Pseudorabies virus immediate-early gene overlaps with an oppositely oriented open reading frame: characterization of their promoter and enhancer regions

Effects of deletion of the early protein 0 gene of pseudorabies virus on the overall viral gene expression

Real-time RT-PCR analysis was applied to evaluate the impact of deletion of the early protein 0 (EP0) gene of pseudorabies virus (PRV) on the global expression of the viral transcripts during lytic infection in cultured porcine kidney cells. Our analysis showed that EP0 exerted an inhibitory effect on the transcription of the PRV genes in the early stage of infection, and alternating stimulatory and inhibitory effects on the viral gene expressions in the late stage of infection. The data also suggested that a general function of EP0 might be to reverse the kinetics of expression of early viral genes. We also observed that EP0 facilitated the development of correlations in the transcription kinetics between the immediate early 180 gene and the PRV transcripts, indicating that a major function of EP0 could be to modify the effects of the IE180 protein on the PRV transcriptome.

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

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

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

The effects of viral load on pseudorabies virus gene expression

Background

Herpesvirus genes are classified into distinct kinetic groups on the basis of their expression dynamics during lytic growth of the virus in cultured cells at a high, typically 10 plaque-forming units/cell multiplicity of infection (MOI). It has been shown that both the host response and the success of a pathogen are dependent on the quantity of particles infecting an organism. This work is a continuation of an earlier study [1], in which we characterized the overall expression of PRV genes following low-MOI infection. In the present study, we have addressed the question of whether viral gene expressions are dependent on the multiplicity of infection by comparing gene expressions under low and high-MOI conditions.

Results

In the present study, using a real-time RT-PCR assay, we address the question of whether the expression properties of the pseudorabies virus (PRV) genes are dependent on the number of virion particles infecting a single cell in a culture. Our analysis revealed a significant dependence of the gene expression on the MOI in most of these genes. Specifically, we found that most of the examined viral genes were expressed at a lower level at a low MOI (0.1) than at a high MOI (10) experiment in the early stage of infection; however, this trend reversed by six hour post-infection in more than half of the genes. Furthermore, in the high-MOI infection, several PRV genes substantially declined within the 4 to 6-h infection period, which was not the case in the low-MOI infection. In the low-MOI infection, the level of antisense transcript (AST), transcribed from the antiparallel DNA strand of the immediate-early 180 (ie180) gene, was comparable to that of ie180 mRNA, while in the high-MOI experiment (despite the 10 times higher copy number of the viral genome in the infected cells) the amount of AST dropped by more than two log values at the early phase of infection. Furthermore, our analysis suggests that adjacent PRV genes are under a common regulation. This is the first report on the effect of the multiplicity of infection on genome-wide gene expression of large DNA viruses, including herpesviruses.

Conclusion

Our results show a strong dependence of the global expression of PRV genes on the MOI. Furthermore, our data indicate a strong interrelation between the expressions of ie180 mRNA and AST, which determines the expression properties of the herpesvirus genome and possibly the replication strategy (lytic or latent infection) of the virus in certain cell types.

Identification of transcripts and protein products of the UL31, UL37, UL46, UL47, UL48, UL49 and US4 gene homologues of avian infectious laryngotracheitis virus

In the present study, the transcription and protein expression of seven genes of infectious laryngotracheitis virus (ILTV) were investigated: UL31 and UL37 possess homologues in all known avian and mammalian herpesviruses, whereas UL46–UL49 and US4 are only conserved in most alphaherpesviruses. A peculiarity of the ILTV genome is the translocation of UL47 from the unique long region to a position upstream of US4 within the unique short region. Northern blot analyses revealed that all of the analysed genes were transcribed most abundantly during the late (γ) phase of replication, but the only true late (γ2) gene was UL47. Using monospecific rabbit antisera, the protein products of all of the genes could be detected and localized in ILTV-infected cells. Considerable amounts of the UL31, UL47 and UL48 gene products were found in the cell nuclei, whereas the other proteins were restricted largely to the cytoplasm. Like the respective tegument proteins of other herpesviruses, the UL37 and UL46–UL49 gene products of ILTV were incorporated into virus particles, whereas the UL31 protein and the glycoprotein encoded by US4 (gG) were not detectable in purified virions. It was also demonstrated that the UL48 protein of ILTV is able to activate an alphaherpesvirus immediate-early gene promoter, which is also a typical feature of other UL48 homologues. Taken together, these results indicate that the functions of all of the investigated ILTV proteins are related to those of their homologues in other alphaherpesviruses.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.

Novel tracing paradigms--genetically engineered herpesviruses as tools for mapping functional circuits within the CNS: present status and future prospects

The mammalian CNS is composed of an extremely complex meshwork of highly ordered interconnections among billions of neurons. To understand the diverse functions of this neuronal network we need to differentiate between functionally related and nonrelated elements. A powerful labeling method for defining intricate neural circuits is based on the utilization of neurotropic herpesviruses, including pseudorabies virus and herpes simplex virus type 1. The recent development of genetically engineered tracing viruses can open the way toward the conception of novel tract-tracing paradigms. These new-generation tracing viruses may facilitate the clarification of problems, which were inaccessible to earlier approaches. This article first presents a concise review of the general aspects of neuroanatomical tracing protocols. Subsequently, it discusses the molecular biology of alpha-herpesviruses, and the genetic manipulation and gene expression techniques that are utilized for the construction of virus-based tracers. Finally, it describes the current utilization of genetically modified herpesviruses for circuit analysis, and the future directions in their potential applications.

