Promoter, spliced leader, and coding sequence for BICP4, the largest of the immediate-early proteins of bovine herpesvirus 1

Stress Triggers Expression of Bovine Herpesvirus 1 Infected Cell Protein 4 (bICP4) RNA during Early Stages of Reactivation from Latency in Pharyngeal Tonsil

Bovine herpesvirus 1 (BoHV-1), an important pathogen of cattle, establishes lifelong latency in sensory neurons within trigeminal ganglia (TG) after acute infection. The BoHV-1 latency-reactivation cycle, like other alphaherpesvirinae subfamily members, is essential for viral persistence and transmission. Notably, cells within pharyngeal tonsil (PT) also support a quiescent or latent BoHV-1 infection. The synthetic corticosteroid dexamethasone, which mimics the effects of stress, consistently induces BoHV-1 reactivation from latency allowing early stages of viral reactivation to be examined in the natural host. Based on previous studies, we hypothesized that stress-induced cellular factors trigger expression of key viral transcriptional regulatory genes. To explore this hypothesis, RNA-sequencing studies compared viral gene expression in PT during early stages of dexamethasone-induced reactivation from latency. Strikingly, RNA encoding infected cell protein 4 (bICP4), which is translated into an essential viral transcriptional regulatory protein, was detected 30 min after dexamethasone treatment. Ninety minutes after dexamethasone treatment bICP4 and, to a lesser extent, bICP0 RNA were detected in PT. All lytic cycle viral transcripts were detected within 3 h after dexamethasone treatment. Surprisingly, the latency related (LR) gene, the only viral gene abundantly expressed in latently infected TG neurons, was not detected in PT during latency. In TG neurons, bICP0 and the viral tegument protein VP16 are expressed before bICP4 during reactivation, suggesting distinct viral regulatory genes mediate reactivation from latency in PT versus TG neurons. Finally, these studies confirm PT is a biologically relevant site for BoHV-1 latency, reactivation from latency, and virus transmission. IMPORTANCE BoHV-1, a neurotropic herpesvirus, establishes, maintains, and reactivates from latency in neurons. BoHV-1 DNA is also detected in pharyngeal tonsil (PT) from latently infected calves. RNA-sequencing studies revealed the viral infected cell protein 4 (bICP4) RNA was expressed in PT of latently infected calves within 30 min after dexamethasone was used to initiate reactivation. As expected, bICP4 RNA was not detected during latency. All lytic cycle viral genes were expressed within 3 h after dexamethasone treatment. Conversely, bICP0 and the viral tegument protein VP16 are expressed prior to bICP4 in trigeminal ganglionic neurons during reactivation. The viral latency related gene, which is abundantly expressed in latently infected neurons, was not abundantly expressed in PT during latency. These studies provide new evidence PT is a biologically relevant site for BoHV-1 latency and reactivation. Finally, we predict other alphaherpesvirinae subfamily members utilize PT as a site for latency and reactivation.

Time-course profiling of bovine alphaherpesvirus 1.1 transcriptome using multiplatform sequencing

Long-read sequencing (LRS) has become a standard approach for transcriptome analysis in recent years. Bovine alphaherpesvirus 1 (BoHV-1) is an important pathogen of cattle worldwide. This study reports the profiling of the dynamic lytic transcriptome of BoHV-1 using two long-read sequencing (LRS) techniques, the Oxford Nanopore Technologies MinION, and the LoopSeq synthetic LRS methods, using multiple library preparation protocols. In this work, we annotated viral mRNAs and non-coding transcripts, and a large number of transcript isoforms, including transcription start and end sites, as well as splice variants of BoHV-1. Our analysis demonstrated an extremely complex pattern of transcriptional overlaps.

Distinctive features of bovine alphaherpesvirus types 1 and 5 and the virus-host interactions that might influence clinical outcomes

Bovine herpesvirus types 1 (BoHV-1) and 5 (BoHV-5) are two closely related alphaherpesviruses. BoHV-1 causes several syndromes in cattle, including respiratory disease and sporadic cases of encephalitis, whereas BoHV-5 is responsible for meningoencephalitis in calves. Although both viruses are neurotropic, they differ in their neuropathogenic potential. This review summarizes the findings on the specific mechanisms and pathways known to modulate the pathogenesis of BoHV-1 and BoHV-5, particularly in relation to respiratory and neurological syndromes, which characterize BoHV-1 and BoHV-5 infections, respectively.

