A vaccine strain of pseudorabies virus with deletions in the thymidine kinase and glycoprotein X genes

A pseudorabies virus (PRV) mutant with deletions in genes for glycoprotein X (gX) and thymidine kinase, designated delta GX delta TK, was constructed and evaluated as a vaccine for protecting swine against PRV-induced mortality. Doses greater than or equal to 10(3) plaque-forming units (PFU) of this strain given to mice provided protection from challenge exposure with virulent PRV. Sera tested from mice inoculated with delta GX delta TK had high titers of neutralizing antibody to PRV, but reactivity in the same sera was not significantly different from that in sera from noninoculated mice (controls) when sera from both groups were evaluated by use of an ELISA with gX antigen produced in Escherichia coli. Compared with noninoculated pigs (controls), those given delta GX delta TK (greater than or equal to 10(2) PFU) were protected completely from lethal challenge exposure, without experiencing adverse effects on weight gain and with reduction of shedding of virulent challenge virus. Serotest results indicated that, although inoculated pigs responded with strong neutralizing antibody titers, the response of delta GX delta TK-inoculated pigs to gX, as determined by ELISA before challenge exposure, was not significantly greater than the ELISA values obtained from control pigs. The ELISA values from a group of pigs inoculated with a commercially available vaccine were significantly (P less than 0.05) higher than those of control pigs. The experimental vaccine, delta GX delta TK, was avirulent for mice, swine, and sheep, but was mildly virulent for calves (mortality, 1 of 12) and more virulent for dogs (mortality, 3 of 6) and cats (mortality, 2 of 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of pseudorabies virus glycoprotein gIII localization and modification by using novel infectious viral mutants carrying unique EcoRI sites

We have constructed two pseudorabies virus (PRV) mutants, each with a unique EcoRI restriction site in the nonessential gIII envelope glycoprotein gene. Since no natural PRV isolate has been reported to contain EcoRI sites, the isolation and single-step growth curve analysis of these mutants established that PRV can carry such a site with little ill effect in tissue culture. Virus carrying these defined mutations produced novel gIII proteins that enabled us to begin functional assignment of protein localization information within the gIII gene. Specifically, one viral mutant contained an in-frame synthetic EcoRI linker sequence that was flanked on one side by the first one-third of the gIII gene and on the other side by the last one-third of the gene. The resulting protein lacked the middle one-third of the parental species, including five of eight putative N-linked glycosylation signals, but was still glycosylated and found in enveloped virions; it was not secreted into the medium. A second viral mutant contained an in-frame synthetic EcoRI linker sequence that additionally specified a nonsense codon at position 158, producing a gIII protein that was glycosylated and secreted into the medium; the fragment was not found in enveloped virions. By endoglycosidase and pulse-chase analyses, we established a precursor-product relationship between the various forms of gIII expressed in the parental and mutant strains, and perhaps determined certain features of the gIII protein that are required for its efficient export within the cell.

The pseudorabies virus gII gene is closely related to the gB glycoprotein gene of herpes simplex virus

We have looked for conserved DNA sequences between four herpes simplex virus type 1 (HSV-1) glycoprotein genes encoding gB, gC, gD, and gE and pseudorabies virus (PRV) DNA, HSV-1 DNA fragments representing these four glycoprotein-coding sequences were hybridized to restriction enzyme fragments of PRV DNA by the Southern blot procedure. Specific hybridization was observed only when HSV-1 gB DNA was used as probe. This region of hybridization was localized to a 5.2-kilobase (kb) region mapping at approximately 0.15 map units on the PRV genome. Northern blot (RNA blot) analysis, with a 1.2-kb probe derived from this segment, revealed a predominant hybridizing RNA species of approximately 3 kb in PRV-infected PK15 cells. DNA sequence analysis of the region corresponding to this RNA revealed a single large open reading frame with significant nucleotide homology with the gB gene of HSV-1 KOS 321. In addition, the beginning of the sequenced PRV region also contained the end of an open reading frame with amino acid homology to HSV-1 ICP 18.5, a protein that may be involved in viral glycoprotein transport. This sequence partially overlaps the PRV gB homolog coding sequence. We have shown that the PRV gene with homology to HSV-1 gB encoded the gII glycoprotein gene by expressing a 765-base-pair segment of the PRV open reading frame in Escherichia coli as a protein fused to beta-galactosidase. Antiserum, raised in rabbits, against this fusion protein immunoprecipitated a specific family of PRV glycoproteins of apparent molecular mass 110, 68, and 55 kilodaltons that have been identified as the gII family of glycoproteins. Analysis of the predicted amino acid sequence indicated that the PRV gII protein shares 50% amino acid homology with the aligned HSV-1 gB protein. All 10 cysteine residues located outside of the signal sequence, as well as 4 of 6 potential N-linked glycosylation sites, were conserved between the two proteins. The primary protein sequence for HSV-1 gB regions known to be involved in the rate of virus entry into the cells and cell-cell fusion, as well as regions known to be associated with monoclonal antibody resistance, were highly homologous with the PRV protein sequence. Furthermore, monospecific antibody made against PRV gII immunoprecipitated HSV-1 gB from infected cells. Taken together, these findings suggest significant conservation of structure and function between the two proteins and may indicate a common evolutionary history.

Role of pseudorabies virus glycoprotein gI in virus release from infected cells

The Bartha vaccine strain of pseudorabies virus has a deletion in the short unique (Us) region of its genome which includes the genes that code for glycoproteins gI and gp63 (E. Petrovskis, J. G. Timmins, T. M. Gierman, and L. E. Post, J. Virol. 60:1166-1169, 1986). Restoration of an intact Us to the Bartha strain enhances its ability to be released from infected rabbit kidney cells and increases the size of the plaques formed on these cells (T. Ben-Porat, J. M. DeMarchi, J. Pendrys, R. A. Veach, and A. S. Kaplan, J. Virol. 57:191-196, 1986). To determine which gene function plays a role in virus release from rabbit kidney cells, deletions were introduced into the genomes of both wild-type virus and the "rescued" Bartha strain (Bartha strain to which an intact Us had been restored) that abolish the expression of either the gI gene alone or both gI and gp63 genes. The effect of these deletions on the phenotype of the viruses was studied. Deletion mutants of wild-type virus defective in either gI or gI and gp63 behave like wild-type virus with respect to virus release and plaque size on rabbit kidney cells. Deletion of gI from the rescued Bartha strain, however, strongly affects virus release and causes a decrease in plaque size. We conclude that gI affects virus release but that at least one other viral function also affects this process. This function is defective in the Bartha strain but not in wild-type virus; in its absence gI is essential to efficient release of the virus from rabbit kidney cells.

