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

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.

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.

The EICP27 protein of equine herpesvirus 1 is recruited to viral promoters by its interaction with the immediate-early protein

The equine herpesvirus 1 (EHV-1) EICP27 protein cooperates with either the immediate-early (IE) or the EICP0 protein to synergistically trans-activate viral promoters. GST-pulldown and co-immunoprecipitation assays revealed that the EICP27 protein's cooperation with the IE or the EICP0 protein involves its physical interaction with these viral proteins. In the case of the IE-EICP27 protein interaction, IE residues 424 to 826 and EICP27 residues 41 to 206 harbor the interactive domains. Electrophoretic mobility shift assays (EMSA) suggested that the EICP27 protein is not a sequence-specific DNA-binding protein as it fails to directly bind to the IE promoter, the early EICP27, EICP0, and TK promoters, or the late gD and IR5 promoters. However, EMSA studies also showed that the interaction of the IE and EICP27 proteins results in the recruitment of the EICP27 protein to representative early promoters. These results support our hypothesis that the EICP27 protein participates in the trans-activation of EHV-1 promoters, and suggest its presence within RNA polymerase II preinitiation complexes that assemble at viral promoters.

The immediate early gene of canine herpesvirus is transcribed through early and late phases

The immediate early (IE) gene of canine herpesvirus (CHV), homologue of the infected cell protein 4 (ICP4) gene of herpes simplex virus 1, is transcribed as a 4.9kb mRNA during IE phase. The IE gene was further transcribed as a 4.8kb mRNA through early (E) and late (L) phases of productive infection. Transcription of the 4.8kb mRNA initiated from downstream of the TATA box in an intron which was spliced out during IE phase. The reverse transcription-polymerase chain reaction revealed that the IE promoter was turned off during L phase at a permissive temperature. We, thus, propose to redesignate the IE gene of CHV as CICP4 gene.

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.

Molecular characterizations of the equine herpesvirus 1 ETIF promoter region and translation initiation site

The equine herpesvirus 1 (EHV-1) homolog of the herpes simplex virus type 1 (HSV-1) tegument phosphoprotein, alphaTIF (Vmw65; VP16), was identified previously as the product of open reading frame 12 (ORF12), was shown to trans-activate immediate-early (IE) gene promoters, and was described as a 60-kDa virion component designated ETIF. However, the ETIF promoter region and transcription initiation site were not identified. The poly(A) signal of the gene 11 (UL49 homolog) lies just upstream of the first ETIF translation initiation codon, indicating that the first ATG may not be used for initiating ETIF translation. Another in-frame translation initiation codon (ATG2) is located 88 bp downstream of the first ETIF initiation codon (ATG1). Western blot analysis showed that the expressed ETIF protein migrated in SDS-PAGE with an apparent molecular mass of approximately 56 kDa, the same molecular weight identified in SDS-PAGE analysis of the KyD EHV-1 virion preparations. The ETIF expression vector pCETIF, which contains ATG2, trans-activated the IE promoter more efficiently than the pC12 containing both ATG1 and ATG2. S1 nuclease analyses mapped the 5' initiation site of the 1.4-kb transcript approximately 17 to 21 nt downstream of the ATG1. The nucleotide sequence upstream of the ATG1 did not have any promoter activity, while the nucleotide sequence upstream of the ATG2 had promoter activity. In transient transfection assays, the pETIFM2 vector, which was mutated in the ATG2, did not trans-activate the IE promoter; however, the pETIFM1 vector, which was mutated in the ATG1, trans-activated the IE promoter. These results demonstrated that the ATG2 of the ETIF ORF is the ETIF translation initiation codon. ETIF trans-activated only the IE promoter, not early (EICP0, EICP22, EICP27, and thymidine kinase) or late (IR5) promoters, confirming that EICP0, EICP22, and EICP27 are early genes.

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.

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.

Detection of latency-associated transcripts of equid herpesvirus 1 in equine leukocytes but not in trigeminal ganglia

Results from Southern hybridization and PCR amplification experiments using a randomly synthesized reverse transcription-PCR product showed that peripheral blood leukocytes from horses showing no clinical signs of disease expressed a putative latency-associated transcript antisense to and overlapping the 3' end of the equid herpesvirus 1 (EHV-1) immediate-early gene (gene 64). A PCR product derived from this transcript has > or =96% identity with the published EHV-1 sequence. In situ hybridization studies of equine bronchial lymph nodes corroborated these findings and are consistent with reactivation data (D. A. Smith, A. Hamblin, and N. Edington, unpublished data), indicating that EHV-1 latency is established predominantly in CD5+/CD8+ leukocytes.

