Characterization of the murine cytomegalovirus genes encoding the major DNA binding protein and the ICP18.5 homolog

Establishment of a novel mouse model of ulcerative colitis with concomitant cytomegalovirus infection: in vivo identification of cytomegalovirus persistent infected cells

BACKGROUND

Human cytomegalovirus (HCMV) infection is considered to be an exacerbating factor in patients with ulcerative colitis (UC). However, the pathogenicity of HCMV in the exacerbation of UC remains unclear. The lack of a model mimicking UC with HCMV infection has posed a challenge for research into the pathogenic mechanism of HCMV in flare of UC. Therefore, the aim of our study was to establish a new mouse model of UC with HCMV infection.

METHODS

We established latent murine CMV (MCMV) infection in T-cell receptor α knockout (TCR-α KO) mice at an early age by adjustment of viral dose. Next, we performed immunohistochemical analysis in various organs of infected adult TCR-α KO mice to prove the correlation between MCMV infection and development of colitis. We then assessed colitis histologically and cytokine expression in the colon of infected and uninfected TCR-α KO mice. Finally, the types of MCMV-infected cells in the inflamed colon were examined by immunohistochemical analysis.

RESULTS

MCMV antigen-positive cells reappeared predominantly in the inflamed colon of TCR-α KO mice. Severe colitis developed in the infected TCR-α KO mice compared with uninfected mice, and Th1/Th17 and Th2 responses were strongly induced. MCMV-infected cells were mainly perivascular stromal cells including pericytes, expressing platelet-derived growth factor receptor-beta (PDGFR-β) and CXC chemokine ligand 12 (CXCL12).

CONCLUSIONS

In this study, we established, to our knowledge, the first mouse model of UC with HCMV infection. This model is an excellent tool for clarifying the detailed pathogenicity of HCMV in the exacerbation of UC and developing new treatment strategy for active UC with HCMV infection.

Putative Functional Domains of Human Cytomegalovirus pUL56 Involved in Dimerization and Benzimidazole D-Ribonucleoside Activity

Background

Benzimidazole d-ribonucleosides inhibit DNA packaging during human cytomegalovirus (HCMV) replication. Although they have been shown to target pUL56 and pUL89 (the large and small subunits of the HCMV terminase, respectively) their mechanism of action is not yet fully understood. We aimed here to better understand HCMV DNA maturation and the mechanism of action of benzimidazole derivatives.

Methods

The HCMV pUL56 protein was studied by sequence analysis of the HCMV UL56 gene and herpesvirus counterparts combined with primary structure analysis of the corresponding amino acid sequences.

Results

The UL56 sequence analysis of 45 HCMV strains and counterparts among herpesviruses allowed the identification of 12 conserved regions. Moreover, comparison with the product of gene 49 (gp49) of bacteriophage T4 suggested that the pUL56 zinc finger is localized close to the dimerization site of pUL56, providing a spatial organization of the catalytic site that allows recognition and cleavage of DNA.

Conclusions

This study provides a basis to investigate the mechanism of concatemeric DNA cleavage and a biochemical basis for DNA packaging inhibition by benzimidazole derivatives.

Coordinated function of murine cytomegalovirus genes completely inhibits CTL lysis

Murine CMV (MCMV) encodes three viral genes that interfere with Ag presentation (VIPRs) to CD8 T cells, m04, m06, and m152. Because the functional impact of these genes during normal infection of C57BL/6 mice is surprisingly modest, we wanted to determine whether the VIPRs are equally effective against the entire spectrum of H-2(b)-restricted CD8 T cell epitopes. We also wanted to understand how the VIPRs interact at a functional level. To address these questions, we used a panel of MCMV mutants lacking each VIPR in all possible combinations, and CTL specific for 15 H-2(b)-restricted MCMV epitopes. Only expression of all three MCMV VIPRs completely inhibited killing by CTL specific for all 15 epitopes, but removal of any one VIPR enabled lysis by at least some CTL. The dominant interaction between the VIPRs was cooperation: m06 increased the inhibition of lysis achieved by either m152 or m04. However, for 1 of 15 epitopes m04 functionally antagonized m152. There was little differential impact of any of the VIPRs on K(b) vs D(b), but a surprising degree of differential impact of the three VIPRs for different epitopes. These epitope-specific differences did not correlate with functional avidity, or with timing of VIPR expression in relation to Ag expression in the virus replication cycle. Although questions remain about the molecular mechanism and in vivo role of these genes, we conclude that the coordinated function of MCMV's three VIPRs results in a powerful inhibition of lysis of infected cells by CD8 T cells.

