Demonstration of three major species of pseudorabies virus glycoproteins and identification of a disulfide-linked glycoprotein complex

The glycoproteins of pseudorabies virus (PRV) Phylaxia were characterized with monoclonal antibodies as specific reagents. Three major structural glycoproteins with molecular weights of 155,000 (155K) (gC), 122K (gA), and 90K (gB) could be identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions. We investigated the processing of glycoproteins gA, gB, and gC by in vitro translation, pulse-chase experiments, and in the presence of the ionophore monensin which inhibits glycosylation. gA and gB were found to compose a single polypeptide, whereas gC was found to be a disulfide-linked glycoprotein complex. Immunoprecipitates formed with the aid of anti-gC monoclonal antibodies gave rise to three glycoprotein bands (gC0 [120K], gC1 [67K], and gC2 [58K]) by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Limited proteolysis of gC0, gC1, and gC2 resulted in peptide maps of gC0 related to those of both gC1 and gC2. No common peptide bands between gC1 and gC2, however, were seen. We suggest that (i) gC1 and gC2 arise by proteolytic cleavage from the same precursor molecule and stay joined via disulfide bridges and (ii) gC0 is an uncleaved precursor.

Differential association with cellular substructures of pseudorabies virus DNA during early and late phases of replication

Pseudorabies virus DNA synthesis can be divided into two phases, early and late, which can be distinguished from each other on the basis of the structures of the replicating DNA. The two types of replicating virus DNA can also be distinguished from each other on the basis of the cellular substructures with which each is associated. Analysis by electron microscopic autoradiography showed that during the first round of replication, nascent virus DNA was found in the vicinity of the nuclear membrane; during later rounds of replication the nascent virus DNA was located centrally within the nucleus. The degree of association of virus DNA synthesized at early and late phases with the nuclear matrix fractions also differed; a larger proportion of late than of early nascent virus DNA was associated with this fraction. While nascent cellular DNA only was associated in significant amounts with the nuclear matrix fraction, a large part (up to 40%) of all the virus DNA remained associated with this fraction. However, no retention of specific virus proteins in this fraction was observed. Except for two virus proteins, which were preferentially extracted from the nuclear matrix, approximately 20% of all virus proteins remained in the nuclear matrix fraction. The large proportion of virus DNA associated with the nuclear fraction indicated that virus DNA may be intimately associated with some proteins. Indeed, protease-treated, "purified" DNA preparations contained two proteins (15K and 10K) with histone-like properties which were protected by the DNA from deproteinization, probably by virtue of being at the center of the concatemeric tangles of virus DNA. It is possible that these proteins play a role in anchoring virus DNA to the nuclear matrices.

Comparison of the genomes of pseudorabies (Aujeszky's disease) virus strains by restriction endonuclease analysis

The DNA of pseudorabies virus (PRV) strains from the United States and Taiwan and attenuated vaccine strains from Romania were compared by restriction endonuclease analysis. Electrophoretically separated PRV DNA fragments of KpnI and BamHI digests demonstrated cleavage pattern variations which clearly distinguished the 3 Taiwan isolates from all other strains, as well as from each other. One type of variation involved the loss or gain of restriction endonuclease cleavage sites. Examples of this type of variation were clearly observed in fragment patterns of the Taiwan isolates. Another type of variation that occurred at higher frequency for fragments mapping in the repeat and repeat-unique joint regions of the PRV genome involved sequence additions or deletions from existing fragments. This second type of variation occurred in most of the strains analyzed.

