Electron microscopic studies of temperature-sensitive mutants of herpes simplex virus type 1

Characterization of mre11 loss following HSV-1 infection

Herpes simplex virus induces the activation of the cellular DNA double strand break response pathway dependent upon initiation of viral DNA replication. The MRN complex, consisting of Mre11, Rad50 and Nbs1, is an essential component of the DNA double strand break response and other reports have documented its presence at sites of viral DNA replication, interaction with ICP8 and its contribution to efficient viral DNA replication. During our characterization of the DSB response following infection of normal human fibroblasts and telomerase-immortalized keratinocytes, we observed the loss of Mre11 protein at late times following infection. The loss was not dependent upon ICP0, the proteasome or lysosomal protease activity. Like activation of the DSB response pathway, Mre11 loss was prevented under conditions which inhibited viral DNA replication. Analysis of a series of mutant viruses with defects in cleavage and packaging (UL6, UL15, UL17, UL25, UL28, UL32) of viral DNA or in the maturational protease (UL26) failed to identify a viral gene product necessary for Mre11 loss. Inactivation of ATM, a key effector kinase in the DNA double strand break response, had no effect on Mre11 loss and only a moderate effect on HSV yield. Finally, treatment of uninfected cells with the topoisomerase I inhibitor camptothecin, to induce generation of free DNA ends, also resulted in Mre11 loss. These results suggest that Mre11 loss following infection is caused by the generation of free DNA ends during or following viral DNA replication.

Properties of the protein encoded by the UL32 open reading frame of herpes simplex virus 1

The functions previously assigned to the essential herpes simplex virus 1 UL32 protein were in cleavage and/or packaging of viral DNA and in maturation and/or translocation of viral glycoproteins to the plasma membrane. The amino acid sequence predicts N-linked glycosylation sites and sequences conserved in aspartyl proteases and in zinc-binding proteins. We report the following. (i) The 596-amino-acid UL32 protein accumulated predominantly in the cytoplasm of infected cells but was not metabolically labeled with glucosamine and did not band with membranes containing a known glycoprotein in flotation sucrose density gradients. The UL32 protein does not, therefore, have the properties of an intrinsic membrane protein. (ii) Experiments designed to demonstrate aspartyl protease activity in a phage display system failed to reveal proteolytic activity. Moreover, substitution of Asp-110 with Gly in the sequence Asp-Thr-Gly, the hallmark of aspartyl proteases, had no effect on viral replication in Vero and SK-N-SH cell lines or in human foreskin fibroblasts. Therefore, if the UL32 protein functions as a protease, this function is not required in cells in culture. (iii) Both the native UL32 protein and a histidine-tagged UL32 protein made in recombinant baculovirus-infected insect cells bound zinc. The consensus sequence is conserved in the UL32 homologs from varicella-zoster virus and equine herpesvirus 1. UL32 protein is therefore a cysteine-rich, zinc-binding essential cytoplasmic protein whose function is not yet clear.

Assembly of the herpes simplex virus capsid: requirement for the carboxyl-terminal twenty-five amino acids of the proteins encoded by the UL26 and UL26.5 genes