Specific transcriptional and post-transcriptional regulation of the major immediate early ICP4 gene of GaHV-2 during the lytic, latent and reactivation phases

Transcriptional and post-transcriptional mechanisms are involved in the switch between the lytic, latent and reactivation phases of the viral cycle in herpesviruses. During the productive phases, herpesvirus gene expression is characterized by a temporally regulated cascade of immediate early (IE), early (E) and late (L) genes. In alphaherpesviruses, the major product of the IE ICP4 gene is a transcriptional regulator that initiates the cascade of gene expression that is essential for viral replication. In this study, we redefine the infected cell protein 4 (ICP4) gene of the oncogenic Marek's disease virus (MDV or gallid herpesvirus 2) as a 9438 nt gene ended with four alternative poly(A) signals and controlled by two alternative promoters containing essentially ubiquitous functional response elements (GC, TATA and CCAAT boxes). The distal promoter is associated with ICP4 gene expression during the lytic and the latent phases, whereas the proximal promoter is associated with the expression of this gene during the reactivation phase. Both promoters are regulated by DNA methylation during the viral cycle and are hypermethylated during latency. Transcript analyses showed ICP4 to consist of three exons and two introns, the alternative splicing of which is associated with five predicted nested ICP4ORFs. We show that the ICP4 gene is highly and specifically regulated by transcriptional and post-transcriptional mechanisms during the three phases of the GaHV-2 viral cycle, with a clear difference in expression between the lytic phase and reactivation from latency in our model.

Functional Characterization of the Serine-Rich Tract of Varicella-Zoster Virus IE62

The immediate early 62 protein (IE62) of varicella-zoster virus (VZV), a major viral trans-activator, initiates the virus life cycle and is a key component of pathogenesis. The IE62 possesses several domains essential for trans-activation, including an acidic trans-activation domain (TAD), a serine-rich tract (SRT), and binding domains for USF, TFIIB, and TATA box binding protein (TBP). Transient-transfection assays showed that the VZV IE62 lacking the SRT trans-activated the early VZV ORF61 promoter at only 16% of the level of the full-length IE62. When the SRT of IE62 was replaced with the SRT of equine herpesvirus 1 (EHV-1) IEP, its trans-activation activity was completely restored. Herpes simplex virus 1 (HSV-1) ICP4 that lacks a TAD very weakly (1.5-fold) trans-activated the ORF61 promoter. An IE62 TAD-ICP4 chimeric protein exhibited trans-activation ability (10.2-fold), indicating that the IE62 TAD functions with the SRT of HSV-1 ICP4 to trans-activate viral promoters. When the serine and acidic residues of the SRT were replaced with Ala, Leu, and Gly, trans-activation activities of the modified IE62 proteins IE62-SRTΔSe and IE62-SRTΔAc were reduced to 46% and 29% of wild-type activity, respectively. Bimolecular complementation assays showed that the TAD of IE62, EHV-1 IEP, and HSV-1 VP16 interacted with Mediator 25 in human melanoma MeWo cells. The SRT of IE62 interacted with the nucleolar-ribosomal protein EAP, which resulted in the formation of globular structures within the nucleus. These results suggest that the SRT plays an important role in VZV viral gene expression and replication.

IMPORTANCE

The immediate early 62 protein (IE62) of varicella-zoster virus (VZV) is a major viral trans-activator and is essential for viral growth. Our data show that the serine-rich tract (SRT) of VZV IE62, which is well conserved within the alphaherpesviruses, is needed for trans-activation mediated by the acidic trans-activation domain (TAD). The TADs of IE62, EHV-1 IEP, and HSV-1 VP16 interacted with cellular Mediator 25 in bimolecular complementation assays. The interaction of the IE62 SRT with nucleolar-ribosomal protein EAP resulted in the formation of globular structures within the nucleus. Understanding the mechanisms by which the TAD and SRT of IE62 contribute to the function of this essential regulatory protein is important in understanding the gene program of this human pathogen.