Alterations in the equine herpesvirus type-1 (EHV-1) strain RacH during attenuation

The equine herpesvirus type-1 modified live-vaccine strain RacH (256th passage on porcine embryonic kidney cells) was investigated by restriction-enzyme analysis and compared to representative plaque isolates of the 12th passage (RacL11, RacL22) and 185th passage (RacM24, RacM36). The restriction patterns of all Rac plaque isolates differed compared with reference strain Ab4. The left UL terminus was shortened by 0.1 kbp and a missing BamHI site led to the fusion of the f and t fragments. In some Rac derivatives, losses of restriction sites without deletions were observed: 1. One BamHI site located in the ribosyl reductase gene was missing in RacH, RacM24, RacM36, and RacL22; and 2. An SalI site mapping to the gp14 (gB) gene was absent in RacM24, RacM36 and RacH. An identical deletion of 0.85 kbp in size was found in both copies of the inverted repeat (IR) regions of RacH. The deletion was present only in the terminal IR of the medium-passage derivative RacM36. By contrast, in the genomes of the apathogenic RacM24, as well as the pathogenic plaque isolates RacL11 and RacL22, no deletions in the IRs were detectable. Nucleotide-sequence and Northern-blot analyses revealed that the deletions led to the elimination of one or both copies of the gene 67 (IR6) open-reading frame in RacM36 and RacH and affected the gene 68 (EUS1) in RacH.

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.

Regulatory function of the equine herpesvirus 1 ICP27 gene product

The UL3 protein of equine herpesvirus 1 (EHV-1) KyA strain is a homolog of the ICP27 alpha regulatory protein of herpes simplex virus type 1 (HSV-1) and the ORF 4 protein of varicella-zoster virus. To characterize the regulatory function of the UL3 gene product, a UL3 gene expression vector (pSVUL3) and a vector expressing a truncated version of the UL3 gene (pSVUL3P) were generated. These effector plasmids, in combination with an EHV-1 immediate-early (IE) gene expression vector (pSVIE) and chimeric EHV-1 promoter-chloramphenicol acetyltransferase (CAT) reporter constructs, were used in transient transfection assays. These assays demonstrated that the EHV-1 UL3 gene product is a regulatory protein that can independently trans activate the EHV-1 IE promoter; however, this effect can be inhibited by the repressive function of the IE gene product on the IE promoter (R. H. Smith, G. B. Caughman, and D. J. O'Callaghan, J. Virol. 66:936-945, 1992). In the presence of the IE gene product, the UL3 gene product significantly augments gene expression directed by the promoters of three EHV-1 early genes (thymidine kinase; IR4, which is the homolog of HSV-1 ICP22; and UL3 [ICP27]) and the promoter of the EHV-1 late gene IR5, which is the homolog of HSV-1 US10. Sequences located at nucleotides -123 to +20 of the UL3 promoter harbor a TATA box, SP1 binding site, CAAT box, and octamer binding site and, when linked to the CAT reporter gene, are trans activated to maximal levels by the pSVIE construct in transient expression assays. Results from CAT assays also suggest that the first 11 amino acids of the UL3 protein are not essential for the regulatory function of the UL3 gene product.

