Characterization of equine herpesvirus type 1 immediate early proteins

Similar regulation of two distinct UL24 promoters by regulatory proteins of equine herpesvirus type 1 (EHV-1)

To characterise the pattern of the transcriptional regulation of equine herpesvirus type 1 (EHV-1) UL24 by regulatory proteins, we identified two distinct promoter regions and two transcription initiation (Tci) sites located upstream of the UL24 open reading frame (ORF). The ORF proximal promoter exhibited higher cis-activity than that of the distal one. Contrary to the former, the latter performed its function dependent on an initiator (INR) due to its lack of a TATA box. Our results showed that the EHV-1 regulatory proteins EICP0, EICP22 and ETIF trans-activated the two promoters, whereas IEP and IR2P displayed negative regulation. In summary, the regulatory proteins exhibited similar regulatory patterns for the two distinct promoters of EHV-1 UL24.

A comparison of herpes simplex virus type 1 and varicella-zoster virus latency and reactivation

Herpes simplex virus type 1 (HSV-1; human herpesvirus 1) and varicella-zoster virus (VZV; human herpesvirus 3) are human neurotropic alphaherpesviruses that cause lifelong infections in ganglia. Following primary infection and establishment of latency, HSV-1 reactivation typically results in herpes labialis (cold sores), but can occur frequently elsewhere on the body at the site of primary infection (e.g. whitlow), particularly at the genitals. Rarely, HSV-1 reactivation can cause encephalitis; however, a third of the cases of HSV-1 encephalitis are associated with HSV-1 primary infection. Primary VZV infection causes varicella (chickenpox) following which latent virus may reactivate decades later to produce herpes zoster (shingles), as well as an increasingly recognized number of subacute, acute and chronic neurological conditions. Following primary infection, both viruses establish a latent infection in neuronal cells in human peripheral ganglia. However, the detailed mechanisms of viral latency and reactivation have yet to be unravelled. In both cases latent viral DNA exists in an 'end-less' state where the ends of the virus genome are joined to form structures consistent with unit length episomes and concatemers, from which viral gene transcription is restricted. In latently infected ganglia, the most abundantly detected HSV-1 RNAs are the spliced products originating from the primary latency associated transcript (LAT). This primary LAT is an 8.3 kb unstable transcript from which two stable (1.5 and 2.0 kb) introns are spliced. Transcripts mapping to 12 VZV genes have been detected in human ganglia removed at autopsy; however, it is difficult to ascribe these as transcripts present during latent infection as early-stage virus reactivation may have transpired in the post-mortem time period in the ganglia. Nonetheless, low-level transcription of VZV ORF63 has been repeatedly detected in multiple ganglia removed as close to death as possible. There is increasing evidence that HSV-1 and VZV latency is epigenetically regulated. In vitro models that permit pathway analysis and identification of both epigenetic modulations and global transcriptional mechanisms of HSV-1 and VZV latency hold much promise for our future understanding in this complex area. This review summarizes the molecular biology of HSV-1 and VZV latency and reactivation, and also presents future directions for study.

The EHV-1 UL4 protein that tempers viral gene expression interacts with cellular transcription factors

The UL4 gene is conserved within the genome of defective interfering particles of equine herpesvirus type 1 (EHV-1) that mediate persistent infection. Here, we show that the UL4 protein inhibits EHV-1 reporter gene expression by decreasing the level of transcribed mRNA. The UL4 protein did not bind any gene class of EHV-1 promoters in electromobility or chromatin immunoprecipitation assays, but directly interacted with the TATA box-binding protein (TBP) and the carboxy-terminal domain of RNA polymerase II both in vitro (GST-pulldown assays) and in infected cells (coimmunoprecipitation analyses). Microarray analyses of the expression of the 78 EHV-1 genes revealed that viral late genes important for virion assembly displayed enhanced expression in cells infected with UL4-null virus as compared to wild-type or UL4-restored EHV-1. Quantitative PCR analyses showed that viral DNA replication was not retarded in cells infected with the UL4-null virus as compared to wild-type EHV-1.

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.

Initial characterization of 17 viruses harboring mutant forms of the immediate-early gene of equine herpesvirus 1

The sole immediate-early (IE) gene of equine herpesvirus 1 (EHV-1) encodes a major regulatory protein of 1487 amino acids (aa) capable of modulating gene expression from both early and late promoters and also of trans-repressing its own promoter. Using a specially designed recombination system and a library of IE linker-insertion, deletion, point, and nonsense mutant constructs that encode forms of the IE protein (IEP) harboring mutations within all five regions, 17 mutant viruses were generated and characterized. Ribonuclease protection analyses revealed that all 17 mutants synthesize the IE mRNA in RK-13 cells, whereas those that failed to replicate on non-complementing RK-13 cells displayed a defect in the transcription of either an important early gene (EICP0) and/or an essential late gene (glycoprotein D). Western blot analyses showed that the IEP was synthesized and detectable in cells infected with each mutant virus, including those mutants that failed to replicate on non-complementing RK-13 cells. Eleven of the 17 mutants were capable of growth on non-complementing RK-13 cells, whereas mutant viruses with deletions within the serine-rich tract (SRT), nucleus localization signal (NLS), or DNA-binding domain (DBD) were capable of growth only on the IEP-producing cell line (IE13.1). Lastly, temperature shift experiments revealed that mutant viruses containing deletions within the C-terminus (KyAn1029 and KyAn1411) or within the SRT (KyADeltaSRT2) of the IEP exhibited a temperature-sensitive phenotype in that these viruses, in contrast to the parent KyA, failed to replicate at 39 degrees C. Overall, these results indicate that the C-terminus of the IEP is not essential for IEP function in cell culture, but this region contains elements that enhance the function(s) of the IEP. The initial characterization of these 17 EHV-1 mutants has shown that sequences totaling at least 43% of the IEP are not essential for virus replication in cell culture.

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.

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

The early 293 amino acid EICP22 protein (EICP22P) of equine herpesvirus 1 localizes within the nucleus and functions as an accessory regulatory protein (J. Virol. 68 (1994) 4329). Transient transfection assays indicated that although the EICP22P by itself only minimally trans-activates EHV-1 promoters, the EICP22P functions synergistically with the immediate-early protein (IEP) to enhance expression of EHV-1 early genes (J. Virol. 71 (1997) 1004). We previously showed that the EICP22 protein enhances the DNA-binding activity of the EHV-1 IEP and that it also physically interacts with the IEP (J. Virol. 74 (2000) 1425). In this communication, we employed transient trans-activation assays utilizing EICP22P deletion mutants to address whether the sequences required for EICP22P-IEP physical interactions are essential for EICP22P's ability to interact synergistically with the IEP. Assays employing various classes of the EHV-1 promoters fused to the chloramphenicol acetyl-transferase (CAT) reporter gene indicated that: (1) neither full length nor any of the EICP22P mutants tested was able to overcome repression of the IE promoter elicited by the IEP, (2) the full-length EICP22P interacted synergistically with the IEP to trans-activate the early and late promoters tested, and (3) all of the EICP22P mutants, including those that were able to physically interact with IEP and itself, failed to function synergistically with the IEP to trans-activate representative EHV-1 early and late promoters. The results suggest that EICP22P sequences required for its interaction with the IE protein are not sufficient to mediate its synergistic effect on the trans-activation function of the IEP. The possible explanations as to why sequences in addition to those that mediate EICP22P-IEP interaction and EICP22P self-interactions are essential for the synergistic function of EICP22P are discussed.