A small open reading frame in pseudorabies virus and implications for evolutionary relationships between herpesviruses

An open reading frame coding for an 11-kDa protein was located downstream from the gI gene of pseudorabies virus (PRV). This open reading frame is homologous to an open reading frame (US9) in an analogous position in herpes simplex virus and to an open reading frame (US1) in a different position in varicella zoster virus. The open reading frame encoding the 11-kDa protein is in a region known to be deleted in live attenuated vaccine strains of PRV.

Characterization of virulent and attenuated strains of pseudorabies virus for thymidine kinase activity, virulence and restriction patterns

A pseudorabies virus mutant lacking thymidine kinase activity (TK-) was isolated and characterized. The mutant replicated as well in cell culture as the parental TK+ strain, was not temperature sensitive at 38.5 degrees C, and did not revert to TK+. Two pseudorabies virus field isolates and three commercial modified live virus vaccine strains were compared for TK activity and virulence for the mouse; all strains expressed TK: Km values for thymidine of the viral TKs ranged from 2.9 to 3.9 microns; the commercial modified live virus vaccine strains were reduced in virulence for the mouse two to ten-fold. The TK- mutant was avirulent for the mouse. Restriction enzyme analysis of the pseudorabies virus DNA from the strains under study revealed that two of the modified live virus vaccine strains are closely related and that all three modified live virus vaccine strains lack the largest PstI fragment characteristic of the other strains included in the study.

Evolution of pseudorabies virions containing genomes with an invertible long component after repeated passage in chicken embryo fibroblasts

The genome of pseudorabies virus consists of two components, short (S) and long (L). Only the S component is bracketed by inverted repeats, and only the S component inverts itself relative to the L component, giving rise to two isomeric forms of the genome. An attenuated vaccine strain of pseudorabies virus (Norden), however, has a genome which is found in four isomeric forms (B. Lomniczi, M. L. Blankenship, and T. Ben-Porat, J. Virol. 49:970-979, 1984). To determine the basis for the atypical structure of the genome of the Norden strain, we examined more than 40 field isolates of pseudorabies virus; all contained genomes in which the L component was fixed in only one orientation relative to the S component. Several independently generated vaccine strains which have been passaged extensively in chicken embryos and chicken embryo fibroblast (CEF) cell cultures were also analyzed; they possessed an invertible L component. Furthermore, emergence of pseudorabies virus variants with an invertible L component was observed after passage of the virus in CEF, but not in rabbit kidney or pig kidney, cells. The invertibility of the L component was associated consistently with a translocation of sequences from the left end of the genome to a position next to the inverted repeat sequence of the S component. Three observations indicate that genomes with an invertible L component (and the translocation) have a selective growth advantage over standard pseudorabies virus when grown in CEF. The proportion of virions with such genomes does not increase linearly as would be expected if the translocation events occurred repeatedly, most genomes eventually experiencing the translocation. Instead, after a lag, the proportion of such virions in the population increases relatively rapidly. The genome structures that are generated upon independent passage in CEF of each virion population were relatively homogeneous. Some heterogeneity was observed at relatively early stages of the emergence of the genomes carrying the translocation; at later stages, virions with genomes with a specific size translocation predominated in the virus population. Parallel passages in CEF of the same pseudorabies virus strain resulted in the emergence of populations of virions with genomes with different size translocations. However, in each of the passaged populations of virions the majority of virions had genomes with the same size translocation. The most likely interpretation of these results is that virions with genomes carrying the translocations that emerge upon passage of the virus in CEF have a selective advantage when grown in these cells.

Variability of pseudorabies virus glycoprotein I expression

The 130,000 mol wt glycoprotein I (gI) derived from two approx 80-kDa precursors is one of the major constituents of the envelope of pseudorabies virus (PRV) strain Phylaxia. Recently, gI has been shown to be nonessential for PRV replication since several PRV vaccine strains with deletions in the region of the genome encoding the gI gene have been described. In this paper we demonstrate that other alterations affecting gI expression can occur. We describe a PRV field isolate which expresses a single gI precursor molecule pgI of 64,000 mol wt. This precursor is processed into 60,000 mol wt gI. In contrast to PRV Phylaxia, the gI-expressing isolate is not neutralized by anti-gI monoclonal antibodies. Virions expressing the pgI also emerged after serial in vitro passages of the wild-type PRV strain NIA-5 which initially expressed wild-type pgI. Concomitant with the appearance of pgI the pgI disappeared and the resistance of the virus population to neutralization by anti-gI monoclonal antibodies increased. Furthermore, the amount of expression of gI and pgI in single plaque isolates of the PRV strain Ka was found to be highly variable among different plaque isolates and correlated with a different susceptibility to neutralization by anti-gI monoclonal antibodies. In single plaque isolates of strain Phylaxia, however, gI expression appeared to be stable. In all cases, no genomic or transcriptional alterations could be observed. Thus, viruses resistant to anti-gI antibodies occur spontaneously in vivo and in vitro, which argues against the use of gI as a subunit vaccine.

Second-generation pseudorabies virus vaccine with deletions in thymidine kinase and glycoprotein genes

A modified-live pseudorabies virus (PRV) vaccine, designated PRV(dlg92/d1tk), with deletions in the thymidine kinase (tk) and glycoprotein-gIII (g92) genes, was derived from the PRV (Bucharest [BUK]-d13) vaccine strain. The vaccine virus also contained a deletion in glycoprotein gI. Despite 3 deletions, PRV(dlg92/d1tk) replicated to high titers in cell culture from 30 C to 39.1 C. Enzyme assays and autoradiography revealed that PRV(dlg92/d1tk) did not induce a functional tk activity in infected tk- RAB(BU) cells (rabbit skin). Rabbit skin cells were infected with PRV(dlg92/d1tk), with vaccine strains derived from BUK or Bartha K strains of PRV or with the virulent Illinois (ILL), Indiana-Funkhauser (IND-F), and Aujeszky (Auj) strains of PRV and were labeled with [3H]mannose from 4 or 5 to 24 hours after infection to investigate whether these viruses induced the synthesis of glycoprotein gIII. Nonionic detergent extracts were prepared and immunoprecipitated with antisera from pigs vaccinated with tk(-)-PRV(BUK-d13) or tk+-Bartha K, pigs vaccinated with tk+-PRV(BUK) strains and then challenge exposed to tk+-PRV(IND-F), naturally infected domestic or feral pigs, and pigs vaccinated with tk-)-PRV(dlg92/d1tk). Mouse monoclonal antibodies against PRV glycoproteins gIII, gp50, and gII were also studied. After immunoprecipitation, labeled PRV-specific proteins were analyzed by sodium dodecylsulfate-polyacrylamide gel electrophoresis and autoradiography. The PRV glycoprotein-gII complex, but not glycoprotein gIII, was synthesized in PRV(dlg92/d1tk)-infected cells. Glycoprotein gII and gIII were made in cells infected with PRV vaccine strains BUK, Bartha K, and BUK-d13 and with virulent PRV strains ILL, IND-F, and Auj. Cells infected with PRV(dlg92/d1tk) and with PRV strains ILL, IND-F, Auj, Bartha K, BUK, and BUK-d13, excreted into the cell culture medium a highly sulfated glycoprotein gX of about 90 kilodaltons. Antibodies to glycoprotein gIII were not detected in the sera of pigs inoculated with PRV(dlg92/d1tk), but were found in all other swine sera.