Identification of a monoclonal antibody from Peromyscus maniculatus (deer mouse) cytomegalovirus (PCMV) which binds to a protein with homology to the human CMV matrix protein HCMV pp71

In this study we identified and characterized a monoclonal antibody against the matrix protein of a cytomegalovirus isolated from the common deer mouse (Peromyscus maniculatus) (PCMV). The monoclonal antibody was isolated using previously described technology which could be applied to the production of monoclonal antibodies against zoonotic disease. The antibody was found to react with a protein homologous to the human cytomegalovirus (HCMV) matrix protein (pp71), the product of the UL82 open reading frame (ORF). mAbs were generated from heterologous fusion of spleen cells from PCMV-positive mice and Balb/C P3X63-Ag8.653 myeloma cells. Using this approach, four monoclonal antibodies: B8C4, C12E8, G6A2 and P4E5 were generated. Antibody G6A2 reacted strongly with PCMV-infected cells as well as purified virions on ELISA and immunofluorescence. Western blot analysis, using sucrose gradient-purified virions, demonstrated that this mAb reacted specifically to a single protein with an apparent molecular weight of 71 kDa. The protein band was excised from the gel, purified and subjected to trypsin digestion followed by mass spectrometry. The protein sequences obtained were found to have identity to HCMV UL82 gene product. Sequence analysis indicated that it is the putative HCMV pp71 protein homolog of PCMV. G6A2 mAb did not cross-react with either human or murine recombinant pp71 proteins expressed in mammalian cells.

Functional interactions between dendritic cells and NK cells during viral infection

Ly49H(+)NK1.1(+) natural killer (NK) cells are essential for the control of murine cytomegalovirus (MCMV) during the acute stage of infection. This cell subset expands at the later stages of infection in an MCMV-specific fashion. Here we demonstrate a critical interaction between Ly49H(+) NK cells and CD8alpha(+) dendritic cells (DCs) whereby the presence of Ly49H(+) NK cells results in maintenance of CD8alpha(+) DCs in the spleen during acute MCMV infection. Reciprocally, CD8alpha(+) DCs are essential for the expansion of Ly49H(+) NK cells by a mechanism involving interleukin 18 (IL-18) and IL-12. This study provides evidence for a functional interrelationship between DCs and NK cells during viral infection and defines some of the critical cytokines.

Expression kinetics of the transcript and product of the UL28 homologue of bovine herpesvirus 1

We report that the bovine herpesvirus 1 (BHV1) UL28 ORF, a homologue of the herpes simplex virus (HSV) UL28 gene, represents a functional gene encoding a viral specific protein. The BHV1 UL28 ORF, located at positions 53058-->55538 of the viral genome, encodes a viral specific transcript of 3.4 kb detected at 6 h post-infection (p.i.) after which its levels accumulated up to 12 h p.i. and then remained constant up to 24 h p.i. Transcription of the BHV1 UL28 was determined to initiate 95 bases upstream from the ORF's initiating codon, which corresponds to 33 nucleotides downstream from a putative TATA box. A BHV1 UL28 specific antiserum, generated against a T7-Tag/UL28 fusion protein expressed in E. coli, specifically reacted with a 100 kDa protein in Western blots of BHV1-infected protein cell lysates. The expression kinetics of the protein was delayed by 6 h relative to that of its transcript suggesting that the gene is regulated at the translational level. In contrast to the HSV and pseudorabies virus UL28 genes, which belong to viral genes of the early (beta) class, that of BHV1 was unambiguously classified as a gamma2 gene. Further studies will be required to determine whether these kinetic differences have any functional implications.

