EHV-1 EICP22 protein sequences that mediate its physical interaction with the immediate-early protein are not sufficient to enhance the trans-activation activity of the IE protein

Unmasking the ongoing challenge of equid herpesvirus- 1 (EHV-1): A comprehensive review

Equid herpesviruses (EHVs) are a group of highly impactful viral pathogens that affect horses, presenting a substantial risk to the global equine industry. Among these, equid herpesvirus-1 (EHV-1) primarily causes respiratory infections. However, its ability to spread to distant organs can lead to severe consequences such as abortion and neurological diseases. These viruses can enter a dormant phase, with minimal activity, and later reactivate to trigger active infections at any time. Recently, there has been a notable rise in the prevalence of a particularly devastating strains of EHV-1 known as equid herpesviral myeloencephalopathy (EHM). In the light of dynamic nature of EHV-1, this review provides a thorough overview of EHV-1 and explores how advances in viral biology affect the pathophysiology of viral infection. The information presented here is crucial for understanding the dynamics of EHV-1 infections and creating practical plans to stop the virus's global spread among equid populations.

Hybrid sequencing discloses unique aspects of the transcriptomic architecture in equid alphaherpesvirus 1

This study employed both short-read sequencing (SRS, Illumina) and long-read sequencing (LRS Oxford Nanopore Technologies) platforms to conduct a comprehensive analysis of the equid alphaherpesvirus 1 (EHV-1) transcriptome. The study involved the annotation of canonical mRNAs and their transcript variants, encompassing transcription start site (TSS) and transcription end site (TES) isoforms, in addition to alternative splicing forms. Furthermore, the study revealed the presence of numerous non-coding RNA (ncRNA) molecules, including intergenic and antisense transcripts, produced by EHV-1. An intriguing finding was the abundant production of chimeric transcripts, some of which potentially encode fusion polypeptides. Moreover, EHV-1 exhibited a greater incidence of transcriptional overlaps and splicing compared to related viruses. It is noteworthy that many genes have their unique TESs along with the co-terminal transcription ends, a characteristic scarcely seen in other alphaherpesviruses. The study also identified transcripts that overlap the replication origins of the virus. Moreover, a novel ncRNA, referred to as NOIR, was found to intersect with the 5'-ends of longer transcript isoform specified by the major transactivator genes ORF64 and ORF65, surrounding the OriL. These findings together imply the existence of a key regulatory mechanism that governs both transcription and replication through, among others, a process that involves interference between the DNA and RNA synthesis machineries.

EHV-1: A Constant Threat to the Horse Industry

Equine herpesvirus-1 (EHV-1) is one of the most important and prevalent viral pathogens of horses and a major threat to the equine industry throughout most of the world. EHV-1 primarily causes respiratory disease but viral spread to distant organs enables the development of more severe sequelae; abortion and neurologic disease. The virus can also undergo latency during which viral genes are minimally expressed, and reactivate to produce lytic infection at any time. Recently, there has been a trend of increasing numbers of outbreaks of a devastating form of EHV-1, equine herpesviral myeloencephalopathy. This review presents detailed information on EHV-1, from the discovery of the virus to latest developments on treatment and control of the diseases it causes. We also provide updates on recent EHV-1 research with particular emphasis on viral biology which enables pathogenesis in the natural host. The information presented herein will be useful in understanding EHV-1 and formulating policies that would help limit the spread of EHV-1 within horse populations.

Pathogenicity and Whole Genome Sequence Analysis of a Pseudorabies Virus Strain FJ-2012 Isolated from Fujian, Southern China

The outbreaks of pseudorabies have been frequently reported in Bartha-K61-vaccinated farms in China since 2011. To study the pathogenicity and evolution of the circulating pseudorabies viruses in Fujian Province, mainland China, we isolated and sequenced the whole genome of a wild-type pseudorabies virus strain named "FJ-2012." We then conducted a few downstream bioinformatics analyses including phylogenetic analysis and pathogenic analysis and used the virus to infect 6 pseudorabies virus-free piglets. FJ-2012-infected piglets developed symptoms like high body temperature and central nervous system disorders and had high mortality rate. In addition, we identified typical micropathological changes such as multiple gross lesions in infected piglets through pathological analysis and conclude that the FJ-2012 genome is significantly different from known pseudorabies viruses, in which insertions, deletions, and substitutions are observed in multiple immune and virulence genes. In summary, this study shed lights on the molecular basis of the prevalence and pathology of the pseudorabies virus strain FJ-2012. The genome of FJ-2012 could be used as a reference to study the evolution of pseudorabies viruses, which is critical to the vaccine development of new emerging pseudorabies viruses.