DNA sequences which regulate the expression of the pseudorabies virus major immediate early gene

It has been shown previously that the transcription of herpes simplex virus (HSV) immediate early (IE) genes is transactivated by a component of the virus particle. The trans-inducing factor (TIF) is known to be polypeptide Vmw65. Infection with pseudorabies virus (PRV), a related herpesvirus, does not increase expression from HSV IE regulatory sequences (W. Batterson and B. Roizman, 1983, J. Virol. 46, 371-377). To examine the control of the PRV IE gene and possible sequence specificity of a TIF, the 5' terminus of the PRV major IE transcript was mapped and hybrid plasmids containing PRV upstream sequences linked to the HSV-1 TK gene were constructed. Gene expression under the control of PRV IE or HSV-1 IE gene 3 upstream regions were compared using transient expression assays. It was found that infection with uv-irradiated PRV did not stimulate expression from PRV IE or from HSV-1 IE gene 3 upstream regions, indicating that PRV did not possess an effective TIF. Infection with uv-treated HSV-1, or cotransfection with a plasmid which encodes Vmw65, stimulated expression from both PRV and HSV IE gene upstream regions. The nucleotide sequence of the 5' end of the PRV transcript and its upstream region was determined. This region was, in overall structure, unlike the upstream regions of HSV IE genes but showed a strong similarity to the enhancers of human and murine cytomegaloviruses (HCMV and MCMV). In particular, a reiterated 15-bp element of the PRV upstream region was homologous to a conserved, repeated sequence element found in both HCMV and MCMV enhancer regions and was also related to the "TAATGARATTC" motif found upstream of all HSV IE genes. Thus a conserved sequence element occurs upstream of IE genes in four herpesviruses with different genome structures and diverse biological properties.

Effects of a temperature-sensitive mutation in the immediate-early gene of pseudorabies virus on class II and class III gene transcription

The pseudorabies virus immediate-early protein activates transcription of both class II and class III genes. At present it is not known whether the activation of class II genes occurs through an activation of cellular factors or by a direct interaction of the immediate-early protein with factors or DNA. It is also not known whether the activation of class II and class III genes occurs by a similar or different mechanism. We utilized tsG, a temperature-sensitive mutation in the immediate-early gene of pseudorabies virus, to study the activation of viral genes transcribed by RNA polymerases II and III. Previous studies have shown that tsG inhibits wild-type adenovirus early gene transcription in coinfections at the nonpermissive temperature (L. T. Feldman and S. E. Ahlers, J. Virol. 57:13-17, 1986). Using this system of mixed infections as an assay, we obtained several results which allowed us to draw certain conclusions about the mode of action of the pseudorabies virus immediate-early protein (IEP). First, the tsG mutation inhibits the formation of new transcription complexes on class II genes, but does not affect transcription from preestablished transcription complexes formed in the presence or absence of the adenovirus E1A protein. Second, tsG does not inhibit transcription from class III genes, suggesting that the activation by IEP of class II and class III genes occurs by different mechanisms. Third, activation of transcription by the adenovirus E1A protein is not dominant to inhibition by tsG, suggesting that the temperature-sensitive IEP is involved in some physical interaction which actively inhibits transcription of viral genes.

Immediate-early protein of pseudorabies virus is not continuously required to reinitiate transcription of induced genes

We examined the role of herpesvirus immediate-early proteins in inducing and maintaining transcription by using tsG, a temperature-sensitive mutant in the immediate-early gene of pseudorabies virus. Cells infected at the permissive temperature were shifted to the nonpermissive temperature. Initially, early gene transcription rates were similar at both temperatures. Early gene transcription subsequently decreased slowly over several hours. Our results suggest that immediate-early protein function is required only for the initial activation of early gene transcription and is not required for multiple reinitiation events from activated early genes.

Genome location and identification of functions defective in the Bartha vaccine strain of pseudorabies virus

We have shown previously (Lomniczi et al., J. Virol. 52:198-205, 1984) that the Bartha vaccine strain of pseudorabies virus has a deletion in the short unique (Us) region of its genome--a deletion that is related to the absence of virus virulence. This strain is, however, also defective in other genes involved in virulence. We show here that virulence can be restored by marker rescue of the Bartha strain to which an intact Us has been restored (but not to the parental Bartha strain) by sequences derived from approximate map units 0.460 and 0.505 of the wild-type virus genome. No difference in the ability to grow in cell culture was observed between parental Bartha, Bartha 43/25a (Bartha to which an intact Us has been restored), or the doubly rescued Bartha strains. However, only the doubly rescued Bartha strain was virulent for both chickens and pigs and replicated to high titers when inoculated directly into the brains of chickens. The sequences that could restore virulence to the Bartha 43/25a strain encode four genes, all of which are involved in processes leading to the assembly of nucleocapsids. Since these sequences rescue virulence, it appears that a function that plays a role in nucleocapsid assembly is defective in the Bartha strain and that this defect contributes to the lack of virulence of this virus.

