Patterns of transcription of human cytomegalovirus in permissively infected cells

DNA damage response(DDR): a link between cellular senescence and human cytomegalovirus

The DNA damage response (DDR) is a signaling cascade that is triggered by DNA damage, involving the halting of cell cycle progression and repair. It is a key event leading to senescence, which is characterized by irreversible cell cycle arrest and the senescence-associated secretory phenotype (SASP) that includes the expression of inflammatory cytokines. Human cytomegalovirus (HCMV) is a ubiquitous pathogen that plays an important role in the senescence process. It has been established that DDR is necessary for HCMV to replicate effectively. This paper reviews the relationship between DDR, cellular senescence, and HCMV, providing new sights for virus-induced senescence (VIS).

Temporal dynamics of HCMV gene expression in lytic and latent infections

During productive human cytomegalovirus (HCMV) infection, viral genes are expressed in a coordinated cascade that conventionally relies on the dependencies of viral genes on protein synthesis and viral DNA replication. By contrast, the transcriptional landscape of HCMV latency is poorly understood. Here, we examine viral gene expression dynamics during the establishment of both productive and latent HCMV infections. We redefine HCMV gene expression kinetics during productive infection and reveal that viral gene regulation does not represent a simple sequential cascade; many viral genes are regulated by multiple independent modules. Using our improved gene expression classification combined with transcriptome-wide measurements of the effects of a wide array of epigenetic inhibitors on viral gene expression during latency, we show that a defining feature of latency is the unique repression of immediate-early (IE) genes. Altogether, we recharacterize HCMV gene expression kinetics and reveal governing principles of lytic and latent gene expression.

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Redefining HCMV gene expression cascade during productive infection

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Many viral genes are regulated by multiple independent modules

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Diverse inhibitors induce broad viral gene expression in monocytes

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In monocytes, immediate-early (IE) genes are repressed compared to all other HCMV genes

Vaccine Vectors Harnessing the Power of Cytomegaloviruses

Cytomegalovirus (CMV) species have been gaining attention as experimental vaccine vectors inducing cellular immune responses of unparalleled strength and protection. This review outline the strengths and the restrictions of CMV-based vectors, in light of the known aspects of CMV infection, pathogenicity and immunity. We discuss aspects to be considered when optimizing CMV based vaccines, including the innate immune response, the adaptive humoral immunity and the T-cell responses. We also discuss the antigenic epitopes presented by unconventional major histocompatibility complex (MHC) molecules in some CMV delivery systems and considerations about routes for delivery for the induction of systemic or mucosal immune responses. With the first clinical trials initiating, CMV-based vaccine vectors are entering a mature phase of development. This impetus needs to be maintained by scientific advances that feed the progress of this technological platform.

Murine Cytomegalovirus Protein pM91 Interacts with pM79 and Is Critical for Viral Late Gene Expression

Viral gene expression is tightly regulated during cytomegalovirus (CMV) lytic replication, but the detailed mechanism of late gene transcription remains to be fully understood. Previous studies reported that six viral proteins (named viral transactivation factors [vTFs]) supporting late gene expression were conserved in beta- and gammaherpesviruses but not in alphaherpesviruses. Here, we performed coimmunoprecipitation experiments to elucidate the organization of these six proteins in murine CMV. Our results showed that these proteins formed a complex by both direct and indirect interactions. Specifically, pM91 strongly bound to pM79 even in the absence of other vTFs. Similar to pM79, pM91 exhibited early-late expression kinetics and localized within nuclear viral replication compartments during infection. Functional analysis was also performed using the pM91-deficient virus. Real-time PCR results revealed that abrogation of M91 expression markedly reduced viral late gene expression and progeny virus production without affecting viral DNA synthesis. Using mutagenesis, we found that residues E61, D62, D89, and D96 in pM91 were required for the pM91-pM79 interaction. Disruption of the interaction via E61A/D62A or D89A/D96A double mutation in the context of virus infection inhibited progeny virus production. Our data indicate that pM91 is a component of the viral late gene transcription factor complex and that the pM91-pM79 interaction is essential for viral late gene expression.IMPORTANCE Cytomegalovirus (CMV) infection is the leading cause of birth defects and causes morbidity and mortality in immunocompromised patients. The regulation of viral late gene transcription is not well elucidated, and understanding of this process benefits the development of novel therapeutics against CMV infection. This study (i) identified that six viral transactivation factors encoded by murine CMV form a complex, (ii) demonstrated that pM91 interacts with pM79 and that pM91 and pM79 colocalize in the nuclear viral replication compartments, (iii) confirmed that pM91 is critical for viral late gene expression but dispensable for viral DNA replication, and (iv) revealed that the pM91-pM79 interaction is required for progeny virus production. These findings give an explanation of how CMV regulates late gene expression and have important implications for the design of antiviral strategies.

The DNA damage response induced by infection with human cytomegalovirus and other viruses

Viruses use different strategies to overcome the host defense system. Recent studies have shown that viruses can induce DNA damage response (DDR). Many of these viruses use DDR signaling to benefit their replication, while other viruses block or inactivate DDR signaling. This review focuses on the effects of DDR and DNA repair on human cytomegalovirus (HCMV) replication. Here, we review the DDR induced by HCMV infection and its similarities and differences to DDR induced by other viruses. As DDR signaling pathways are critical for the replication of many viruses, blocking these pathways may represent novel therapeutic opportunities for the treatment of certain infectious diseases. Lastly, future perspectives in the field are discussed.