Full trans-activation mediated by the immediate-early protein of equine herpesvirus 1 requires a consensus TATA box, but not its cognate binding sequence

The immediate-early protein (IEP) of equine herpesvirus 1 (EHV-1) has extensive homology to the IEP of alphaherpesviruses and possesses domains essential for trans-activation, including an acidic trans-activation domain (TAD) and binding domains for DNA, TFIIB, and TBP. Our data showed that the IEP directly interacted with transcription factor TFIIA, which is known to stabilize the binding of TBP and TFIID to the TATA box of core promoters. When the TATA box of the EICP0 promoter was mutated to a nonfunctional TATA box, IEP-mediated trans-activation was reduced from 22-fold to 7-fold. The IEP trans-activated the viral promoters in a TATA motif-dependent manner. Our previous data showed that the IEP is able to repress its own promoter when the IEP-binding sequence (IEBS) is located within 26-bp from the TATA box. When the IEBS was located at 100 bp upstream of the TATA box, IEP-mediated trans-activation was very similar to that of the minimal IE(nt -89 to +73) promoter lacking the IEBS. As the distance from the IEBS to the TATA box decreased, IEP-mediated trans-activation progressively decreased, indicating that the IEBS located within 100 bp from the TATA box sequence functions as a distance-dependent repressive element. These results indicated that IEP-mediated full trans-activation requires a consensus TATA box of core promoters, but not its binding to the cognate sequence (IEBS).

Characterization of the nuclear import and export mechanisms of bovine herpesvirus-1 infected cell protein 27

In previous study, we have identified a nuclear localization signal (NLS) and a nucleolar localization signal (NoLS) in bovine herpesvirus-1 (BHV-1) infected cell protein 27 (BICP27), which targets predominantly to the nucleolus. Furthermore, the C-terminal 300 amino acid residues targets exclusively to the cytoplasm, suggesting that BICP27 might contain a nuclear export signal (NES). Amino acid sequence analysis revealed that there is a cluster of leucine-rich residues resembling a NES. Heterokaryon assays demonstrated that BICP27 is capable of shuttling between the nucleus and the cytoplasm of the BHV-1 infected, BICP27 and BICP27-EYFP transfected cells. Deletion mutant analysis revealed that this property is attributed to the leucine-rich NES 299LEELCAARRLSL310. Moreover, the functional NES could mediate transport of a monomer EYFP and a dimer EYFP to the cytoplasm. The nucleocytoplasmic shuttling of BICP27 and the nuclear export of NES-EYFP and NES-dEYFP could be blocked by leptomycin LMB, an inhibitor of the chromosomal region maintenance 1 (CRM1), which is the receptor for exportin-1-dependent nuclear export. In addition, the nuclear import of BICP27 was inhibited by a dominant negative Ran-GTP, namely Ran-GTP Q69L, indicating that BICP27 localized to the nucleus by means of a classic Ran dependent nuclear import mechanism. In conclusion, these results demonstrate that BICP27 shuttles between the nucleus and the cytoplasm by the functional NES and NLS through a CRM1-dependent nuclear export pathway and a Ran dependent nuclear import pathway.

Characterization of the nuclear and nucleolar localization signals of bovine herpesvirus-1 infected cell protein 27

Bovine herpesvirus-1 infected cell protein 27 (BICP27) was detected predominantly in the nucleolus. The open reading frame of BICP27 was fused with the enhanced yellow fluorescent protein (EYFP) gene to investigate its subcellular localization in live cells and BICP27 was able to direct monomeric, dimeric or trimeric EYFP exclusively to the nucleolus. By constructing a series of deletion mutants, the putative nuclear localization signal (NLS) and nucleolar localization signal (NoLS) were mapped to (81)RRAR(84) and (86)RPRRPRRRPRRR(97) respectively. Specific deletion of the putative NLS, NoLS or both abrogated nuclear localization, nucleolar localization or both respectively. Furthermore, NLS was able to direct trimeric EYFP predominantly to the nucleus but excluded from the nucleolus, whereas NoLS targeted trimeric EYFP primarily to the nucleus, and enriched in the nucleolus with faint staining in the cytoplasm. NLS+NoLS directed trimeric EYFP predominantly to the nucleolus with faint staining in the nucleus. Moreover, deletion of NLS+NoLS abolished the transactivating activity of BICP27 on gC promoter, whereas deletion of either NLS or NoLS did not. The study demonstrated that BICP27 is a nucleolar protein, adding BICP27 to the growing list of transactivators which localize to the nucleolus.

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.