Mapping the sequences that mediate interaction of the equine herpesvirus 1 immediate-early protein and human TFIIB

The sole immediate-early (IE) gene of equine herpesvirus 1 encodes a 1,487-amino-acid (aa) regulatory phosphoprotein that independently activates expression of early viral genes. Coimmunoprecipitation assays demonstrated that the IE protein physically interacts with the general transcription factor TFIIB. Using a variety of protein-binding assays that employed a panel of IE truncation and deletion mutants expressed as in vitro-synthesized or glutathione S-transferase fusion proteins, we mapped a TFIIB-binding domain to aa 407 to 757 of the IE protein. IE mutants carrying internal deletions of aa 426 to 578 and 621 to 757 were partially defective for TFIIB binding, indicating that aa 407 to 757 may harbor more than one TFIIB-binding domain. The interaction between the IE protein and TFIIB is of physiological importance, as evidenced by transient-cotransfection assays. Partial deletion of the TFIIB-binding domain within the IE protein inhibited its ability to activate expression of the viral thymidine kinase gene, a representative early promoter, and of the IR5 gene, a representative late promoter, by greater than 20 and 50%, respectively. These results indicate that the interaction of the IE protein with TFIIB is necessary for its full transactivation function and that the IE-TFIIB interaction may be part of the mechanism by which the IE protein activates transcription.

The EICP22 protein of equine herpesvirus 1 physically interacts with the immediate-early protein and with itself to form dimers and higher-order complexes

The EICP22 protein (EICP22P) of Equine herpesvirus 1 (EHV-1) is an early protein that functions synergistically with other EHV-1 regulatory proteins to transactivate the expression of early and late viral genes. We have previously identified EICP22P as an accessory regulatory protein that has the ability to enhance the transactivating properties and the sequence-specific DNA-binding activity of the EHV-1 immediate-early protein (IEP). In the present study, we identify EICP22P as a self-associating protein able to form dimers and higher-order complexes during infection. Studies with the yeast two-hybrid system also indicate that physical interactions occur between EICP22P and IEP and that EICP22P self-aggregates. Results from in vitro and in vivo coimmunoprecipitation experiments and glutathione S-transferase (GST) pull-down studies confirmed a direct protein-protein interaction between EICP22P and IEP as well as self-interactions of EICP22P. Analyses of infected cells by laser-scanning confocal microscopy with antibodies specific for IEP and EICP22P revealed that these viral regulatory proteins colocalize in the nucleus at early times postinfection and form aggregates of dense nuclear structures within the nucleoplasm. Mutational analyses with a battery of EICP22P deletion mutants in both yeast two-hybrid and GST pull-down experiments implicated amino acids between positions 124 and 143 as the critical domain mediating the EICP22P self-interactions. Additional in vitro protein-binding assays with a library of GST-EICP22P deletion mutants identified amino acids mapping within region 2 (amino acids [aa] 65 to 196) and region 3 (aa 197 to 268) of EICP22P as residues that mediate its interaction with IEP.

Characterization of the trans-activation properties of equine herpesvirus 1 EICP0 protein

The EICP0 protein of equine herpesvirus 1 (EHV-1) is an early, viral regulatory protein that independently trans-activates EHV-1 immediate-early (IE), early, gamma1 late, and gamma2 late promoters. To assess whether this powerful trans-activator functions in conjunction with three other EHV-1 regulatory proteins to activate expression of the various classes of viral promoters, transient cotransfection assays were performed in which effector plasmids expressing the EICP22, EICP27, and IE proteins were used either singly or in combination with an EICP0 effector construct. These analyses revealed that (i) independently, the EICP0 and IE proteins are powerful trans-activators but do not function synergistically, (ii) the IE protein inhibits the ability of the EICP0 protein to trans-activate the IE, gamma1 late, and gamma2 late promoters, (iii) the EICP22 and EICP0 proteins do not function together to significantly trans-activate any EHV-1 promoter, and (iv) the EICP27 and EICP0 proteins function synergistically to trans-activate the early and gamma1 late promoters. A panel of EICP0 truncation and deletion mutant plasmids was generated and used in experiments to define the domains of the 419-amino-acid (aa) EICP0 protein that are important for the trans-activation of each class of EHV-1 promoters. These studies revealed that (i) carboxy-terminal truncation mutants of the EICP0 protein exhibited a progressive loss of trans-activating ability as increasing portions of the carboxy terminus were removed, (ii) the amino terminus of the EICP0 protein containing the RING finger (aa 8 to 46) and the acidic region (aa 71 to 84) was necessary but not sufficient for activation of all classes of EHV-1 promoters, (iii) the RING finger was absolutely essential for activation of EHV-1 promoters, since deletion of the entire RING finger motif (aa 8 to 46) or a portion of it (aa 19 to 30) completely abrogated the ability of these mutants to activate any promoter tested, (iv) the acidic region contributed to the ability of the EICP0 protein to activate the early and gamma1 late promoters, and deletion of the acidic region enhanced the ability of this mutant to activate the IE promoter, (v) the carboxy terminus (aa 325 to 419), which is rich in glutamine residues, was dispensable for the EICP0 trans-activation function, (vi) a motif resembling a nuclear localization signal (aa 289 to 293) was unnecessary for the EICP0 protein to trans-activate promoters of any temporal class, and (vii) the EICP0 protein was phosphorylated during infection, and deletion of the serine-rich region (aa 210 to 217), a potential site for phosphorylation, reduced by more than 70% the ability of the EICP0 protein to activate the gamma2 late class of promoters.