Characterization of cis-acting elements required for autorepression of the equine herpesvirus 1 IE gene

The immediate-early protein (IEP), the major regulatory protein encoded by the IE gene of equine herpesvirus 1 (EHV-1), plays a crucial role as both transcription activator and repressor during a productive lytic infection. To investigate the mechanism by which the EHV-1 IEP inhibits its own promoter, IE promoter-luciferase reporter plasmids containing wild-type and mutant IEP-binding site (IEBS) were constructed and used for luciferase reporter assays. The IEP inhibited transcription from its own promoter in the presence of a consensus IEBS (5'-ATCGT-3') located near the transcription initiation site but did not inhibit when the consensus sequence was deleted. To determine whether the distance between the TATA box and the IEBS affects transcriptional repression, the IEBS was displaced from the original site by the insertion of synthetic DNA sequences. Luciferase reporter assays revealed that the IEP is able to repress its own promoter when the IEBS is located within 26-bp from the TATA box. We also found that the proper orientation and position of the IEBS were required for the repression by the IEP. Interestingly, the level of repression was significantly reduced when a consensus TATA sequence was deleted from the promoter region, indicating that the IEP efficiently inhibits its own promoter in a TATA box-dependent manner. Taken together, these results suggest that the EHV-1 IEP delicately modulates autoregulation of its gene through the consensus IEBS that is near the transcription initiation site and the TATA box.

A wide extent of inter-strain diversity in virulent and vaccine strains of alphaherpesviruses

Alphaherpesviruses are widespread in the human population, and include herpes simplex virus 1 (HSV-1) and 2, and varicella zoster virus (VZV). These viral pathogens cause epithelial lesions, and then infect the nervous system to cause lifelong latency, reactivation, and spread. A related veterinary herpesvirus, pseudorabies (PRV), causes similar disease in livestock that result in significant economic losses. Vaccines developed for VZV and PRV serve as useful models for the development of an HSV-1 vaccine. We present full genome sequence comparisons of the PRV vaccine strain Bartha, and two virulent PRV isolates, Kaplan and Becker. These genome sequences were determined by high-throughput sequencing and assembly, and present new insights into the attenuation of a mammalian alphaherpesvirus vaccine strain. We find many previously unknown coding differences between PRV Bartha and the virulent strains, including changes to the fusion proteins gH and gB, and over forty other viral proteins. Inter-strain variation in PRV protein sequences is much closer to levels previously observed for HSV-1 than for the highly stable VZV proteome. Almost 20% of the PRV genome contains tandem short sequence repeats (SSRs), a class of nucleic acids motifs whose length-variation has been associated with changes in DNA binding site efficiency, transcriptional regulation, and protein interactions. We find SSRs throughout the herpesvirus family, and provide the first global characterization of SSRs in viruses, both within and between strains. We find SSR length variation between different isolates of PRV and HSV-1, which may provide a new mechanism for phenotypic variation between strains. Finally, we detected a small number of polymorphic bases within each plaque-purified PRV strain, and we characterize the effect of passage and plaque-purification on these polymorphisms. These data add to growing evidence that even plaque-purified stocks of stable DNA viruses exhibit limited sequence heterogeneity, which likely seeds future strain evolution.

Alphaherpesviruses such as herpes simplex virus (HSV) are ubiquitous in the human population. HSV causes oral and genital lesions, and has co-morbidities in acquisition and spread of human immunodeficiency virus (HIV). The lack of a vaccine for HSV hinders medical progress for both of these infections. A related veterinary alphaherpesvirus, pseudorabies virus (PRV), has long served as a model for HSV vaccine development, because of their similar pathogenesis, neuronal spread, and infectious cycle. We present here the first full genome characterization of a live PRV vaccine strain, Bartha, and reveal a spectrum of unique mutations that are absent from two divergent wild-type PRV strains. These mutations can now be examined individually for their contribution to vaccine strain attenuation and for potential use in HSV vaccine development. These inter-strain comparisons also revealed an abundance of short repetitive elements in the PRV genome, a pattern which is repeated in other herpesvirus genomes and even the unrelated Mimivirus. We provide the first global characterization of repeats in viruses, comparing both their presence and their variation among different viral strains and species. Repetitive elements such as these have been shown to serve as hotspots of variation between individuals or strains of other organisms, generating adaptations or even disease states through changes in length of DNA-binding sites, protein folding motifs, and other structural elements. These data suggest for the first time that similar mechanisms could be widely distributed in viral biology as well.