Genetic basis of the neurovirulence of pseudorabies virus

Lomniczi et al. (J. Virol. 49:970-979, 1984) have shown previously that two attenuated vaccine strains of pseudorabies virus have a similar deletion in the short unique (US) region of the genome. The region which is deleted normally codes for several translationally competent mRNAs. As expected, these mRNAs are not formed in the cells infected with the vaccine strains. The function specified by these mRNAs is thus not necessary for growth in cell culture. Using intracerebral inoculation of 1-day-old chicks as a test system, we have attempted to determine whether a gene within the region that is missing from the attenuated strains specifies functions that are required for the expression of virulence. An analysis of recombinants between the Bartha vaccine strain and a virulent pseudorabies virus strain (having or lacking a thymidine kinase gene [TK+ or TK-]) revealed the following. None of the recombinant plaque isolates that were either TK- or which had a deletion in the US was virulent. Not all recombinant plaque isolates which were both TK+ and had an intact US were virulent. These results indicate that both thymidine kinase activity and an intact US were necessary but not sufficient for the expression of virulence. Marker rescue experiments involving cotransfection of the Bartha strain DNA and a restriction fragment spanning the region of the genome that was missing from the Bartha strain resulted in the isolation of virions to which an intact US had been restored. These virions were not virulent but had an improved ability to replicate in the brains of chicks compared with that of the parental nonrescued Bartha strain. Our results show that genes in the US region, which are missing from the Bartha strain, were necessary for virulence but that this strain was also defective in other genes required for the expression of virulence. Thus, the virulence of pseudorabies virus, as measured by intracerebral inoculation into chicks, appears to be controlled multigenically.

Synthesis of alpha (immediate-early) proteins in Vero cells infected with pseudorabies virus

The synthesis of alpha (immediate-early) polypeptides in Vero cells infected with pseudorabies virus was studied. Cycloheximide was added at the beginning of infection and removed several hours later. The accumulated alpha mRNA was translated either in vivo in the presence of actinomycin D to prevent further mRNA synthesis, or in vitro. In intact cells three electrophoretically distinct virus-specific proteins were synthesized, with apparent molecular weights of approximately 180 000 (A), 190 000 (B) and 200 000 (C). The accumulation of B and C was prevented by the proline analogue azetidine. Only protein A was detected in vitro. Proteins B and C were not detected in normally infected cells. All three were associated with the nuclear fraction of cell homogenates and A and B were phosphorylated. The radioactivity of B and C declined during a chase period while that of A increased. This change was prevented by adding cycloheximide during the chase. The pattern of chymotrypsin digestion products suggested that A and B at least were similar proteins. It is presumed that protein A is the single immediate-early protein previously described and analogous to ICP 4 of herpes simplex virus. The significance and function, if any, of proteins B and C is not known but it is possible that they represent stages in the formation or transport of A within the cell and that the progression depends on an unstable protein which is depleted in cells treated with cycloheximide.

Some characteristics of four attenuated vaccine virus strains and a virulent strain of Aujeszky's disease virus

In the present study four attenuated virus strains, used as vaccines, and a virulent strain of Aujeszky's disease virus (ADV) were compared with respect to their virulence in mice, their ability to induce virus-specified thymidine kinase (TK) in infected cells, and their cleavage profiles of viral DNA's after treatment with the restriction endonuclease KpnI. The survival time of mice inoculated with the B-KAL or the virulent NIA-3 strain was comparable, whereas the Bartha and BUK strains required significantly longer periods to kill mice. Mice were resistant to the MK-25 strain of ADV. The strains were assayed for TK phenotype by plaque autoradiography after 3H-thymidine labelling of infected cells. MK-25 proved to be the only strain defective in induction of TK in pig kidney cells. Restriction endonuclease analysis of viral DNA's revealed that each vaccine strain showed a characteristic fragment pattern that could easily be differentiated from that of other vaccine and field strains of ADV. The present results demonstrate that the mouse virulence test and the TK assay detect differences in biological properties of ADV strains, but that restriction endonuclease analysis is required for unambiguous identification of vaccine and field strains of ADV.

Stability of the pseudorabies virus genome after in vivo serial passage

Restriction endonuclease patterns of pseudorabies virus (PRV) DNA were examined after each of 11 serial passages of the virus through pigs. Minor variations in the electrophoretic mobility of certain restriction enzyme fragments were observed by the sixth passage. This variability was similar to some of the minor variability observed in field isolates. The variable fragments were mapped to three locations on the PRV genome: the junction between the short unique sequences and the repeat sequences, the terminus of the long unique region, and an internal area of the long unique region.