Complementation of a replication-defective mutant of equine herpesvirus type 1 by a cell line expressing the immediate-early protein

Equine herpesvirus type 1 (EHV-1) possesses a sole, diploid immediate-early (IE) gene that encodes a major regulatory protein of 1487 amino acids capable of modulating expression of both early and late EHV-1 promoters and capable of trans-repressing its own promoter. In this study, a rabbit kidney cell line (IE13.1) that constitutively expresses the EHV-1 IE protein was generated by cotransfection of rabbit kidney (RK-13) cells with the viral IE gene and a neomycin resistance marker. The IE protein expressed by this cell line was shown (1) to be expressed by and to localize to the nucleus of virtually all cells as demonstrated by indirect immunofluorescence, (2) to be the full-size IE polypeptide as judged by Western immunoblot analyses with an anti-IE protein-specific antibody, and (3) to be functional as shown by the transactivation of two representative EHV-1 early promoters linked to the chloramphenicol acetyltransferase reporter gene in transient transfection assays. The IE13.1 cell line was able to complement a recombinant virus in which both copies of the IE gene were replaced by insertion of the Escherichia coli lacZ gene. This IE deletion mutant, designated KyADeltaIE, was not able to replicate in equine, rabbit, or mouse cells but was capable of replication in the IE13.1 cells that provided the IE protein in trans. Rescue of the KyADeltaIE virus was achieved by recombination with a marker plasmid that harbors the wild-type IE gene, and the rescued virus (KyADeltaIER) was able to grow on noncomplementary cells. Overall, these results offer direct evidence that the IE gene is essential for EHV-1 replication and provide reagents useful for the analysis of IE protein function.

Structural and antigenic identification of the ORF12 protein (alpha TIF) of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) homolog of the herpes simplex virus type 1 (HSV-1) tegument phosphoprotein, alpha TIF (Vmw65; VP16), was identified previously as the product of open reading frame 12 (ORF12) and shown to transactivate immediate early (IE) gene promoters. However, a specific virion protein corresponding to the ORF12 product has not been identified definitively. In the present study the ORF12 protein, designated ETIF, was identified as a 60-kDa virion component on the basis of protein fingerprint analyses in which the limited proteolysis profiles of the major 60-kDa in vitro transcription/ translation product of an ORF12 expression vector (pT7-12) were compared to those of purified virion proteins of similar size. ETIF was localized to the viral tegument in Western blot assays of EHV-1 virions and subvirion fractions using polyclonal antiserum and monoclonal antibodies generated against a glutathione-S-transferase-ETIF fusion protein. Northern and Western blot analyses of EHV-1-infected cell lysates prepared under various metabolic blocks indicated that ORF12 is expressed as a late gene, and cross reaction of polyclonal anti-GST-ETIF with a 63.5-kDa HSV-1 protein species suggested that ETIF and HSV-1 alpha TIF are antigenically related. Last, DNA band shift assays used to assess ETIF-specific complex formation indicated that ETIF participates in an infected cell protein complex with the EHV-1 IE promoter TAATGARAT motif.

Detection and intracellular localization of equine herpesvirus 1 IR1 and IR2 gene products by using monoclonal antibodies

During lytic infection, two transcripts arise from the equine herpesvirus 1 (EHV-1) immediate-early (IE) gene (IR1): a single, spliced 6.0-kb IE mRNA and a 3'-coterminal 4.4-kb early mRNA (IR2). Previous studies demonstrated that transiently expressed IR1 and IR2 gene products are potent transcriptional regulators: IR1 proteins are capable of trans activating representative EHV-1 early and late promoters, while both IR1 proteins and the IR2 product, which lacks IR1 amino acid residues 1 to 322, trans repress the IR1 promoter. In the present study, monoclonal antibodies (MAbs) against the major IE protein, IE1, were developed, characterized as to their ability to detect IR1 and IR2 products, and used to examine extracellular virions for the presence of IE1-related proteins and to define the IR1 and IR2 protein synthesis and intracellular distribution in EHV-1-infected cells. The results demonstrated that (i) anti-IE1 MAbs representing three noncompetitive epitope-binding groups reacted with multiple IE protein species, as well as with a 146-kDa early protein identified as the putative IR2 gene product; (ii) the three reactive epitopes mapped to a region spanning amino acids 323 to 552 of IR1; (iii) anti-IE1 MAbs reacted with the 144-kDa in vitro-translated IR2 product and with a transiently expressed IR2 product similar in size; (iv) small amounts of IE1 and the 146-kDa protein were associated with the nucleocapsid-tegument fraction of mature virions; (v) in immunofluorescence assays of lytically infected cells, IR1-IR2 gene products were first detectable between 1 and 2 h postinfection as discrete, punctate, intranuclear foci; (vi) as the infection progressed, the intranuclear reactivity increased and redistributed into large, intensely stained nuclear compartments which corresponded to the sites of active viral DNA synthesis; (vii) fibrillar, as well as more generalized cytoplasmic staining, first observed at about 5 h postinfection, increased throughout infection; and (viii) while viral DNA synthesis was required for the progressive intranuclear redistribution, the cytoplasmic accumulation of IR1-IR2 proteins occurred subsequent to early infection events.

Identification and characterization of the ICP22 protein of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) homolog of herpes simplex virus type 1 ICP22 is differently expressed from the fourth open reading frame of the inverted repeat (IR4) as a 1.4-kb early mRNA and a 1.7-kb late mRNA which are 3' coterminal (V. R. Holden, R. R. Yalamanchili, R. N. Harty, and D. J. O'Callaghan, J. Virol. 66:664-673, 1992). To extend the characterization of IR4 at the protein level, the synthesis and intracellular localization of the IR4 protein were investigated. Antiserum raised against either a synthetic peptide corresponding to amino acids 270 to 286 or against a TrpE-IR4 fusion protein (IR4 residues 13 to 150) was used to identify the IR4 protein. Western immunoblot analysis revealed that IR4 is expressed abundantly from an open reading frame composed of 293 codons as a family of proteins that migrate between 42 to 47 kDa. The intracellular localization of IR4 was examined by cell fractionation, indirect immunofluorescence, and laser-scanning confocal microscopy. These studies revealed that IR4 is localized predominantly in the nucleus and is dispersed uniformly throughout the nucleus. Interestingly, when IR4 is expressed transiently in COS-1 or LTK- cells, a punctate staining pattern within the nucleus is observed by indirect immunofluorescence. Cells transfected with an IR4 mutant construct that encodes a C-terminal truncated (19 amino acids) IR4 protein exhibited greatly reduced intranuclear accumulation of the IR4 protein, indicating that this domain possesses an important intranuclear localization signal. Western blot analysis of EHV-1 virion proteins revealed that IR4 proteins are structural components of the virions. Surprisingly, the 42-kDa species, which is the least abundant and the least modified form of the IR4 protein family in infected cell extracts, was the most abundant IR4 protein present in purified virions.

