection. Based on these findings; we hypothesize that the exaggerated multiplicity-dependent growth phenotype characteristic of ICP0 null mutants reflects the requirement for ICP0 under conditions where the steady-state level of mRNA is rate limiting, such as during low-multiplicity infection and reactivation from latency.

Transcriptional analysis of the region of the herpes simplex virus type 1 genome containing the UL8, UL9, and UL10 genes and identification of a novel delayed-early gene product, OBPC

The region of the UL component of the herpes simplex virus type 1 genome between nucleotides 17,793 and 25,150 includes three open reading frames that code for the protein products of the UL8, UL9, and UL10 genes (D.J. McGeogh, M.A. Dalrymple, A.J. Davison, A. Dolan, M.C. Frame, D. McNab, L.J. Perry, J.E. Scott, and P. Taylor, J. Gen. Virol. 69:1531-1574, 1988). We have mapped and characterized the overlapping transcripts in this region and have found that, in addition to the low-abundance UL8 and UL9 transcripts and the abundant UL10 transcript, at least two additional transcription units, designated UL8.5 and UL9.5, are specified by this region of the genome. The 5' ends of the UL8, UL8.5, and UL9 transcripts were mapped to nucleotides 20,682, 22,351, and 23,381, respectively. The 5' terminus of the UL9.5 transcript has not yet been mapped. The 3' ends of the UL8, UL8.5, UL9, and UL9.5 transcripts are coterminal at nucleotide 18,197. The 5' end of the UL10 mRNA, which is transcribed from the strand opposite that specifying the UL8, UL8.5, UL9, and UL9.5 transcripts, lies within the UL9 open reading frame at nucleotide 22,944, while the 3' terminus was mapped to nucleotide 24,666. Time course studies demonstrated that the UL8 and UL9 transcripts are members of the early kinetic class, the UL8.5 mRNA is a delayed-early transcript, and the UL9.5 and UL10 transcripts belong to the true-late kinetic class. Examination of the nucleotide sequence of the UL8.5 transcript revealed a potential open reading frame that overlaps and is in frame with the C-terminal half of the open reading frame encoding the origin-binding protein (OBP), the product of the UL9 gene. In vitro translation of the UL8.5 transcript demonstrated that it encodes a protein with an apparent molecular mass of 53 kDa. This protein was recognized by antibody directed against the C-terminal region of OBP and has thus been designated OBPC. A protein with an identical apparent molecular mass was also recognized by this antibody in infected-cell lysates, indicating that OBPC is synthesized during viral infection.

Varicella-zoster virus DNA polymerase and major DNA-binding protein genes have overlapping divergent promoters

A detailed analysis of the transcriptionally divergent promoters of varicella-zoster virus (VZV) open reading frames (ORFs) 28 and 29, encoding the DNA polymerase and major DNA-binding proteins, respectively, was performed. We found that the 221-bp ORF 28-29 intergenic domain contains overlapping divergent promoters; these promoters have TATA boxes and cap sites arranged closely back-to-back, have highly concordant patterns of responsiveness to transactivation by VZV ORFs 4 and/or 62, and could not be separated without abolishing the effects that VZV trans activators imparted to them. Mutation of the ORF 28 TATA box rendered this promoter unresponsive to ORF 62 and the combination of ORFs 4 and 62 without altering ORF 29 promoter activity. Mutations of all potential ORF 29 TATA boxes collectively failed to abolish this promoter's responsiveness to either ORF 4 or ORF 62, suggesting a mechanism of gene regulation for ORF 29 that differs from that of ORF 28. These findings are concordant with the observation that both genes are expressed in productive infection, but only ORF 29 expression has been identified in latency.