Lytic infection of pseudorabies virus in the presence of spermine, spermidine, or DFMO

The effects of polyamines (spermine or spermidine) and DFMO (an ornithine decarboxylase inhibitor) on the infection of LM (tk-) cells by pseudorabies virus (PRV) were investigated. Results from radioactive methionine labeling showed that the synthesis of viral proteins was not affected; however, the expression of a distinctive cellular protein ( approximately 27 kDa) was induced after the treatment of spermine or spermidine. Using plaque assay, we found that the plaque formation of PRV was not affected by these three reagents either. Furthermore, the effects of these drugs on the transcription of PRV immediate-early gene (IE) promoter were examined by CAT assay, and results showed weak stimulation of transcription by these drugs. Taken together, our results demonstrated that lytic infection of PRV was not influenced by addition of exogenous polyamines or depletion of endogenous polyamines; this conclusion was similar to earlier studies by using herpes simplex virus.

Temperature-dependent conformational changes in herpes simplex virus ICP4 that affect transcription activation

The C-terminal 500 amino acids of herpes simplex virus type 1 ICP4 are required for full activator function and viral growth and are known to participate in interactions consistent with the role of ICP4 as an activator of transcription. Oligonucleotide mutagenesis was used to target stretches of amino acids that are conserved with the ICP4 analogs of other alphaherpesviruses and were also predicted to be exposed on the surface of the molecule. Seven mutants were isolated that possessed one to three amino acid changes to the residue alanine in four regions between residues 1000 and 1200. The mutants generated were analyzed first in transfection assays and subsequently after introduction into the viral genome. A number of phenotypes representing different degrees of functional impairment were observed. In transient assays conducted at 37 degrees C, mutant M2 was indistinguishable from wild-type ICP4. Mutants M6 and M7 were marginally impaired. M3, M4, and M5 were more significantly impaired but still able to activate transcription, and M1 was completely impaired. In the context of the viral genome, M1, M3, and M7 were found to be temperature sensitive for growth. All three overproduced immediate-early (IE) proteins at the nonpermissive temperature (NPT). M3 and M7 produced early but not late proteins, and M1 produced neither early nor late proteins, at the NPT. The ICP4 proteins synthesized by all of the mutants tested were able to bind to specific ICP4 binding sites in electrophoretic mobility shift experiments. However, the DNA-protein complexes formed with the ICP4 from M1, M3, or M7 produced at the NPT possessed altered mobility. These complexes were not supershifted by a monoclonal antibody that recognizes an epitope in the C terminus; however, they were supershifted by a monoclonal antibody that recognizes the N terminus. The results suggest that the mutant forms of ICP4, while able to bind to DNA, are conformationally altered at the NPT, thus impairing the ability of the protein to activate transcription to different extents. The complete lack of ICP4 function characteristic of the M1 protein, and the inability of all the mutants to attenuate IE gene expression, suggest that the mutations additionally affect functions of the N terminus to different extents.

Construction of recombinant pseudorabies viruses optimized for labeling and neurochemical characterization of neural circuitry

In this study we have modified the neuroinvasiveness of pseudorabies virus strain Bartha, a commonly utilized trans-synaptic tract-tracer. In addition, we sought to facilitate detection of cellular mRNAs in neurons infected with the virus. In order to modify spreading characteristics, we inserted the lacZ or the GFP (green fluorescent protein) genes into the genomic loci containing the putative latency-associated transcript promoter (P(LAT2)), resulting in the disruption of the promoter function. Following rat kidney injection, mutant viruses labeled central autonomic neurons in a slower and much more restricted manner than the parent Bartha strain. Since both reporter genes were controlled by the human cytomegalovirus immediate early (IE) 1 promoter, they exhibited IE expression kinetics. This property proved to be important for the co-detection of reporter proteins with neuronal mRNAs, readily detected at early but not at late stage of infection, as shown in tyrosine-hydroxylase expressing A5 catecholaminergic neurons and in serotonin transporter expressing raphe magnus neurons.