The equine herpesvirus-1 (EHV-1) IR3 transcript downregulates expression of the IE gene and the absence of IR3 gene expression alters EHV-1 biological properties and virulence

The IR3 transcript of equine herpesvirus-1 (EHV-1) harbors 117 nts antisense to the immediate-early (IE) mRNA, suggesting it plays a regulatory role. Here, we show that the IR3 transcript downregulates IE gene expression and that the absence of IR3 expression altered EHV-1 biological properties and virulence in mice. Reporter assays revealed that the IR3/IE overlapping sequences [IR3(+226/+342)] and an additional IR3(+343/+433) region are necessary for the IR3 RNA to downregulate IE expression. Experiments with the DeltaIR3 EHV-1 showed that the IR3 gene is dispensable for EHV-1 replication. Protein expression of the IE and representative EHV-1 genes was increased in cells infected with DeltaIR3 EHV-1 as compared to that of cells infected with wt EHV-1. The DeltaIR3 EHV-1 exhibited increased virulence in mice as compared to the parent virus. The finding that the IR3 transcript affects IE gene expression extends the role of RNA as a regulatory molecule in alphaherpesvirus infection.

The IR4 auxiliary regulatory protein expands the in vitro host range of equine herpesvirus 1 and is essential for pathogenesis in the murine model

IR4, an early regulatory protein of equine herpesvirus 1 (EHV-1), is not a DNA-binding protein, but interacts with the sole immediate-early protein (IEP) to increase both IEP site-specific DNA-binding and IEP-mediated trans-activation of EHV-1 promoters. To investigate the biological properties of IR4 and ascertain whether this regulatory protein is essential for virus growth, bacterial artificial chromosome methods were employed to generate an IR4-null EHV-1. The IR4 gene was dispensable for EHV-1 growth in non-immortalized equine NBL-6 cells, but virus replication was delayed and was reduced by greater than 10-fold. In addition, replication of the IR4 mutant was abrogated in all other cell types tested, including equine ETCC tumor cells and cells of mouse, rabbit, monkey, and human origin. Further, in contrast to the highly pathogenic parent virus, the IR4 deletion mutant failed to cause disease in the CBA mouse as judged by assessing body weight and clinical signs and was unable to replicate in the murine lung. To define the nature of the block in the replication of the IR4-null virus, molecular analyses were carried out in RK-13 rabbits' cells infected with the IR4-deleted virus and revealed that: 1) the synthesis of the sole IEP was not inhibited; 2) the synthesis of early viral proteins examined was either not affected or was delayed to late times; 3) viral DNA replication was inhibited by more than 99.9%; and 4) synthesis of essential late proteins such as glycoprotein D and glycoprotein K was prevented. These findings indicate that the IR4 protein is required for EHV-1 DNA replication in non-permissive cells, and, like its homologues in other alphaherpesviruses, contributes a function required for virus replication in a variety of cell types.

Biological and genotypic properties of defective interfering particles of equine herpesvirus 1 that mediate persistent infection