Unusual regulation of expression of the herpes simplex virus DNA polymerase gene

During herpes simplex virus infection, expression of the viral DNA polymerase (pol) gene is regulated temporally as an early (beta) gene and is additionally down-regulated at late times at the level of translation (D. R. Yager, A. I. Marcy, and D. M. Coen, J. Virol. 64:2217-2225, 1990). To examine the role of viral DNA synthesis in pol regulation, we studied pol expression during infections in which viral DNA synthesis was blocked, either by using drugs that inhibit Pol or ribonucleotide reductase or by using viral mutants with lesions in either the pol or a primase-helicase subunit gene. Under any of these conditions, the level of cytoplasmic pol mRNA was reduced. This reduction was first seen at approximately the time DNA synthesis begins and, when normalized to levels of other early mRNAs, became as great as 20-fold late in infection. The reduction was also observed in the absence of the adjacent origin of replication, oriL. Thus, although pol mRNA accumulated as expected for an early gene in terms of temporal regulation, it behaved more like that of a late (gamma) gene in its response to DNA synthesis inhibition. Surprisingly, despite the marked decrease in pol mRNA in the absence of DNA synthesis, the accumulation of Pol polypeptide was unaffected. This was accompanied by loss of the normal down-regulation of translation of pol mRNA at late times. We suggest a model to explain these findings.

Nucleotide sequence of the major DNA-binding protein gene of herpes simplex virus type 2 and a comparison with the type 1

The nucleotide sequence of a region encompassing about 5,200 base pairs (bp) of the left side of the origin of replication in the long unique region of the herpes simplex virus type 2 (HSV-2) has been determined. This region contained the major DNA-binding protein or the infected-cell protein 8 (ICP 8) gene and 5'-part of the counterpart of HSV-1 ICP 18.5 gene. A comparison of the nucleotide sequence of the ICP8 gene between HSV-1 and HSV-2 showed an 89.8% homology. A primer extension analysis for the HSV-2 ICP 8 mRNA showed that the major transcriptional start site was mapped at 315 bp upstream of the initiation codon. A comparison of the predicted functional amino acid sequence of the ICP 8 between HSV-1 and HSV-2 revealed a striking homology (97.2%), the value of which was the highest among those of the other polypeptides encoded by HSV-1 and HSV-2. Some domains, which were shown to be required for the nuclear function, the binding to single-stranded DNA and the nuclear localization were well conserved. In addition, the nucleotide and the functional amino acid sequences of a part of the HSV-2 counterpart of the HSV-1 ICP 18.5 gene were also compared, demonstrating an 88.4% and 95.9% homology, respectively.

Substitution of a TATA box from a herpes simplex virus late gene in the viral thymidine kinase promoter alters ICP4 inducibility but not temporal expression

The role of cis-acting promoter elements associated with herpes simplex virus type 1 (HSV-1) early and late genes was evaluated during productive infection with regard to activation of gene expression by the HSV-1 transactivator ICP4 and control of temporal regulation. A set of recombinant viruses was constructed such that expression of an HSV-1 early gene, thymidine kinase (tk), was placed under the control of either the tk TATA box or the TATA box from the late gene, glycoprotein C (gC), in the presence or absence of the upstream Sp1 and CCAAT sites normally found in the tk promoter. The presence of Sp1 sites in the promoter or replacement of the tk TATA box with the gC TATA box resulted in a decreased activation of tk mRNA expression by ICP4. Substitution of the A + T-rich region from the gC TATA box in the context of the remainder of the surrounding tk sequences resulted in a promoter that bound recombinant TATA-binding protein (TBP) better at lower concentrations than the wild-type tk promoter did. These results indicate that tk promoters that are better able to utilize TBP are less responsive to ICP4 activation and suggest that activation by ICP4 involves the general transcription factors that interact with TBP or TBP itself. Additionally, all of the viruses expressed tk at early times postinfection, indicating that cis-acting promoter elements that control the level of expression of HSV-1 early and late genes do not determine temporal regulation.

Analysis of the HSV-1 strain 17 DNA polymerase gene reveals the expression of four different classes of pol transcripts