In vivo acute depletion of CD8(+) T cells before murine cytomegalovirus infection upregulated innate antiviral activity of natural killer cells

In this study, we investigated the effect of depletion of CD8(+) T cells on the activity of natural killer (NK) cells at an early phase of murine cytomegalovirus (MCMV) infection. For CD8(+) T cell depletion, mice were intraperitoneally treated with anti-CD8 mAb, purified from 2.43 hybridoma, for 2 consecutive days before or after infection. Three days after infection, we found that an acute depletion of CD8(+) T cells before infection caused a significant decrease in the viral load in liver and spleen. This effect coincided with an increase in numbers of CD3(-) NK1.1(+) cells in spleen and their expression of the early activation molecule CD69. Although cytolytic activity of NK cells increased on day 3 of infection in CD8-depleted mice, the level of IFN-gamma decreased in serum and supernatant of cultured spleen cells. In contrast to the effect of acute depletion of CD8(+) T cells before infection, the depletion after infection had no effect on the viral load or number and cytolytic function of NK cells. Lack of effects of CD8(+) T cell depletion on the viral load and NK cytolytic activity is also observed in CD8(+) knockout mice. In conclusion, the results suggest that an acute depletion of CD8(+) T cells before MCMV infection effectively upregulated the antiviral activity of NK cells. This effect appears to be mediated through an increase in numbers, activation and cytolytic activity of NK cells.

Protective effect of black seed oil from Nigella sativa against murine cytomegalovirus infection

In this study, antiviral effect of black seed oil (BSO) from Nigella sativa was investigated using murine cytomegalovirus (MCMV) as a model. The viral load and innate immunity mediated by NK cells and Mφ during early stage of the infection were analyzed. Intraperitoneal (i.p.) administration of BSO to BALB/c mice, a susceptible strain of MCMV infection, strikingly inhibited the virus titers in spleen and liver on day 3 of infection with 1x10(5) PFU MCMV. This effect coincided with an increase in serum level of IFN-gamma. Although BSO treatment decreased both number and cytolytic function of NK cells on day 3 of infection, it increased numbers of Mφ and CD4(+) T cells. On day 10 of infection, the virus titer was undetectable in spleen and liver of BSO-treated mice, while it was detectable in control mice. Although spleen of both control and BSO-treated mice showed similar CTL activities on day 10 after infection, serum level of IFN-gamma in BSO-treated mice was higher. Furthermore, BSO treatment upregulated suppressor function of Mφ in spleen. These results show that BSO exhibited a striking antiviral effect against MCMV infection which may be mediated by increasing of Mφ number and function, and IFN-gamma production.

The RGD sequence in the cytomegalovirus DNA polymerase accessory protein can mediate cell adhesion

The murine cytomegalovirus (MCMV) polymerase processivity factor ppM44 (also referred to as pp50) is an abundant phosphoprotein found in MCMV-infected cells. Sequence analysis of the MCMV M44 open reading frame revealed an "RGD" motif that is also present in the human cytomegalovirus (HCMV) UL44 open reading frame. In this report, histidine-tagged M44 protein produced in Escherichia coli or the vaccinia/T7 expression system was purified to near homogeneity by metal chelation affinity chromatography using His*Bind resins. We demonstrated that recombinant M44 protein could mediate cell adhesion via its conserved "RGD" motif, because a single amino acid change (RGD to RGE) abolished cell attachment. In addition, cell adhesion was abolished in the presence of EDTA. We next showed that recombinant HCMV UL44, but not human herpesvirus type 6 p41, which lacks the RGD motif, could mediate cell adhesion in a similar manner. We also provided evidence that ppM44 was present in the culture medium during virus infection. Thus these results suggested that in addition to its primary role as the polymerase processivity factor, MCMV ppM44 may serve as a substrate for integrin-binding via its conserved RGD motif, with the potential for a novel role in the MCMV replication cycle.