Construction and characterization of deletion mutants of pseudorabies virus: a new generation of 'live' vaccines

Various deletions were introduced into a cloned subgenomic fragment (BamHI-7), located in the unique short (US) region of the DNA from the virulent Northern Ireland Aujeszky-3 (NIA-3) strain of pseudorabies virus (PRV). In the cloned HindIII-B fragment, the MluI-BglII fragment was replaced by different MluI-BglII fragments of the deleted BamHI-7 clones. Transfection of the deleted HindIII-B fragments together with the HindIII-A fragment of either the NIA-3 or the non-virulent NIA-4 strain yielded replication-competent deletion mutants. The region in US in which sequences were deleted specified several mRNAs. Some of the mRNAs present in cells infected with NIA-3 were absent from cells infected with the deletion mutants, whereas other differently sized mRNAs were generated. The mutants were examined with respect to their biological properties in cell culture, mice and pigs. The results showed that the type of cytopathic effect induced in cell culture seemed to be determined by the UL region, using the mean time to death in mice as a parameter, markers for virulence were present in the US and UL regions and the introduction of deletions in US strongly reduced the virulence of PRV for pigs. Despite the impaired capacity of the deletion mutants to induce high titres of neutralizing antibodies in the serum, inoculation with mutants derived from NIA-3 prevented clinical disease in pigs upon challenge with the virulent parent strain. These deletion mutants provide a good basis for the production of bioengineered live PRV vaccines.

Pseudorabies virus as a live virus vector for expression of foreign genes

The cDNA coding for human tissue plasminogen activator (tPA) was cloned downstream from the promoter for pseudorabies virus (PRV) glycoprotein and flanked by downstream PRV DNA. After co-transfection with PRV DNA, this plasmid recombined to insert the tPA cDNA into the viral genome. In cells infected by this recombinant virus, tPA was detected by immunoprecipitation analysis and by enzymatic activity. Since it has a wide host range but does not infect humans, PRV is a possible vaccine vector for genes from animal pathogens.

A necrotizing pneumonia in lambs caused by pseudorabies virus (Aujesky's disease virus)

An outbreak of pseudorabies occurred in sheep housed with swine in the same building. Although the sheep and swine were not in physical contact, the lambs and ewes were exposed to air from the sows' section. Three dead lambs were submitted to the Iowa State University Veterinary Diagnostic Laboratory for necropsy. Grossly there were pulmonary congestion and multifocal pulmonary hemorrhages. Microscopic lesions were severe acute multifocal necrotizing bronchopneumonia with necrotizing vasculitis and intranuclear inclusion bodies within the neurons of the parabronchial ganglia. Bacterial cultures were negative for pathogenic agents; pseudorabies virus was isolated from ovine brain tissue. Viral antigen was demonstrated in the neurons of the parabronchial ganglia by immunoperoxidase staining. Electron microscopy revealed nucleocapsids in the parabronchial ganglionic neurons which contained basophilic intranuclear inclusion bodies. Viral DNA prepared from the ovine pseudorabies virus isolate was found by restriction endonuclease analysis to be related to the Indiana Funkhauser strain of pseudorabies virus.

Replication and virulence of pseudorabies virus mutants lacking glycoprotein gX

Pseudorabies virus (PRV) glycoprotein gX accumulates in the medium of infected cells. In an attempt to study the function of gX, two viruses were constructed that lacked a functional gX gene. One virus, PRV delta GX1, was derived by insertion of the herpes simplex virus thymidine kinase gene into the gX-coding region. The other virus, PRV delta GXTK-, was derived by subsequent deletion of the inserted herpes simplex virus thymidine kinase gene. Both viruses replicated in cell cultures but produced no gX. Furthermore, PRV delta GX1 was capable of killing mice with a 50% lethal dose of less than 100 PFU.

Deletions in vaccine strains of pseudorabies virus and their effect on synthesis of glycoprotein gp63

The pseudorabies virus vaccine strains Norden and Bartha each have been reported to have deletions in the small unique component of the genome (B. Lomniczi, M. L. Blankenship, and T. Ben-Porat, J. Virol. 49:970-979, 1984). The deletion in Norden was shown to delete the entire coding region for gI but not any of the coding sequences for gp63. However, gp63 in Norden-infected cells was only 36 kilodaltons, and a 44-kilodalton form of gp63 was released into the medium. In Bartha, the deletion removed the coding region for all but 89 amino acids of gp63, and no gp63 was detected in either Bartha-infected cells or medium.

Herpesvirus (pseudorabies virus) latency in swine: occurrence and physical state of viral DNA in neural tissues

The occurrence of the pseudorabies virus (PRV, herpes suis 1) genome in various neural tissues of latently infected pigs was investigated. During the latent phase of infection, between 7 and 52 weeks p.i., the average amount of PRV DNA ranged between 0.3 and 0.05 genome copies per cell. The results obtained by in situ cytohybridization and reassociation kinetic experiments indicated that each latently infected cell harbored at least 30 viral genome copies. PRV DNA could be demonstrated in similar frequencies (about 30% of cases) in the trigeminal ganglia, the olfactory bulb, and the medulla oblongata, and less frequently in the brain stem and the spinal cord. Southern blot analysis showed that in general the physical state of the latent genome was linear and nonintegrated. Only in 2 of 15 animals could the presence of circular or concatemeric viral DNA be observed. Thus, we could show that over a period of 13 months after infection the PRV genome persisted both qualitatively and quantitatively in a stable state in different areas of both the peripheral and the central nervous system.

Sequence of the genome ends and of the junction between the ends in concatemeric DNA of pseudorabies virus

The nucleotide sequences of the termini of the mature pseudorabies virus genome and of the junction between these termini in concatemeric DNA were compared. To ensure conservation of unmodified 5' and 3' termini, the end fragments obtained directly (uncloned) from mature viral DNA were sequenced. The sequence obtained from 5' and 3' end labeling revealed that whereas the L terminus was blunt ended, the S terminus had a 2-base (GG) 3' overhang. The sequences spanning the junction between the termini present in concatemeric DNA was also determined and compared with that expected when the two ends of the mature DNA were juxtaposed. This comparison showed that in concatemeric DNA the ends of the mature genome had become joined by blunt-end ligation of one of the strands and that the 2-nucleotide gap on the other strand had been repaired. A significant degree of homology between the sequences spanning the junction between the ends of the varicella-zoster virus and pseudorabies virus genomes was found.