Isolation, characterization, and physical mapping of a pseudorabies virus mutant containing antigenically altered gp50

A pseudorabies virus variant ( mar197 -1) containing a mutation in a viral glycoprotein with a molecular weight of 50,000 ( gp50 ) was isolated by selecting for resistance to a neurtralizing monoclonal antibody ( MCA50 -1) directed against gp50 . This mutant was completely resistant to neutralization with MCA50 -1 in the presence or absence of complement, and was therefore defined as a mar (monoclonal-antibody-resistant) mutant. The mutation did not affect neutralization with polyvalent immune serum. The mar197 -1 mutant synthesized and processed gp50 normally, but the mutation prevented the binding and immunoprecipitation of gp50 by MCA50 -1. Thus, the mutation was within the structural portion of the gp50 gene affecting the epitope of the monoclonal antibody. The mutation was mapped by marker rescue with cloned pseudorabies restriction enzyme fragments to the short region of the pseudorabies genome between 0.813 and 0.832 map units. This is equivalent to a 2.1-kilobase-pair region.

Stability of virulent pseudorabies (Aujeszky's disease) viral genome after single passage through nonswine animal species

This experiment was done to determine the effect(s) of single passage of pseudorabies virus in dead-end hosts on the stability of the pseudorabies virus genome. Calves, dogs, rabbits and cats were inoculated with a virulent strain of pseudorabies virus and the virus was reisolated from each animal and restriction endonuclease analysis was used to determine possible alterations in the DNA banding patterns. The restriction fragment migration profile of the pseudorabies virus DNA isolated from the animals was indistinguishable from the DNA profile of the original pseudorabies virus inoculum. The restriction endonuclease viral DNA profile appears to be relatively stable after a single passage in dead-end hosts, although minor changes in the viral genome that are not detectable in the DNA banding pattern may have occurred.

The large late virus transcripts synthesized in herpesvirus suis (pseudorabies) virus-infected cells are not precursors of mRNA

The sizes of early and late pseudorabies virus transcripts were compared to those of early and late mRNA. While the early primary transcripts were of approximately the same size as early mRNA, a large proportion of late primary transcripts was much larger than late mRNA. Furthermore, most early transcripts were transported efficiently to the cytoplasm and were relatively stable. In contrast, a large proportion of the late transcripts were retained in the nucleus and turned over rapidly. Specific retention in the nucleus of transcripts originating from some regions of the genome could be detected. These were, however, not preferentially degraded; degradation of transcripts originating from all regions of the genome, including those from which late mRNA originates, occurred. Experiments designed to determine whether part of the large transcripts are processed into mRNA revealed that most of the large late transcripts synthesized by the infected cells bear no precursor relationship to mRNA. Thus, during late phase of infection, most regions of the genome are abundantly transcribed as large RNA molecules; these are not destined to be processed into mRNA.

Deletions in the genomes of pseudorabies virus vaccine strains and existence of four isomers of the genomes

As part of a study designed to identify the genes responsible for the virulence of pseudorabies virus, we have mapped the genomes of two independently derived vaccine strains (Bartha and Norden) by restriction enzyme analysis. The structures of these genomes have been compared with that of the genome of a laboratory strain previously mapped, of restriction fragments which had been cloned. The genome of the Bartha strain was found to be very similar to that of other pseudorabies virus strains, except that a deletion of approximately 2.7 X 10(6) daltons was found in the unique short (US) region. This deletion was also observed in the genome of the Norden vaccine strain but was not observed in the genomes of any other pseudorabies virus strains that have been studied (more than 20). The genome of the Norden strain differs from that of other pseudorabies virus strains in several other respects as well. The most important difference is that in contrast to all other pseudorabies virus strains analyzed to date, which contain a type 2 herpesvirus DNA molecule (in which the US region only inverts itself relative to the unique long [UL] region), the genome of the Norden strain is a type 3 molecule in which both the US and the UL regions of the genome invert themselves, giving rise to four isomeric forms of the genome. The ability of the UL region to invert itself is probably related to the fact that a sequence normally present in all other pseudorabies virus strains at the end of the UL only is found also in inverted form at the junction of the UL and the internal inverted repeat in the Norden strain.