We have investigated the structure and the expression of transcripts of the HSV-1 strain 17 DNA polymerase gene (pol) by various mapping methods including cDNA cloning. The majority of mature pol transcripts is strictly colinear with the pol gene. But additionally, pol cDNAs show a defined heterogeneity in respect to their 5'-terminal regions and can be divided into four classes with characteristic differences; (i) class 1 represents the major transcript (pol-R1) with initiation at HSV-1 positions 62,605-62,610, (ii) class 2 initiates about 70 bp downstream, (iii) class 3 is generated by splicing the short open reading frame (SORF) to a 5'-truncated part of the long open reading frame (LORF) which results in a partially different coding potential, and (iv) class 4 starts 120 bp upstream of the major initiation site in the central part of the origin of replication (oriL). S1 and Exo VII nuclease and RNase protection assays as well as primer extension analyses confirm the classification regarding the genuine structure of pol mRNAs and the differential usage of transcriptional start sites. Furthermore, the transcript classes can be distinguished from each other by their kinetics of appearance/disappearance in the cytoplasm: The first transcription of the pol gene is indicated by the predominant presence of class 2 and class 4 mRNAs at 2 hr postinfection (h.p.i.), followed by an increase of class 1 transcripts up to 4 h.p.i. and a parallel decrease of class 2 mRNAs. These data suggest that expression of the pol gene is finely regulated already at the transcriptional and/or posttranscriptional level prior to the translation of pol mRNAs.

Herpes simplex virus transactivator ICP4 operationally substitutes for the cellular transcription factor Sp1 for efficient expression of the viral thymidine kinase gene

The herpes simplex virus type 1 (HSV-1) ICP4 protein is a transcriptional activator of many eucaryotic RNA polymerase II promoters. The HSV-1 thymidine kinase gene (tk) promoter is induced by ICP4 and contains binding sites for the cellular transcription factors TFIID, Sp1, and CCAAT-binding proteins, each of which affects expression of the tk gene. In this study, the effects of mutations in these sites on the transcription of tk in the presence and absence of ICP4 were determined during viral infection. Only the TATA box was necessary for efficient expression in the presence of ICP4; however, ICP4 apparently can still induce tk transcription even when the TATA box is disrupted. Alteration of the Sp1 sites had a minor effect on ICP4-induced expression in comparison to a large effect in the absence of ICP4, indicating that ICP4 can operationally substitute for the function of the transcription factor Sp1. In addition, tk was still expressed with the kinetics of an early gene in the absence of binding sites for Sp1 and CCAAT-binding proteins.

Analysis of the herpes simplex virus type 1 promoter controlling the expression of UL38, a true late gene involved in capsid assembly

The cistrons encoding the herpes simplex virus type 1 (HSV-1) UL37 and UL38 genes are adjacent to one another but are transcribed from opposite strands of the viral DNA. The UL37 gene encodes a 1,123-amino-acid protein of unknown function, while the 465-amino-acid UL38 protein is involved in capsid assembly. Previous work from our laboratory indicated that the transcripts encoding these proteins are expressed with significantly different kinetics in productive infection. In the present communication we confirm the kinetic classes and precisely map the cap sites of the UL37 and UL38 mRNAs. A bifunctional reporter gene vector was used to demonstrate that divergent promoters control the expression of these reporter genes in trans-activation assays. The UL38 promoter is functionally separable from that controlling UL37 in a recombinant virus. We used deletion analysis to demonstrate that as few as 29 bases 5' of the mRNA cap site are adequate for full activity of the UL38 promoter in trans-activation assays. Finally, we analyzed the protein-binding properties of the UL38 promoter; several sites that form complexes containing ICP4, with clear homology to those identified in the HSV-1 gamma 42 promoter, are present. Thus, in general, the properties of this promoter are quite similar to those of other gamma promoters.

Characterization of the murine cytomegalovirus early transcription unit e1 that is induced by immediate-early proteins

The regulation of murine cytomegalovirus early (E) gene expression was studied in the cell line B25, which is stably transfected with the immediate-early ie1/ie3 gene complex. Infection of B25 cells in the presence of the protein synthesis inhibitor cycloheximide resulted in the expression of some E genes, whereas for the expression of other E genes prior protein synthesis was still mandatory, thus showing differences in the expression requirements of individual E genes. Transcription unit e1, a member of the E genes induced by immediate-early products of the ie1/ie3 gene complex, was characterized. It is located between map units 0.709 and 0.721 of the genome of murine cytomegalovirus strain Smith. A 2.6-kilobase RNA specified in this region is spliced from three exons of 912, 177, and 1,007 or 1,020 nucleotides, which are separated by introns of 93 and 326 nucleotides. The second AUG located in the first exon 119 nucleotides downstream of the 5' cap site is followed by an open reading frame of 990 nucleotides. The predicted polypeptide of 330 amino acids has a calculated molecular mass of 36.4 kilodaltons. Transfection with e1 revealed three antigenically related proteins of 36, 37, and 38 kilodaltons; these proteins probably represent differently modified forms of the predicted protein. These three proteins are phosphorylated and are associated with intranuclear inclusion bodies. A 33-kilodalton protein also derived from e1 was identified as a product of nonspliced transcripts. Comparison of amino acid sequences revealed homology between the murine cytomegalovirus transcription unit e1 and a human cytomegalovirus E transcription unit.