Development of an effective polyvalent vaccine against both Marek's and Newcastle diseases based on recombinant Marek's disease virus type 1 in commercial chickens with maternal antibodies

An earlier report (M. Sakaguchi et al., Vaccine 16:472-479, 1998) showed that recombinant Marek's disease virus type 1 (rMDV1) expressing the fusion (F) protein of Newcastle disease virus (NDV-F) under the control of the simian virus 40 late promoter [rMDV1-US10L(F)] protected specific pathogen-free chickens from NDV challenge, but not commercial chickens with maternal antibodies against NDV and MDV1. In the present study, we constructed an improved polyvalent vaccine based on MDV1 against MDV and NDV in commercial chickens with maternal antibodies. The study can be summarized as follows. (i) We constructed rMDV1 expressing NDV-F under the control of the MDV1 glycoprotein B (gB) promoter [rMDV1-US10P(F)]. (ii) Much less NDV-F protein was expressed in cells infected with rMDV1-US10P(F) than in those infected with rMDV1-US10L(F). (iii) The antibody response against NDV-F and MDV1 antigens of commercial chickens vaccinated with rMDV1-US10P(F) was much stronger and faster than with rMDV1-US10L(F), and a high level of antibody against NDV-F persisted for over 80 weeks postvaccination. (iv) rMDV1-US10P(F) was readily reisolated from the vaccinated chickens, and the recovered viruses were found to express NDV-F. (v) Vaccination of commercial chickens having maternal antibodies to rMDV1-US10P(F) completely protected them from NDV challenge. (vi) rMDV1-US10P(F) offered the same degree of protection against very virulent MDV1 as the parental MDV1 and commercial vaccines. These results indicate that rMDV1-US10P(F) is an effective and stable polyvalent vaccine against both Marek's and Newcastle diseases even in the presence of maternal antibodies.

The pathogenicity of cytomegalovirus

Human cytomegalovirus is ubiquitous, yet causes little illness in immunocompetent individuals. Disease is evident in immunodeficient groups such as neonates, transplant recipients and AIDS patients either following a primary infection or reactivation of a latent infection. Little is known of the mechanisms underlying the pathogenicity of the virus. The recent determination of the nucleotide sequence of both human cytomegalovirus (strain AD169) and murine cytomegalovirus (murine cytomegalovirus strain Smith) has allowed an analysis of the biological importance of several virus genes. Studies with human cytomegalovirus have indicated that many viral genes are non-essential for replication in vitro which are thus assumed to be important in the pathogenesis of the virus. This is being examined in the murine model where the role of the gene and its product in disease can be directly examined in vivo using viral mutants in which the relevant gene has been interrupted or deleted. Current information on the role of cytomegalovirus genes in tissue tropism, immune evasion, latency, reactivation from latency and damage is described.

Protective effects of hochu-ekki-to, a Chinese traditional herbal medicine against murine cytomegalovirus infection

The innate immunity against murine cytomegalovirus (MCMV) at the early phase of infection is mediated by NK cells and macrophages. We studied the effects of hochu-ekki-to (HET), a traditional Chinese herbal medicine, on the regulation of innate immunity mediated by NK cells and macrophages. We found the oral administration of HET to increase both the number of leukocytes in the spleen and liver and the splenic NK cell cytotoxicity associated with the increased induction of serum IFN-alpha/beta after an MCMV infection but it had no effect on liver NK cells. However, no differences were found in the serum IL-12, IFN-gamma, TNF-alpha and nitric oxide (NO) production in the culture of macrophages between the HET- and PBS-treated mice on day 2 after MCMV infection. In addition, HET-treated splenic and peritoneal macrophages were found to show a higher intrinsic resistance against in vitro MCMV infection than that of PBS-treated mice. Therefore, the HET-induced effects on NK cells and macrophages selectively reduced the viral load in the spleen but not in the liver at an early phase of MCMV infection. HET may thus be useful in the treatment of human cytomegalovirus infection which commonly occurs in HIV-infected AIDS patients.