Equalization of the inverted repeat sequences of the pseudorabies virus genome by intermolecular recombination

During a study designed to identify changes in the genomes that are observed in mutant populations of pseudorabies virions, a thymidine kinase-defective population of virions which contains genomes that possess inverted repeated sequences of unequal sizes has been identified. This population of virions has been used to ascertain the mechanism and rates of equalization of the repeats. Results showed that when the virions were passed in cell cultures at high multiplicities of infection (either once or several times), one-half of the molecules had repeats of equal size and one-half had repeats of unequal size. This result is to be expected if conditions allowing a high degree of intermolecular recombination involving exchange of repeats exist. The process of intermolecular exchange of inverted repeats is rapid and the DNA molecules were at equilibrium by the time a virion had produced a small plaque. If the virions were passed in cell culture at low multiplicities of infection, they acquired genomes with equal-size repeats. This is probably due to segregation of the virions which had acquired genomes with equal repeats by recombination; in the absence of coinfection of the cells with virions containing genomes with different repeat sizes, heterogeneity of the repeats could not be generated. The fact that virions containing genomes with equal repeats are usually isolated from nature can thus be attributed to equalization of the repeats which results from intermolecular recombination, followed by segregation. A special mechanism ensuring equalization of repeats, such as one repeat acting as a template for the synthesis (or repair) of the other, need therefore not be invoked.

Characterization of the immediate-early functions of pseudorabies virus

The immediate-early transcripts of pseudorabies virus have been located in a region of the genome situated internally within the inverted repeat between map positions 0.99 and 0.95. A single immediate-early transcript (approximately 6 kb) can be detected both in the cytoplasmic and nuclear fractions of infected, cycloheximide-treated cells. Analysis of the proteins synthesized after removal of cycloheximide from infected cells or after translation in vitro of the RNA isolated from these cells revealed the presence of a single protein (180K) not present in similarly treated, uninfected cells. That this is a virus protein and is specified by the immediate-early region of the genome was shown by selection and translation of mRNA hybridizing with virus DNA from the appropriate region of the genome. The effects of infection of cells with a temperature-sensitive mutant (tsG1) defective in the 180K protein were studied. At the nonpermissive temperature only immediate-early RNA was transcribed and only one virus protein, the 180K protein was synthesized. Inhibition of cellular protein and DNA synthesis was also observed. After shift down of tsG1-infected cells from the nonpermissive to the permissive temperature at 3 hr post infection, a transition to early RNA transcription occurred. However, if the shift down was delayed until 5 hr post infection, transcription of the virus genome was completely inhibited and an abortive infection ensued. Shift of the mutant-infected cells from the permissive to the nonpermissive temperature resulted in a decrease in the rate of accumulation of early and late transcripts, and a resumption of the synthesis of immediate-early RNA and protein. From these as well as from previous results, it is concluded that pseudorabies virus codes for a single multifunctional immediate-early protein which is essential for the transcription of immediate-early to early RNA and is required for the continuous transcription of early (and late) RNA. The immediate-early protein is also self-regulatory; the presence of the functional immediate-early protein represses the transcription of its RNA. In addition, the immediate-early protein of pseudorabies virus appears to play a direct role, under certain conditions, in the inhibition of cellular macromolecular synthesis.

Transcriptional activation of cloned human beta-globin genes by viral immediate-early gene products

When the human beta-globin gene is transfected into Hela cells, no beta-globin RNA is detected unless the gene is linked to a viral transcription enhancer. In this paper we show that trans-acting adenovirus and herpesvirus (pseudorabies) transcriptional regulatory proteins can circumvent this enhancer requirement for detectable beta-globin transcription in transient expression assays. The viral gene products can be provided by constitutively expressed, integrated viral genes in established cell lines, by viral infection of permissive cells, or by transfection of cells with bacterial plasmids carrying the viral immediate-early genes. These results demonstrate the utility of transient expression assays for studying regulatory mechanisms involving trans-acting factors. Analysis of beta-globin promoter mutants indicates that between 75 and 128 bp of sequence 5' to the mRNA cap site is required for enhancer-dependent transcription in Hela cells. In contrast, beta-globin transcription in the presence of viral immediate-early gene products requires only 36 bp of 5'-flanking sequence, which includes the TATA box. Thus both cis and trans-acting viral factors activate beta-globin gene transcription in transient expression experiments, but the mechanisms by which they act appear to be fundamentally different.