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

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

Genome of bovine herpesvirus 5

Here we present the complete genomic sequence of bovine herpesvirus 5 (BHV-5), an alphaherpesvirus responsible for fatal meningoencephalitis in cattle. The 138390-bp genome encodes 70 putative proteins and resembles the alpha2 subgroup of herpesviruses in genomic organization and gene content. BHV-5 is very similar to BHV-1, the etiological agent of infectious bovine rhinotracheitis, as reflected by the high level of amino acid identity in their protein repertoires (average, 82%). The highest similarity to BHV-1 products (>or=95% amino acid identity) is found in proteins involved in viral DNA replication and processing (UL5, UL15, UL29, and UL39) and in virion proteins (UL14, UL19, UL48, and US6). Among the least conserved ( Collapse

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MESH Headings

• Amino Acid Sequence
• Base Sequence
• Genes, Immediate-Early
• Genome, Viral
• Herpesvirus 5, Bovine/genetics
• Molecular Sequence Data
• Promoter Regions, Genetic
• Repetitive Sequences, Nucleic Acid
• Virus Latency/genetics

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Affiliation(s)

G Delhon Plum Island Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Greenport, New York 11944, USA
M P Moraes
Z Lu
C L Afonso
E F Flores
R Weiblen
G F Kutish
D L Rock

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Defective infection of bovine herpesvirus 1 in non-permissive murine cells

The defective growth of bovine herpesvirus I (BHV-1) was analyzed in non-permissive murine embryo fibroblast, BALB/3T3 A31-1-1 (A31) cells. BHV-1 was able to attach and penetrate into A31 cells at similar levels that were seen in semi-permissive cells. Once penetrated into A31 cells, BHV-1 was efficiently transported to nuclei, but the onset of expression of immediate early (IE) protein and viral DNA replication was not observed. These data suggest that the viral replication of BHV-1 in A31 cells is arrested at the point prior to the expression of IE proteins.

Growth behavior of bovine herpesvirus-1 in permissive and semi-permissive cells

Bovine herpesvirus-1 (BHV-1) can replicate well in bovine-derived cell lines such as Madin Darby bovine kidney (MDBK) but grows poorly in hamster lung (HmLu-1). Virus replication, DNA synthesis, and immediate-early gene expression are severely restricted in HmLu-1. We compared adsorption and penetration of BHV-1 in permissive MDBK and semi-permissive HmLu-1 cells. At a low multiplicity of infection, BHV-1 attached to permissive MDBK cells twice as much as to HmLu-1. The presence of heparin inhibited the attachment of BHV-1 to MDBK cells by about 60%, but over 90% of the attachment was inhibited in HmLu-1. To investigate the penetration of BHV-1, we performed the quantitative measurement of viral DNA by quantitative competitive (QC)PCR in infected cells. In MDBK cells, virions attached to the cell surface, penetrated into the cells and were transported to the nucleus. However in HmLu-1, only a small fraction of the virions attached to the cell surface were allowed to penetrate. Our results indicated that the replication of BHV-1 in semi-permissive HmLu-1 was not dramatically restricted at one certain point but at some various stages including adsorption and penetration.

Characterization of promoters integrated in the genome of bovine herpesvirus-1 (BHV-1)

Bovine herpesvirus-1 (BHV-1) has been used as a vector of live recombinant vaccines for cattle which express the genes of other pathogens. Because of the importance of the choice of the promoter which allows the efficient expression of the foreign genes in the BHV-1 vector, we compared the relative efficacy of various promoters integrated in the BHV-1 genome. The promoter sequences of the BHV-1 thymidine kinase (tk), gB, gC, SV40 early, and pseudorabies virus (PRV) immediate early (IE) genes were placed at the upstream of the open reading frame of the chloramphenycol acetyl transferase (CAT) gene and the promoter-CAT sequences were integrated into the tk gene of BHV-1 by homologous recombination. The promoter activity was assayed by measuring the CAT activity in the extracts of Madin Darby bovine kidney (MDBK) cells infected with the recombinant BHV-1. The PRV IE promoter was activated earlier and maintained at a higher level activity than the BHV-1 gB or gC promoters throughout the most of the growth phase of BHV-1. At the late phase, however, the activities of the BHV-1 gB and gC promoters reached the higher level. The BHV-1 tk promoter activity was low and the SV40 early promoter was hardly activated when integrated into the BHV-1 genome. promoter, recombinant BHV-1.