Murine cytomegalovirus induces a Sjögren's syndrome-like disease in C57Bl/6-lpr/lpr mice

OBJECTIVE

To analyze Fas and tumor necrosis factor receptor I (TNFRI) apoptosis pathways in salivary gland inflammatory disease induced by murine cytomegalovirus (MCMV) infection.

METHODS

Four different strains of mice (C57BI/6 [B6]-+/+, Fas-deficient B6-lpr/lpr, TNFRI-deficient B6-tnfr1(0/0), and B6-tnfr1(0/0)-lpr/lpr mice) were infected intraperitoneally with the Smith strain of MCMV (1 x 10(5) plaque-forming units). Viral load was determined by a plaque assay, inflammation and apoptosis by immunohistochemistry and staining with terminal dUTP nickend labeling, and autoantibodies by enzyme-linked immunosorbent assay.

RESULTS

Infectious MCMV was not detectable by day 100. Although all MCMV-infected mice developed acute sialadenitis by day 28, a chronic (>100 days), severe salivary gland inflammation and anti-Ro and anti-La antibodies developed only in the B6-lpr/lpr mice. Apoptotic cells were detected during the acute, but not the chronic, phase of inflammation.

CONCLUSION

Both Fas- and TNFRI-mediated apoptosis contribute to the clearance of MCMV-infected cells in the salivary glands. However, because Fas-mediated apoptosis is necessary for the down-modulation of the immune response, a defect in this process can lead to a postinfection, chronic inflammatory response that resembles Sjögren's syndrome.

Apoptosis mediated by Fas but not tumor necrosis factor receptor 1 prevents chronic disease in mice infected with murine cytomegalovirus

The role of Fas- and TNF-receptor 1 (TNF-R1)-mediated apoptosis in the clearance of virally infected cells and in the regulation of the immune response was analyzed after murine cytomegalovirus (MCMV) infection of C57BL/6 (B6)-+/+ mice, Fas-mutant B6-lpr/lpr mice, TNF-R1 knockout B6-tnfr0/0 mice, and double-deficient B6-tnfr0/0 lpr/lpr mice. There was approximately equivalent clearance of MCMV in B6-+/+, B6-tnfr0/0, and B6-lpr/lpr mice, and by day 28 no infectious virus could be detected in the liver, kidney, lung, or peritoneal exudate. However, delayed virus clearance was observed in B6-tnfr0/0 lpr/lpr mice. An acute inflammatory response occurred in the liver, lung, and kidney of all mice, which was most severe 7 d after MCMV infection, but resolved by day 28 in B6-+/+ and B6-tnfr0/0 mice, but not in B6-lpr/lpr or B6-tnfr0/0 lpr/lpr mice. These results indicate that apoptosis mediated by either Fas or TNF-R1 is sufficient for rapid clearance of the virus. However, apoptosis induced by Fas, but not TNF-R1, is required for the downmodulation of the immune response to the virus and prevention of a chronic inflammatory reaction.

Physical and genetic maps of the human herpesvirus 7 strain SB genome

Human herpesvirus 7 (HHV-7) is a close relative of human herpesvirus 6A (HHV-6A) and human herpesvirus 6B (HHV-6B) based on limited biologic and genetic data. In this work we describe physical and genetic maps for HHV-7 strain SB [HHV-7(SB)], which was obtained from the saliva of a healthy adult. The HHV-7(SB) genome length is approximately 144 kb by clamped homogeneous electric field gel electrophoresis and approximately 135 kb by summation of restriction endonuclease fragments. We constructed plasmid clones and PCR amplimers that span the HHV-7 genome, except for the genomic termini, and determined the maps of the restriction endonuclease cleavage sites for BamHI, PstI, and SacI. The HHV-7(SB) genome is composed of a single unique region of approximately 122 kb bounded at each end by a 6 kb direct repeat. Homologs to thirty-five herpesvirus genes were identified. The highest similarity was with the HHV-6 genes, with an average amino acid identity of 50%, followed by the human cytomegalovirus counterpart. The genomic and genetic maps indicated that the HHV-7 and HHV-6 genomes are colinear. There was no sequence variation in a segment of the gene encoding the DNA polymerase-associated factor homolog among six HHV-7 isolates, while the corresponding segment of the HHV-6A and HHV-6B counterparts differed by 4.6%. These data support previous observations that the closest genetic relatives of HHV-7 are betaherpesviruses.