Genetic evidence for multiple nuclear functions of the herpes simplex virus ICP8 DNA-binding protein

We have isolated several mutant herpes simplex viruses, specifically mutated in the infected cell protein 8 (ICP8) gene, to define the functional domains of ICP8, the major viral DNA-binding protein. To facilitate the isolation of these mutants, we first isolated a mutant virus, HD-2, with the lacZ gene fused to the ICP8 gene so that an ICP8-beta-galactosidase fusion protein was expressed. This virus formed blue plaques on ICP8-expressing cell lines in the presence of 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside. Mutated ICP8 gene plasmids cotransfected with HD-2 DNA yielded recombinant viruses with the mutant ICP8 gene incorporated into the viral genome. These recombinants were identified by formation of white plaques. Four classes of mutants were defined: (i) some expressed ICP8 that could bind to DNA but could not localize to the cell nucleus; (ii) some expressed ICP8 that did not bind to DNA but localized to the nucleus; (iii) some expressed ICP8 that neither bound to DNA nor localized to the nucleus; and (iv) one expressed ICP8 that localized to the cell nucleus and bound to DNA in vitro, but the mutant virus did not replicate its DNA. These classes of mutants provide genetic evidence that DNA binding and nuclear localization are distinct functions of ICP8 and that ICP8 has nuclear functions other than binding to DNA. Furthermore, the portion of ICP8 needed for a nuclear function(s) distinct from DNA binding is the part of ICP8 showing sequence similarity to that of the cellular protein cyclin or proliferating cell nuclear antigen.

Examination of the roles of transcription factor Sp1-binding sites and an octamer motif in trans induction of the herpes simplex virus thymidine kinase gene

Herpes simplex virus mutants with both Sp1-binding sites in the thymidine kinase (tk) promoter inactivated or an octamer motif deleted were at most modestly impaired for tk expression. Thus, no cellular transcription factor that binds upstream of the tk TATA box is solely required for trans induction of this gene.

Herpes simplex virus alpha protein ICP27 can inhibit or augment viral gene transactivation

Three of the five alpha (immediate early) gene products of herpes simplex virus, infected cell proteins (ICPs) 4, 0, and 27 play a role in the control of expression of viral beta (delayed-early) and gamma (late) genes. We report here that ICP27 can inhibit or augment the individual or combined abilities of ICP4 and ICP0 to stimulate expression of chimeric genes containing viral gene promoters in a transient expression system. The specific effect of ICP27 was dependent on the viral gene promoter in the chimeric gene. ICP27 inhibited the ability of ICP4 and ICP0 to activate some beta gene promoters but augmented their ability to activate other beta or gamma 1 gene promoters when they were used in the target genes. Activation of the target genes by adenovirus E1A was not affected by ICP27 under the same conditions. ICP27 also repressed the ability of ICP0 to stimulate expression of a chimeric gene containing an alpha gene promoter. Insertion of a termination codon in the middle of the ICP27 coding region severely reduced the inhibitory effect of the plasmid, indicating that this activity requires expression of functional ICP27 polypeptide. This report focuses on the ICP27 activity that negatively regulates ICP4 transactivation of a chimeric gene containing the upstream sequences of the HSV beta gene ICP8. ICP27 decreased the level of mRNA initiated at the transcriptional start site of the ICP8 gene. The level of expression of the ICP4 gene was not changed by ICP27 but an alteration in the electrophoretic mobility of ICP4 expressed was observed. The modulatory effect of ICP27 on HSV transactivators may control the progress of the lytic cycle or provide a balance that varies in different cell types to affect whether lytic or latent infection ensues.