Herpes simplex virus type 1 (HSV-1) intermediate capsids are composed of seven proteins, VP5, VP19C, VP21, VP22a, VP23, VP24, and VP26, and the genes that encode these proteins, UL19, UL38, UL26, UL26.5, UL18, UL26, and UL35, respectively. The UL26 gene encodes a protease that cleaves itself and the product of the UL26.5 gene at a site (M site) 25 amino acids from the C terminus of these two proteins. In addition, the protease cleaves itself at a second site (R site) between amino acids 247 and 248. Cleavage of the UL26 protein gives rise to the capsid proteins VP21 and VP24, and cleavage of the UL26.5 protein gives rise to the capsid protein VP22a. Previously we described the production of HSV-1 capsids in insect cells by infecting the cells with recombinant baculoviruses expressing the six capsid genes (D. R. Thomsen, L. L. Roof, and F. L. Homa, J. Virol. 68:2442-2457, 1994). Using this system, we demonstrated that the products of the UL26 and/or UL26.5 genes are required as scaffolds for assembly of HSV-1 capsids. To better understand the functions of the UL26 and UL26.5 proteins in capsid assembly, we constructed baculoviruses that expressed altered UL26 and UL26.5 proteins. The ability of the altered UL26 and UL26.5 proteins to support HSV-1 capsid assembly was then tested in insect cells. Among the specific mutations tested were (i) deletion of the C-terminal 25 amino acids from the proteins coded for by the UL26 and UL26.5 genes; (ii) mutation of His-61 of the UL26 protein, an amino acid required for protease activity; and (iii) mutation of the R cleavage site of the UL26 protein. Analysis of the capsids formed with wild-type and mutant proteins supports the following conclusions: (i) the C-terminal 25 amino acids of the UL26 and UL26.5 proteins are required for capsid assembly; (ii) the protease activity associated with the UL26 protein is not required for assembly of morphologically normal capsids; and (iii) the uncleaved forms of the UL26 and UL26.5 proteins are employed in assembly of 125-nm-diameter capsids; cleavage of these proteins occurs during or subsequent to capsid assembly. Finally, we carried out in vitro experiments in which the major capsid protein VP5 was mixed with wild-type or truncated UL26.5 protein and then precipitated with a VP5-specific monoclonal antibody.(ABSTRACT TRUNCATED AT 400 WORDS)

The size and symmetry of B capsids of herpes simplex virus type 1 are determined by the gene products of the UL26 open reading frame

Herpes simplex virus type 1 (HSV-1) B capsids are composed of seven proteins, designated VP5, VP19C, 21, 22a, VP23, VP24, and VP26 in order of decreasing molecular weight. Three proteins (21, 22a, and VP24) are encoded by a single open reading frame (ORF), UL26, and include a protease whose structure and function have been studied extensively by other investigators. The protease encoded by this ORF generates VP24 (amino acids 1 to 247), a structural component of the capsid and mature virions, and 21 (residues 248 to 635). The protease also cleaves C-terminal residues 611 to 635 of 21 and 22a, during capsid maturation. Protease activity has been localized to the N-terminal 247 residues. Protein 22a and probably the less abundant protein 21 occupy the internal volume of capsids but are not present in virions; therefore, they may form a scaffold that is used for B capsid assembly. The objective of the present study was to isolate and characterize a mutant virus with a null mutation in UL26. Vero cells were transformed with plasmid DNA that encoded ORF UL25 through UL28 and screened for their ability to support the growth of a mutant virus with a null mutation in UL27 (K082). Four of five transformants that supported the growth of the UL27 mutant also supported the growth of a UL27-UL28 double mutant. One of these transformants (F3) was used to isolate a mutant with a null mutation in UL26. The UL26 null mutation was constructed by replacement of DNA sequences specifying codons 41 through 593 with a lacZ reporter cassette. Permissive cells were cotransfected with plasmid and wild-type virus DNA, and progeny viruses were screened for their ability to grow on F3 but not Vero cells. A virus with these growth characteristics, designated KUL26 delta Z, that did not express 21, 22a, or VP24 during infection of Vero cells was isolated. Radiolabeled nuclear lysates from infected nonpermissive cells were layered onto sucrose gradients and subjected to velocity sedimentation. A peak of radioactivity for KUL26 delta Z that sedimented more rapidly than B capsids from wild-type-infected cells was observed. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the gradient fractions showed that the peak fractions contained VP5, VP19C, VP23, and VP26. Analysis of sectioned cells and of the peak fractions of the gradients by electron microscopy revealed sheet and spiral structures that appear to be capsid shells.(ABSTRACT TRUNCATED AT 400 WORDS)

Assembly of herpes simplex virus (HSV) intermediate capsids in insect cells infected with recombinant baculoviruses expressing HSV capsid proteins