Suppression of promoter activity of the LAT gene by IE180 of pseudorabies virus

The latency-associated transcript (LAT) gene is the only viral genomic region that is abundantly transcribed during pseudorabies virus (PrV) latent infection. The mechanism of reactivation of PrV from latency remains unknown. To analyze the regulation mechanism of the LAT promoter, we constructed a series of recombinant vectors in which various sequences upstream of LAT were linked to the chloramphenicol acetyltransferase (CAT) gene. Transcriptional efficiency was examined by cotransfection with plasmids carrying the PrV IE, EP0, or gD gene, respectively. Results showed that the activity of PrV LAT promoter was dramatically repressed by the IE180 protein and a TATA box and a putative IE180 binding site within the promoter were involved in this repression. To dissect the functional domains of IE180, we compared the relative repressive abilities of IE180 variants to the LAT promoter by transient transfection assays. Mutational analysis demonstrated that almost the whole IE180 (amino acid residues 1-1440) are essential for its repression to LAT promoter. To explore the possible mechanism of repression, an electrophoretic mobility shift assay (EMSA) using nuclear extracts from neuronal cells was performed and formation of protein-DNA complexes between IE180 and the oligonucleotide probe (-46 to -19, relative to the start site of LAT transcription) was demonstrated. The association of IE180 with the region encompassing the putative IE180 binding site and the TATA box upstream of PrV LAT gene was further confirmed by supershift of EMSA complexes using IE180 specific antibody. Thus, our results suggested that IE180 repressed the LAT promoter via an interaction between IE180, LAT promoter and cellular protein(s).

The UL48 tegument protein of pseudorabies virus is critical for intracytoplasmic assembly of infectious virions

The pseudorabies virus (PrV) homolog of the tegument protein encoded by the UL48 gene of herpes simplex virus type 1 (HSV-1) was identified by using a monospecific rabbit antiserum against a bacterial fusion protein. UL48-related polypeptides of 53, 55, and 57 kDa were detected in Western blots of infected cells and purified virions. Immunofluorescence studies demonstrated that the PrV UL48 protein is predominantly localized in the cytoplasm but is also found in the nuclei of infected cells. Moreover, it is a constituent of extracellular virus particles but is absent from primary enveloped perinuclear virions. In noncomplementing cells, a UL48-negative PrV mutant (PrV-DeltaUL48) exhibited delayed growth and significantly reduced plaque sizes and virus titers, deficiencies which were corrected in UL48-expressing cells. RNA analyses indicated that, like its HSV-1 homolog, the PrV UL48 protein is involved in regulation of immediate-early gene expression. However, the most salient effect of the UL48 gene deletion was a severe defect in virion morphogenesis. Late after infection, electron microscopy of cells infected with PrV-DeltaUL48 revealed retention of newly formed nucleocapsids in the cytoplasm, whereas enveloped intracytoplasmic or extracellular complete virions were only rarely observed. In contrast, capsidless particles were produced and released in great amounts. Remarkably, the intracytoplasmic capsids were labeled with antibodies against the UL36 and UL37 tegument proteins, whereas the capsidless particles were labeled with antisera directed against the UL46, UL47, and UL49 tegument proteins. These findings suggested that the UL48 protein is involved in linking capsid and future envelope-associated tegument proteins during virion formation. Thus, like its HSV-1 homolog, the UL48 protein of PrV functions in at least two different steps of the viral life cycle. The drastic inhibition of virion formation in the absence of the PrV UL48 protein indicates that it plays an important role in virion morphogenesis prior to secondary envelopment of intracytoplasmic nucleocapsids. However, the UL48 gene of PrV is not absolutely essential, and concomitant deletion of the adjacent tegument protein gene UL49 also did not abolish virus replication in cell culture.

Cloning and regulation of the promoter of pseudorabies virus (TNL strain) glycoprotein E gene

The nucleotide sequence upstream to the glycoprotein E (gE) gene of pseudorabies virus (PrV, TNL strain) was cloned from the genomic virus DNA by polymerase chain reaction (PCR) and its DNA sequences were determined. The DNA segment, which was supposed to contain the gE promoter, was subcloned into a chloramphenicol acetyltransferase (CAT) reporter gene and the resulting plasmid was named pgEp-B-CAT. To examine the promoter function of this upstream sequence of gE gene, we transfected pgEp-B-CAT DNA into L-M cells and the promoter activity was analyzed by CAT assay. Results showed that our DNA fragment could exhibit promoter activity. Furthermore, we transfected L-M cells with pgEp-B-CAT for 48 h, then superinfected cells with pseudorabies virus, and performed CAT assay. It was found that PrV superinfection could slightly enhance the activity of gE promoter, suggesting that factors produced during viral infection could stimulate the promoter. To explore the possible mechanism of regulation at transcriptional level, the pgEp-B-CAT plasmid were cotransfected with eukaryotic vectors expressing viral regulatory proteins IE or EP0, and results indicated that the gE promoter was activated by IE protein whereas it was inhibited by EP0 protein. Moreover, the effect of exogenous IE or EP0 on the protein level of gE in PrV-infected cells was examined; conclusion similar to that of CAT assay were obtained.

Pseudorabies virus-based gene delivery to rat embryonic spinal cord grafts

The construction and application of recombinant pseudorabies viruses (PrVs) for the delivery of beta-galactosidase and/or green fluorescent protein (GFP) genes to rat embryonic spinal cord cells are reported here. These viruses were specifically designed to infect embryonic spinal cord neurons, which can be grafted into a lesioned spinal cord in order to restore the lost functions of the host cord. The recombinant viruses were constructed in two steps. The small subunit of the ribonucleotide reductase (RR) gene was first abolished by a frameshift mutation and an expression cassette containing the lacZ gene alone or together with the GFP gene was then inserted in place of the early protein 0 (EP0) gene of PrV. The reporter gene cassettes were positioned downstream from the PrV latency-associated promoter. Using an ex vivo system, we infected embryonic spinal cord explants with these viruses and found that neither vRREP0lac nor vRREP0lacgfp exerted any cytotoxic effect at all. It was also revealed that these viruses infect embryonic cells with high efficiency, and that infected neurons grafted into the spinal cord express the inserted reporter genes for periods of up to 12 weeks. This system offers a new approach for foreign gene transfer to neurons grafted into the CNS.