The role of viral and cellular nuclear proteins in herpes simplex virus replication

Following infection of cells by herpes simplex virus, the cell nucleus is subverted for transcription and replication of the viral genome and assembly of progeny nucleocapsids. The transition from host to viral transcription involves viral proteins that influence the ability of the cellular RNA polymerase II to transcribe a series of viral genes. The regulation of RNA polymerase II activity by viral gene products seems to occur by several different mechanisms: (1) viral proteins complex with cellular proteins and alter their transcription-promoting activity (e.g., alpha TIF), (2) viral proteins bind to specific DNA sequences and alter transcription (e.g., ICP4), and (3) viral proteins affect the posttranslational modification of viral or cellular transcriptional regulatory proteins (e.g., possibly ICP27). Thus, HSV may utilize several different approaches to influence the ability of host-cell RNA polymerase II to transcribe viral genes. Although it is known that viral transcription uses the host-cell polymerase II, it is not known whether viral infection causes a change in the structural elements of the nucleus that promote transcription. In contrast, HSV encodes a new DNA polymerase and accessory proteins that complex with and reorganize cellular proteins to form new structures where viral DNA replication takes place. HSV may encode a large number of DNA replication proteins, including a new polymerase, because it replicates in resting cells where these cellular gene products would never be expressed. However, it imitates the host cell in that it localizes viral DNA replication proteins to discrete compartments of the nucleus where viral DNA synthesis takes place. Furthermore, there is evidence that at least one specific viral gene protein can play a role in organizing the assembly of the DNA replication structures. Further work in this system may determine whether assembly of these structures is essential for efficient viral DNA replication and if so, why assembly of these structures is necessary. Thus, the study of the localization and assembly of HSV DNA replication proteins provides a system to examine the mechanisms involved in morphogenesis of the cell nucleus. Therefore, several critical principles are apparent from these discussions of the metabolism of HSV transcription and DNA replication. First, there are many ways in which the activity of RNA polymerase II can be regulated, and HSV proteins exploit several of these in controlling the transcription of a single DNA molecule. Second, the interplay of these multiple regulatory pathways is likely to control the progress of the lytic cycle and may play a role in determining the lytic versus latent infection decision.(ABSTRACT TRUNCATED AT 400 WORDS)

A carboxyl-terminal peptide of the DNA-binding protein ICP8 of herpes simplex virus contains a single-stranded DNA-binding site

The DNA-binding protein ICP8 of herpes simplex virus is a multifunctional protein which is required for viral replication. To identify the single-stranded DNA-binding domain of the protein, recombinant plasmids containing the 5' or 3' coding portion of the ICP8 gene or the intact gene were constructed and transcribed using SP6 RNA polymerase. The resulting RNA was translated in vitro to produce a 62,000-Da amino-terminal peptide, a 69,000-Da carboxyl-terminal peptide, or the intact protein. When these were analyzed by single-stranded DNA-cellulose column chromatography, large amounts of the intact ICP8 bound to the columns while small amounts of the carboxyl-terminal peptide and undetectable amounts of the amino-terminal peptide bound. The majority of the carboxyl-terminal peptide which bound eluted from the columns with the same salt concentration as the intact ICP8. The in vitro synthesized intact protein had the same affinity for single-stranded DNA-cellulose as ICP8 purified from infected cells. These results suggest that the carboxyl-terminal portion of ICP8 contains a single-stranded DNA-binding site.

Analysis of the transcript of the herpes simplex virus DNA polymerase gene provides evidence that polymerase expression is inefficient at the level of translation

We have mapped the termini and determined the relative abundance and ribosome density of the major cytoplasmic transcript of the DNA polymerase (pol) gene of herpes simplex virus type 1. Nuclease protection and primer extension analyses located the 5' end of the major pol transcript at two closely spaced sites 51 and 57 nucleotides to the left of a BamHI site at map position 0.413. S1-sensitive sites corresponding to additional minor transcripts were found to map further upstream within a palindromic sequence that contains a viral replication origin. The major 3' end was found to map 90 nucleotides upstream of a KpnI site at map position 0.439. Quantitative S1 nuclease assays revealed that pol transcripts were nearly as abundant as transcripts encoded by the viral thymidine kinase gene. However, relatively few pol transcripts were found on large polysomes at 5.5 h after infection, when pol transcripts were most abundant. This was in marked contrast to the polyribosome distribution of transcripts from the thymidine kinase gene and the major DNA-binding protein gene. These results and sequence features of the pol transcript suggest that pol expression is regulated, in part, at the level of translation.