Transcriptional analyses of the unique short segment of EHV-1 strain Kentucky A

The unique short (Us) segment of the genome of equine herpesvirus type 1 (EHV-1) strain KyA is comprised of six open reading frames (ORFs) that encode: a) a homolog of the Us2 protein of herpes simplex virus type 1 (HSV-1); b) a serine threonine protein kinase that is a homolog of the HSV-1 Us3 protein; c) a homolog of pseudorabies virus glycoprotein gX and HSV-2 gG; d) a novel glycoprotein, EUS4, not encoded by other herpesviruses sequenced to date; e) a homolog of HSV-1 gD; and f) a homolog of HSV-1 Us9. The KyA strain is a deletion mutant that lacks Us sequences encoding gI, gE, and a potential 10 kD polypeptide, and thus may be useful as a parent virus for the generation of live virus vaccines. To complete the elucidation of the transcriptional program of the Us segment, Northern blot hybridization and S1 nuclease analyses were performed on poly(A)(+)-selected RNA isolated from infected cells maintained under early (phosphonoacetic acid-block) and late conditions. The findings revealed that the gene (EUS2 ORF) encoding the protein kinase is expressed as an early 2.9 kb transcript that overlaps and is 3' coterminal with a 1.6 kb early transcript that encodes the gG/gX homolog (EUS3 ORF). Two transcripts of 1.6 kb and 5.8 kb are 5' coterminal and may both encode the novel glycoprotein gene EUS4. The 1.6 kb transcript terminates at a poly(A) signal site downstream of the EUS4 ORF, and the 5.8 kb transcript terminates within the inverted repeat (IR) segment. Overall, the transcriptional program of the EHV-1 KyA Us segment is complex and exhibits similarities to that of HSV-1 Us segment: a) transcripts arise from both DNA strands; b) some transcripts, including those mapping at the termini of the Us segment, extend into the IR segments and are 3' coterminal with the 1.2 kb IR6 transcript; c) at least one transcript reads through a functional polyadenylation signal; d) some transcripts encoding genes that lie in different reading frames exist as a family of overlapping mRNAs, some in an anti-sense manner. Lastly, of the six Us genes of the EHV-1 KyA strain, only those encoding the EHV-1 protein kinase and the HSV-2 gG/gX homolog are members of the early kinetic class.

Nucleotide sequence of infectious laryngotracheitis virus (gallid herpesvirus 1) ICP4 gene

The infectious laryngotracheitis virus (ILTV) gene encoding a homologue to the ICP4 protein of herpes simplex virus (HSV) has been mapped to the inverted repeat region. The complete nucleotide sequence of ILTV ICP4 has been determined. The ILTV ORF encoding ICP4 is 4386 nucleotides long, calculated from the first of four ATG codons, and has an overall G+C content of 59%. The ILTV ICP4 contains two domains of high homology which have been reported in other studies to be conserved in the ICP4 homologues of alphaherpesviruses, and to be functionally important. Several regulatory features were identified including a serine-rich domain in region one. A more extensive serine-rich domain was located in region five which is also found in varicella-zoster virus (VZV) and bovine herpesvirus 1. A 5.4 kb immediate early transcript was identified in infected primary kidney cells.

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.

Transcriptional and translational analyses of the UL2 gene of equine herpesvirus 1: a homolog of UL55 of herpes simplex virus type 1 that is maintained in the genome of defective interfering particles

Defective interfering particles (DIPs) of equine herpesvirus 1 (EHV-1; Kentucky A strain) mediate persistent infection. DNA sequences at the L terminus, which contain the UL2 gene (homolog of UL55 of herpes simplex virus type 1 and open reading frame 3 of varicella-zoster virus) of standard EHV-1, have been shown to be highly conserved in all clones of the EHV-1 DIP genome. The UL2 mRNA was characterized by S1 nuclease analyses, which mapped the 5' and 3' termini of the 0.9-kb early UL2 mRNA to approximately 26 and 16 nucleotides downstream of a TTTAAA box and polyadenylation signal, respectively. The UL2 open reading frame, present within both the EHV-1 standard and DIP genomes, was inserted into the transcription expression vector pGEM-3Z to yield constructs pGEML2 and pDIL2, respectively. After in vitro transcription and translation, both constructs yielded a comigrating 23-kDa protein, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polyclonal antiserum was raised against the UL2 protein by injecting rabbits with a TrpE/UL2 fusion protein expressed from plasmid pATH23L2 in Escherichia coli. The UL2-specific antiserum reacted in Western immunoblot and immunoprecipitation analyses with a 23-kDa polypeptide synthesized in cells infected with standard EHV-1 or DIP-enriched virus. These data also indicated that the UL2 polypeptide was more abundant in DIP-infected cells than in standard EHV-1-infected cells. Results from time course and pulse-chase analyses suggested that the UL2 polypeptide has a rapid turnover rate in DIP-infected cells.

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.