The location and sequence composition of the murine cytomegalovirus replicator (oriLyt)

DNA replication during human or simian cytomegalovirus (CMV) infection has been shown to be under control of a replicator region referred to as oriLyt. The murine CMV oriLyt has been mapped to a region of the genome located upstream of the gene encoding the herpesvirus-conserved single-stranded DNA binding protein, analogous to human and simian CMV oriLyts. A minimal oriLyt of approximately 1.7 kbp has been identified using a transient replication system. Like occurs with human and simian CMV counterparts, addition of flanking sequences to this minimal origin-stimulated replication efficiency. Analysis of the DNA sequence in this region shows that murine CMV oriLyt is complex and exhibits an asymmetric distribution of nucleotides as well as many repeat sequence elements, including distinct AT- and GC-rich regions and region with arrays of closely spaced direct repeats. Despite similarities in organization of all three CMV oriLyts, no sequence identity and only limited DNA sequence similarity was detectable. Consistent with this sequence divergence, the human and murine CMV oriLyts were unable to substitute for one another in transient replication assays.

Analysis of the complete DNA sequence of murine cytomegalovirus

The complete DNA sequence of the Smith strain of murine cytomegalovirus (MCMV) was determined from virion DNA by using a whole-genome shotgun approach. The genome has an overall G+C content of 58.7%, consists of 230,278 bp, and is arranged as a single unique sequence with short (31-bp) terminal direct repeats and several short internal repeats. Significant similarity to the genome of the sequenced human cytomegalovirus (HCMV) strain AD169 is evident, particularly for 78 open reading frames encoded by the central part of the genome. There is a very similar distribution of G+C content across the two genomes. Sequences toward the ends of the MCMV genome encode tandem arrays of homologous glycoproteins (gps) arranged as two gene families. The left end encodes 15 gps that represent one family, and the right end encodes a different family of 11 gps. A homolog (m144) of cellular major histocompatibility complex (MHC) class I genes is located at the end of the genome opposite the HCMV MHC class I homolog (UL18). G protein-coupled receptor (GCR) homologs (M33 and M78) occur in positions congruent with two (UL33 and UL78) of the four putative HCMV GCR homologs. Counterparts of all of the known enzyme homologs in HCMV are present in the MCMV genome, including the phosphotransferase gene (M97), whose product phosphorylates ganciclovir in HCMV-infected cells, and the assembly protein (M80).

Investigation of murine cytomegalovirus latency and reactivation in mice using viral mutants and the polymerase chain reaction

Studies with 6 ts mutants of mouse cytomegalovirus indicated that mutants tsm1, tsm2, tsm3, and tsm6, like wild-type (wt) virus, produced acute infection in mice, became latent, and were reactivated as infectious virus immunosuppression. Using PCR, all five viruses expressed immediate-early (IE)-1, early (E)-1, and late (L, gB) genes during acute infection in all tissues examined (salivary glands, lung, spleen, liver, kidney, and heart). DNA was present in most tissues during latent infection with all five viruses, but transcription was restricted to the IE-1 gene in the salivary glands of wt infected mice only, suggesting true molecular latency rather than low level virus persistence. Similarly, mutant tsm5 expressed all three genes following primary inoculation. Although no detectable virus was produced, tsm5 subsequently entered the latent state as evidenced by DNA detection without RNA transcription indicating that productive infection is not required to initiate latency. This mutant also failed to reactivate from latency, although all three marker genes were expressed in most tissues. In contrast, tsm4 expressed all three marker genes and produced infectious virus during acute infection, then became latent. However, upon immunosuppression to reactivate tsm4, IE-1 and E-1 transcription occurred but neither gB transcription nor infectious virus was detectable in salivary glands, lung, spleen, liver, kidney, heart, or blood. The significance of this with regard to reactivation from latency is discussed.

