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

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

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

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

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

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

Identification and nucleotide sequence of a gene in equine herpesvirus 1 analogous to the herpes simplex virus gene encoding the major envelope glycoprotein gB

A gene in equine herpesvirus 1 (EHV-1; equine abortion virus) equivalent to the gB glycoprotein gene of herpes simplex virus (HSV) has been identified by DNA hybridization and nucleotide sequencing. A 4.3 kbp EHV-1 PstI-ClaI sequence (0.40 to 0.43 map units) contained an open reading frame flanked by appropriate control elements and was capable of encoding a polypeptide of 980 amino acids. This had 50 to 60% identity over a 617 amino acid conserved region with the gB gene products of HSV and three other alphaherpesviruses, and 20 to 30% identity with those of human cytomegalovirus and Epstein-Barr virus. Analysis of the amino acid sequence predicts a long signal peptide, hydrophobic and hydrophilic domains and N-glycosylation sites, and has identified a probable internal proteolytic cleavage site. The EHV-1 gB open reading frame appears to be overlapped at its 5' end by 135 nucleotides of the 3' end of an upstream open reading frame the potential translation product of which has approximately 50% identity with HSV gene ICP 18.5 and VZV gene 30 products.

Evolution of the herpes thymidine kinase: identification and comparison of the equine herpesvirus 1 thymidine kinase gene reveals similarity to a cell-encoded thymidylate kinase

We have identified the equine herpesvirus 1 (EHV-1) thymidine kinase gene (TK) by DNA-mediated transformation and by DNA sequencing. Alignment of the amino acid sequence of the EHV-1 TK with the TKs from 3 other herpesviruses revealed regions of homology, some of which correspond to the previously identified substrate binding sites, while others have as yet, no assigned function. In particular, the strict conservation of an aspartate within the proposed nucleoside binding site suggests a role in ATP binding for this residue. Comparison of 5 herpes TKs with the thymidylate kinase of yeast revealed significant similarity which was strongest in those regions important to catalytic activity of the herpes TKs, and, therefore we propose that the herpes TK may be derived from a cellular thymidylate kinase. The implications for the evolution of enzyme activities within a pathway of nucleotide metabolism are discussed.

Isolation and comparative restriction endonuclease DNA fingerprinting of equine herpesvirus-1 from cattle

Deoxyribonucleic acid fingerprinting analyses with 4 restriction endonucleases (EcoRI, BamHI, BglII, and HindIII) and serotest results have definitively indicated that 5 herpesviruses isolated from 1974 to 1986 from aborted bovine fetuses and from bovine tissues and nasal secretions were abortigenic subtypes of equine herpesvirus type 1 (EHV-1). The herpesviruses, designated BH1247, 3M20-3, G118, H1753, and 9BSV4, were neutralized by EHV-1-specific antiserum and could be propagated in cultures of either bovine or equine cells. Only minor differences in restriction endonuclease patterns were detected from the pattern of an Army 183 isolate of EHV-1 subtype 1 that had been passaged only in equine cells and from that of an attenuated EHV-1 subtype 1 (RQ) strain that had been passaged several hundred times in non-equine cells. The individual differences in the restriction endonuclease fragments of the 5 bovine isolates and the Army 183 and RQ strains mainly were attributable to alterations in the terminally repeated and the unique short nucleotide sequences of the EHV-1 genomes, which are known to be hot spots for deletions and tandem repeats. The BamHI restriction endonuclease pattern of the 1977 bovine isolate H1753 was identical to that of EHV-1 subtype-1 strains responsible for most of the virus abortions in vaccinated horses since 1981. Abortigenic EHV-1 strains have the ability to infect cattle and cause disease under natural conditions.