Equine herpesvirus 1 glycoprotein D: mapping of the transcript and a neutralization epitope

Studies with molecular and immunological techniques identified and mapped the transcript encoding glycoprotein D (gD) of equine herpesvirus 1 KyA, as well as two continuous gD antigenic determinants. Three mRNA species of 5.5, 3.8, and 1.7 kb overlap the gD open reading frame and are transcribed from the DNA strand encoding gD. Northern (RNA) blot hybridization with both DNA clones and riboprobes, as well as S1 nuclease analyses, showed the 3.8-kb mRNA to encode gD and to be synthesized as a late (beta-gamma) transcript. The 3.8-kb gD mRNA initiates within the US segment 91 and 34 nucleotides downstream of the CCAAT and TATA elements, respectively, and encodes a potential polypeptide of 392 amino acids. The termination site of this transcript maps within the terminal repeat at a site also used by the 5.5-kb mRNA and the IR6-encoded 1.2-kb mRNA, such that these three transcripts form a 3'-coterminal nested set. The extended size (2,250 nucleotides) of the 3' untranslated region of the gD transcript and its termination within the terminal repeat may result from the deletion of 3,859 bp, which eliminates two consensus polyadenylation signals downstream of the gD open reading frame of EHV-1 KyA. Use of antisera to synthetic peptides of 19 amino acids (residues 4 to 22) and 20 amino acids (residues 267 to 285) in Western immunoblot analyses revealed that gD is present in EHV-1 virions as a 55-kDa polypeptide. In addition, these antisera detected the 55-kDa protein as well as 58- and 47-kDa polypeptides in infected-cell extracts at late times of infection. Residues 4 to 22 make up a continuous neutralizing epitope of gD, since incubation of equine herpesvirus 1 with the anti-19-mer serum prior to infection results in reduced numbers of plaques and reduced levels of virus-encoded thymidine kinase. Complement is not required for neutralization mediated by the anti-19-mer serum.

Identification and transcriptional analyses of the UL3 and UL4 genes of equine herpesvirus 1, homologs of the ICP27 and glycoprotein K genes of herpes simplex virus

The DNA sequence of 3,240 nucleotides of the XbaI G fragment located in the unique long (UL) region of the equine herpesvirus 1 genome revealed two major open reading frames (ORFs) designated UL3 and UL4. The UL3 ORF of 470 amino acids (aa) maps at nucleotides (nt) 4450 to 3038 from the long terminus, and its predicted 51.4-kDa protein product exhibits significant homology to the ICP27 alpha regulatory protein of herpes simplex virus type 1 (HSV-1; 32% identity) and to the ORF4 protein of varicella-zoster virus (13% identity). Interestingly, a zinc finger motif is conserved in the C-terminal domains of both ICP27 of HSV-1 (aa 483 to 508) and UL3 of equine herpesvirus 1 (aa 441 to 466). The UL4 ORF of 343 aa maps at nt 5618 to 4587 and could encode a protein of 38.1 kDa which exhibits significant homology to the UL53 protein (cell fusion protein or glycoprotein K) of HSV-1 (26% identity) and to the ORF5 protein of varicella-zoster virus (33% identity). Analyses of the UL4 amino acid sequence revealed domains characteristic of a membrane-bound glycoprotein and included potential signature sequences for (i) a signal sequence, (ii) two N-linked glycosylation sites, and (iii) four transmembrane domains. Nucleotide sequence analyses also revealed potential TATA boxes located upstream of the UL3 and UL4 ORFs. However, only a single polyadenylation signal (nt 2988 to 2983) was detected downstream of the UL3 ORF. Northern (RNA) blot hybridization and S1 nuclease analyses were used to map and characterize the UL3 and UL4 mRNAs. Metabolic inhibitors were used to identify the kinetic class of these two genes. The data revealed that UL3 is an early gene that encodes a 1.6-kb mRNA, while UL4 is a late gene encoding a 3.8-kb mRNA that overlaps the UL3 transcript. Both transcripts were shown by S1 nuclease analyses to initiate 24 to 26 nt downstream of their respective TATA boxes and to have a common transcription termination signal as a pair of 3'-coterminal mRNAs.