Analysis of regulatory functions for the region located upstream from the latency-associated transcript (LAT) promoter of pseudorabies virus in cultured cells

The latency-associated transcript (LAT) promoter of pseudorabies virus (PrV) is unique among the many promoters of the viral genome in that it remains active during the latent state. The regulatory mechanism of PrV LAT gene expression is complex and different between latency and lytic infection of cultured cells. Although two different sequences, LAP1 and LAP2, are thought to be involved in LAT gene expression, the function of the upstream region of the LAT promoter (LAP1 and LAP2) remains an enigma, even in cultured cells. To analyze the function of the upstream region, it is necessary to examine the effects of the upstream sequence on LAT gene expression in the absence of other viral proteins. Transient expression assays were performed by employing a series of reporter plasmids in which various sequences upstream of the LAT promoter (from nucleotide positions -592 to +423 relative to the transcriptional start site of the large latency transcript (LLT)) were linked to the chloramphenicol acetyltransferase (CAT) gene in cells of neuronal and non-neuronal origin. We identified a region (from nucleotide positions -3606 to -1386) that was capable of repressing the LAT promoter activity in Vero cells by analyzing CAT gene expression of the series of reporter plasmids. This effect was not observed in Neuro-2a cells. We have also shown that the LAT promoter activity of the reporter plasmid containing the upstream region was repressed by the immediate-early gene product IE180 in Vero cells, but not in Neuro-2a cells. These results suggest that the upstream region of the LAT promoter may have a role in repressing LAT gene expression in cultured non-neuronal cells.

Replication and virulence of early protein 0 and long latency transcript deficient mutants of the Aujeszky's disease (pseudorabies) virus

Early protein 0 (EP0)-deficient recombinant Aujeszky's disease viruses, Ka-ep0lac and Ba-ep0lac derived from strains Kaplan and Bartha, respectively, were constructed to explore the impact of the mutation on replication, virulence and latency of the virus. Inactivation of the EP0 gene resulted in a mutation of long latency transcript (Cheung et al., 1991) that is located on the complementary DNA strand of EP0 and immediate early protein (IE)175 genes. In infection of immortalized porcine kidney cells, the growth rate and yield of both EP0(-) mutant strains were significantly smaller than that of wild-type virus. Ka-ep0lac was found to be highly virulent, while Ba-ep0lac showed an attenuated phenotype in mice. PCR assay and immunohistochemistry showed that the Ba-ep0lac virus was able to establish latency in the mouse trigeminal ganglia. However, latent virus was not able to reactivate in explant reactivation assays. Accordingly, latent Ba-ep0lac has the potential to be exploited as vectors for the delivery of foreign genes to the nervous system.

Characterization of the replication origin (Ori(S)) and adjoining parts of the inverted repeat sequences of the pseudorabies virus genome

The DNA sequence of a 2.4 kbp fragment located in the internal and terminal inverted repeat sequences of the pseudorabies virus genome determined in this study closes a gap between the previously described genes for the ICP4 and ICP22 homologues. The novel sequence contains no conserved herpesvirus open reading frames. Northern blot and cDNA analyses revealed a viral immediate-early transcript of 1.8 kb, which is spliced by the removal of two small introns close to its 5' end and which presumably represents the mRNA of the downstream open reading frame encoding the ICP22 homologue. Upstream of the transcribed region, an imperfect set of three directly repeated sequences was identified. Each of them contains a complementary pair of the alphaherpesvirus origin-binding protein recognition motif GTTCGCAC, spaced by AT-rich sequences. In vitro studies confirmed that the DNA fragment analysed includes a functional origin of viral DNA replication.

A putative latency promoter/enhancer (P(LAT2)) region of pseudorabies virus contains a virulence determinant

Contradictory data have recently been reported on the role of the unique long-internal repeat junction area of pseudorabies (Aujeszky's disease) virus (PrV) genome in the virulence of the virus. To investigate the basis of the difference, four recombinant PrVs mutated at the outer region of inverted repeats that involved a putative latency promoter (P(LAT2)) were constructed in this study. Propagation characteristics of mutant viruses in cultured cells were similar to those of the wild-type virus. However, a 757 bp deletion at this location caused significant reduction in the virulence of PrV after intraperitoneal inoculation of mice and a moderate decrease in the virulence after intracranial inoculation. These results indicate that the P(LAT2) region is an important virulence determinant that may be implicated in the neuroinvasive capability of the virus.