The capsid of herpes simplex virus type 1 (HSV-1) is composed of seven proteins, VP5, VP19C, VP21, VP22a, VP23, VP24, and VP26, which are the products of six HSV-1 genes. Recombinant baculoviruses were used to express the six capsid genes (UL18, UL19, UL26, UL26.5, UL35, and UL38) in insect cells. All constructs expressed the appropriate-size HSV proteins, and insect cells infected with a mixture of the six recombinant baculoviruses contained large numbers of HSV-like capsids. Capsids were purified by sucrose gradient centrifugation, and electron microscopy showed that the capsids made in Sf9 cells had the same size and appearance as authentic HSV B capsids. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated that the protein composition of these capsids was nearly identical to that of B capsids isolated from HSV-infected Vero cells. Electron microscopy of thin sections clearly demonstrated that the capsids made in insect cells contained the inner electron-translucent core associated with HSV B capsids. In infections in which single capsid genes were left out, it was found that the UL18 (VP23), UL19 (VP5), UL38 (VP19C), and either the UL26 (VP21 and VP24) or the UL26.5 (VP22a) genes were required for assembly of 100-nm capsids. VP22a was shown to form the inner core of the B capsid, since in infections in which the UL26.5 gene was omitted the 100-nm capsids that formed lacked the inner core. The UL35 (VP26) gene was not required for assembly of 100-nm capsids, although assembly of B capsids was more efficient when it was present. These and other observations indicate that (i) the products of the UL18, UL19, UL35, and UL38 genes self-assemble into structures that form the outer surface (icosahedral shell) of the capsid, (ii) the products of the UL26 and/or UL26.5 genes are required (as scaffolds) for assembly of 100-nm capsids, and (iii) the interaction of the outer surface of the capsid with the scaffolding proteins requires the product of the UL18 gene (VP23).

Herpes simplex virus type 1 DNA cleavage and encapsidation require the product of the UL28 gene: isolation and characterization of two UL28 deletion mutants

The herpes simplex virus type 1 UL28 gene contains a 785-amino-acid open reading frame that codes for an essential protein. Studies with temperature-sensitive mutants which map to the UL28 gene indicate that the UL28 gene product (ICP18.5) is required for packaging of viral DNA and for expression of viral glycoproteins on the surface of infected cells (C. Addison, F. J. Rixon, and V. G. Preston, J. Gen. Virol. 71:2377-2384, 1990; B. A. Pancake, D. P. Aschman, and P. A. Schaffer, J. Virol. 47:568-585, 1983). In this study, we describe the isolation of two UL28 deletion mutants that were constructed and propagated in Vero cells transformed with the UL28 gene. The mutants, gCB and gC delta 7B, contained deletions of 1,881 and 537 bp, respectively, in the UL28 gene. Although the mutants synthesize viral DNA, they fail to form plaques or produce infectious virus in cells that do not express the UL28 gene. Transmission electron microscopy and Southern blot analysis demonstrated that both mutants are defective in cleavage and encapsidation of viral DNA. Analysis by cell surface immunofluorescence showed that the UL28 gene is not required for expression of viral glycoproteins on the surface of infected cells. A rabbit polyclonal antiserum was made against an Escherichia coli-expressed Cro-UL28 fusion protein. This antibody reacted with an infected-cell protein having an apparent molecular mass of 87 kDa. The 87-kDa protein was first detected at 6 h postinfection and was expressed as late as 24 h postinfection. No detectable UL28 protein was synthesized in gCB- or gC delta 7B-infected Vero cells.

Mutations in herpes simplex virus type 1 genes encoding VP5 and VP23 abrogate capsid formation and cleavage of replicated DNA