Infection with equine herpesvirus 1 (EHV-1) preparations enriched for defective interfering particles (DIP) leads to a state of persistent infection in which infected cells become lysis resistant and release both infectious (standard) virus and DIP. EHV-1 DIP are unique in that the recombination events that generate DIP genomes produce new open reading frames (ORFs; Hyb1.0 and Hyb2.0) consisting of 5' sequences of varying lengths of the early regulatory gene IR4 fused to 3' sequences of varying lengths of the UL5 regulatory gene. Only two additional ORFs (UL3 and UL4) are conserved. Because persistently infected cells release a heterogeneous mixture of DIP, characterization of the elements responsible for this altered state of infection has proved difficult. Here we describe a method for studying persistent infection using recombinant DIP (rDIP). Infection with rDIP resulted in the production of recombinant DIP that replicated faithfully to, at least, five passages and mediated a rapid progression to persistent infection as measured by: 1) production of cells resistant to lysis by the standard virus; and 2) infected cells that released both standard virus and DIP. High concentrations of rDIP also resulted in interference with the standard virus replication, another hallmark of persistent infection. rDIP deleted of UL3, UL4, and either Hyb gene, the only functional genes conserved in the DIP genome, replicated but exhibited markedly reduced ability to interfere with standard virus replication. Restoring only the Hyb genes (either Hyb1.0 or Hyb2.0), the IR4 gene, or specific portions of the IR4 gene restored interference. These data suggest that residues 144 to 196 of the IR4 protein within the HYB proteins are important for DIP interference and that persistent infection results from recombination events that produce DIP genomes.

The equine herpesvirus-1 IR3 gene that lies antisense to the sole immediate-early (IE) gene is trans-activated by the IE protein, and is poorly expressed to a protein

The unique IR3 gene of equine herpesvirus 1 (EHV-1) is expressed as a late 1.0-kb transcript. Previous studies confirmed the IR3 transcription initiation site and tentatively identified other cis-acting elements specific to IR3 such as a TATA box, a 443 base pair 5'untranslated region (UTR), a 285 base pair open reading frame (ORF), and a poly adenylation (A) signal [Holden, V.R., Harty, R.N., Yalamanchili, R.R., O'Callaghan, D.J., 1992. The IR3 gene of equine herpesvirus type 1: a unique gene regulated by sequences within the intron of the immediate-early gene. DNA Seq. 3, 143-152]. Transient transfection assays revealed that the IR3 promoter is strongly trans-activated by the IE protein (IEP) and that coexpression of the IEP with the early EICP0 and IR4 regulatory proteins results in maximal trans-activation of the IR3 promoter. Gel shift assays revealed that the IEP directly binds to the IR3 promoter region. Western blot analysis showed that the IR3 protein produced in E. coli was detected by antibodies to IR3 synthetic peptides; however, the IR3 protein was not detected in EHV-1 infected cell extracts by these same anti-IR3 antibodies, even though the IR3 transcript was detected by northern blot. These findings suggest that the IR3 may not be expressed to a protein. Expression of an IR3/GFP fusion gene was not observed, but expression of a GFP/IR3 fusion gene was detected by fluorescent microscopy. In further attempts to detect the IR3/GFP fusion protein using anti-GFP antibody, western blot analysis showed that the IR3/GFP fusion protein was not detected in vivo. Interestingly, a truncated form of the GFP/IR3 protein was synthesized from the GFP/IR3 fusion gene. However, GFP/IR3 and IR3/GFP fusion proteins of the predicted sizes were synthesized by in vitro coupled transcription and translation of the fusion genes, suggesting poor expression of the IR3 protein in vivo. The possible role of the IR3 transcript in EHV-1 infection is discussed.

The unique IR2 protein of equine herpesvirus 1 negatively regulates viral gene expression

The IR2 protein (IR2P) is a truncated form of the immediate-early protein (IEP) lacking the essential acidic transcriptional activation domain (TAD) and serine-rich tract and yet retaining binding domains for DNA and TFIIB and nuclear localization signal (NLS). Analysis of the IR2 promoter indicated that the IR2 promoter was upregulated by the EICP0P. The IR2P was first detected in the nucleus at 5 h postinfection in equine herpesvirus 1 (EHV-1)-infected HeLa and equine NBL6 cells. Transient-transfection assays revealed that (i) the IR2P by itself downregulated EHV-1 early promoters (EICP0, TK, EICP22, and EICP27) in a dose-dependent manner; (ii) the IR2P abrogated the IEP and the EICP27P (UL5) mediated transactivation of viral promoters in a dose-dependent manner; and (iii) the IR2P, like the IEP itself, also downregulated the IE promoter, indicating that the IEP TAD is not necessary to downregulate the IE promoter. In vitro interaction assays revealed that the IR2P interacts with TATA box-binding protein (TBP). The essential domain(s) of the IR2P that mediate negative regulation were mapped to amino acid residues 1 to 706, indicating that the DNA-binding domain and the NLS of the IR2P may be important for the downregulation. In transient-transfection and virus growth assays, the IR2P reduced EHV-1 production by 23-fold compared to virus titers achieved in cells transfected with the empty vector. Overall, these studies suggest that the IR2P downregulates viral gene expression by acting as a dominant-negative protein that blocks IEP-binding to viral promoters and/or squelching the limited supplies of TFIIB and TBP.