Specific amino acid content and codon usage account for the existence of overlapping ORFS

Here we present a novel hypothesis for the origin of overlapping open reading frames (O-ORFs) observed in the 'non-coding frames' of several genes of yeast chromosome II. By computer analysis it was found that the specific amino acid content and base distribution pattern at certain genomic locations and the presence of O-ORFs were related. This observation prompt us to conclude that these O-ORFs are mere statistical curiosities without any biological function, which is in contrast to the hypotheses proposed by other authors.

Construction of a recombinant herpesvirus expressing the jellyfish green fluorescent protein

Here we report the insertion of a synthetic version of the cDNA encoding the jellyfish (Aequorea victoria) green fluorescent protein (gfph ) into the genome of pseudorabies (Aujeszky's disease) virus (PrV). A putative latency promoter (PLAT) located at the inverted repeat region of the PrV genome was chosen as the target site for the insertion. Recombinant viral DNA designated as vLAT-gfp was generated as a result of homologous recombination between the transfected viral DNA and a plasmid containing the GFP-expression cassette flanked by viral sequences homologous to the target region. Plaques containing recombinant virus were selected visually using a fluorescent microscope. We demonstrated a GFP-expression in infected neurons of rat brain which showed normal morphology at early stage of viral infection by monitoring fluorescent light emission.

Analysis of the equalization of inverted repeats and neurovirulence using a pseudorabies virus mutant strain altered at the Ul/Ir junction

A recombinant pseudorabies (Aujeszky's disease) virus (PrV) designated as vE16lac was constructed by deleting a 3-kbp DNA segment spanning the junction of long and short components of the viral genome, and by replacing the deleted segment with a lacZ-expression cassette. The aim of constructing this mutant was (a) to determine whether the terminal repeat (Tr) can serve as a template for the regeneration of the internal repeat (Ir), and (b) whether this deletion causes a reduction in the neuroinvasiveness of the virus. To analyze the mechanism of equalization, revertant viruses were selected and structurally characterized from vE16lac infection of PK-15 cells, mice and pigs. Because all revertants acquired Ir sequences identical to that of the wild-type virus, the equalization process occurred using the Tr as a template to reconstitute the Ir. We also found that the recombinant virus vE16lac was virulent in both pigs and mice. The data are discussed in view of studies performed with similar PrV mutants by other authors (Rall et al., 1992, Dean and Cheung, 1995 and Dean et al., 1996).

Promoter activity of sequence located upstream of the pseudorabies virus early protein 0 gene

Promoter activity of the 5'-flanking region of the pseudorabies virus (PRV) early protein 0 (EP0) gene was analysed by transient transfection assays employing chloramphenicol acetyl transferase (CAT) reporter constructs. We identified a 213 bp segment of the viral genome that was capable of efficiently driving expression of the EPO gene and a linked reporter gene upon transient transfection into Vero cells. This segment lacked the typical TATA element, and possessed an initiator element and the putative binding sites for the transcription factor Sp1 and immediate-early protein IE180, a strong transactivator of PRV. By analysing 5'-deletion mutants of the segment, a 48 bp segment (from nucleotide positions -65 to -17), which possessed three Sp1 binding sites, was identified to be critical for the promoter activity. Cotransfection of Vero cells with the mutant constructs and an IE180 expression plasmid resulted in transactivation of only those constructs in which the Sp1 sites were present. These results indicate that the EP0 gene may be transcribed from the TATA-less promoter that responds to Sp1.

A restriction cleavage and transfection system for introducing foreign DNA sequences into the genome of a herpesvirus

This report describes a simple and efficient system for construction of recombinant pseudorabies (Aujeszky's disease) virus (PrV) which is based on the use of a unique restriction site inserted into the viral genome. This system enables the recovery of genetically modified viruses without screening or selection for a specific phenotype, since practically all mature viral particles obtained carry the foreign sequences. To demonstrate, we introduced the tumour suppressor protein-53 (p53) gene into two different intergenic locations of PrV: the ribonucleotide reductase (rr) gene and the promoter of a putative latency gene (PLAT), located at the inverted repeat (IR) region of the viral genome. As a first step, we engineered a unique EcoRI recognition site into the rr gene or into both copies of PLAT with the help of marker transfer using the bacterial lacZ gene. Then, in both cases viral DNAs were cut with the restriction endonuclease EcoRI followed by treatment with calf intestinal phosphatase and used for cotransfection into porcine kidney cells with a plasmid containing the p53 gene flanked by viral DNAs homologous to the target region. As a result of this process, in most of the experiments, we obtained recombinant viruses without the background of parental viruses. Here we show that this method can be used for directional insertion of exogenous sequences into either the unique or the IR region of the PrV chromosome. In principle, this system should be applicable to the construction of recombinant derivatives of any viruses having infectious DNA.