The polyserine tract of herpes simplex virus ICP4 is required for normal viral gene expression and growth in murine trigeminal ganglia

ICP4 of herpes simplex virus (HSV) is essential for productive infection due to its central role in the regulation of HSV transcription. This study identified a region of ICP4 that is not required for viral growth in culture or at the periphery of experimentally inoculated mice but is critical for productive growth in the trigeminal ganglia. This region of ICP4 encompasses amino acids 184 to 198 and contains 13 nearly contiguous serine residues that are highly conserved among the alphaherpesviruses. A mutant in which this region is deleted (DeltaSER) was able to grow on the corneas of mice and be transported back to the trigeminal ganglia. DeltaSER did not grow in the trigeminal ganglia but did express low levels of several immediate-early (ICP4 and ICP27) and early (thymidine kinase [tk] and UL42) genes. It expressed very low levels of the late gC gene and did not appear to replicate DNA. This pattern of gene expression was similar to that observed for a tk mutant, dlsptk. Both DeltaSER and dlsptk expressed higher levels of the latency-associated transcript (LAT) per genome earlier in infected ganglia than did the wild-type virus, KOS. However, infected ganglia from all three viruses accumulated the same level of LAT per genome at 30 days postinfection (during latency). The data suggest that the polyserine tract of ICP4 provides an activity that is required for lytic infection in ganglia to progress to viral DNA synthesis and full lytic gene expression. In the absence of this activity, higher levels of LAT per genome accumulate earlier in infection than with wild-type virus.

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.

Analysis of bovine herpesvirus 1 transcripts during a primary infection of trigeminal ganglia of cattle

During an infection of nonneuronal cells, bovine herpesvirus 1 (BHV-1) gene expression proceeds in a well-defined cascade. Products of immediate-early (IE) genes are expressed first, and they activate expression of early (E) and late (L) genes. Although the same cascade is assumed to occur during an infection of neurons in trigeminal ganglia (TG) of cattle, no experimental data is available to support this hypothesis. Consequently, we analyzed BHV-1 gene expression in bovine TG at 1, 2, 4, 7, and 15 days postinfection (dpi). Infectious virus was detected in ocular swabs from 1 to 7 dpi but not 15 dpi. By reverse transcription (RT)-PCR, IE (bICP4), E (thymidine kinase, ribonucleotide reductase [RR]), L (glycoprotein C, and alpha trans-inducing factor), and dual-kinetic (bICP0 and bICP22) transcripts were analyzed. When cDNA synthesis was primed with random hexamers, IE and E transcripts were detected at the same time. However, full-length and poly(A)+ (FL&P) RR or bICP22 RNAs were detected before FL&P IE RNAs. Furthermore, FL&P IE transcripts were not detected until viral DNA increased in TG. IE transcripts were detected before E or L RNAs when rabbit kidney cells were infected with a low multiplicity of infection and the same RT-PCR detection method was used. These studies suggested that expression of full-length and polyadenylated IE transcripts in trigeminal ganglia was not efficient compared to that of RR and bICP22 transcripts.

Identification of a promoter-specific transactivation domain in the herpes simplex virus regulatory protein ICP4

ICP4 is expressed during the immediate-early phase of infection by herpes simplex virus (HSV) and activates transcription of viral genes during subsequent phases of productive infection. Several members of the alpha-herpesvirus family encode regulatory proteins that have extensive homology with ICP4 and exhibit a transactivation domain (TAD) at the N terminus. The portions of ICP4 required for nuclear localization, DNA binding, and dimerization have been defined, but a domain that is specifically required for transactivation has not been identified. We have defined a promoter-specific ICP4 TAD by analysis of the activity of GAL4-ICP4 fusion proteins cotransfected into HeLa cells with a luciferase reporter gene linked to a promoter with five GAL4 binding sites. The transactivation activity of GAL4-ICP4 hybrids is located entirely within the first 139 residues of ICP4 and is significantly less potent than the activity of GAL4-TAD hybrids derived from ICP4 homologs. ICP4 residues 97 to 109 are a critical component of this N-terminal TAD. Transient transfection assays performed with nonfusion forms of ICP4 and luciferase genes linked to the HSV glycoprotein D (gD) or thymidine kinase (tk) promoter revealed that ICP4 residues 97 to 109 are required for induction of the gD promoter but are not required for induction of the tk promoter. Comparative experiments with ICP4 homologs revealed that the pseudorabies virus TAD is a potent activator of the gD promoter and a weak activator of the tk promoter. Complementation assays revealed that loss of ICP4 residues 97 to 109 reduced the yield of virus from infected cells nearly 500-fold compared to wild-type ICP4. We conclude that ICP4 residues 97 to 109 are a core component of a promoter-specific transactivation domain that is required for efficient replication of herpes simplex virus.