Activation of gene expression by adenovirus and herpesvirus regulatory genes acting in trans and by a cis-acting adenovirus enhancer element

A plasmid containing the adenovirus E2 gene, a gene normally requiring E1A-mediated induction during viral infection, is expressed very poorly upon transfection into mouse L cells. If the same plasmid is transfected into 293 cells, which constitutively express the adenovirus E1A gene, or into L cells together with a plasmid containing the E1A gene, the E2 gene is expressed at higher levels. Cotransfection of the E2 plasmid with a plasmid containing the pseudorabies virus (a herpesvirus) immediate early gene results in an even higher increase in the level of E2 expression. In addition, efficient E2 expression in the absence of trans induction was obtained by inserting E1A upstream promoter sequences at the 5' or 3' end of the E2 gene, indicating that these E1A sequences possess enhancer properties. Thus the efficient expression of the E2 gene can be obtained either by a structural change in the gene itself or by a trans-acting induction.

Restriction endonuclease analysis of the pseudorabies (Aujeszky's disease) virus before and after serial passage in vivo and in vitro

The deoxyribonucleic acid (DNA) of pseudorabies virus (PRV) was examined by restriction endonuclease analysis before and after various treatments: namely 1. plaque purification of stock virus, 2. serial passage of virus in cell culture at high (H) and low (L) multiplicity of infection (MOI), and 3. serial passage of virus in swine. The objective of the study was to determine if such treatments would either select or induce virus populations with a different predominant number or location of enzyme cleavage sites. Heterogeneity of the DNA of stock virus was revealed by differences among restriction patterns of 10 plaque-purified populations. All of these populations were then relatively stable through an additional 9 plaque purifications and passages in cell culture. Without plaque purification, heterogeneity was not evident. The predominant restriction pattern of stock virus appeared unaltered by 10 serial passages in cell culture at either HMOI or LMOI, except for the appearance in the HMOI series of several minor bands. Conversely, 10 serial passages of stock virus in swine resulted in either selection or induced changes or both. Most differences were slightly altered migration rates of relatively small (less than or equal to 5 Kbp) fragments. However, after 10 passages in swine, there were also changes in the migration rates of 2 large fragments (greater than 10 Kbp) of the viral genome.

The role of pseudorabies virus thymidine kinase expression in trigeminal ganglion infection

The role of pseudorabies virus (PRV) thymidine kinase (TK) expression in the pathogenesis of PRV infection of mice was studied with TK-negative (TK-) mutants. Thymidine phosphorylation and arabinosylthymine inhibition of PRV replication and efficiency of plating were used to characterize TK+ and TK- PRV. In addition, a plaque autoradiography procedure was utilized to determine the TK phenotype of individual plaques. TK+ and TK- PRV replicated well in ocular tissues, while TK+ but not TK- did so in ganglion tissue. Mortality was absent after TK- PRV inoculation and widespread after inoculation of similar amounts of TK+ PRV. Latent infection in mice was not detected with either TK+ or TK- PRV. This study indicated the probable importance of PRV TK expression in acute trigeminal ganglion infection.

Localization of the regions of homology between the genomes of herpes simplex virus, type 1, and pseudorabies virus

Only 8% of the sequences of the genomes of pseudorabies (PRV) and herpes simplex (type 1) (HSV) viruses are homologous. These homologous sequences have been shown previously to be distributed throughout most of the genomes of the two viruses. By means of blot hybridization of restriction fragments of HSV-1 DNA to cloned, nick-translated restriction fragments of PRV DNA, it was possible to compare the location on the genomes of these viruses of the homologous regions. The results showed that the genome of PRV is, for the most part, colinear with the IL arrangement of the genome of HSV-1. An inversion or translocation of sequences mapping on the PRV genome between 0.07 and 0.39 map units was observed on the genome of one of these viruses. A comparison of the map positions of five genes with known functions confirmed these findings. The genes coding for the major immediate-early protein, the major capsid protein, and the thymidine kinase occupy similar positions on the genome of PRV and on the genome of HSV-1 in the IL arrangement. However, the genes for DNA polymerase and for the major DNA binding protein appear to be inverted relative to one another on the genomes of the two viruses.