Suppression of pseudorabies virus replication by a mutant form of immediate-early protein IE180 repressing the viral gene transcription

A mutant form of the immediate-early (IE) protein IE180 of pseudorabies virus (PRV), dIN454-C1081 is a strong repressor of the PRV IE gene promoter. In order to assess the antiviral potential of the IE180 mutant, HeLa cells were transformed with the mutant gene and then infected with PRV and herpes simplex virus type 1 (HSV-1). The transformed cell lines showed marked resistance to PRV infection, but were susceptible to infection with HSV-1, indicating that the IE180 mutant expressed in the stable cell line specifically inhibited PRV growth. In those cells infected with PRV, transcription of the PRV IE gene was repressed. In addition, the IE180 mutant exhibited a dominant-negative property in transient expression assay. The present results indicate that the resistance of the cells to PRV infection was due to repression of the IE gene transcription by the IE 180 mutant.

Negative regulation of immediate-early gene expression of pseudorabies virus by interferon-alpha

Pseudorabies rabies (PrV) replication in Vero cells was suppressed by treatment with human natural interferon-alpha (IFN-alpha). Messenger RNA transcribed from the PrV immediate-early (IE) gene was reduced in the IFN-alpha-treated cells. Transient expression assays showed that transcription from the PrV IE promoter was selectively inhibited in the IFN-alpha-treated cells. Analysis of deletion mutants of the PrV IE promoter sequence suggested that at least one element between the transcription initiation site (+1) and -90 in the PrV IE promoter was concerned with the negative regulation.

Mapping of transregulatory domains of pseudorabies virus early protein 0 and identification of its dominant-negative mutant

Pseudorabies virus (PRV) early protein 0 (EP0) is a transactivator containing the RING finger domain. Analysis of transactivating activity of truncated forms of the EP0 molecule consisting of 410 amino acids revealed that amino-terminal region containing the RING finger domain, amino acids 1 to 84, and the region between amino acids 114 to 242 containing acidic amino acid sequences were required for the transactivation. On the other hand, the mutant consisting of amino acids 1 to 113 exhibited a dominant-negative property.

Construction and properties of pseudorabies virus recombinants with altered control of immediate-early gene expression

To investigate how altered control of expression of the essential immediate-early (IE) gene of pseudorabies virus influences virus replication and virulence, we replaced the IE promoter with the tissue-specific promoters of the bovine cytokeratin IV gene (CKIV), the bovine cytokeratin VIb gene (CKVIb), or the inducible promoter of Drosophila heat shock gene HSP70. We compared expression of the IE gene of the wild-type virus and recombinant viruses in different cell types and at different temperatures and found that IE expression had become cell type or temperature dependent. When a recombinant virus was titrated on nonpermissive cells or was titrated at nonpermissive temperatures in vitro, the plating efficiency was reduced by more than 99%. Mice were inoculated subcutaneously (s.c.), intraperitoneally (i.p.), or intranasally (i.n.) with a dose equal to 100 times the 50% lethal dose of the wild-type virus. After inoculation with temperature-sensitive recombinant N-HSP, two (s.c.), two (i.p.), and four (i.n.) of five mice died. However, at this dose, recombinant N-CKIV, which contains a promoter specific for stratified epithelial tissue of the tongue mucosa, was not lethal when inoculated s.c. or i.p. but killed four mice when inoculated i.n. Recombinant N-CKVIb, which contains a promoter specific for the suprabasal layers of the epidermis, was not lethal after inoculation by any of the three routes. In explant cultures of nasal mucosa of pigs, replication of N-CKIV and N-CKVIb was not markedly reduced in the epithelium. However, in contrast to results obtained with wild-type virus, infection of the stroma was not observed. We conclude that the replicative ability and virulence of pseudorabies virus can be influenced by altering control of expression of the IE gene.

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.

A novel explanation for the existence of open reading frames on latency-associated transcripts of alphaherpesviruses

Recent studies indicate that alphaherpesviruses express latency associated transcripts (LATs) from the antisense strand of immediate early (IE) genes of the viral genome. It has been discussed that LATs containing extended open reading frames (ORFs) might be translated into protein products. We found that a salient feature of some herpesvirus DNAs is a high GC preference at the third codon positions. As a consequence, the probability of a stop codon arising at two of the six frames of the DNA strand is very low. The regions missing stop codons frequently start with ATG, resulting in extended ORFs. Therefore, the presence of a gene-long ORF does not necessarily mean that it is relevant to real translation.

Mapping of transcriptional regulatory domains of pseudorabies virus immediate-early protein

The 180 kilodalton immediate-early protein (IE180) of pseudorabies virus functions as a strong transactivator of several different promoters and also as a repressor of its own transcription. To map the functional domains of IE180, we prepared various truncated mutants and analyzed their transcriptional regulatory activities using the chloramphenicol acetyl transferase (CAT) assay. Analysis of mutants truncated from the carboxy-terminal end of the 1,460-amino acid polypeptide showed that a polypeptide possessing amino acids 1 to 1,081 retained significant functions of transactivation and autoregulation potential. On the other hand, removing amino acids 1 to 131 resulted in a complete loss of transactivation potential, indicating that the domain responsible for transactivation is located in the amino-terminal end of IE180. Additional amino-terminal truncation up to amino acid 453 did not affect the autoregulation activity, indicating that the region between amino acids 454 and 1081 has autoregulation potential.