Characterization of the myristylated polypeptide encoded by the UL1 gene that is conserved in the genome of defective interfering particles of equine herpesvirus 1

Equine herpesvirus 1 (EHV-1, Kentucky A strain) preparations enriched for defective interfering particles (DIPs) can readily establish persistent infection. The UL1 gene, which is conserved in the genome of DIPs that mediate persistent infection, maps between nucleotides 1418 and 2192 (258 amino acids) from the L (long) terminus. UL1 has no homology with any known gene encoded by herpes simplex virus type 1 but has limited homology to open reading frame 2 of varicella-zoster virus and the "circ" gene of bovine herpesvirus type 1. Previous work showed that the EHV-1 UL1 gene belongs to the early kinetic class and is transcribed as a 1.2-kb polyadenylated mRNA (R. N. Harty, R. R. Yalamanchili, and D. J. O'Callaghan, Virology 183:830-833, 1991). In this report, the UL1 protein was identified and characterized as a 33-kDa polypeptide in EHV-1-infected cells by using rabbit polyclonal antiserum raised against a TrpE-UL1 fusion protein (amino acids 7 to 258 of UL1) synthesized in Escherichia coli. Results from Western blot (immunoblot), immunoprecipitation, indirect immunofluorescence, and biochemical analyses indicated that the UL1 polypeptide (i) is more abundant in cells infected with DIP-enriched virus than in cells infected with standard EHV-1, (ii) is synthesized as early as 3 h postinfection (p.i.) in infection with standard virus or in infection with DIP-enriched virus preparations and increases in abundance up to 12 h p.i., (iii) appears to be associated with the rough endoplasmic reticulum-Golgi apparatus early in infection (3 to 4 h p.i.), while a diffuse cytoplasmic pattern of fluorescence is observed late in infection (7 to 8 h p.i.), (iv) is modified by myristic acid as it contains a consensus N-terminal myristylation site and is readily labeled with [3H]myristic acid, and (v) is associated with mature EHV-1 virions.

The equine herpesvirus 1 (EHV-1) UL3 gene, an ICP27 homolog, is necessary for full activation of gene expression directed by an EHV-1 late promoter

We have previously reported that the equine herpesvirus 1 (EHV-1) XbaI G restriction fragment (nucleotides 1436 to 7943 relative to the left terminus of the EHV-1 genome [Kentucky A strain]) is required in combination with the EHV-1 immediate-early (IE) gene to achieve significant activation of two representative EHV-1 late promoter-chloramphenicol acetyltransferase (CAT) recombinants in transient expression assays. In this report, we demonstrate that the XbaI G-encoded UL3 gene (an ICP27 homolog) provides a trans-acting factor which acts (in combination with the EHV-1 IE gene product) to increase reporter gene expression directed by an EHV-1 late promoter-CAT recombinant plasmid. We show that cloned copies of UL3 can successfully substitute for the XbaI G fragment in CAT assays and that stop codon insertion within the UL3 open reading frame inhibits the ability of UL3 to activate reporter gene expression in trans.

Identification and transcriptional mapping of genes encoded at the IR/Us junction of equine herpesvirus type 1

Two open reading frames (ORFs) encoded at the inverted repeat unique short (Us) junction of the Short (S) region of the equine herpesvirus type 1 genome were identified by DNA sequencing of a 2876 base pair (bp) genomic segment, and transcripts encoding these ORFs were characterized by Northern blot, S1 nuclease, and primer extension analyses. These studies also established the size of each inverted repeat to be 12,768 nucleotides (nts). The IR6 ORF (816 bp), mapping at nts 12,317-11,502 of the S region, is the last gene completely encoded within each inverted repeat and encodes a predicted 30.1-kDa protein of 272 amino acids, which does not exhibit homology to other alphaherpesvirus proteins. IR6 is expressed as an early transcript of 1.2 kb which is detected initially at 1.5 hr p.i. and up to 12 hr p.i. The transcription initiation and termination sites of IR6 were mapped by primer extension and S1 nuclease analyses to nts 12,465 and 11,408, respectively. The first ORF encoded within the Us segment (909 bp; EUS1), mapping at nts 13,397-12,489, encodes a predicted 33.5-kDa protein of 303 amino acids that exhibits 29% identity to the US2 protein of herpes simplex virus 1. EUS1 is expressed as a 2.3-kb mRNA of the gamma-1 class, as its synthesis begins prior to viral DNA replication at 4 hr p.i. but is retarded by phosphonoacetic acid, an inhibitor of viral DNA replication. The Tci and Tct sites of EUS1 were mapped by S1 nuclease analyses to nts 13,637 and 11,408, respectively. Interestingly, this termination site is also utilized by three late mRNAs of 5.8, 3.8, and 1.7 kb which originate within the Us and overlap the IR6 mRNA encoded in the terminal inverted repeat (TR) of the prototype genomic isomer. EUS1 is 3' coterminal with IR6 in the inverted repeat, whereas, the 5.8, 3.8, and 1.7 kb transcripts are 3' coterminal with IR6 of the TR.

Identification and characterization of an equine herpesvirus 1 late gene encoding a potential zinc finger

In this report, we present the DNA sequence and transcriptional characterization of a gene (IR5) that maps within each of the inverted repeat (IR) segments of the equine herpesvirus type 1 (EHV-1) genome. The IR5 open reading frame (ORF) is located within both IR sequences (nucleotides 9932-10,642 of the IR). DNA sequence analyses of the IR5 gene region revealed an ORF of 236 amino acids (24,793 Da) that showed significant homology to ORF64 of varicella-zoster virus and ORF3 of EHV-4 both of which map within the inverted repeats and to the US10 ORF of herpes simplex virus type 1 (HSV-1) which maps within the unique short segment. Additional analyses of the nucleotide sequence failed to reveal any overlapping ORFs that would correspond to US11 or US12 of HSV-1. Interestingly, the IR5 ORF of EHV-1 possesses a sequence of 13 amino acids (CAYWCCLGHAFAC) that is a perfect match to the consensus zinc finger motif (C-X2-4-C-X2-15-C/H-X2-4-C/H). Putative cis-acting elements flanking the IR5 ORF include a TATA box (nucleotides 9864-9870), a CAAT box (nucleotides 9709-9714), and a polyadenylation signal (nucleotides 10,645-10,650). Northern blot and S1 nuclease analyses identified a single 0.9-kb mRNA species that first appears at 2 hr postinfection, and whose synthesis is reduced in the presence of phosphonoacetic acid, an inhibitor of EHV-1 DNA synthesis. Thus, the IR5 gene of EHV-1 exhibits characteristics representative of a late gene of the gamma-1 class. The characterization of the IR5 gene at the DNA and RNA levels will facilitate ongoing studies to identify and characterize the IR5 polypeptide.