Genetic identification of a portion of the herpes simplex virus ICP8 protein required for DNA-binding

The major DNA-binding protein or infected cell protein 8 (ICP8) encoded by herpes simplex virus exhibits multiple interactions with the cell nucleus in that it interacts with the host cell nuclear matrix and viral DNA molecules as sequential stages in its maturational process (M. P. Quinlan, L. B. Chen, and D. M. Knipe (1984), Cell 36, 857-868). To define the portion(s) of ICP8 required for DNA binding, we have fine-mapped and identified the sequence changes in mutant genes causing changes in the protein that affect DNA binding. These mutations lead to amino acid changes between residues 348 and 450 of ICP8. Construction of a mutant ICP8 gene specifically altered at residues 499 and 502 led to a gene product that was also defective in a nuclear function. Thus, at least part of the region of ICP8 from residues 348 to 450 is required for DNA binding by ICP8. This portion of the protein may be involved in binding to DNA or forming intermolecular contacts needed for cooperative DNA binding. If this region is directly involved in binding of the protein to DNA, the most likely structure predicted for this region involves folding of beta-strands to form a channel for binding to a nucleotide chain.

Alternative mechanisms for activation of human immunodeficiency virus enhancer in T cells

The expression of human immunodeficiency virus (HIV) after T cell activation is regulated by NF-kappa B, an inducible DNA-binding protein that stimulates transcription. Proteins encoded by a variety of DNA viruses are also able to activate expression from the HIV enhancer. To determine how this activation occurs, specific genes from herpes simplex virus type 1 and adenovirus that activate HIV in T lymphoma cells have been identified. The cis-acting regulatory sequences in the HIV enhancer that mediate their effect have also been characterized. The relevant genes are those for ICP0-an immediate-early product of herpes simplex virus type 1-and the form of E1A encoded by the 13S messenger RNA of adenovirus. Activation of HIV by adenovirus E1A was found to depend on the TATA box, whereas herpesvirus ICP0 did not work through a single defined cis-acting element. These findings suggest multiple pathways that can be used to bypass normal cellular activation of HIV, and they raise the possibility that infection by herpes simplex virus or adenovirus may directly contribute to the activation of HIV in acquired immunodeficiency syndrome by mechanisms independent of antigenic stimulation in T cells.

Herpes simplex virus type 1 oriL is not required for virus replication or for the establishment and reactivation of latent infection in mice

During the course of experiments designed to isolate deletion mutants of herpes simplex virus type 1 in the gene encoding the major DNA-binding protein, ICP8, a mutant, d61, that grew efficiently in ICP8-expressing Vero cells but not in normal Vero cells was isolated (P. K. Orberg and P. A. Schaffer, J. Virol. 61:1136-1146, 1987). d61 was derived by cotransfection of ICP8-expressing Vero cells with infectious wild-type viral DNA and a plasmid, pDX, that contains an engineered 780-base-pair (bp) deletion in the ICP8 gene, as well as a spontaneous approximately 55-bp deletion in oriL. Gel electrophoresis and Southern blot analysis indicated that d61 DNA carried both deletions present in pDX. The ability of d61 to replicate despite the deletion in oriL suggested that a functional oriL is not essential for virus replication in vitro. Because d61 harbored two mutations, a second mutant, ts+7, with a deletion in oriL-associated sequences and an intact ICP8 gene was constructed. Both d61 and ts+7 replicated efficiently in their respective permissive host cells, although their yields were slightly lower than those of control viruses with intact oriL sequences. An in vitro test of origin function of isolated oriL sequences from wild-type virus and ts+7 showed that wild-type oriL, but not ts+7 oriL, was functional upon infection with helper virus. In an effort to determine the requirement for oriL in latency, ts+7 was compared with wild-type virus for its ability to establish, maintain, and be reactivated from latent infection in a murine eye model. The mutant was reactivated as efficiently as was wild-type virus from trigeminal ganglia after cocultivation with permissive cells, and each of the seven reactivated isolates was shown to carry the approximately 150-bp deletion characteristic of ts+7. These observations demonstrate that oriL is not required for virus replication in vitro or for the establishment and reactivation of latent infection in vivo.