The herpes simplex virus type 1 capsid is composed of seven capsid proteins which are termed VP5, VP19c, VP21, VP22a, VP23, VP24, and VP26. Major capsid protein VP5 is encoded by the gene UL19. UL18, whose transcript is 3' coterminal with that of VP5, specifies capsid protein VP23. Vero cell lines have been isolated that are transformed with either the BglII N (UL19) or EcoRI G (UL16 to UL21) fragment of KOS. These cell lines, selected for the ability to support the replication of a temperature-sensitive VP5 mutant, were used to isolate VP5 and VP23 null mutants. The mutations in VP5 (K5 delta Z) and VP23 (K23Z) were generated by insertion of the lacZ gene at the beginning of the coding sequences of the genes. Both mutants failed to form plaques on the nonpermissive cell line, and therefore, VP23, like VP5, is an essential gene product for virus replication. Both mutants expressed wild-type levels of infected-cell proteins upon infection of permissive and nonpermissive cell lines. However, the VP5 (150-kDa) and VP23 (33-kDa) polypeptides were absent in lysates prepared from K5 delta Z- and K23Z-infected Vero cells, respectively. No capsid structures were observed by electron microscopic analysis of thin sections of K5 delta Z- and K23Z-infected Vero cells. Following sedimentation of lysates from cells infected by the mutants, capsid proteins were not observed in the fractions where capsids normally sediment. The amounts of DNA replicated in the VP5 and VP23 mutant and in KOS-infected Vero cells were the same as in permissive cells. However, genomic ends were not evident in Vero cells infected with the mutants, suggesting that the DNA remains in concatemers and is not processed into unit length genomes.

Temperature sensitivity of herpes simplex virus type 1 is a tissue-dependent phenomenon

The temperature sensitivity of herpes simplex virus type 1 (HSV-1) was assessed in primary cultures of mouse central nervous system (MNS) cells and mouse embryo cells (MEC). Infectious yields were determined and the ultrastructural morphogenesis of HSV-1 particles was compared following incubation at 37 or 40.5 degrees C. Yields of infectious virus were significantly reduced for both types of cell cultures following incubation at 40.5 degrees C. However, the effect of supraoptimal temperature (40.5 degrees C) on HSV-1 replication in MEC was significantly greater than the effect of supraoptimal temperature on virus replication in MNS cells. With respect to viral morphogenesis, no significant differences were found in either the quantity or the appearance (empty versus electron opaque core) of intranuclear particles present per infected nucleus, regardless of cell type or incubation temperature. However, complete virus particles (enveloped capsids with dense cores) were never observed in MEC at 40.5 degrees C, either intracytoplasmically or extracellularly. In contrast, complete virus particles were observed in MNS cell cultures at 40.5 degrees C, albeit in reduced numbers. At the permissive temperature (37 degrees C), complete intracytoplasmic and/or extracellular virus particles were associated with every infected cell in the MNS cell or MEC cultures. Thus an interactional effect on HSV-1 replication was found between cell culture type and incubation temperature.

Structure of the herpes simplex virus capsid: effects of extraction with guanidine hydrochloride and partial reconstitution of extracted capsids

Viral B capsids were purified from cells infected with herpes simplex virus type 1 and extracted in vitro with 2.0 M guanidine hydrochloride (GuHCl). Sodium dodecyl sulfate-polyacrylamide gel analyses demonstrated that extraction resulted in the removal of greater than 95% of capsid proteins VP22a and VP26 while there was only minimal (less than 10%) loss of VP5 (the major capsid protein), VP19, and VP23. Electron microscopic analysis of extracted capsids revealed that the pentons and the material found inside the cavity of B capsids (primarily VP22a) were removed nearly quantitatively, but extracted capsids remained otherwise structurally intact. Few, if any, hexons were lost; the capsid diameter was not greatly affected; and its icosahedral symmetry was still clearly evident. The results demonstrate that neither VP19 nor VP23 could constitute the capsid pentons. Like the hexons, the pentons are most likely composed of VP5. When B capsids were treated with 2.0 M GuHCl and then dialyzed to remove GuHCl, two bands of viral material were separated by sucrose density gradient ultracentrifugation. The more rapidly migrating of the two consisted of capsids which lacked pentons and VP22a but had a full complement of VP26. Thus, VP26 must have reassociated with extracted capsids during dialysis. The more slowly migrating band consisted of torus-shaped structures approximately 60 nm in diameter which were composed entirely of VP22a. These latter structures closely resembled torus-shaped condensates often seen in the cavity of native B capsids. The results suggest a similarity between herpes simplex virus type 1 B capsids and procapsids of Salmonella bacteriophage P22. Both contain an internal protein (VP22a in the case of HSV-1 B capsids and gp8 or "scaffolding" protein in phage P22) that can be extracted in vitro with GuHCl and that is absent from mature virions.