Open reading frames encoding a protein kinase, homolog of glycoprotein gX of pseudorabies virus, and a novel glycoprotein map within the unique short segment of equine herpesvirus type 1

DNA sequence analysis of the unique short (Us) segment of the genome of equine herpesvirus type 1 Kentucky A strain (EHV-1) by our laboratory and strains Kentucky D and AB1 by other workers identifies a total of nine open reading frames (ORF). In this report, we present the DNA sequence of three of these newly identified ORFs, designated EUS 2, EUS 3, and EUS 4. The EUS 2 ORF is 1146 nucleotides (nt) in length and encodes a potential protein of 382 amino acids. Cis-regulatory sequences upstream of the putative ATG start codon include a G/C box 112 nt upstream and two potential TATA-like elements located between 15 and 90 nt before the ATG. The EUS 2 translation product exhibits significant homology to Ser/Thr protein kinases encoded within the Us segments of other herpesviruses, such as herpes simplex virus (26% homology) and pseudorabies virus (PRV), (45% homology), and possesses sequence domains conserved in protein kinases of cellular and viral origin. The EUS 3 ORF begins 127 nt downstream from the EUS 2 stop codon and ends at a stop codon 1119 nt further downstream. A single TATA-like element maps 61 nt upstream of the ORF. This ORF encodes a potential protein of 373 amino acids and is a homolog of glycoprotein gX of PRV, as judged by overall homology of amino acid residues, cysteine displacement, and presence of potential glycosylation sites and signal sequence. Interestingly, the EUS 4 ORF encodes a potential membrane glycoprotein that does not exhibit homology to any reported protein sequence. The EUS 4 ORF encodes a 383 amino acid polypeptide with a sequence indicative of a signal sequence at its amino terminal end, glycosylation sites for N-linked oligosaccharides, and a transmembrane domain near its carboxyl terminus. Several cis-acting regulatory sequences lie upstream of this ORF. These findings support the observation that the short region of alphaherpesviruses show considerable variation in their genetic content and gene organization.

ICP22 homolog of equine herpesvirus 1: expression from early and late promoters

The complete nucleotide sequence of the short region, made up of a unique segment (Us; 6.5 kb) bracketed by a pair of inverted repeat sequences (IR; 12.8 kb each), of the equine herpesvirus 1 (EHV-1) genome has been determined recently in our laboratory. Analysis of the IR segment revealed a major open reading frame (ORF) designated IR4. The IR4 ORF exhibits significant homology to the immediate-early gene US1 (ICP22) of herpes simplex virus type 1 and to the ICP22 homologs of varicella-zoster virus (ORF63), pseudorabies virus (RSp40), and equine herpesvirus 4 (ORF4). The IR4 ORF is located entirely within each of the inverted repeat sequences (nucleotides [nt] 7918 to 9327) and has the potential to encode a polypeptide of 469 amino acids (49,890 Da). Within the IR4 ORF are two reiterated sequences: a 7-nt sequence tandemly repeated 17 times and a 25-nt sequence tandemly repeated 13 times. Nucleotide sequence analyses of IR4 also revealed several potential cis-regulatory sequences, two TATA sequences separated by 287 nt, an in-frame translation initiation codon following each TATA sequence, and a single polyadenylation site. To address the nature of the mRNA species encoded by IR4, we used Northern (RNA) blot and S1 nuclease analyses. RNA mapping data revealed that IR4 has two promoters that are regulated differentially during a lytic infection. A 1.4-kb mRNA appears initially at 2 h postinfection and is an early transcript since its synthesis is not affected by the presence of phosphonoacetic acid, an inhibitor of EHV-1 DNA replication. In contrast, a 1.7-kb mRNA appears at later times postinfection and is designated as a gamma-1 transcript, since its synthesis is significantly reduced by phosphonoacetic acid. These IR4-specific mRNAs are 3' coterminal, have unique 5' termini, and would code for in-frame, overlapping, carboxy-coterminal proteins of 293 and 469 amino acids, respectively. Interestingly, the site of homologous recombination to generate the genome of EHV-1 defective interfering particles that initiate persistent infection occurs between nt 3244 and 3251 of UL3 (ICP27 homolog) and nt 9027 and 9034 of IR4 (ICP22 homolog). Thus, this recombination event would generate a unique ORF that would encode a potential protein whose amino end was derived from the N-terminal 193 amino acids of the ICP22 homolog and whose carboxyl end was derived from the C-terminal 68 amino acids of the ICP27 homolog.

Characterization of the regulatory functions of the equine herpesvirus 1 immediate-early gene product

Use of the translation-inhibiting drug cycloheximide has indicated that the equine herpesvirus 1 (EHV-1) immediate-early (IE) gene, the sole EHV-1 IE gene, encodes a major viral regulatory protein since IE mRNA translation is a prerequisite for all further viral gene expression (W.L. Gray, R. P. Baumann, A. T. Robertson, G. B. Caughman, D. J. O'Callaghan, and J. Staczek, Virology 158:79-87, 1987). An EHV-1 IE gene expression vector (pSVIE) in combination with chimeric EHV-1 promoter-chloramphenicol acetyltransferase (CAT) reporter constructs was used in transient transfection assays to characterize the regulatory functions of the IE gene product. These experiments demonstrated that (i) the EHV-1 IE gene product is a bifunctional protein capable of both positive and negative modulation of gene expression; (ii) the IE gene product possesses an autoregulatory function which represses the IE promoter; (iii) IE autoregulation is dependent on IE promoter sequences mapping within positions -288 to +73 relative to the transcription initiation site (+1) of the IE gene; (iv) the IE gene product can independently activate the EHV-1 tk promoter (an early promoter) by as much as 60-fold; (v) two EHV-1 beta-gamma (leaky late) promoters, those of IR5 (gene 5 in the inverted repeat) and the glycoprotein D gene, demonstrate a requirement for both the IE gene product as well as a gene product encoded within the EHV-1 XbaI G fragment for significant activation; and (vi) the IE gene product is capable of activating heterologous viral promoters.

The IR3 gene of equine herpesvirus type 1: a unique gene regulated by sequences within the intron of the immediate-early gene

The complete nucleotide sequence of the inverted repeat component (IR; 12,776 bp each) of the genome of equine herpesvirus type 1 (EHV-1) has been determined. Transcription analyses have revealed that the EHV-1 IR sequence encodes at least 6 genes. In this report, we present the DNA sequence and transcriptional characterization of a gene (IR3) that maps entirely within the IR sequences. The IR3 open reading frame (ORF) is located between nucleotides (nt) 6123-6411 of the IR sequence and possesses an ORF of 95 amino acids. Interestingly, this ORF does not show homology to any known herpesvirus gene, suggesting that the IR3 gene is unique to EHV-1. Moreover, the location of the IR3 gene between the immediate-early (IR1) gene and the origin of replication is unique in comparison to the IR gene arrangement of other alphaherpesviruses such as herpes simplex virus type 1 and varicella zoster virus. Putative cis-acting elements flanking the IR3 ORF include a TATA box (nt 5648-5652), a GC box (nt 5600-5605), and three polyadenylation signals (nt 6533-6538, 6648-6653, and 6663-6668). Northern blot analyses identified a 1.0 kb mRNA that exhibits characteristics of a late gene of the gamma-1 class. Northern blot, S1 nuclease, and primer extension analyses revealed that transcription of IR3 initiates within the intron of the immediate-early gene (IR1) on the opposite stand of the genome. Thus, the 5' end of IR3 transcript is antisense to the 5' end of the IR1 mRNA and promoter, and IR3 transcription may regulate the expression of IR1 during late times of infection.