Recombinant bovine herpesvirus-1 (BHV-1) lacking transactivator protein BICPO entails lack of glycoprotein C and severely reduced infectivity

The immediate-early transactivator protein BICPO is a key regulatory element of bovine herpesvirus 1 (BHV-1) replication based on transient expression assays. To examine BICPO function in the context of the viral genome, we created recombinant BHV-1 expressing beta-galactosidase instead of BICPO. To complement the defect, a neomycin resistant MDBK cell line (M164) expressing BICPO was established, permitting selection of a blue-staining BHV-1 recombinant (A2G2). Southern blot and PCR analysis confirmed that the BICPO gene was interrupted by the beta-galactosidase gene and that wt progeny was absent. Compared with wt BHV-1, A2G2 reached lower titers in M164 cells but replicated with similar kinetics. Once isolated, A2G2 also grew in MDBK cells although the titer was reduced a further 10-fold and the virus remained strongly cell-associated. Thus, BICPO is not absolutely required for replication in cell culture. Gene expression of A2G2 was investigated by Western blots and immunofluorescence. Surprisingly, not only was BICPO absent, but glycoprotein C (gC) was also missing. Other viral genes were expressed normally. Semiquantitative PCR showed that A2G2 produced similar amounts of viral DNA as wt but a much smaller number of infectious particles. Cotransfection of A2G2 DNA and a plasmid containing the BICPO gene yielded revertant virus with fully restored wt properties. We conclude that BICPO is required for gC expression, and that the missing gC partly accounts for the reduced A2G2 infectivity.

Molecular virology of ruminant herpesviruses

Molecular virology has served to establish bovine herpesvirus 1 (BHV-1) as the prototype member of ruminant herpesviruses. Based on the genomic sequence of the virus, we aim to identify and characterize virus-specified components, to explain their concerted action, and to predict how the chain of events during the lytic and latent phases of the viral life cycle may be interrupted. The nucleotide sequence of the BHV-1 genome (136 kb) has just been completed by international cooperation (July 1995; except for a small gap in UL36). It comprises 67 unique genes and 2 genes, both duplicated, in the inverted repeats. In general, these genes exhibit strong homology at the amino acid sequence level to those of other alphaherpesviruses (HSV-1, VZV, EHV-1) and are arranged in similar order. A few genes are peculiar to only one or two herpesviruses, e.g. in BHV-1 the circ, UL0.5, UL3.5 and US1.5 genes. Not long ago, the repertoire of BHV-1 proteins under study was restricted to the three major glycoproteins (gB, gC, and gD) and thymidine kinase. The repertoire is now growing rapidly and includes 7 additional glycoproteins (gE, gI, gH, gL, gG, gK and gM), a number of enzymes (e.g. ribonucleotide reductase, DNA Polymerase, dUTPase), and a group of regulatory proteins (BICPO, 4, 22, and 27, alpha TIF). Investigations into the functions of these proteins and comparison with their counterparts in other herpesviruses should reveal which are useful targets for diagnosis, prevention or antiviral treatment. Recombinant viruses containing deletions or replacements of individual genes are being created, aiming at vaccine development and insights into pathogenesis, notably latency, neurotropism, and interference with host functions. Molecular analysis of other ruminant herpesviruses is much less advanced. Over a dozen virus species have been described; most share basic properties with BHV-1 and may be classified as alphaherpesviruses. The gammaherpesviruses are represented by the proposed agent of malignant catarrhal fever, alcelaphine herpesvirus 1, and by bovine herpesvirus 4, whose partial sequences exhibit similarity to herpesvirus saimiri.