Intracellular organization of herpes simplex virus type 1 DNA assayed by staphylococcal nuclease sensitivity

The nucleoprotein organization of herpes simplex virus type 1 (HSV-1) DNA during productive infection was analyzed using staphylococcal nuclease. Both prior to and during the genome replication phase of infection, digestion of nuclei revealed two readily discernible forms of viral DNA, resistant and sensitive. The identity of these forms was established by the use of a variety of assays, including velocity sedimentation, nucleic acid hybridization and restriction endonuclease digestion and by employing temperature sensitive (ts) mutants impaired in either DNA replication or encapsidation of progeny DNA. Thus, nuclease resistant DNA was derived from encapsidated unit length genomes while sensitive DNA represented digestion products of replicating viral genomes. Importantly, no evidence was obtained for the arrangement of either parental or progeny viral DNA in nucleosomes. These findings are discussed with regard to the nucleoprotein structure of replicating viral DNA.

Fine structure of EBV-infected keratinocytes in oral hairy leukoplakia

We evaluated biopsy specimens of 42 cases of clinically suspected oral hairy leukoplakia for the pattern and frequency of ultrastructural alterations specific to epithelial cells infected with Epstein-Barr virus. Some structures could clearly be identified as Epstein-Barr virus at different stages of assembly, but other intranuclear and cytoplasmic alterations were not conclusively identifiable as any known structure. Keratinocytes producing Epstein-Barr virus contained intranuclear particles of different size and shape; some of them were arranged in a monodispersed pattern and others formed arrays. In contrast, both lesional keratinocytes not producing virus and keratinocytes in uninvolved mucosa contained intranuclear particles reminiscent of perichromatin granules. The nuclei of productive cells also contained marginated chromatin, tubular structures, and, occasionally, crystalline and fibrillar formations as well as enveloped virus. Formations of electron-dense bilayers were seen on both sides of the nuclear membrane. In the cytoplasm of productive cells we observed aggregates of parallel tubules and enveloped electron-dense bodies. Although many of these observations are of diagnostic and pathobiological significance, the morphogenesis, composition, and function of alterations with uncertain morphological identification remain unclear.

Physical mapping and nucleotide sequence of a herpes simplex virus type 1 gene required for capsid assembly

In this report, we describe some phenotypic properties of a temperature-sensitive mutant of herpes simplex type 1 (HSV-1) and present data concerning the physical location and nucleotide sequence of the genomic region harboring the mutation. The effect of shifts from the permissive to the nonpermissive temperature on infectious virus production by the mutant A44ts2 indicated that the mutated function is necessary throughout, or late in, the growth cycle. At the nonpermissive temperature, no major differences were detected in viral DNA or protein synthesis with respect to the parent A44ts+. On the other hand, electron microscopy of mutant-infected cells revealed that neither viral capsids nor capsid-related structures were assembled at the nonpermissive temperature. Additional analyses employing the Hirt extraction procedure showed that A44ts2 is also unable to mature replicated viral DNA into unit-length molecules under nonpermissive conditions. The results of marker rescue experiments with intact A44ts2 DNA and cloned restriction fragments of A44ts+ placed the lesion in the coordinate interval 0.553 to 0.565 (1,837 base pairs in region UL) of the HSV-1 physical map. No function has previously been assigned to this region, although it is known to be transcribed into two 5' coterminal mRNAs which code in vitro for a 54,000-molecular-weight polypeptide (K. P. Anderson, R. J. Frink, G. B. Devi, B. H. Gaylord, R. H. Costa, and E. K. Wagner, J. Virol. 37:1011-1027, 1981). We sequenced the interval 0.551 to 0.565 and found an open reading frame (ORF) for a 50,175-molecular-weight polypeptide. The predicted product of this ORF exhibits strong homology with the product of varicella-zoster virus ORF20 and lower, but significant, homology with the product of Epstein-Barr virus BORF1. For the three viruses, the corresponding ORFs lie just upstream of the gene coding for the large subunit of viral ribonucleotide reductase. The ORF described here corresponds to the ORF designated UL38 in the recently published nucleotide sequence of the HSV-1 UL region (D. J. McGeoch, M. A. Dalrymple, A. J. Davison, A. Dolan, M. C. Frame, D. McNab, L. J. Perry, J. E. Scott, and P. Taylor, J. Gen. Virol. 69:1531-1574, 1988).