Transcriptional analysis of the UL1 gene of equine herpesvirus 1: a gene conserved in the genome of defective interfering particles

Defective interfering particles (DIPs) of equine herpesvirus type 1 (EHV-1) are biologically active, in that they mediate the coestablishment of oncogenic transformation and persistent infection in permissive, primary hamster embryo fibroblasts. The DIP genome is composed of EHV-1 sequences originating from the L-terminus (mapping units (m.u.) 0.00-0.023), the junction of the unique long (UL) region and the internal inverted repeat (IR) (m.u. 0.78-0.79 and 0.99-1.00), and the central portion of the IR (m. u. 0.83-0.87 and 0.91-0.95). The nature of one of the genes (UL1) mapping at the L-terminus was analyzed at the RNA level by Northern blot hybridization and S1 nuclease analyses. These data, and DNA sequencing analyses reported previously revealed that the UL1 gene: (1) contains a major open reading frame (ORF) of 258 amino acids, (2) is a homologue of the ORF2 gene of varicella zoster virus (VZV), (3) is conserved in the genome of DIPs of EHV-1, (4) encodes a 1.2-kb early (E) mRNA that is transcribed toward the short region of the genome, (5) utilizes a transcription initiation site approximately 1,120 nucleotides from the L-terminus, and (6) utilizes a transcription termination site approximately 2211 nucleotides from the L-terminus. These initial studies serve as the basis of future work to determine the function of the UL1 gene in cytolytic infection, and its potential role in EHV-1 persistent infection.

An early gene maps within and is 3' coterminal with the immediate-early gene of equine herpesvirus 1

The immediate-early (IE) gene (IR1 gene) of equine herpesvirus 1 (EHV-1) encodes a single, spliced 6.0-kb mRNA during cytolytic infection. However, under early (in the presence of phosphonoacetic acid) and late (8 h postinfection; no metabolic inhibitors) conditions, in addition to the 6.0-kb IE mRNA, a 4.4-kb early (E) mRNA is transcribed from the IE gene region beginning at approximately 4 h postinfection. To map and characterize the 4.4-kb E mRNA and the protein product of this early gene (IR2 gene), Northern (RNA) blot hybridization, S1 nuclease, primer extension, and in vitro transcription and translation analyses were used. The data from RNA mapping analyses revealed that the 4.4-kb E IR2 mRNA (i) maps at nucleotides 4481 to 635 within each of the inverted repeats of the short region and thus is encoded by sequences that lie entirely within the IE gene, (ii) is transcribed in the same direction as the IE mRNA, initiating at nucleotide 4481, which lies 25 bp downstream of a putative TATA-like sequence and 1,548 bp downstream of the transcription initiation site of the IE mRNA, and (iii) is 3' coterminal with the IE mRNA which terminates at nucleotide 635 of the inverted repeats. The IR2 open reading frame was inserted into the transcription expression vector pGEM-3Z, and the RNA transcribed from this construct (pGEM44) was shown to be a 4.2-kb transcript that contained all IR2 sequences. In vitro translation of the 4.2-kb RNA yielded a major protein of approximately 130 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. This protein corresponds to the predicted IR2 product of 1,165 amino acids that would be in frame with the major IE polypeptide (IE1 = 200 kDa; 1,487 amino acids) and thus would be a 5'-truncated form of the IE1 polypeptide. The presence and potential role of the IR2 gene embedded within the IR1 gene increase the complexity of the regulation of the IE gene region during various stages of a productive infection.

Translational control of equine herpesvirus type 1 gene expression

Translational control mechanisms modulate gene expression in a variety of cellular and viral systems. Using hypertonic conditions to block protein synthesis in vivo, we observed that the synthesis of several major equine herpesvirus type 1 proteins was selectively inhibited. Although sensitivity to hypertonic conditions was graded across a continuum, messages coding for proteins of 203, 130.5, and 31.5 kDa were significantly more resistant to higher salt concentrations in vivo than those coding for polypeptides of 148, 116, and 74 kDa. Similar results were observed in vitro when potassium acetate was used to block initiation. In addition, Northern blot analyses demonstrated that steady-state levels of cellular mRNAs declined beginning at about 6 hr after infection. Taken together, these results indicate that the expression of several major equine herpesvirus type 1 genes was controlled in part at the post-transcriptional level.

The three major immediate-early transcripts of bovine herpesvirus 1 arise from two divergent and spliced transcription units

Among 54 transcripts expressed in a temporal cascade during lytic infection with bovine herpesvirus 1, we have previously identified three major immediate-early (IE) RNAs, IER4.2 (4.2 kb), IER2.9 (2.9 kb), and IER1.7 (1.6 to 1.8 kb depending on the virus strain) transcribed from the HindIII C genome region (U. V. Wirth, K. Gunkel, M. Engels, and M. Schwyzer, J. Virol. 63:4882-4889, 1989). Northern (RNA) blot, S1 nuclease protection, and primer extension analysis used in the present study demonstrated that all three IE transcripts were spliced and originated from two divergent transcription units with start sites located in the inverted repeat. Transcription unit 1 encoded two alternative spliced transcripts, IER4.2 and IER2.9, with a common exon 1 located at 0.797 to 0.795 map units (m.u.) and an exon 2 for IER4.2 (0.792 to 0.762 m.u.) in the inverted repeat; exon 2 for IER2.9 (0.754 to 0.738 m.u.) was located in the unique long sequence and transcribed in antisense orientation to latency-related RNA. Transcription unit 2 (0.818 to 0.836 m.u.), further characterized by cDNA cloning, encoded the spliced IER1.7 with three exons in the inverted repeat. Additional minor IE transcripts were interpreted as unspliced precursors and splicing variants. With regard to the number and layout of IE genes, bovine herpesvirus 1 occupies an intermediate position between pseudorabies virus and equine herpesvirus 1 on the one hand and varicella-zoster virus and herpes simplex virus type 1 on the other.