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

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

Molecular biological characterization of equine herpesvirus type 1 (EHV-1) isolates from ruminant hosts

Isolates of equine herpesvirus type 1 (EHV-1) originating from affected antelope and cattle were compared with reference EHV-1 and EHV-4 isolates and were characterized. Based on cross-neutralization, DNA restriction profiles and blot-hybridization data these isolates could be characterized as EHV-1. One isolate (from an antelope) with a different restriction profile showed significant DNA homology with EHV-1, partial homology with EHV-4, and little or no homology with EHV-2 and HSV-1 DNAs. Blot hybridization revealed differences in DNA restriction fragments located at the termini of two isolates and size heterogeneity in the unique long/internal repeat junction fragment (UL/IR) of one isolate.

Characterization of the genome of equine herpesvirus 1 subtype 2

The genome structure of equine herpesvirus 1 (EHV-1) subtype 2 was shown by electron microscopic studies and restriction endonuclease site mapping to comprise two covalently linked segments (L, 109 kbp; S, 35 kbp). The S segment contains a unique sequence (US) flanked by a substantial inverted repeat (TRS/IRS). Thus, the genome structure of EHV-1 subtype 2 is similar to that published previously for EHV-1 subtype 1, but the two subtypes differ in the occurrences of EcoRI and BamHI restriction sites. Hybridization studies using cloned EHV-1 DNA showed that the genome of EHV-1 subtype 2 is colinear with the genomes of EHV-1 subtype 1 and herpes simplex virus type 1. DNA sequence data for four EHV-1 subtype 2 genes, including one potentially encoding a glycoprotein, were obtained by sequencing a 4574 bp BamHI fragment containing the junction between US and TRS. The genome structure, hybridization and sequence data confirm that EHV-1 subtype 2 is of the alphaherpesvirus lineage.

Characterization of equine herpesvirus type 1 immediate early proteins

EHV-1 immediate early (IE) gene expression in lytic infection results in the production of four high mol wt immediate early polypeptides (IEPs), designated IE1, IE2, IE2, and IE4; however, IE transcription is limited to the synthesis of a single 6-kb mRNA. Together, these findings raised questions as to whether the four IEPs were related products of the same gene. In the present study the IEPs were characterized with respect to their structural similarities, antigenic relatedness, and postsynthetic modifications. IE1 was the most abundant IEP, in that it accounted for approximately 80% of the IEP-incorporated radiolabel in infected rabbit kidney cells labeled under IE conditions with [35S]methionine or 14C-labeled amino acid mixtures. IE1 also was the major phosphorylated species. Limited proteolytic digestion of isolated radiolabeled IEP bands with Staph V8 protease yielded virtually identical fragment profiles in SDS-PAGE, as did digestions with chymotrypsin and N-chlorosuccinimide. Monospecific rabbit antisera raised against each of the four isolated IEPs reacted with all the IEP species in immunoblotting assays. Pulse-chase experiments indicated that all the IEPs were detectable immediately after a 15-min pulse and that several alterations in the IEP profile occurred during subsequent chase periods. Thus, the EHV-1 IEPs are closely related structurally and antigenically and appeared to be either produced simultaneously or processed to yield the individual forms immediately.

Characterization and mapping of equine herpesvirus type 1 immediate early, early, and late transcripts

Northern blot analysis was used to characterize and map equine herpesvirus type 1 (EHV-1) immediate early (IE), early, and late transcripts. Genomic EHV-1 DNA and cloned EHV-1 restriction endonuclease fragments, representing the entire genome, were 32P-labeled and hybridized to immobilized total cell RNA isolated from EHV-1 infected rabbit kidney cells incubated in the presence or absence of metabolic inhibitors. A single 6.0 kilobase (kb) IE transcript mapped to viral inverted repeat sequences. Approximately 41-45 early transcripts ranging in size from 0.8 to 6.4 kb and 18-20 late transcripts ranging in size from 0.8 to 10.0 kb were identified. These findings demonstrate that EHV-1 gene expression is regulated at the level of transcription, although regulation at the level of translation is also possible. The results provide a basis for examining alterations in viral gene expression in EHV-1 oncogenically transformed and persistently infected cells.