Vaccine properties of pseudorabies virus strain 783 are not affected by a deletion of 71 base pairs in the promoter/enhancer region of the viral immediate early gene

Strain 783 of pseudorabies virus (PRV) is a genetically engineered vaccine which contains three deletions. The purpose of this study was to examine the effect of one of the deletions, which until now has not been characterized. The deletion occurs within the inverted repeats. Seventy-one base pairs (bp) were deleted, including one of the repeat sequence elements related to the TAATGARATTC boxes detected within the promoter and enhancer region of the immediate early (IE) genes of herpes simplex virus. The deletion affected neither the transcription of the IE gene nor viral growth in vitro. In our animal experiments, one group of pigs was inoculated with the original strain 783 and another with strain 783 which had had the repeat sequences restored. These two groups were then compared to determine the protective efficacy of the two vaccine strains against PRV infection. The deletion in the inverted repeats does not affect the vaccine properties of PRV strain 783: strain 783, with and without the 71 bp deletion in the repeats, protected pigs equally well.

A novel intergenic site for integration and expression of foreign genes in the genome of pseudorabies virus

Restriction enzyme analysis of DNA of a number of Pseudorabies virus (PRV) single plaque isolates revealed in several cases the existence of a unique EcoRI cleavage site, which has not been observed in PRV DNA before. This EcoRI site was mapped to the right end of the unique long region of the PRV genome, in BamHI-fragment 6. Sequence analysis of this region demonstrated the presence of an 11 bp tandem repeat in variable copy numbers in different PRV strains, suggesting the creation of the EcoRI recognition site by a recombinational event. The occurrence of variable reiterations and Northern blot analysis indicated an intergenic region. We therefore, used this site for integration and expression of heterologous DNA (the multiple cloning site of phage M13 and the E. coli lacZ gene). Viable PRV recombinants could be obtained which showed no detectable differences in virus growth in vitro compared to wild-type PRV. The novel insertion site can be used for the construction of PRV recombinants expressing foreign genes without apparent impairment of PRV genes.

Effects of replacing the promoter of the immediate early gene with the promoter of Drosophila heat-shock gene HSP70 on the growth and virulence of pseudorabies virus

We investigated whether altering control of expression of an essential gene of pseudorabies virus (PRV) influences virus replication and virulence. The PRV immediate early (IE) gene was selected as a target, and its promoter was replaced with the promoter of the heat-shock gene HSP70 of the fruit fly Drosophila. The HSP70 promoter was selected because it is well characterized and can be induced in a broad range of eukaryotic cell lines at temperatures around 42 degrees C. Overlap recombination was used to construct the NIA3-HSP mutant virus. When stocks of the recombinant virus were titrated at 42 degrees C, virus titres were 100 times higher than titres obtained at 37 degrees C. Once replication began, however, the rate of growth of the mutant NIA3-HSP was equal at both temperatures. When wild-type virus was titrated at both temperatures, titres were identical. Mice that were infected with the mutant virus had a longer mean-time-to-death than those infected with the wild-type virus. Thus, the mutant virus was considered to be less virulent. We conclude that replication and virulence of PRV can be modified by altering control of expression of the viral IE gene.

The DNA sequence of equine herpesvirus-1

The complete DNA sequence was determined of a pathogenic British isolate of equine herpesvirus-1, a respiratory virus which can cause abortion and neurological disease. The genome is 150,223 bp in size, has a base composition of 56.7% G + C, and contains 80 open reading frames likely to encode protein. Since four open reading frames are duplicated in the major inverted repeat, two are probably expressed as a spliced mRNA, and one may contain an internal transcriptional promoter, the genome is considered to contain 76 distinct genes. The genes are arranged collinearly with those in the genomes of the two previously sequenced alphaherpesviruses, varicella-zoster virus, and herpes simplex virus type-1, and comparisons of predicted amino acid sequences allowed the functions of many equine herpesvirus 1 proteins to be assigned.

Immediate-early RNA 2.9 and early RNA 2.6 of bovine herpesvirus 1 are 3' coterminal and encode a putative zinc finger transactivator protein

Bovine herpesvirus 1 (BHV-1) contains three major immediate-early (IE) genes involved in regulation of the productive cycle of replication. Two spliced IE RNAs, IER4.2 (4.2 kb) and IER2.9 (2.9 kb), are under the control of a single promoter; IER1.7 (1.7 kb) is transcribed from a different promoter in the opposite direction. Examining the kinetics of transcription, we found that the IER4.2/2.9 promoter was turned off at the end of the IE period. An alternative promoter became active, directing synthesis of an unspliced early RNA, ER2.6 (2.6 kb), which was colinear with the second exon of IER2.9 except for its 5' end in the intron about 10 bases upstream of the splice site. Sequence analysis revealed a single open reading frame common to IER2.9 and ER2.6 with a coding potential of 676 amino acids. The putative protein, named p135, contained a cysteine-rich zinc finger domain near the N terminus with homology to ICP0 of herpes simplex virus type 1, to protein 61 of varicella-zoster virus, to early protein 0 of pseudorabies virus, and to other viral and cellular proteins. The remaining parts of p135 exhibited only limited homology, mainly with pseudorabies virus protein 0, but the entire sequence was highly conserved between two strains of BHV-1 (K22 and Jura). The latency-related antisense transcript covered a large portion of ER2.6 excluding the zinc finger coding region. In transient expression assays, p135 activated a variety of promoters, including that for ER2.6, but repressed the IER1.7 promoter. Thus, p135 combines functional characteristics of ICP0, a strong transactivator, and of protein 61, a repressor. BHV-1 seems to have evolved a subtle mechanism to ensure the continued synthesis of p135 while turning off IER4.2, which encodes p180, the herpes simplex virus type 1 ICP4 homolog.