Murine cytomegalovirus with a deletion of genes spanning HindIII-J and -I displays altered cell and tissue tropism

Murine cytomegalovirus (MCMV) gene products dispensable for growth in cell culture are likely to have important functions within the infected host, influencing tissue tropism, dissemination, or immunological responses against the virus. To identify such genes, our strategy was to delete large regions of the MCMV genome likely to contain genes nonessential for virus replication in NIH 3T3 cells. Mutant virus RV7 contained a deletion of 7.7 kb spanning portions of MCMV HindIII-J and -I. This virus grew comparably to wild-type (WT) virus in NIH 3T3 fibroblasts, primary embryo fibroblasts, and bone marrow macrophages. However, RV7 failed to replicate in target organs of immunocompetent BALB/c mice and severe combined immunodeficient mice, which are exquisitely sensitive to MCMV infection. This defect in vivo growth may be related to the observation that RV7 grew poorly in the peritoneal macrophage cell line IC-21, which is highly permissive for growth of WT MCMV. Two other mutant viruses with an insertion or smaller deletion in the region common to the RV7 deletion grew comparably to WT virus in the macrophage cell line and replicated in salivary gland tissue. The poor growth of RV7 in IC-21 cells was due to a block in immediate-early gene expression, as levels of RNA from immediate-early gene IE1 were reduced eightfold compared with levels for WT virus in macrophages infected with RV7. Consequently, levels of RNA from early and late genes were also reduced. The lower expression of IE1 in RV7-infected IC-21 macrophages was not due to defective entry of virus into the cells, as equal amounts of viral DNA were present in cells 3 h after infection with RV7 or WT MCMV. These studies demonstrate that deletion of sequences in HindIII-J and -I confer altered cell and tissue tropism.

Identification and characterization of the bovine herpesvirus 1 UL7 gene and gene product which are not essential for virus replication in cell culture

The UL7 gene of bovine herpesvirus 1 (BHV-1) strain Schönböken was found at a position and in a context predicted from the gene order in the prototype alphaherpesvirus herpes simplex virus type 1. The gene and flanking regions were sequenced, the UL7 RNA and protein were characterized, and 98.3% of the UL7 open reading frame was deleted from the viral genome without destroying productive virus replication. Concomitant deletion of nine 3' codons from the BHV-1 UL6 ORF and 77 amino acids from the carboxy terminus of the predicted BHV-1 UL8 protein demonstrated that these domains are also not essential for function of the respective proteins. The UL7 open reading frame encodes a protein of 300 amino acids with a calculated molecular mass of 32 kDa. Comparison with UL7 homologs of other alphaherpesviruses revealed a high degree of homology, the most prominent being to the predicted UL7 polypeptide of varicella-zoster virus, with 43.3% identical amino acids. A monospecific anti-UL7 serum identified the 33-kDa (apparent-molecular-mass) UL7 polypeptide which is translated from an early-expressed 1.7-kb RNA. The UL7 protein was localized in the cytoplasm of infected cells and could not be detected in purified virions. In summary, we describe the first identification of an alphaherpesviral UL7-encoded polypeptide and demonstrate that the UL7 protein is not essential for replication of BHV-1 in cell culture.