Use of lambda gt11 and monoclonal antibodies to map the genes for the six major glycoproteins of equine herpesvirus 1

To localize the genes for the major glycoproteins of equine herpesvirus 1 (EHV-1), a library of the EHV-1 genome was constructed in the lambda gt11 expression vector. Recombinant bacteriophage expressing EHV-1 glycoprotein epitopes as fusion products with beta-galactosidase were detected by immunoscreening with monoclonal antibodies specific for each of six EHV-1 glycoproteins. Seventy-four recombinant lambda gt11 clones reactive with EHV-1 monoclonal antibodies were detected among 4 X 10(5) phage screened. Phage expressing determinants on each of the six EHV-1 glycoproteins were represented in the library. Herpesviral DNA sequences contained in lambda gt11 recombinants expressing epitopes of EHV-1 glycoproteins were used as hybridization probes for mapping insert sequences on the viral genome. Genes for five EHV-1 glycoproteins (gp2, gp10, gp13, gp14, and gp21/22a) mapped to the genome L component; only one EHV-1 glycoprotein (gp17/18) was expressed from the unique S region of the genome where genes of several major glycoproteins of other herpesviruses have been located. Two glycoproteins of EHV-1, gp13 and gp14, mapped to positions colinear with genes of major glycoproteins identified in several other alphaherpesviruses (gC- and gB-like glycoproteins, respectively). The genomic locations of other EHV-1 glycoproteins indicated the existence of major glycoproteins of EHV-1 (gp2, gp10, and gp21/22a) for which no genetic homologs have yet been detected in other herpesviruses. The results confirm the general utility of the lambda gt11 expression system for localizing herpesvirus genes and suggest that the genomic positioning of several high-abundance glycoproteins of EHV-1 may be different from that of the prototype alphaherpesvirus, herpes simplex virus.

Equine herpesvirus type 1 defective-interfering (DI) particle DNA structure: the central region of the inverted repeat is deleted from DI DNA

The inverted repeat (IRs) component of the genome of equine herpesvirus type 1 (EHV-1) is an important region of structure and function. It is a major constituent of the DNA of EHV-1 defective-interfering (DI) particles which have been shown to mediate the coestablishment of oncogenic transformation and persistent infection of hamster embryo cells. In addition, the IRs encodes the single EHV-1 immediate early gene and the 31.5K very early protein. DNA sequences encompassing EHV-1 internal IRs and the joint between the long (L) and short (S) regions were subcloned into the plasmid vectors pBR322 and pUC12. A total of 22 subclones were derived, including six Sa/l subclones in pBR322 and 12 SmaI subclones in pUC12. Individual subclones were employed in Southern blot hybridizations to define subclone homology to repeated, unique, or heterogeneous (het) DNA sequences within the EHV-1 genome. These studies revealed that the EHV-1 het region is contained entirely within the unique long region of the viral genome and is separated from the L/S junction by approximately 1.8 MDa of completely unique DNA sequences. Furthermore, these IRs subclones were employed in blot hybridizations to analyze the integrity of IRs DNA sequences within the cloned DNA of EHV-1 DI particles. These analyses demonstrated that IRs DNA sequences present in DI DNA were extensively rearranged and contained major deletions (0.80-0.83 map units) which removed a large portion of the single EHV-1 immediate early gene (0.78-0.83 and 0.95-1.00 map units) located in the IRs. Thus, these data and those previous studies (R. P. Baumann et al., 1984, J. Virol. 50, 13-21; R. P. Baumann, J. Staczek, and D. J. O-Callaghan, 1986, Virology 153, 188-200) indicate that the major subunits of the DI DNA molecule are comprised of selected sequences from the IRs component and a highly conserved short sequence located at the terminus of the L region of the standard viral genome.