Detection of latent pseudorabies virus in porcine tissue, using a DNA hybridization dot-blot assay

A DNA-hybridization dot-blot technique was developed to detect the presence of pseudorabies virus (PRV) DNA in porcine tissue. Seven 32P-nick translated probes of high specific activity were prepared from transformed Escherichia coli plasmids into which Bacillus amyloliquefaciens H (Bam HI) restriction fragments of PRV-DNA had been inserted. Samples of DNA that had been extracted from porcine tissue or from PRV grown in tissue culture were transferred to nitrocellulose paper, using a microsample filtration manifold and were hybridized to the probes under high-stringency conditions. Under optimal hybridization conditions, the minimum detection amount of PRV-DNA was 10(-11) g, which is equivalent to 1 copy of the PRV genome/80 host cells. Four probes did not show cross hybridization with DNA extracted from tissues of known PRV-negative swine, and these were subsequently used to detect PRV-DNA in infected porcine tissues. Generally, correlation between virus isolation and hybridization data was good for tissues from swine that had died of acute PRV infection. Furthermore, PRV-DNA was present in specific tissues of all 4 seropositive swine that had recovered from pseudorabies and in which no infective virus or viral products were detected at necropsy. Pseudorabies virus DNA was present in the rostralis cerebral cortex (n = 2) or in the medulla oblongata (n = 1) and trigeminal ganglion (n = 1). This probably indicated the portal of entry of the virus into the CNS. In another seropositive pig, there was evidence of a productive infection in the tonsils, although virus was not isolated in a tissue culture system.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of lambda gt11 to isolate genes for two pseudorabies virus glycoproteins with homology to herpes simplex virus and varicella-zoster virus glycoproteins

A library of pseudorabies virus (PRV) DNA fragments was constructed in the expression cloning vector lambda gt11. The library was screened with antisera which reacted with mixtures of PRV proteins to isolate recombinant bacteriophages expressing PRV proteins. By the nature of the lambda gt11 vector, the cloned proteins were expressed in Escherichia coli as beta-galactosidase fusion proteins. The fusion proteins from 35 of these phages were purified and injected into mice to raise antisera. The antisera were screened by several different assays, including immunoprecipitation of [14C]glucosamine-labeled PRV proteins. This method identified phages expressing three different PRV glycoproteins: the secreted glycoprotein, gX; gI; and a glycoprotein that had not been previously identified, which we designate gp63. The gp63 and gI genes map adjacent to each other in the small unique region of the PRV genome. The DNA sequence was determined for the region of the genome encoding gp63 and gI. It was found that gp63 has a region of homology with a herpes simplex virus type 1 (HSV-1) protein, encoded by US7, and also with varicella-zoster virus (VZV) gpIV. The gI protein sequence has a region of homology with HSV-1 gE and VZV gpI. It is concluded that PRV, HSV, and VZV all have a cluster of homologous glycoprotein genes in the small unique components of their genomes and that the organization of these genes is conserved.

Pseudorabies virus gene encoding glycoprotein gIII is not essential for growth in tissue culture

We have established that in the Becker strain of pseudorabies virus (PRV), the glycoprotein gIII gene is not essential for growth in cell culture. This was accomplished by construction and analysis of viral mutants containing two defined deletion mutations affecting the gIII gene. These mutations were first constructed in vitro and introduced into Escherichia coli expression plasmids to verify structure and protein production. Each mutation was then crossed onto PRV by cotransfection of plasmid DNA and parental viral DNA by using gIII-specific monoclonal antibodies as selective and screening reagents. One resultant virus strain, PRV-2, contained an in-frame deletion of a 402-base-pair (bp) SacI fragment contained within the gIII gene. Another virus strain, PRV-10, contained a deletion of a 1,480-bp XhoI fragment removing 230 bp of the upstream, putative transcriptional control sequences and 87% of the gIII coding sequence. The deletion mutants were compared with parental virus by analysis of virion DNA, gIII specific RNA, and proteins reacting with gIII specific antibodies. Upon infection of PK15 cells, the deletion mutants did not produce any proteins that reacted with two gIII specific monoclonal antibodies. However, two species of truncated glycosylated proteins were observed in PRV-2 infected cells that reacted with antiserum raised against bacterially produced gIII protein. PRV-10 produced no detectable gIII-specific RNA or protein. PRV-10 could be propagated without difficulty in tissue culture. Virus particles lacking gIII were indistinguishable from parental PRV virus particles by analysis of infected-cell thin sections in the electron microscope. We therefore conclude that expression of the gIII gene was not absolutely essential for PRV growth in tissue culture.

cis Functions involved in replication and cleavage-encapsidation of pseudorabies virus

Serial passage at high multiplicity of pseudorabies virus generates defective interfering particles (DIPs) whose genomes consist at least in part of reiterations of segments of DNA in which sequences originating from different regions of the genome have become covalently linked (F. J. Rixon and T. Ben-Porat, Virology 97:151-163). To determine whether some cis functions present in these reiterated DNA sequences may be responsible for the amplification of DIP DNA, BamHI restriction fragments of this DNA were cloned. These fragments were analyzed and tested for their ability to promote the amplification of covalently linked pBR325 DNA when cotransfected into cells with helper pseudorabies virus DNA. The cloned DIP BamHI DNA fragments consisted of various combinations of sequences originating from either one or both ends as well as sequences from the middle of the unique long (UL) segment of the genome. Only plasmids with inserts consisting of segments of defective DNA originating from the middle of the UL, as well as from both ends of the genome, were able to replicate and be encapsidated autonomously. This finding indicated that signals present at both ends of the genome may be necessary for efficient cleavage-encapsidation. To confirm this observation, we constructed plasmids in which DNA segments containing an origin of replication and sequences from either one or both ends of the virus genome were linked. These experiments showed that efficient cleavage-encapsidation requires the presence of sequences derived from both ends of the genome. Two origins of replication, one at the end of the UL segment and one in the middle of the UL segment, were also identified.

DNA sequence of the gene for pseudorabies virus gp50, a glycoprotein without N-linked glycosylation

The DNA sequence was determined for a region of the pseudorabies virus (PRV) genome to which a mutation defining resistance to a monoclonal antibody has been mapped (M. W. Wathen and L. M. K. Wathen, J. Virol., 51:57-62, 1984). This sequence was found to contain an open reading frame that did not include an amino acid sequence directing N-linked glycosylation. This open reading frame was expressed in uninfected Chinese hamster ovary cells to produce the PRV glycoprotein gp50. When PRV-infected Vero cells were incubated in the presence of tunicamycin, the gp50 that was produced had an identical molecular weight to that produced in the absence of drug. When infected cells were incubated in the presence of monensin, the molecular weight of gp50 was reduced from 60,000 to 45,000, but was not sensitive to endo-beta-N-acetylglucosaminidase H. These observations led to the conclusion that gp50 does not contain N-linked carbohydrate, as predicted from the DNA sequence. A region of the amino acid sequence and the positions of the cysteine residues of PRV gp50 are homologous to glycoprotein D of herpes simplex virus.