Functions of the major nonstructural DNA binding protein of a herpesvirus (pseudorabies)

Eight mutants of pseudorabies virus belonging to complementation group 3 and situated between 0.14 and 0.18 units on the physical map of the genome were analyzed. All the mutants tested in this respect (seven) recombined with one another, indicating that the mutations were located in different regions of the gene. All mutants were DNA-; the first round, as well as subsequent rounds, of DNA replication was completely blocked at the nonpermissive temperature in the mutant-infected cells. After shift-up from the permissive to the nonpermissive temperature, viral DNA synthesis continued for a short period of time only and viral DNA which had accumulated at the permissive temperature became degraded. Parental viral DNA, however, retained its integrity at the nonpermissive temperature and viral DNA synthesis started immediately after shift-down of the mutant-infected cells from the nonpermissive to the permissive temperature (even in the absence of protein synthesis). All mutants belonging to complementation group 3 tested (5 out of 8) produced a thermolabile nonstructural DNA binding protein (136K). In some of the mutant virus-infected cells this protein failed to migrate to the nucleus. We conclude that the pseudorabies virus mutants in complementation group 3 code for a defective 136K protein and that this protein is not only essential to the process of viral DNA synthesis but also plays a role in the stabilization of progeny DNA (but not of nonreplicating parental DNA) within the infected cells.

Recombination occurs mainly between parental genomes and precedes DNA replication in pseudorabies virus-infected cells

The experiments described in this paper were part of an attempt to determine the mechanisms involved in the isomerization of the pseudorabies virus genome. To this end, [(14)C]thymidine-labeled parental virus DNA that was transferred to progeny virions produced by cells incubated in medium containing bromodeoxy-uridine was analyzed in neutral and alkaline CsCl density gradients. The buoyant density of the (14)C-labeled DNA indicated that the parental DNA strands had retained their integrity and had not undergone breakage and reunion with progeny DNA strands; neither massive intermolecular nor intramolecular recombination had occurred after replication of the DNA. Whereas breakage and reunion between parental and progeny virus DNA strands were not detectable, these processes were observed between differentially density-labeled parental DNAs. Furthermore, the frequency of recombination between progeny DNAs accumulating in the cells was low. These results indicate that in pseudorabies virus-infected rabbit kidney cells recombination occurs mainly between parental genomes and precedes DNA replication. An analysis of the kinetics of appearance of recombinants between pairwise combinations of temperature-sensitive mutants also indicated that recombination is an early event. The ratio between the number of recombinant virions and the number of temperature-sensitive mutant virions produced by the cells remained the same throughout infection. Since the relative amounts of viral DNAs synthesized early and late during the infective process that were integrated into virions were approximately the same, it appears that late viral DNA did not experience an increased number of recombinational events compared with early viral DNA. These results, which reinforce the conclusion reached from the results of the analysis of the behavior of the parental DNA molecules in density shift experiments, indicate that recombination is an early event.

Activation of early adenovirus transcription by the herpesvirus immediate early gene: evidence for a common cellular control factor

Adenovirus mutants carrying a defective E1A gene, such as dl312, are unable to express any of the early viral genes upon infection of HeLa cells. However, efficient expression of the other early adenovirus genes was obtained when dl312-infected HeLa cells were coinfected with pseudorabies virus, a herpesvirus. By employing a temperature-sensitive pseudorabies mutant (tsG1) it was demonstrated that the herpesvirus function responsible for the induction of adenovirus transcription was the immediate early gene, a gene required for the activation of herpesvirus early gene expression and the maintenance of early and late herpesvirus transcription. Specifically, HeLa cells coinfected with dl312 and tsG1, when shifted to the nonpermissive temperature, lost their capacity to express the early adenovirus genes. Furthermore, activation of early adenovirus gene expression in herpesvirus coinfection occurred earlier and at a higher level than in wild-type adenovirus infection. Therefore, the herpesvirus immediate early protein not only activates the early adenovirus transcription units but apparently does so more efficiently than the adenovirus E1A gene product. Because of this fact, we argue that the activation, either by the E1A protein or the herpesvirus immediately early protein, most likely occurs indirectly through interaction with a cellular protein rather than by a direct recognition of regulatory sequences at the adenovirus promoters.