Regulation of equine herpesvirus type 1 gene expression: characterization of immediate early, early, and late transcription

The regulation of equine herpesvirus type 1 (EHV-1) transcription was examined in infected rabbit kidney cells using metabolic inhibitors. In order to map EHV-1 immediate early, early, and late transcripts, viral RNA was 32P-labeled in vivo and hybridized to EHV-1 DNA restriction fragments immobilized on nitrocellulose filters. Immediate early viral RNA was mapped to one region of the viral genome within the inverted repeat DNA sequences (map units 0.78-0.83 and 0.95-1.0). Northern blot hybridization analysis using a 32P-labeled cloned DNA probe from this region identified a single immediate early viral transcript (approximately 6 kb). Transcription of early and late genes was not restricted to any specific region on the viral genome as indicated by the ability of 32P-labeled early and late RNA to hybridize to EHV-1 restriction endonuclease fragments from both the long and short components of EHV-1 DNA. Additional experiments performed without the use of metabolic inhibitors confirmed that EHV-1 transcription is temporally regulated. The characterization of EHV-1 transcription during productive infection will serve as a reference for the analysis of viral transcripts in oncogenically transformed and persistently infected cells.

Phosphorylation of nucleoside analogs by equine herpesvirus type 1 pyrimidine deoxyribonucleoside kinase

Replication of equine herpesvirus type 1 (EHV-1) was sensitive to 9-(1,3-dihydroxy-2-propoxymethyl)guanine(DHPG) but relatively resistant to E-5-(2-bromovinyl)-2'-deoxyuridine (BVDU). Likewise, plaque formation by EHV-1 was inhibited by DHPG, but not by BVDU. Plaque formation by a thymidine kinase-negative (tk-) mutant of EHV-1 was not inhibited by DHPG. In order to investigate biochemical mechanisms determining the differential sensitivity of EHV-1 to these drugs, the EHV-1-encoded thymidine kinase enzyme activity (TK)1 was partially purified from EHV-1-infected cells and analyzed. The EHV-1-induced enzyme utilized both ATP and CTP as phosphate donors and differed in relative electrophoretic mobility from the TKs of mock-infected and HSV-1-infected cells. Phosphorylation of 3H-dThd by the EHV-1 TK was inhibited by AraT, IdUrd, BVDU, and DHPG. The EHV-1 TK phosphorylated 125I-dCyd and 3H-ACV. The results indicate that EHV-1 encodes a pyrimidine deoxyribonucleoside kinase with broad nucleoside substrate specificity. These observations suggest that the failure of BVDU to inhibit EHV-1 replication is not attributable to an inability of the EHV-1 TK to phosphorylate BVDU, but may result from the incapacity of the viral TK to convert BVDU monophosphate to the triphosphate or from lack of inhibitory effect of BVDU triphosphate on viral DNA polymerase reactions.

Rapid method for the identification and screening of herpesviruses by DNA fingerprinting combined with blot hybridization

Rapid characterization of herpesvirus isolates exemplified by equine herpesviruses is described. Total DNAs were isolated from virus infected small scale cell cultures. The DNA fragments obtained after restriction enzyme digestion were separated on agarose gels, transferred and immobilized on filter membranes. A radioactively labelled probe derived from the purified DNA of an EHV-1 reference strain was used for hybridization in order to detect the restriction fragments of different EHV-1 field isolates. This method allows the typing of many isolates within a short period of time.

Cloning and fine mapping the DNA of equine herpesvirus type one defective interfering particles

Equine herpesvirus type one (EHV-1) defective interfering (DI) particle DNA fragments were inserted into the XbaI site of the plasmid vector pACYC184. Five DI XbaI fragments, which ranged in molecular weight from 4.5 to 6.7 MDa, were selected for detailed analysis. Each DI DNA clone was labeled with 32P-deoxynucleotides by nick translation and hybridized to genomic digests of EHV-1 standard (STD) DNA bound to nitrocellulose. All five clones were shown to hybridize to DNA sequences derived from the left terminus (0.0-0.04 map units) of the long (L) region and from the short (S) region inverted repeats (IRs, 0.79-0.86 and 0.93-1.00 map units) of the STD EHV-1 genome. Restriction enzyme mapping studies and Southern blot hybridizations employing cloned STD virus DNA fragments as probes revealed that these EHV-1 DI clones contain two major domains: (1) an L terminal region which maps to 0.01-0.04 map units and is highly conserved among all five clones, and (2) a region homologous to the IRs which appears to vary between individual clones.