Molecular basis for interference of defective interfering particles of pseudorabies virus with replication of standard virus

Serial passage of pseudorabies virus (PrV) at high multiplicity yields defective interfering particles (DIPs), but the sharp cyclical increases and decreases in titer of infectious virus that are observed upon continued passage at high multiplicity of most DIPs of other viruses are not observed with DIPs of PrV (T. Ben-Porat and A. S. Kaplan, Virology 72:471-479). We have studied the dynamics of the interactions of the virions present in a population of DIPs to assess the cis functions for which the genomes of the DIPs are enriched. The defective genomes present in one population of DIPs, [PrV(1)42], replicate preferentially over the nondefective genomes present in that virion population at early stages of infection, indicating that the DIP DNA is enriched for sequences that can serve as origins of replication at early stages of infection. This replicative advantage of the DIP DNA is transient and disappears at later stages of infection. The defective DNA does not appear to be encapsidated preferentially over the nondefective DNA present in this virion population, which might indicate that it is not enriched for cleavage-encapsidation sites. However, the nondefective DNA in the DIP virion population has become modified and has acquired reiterations of sequences originating from the end of the unique long (UL) region of the genome. Furthermore, both the infectious and defective genomes present in the DIP population compete for encapsidation more effectively than do the genomes of standard PrV. These results indicate that the defective genomes in the population of virions studied are enriched not only for an origin of replication but probably also for sequences necessary for efficient cleavage-encapsidation. Furthermore, the nondefective genomes present in this population of DIPs have also been modified and have acquired the ability to compete with the defective genomes for cleavage-encapsidation.

Location of the structural gene of pseudorabies virus glycoprotein complex gII

The glycoprotein gII, one of the major glycoproteins of pseudorabies virus (PRV), is represented by a complex of three related glycopolypeptides. There is evidence that two of them, linked by disulfide bonds, arise by proteolytic cleavage of the larger precursor glycoprotein. Using specific antisera and a monoclonal antibody against the glycoprotein complex one single nonglycosylated in vitro translated precursor polypeptide with mol wt 110,000 was identified. Mapping of the gene coding for this polypeptide was achieved by hybrid selection of late viral RNA on cloned DNA fragments. The structural gene for the gII complex was found to reside in the long unique part of the PRV genome on BamHI fragment 1 and SalI subfragments 1A and G (map units 0.105 to 0.130). A 3.5-kb mRNA was identified as the probable gII-specific transcript. In addition, further polypeptides encoded in the BamHI fragment 1 were described and RNAs were characterized by Northern blot hybridizations with the cloned SalI subfragments.

Identification and genomic localization of a pseudorabies glycoprotein via expression in Escherichia coli

A glycoprotein (gp-88) secreted by pseudorabies virus (PRV) infected cells was isolated and purified. Anti gp-88 antibodies were used to screen an expression library constructed in Escherichia coli using genomic PRV DNA. Two positive clones were identified and the cloned genetic information was used to localize the corresponding gene in the unique short region of the PRV genome. Antibodies to the glycoprotein were also used to identify the unglycosylated precursor as synthesized in an in vitro translation system. A major precursor of 65 kDa was detected. Although the glycoprotein described here was not found as a major structural glycoprotein of the virion, antibodies to gp-88 are able to neutralise viral activity in vitro. The possible relationship with known glycoproteins of PRV is discussed.

Characterization of a pseudorabies virus glycoprotein gene with homology to herpes simplex virus type 1 and type 2 glycoprotein C

A pseudorabies virus (Becker strain) glycoprotein gene was located in the UL region at map position 0.40. The gene was identified by using open reading frame Escherichia coli plasmid expression vectors and specific antibody reagents. A 1.55-kilobase unspliced transcript from the gene was detected in pseudorabies virus-infected tissue culture cells. The DNA sequence revealed a single open reading frame of 1,437 base pairs encoding 479 amino acids. The predicted primary translation product has a molecular weight of 50,860 and contains features of a typical herpesvirus glycoprotein. An E. coli expression plasmid was constructed that contained essentially all of the open reading frame for this gene. Antibodies raised in rabbits against the protein expressed in bacteria by this plasmid immunoprecipitated pseudorabies virus-specific glycoproteins of 92,000 and 74,000 daltons from infected cell extracts. It is likely that these two forms represent different glycosylation states of the protein.

Characterization and mapping of a nonessential pseudorabies virus glycoprotein

Antigenic variants of pseudorabies virus (PRV) containing mutations in a viral glycoprotein with a molecular weight of 82,000 (gIII) were isolated by selecting for resistance to a complement-dependent neutralizing monoclonal antibody (MCA82-2) directed against gIII. These mutants were completely resistant to neutralization with MCA82-2 in the presence of complement. Two mutants selected for further studies either did not express gIII or expressed an improperly processed form of the glycoprotein. The mutations were also associated with an altered plaque morphology (syncytium formation). The gIII gene was mapped by marker rescue of a gIII- mutant with cloned restriction enzyme fragments to the long unique region of the PRV genome between 0.376 and 0.383 map units. This corresponds to the map location of a glycoprotein described by Robbins et al. (J. Mol. Appl. Gen. 2:485-496, 1984). Since gIII is nonessential for viral replication in cell culture and has several other characteristics in common with the herpes simplex virus glycoprotein gC, gIII may represent the PRV equivalent to herpes simplex virus gC.

Construction of an infectious pseudorabies virus recombinant expressing a glycoprotein gIII-beta-galactosidase fusion protein

An infectious herpesvirus mutant has been constructed in which a major structural envelope glycoprotein gene was replaced by a hybrid gene encoding a novel fusion protein consisting of the N-terminus of the viral glycoprotein joined to Escherichia coli beta-galactosidase (beta Gal). Specifically, we fused DNA encoding the first 157 amino acids of the structural glycoprotein gIII from pseudorabies virus strain Becker to the E. coli lacZ gene in a bacterial expression vector. The resulting hybrid gene was then used to replace the wild-type gIII gene in the virus by cotransfection of plasmid and viral DNA. The desired viral recombinants were identified by their inability to react with specific monoclonal antibodies that recognized only wild-type gIII protein. One such mutant virus, PRV-Z1, was chosen for further analysis. PRV-Z1 expressed a glycosylated gIII-beta Gal fusion protein after infection of PK15 cells. The fusion protein has no demonstrable beta Gal activity and, although glycosylated, remains sensitive to the enzyme endo-beta-N-acetylglucosaminidase H, unlike the mature gIII gene product, indicating that the fusion protein was incompletely processed.