Identification and expression of the UL1 gene product of equine herpesvirus 1

Sequences encoding the UL1 gene of equine herpesvirus type 1 (EHV-1) are conserved in the genome of EHV-1 defective interfering particles (DIPs) that mediate oncogenic transformation and persistent infection. The UL1 protein was identified by in vitro transcription/translation and hybrid-arrest translation analyses which employed a UL1/pGEM-3Z construct designated pGEML1. SDS-PAGE analyses of in vitro translation products synthesized from UL1-specific RNA revealed that the UL1 ORF encodes a 30 kDa protein which corresponds in size to the 258 amino acid protein predicted by DNA sequence analyses. This result was confirmed by arresting translation of the in vitro transcribed UL1 RNA with an oligodeoxynucleotide complementary to UL1 coding sequences. The UL1 protein is a homolog of the predicted protein encoded by the ORF2 gene of varicella-zoster virus, but UL1 has no homolog in herpes simplex virus type 1. The UL1 protein contains a region conforming to a 'PEST' (Proline, Glutamic acid, Serine, and Threonine) sequence, which is commonly found in proteins with half-lives of less than two hours.

Complete nucleotide sequence of the Marek's disease virus ICP4 gene

The Marek's disease virus (MDV) gene encoding a homologue to the ICP4 protein of herpes simplex virus has been mapped to BamHl fragment A based on the physical map of the MDV genome (Fukuchi et al., 1984). The gene lies completely within the inverted repeat flanking the unique short region of the genome. The complete nucleotide sequence of the MDV ICP4 gene has been determined. The coding region is 4245 nucleotides long and has an overall G+C content of 52%. The MDV ICP4 protein is predicted to have a structure similar to that of ICP4-like proteins of other herpesviruses in that it has five distinct regions, the second and fourth of which are highly conserved. In addition, the protein contains the characteristic run of serine residues located toward its amino terminus. The MDV ICP4 gene is expressed in MDV-infected chicken embryo fibroblasts.

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.

The conserved DNA-binding domains encoded by the herpes simplex virus type 1 ICP4, pseudorabies virus IE180, and varicella-zoster virus ORF62 genes recognize similar sites in the corresponding promoters

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), pseudorabies virus (PRV), varicella-zoster virus (VZV), and equine herpesvirus 1 (EHV-1) are all classified as Alphaherpesvirinae. Each of these five viruses encodes an essential immediate-early (IE) regulatory protein referred to as HSV-1 ICP4, HSV-2 ICP4, PRV IE180, VZV ORF62 protein, and EHV-1 IE1, respectively. These five proteins share extensive homology with each other in domains referred to as regions 2 and 4. The HSV-1 ICP4 region 2 domain contains residues that are required for the DNA-binding capability of ICP4. In this report, we describe the expression of region 2 domains from the ICP4, IE180, and ORF62 genes as fusion proteins in Escherichia coli. DNA-binding assays revealed that each of these region 2 fusion proteins binds to a sequence that overlaps the transcription start site in the promoter for the gene encoding the corresponding protein. Each of the sites with high affinity for one or more of these fusion proteins contains the sequence 5'-ATCGT-3'. This sequence spans the mRNA cap site in the HSV-2 ICP4 gene promoter and is immediately upstream from the transcription start site in the EHV-1 IE1 gene. These results suggest that formation of a specific complex between an IE protein and its own gene promoter may be a common mechanism used by Alphaherpesvirinae to autoregulate transcription of an essential IE gene.

Sequence analysis of a glycoprotein D gene homolog within the unique short segment of the EHV-1 genome

DNA sequence analysis of one-third of the unique short (Us) segment of the equine herpesvirus type 1 (EHV-1) genome revealed an open reading frame (ORF) whose translated sequence exhibits significant homology to glycoprotein D of herpes simplex virus (HSV) types 1 and 2 and to pseudorabies virus (PRV) glycoprotein 50, the gD equivalent. The ORF of the EHV-1 gD homolog lies within the pSZ-4 BamHI/KpnI fragment (map units 0.865 to 0.872 and 0.869 to 0.884) and is capable of encoding a polypeptide of 385 amino acids (43,206 molecular weight). Analysis of the nucleotide sequence revealed a complete transcriptional unit including CAAT and TATA elements and signals for polyadenylation. The predicted protein exhibits features typical of a transmembrane protein: a hydrophobic N-terminal signal sequence followed by a probable cleavage site, four potential N-linked glycosylation sites, and a hydrophobic membrane-spanning domain near the carboxyl terminus followed by a charged membrane anchor sequence.

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.

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.