Behaviour of ts mutants of pseudorabies virus in the range between permissive and nonpermissive temperature

The growth properties of pseudorabies virus (PRV) strain V BUK, wild type and its four ts mutants, were examined using highly accurate temperature measurements in the incubator and on the cell sheet. The transition from permissive to nonpermissive temperature occurred within an interval of 1 degree C. The midpoint of this interval was termed the semipermissive temperature since at this temperature the reproduction and consequently the plaquing of mutant viruses was not completely restricted. The "leakiness" of ts mutants might be due to their incubation at semipermissive temperature. Under semipermissive conditions and at high virus inputs, three mutants exhibited nearly complete inhibition of plaquing. In an experimental mixture of to mutants and PRV-BUK wild type, the latter was not affected by the inhibition. This might be due to an enhanced sensitivity of the mutants, but not of the wild type, to interferon under these conditions.

Isolation and characterization of a temperature-sensitive uncoating mutant of pseudorabies virus

During the course of characterizing a series of temperature-sensitive mutants of pseudorabies virus, we found one (designated tsL) that did not produce cytopathic changes in rabbit kidney cells at the non-permissive temperature (41 degree C). Although the mutant adsorbed to and penetrated the cells in a normal fashion, virus RNA was not synthesized at 41 degree C in the infected cells. However, if the cells were first incubated at the permissive temperature (32 degree C), virus RNA synthesis occurred at the non-permissive temperature. This occurred even if, during the incubation period at 32 degree C, the expression of viral functions was prevented by treatment with an inhibitor of protein synthesis. The DNA in tsL virions did not appear in the cell nucleus at 41 degree C, and full, non-enveloped nucleocapsids could be recovered from the cytoplasm of tsL virus-infected cells. These results show that the nucleocapsids of tsL remain intact at the non-permissive temperature and that tsL is an uncoating mutant.

The characteristics of the cell-free translation of mRNA from cells infected with the herpes virus pseudorabies virus

The translation in vitro of mRNA from pseudorabies virus infected cells was studied using systems derived from wheat germ and from rabbit reticulocyte. The mRNA was shown by molecular hybridization to contain sequences complementary to virus DNA. Products of in vitro translation co-migrating with virus proteins on polyacrylamide gel electropherograms were detected and the major immune precipitation. Optimum conditions for the stimulation of amino acid incorporation in vitro were determined and found to be similar for mRNA from both infected and mock-infected cells.

Replication of herpesvirus DNA. V. Maturation of concatemeric DNA of pseudorabies virus to genome length is related to capsid formation

The maturation of pseudorabies virus DNA from the replicative concatemeric form to molecules of genome length was examined using nine DNA+ temperature-sensitive mutants of pseudorabies virus, each belonging to a different complementation group. At the nonpermissive temperature, cells infected with each of the mutants synthesized concatemeric DNA. Cleavage of the concatemeric DNA to genome-length viral DNA was defective in all the DNA+ ts mutants tested, indicating that several viral gene products are involved in the DNA maturation process. In none of the ts mutant-infected cells were capsids with electron-dense cores (containing DNA) formed. Empty capsids with electron-translucent cores were, however, formed in cells infected with six of the nine temperature-sensitive mutants; in cells infected with three of the mutants, no capsid assembly occurred. Because these three mutants are deficient both in maturation of DNA and in the assembly of viral capsids, we conclude that maturation of viral DNA is dependent upon the assembly of capsids. In cells infected with two of the mutants (tsN and tsIE13), normal maturation of viral DNA occurred after shiftdown of the cells to the permissive temperature in the presence of cycloheximide, indicating that the temperature-sensitive proteins involved in DNA maturation became functional after shiftdown. Furthermore, because cycloheximide reduces maturation of DNA in wild-type-infected cells but not in cells infected with these two mutants, we conclude that a protein(s) necessary for the maturation of concatemeric DNA, which is present in limiting amounts during the normal course of infection, accumulated in the mutant-infected cells at the nonpermissive temperature. Concomitant with cleavage of concatemeric DNA, full capsids with electron-dense cores appeared after shiftdown of tsN-infected cells to the permissive temperature, indicating that there may be a correlation between maturation of DNA and formation of full capsids. The number of empty and full capsids (containing electron-dense cores) present in tsN-infected cells incubated at the nonpermissive temperature, as well as after shiftdown to the permissive temperature in the presence of cycloheximide, was determined by electron microscopy and by sedimentation analysis in sucrose gradients. After shiftdown to the permissive temperature in the presence of cycloheximide, the number of empty capsids present in tsN-infected cells decreased with a concomitant accumulation of full capsids, indicating that empty capsids are precursors to full capsids.