Repression of adenovirus early gene expression by coinfection with a temperature-sensitive mutant in the immediate-early gene of pseudorabies virus

Wild-type adenovirus was coinfected with a mutant temperature sensitive for the immediate-early gene of pseudorabies virus. At the nonpermissive temperature, this mutant, tsG, strongly inhibited the transcription of all adenovirus early genes, including E1A. This inhibition was not observed with wild-type pseudorabies virus coinfection or with tsG coinfection at the permissive temperature. The level of repression was dependent upon the ratio of tsG to adenovirus in the infection. The results suggest that the temperature-sensitive protein may be interacting with transcription factors on the viral DNA or with the DNA itself to inhibit adenovirus transcription.

Proteins specified by the short unique region of the genome of pseudorabies virus play a role in the release of virions from certain cells

Two pseudorabies virus vaccine strains (Bartha and Norden) that have a similar deletion in the short unique (Us) region of the genome have been identified previously (B. Lomniczi, M. L. Blankenship, and T. Ben-Porat, J. Virol. 49:970-979, 1984). These strains do not code for the glycoprotein gI, a glycoprotein that has been mapped on the wild type virus genome by T. C. Mettenleiter, N. Lukacs, and H. J. Rziha (J. Virol. 53:52-57, 1985) to the sequences deleted from the vaccine strain. Restoration of these deleted sequences to the Bartha strain genome restores to the virus the ability to specify the gI glycoprotein. The Bartha vaccine strain grows as well as wild-type virus in pig kidney and in rabbit kidney (RK) cells, but is not released efficiently from and forms small plaques in RK cells. The rescued Bartha 43/25a strain (which has an intact Us) is released considerably more efficiently than the Bartha vaccine strain, but less efficiently than wild-type virus from RK cells; it also forms larger plaques on RK cells than does the parental Bartha vaccine strain. The Norden vaccine strain, which has a deletion in the Us, is released better from RK cells than is the Bartha strain, but not as well as is wild-type virus. We conclude that whereas the sequences in the Us that are deleted from the Bartha and Norden strain genomes specify functions that play a role in the release of virions from some cell types, at least one other function (which is defective in the Bartha strain but not in the Norden strain) also affects release of virus from these cells. Since restoration to the Bartha strain of an intact Us restores to the virus both the ability to grow in chicken brains (B. Lomniczi, S. Watanabe, T. Ben-Porat, and A. S. Kaplan, J. Virol. 52:198-205, 1984) and to be released from RK cells, the possibility that the lack of virulence of the Bartha vaccine strain may be related to its limited release from some target cells is discussed.

Herpesviruses provide helper functions for avian adeno-associated parvovirus

The avian herpesviruses infectious laryngotracheitis virus (ILTV) and herpesvirus of turkeys (HVT), as well as the mammalian herpesvirus pseudorabies virus (PRV) were able to provide complete helper activity for the production of infectious avian adeno-associated virus (AAAV) in chicken cells. The presence of AAAV in the infected chicken cell reduced the multiplication of HVT. ILTV or PRV, however, were not affected if used as helper viruses. Infectious AAAV was determined by an indirect immunofluorescence assay and infectious herpesvirus by plaque assays.

Demonstration of pseudorabies virus DNA in the mouse inner ear by an in situ nucleic acid hybridization technique in plastic embedded bony material

This investigation is concerned with the possibility of identifying viral DNA using the in situ DNA hybridization method in methylmethacrylate-embedded material. As an experimental model we chose viral labyrinthitis produced by intranasal infection of the mouse with pseudorabies virus. Fixation and embedding methods specially adapted to this procedure and bony histology preparation technique (specimens by grinding or micromilling) made it possible to identify viral DNA directly morphologically and virologically in the inner ear. Quantitative microphotometric analyses of trans-sagittal sections of the entire skull after in situ DNA hybridization are presented and discussed here as an explicit method of investigating the path of distribution of viral DNA in the brain and the inner ear.

In vitro stimulation of specific RNA polymerase II-mediated transcription by the pseudorabies virus immediate early protein

Nuclear extracts from human cells infected with pseudorabies virus (PRV) exhibited higher levels of accurate transcription of RNA polymerase II genes than did control extracts from mock-infected cells. Stimulation was maximal at low DNA concentrations and was not gene-specific. It was heat sensitive in extracts from cells infected with a virus containing a temperature sensitive mutation in the immediate early (IE) gene. The stimulatory activity copurified from the IE protein and was also heat sensitive when purified with cells infected with tsG, further indicating that the IE protein was responsible for this stimulation. These results thus demonstrate an in vitro system that mimics, at least in part, the in vivo stimulatory action of the PRV IE protein. They further imply that the IE protein acts not by increasing the amounts of cellular transcription factors, but rather by directly or indirectly altering their activities.

The expression of viral functions is necessary for recombination of a herpesvirus (pseudorabies)

To determine whether viral functions are necessary for recombination of the pseudorabies virus genome in infected cells, we have used as a model system marker rescue at the permissive temperature (PT) and nonpermissive temperature (NPT) of a temperature sensitive mutant (tsG1) deficient in the immediate-early (180K) protein. Two restriction fragments, both of which can rescue tsG1 at the PT but only one of which encompasses the whole immediate-early gene and can complement tsG1, were compared for their ability to rescue the mutant at the NPT. Although both restriction fragments rescued the mutant with equal frequency at the PT, only the fragment which could express the immediate-early 180K protein prior to recombination, i.e. could complement tsG1, rescued the mutant at the NPT. We conclude that the expression of viral functions is necessary for high frequency recombination of the pseudorabies virus genome.

Transcription of class III genes activated by viral immediate early proteins

The adenovirus EIA and pseudorabies virus immediate early (IE) proteins induce transcription from transfected viral and nonviral genes transcribed by RNA polymerase II (class II genes). These proteins have now been shown also to activate transcription of transfected genes transcribed by RNA polymerase III (class III genes). As previously observed for class II genes, this stimulation of class III gene transcription was much greater for transfected genes than for the major endogenous cellular class III genes. Extracts made from cell lines stably expressing a transfected pseudorabies virus IE gene were 10 to 20 times more active in the in vitro transcription of exogenously added class III genes than extracts of the parental cell line. These results indicate that the E1A and IE proteins stimulate the expression of class III genes by a mechanism similar to the mechanism for stimulation of class II gene transcription by these proteins.