Three-dimensional structure of the HSV1 nucleocapsid

The three-dimensional structures of full and empty capsids of HSV1 were determined by computer analysis of low dose cryo-electron images of ice embedded capsids. The full capsid structure is organized into outer, intermediate, and inner structural layers. The empty capsid structure has only one layer which is indistinguishable from the outer layer of the full capsids. This layer is arranged according to T = 16 icosahedral symmetry. The intermediate layer of full capsids appears to lie on a T = 4 icosahedral lattice. The genomic DNA is located inside the T = 4 shell and is the component of the innermost layer of the full capsids. The outer and intermediate layers interact in such a way that the channels along their icosahedral two-fold axis coincide and form a direct pathway between the DNA and the environment outside the capsid.

Primate cytomegalovirus assembly: evidence that DNA packaging occurs subsequent to B capsid assembly

Results presented here show that when cytomegalovirus (strain Colburn)-infected cells are treated with the DNA synthesis inhibitor hydroxyurea or phosphonoformate, one type of intranuclear capsid accumulates. These particles appeared to contain symmetrically organized internal material, and had a protein composition and sedimentation rate characteristic of B capsids. Radiolabeling experiments provided evidence that a population of B capsids lacking DNA is present during the course of a normal infection. These capsids sedimented slightly slower than the peak of viral DNA in the same region of the gradient, and had a ratio of DNA/protein that was estimated to be sevenfold lower than that of the faster sedimenting C capsids. DNA in both the B and C capsid regions of such gradients was found to be relatively resistant to digestion with DNase. The possibility is considered that herpesvirus B capsids lacking DNA may be counterparts of unexpanded proheads in the bacteriophage assembly pathway.

Characterization of intranuclear capsids made by ts morphogenic mutants of HSV-1

We have characterized capsids made by seven temperature-sensitive (ts) mutants of HSV-1 previously shown to be defective in viral DNA processing and packaging at the nonpermissive temperature (NPT). The empty capsids isolated from mutant-infected cells at the NPT were devoid of DNA, cosedimented in sucrose with wt B capsids, and contained the same structural proteins found in wt B capsids (W. Gibson and B. Roizman (1972). J. Virol. 10, 1044-1052). The presence of VP22a in empty capsids suggests that the processing of this protein from higher-molecular-weight precursors and its association with capsids is required, but not sufficient, for DNA encapsidation. Mutants made no detectable A capsids at the NPT, but did so at the permissive temperature (PT), suggesting that A particles are generated during or subsequent to, rather than prior to, encapsidation. In temperature-shift experiments, it was demonstrated that capsids of one of the mutants, F18, made at the NPT did not participate in DNA encapsidation when cells were subsequently shifted to the PT. Only those capsids made after temperature shift to the PT acquired viral DNA, implying that the ts mutation in F18 may lie in a gene coding for a structural protein, or in a protein involved in the processing of viral DNA.

A temperature-sensitive mutation in a herpes simplex virus type 1 gene required for viral DNA synthesis maps to coordinates 0.609 through 0.614 in UL