Distribution of sequences homologous to the DNA of herpes simplex virus, types 1 and 2, in the genome of pseudorabies virus

The distribution of related sequences between the genomes of pseudorabies virus (PRV) and herpes simplex virus, types 1 (HSV-1) and 2 (HSV-2), was determined. Approximately 7% of the sequences in PRV are shared by HSV-1 and HSV-2 DNAs. By means of the Southern blot technique, it was found that the homologous sequences are not sequestered in one region but are distributed throughout the PRV genome. HSV-1 and HSV-2 have the greatest homology with the long unique region of PRV DNA and the least with the inverted repeat regions of the molecule. HSV-1 DNA also has few sequences homologous to the short unique region of the PRV genome; HSV-2 DNA hybridizes well to this region. There was no homology of HSV-1 or HSV-2 DNAs with the extreme ends of the inverted repeat regions of PRV DNA.

Expression of the genome of defective interfering pseudorabies virions in the presence or absence of helper functions provided by standard virus

The synthesis of virus RNA and proteins in cells infected with two populations of defective virions [Pr(1)53 and Pr(2)53] which vary in the overall composition of their DNA, but which share some structural and biological characteristics, have been examined. The experiments were done under two sets of conditions: (1) at high multiplicity of infection. In this case, practically all the cells in the cultures were co-infected with defective and infectious virions; (2) at low multiplicity of infection. In this case, 75% of the cells in the cultures were infected with defective virions only and 25% were co-infected with defective and infectious virions. The relative abundance of RNA classes complementary to different regions of the virus genomes that were synthesized under various conditions of infection were determined by the Southern (1975) technique; the synthesis of virus proteins was determined by polyacrylamide gel electrophoresis (PAGE). In cells co-infected with standard and defective virions, RNA complementary to the regions that are reiterated in the defective genomes is present in larger amounts than in cells infected with standard virions alone, indicating that the genomes of the defective virions are transcribed. Furthermore, the transcriptional controls that operate normally in the infected cells also operate in cells co-infected with standard and defective virions. The over-abundant accumulation of transcripts of some regions of the virus genome in cells co-infected with defective virions is not necessarily accompanied by an overproduction of some virus proteins. No difference in the PAGE profiles of the proteins synthesized was detected in cells co-infected with PR(1)53 and standard virions. However, cells co-infected with standard and Pr(2)53 overproduced three polypeptides. Transcription of the virus genome is detectable in cells infected with Pr(2)53 alone but not in cells infected with Pr(2)53 alone. Virus protein synthesis is also detectable under these conditions in Pr(2)-, but not in Pr(1)-infected cells. Thus, despite the similarities in the biological characteristics of the two populations of defective virions described previously, similarities with respect to the expression of their genomes were not found.

Origin of replication of the DNA of a herpesvirus (pseudorabies)

During the first round of the replication of pseudorabies virus DNA, replicating DNA is mainly in the form of circles. The main origin of replication is located inthe region of the molecule bearing the inverted repeat. Replication proceeds unidirectionally from the origin.

Isolation and preliminary characterization of temperature-sensitive mutants of pseudorabies virus

Nine ts mutants of pseudorabies virus (PRV) strain BUK were isolated following bromodeoxyuridine (BUdR) or NaNO2 mutagenesis by a selection technique based on a shiftdown from nonpermissive to permissive temperature. This technique is described and its advantages are discussed. The nine ts mutants have been assigned to four complementation groups.