Equine herpesvirus genomes: heterogeneity of naturally occurring type 4 isolates and of a type 1 isolate after heterologous cell passage

The restriction endonuclease DNA fingerprints of 20 low passage, epidemiologically unrelated isolates of equine herpesvirus 4 (equine rhinopneumonitis virus) showed considerable heterogeneity in certain fragments, the positions of which were assigned to quite restricted positions on the 141 kilobase (kb) genome. We note that the heterogeneity observed in the restriction endonuclease DNA fingerprints of EHV 1 (equine abortion virus) and of pseudorabies virus also tend to map to these same restricted regions. The restriction endonuclease DNA fingerprints of an EHV 1 strain was invariant using low (less than 1) multiplicity of infection during 20 passages in equine cells but when adapted to hamster cells developed an approximately 0.8 kb deletion in the unique short region of the genome between 8 and 11 passages.

Genomic and antigenic comparison of an equine herpesvirus 1 (EHV 1) isolate from the 1983 Lippizan abortion storm with EHV 1 reference strains

An EHV 1 isolate from the Lippizan Stud at Piber, which caused the abortion and paresis outbreak in 1983, was investigated using 3 known subtype 1 and 2 subtype 2 strains for comparison. Broad-scale restriction enzyme analysis as well as cross-neutralization with hyperimmune sera produced in rabbits were performed, and SDS-PAGE of infected cell proteins was conducted on a limited scale. The Piber isolate was clearly classified as a subtype 1 strain of EHV 1, and showed closest resemblance in its restriction patterns with a British EHV 1 strain, which originated from an outbreak with paretic symptoms. A second Piber isolate from the same outbreak examined to limited extent only was practically indistinguishable from the first one, as could have been expected. A thoroughly controlled systematic vaccination program with existing commercial vaccines against EHV 1 should protect the endangered Lippizan horses population against the abortigenic and less certainly against the paretic syndromes caused by this virus. According to data presented, a protection against respiratory disease is less probable.

Genetic relatedness and colinearity of genomes of equine herpesvirus types 1 and 3

The arrangement and location of homologous DNA sequences within the genomes of equine herpesvirus type 1 (EHV-1) and EHV-3 were investigated by using Southern blot hybridization analyses conducted under stringent conditions. Recombinant plasmid libraries comprising 95 and 84% of the EHV-1 and EHV-3 genomes, respectively, were labeled with 32P-deoxynucleotides by nick translation and were used as probes in filter hybridization studies. The DNA homology between the EHV-1 and EHV-3 genomes was dispersed throughout the genomes in a colinear arrangement. Significant hybridization was detected between the EHV-1 short region inverted repeat sequences, which are known to encode immediate early transcripts, and the corresponding EHV-3 inverted repeat sequences. Interestingly, probes derived from the EHV-1 heterogeneous region, which is adjacent to the EHV-1 short region, hybridized strongly to EHV-3 DNA sequences within a similar genomic location, but did not reveal any corresponding heterogeneity within the EHV-3 genome. Our results demonstrated that there is a highly conserved evolutionary relationship between EHV-1 and EHV-3 and provided the foundation for further investigations to determine whether similarities in protein function underpin the genetic relatedness between these two herpesviruses.