ts701 is a temperature-sensitive mutant of herpes simplex virus type 1 strain KOS induced by hydroxylamine mutagenesis (C.T. Chu, D. S. Parris, R. A. F. Dixon, F. E. Farber, and P. A. Schaffer, Virology 98:168-181, 1979). In the present study, the mutation rendering ts701 temperature sensitive was mapped to coordinates 0.609 through 0.614 in the UL region of the genome. At the nonpermissive temperature, ts701 (i) failed to induce the synthesis of viral DNA, (ii) exhibited a dramatically reduced ability to drive replication of a plasmid containing the herpes simplex virus origin of viral DNA synthesis, oriS, (iii) generated no viral polypeptides of the late (gamma 2) kinetic class, and (iv) produced virions with electron-translucent cores. Northern (RNA) blot hybridization demonstrated that two mRNAs--one of the beta kinetic class and one of the gamma kinetic class--hybridized to a 1.3-kilobase viral DNA fragment that rescued the mutation in ts701. Based on the phenotype and mapping of ts701, it is likely that its mutation lies in the gene specifying the 65,000-Mr DNA-binding protein (65KDBP) recently described by Marsden et al. (H.S. Marsden, M.E.M. Campbell, L. Haarr, M. C. Frame, D. S. Parris, M. Murphy, R. G. Hope, M. T. Muller, and C. M. Preston, J. Virol. 61:2428-2437, 1987).

Genetic and phenotypic characterization of mutants in four essential genes that map to the left half of HSV-1 UL DNA

Several HSV-1 proteins including the major capsid protein (VP5), two minor capsid proteins (VP11-12 and VP18.8), the alkaline nuclease and glycoprotein gH have been reported to be encoded by the left-most one-third of HSV-1 UL DNA. In this paper, we present physical mapping data and phenotypic analysis of six ts mutants whose mutations lie within this region and which collectively represent four functional complementation groups (1-6, 1-7, 1-10, and 1-26). In this study, mutants in complementation group 1-10 were found to be defective in the synthesis of viral DNA, late viral polypeptides, and the formation of mature capsid-like structures--properties characteristic of other ts mutants defective in functions required for viral DNA synthesis. Two DNA-positive mutants in complementation group 1-7 fail to induce capsid formation and probably possess mutations in coding sequences for VP5. Mutants in two other complementation groups (1-6 and 1-26) synthesize significant levels of viral DNA, late polypeptides, and capsids. The functions of the gene products represented by these mutants remain to be determined.

DNA processing in temperature-sensitive morphogenic mutants of HSV-1

The anatomy of DNA synthesized by five HSV-1 mutants previously shown to accumulate predominantly empty capsids at the nonpermissive temperature (NPT) was analyzed with Bg/II restriction digestion. At the NPT, all five generated DNA lacking termini, indicating that in the absence of packaging, viral DNA is not processed to unit length. One mutant, F18, was able to process DNA made at the NPT to unit length molecules during a 6-hr period after shift to the permissive temperature. The appearance of unit length molecules correlated with the appearance of staphylococcal nuclease-resistant F18 DNA.

Electron microscopic observations on primary hepatocyte cultures infected with herpes simplex virus types I and II

The replication cycle of the Herpes simplex virus (HSV) strains I and II has so far been described mainly in established proliferative cell cultures. Most of the biochemical data and ultrastructural cell changes regarding the virus-cell interaction have been obtained from 'permissive' cells which allow almost unrestricted viral multiplication. It seems obvious, however, that the in vivo viral infections are not represented adequately by these experiments. In order to achieve a more realistic view of the ultrastructural events during HSV infection of adult tissue, cell cultures were prepared from adult mouse and rat livers and infected with several HSV strains. Established 'permissive' cell lines (BHK and RK cells), served as controls. Although the main principles of viral attachment, replication and release of viral particles, were similar in hepatocytes and proliferating cells, marked differences were observed regarding the better preservation of the nuclear structures, the lower replication rate of viruses, the hypertrophy of the endoplasmic reticulum, and the changes in the Golgi apparatus. Summarizing, it can be stated that hepatocytes infected by HSV in cell culture display the well known general features of adult cells infected by viruses in vivo.