Genetic Complexity of EHV-1 Defective Interfering Particles and Identification of Novel IR4/UL5 Hybrid Proteins Produced During Persistent Infection

This study examined the genetic complexity of three equine herpesvirus 1 (EHV-1) defective interfering particles (DIP) and found the DIP genomes to range from 5.9 kbp to 7.3 kbp in total size. Each DIP contains an identical 5' end ( approximately 1.9 kb) that harbors UL3 and UL4 genes that are 100% identical to those of the infectious virus. DIP2 and DIP3 contain a previously described unique IR4/UL5 (EICP22/EICP27) hybrid gene (Hyb1.0). The DIP1 genome, however, appears to be generated from a different recombination event which results in the formation of a new distinct hybrid ORF. The new ORF (Hyb2.0) is comprised of 684 bp from the 5' end of IR4 fused to 45 bp from the 3' terminus of UL5. In contrast to Hyb1.0, the UL5 sequences present in Hyb2.0 are not in-frame. Thus, the Hyb2.0 protein is comprised of 228 residues from IR4 linked to a sequence of 15 amino acids that result from a frameshifted reading of UL5 sequences. Western blot analysis confirmed that the Hyb2.0 ORF is expressed during persistent infection to produce a family of proteins that migrate at 36-42 kDa. Fluorescence microscopy revealed that both Hyb proteins display diffuse cytoplasmic localization patterns dissimilar to the nuclear localization patterns of both IR4 and UL5. Neither Hyb protein, however, disrupts the nuclear entry of the EHV-1 immediate-early, IR4, or UL5 proteins or cellular TATA box binding protein (TBP) previously shown to interact with both IR4 or UL5 in productive infection. DIP genomic segments ( approximately 3.5-5.0 kbp) downstream of the 100% conserved origin of replication are highly variable among the three DIP genomes and contain large areas of repetitive sequences. The possibility that the non-coding sequences play a role in viral interference and/or persistent infection remains to be determined.

Host cell targets of immediate-early protein BICP22 of bovine herpesvirus 1

The immediate-early (IE) protein BICP22 of bovine herpesvirus 1 (BHV-1) acts as transrepressor protein on viral promoters of different kinetic classes. In the present work, we looked for host cell targets of BICP22 using a yeast two-hybrid system and identified seven candidates: (1) JIK, a serine/threonine kinase of the sterile 20 protein (STE20) family that inhibits stress-related pathways; (2) cAMP response element binding protein-like 2 (CREBL2), which in its bZip domain shares homology with CREB, modulating transcription of cAMP responsive genes; (3) DNA-dependent ATPase and helicase (ATRX), a protein of the SNF2 family altering nucleosome structure; (4) scaffold attachment factor B (SAF-B), which helps to organize chromatin into topologically separated loops; (5) peptidylglycine alpha-amidating monooxygenase COOH-terminal interactor protein 1 (PAMCIP1), involved in regulation of the secretory pathway in the perinuclear area; (6) zinc finger protein (ZNF38) found in proliferating cells and possibly associated with meiosis in male and female gametogenesis; (7) FLJ22709, hypothetical protein conserved among various species, containing an occludin/ELL domain. To confirm some of the interactions by confocal fluorescence microscopy, BICP22 was tagged with red fluorescent protein in an amplicon, and selected target sequences were tagged with green fluorescent protein in plasmid expression vectors. Upon amplicon transduction of Vero cells and plasmid transfection, CREBL2 and ZNF38 both colocalized with BICP22 in distinct nuclear domains.