Expression of porcine pseudorabies virus genes by a bovine herpesvirus-1 (infectious bovine rhinotracheitis virus) vector

Recombinant DNA techniques were used to insert foreign genes into bovine herpesvirus-1 [infectious bovine rhinotracheitis virus (IBRV)] vectors which were attenuated by deletion and/or insertion mutations in the IBRV thymidine kinase (tk) gene. In one recombinant, the regulatory and coding sequences of the late pseudorabies virus (PRV) glycoprotein gIII gene, were inserted into the early IBRV tk gene. This recombinant efficiently expressed the PRV gIII gene indicating that immediate early IBRV proteins were competent to transactivate the late PRV gIII gene. IBRV vector viruses were also prepared in which the coding sequences of the early PRV tk gene, the late PRV gIII gene, and the E. coli beta-galactosidase gene were ligated to the late IBRV gIII promoter. Genotypes and phenotypes of the recombinant viruses were verified by restriction endonuclease and molecular hybridization experiments, thymidine plaque autoradiography, beta-gal plaque assays, and by immunoprecipitation experiments on extracts from 3H-mannose-labelled cells. The recombinant IBRV expressing beta-gal from the IBRV gIII promoter has been useful as an intermediate in the construction of IBRV vectors harboring foreign DNA sequences. The infectivity of the IBRV recombinant that expressed PRV gIII from the IBRV gIII promoter, was neutralized by polyclonal PRV antisera and by monoclonal antibodies to PRV gIII. The PRV gIII glycoprotein synthesized by the preceding recombinant has been used to coat microtiter test plate wells in a PRV gIII differential diagnostic test kit.

The 5' and 3' limits of transcription in the pseudorabies virus latency associated transcription unit

While latent in sensory neurons of infected pigs, pseudorabies virus expresses transcripts from a limited genomic area. These RNAs are transcribed from the strand opposite to that which encodes the pseudorabies immediate-early protein. Using a combination of in situ nucleic acid hybridization performed on latently infected pig trigeminal ganglia and DNA sequencing, 5' and 3' limits of transcription for the pseudorabies LAT transcription unit have been defined. The 5' limit of transcription has been localized to a NarI-BamHI subfragment of the BamHI-6 fragment. Several promoter elements in the correct orientation for the transcript are present including consensus TATA and CAAT boxes and an SP1 site. The 3' limit of transcription has been localized to a HindIII-KpnI subfragment of the BamHI-5 fragment which contains a consensus polyadenylation signal and two termination codones in the correct orientation. From these results we conclude that the region of pseudorabies virus DNA which is active during latency can be no longer than 12.6 kb and completely overlaps the gene encoding the pseudorabies immediate-early protein.

Multiple sets of adjacent mu E1 and oct-1 binding sites upstream of the pseudorabies virus immediate-early gene promoter

Binding of cellular proteins to specific motifs in the promoter region of the immediate-early gene of pseudorabies virus was studied. The region was dissected into several portions that were used with HeLa cell nuclear proteins in mobility shift assays, DNase I footprinting, and a methylation interference assay. Close to the transcription start site (nucleotide + 1) are a TATA-box (-26 to -29), an Sp 1-binding motif (GGGGCGGGC) (-45 to -54), a CCAAT motif (-66 to -70), and, further upstream, an NF-microE1-binding site (AAGATGGC) (-161 to -168). Binding of a protein to the Sp1 site was demonstrated. Competition experiments show that the CCAAT motif might bind the NF-microE1 factor, rather than any of the known CCAAT-specific factors. Four domains were identified further upstream (nucleotides -200 to -500) from this promoter, each of which contained closely associated motifs where cellular transcription factors NF-microE1 and oct-1 could bind. These domains comprise what we call the upstream element. The orientation of the four NF-microE1 motifs in the upstream element is opposite to the orientation of the two NF-microE1 motifs located closer to the transcription start site. Transient expression of reporter genes was used to study the activity of the upstream element after transfection into 3T3 and HeLa cells. The upstream element was necessary for efficient expression of the pseudorabies virus immediate-early gene and increased somewhat the efficiency of the herpes simplex virus thymidine kinase promoter.

Molecular biology of pseudorabies (Aujeszky's disease) virus

In this review, some of the aspects concerning the molecular biology of pseudorabies virus (PrV), the causative agent of Aujeszky's disease, will be discussed. It will mainly focus on new findings concerning viral glycoproteins, factors determining PrV virulence, the problem of PrV latency and the development regarding genetically engineered vaccines.

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.