Monoclonal antibody to immediate early protein encoded by varicella-zoster virus gene 62

Monoclonal antibodies (mAbs) were prepared against varicella-zoster virus (VZV)-infected cell proteins, and 10 mAbs which reacted with nuclear antigens were selected. These mAbs recognized a major 175-180 kDa and three minor VZV-specific phosphoprotein species. Immunofluorescence staining of VZV-infected cells showed that the 175-180 kDa protein was synthesized within 6 h after infection. The synthesis of this protein was inhibited by cycloheximide (CH); however, reversal of CH treatment and addition of actinomycin D (ActD) resulted in the synthesis of the 175-180 kDa protein. To determine whether the 175-180 kDa protein seen in the infected cells is encoded by VZV immediate early (IE) gene 62, the predicted open reading frames of VZV genes 61 and 62 were cloned into pGEM transcription vectors. RNA was transcribed from each gene, translated in vitro and immunoprecipitated with a mAb which recognizes a major 175-180 kDa and three minor proteins. The reactivity of the in vitro translation products encoded by gene 62 with this mAb suggested that the 175-180 kDa protein is encoded by VZV IE gene 62.

Temporal regulation of equine herpesvirus type 3 transcription

The transcription of equine herpesvirus type 3 (EHV-3; equine coital exanthema virus) has been examined and found to be temporally regulated into three classes: immediate early (IE), early (E), and late (L). Hybridization of in vivo 32PO4-labeled transcripts revealed that IE transcript(s) are derived exclusively from the inverted repeat segments (IRs) of the viral genome, while E and L transcripts are not restricted to any specific region of the genome. Northern blot analysis of EHV-3 IE RNA revealed a single transcript of approximately 5.7 kb (3.8 MDa). We have previously shown that transcription of equine herpesvirus type 1 (EHV-1) DNA is temporally regulated and produces a single 6 kb IE RNA which is derived from the IRs segments. In this paper, we show that the EHV-1 and EHV-3 IE RNA species are homologous, reflecting the colinearity of the genomes of these two related viruses. While four IE polypeptides are synthesized in EHV-1 infected cells in the presence of actinomycin D following the removal of a cycloheximide block, only one major IE polypeptide (180 kDa) is detectable in EHV-3 infected cells under these conditions. However, immunoprecipitation of EHV-3 infected cell extracts with polyvalent rabbit antisera to IE1 of EHV-1 revealed at least two other viral specific IE polypeptides.

Synthesis and processing of equine herpesvirus type 1 glycoprotein 14

Glycoprotein 14 (gp14) of equine herpesvirus type 1 (EHV-1), the homolog of herpes simplex virus (HSV) glycoprotein B (gB), was investigated employing a panel of monoclonal antibodies to ascertain the regulatory class, rate of synthesis, and type of glycosylation of this polypeptide. Application of immunoprecipitation, Western blot, and SDS-PAGE analysis in conjunction with the use of metabolic inhibitors (cycloheximide, antinomycin D, phosphonoacetic acid, tunicamycin, and monensin), and time-course and pulse-chase experiments revealed the following information: (1) Three gp14-related polypeptides with molecular weights of 138 kilodaltons (K), 77-75K, and 55-53K are present in EHV-1-infected cell extracts. (2) All three species are synthesized in the presence of the DNA synthesis inhibitor phosphonoacetic acid although their synthesis is enhanced by DNA replication, indicative of a beta-gamma class molecule. (3) The 138K species is synthesized first as a precursor of the smaller species of gp14, the 77-75K and 55-53K forms. (4) Use of glycosylation inhibitors and digestion of immunoprecipitated gp14 with endoglycosidases indicate that the primary translation product is a 118K molecule which is cotranslationally glycosylated to the 138K form by the addition of high mannose oligosaccharides. (5) The 77-75K species contains both high mannose and hybrid oligosaccharides while the 55-53K form of gp14 contains some complex oligosaccharides. (6) In the absence of a reducing agent, the 138K polypeptide and a large 145K species are observed in both infected cell extracts and purified virions. Thus, EHV-1 gp14 appears to be synthesized as a large precursor molecule of 138K and is proteolytically cleaved to two smaller forms, 77-75K and 55-53K, which are linked by a disulfide bond(s) to form a 145K complex. This model of gp14 synthesis and maturation is similar to those proposed for a number of HSV gB equivalents found in the Alphaherpesvirnae.

Mapping the termini and intron of the spliced immediate-early transcript of equine herpesvirus 1

Equine herpesvirus 1 (EHV-1) has been shown to synthesize a 6.0-kilobase (kb) species of immediate-early (IE) mRNA in productively infected cells. This IE gene region maps within the outer portion (map units 0.79 to 0.83 and 0.96 to 1.00) of the two inverted repeat segments of the short genomic region, and elucidation of its DNA sequence has revealed multiple potential open reading frames (ORFs), including a major ORF of 4,461 nucleotides (F. J. Grundy, R. P. Baumann, and D. J. O'Callaghan, Virology 172:223-236, 1989). Analyses of IE polypeptides synthesized in EHV-1-infected cells (in vivo) and in vitro translation of hybrid-selected IE mRNA indicated that multiple species of IE proteins are encoded by this IE mRNA species. To address the nature of the 6.0-kb IE RNA species, Northern (RNA) blot hybridization, S1 nuclease mapping, and primer extension analyses have been employed. These data revealed that no major introns were detected within the body of the IE transcript. However, the IE mRNA was shown to be spliced at the 5' terminus, such that a 372-base intron containing two small ORFs (19 and 51 amino acids) was removed from the leader region of the transcript. This splicing event reduced the leader region from 625 to 253 bases. S1 and primer extension analyses of the 5' terminus of this transcript revealed that the transcription initiation site is located 24 to 26 bases downstream of the consensus TATAAA motif. The 3' transcription termination site was mapped by S1 nuclease analysis to approximately 10 to 20 bases downstream of the polyadenylation signal, AATAAA. The distance from the stop codon of the major ORF to the polyadenylation site is approximately 300 bases. Results from S1 nuclease experiments indicated that splicing does not occur at the 3' terminus. These studies indicated that the EHV-1 6.0-kb IE mRNA is spliced at the 5' terminus and that alternative splicing of this transcript may function in regulating translation of the IE mRNA species.

Spatial and temporal distribution of bovine herpesvirus 1 transcripts

Northern (RNA) blot analysis was used to determine the spatial and temporal distribution of bovine herpesvirus 1 (BHV-1) transcripts. Total RNA was isolated from Madin-Darby bovine kidney cells which had been infected with BHV-1.2b strain K22 or BHV-1.1 strain Jura in the presence or absence of metabolic inhibitors. Cloned restriction fragments representing the entire genome of strain K22 were labeled with 32P and hybridized to immobilized RNA. A total of 54 BHV-1 transcripts were found, ranging in size from 0.4 to larger than 8 kilobases (kb). The inverted repeat regions and an adjacent segment of the unique large part of the BHV-1 genome encoded three major immediate-early (IE) transcripts and one minor IE transcript enriched after cycloheximide treatment of infected cells. Late transcripts were identified by drastically reduced abundance after cytosine arabinoside (araC) treatment. Twelve late transcripts were encoded mainly by the unique long genome region, with a cluster of four transcripts located on HindIII fragment K (map units 0.677 to 0.733). The 21 transcripts unaffected by araC treatment were defined as early; they showed dispersed locations over the whole genome, with a cluster on the unique short sequence. The 17 remaining transcripts could not be classified unambiguously as early or late by these techniques. The IE transcript with a size of 4.2 kb exhibited homology with the single IE gene of pseudorabies virus, and the IE transcript with a size of 2.9 kb was encoded in part by the genome region known to be transcriptionally active during latency.