A negative regulatory element (base pairs -204 to -177) of the EICP0 promoter of equine herpesvirus 1 abolishes the EICP0 protein's trans-activation of its own promoter

The early EICP0 protein is a powerful trans-activator that activates all classes of equine herpesvirus 1 (EHV-1) promoters but, unexpectedly, trans-activates its own promoter very weakly. Transient transfection assays that employed constructs harboring deletions within the EICP0 promoter indicated that EICP0 cis-acting sequences within bp -224 to -158 relative to the first ATG abolished the EICP0 protein's trans-activation of its own promoter. When inserted into the promoters of other EHV-1 genes, this sequence also downregulated activation of the immediate-early IE(-169/+73), early thymidine kinase TK(-215/+97), and late glycoprotein K gK(-83/+14) promoters, indicating that the cis-acting sequence (-224 to -158) downregulated expression of representative promoters of all classes of EHV-1 genes and contains a negative regulatory element (NRE). To define the cis-acting element(s), three synthetic oligonucleotides (Na [bp -224 to -195], Nb [bp -204 to -177], and Nc [bp -185 to -156]) were synthesized and cloned upstream of the EICP0(-157/-21) promoter. Of the three synthetic sequences, only the Nb oligonucleotide caused the downregulation of the EICP0 promoter. The NRE was identified as a 28-bp element to lie at -204 to -177 that encompassed the sequence of ([-204]AGATACAGATGTTCGATAAATTGGAACC[-177]). Gel shift assays performed with mouse L-M, rabbit RK-13, and human HeLa cell nuclear extracts and gamma-(32)P-labeled wild-type and mutant NREs demonstrated that a ubiquitous nuclear protein(s) (NRE-binding protein, NREBP) binds specifically to a sequence (bp -193 to -183) in the NRE. The NREBP is also present in the nucleus of EHV-1-infected cells; however, the amount of NREBP in EHV-1-infected L-M cells that bound to the Nb oligonucleotide was reduced compared to that in uninfected L-M cells. Transient transfection assays showed that deletions or mutations within the NREBP-binding site abolished the NRE activity of the EICP0 promoter. These results suggested that the NREBP may mediate the NRE activity of the EICP0 promoter and may function in the coordinate expression of EHV-1 genes.

The immediate-early 63 protein of Varicella-Zoster virus: analysis of functional domains required for replication in vitro and for T-cell and skin tropism in the SCIDhu model in vivo

The immediate-early 63-kDa (IE63) protein of varicella-zoster virus (VZV) is a phosphoprotein encoded by open reading frame (ORF) ORF63/ORF70. To identify functional domains, 22 ORF63 mutations were evaluated for effects on IE63 binding to the major VZV transactivator, IE62, and on IE63 phosphorylation and nuclear localization in transient transfections, and after insertion into the viral genome with VZV cosmids. The IE62 binding site was mapped to IE63 amino acids 55 to 67, with R59/L60 being critical residues. Alanine substitutions within the IE63 center region showed that S165, S173, and S185 were phosphorylated by cellular kinases. Four mutations that changed two putative nuclear localization signal (NLS) sequences altered IE63 distribution to a cytoplasmic/nuclear pattern. Only three of 22 mutations in ORF63 were compatible with recovery of infectious VZV from our cosmids, but infectivity was restored by inserting intact ORF63 into each mutated cosmid. The viable IE63 mutants had a single alanine substitution, altering T171, S181, or S185. These mutants, rOKA/ORF63rev[T171], rOKA/ORF63rev[S181], and rOKA/ORF63rev[S185], produced less infectious virus and had a decreased plaque phenotype in vitro. ORF47 kinase protein and glycoprotein E (gE) synthesis was reduced, indicating that IE63 contributed to optimal expression of early and late gene products. The three IE63 mutants replicated in skin xenografts in the SCIDhu mouse model, but virulence was markedly attenuated. In contrast, infectivity in T-cell xenografts was not altered. Comparative analysis suggested that IE63 resembled the herpes simplex virus type 1 U(S)1.5 protein, which is expressed colinearly with ICP22 (U(S)1). In summary, most mutations of ORF63 made with our VZV cosmid system were lethal for infectivity. The few IE63 changes that were tolerated resulted in VZV mutants with an impaired capacity to replicate in vitro. However, the IE63 mutants were attenuated in skin but not T cells in vivo, indicating that the contribution of the IE63 tegument/regulatory protein to VZV pathogenesis depends upon the differentiated human cell type which is targeted for infection within the intact tissue microenvironment.