Nuclear localization and transcriptional activation activities of truncated versions of the immediate-early gene product of equine herpesvirus 1

The equine herpesvirus 1 (EHV-1) immediate-early (IE) gene product encodes a nuclear regulatory protein capable of negatively autoregulating its own promoter, transactivating representative EHV-1 early promoters, and acting in a concerted fashion with accessory EHV-1 regulatory factors to transactivate EHV-1 late promoters. To identify IE amino acid sequences involved in nuclear localization and to examine the contribution of C-terminal portions of the IE polypeptide to transactivation, vectors that express various carboxyterminally truncated IE polypeptides were constructed. It is demonstrated that amino acids 963 through 970 of the 1,487-amino-acid IE protein are required for efficient localization of the truncated IE polypeptides to the nuclei of transfected cells. In addition, it is demonstrated that the first 970 amino acids of the IE gene product are sufficient to transactivate the EHV-1 thymidine kinase promoter to significant levels (i.e., approximately 40% of the level of wild-type activation).

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 of the bovine herpesvirus 1 circ protein, a myristylated and virion-associated polypeptide which is not essential for virus replication in cell culture

We have recently reported immediate-early (IE) transcription over covalently joined genome ends of bovine herpesvirus 1 (BHV-1). A spliced 1.5-kb IE RNA (IER1.5) is coterminal with an unspliced 1.1-kb late RNA (LR1.1) which is transcribed from the left end of the genome. Sequence analysis reveals an open reading frame common to IER1.5 and LR1.1 predicted to encode the 247-amino-acid circ polypeptide. This paper reports on the identification of circ as a protein. Using a rabbit antiserum raised against a synthetic oligopeptide representing the carboxy terminus of the predicted circ polypeptide for Western blot (immunoblot) analyses and immunofluorescence assays, we identified a 34-kDa virion-associated protein which accumulated in the cytoplasm of infected cells. To confirm that LR1.1 indeed encoded the 34-kDa polypeptide, we inserted a DNA fragment containing circ coding sequences into the Autographa californica baculovirus genome. A group of recombinant polypeptides with sizes of 32, 34, and 35 kDa were identified by their reactivity with the antipeptide serum. Chicken egg yolk antibodies raised against total proteins of insect cells infected with the recombinant baculovirus identified the 34-kDa circ protein specified by BHV-1. The recombinant circ polypeptides and the circ protein specified by BHV-1 were both myristylated, as determined by radiolabeling with [3H]myristic acid. It was noted that the circ gene could be deleted from the BHV-1 genome without impairing virus replication in cell culture.

Characterization of Marek's disease virus insertion and deletion mutants that lack US1 (ICP22 homolog), US10, and/or US2 and neighboring short-component open reading frames

We report the characterization of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome. A deletion mutant (GA delta 4.8lac) lacks 4.8 kbp of US region DNA, the deleted segment having been replaced by the lacZ gene of Escherichia coli. This deletion results in the loss of the MDV-encoded US1, US10, and US2 homologs of herpes simplex virus type 1, as well as three putative MDV-specific genes, Sorf1, Sorf2, and Sorf3. Two mutants containing lacZ insertions in the US1 and US10 genes have been constructed, and we have previously reported a US2lac insertion mutant (J. L. Cantello, A. S. Anderson, A. Francesconi, and R. W. Morgan, J. Virol. 65:1584-1588, 1991). The isolation of these mutants indicates that the relevant genes are not required for growth of MDV in chicken embryo fibroblasts. The mutants had early growth kinetics indistinguishable from those of their parent viruses; however, 5 to 7 days after being plated, the US1 insertion mutant (US1lac) and the GA delta 4.8lac deletion mutant showed a 5- to 10-fold decrease in virus growth. This decrease in virus accumulation correlated with a 30 to 50% decrease in plaquing efficiency when these viruses were plated onto established versus fresh chicken embryo fibroblast monolayers compared with a 10 to 15% decrease seen for the parent viruses and for the US10lac or US2lac insertion mutants. Finally, GA delta 4.8lac could be reisolated from chickens, indicating that the deleted genes are not required for the infection of chickens following intra-abdominal inoculation of an attenuated serotype 1 MDV.