Comparative usefulness of tissue fixatives for in situ viral nucleic acid hybridization

Traditionally tissues for in situ hybridization of viral nucleic acid have been small pieces obtained from laboratory rodents, and fixatives that are designed for electron microscopy, such as periodate-lysine-paraformaldehyde (PLP) can handle them adequately. However, these fixatives have limited penetrating ability and may produce no appreciable hardening, so alternative fixation methods were evaluated. The intention was to determine whether fixatives adequate for bulky tissues such as whole or halved pig and cow brains would also be compatible with in situ hybridization. Various fixatives were evaluated using a system of intracranial inoculation of BALB/c mice with pseudorabies virus (PRV) followed by in situ hybridization of brain tissue sections with a 35S-labeled PRV DNA probe. Loss of tissue sections was a major problem, particularly with PLP and formalin, but positive results were obtained with five fixatives tested. Cellular morphology was especially good with PLP and with a modification of Carnoy's fluid, MOCA fixative. An incidental but important observation was that formalin is compatible with in situ hybridization. Retroactive studies of viral diseases using routinely processed blocks of tissue (formalin-fixed, paraffin-embedded) are therefore conceivable.

Restriction endonuclease analysis of Aujeszky's disease (pseudorabies) virus DNA: comparison of Northern Ireland isolates and isolates from other countries

Aujeszky's disease (AD; pseudorabies) viruses isolated in Northern Ireland over a 20 year period were compared with isolates from other parts of the world using restriction endonuclease analysis of virus DNA. When the numbers of Bam H1, Kpn 1 and Sal 1 restriction sites were considered, pathogenic Northern Ireland isolates resembled viruses isolated in England, Hungary and the U.S.A. and could be differentiated from viruses isolated in Denmark, Belgium and the Netherlands. The avirulent Northern Ireland isolate NIA4 and the Bartha vaccine strain were very similar to each other and could be distinguished from pathogenic isolates. While almost all the pathogenic viruses isolated in Northern Ireland from 1963 to 1983 appeared to possess the same number of restriction sites none of the viruses, even those made at the same farm during one outbreak of infection, were identical. The differences were confined to variation in the sizes of certain fragments which map in "variable" regions of the genome.

A comparison of the genomes of bovine herpesvirus type 1 and pseudorabies virus

The DNA sequence homology between bovine herpesvirus type 1 (BHV-1) and pseudorabies virus (PRV) was examined. Reciprocal cross-hybridization of viral DNA labelled by nick translation to Southern blots of restriction endonuclease-digested DNA detected homologous sequences dispersed throughout the genomes of the two viruses. The DNA-DNA hybrids formed were stable under high-stringency wash conditions. Sequences of a 32P-labelled PRV DNA fragment probe were found to hybridize only to a specific region of the BHV-1 genome, suggesting that the detected sequence homology was not due to fortuitous hybridization of G + C-rich sequences. As measured by liquid reassociation kinetics the homology between these two viruses was approximately 8%.

Relation between the levels of mRNA abundance and kinetics of protein synthesis in pseudorabies virus-infected cells

Virus proteins synthesized by pseudorabies virus-infected cells can be classified into five groups on the basis of the kinetics of their synthesis at various stages of the infective process; virus mRNAs can similarly be classified into four groups. To determine whether the kinetics of synthesis of specific proteins are determined solely by the level of abundance in the cells of their mRNAs, we have compared at various times after infection the relative synthesis of these proteins with the relative abundance of their mRNAs. We have focused on two proteins: the 142K major capsid protein, an early-late protein, and the 136K major DNA binding protein, an early protein. The mRNAs encoding these proteins were identified. The relative abundances of these mRNAs in the cytoplasms of the infected cells were found to be the same as those associated with the polysome fractions. The relative amount of the proteins synthesized by the infected cells at a given stage of the infective process closely reflected the relative amount of the mRNA encoding these proteins that was present in the cells at that stage of the infective process. Most virus mRNA species that are present in the cytoplasm of infected cells were represented on polysomes to approximately an equal extent. Some RNA species were, however, significantly underrepresented under certain conditions in the polysomal fractions. We conclude that whereas the amount of many virus proteins synthesized by the infected cells is determined mainly by the level of the abundance of their mRNAs, additional controls operate in the cells that determine the relative rates of synthesis of some other virus proteins.

Genome differences among field isolates and vaccine strains of pseudorabies virus

The DNA of field isolates and vaccine strains of pseudorabies virus (PRV) was analysed by digestion with the restriction endonuclease BamHI. A number of distinct restriction profiles of the field isolates obtained from different locations within Europe were observed. As for herpes simplex virus, the variations could be classified into two types: first, alterations in the mobility of fragments due to the presence of additional sequences and/or the occurrence of deletions, a phenomenon most apparent in fragments containing part or whole of the repeat sequence of PRV DNA; second, generation of differently sized fragments due to loss and/or gain of restriction endonuclease cleavage sites. By analysis of several strains with BamHI a small number of variable cleavage sites were identified within particular regions of the unique long (UL) segment of the PRV genome. Compared to wild-type PRV, the restriction fragment patterns of vaccine strains showed characteristic alterations, including the absence of bands, which were non-variable in wild-type strains, and/or the presence of new bands some of which were submolar. Some of these characteristics could be explained by a deletion in the unique short (US) region of the genome of most vaccine strains and the occurrence of closely related variants in the uncloned vaccine virus stocks.

A method for efficient gene isolation from phage lambda gt11 libraries: use of antisera to denatured, acetone-precipitated proteins

Experience with cloning pseudorabies virus (PRV) DNA in the lambda gt11 phage vector has shown that there are special requirements for the antisera used in screening the libraries, in addition to the requirement that the antisera recognize proteins on a Western blot. Initial screening of a lambda gt11 library of sheared PRV DNA fragments in Escherichia coli for expression of PRV antigens using PRV hyperimmune antisera was unsuccessful. It was only after screening the library with antisera raised against PRV proteins eluted from sodium dodecyl sulfate (SDS)-polyacrylamide (PA) gels that positive results were obtained. These "gel-slice" antisera (GSA) were equivalent in potency to hyperimmune antisera in standard immunoassays (including ELISA, immunoprecipitation, Western blots, and neutralization of virus), but only the GSA could recognize PRV fusion proteins expressed by recombinant lambda gt11 phage. This difference was seen despite the fact that hyperimmune antisera performed satisfactorily on Western blots of denatured PRV-infected cell extracts. These results show that the efficiency of screening expression libraries in E. coli can be improved if antibodies are raised against denatured proteins.