Ultrastructural localization of viral antigen in nuclear inclusions of cytomegalovirus infected guinea pig cells

Intranuclear localization of viral antigens in guinea pig cytomegalovirus (GPCMV) infected guinea pig embryo (GPE) cells was investigated by cross-reactive indirect immunoperoxidase and immunoferritin techniques utilizing guinea pig antisera to GPCMV. Following primary fixation with 4 percent paraformaldehyde, a brief treatment of infected cells with 0.25 percent trypsin was found to enhance penetration of antibodies and the conjugates. Ferritin or horseradish peroxidase conjugated goat anti-rabbit IgG was used as a secondary antibody that cross reacted with guinea pig immunoglobulins in order to reduce non-specific immunochemical reactions. Using light microscopy following immunoperoxidase staining, GPCMV antigens in an intranuclear location were not discernable when the infected cells were stained without pretreatment with trypsin, however intranuclear GPCMV antigens could be visualized after the fixed cells were treated with trypsin for 2-4 minutes prior to addition of the antiserum. Electron microscopic examination following indirect immunoferritin staining revealed viral antigens localized on viral capsids and on scattered electrondense amorphous matrices but not on the surrounding tubular structures on fibrils. The possibility that tubular structures may be a host cell product produced in response to GPCMV infection is discussed.

Morphological observations of the replication of herpesvirus tamarinus in RL-33 cells

The replication in RL-33 cells (rabbit lung cell line) of herpesvirus tamarinus isolated from cotton-topped marmosets (Saguinus oedipus) was investigated by electron microscopy. In the early stages of infection, ring-shaped and granular structures, and fibrillar materials were recognized in the nucleus. Immature particles were often found in such nuclei. The envelope of the virus was formed by budding through intracytoplasmic membranes, the inner nuclear membrane or the membrane of intracytoplasmic vacuoles. Virus particles which appeared to be budding through the plasma membrane were also observed. Aberrant viral forms were produced by independent budding of both the inner and outer nuclear membranes. The mature particles once enveloped acquired a second envelope by budding through intracytoplasmic double membranes or the outer nuclear membrane. Unusual virus-associated structures were observed in the cytoplasm and nucleus. Virus particles appeared to be released by the process of reverse phagocytosis.

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.

Ultrastructural characterization of an early, nonstructural polypeptide of herpes simplex virus type 1

An immunoperoxidase procedure was employed to study the expression of a large-molecular-weight, virus-induced polypeptide (VP175; molecular weight, 175,000) at the light and electron microscopic levels in Vero cells infected with herpes simplex virus type 1 or with tsB2, a DNA-negative, temperature-sensitive mutant of herpes simplex virus type 1. In cells infected with herpes simplex virus type 1 and in cells infected with tsB2 at the permissive temperature (34 degrees C), VP175 was found within the nucleus. The protein was detected as early as 2 h postinfection and, by 3 h postinfection, was generally distributed in a marginated pattern contiguous with, and extending from, the inner lamella of the nuclear membrane. At 6 h postinfection, protein accumulations were dispersed throughout the nucleus, and, by 9 h postinfection, these accumulations tended to be localized in a marginated pattern near the nuclear membrane. It was also noted that, at 9 h postinfection, under permissive conditions, VP175 was not found in association with nucleocapsids or enveloped particles. In contrast, in cells infected with tsB2 at the nonpermissive temperature (39 degrees C) and harvested at 6 or 9 h postinfection, accumulations of VP175 were identified not only within the nucleus, but also within the cytoplasm in the form of annular or globular aggregates. These aggregates consisted of a granular matrix and were not bound by membranes.

A nonlytic transforming mutant of herpes simplex virus type 2

A small-plaque mutant (NO.69) of herpes simplex virus type 2 (HSV-2) strain 333 has been previously isolated and characterized in this laboratory. This mutant was shown to produce a high ratio of noninfectious to infectious particles when grown at the nonpermissive temperature in hamster embryo fibroblasts [Westmoreland D. and Rapp F. (1976). Journal of Virology [8:92--102]. In this study, we have demonstrated that it is possible to obtain noninfectious stocks of this virus which retain transforming ability in a biochemical transformation assay specific for detection of the HSV gene for thymidine kinase. This mutant contains a DNA genome that has a density identical to the DNA of wild-type virus. Virus and cell DNA synthesis after infection with the mutant at both the permissive and nonpermissive temperature are similar to that observed in cultures infected with the parental virus. Clones of mouse cells biochemically transformed by this virus contain HSV antigens and are presently being examined for oncogenicity.