Viral transcripts in cells infected with defective interfering particles of equine herpesvirus type 1

Equine herpesvirus type 1 (EHV-1) preparations enriched in defective interfering particles (DIPs) have previously been demonstrated to mediate the coestablishment of persistent infection and oncogenic transformation in primary hamster embryo fibroblasts. In this study, it was demonstrated that infection of a rabbit kidney (RK) cell line with EHV-1 DIP-enriched preparations also results in the establishment of persistent infection. Viral transcription was characterized in RK cells infected with DIP-enriched stocks and compared to viral transcription in RK cells infected with standard (STD) EHV-1. During the first 8 hr of infection with the DIP-enriched EHV-1 preparation, viral DNA sequences which are conserved in the DIP genome were predominantly expressed. Thus, these transcripts originate from DNA sequences that contain the components of the defective genome which originates from DNA sequences mapping at 0.00-0.04 of the Long region terminus and within two portions of the Short region inverted repeats (IR), 0.78-0.79 and 0.83-0.865 of the internal IRs and 0.99-1.00 and 0.915-0.95 of the terminal IRs. The overwhelming majority of viral transcripts that were synthesized in the DIP-enriched infections appeared to correspond to transcripts expressed in STD infection as assessed by Northern hybridization analysis but the synthesis of transcripts originating from sequences not conserved in the defective genome was significantly delayed. However, some high molecular weight RNA species that were synthesized in STD infections were not detected in DIP-enriched infections. Studies utilizing metabolic inhibitors indicated that viral transcription in DIP-enriched infections, like that of STD cytocidal infection, is regulated in an immediate early, early and late manner.

DNA sequence and comparative analyses of the equine herpesvirus type 1 immediate early gene

The immediate early (IE) proteins of herpesviruses are important regulatory factors which control the expression of genes at the transcriptional level. We report the DNA sequence of the immediate early gene of the alphaherpesvirus equine herpesvirus type 1 (EHV-1). This sequence is shown to be extremely rich in guanine and cytosine, resulting in a highly biased codon usage. The IE gene region possesses 38 open reading frames (ORFs) greater than 300 bp in length, 11 of which have coding regions of at least 100 amino acids (aa) following potential translation initiator codons. The largest ORF consists of 1487 codons (4461 bp) starting with the first ATG and would encode a protein of MW 155,000. TATA and CCAAT sequences as well as several potential cis-acting elements lie upstream to the major ORF. The deduced amino acid sequence for the 155,000 protein has a high degree of homology to the herpes simplex virus type 1 (HSV-1) ICP4 protein and its varicella-zoster virus (VZV) homolog. The regions of the EHV-1 IE protein that are homologous with these proteins correspond to the previously determined pattern of homology between the HSV and VZV IE polypeptides. However, there are are a number of differences within these broadly defined regions. It is therefore expected that this comparative study will facilitate the identification of functionally important residues within the amino acid sequence of IE proteins.

Functional mapping and DNA sequence of an equine herpesvirus 1 origin of replication

The genome of equine herpesvirus 1 (EHV-1) defective interfering (DI) particle DNA originates from discrete regions within the standard (STD) EHV-1 genome: the left terminus (0.0 to 0.04 map units) and the inverted repeats (0.78 to 0.79 and 0.83 to 0.87 map units of the internal inverted repeat; 0.91 to 0.95 and 0.99 to 1.00 map units of the terminal inverted repeat). Since DI DNA must contain cis-acting DNA sequences, such as replication origins, which cannot be supplied in trans by the STD EHV-1 virus, regions of the EHV-1 genome shown to be in DI DNA were assayed for the presence of a viral origin of DNA replication. Specifically, STD EHV-1 DNA fragments encompassing the genomic regions present in DI particle DNA were inserted into the vector pAT153, and individual clones were tested by transfection assays for the ability to support the amplification and replication of plasmid DNA in EHV-1-infected cells. The Sma-1 subfragment of the internal inverted repeat sequence (0.83 to 0.85 map units) was shown to contain origin of replication activity. Subcloning and BAL 31 deletion analysis of the 2.35-kilobase-pair (kbp) Sma-1 fragment delineated a 200-bp fragment that contained origin activity. The origin activities of all EHV-1 clones which were positive by the transfection assay were confirmed by methylation analysis by using the methylation-sensitive restriction enzymes DpnI and MboI. DNA sequencing of the 200-bp fragment which contained an EHV-1 origin of replication indicated that this region has significant homology to previously characterized origins of replication of human herpesviruses. Furthermore, comparison of known origin sequences demonstrated that a 9-bp sequence, CGTTCGCAC, which is conserved among all origins of replication of human lytic herpesviruses and which is contained within the 18-bp region in herpes simplex virus type 1 origins shown by others to be protected by an origin-binding protein (P. Elias, M. E. O'Donnell, E. S. Mocarski, and I. R. Lehman, Proc. Natl. Acad. Sci. USA 83:6322-6326) is also conserved across species in the EHV-1 origin of replication.

Analysis of the in vitro translation products of the equine herpesvirus type 1 immediate early mRNA

Equine herpesvirus type 1 (EHV-1) gene expression is coordinately regulated in an alpha, beta, gamma fashion. Viral alpha gene products include a 6.0-kb immediate early (IE) mRNA species (W. L. Gray et al., 1987, Virology 158, 79-87) and at least four closely related IE polypeptides (IEPs) (G.B. Caughman et al., 1985, Virology 145, 49-61). In this report, we describe results obtained from a series of in vitro translation experiments which were performed in an effort to characterize the IEPs and identify the mechanism by which individual IE protein species are generated. Our data indicate that a family of IEPs is generated in vitro from the 6.0-kb mRNA size class and that these IEPs correspond in overall size and antigenicity to those synthesized in infected cells. Using time-course/pulse-chase analyses, we show that production of three of the major IEPs [IE1' (193 kDa), IE3' (166 kDa), and IE4' (130 kDA)] occurs concomitantly, that none of these protein species can be chased completely into another, and that at least two additional minor species appear to be processed following synthesis. Finally, we show that the 6.0-kb mRNA species isolated during early or late stages of the infection cycle can be translated to yield all of the major IE proteins, indicating that production of the family of IEPs is not dependent upon accumulation of the IE mRNA which occurs during a cycloheximide blocked infection cycle. The implications of these findings are discussed as they relate to the origin and production of the IEPs both in vivo and in vitro.