Identification and molecular characterization of the murine cytomegalovirus homolog of the human cytomegalovirus UL100 gene

The UL100 gene of human cytomegalovirus (HCMV) has been shown to encode an envelope glycoprotein that might play an important role in HCMV infection. Using the HCMV UL100 gene as a probe in low stringency hybridization studies, we were able to identify the putative UL100 homologous gene of murine cytomegalovirus (MCMV), strain Smith. The MCMV gene was sequenced and localized to the left end of the EcoRI fragment H on the MCMV physical map. This genomic location is similar to that found for the HCMV UL100 gene. The MCMV UL100 mRNA is 1.6 kb in size and is expressed exclusively in the late stages of infection. The 5' and 3' boundaries of the transcript were determined. The open reading frame (ORF) of the UL100 gene could encode a protein of 371 amino acid residues with a calculated molecular mass of 42 kDa. Computer analysis of the deduced amino acid sequence of this gene predicted the presence of eight transmembrane domains and four N-linked glycosylation sites in the protein. Sequence comparison revealed that this putative protein shares similarity with the predicted UL100 homologs of several other herpesviruses, and is most similar to the HCMV UL100 protein (47% identity).

Characterization of a conserved gene block in the murine cytomegalovirus genome

The nucleotide sequence between the lytic origin and the gH gene of the murine cytomegalovirus genome (map units 0.416-0.455) was determined. Five of the 15 open reading frames identified encode polypeptides, which exhibit significant homology to polypeptides of human cytomegalovirus (HCMV) (UL69, UL71, and UL73) and to polypeptides of several other herpesviruses (dUTPase and helicase/primase). The location in the genome of the five open reading frames and their direction of transcription is perfectly conserved between murine cytomegalovirus (MCMV) and HCMV. These data suggest that MCMV and HCMV have a highly related genome organization. Thus, MCMV offers a good model for molecular and functional analysis of cytomegalovirus genes in vitro and in vivo.

Structure and function of the murine cytomegalovirus sgg1 gene: a determinant of viral growth in salivary gland acinar cells

The salivary gland has long been recognized as an important target organ for cytomegalovirus replication in the infected host. A viral gene, denoted sgg1, plays an important role for replication in the salivary gland even though it is dispensable for growth in other organs or in cultured cells. The nucleotide sequence of this gene and of cDNA clones representing two spliced transcripts (1.5 and 1.8 kb in size) has been determined. The more abundant 1.5-kb transcript contains a 312-amino-acid (aa) open reading frame (ORF) and encodes the corresponding 37-kDa protein (Sgg1) when expressed in transfected COS-7 cells. The 1.8-kb transcript initiates upstream of the 1.5-kb transcript and contains a 108-aa ORF in addition to the 312-aa ORF. This longer cDNA also encodes the 37-kDa protein Sgg1, although at lower abundance than the 1.5-kb cDNA. Sgg1 localizes to the cytoplasm of COS-7 cells, which is consistent with the predicted structural characteristics of the 312-aa ORF: a type 1 integral membrane protein. During viral infection, expression of both sgg1 transcripts is highest at early times (8 to 12 h) after infection; only the 1.5-kb transcript is present, at low levels, late in infection. A recombinant virus, RM868, carrying a lacZ-gpt insertion within sgg1, fails to express Sgg1 protein and exhibits reduced growth in the salivary gland. RM868 retains the capacity to disseminate in the infected mouse and to enter serous acinar cells, although it fails to replicate efficiently in this cell type. These results suggest that sgg1 is critical for high levels of viral replication in the salivary gland.

Identification of genes involved in DNA replication of the Autographa californica baculovirus

By use of a transient replication assay, nine genes involved in DNA replication were identified in the genome of the Autographa californica baculovirus. Six genes encoding helicase, DNA polymerase, IE-1, LEF-1, LEF-2, and LEF-3 are essential for DNA replication while three genes encoding P35, IE-2, and PE38 stimulate DNA replication. No stimulation by the AcMNPV pcna gene, encoding a protein with sequence homology to proliferating-cell nuclear antigen, was observed. A pattern of amino acids found in a number of single-stranded-DNA-binding proteins was identified in the carboxyl-terminal region of IE-1.