Direct interaction of TFIIB and the IE protein of equine herpesvirus 1 is required for maximal trans-activation function

Recently, we reported that the immediate-early (IE) protein of equine herpesvirus 1 (EHV-1) associates with transcription factor TFIIB [J. Virol. 75 (2001), 10219]. In the current study, the IE protein purified as a glutathione-S-transferase (GST) fusion protein was shown to interact directly with purified TFIIB in GST-pulldown assays. A panel of TFIIB mutants employed in protein-binding assays revealed that residues 125 to 174 within the first direct repeat of TFIIB mediate its interaction with the IE protein. This interaction is physiologically relevant as transient transfection assays demonstrated that (1). exogenous native TFIIB did not perturb IE protein function, and (2). ectopic expression of a TFIIB mutant that lacked the IE protein interactive domain significantly diminished the ability of the IE protein to trans-activate EHV-1 promoters. These results suggest that an interaction of the IE protein with TFIIB is an important aspect of the regulatory role of the IE protein in the trans-activation of EHV-1 promoters.

Generation and characterization of an EICP0 null mutant of equine herpesvirus 1

The EICP0 gene (gene 63) of equine herpesvirus 1 (EHV-1) encodes an early regulatory protein that is a promiscuous trans-activator of all classes of viral genes. Bacterial artificial chromosome (BAC) technology and RecE/T cloning were employed to delete the EICP0 gene from EHV-1 strain KyA. Polymerase chain reaction, Southern blot analysis, and DNA sequencing confirmed the deletion of the EICP0 gene and its replacement with a kanamycin resistance gene in mutant KyA. Transfection of rabbit kidney cells with the EICP0 mutant genome produced infectious virus, indicating that the EICP0 gene is not essential for KyA replication in cell culture. Experiments to assess the effect of the EICP0 deletion on EHV-1 gene programming revealed that mRNA expression of the immediate-early gene and representative early and late genes as well as the synthesis of these viral proteins were reduced as compared to the kinetics of viral mRNA and protein synthesis observed for the wild type virus. However, the transition from early to late viral gene expression was not prevented or delayed, suggesting that the absence of the EICP0 gene did not disrupt the temporal aspects of EHV-1 gene regulation. The extracellular virus titer and plaque areas of the EICP0 mutant virus KyADeltaEICP0, in which the gp2-encoding gene 71 gene that is absent in the KyA BAC was restored, were reduced by 10-fold and 19%, respectively, when compared to parental KyA virus; while the titer and plaque areas of mutant KyADeltaEICP0Deltagp2 that lacks both the EICP0 gene and gene 71 were reduced more than 50-fold and 67%, respectively. The above results show that the EICP0 gene is dispensable for EHV-1 replication in cell culture, and that the switch from early to late viral gene expression for the representative genes examined does not require the EICP0 protein, but that the EICP0 protein may be structurally required for virus egress and cell-to-cell spread.

Interaction of the equine herpesvirus 1 EICP0 protein with the immediate-early (IE) protein, TFIIB, and TBP may mediate the antagonism between the IE and EICP0 proteins

The equine herpesvirus 1 (EHV-1) immediate-early (IE) and EICP0 proteins are potent trans-activators of EHV-1 promoters; however, in transient-transfection assays, the IE protein inhibits the trans-activation function of the EICP0 protein. Assays with IE mutant proteins revealed that its DNA-binding domain, TFIIB-binding domain, and nuclear localization signal may be important for the antagonism between the IE and EICP0 proteins. In vitro interaction assays with the purified IE and EICP0 proteins indicated that these proteins interact directly. At late times postinfection, the IE and EICP0 proteins colocalized in the nuclei of infected equine cells. Transient-transfection assays showed that the EICP0 protein trans-activated EHV-1 promoters harboring only a minimal promoter region (TATA box and cap site), suggesting that the EICP0 protein trans-activates EHV-1 promoters by interactions with general transcription factor(s). In vitro interaction assays revealed that the EICP0 protein interacted directly with the basal transcription factors TFIIB and TBP and that the EICP0 protein (amino acids [aa] 143 to 278) mediated the interaction with aa 125 to 174 of TFIIB. Our unpublished data showed that the IE protein interacts with the same domain (aa 125 to 174) of TFIIB and with TBP. Taken together, these results suggested that interaction of the EICP0 protein with the IE protein, TFIIB, and TBP may mediate the antagonism between the IE and EICP0 proteins.