Strong transcriptional activators isolated from viral DNA by the 'activator trap', a novel selection system in mammalian cells

Transcription factors often contain activation domains that interact with the basic transcription machinery. We have developed a functional screening strategy in mammalian cells to selectively isolate activation domains from a library of random DNA inserts. For this, sonicated DNA fragments are cloned next to the DNA binding domain of GAL4 factor in a plasmid that also contains the SV40 origin of replication. Pools of fusion protein clones are transfected into CV-1-5GT monkey cells containing an SV40 T antigen gene under the control of a promoter with GAL4 binding sites. Plasmids that express functional transactivating fusion proteins activate the T antigen gene, thus promoting selective amplification of the plasmid in the mammalian host cell line. Using this method, we were able to select strong enhancer-type activation domains from the immediate early regions of two herpesviruses, namely pseudorabies virus and bovine herpesvirus 1. In both cases, the activation domains selected were homologues of the ICP4 regulatory protein of herpes simplex virus. The activation domain from pseudorabies virus is four times stronger than the activation domain of herpes simplex virus protein VP16 (Vmw65), making it the strongest activation domain characterized so far. This activator trap method should be useful for precisely localizing activation domain(s) in known factors, or to identify mammalian transcriptional adaptors that do not bind DNA and which may escape conventional detection methods.

Protein and DNA elements involved in transactivation of the promoter of the bovine herpesvirus (BHV) 1 IE-1 transcription unit by the BHV alpha gene trans-inducing factor

In herpes simplex virus (HSV)-infected cells, the transcription of immediate-early (alpha) genes is regulated by a virion component, the alpha gene trans-inducing factor (alpha TIF). This protein forms a complex with cellular factors and TAATGARAT motifs present in one or more copies in the promoters of all alpha genes. We have characterized the bovine herpesvirus 1 (BHV-1) homolog of this protein. Like its HSV counterpart, the BHV alpha TIF was synthesized in the later stages of infection and could be demonstrated to be a component of purified virions. In transient expression assays, BHV alpha TIF was a strong transactivator and stimulated the activity of IE-1, the major BHV-1 alpha gene promoter, with an efficiency comparable to that of HSV alpha TIF. This stimulation was largely dependent on a TAATGAGCT sequence present in a single copy in IE-1, and BHV alpha TIF, in conjunction with cellular factors, formed a complex with oligonucleotides containing this sequence. Despite these similarities between the two alpha TIFs, our preliminary observations suggest that the proteins may activate transcription by different mechanisms. Although BHV alpha TIF strongly transactivated IE-1, it differed from its HSV counterpart in that the carboxyl terminus of BHV alpha TIF, when fused to the DNA-binding domain of GAL4, was a relatively poor stimulator of a promoter containing GAL4-binding sites. Also unlike HSV alpha TIF, removal of the carboxyl terminus of BHV alpha TIF reduced but did not eliminate the ability of the protein to transactivate IE-1. These results are discussed in view of the structural similarities and differences among the alpha TIFs of alphaherpes-viruses.

Identification and zinc dependence of the bovine herpesvirus 1 transactivator protein BICP0

Bovine herpesvirus 1 (BHV-1) specifies and unspliced early 2.6-kb RNA (ER2.6) which is 3' coterminal with exon 2 of the 2.9-kb immediate-early (IE) RNA. The two transcripts have a common open reading frame (676 codons). The predicted protein, designated BHV-1 infected cell protein 0 (BICP0), contains a zinc finger domain with homology to ICP0 of herpes simplex virus type 1 and protein 61 of varicella-zoster virus, and depending on the promoter, it acts as a strong activator or as a repressor in transient expression assays. In situ immunoadsorbent assays using antisera against synthetic oligopeptides demonstrated that BICP0 accumulates in nuclei of BHV-1-infected cells, as expected for an IE gene product involved in gene regulation. Western blots (immunoblots) revealed a BHV-1-specific 97-kDa protein which was detectable during the IE phase and also at later periods of infection, indicating that the kinetics of BICP0 synthesis is consistent with the switch from IER2.9 to ER2.6. To confirm that ER2.6 encoded the 97-kDa BICP0 protein, a DNA fragment containing BICP0-coding sequences was inserted into the Autographa californica baculovirus genome. A recombinant protein, identified by its reactivity with antipeptide sera, exhibited the same electrophoretic mobility as BICP0 specified by BHV-1. We microinjected Xenopus oocytes with a BICP0 effector plasmid and a promoter-chloramphenicol acetyltransferase plasmid. BICP0-induced stimulation of this promoter was strongly reduced when intracellular zinc was chelated by thionein, indicating that the effect of BICP0 is zinc dependent.