Equine herpesvirus type 1 infected cell polypeptides: evidence for immediate early/early/late regulation of viral gene expression

EHV-1 polypeptide synthesis was examined in productively infected rabbit kidney and hamster embryo cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analyses of extracts from [35S]methionine- and 3H-amino acid-labeled-infected and mock-infected cultures revealed the presence of 30 infected cell-specific polypeptides (ICPs) which ranged in apparent molecular weights from 16.5K to 213K. Twenty-two of these ICPs comigrated with virion structural proteins. Four ICPs (203K, 176K, 151K, 129K) were detected in extracts of infected cultures labeled in the presence or absence of actinomycin D (Act D) immediately after release from a 4-hr treatment with cycloheximide (CH). These polypeptides, which were designated as EHV-1 immediate early (alpha) ICPs, were not detected in unblocked (non-CH-treated) infected cells. The most abundant ICP was a 31.5K nonstructural protein which, in addition to a 74K protein, was detected in unblocked infected cells at 2-3 hr postinfection. These proteins appeared to be regulated as early (beta) ICPs, since neither protein was observed in Act D-treated cultures released from CH block. Twelve ICPs were classified as late (gamma) polypeptides on the basis of their reduced synthesis in cultures in which viral DNA replication was inhibited by phosphonoacetic acid. All but one (40K) of these late ICPs corresponded to virion structural proteins.

Structure and genetic complexity of the genomes of herpesvirus defective-interfering particles associated with oncogenic transformation and persistent infection

The complexity and structural organization of defective-interfering (DI) particle DNA of equine herpesvirus type 1 (EHV-1) have been elucidated by using restriction enzyme and Southern blot hybridization analyses. DI particles were generated by serial high-multiplicity passage of EHV-1 in L-M cells, and total viral DNA was extracted from virus purified from supernatants of these serial passages. EHV-1 DI particle DNA was quantitatively separated from standard (STD) DNA by several cycles of CsCl isopycnic banding in a vertical rotor. Restriction endonuclease digestion profiles of pure DI DNA were completely different from the mapped patterns observed for EHV-1 STD DNA. Digestion of pure defective DNA with restriction enzymes (Bg/II, EcoRI, and XbaI), for which there are few or no cleavage sites within the S (short) region of the EHV-1 STD genome, yielded high-molecular-weight supermolar DNA bands, suggesting that a large subgenomic repeat unit was present in defective DNA. DNA blot hybridization analysis with the Bg/II supermolar fragment of defective DNA, intact DI particle genomic DNA, and EHV-1 STD DNA restriction enzyme fragments as 32P-labeled probes indicated that the EHV-1 DI particle genome originates predominately from the STD DNA S region (0.77 to 1.00 map units) and to a lesser extent from the left terminus of the unique long (UL) region (0.00 to 0.05 map units). None of the EHV-1 DNA sequences associated to date with EHV-1 oncogenesis (0.32 to 0.38 map units; O'Callaghan et al. in B. Roizman [ed.], Herpesviruses, in press; Robinson et al., Cell 32:204-219, 1983, and Proc. Natl. Acad. Sci., U.S.A., 78:6684-6688, 1981) were detected in the DI particle DNA. The importance of these data with regard to DNA replication of DI particles and the role of DI particles in one model system of EHV-1 oncogenic transformation are discussed.

Alterations in the equine herpesvirus 1 genome after in vitro and in vivo virus passage

The effect of in vitro and in vivo serial virus passage on the genetic stability of equine herpesvirus 1 (EHV-1) was investigated by restriction endonuclease analysis of the viral DNA. DNAs of EHV-1 isolates at different passage levels in cultured cells or in Syrian hamsters were compared by electrophoresis of the DNA cleavage fragments produced by restriction endonuclease digestion. No changes were observed in the restriction profile of the DNAs of EHV-1 strains after 100 sequential passages in cultured equine cells. However, serial passage of the virus in hamsters or in cells of non-equine origin quickly gave rise to alterations in the viral DNA. These changes occurring in the restriction endonuclease profiles of EHV-1 DNA during serial virus passage in non-equine cells or animals hosts could be explained by sequence additions or deletions to preexisting restriction fragments resulting in variation in their electrophoretic mobilities.