History of the molecular biology of cytomegaloviruses

The history of the molecular biology of cytomegaloviruses from the purification of the virus and the viral DNA to the cloning and expression of the viral genes is reviewed. A key genetic element of cytomegalovirus (the CMV promoter) contributed to our understanding of eukaryotic cell molecular biology and to the development of lifesaving therapeutic proteins. The study of the molecular biology of cytomegaloviruses also contributed to the development of antivirals to control the viral infection.

Inhibition of p53 transcriptional activity by human cytomegalovirus UL44

Human cytomegalovirus (HCMV) stimulates cellular synthesis of DNA and proteins and induces transition of the cell cycle from G(1) to S and G(2) /M phase, in spite of increased amounts of p53 in the infected cells. The immediate early protein IE2-86  kDa (IE86) tethers a transcriptional repression domain to p53; however, its repression of p53 function is not enough to abrogate the G(1) checkpoint function of p53. Other HCMV proteins that suppress the activity of p53 were investigated in this study. Of the HCMV proteins that bind to p53 when assessed by immunoprecipitation and immunoblot analysis, HCMV UL44 was chosen as a candidate protein. It was found that reporter gene containing p53 consensus sequence was activated by transfection with wild type p53, but when plasmids of p53 with IE86 or UL44 were co-transfected, p53 transcriptional activity was decreased to 3-7% of the p53 control in a dose-dependent manner. When the deletion mutant of UL44 was co-transected with p53, the carboxyl one-third portion of UL44 had little effect on inhibition of p53 transcriptional activity. The amount of mRNA p21 was measured in H1299 by real time PCR after transfection of the combination of p53 and UL44 vectors and it was found that p21 transcription by p53 was inhibited dose-dependently by UL44. Increased G0/G1 and decreased S phases in p53 wild type-transfected H1299 cells were recovered to the level of p53 mutant type-transfected ones by the additional transfection of UL44 in a dose-dependent manner. In conclusion, the transcriptional activity of p53 is suppressed by UL44 as well as by IE86.

Whole-genome analysis of pseudorabies virus gene expression by real-time quantitative RT-PCR assay

Background

Pseudorabies virus (PRV), a neurotropic herpesvirus of pigs, serves as an excellent model system with which to investigate the herpesvirus life cycle both in cultured cells and in vivo. Real-time RT-PCR is a very sensitive, accurate and reproducible technique that can be used to detect very small amounts of RNA molecules, and it can therefore be applied for analysis of the expression of herpesvirus genes from the very early period of infection.

Results

In this study, we have developed and applied a quantitative reverse transcriptase-based real-time PCR technique in order to profile transcription from the whole genome of PRV after lytic infection in porcine kidney cells. We calculated the relative expression ratios in a novel way, which allowed us to compare different PRV genes with respect to their expression dynamics, and to divide the PRV genes into distinct kinetic classes. This is the first publication on the whole-genome analysis of the gene expression of an alpha-herpesvirus by qRT2-PCR. We additionally established the kinetic properties of uncharacterized PRV genes and revised or confirmed data on PRV genes earlier examined by traditional methods such as Northern blot analysis. Our investigations revealed that genes with the same expression properties form clusters on the PRV genome: nested overlapping genes belong in the same kinetic class, while most convergent genes belong in different kinetic classes. Further, we detected inverse relationships as concerns the expressions of EP0 and IE180 mRNAs and their antisense partners.

Conclusion

Most (if not all) PRV genes begin to be expressed from the onset of viral expression. No sharp boundary was found between the groups of early and late genes classified on the basis of their requirement for viral DNA synthesis. The expressions of the PRV genes were analyzed, categorized and compared by qRT2-PCR assay, with the average of the minimum cycle threshold used as a control for the calculation of a particular R value. In principle, this new calculation technique is applicable for the analysis of gene expression in all temporally changing genetic systems.

Detection of human cytomegalovirus in atherosclerotic carotid arteries in humans

BACKGROUND/PURPOSE

Atherosclerosis plays an important role in stroke. A microbiological infection has been suggested to be involved in the pathogenesis of atherosclerosis, in particular the human cytomegalovirus (HCMV). The objective of this study was to determine the association between HCMV infection and atherosclerosis of the internal carotid arteries in patients of Chinese Han ethnicity with ischemic stroke.

METHODS

HCMV DNA and antigen were detected in atherosclerotic internal carotid arteries from 35 patients with ischemic stroke and 20 controls from a Chinese Han ethnic population. Immunohistochemistry, in situ hybridization and polymerase chain reaction were used to detect the HCMV immediate early (IE) and late (L) antigen as well as DNA in the vascular walls.

RESULTS

We observed that the proportion of cases that tested positive for HCMV IE but not L antigen and DNA was statistically greater in stroke patients compared with the control population.

CONCLUSION

HCMV IE antigen and DNA were associated with the pathological process of atherosclerosis. The vessel wall might be the infection site of the dormant virus.

Repression of human cytomegalovirus major immediate early gene expression by the cellular transcription factor CCAAT displacement protein

Initiation of human cytomegalovirus (HCMV) productive infection is dependent on the major immediate early (MIE) genes ie1 and ie2. Several putative binding sites for CCAAT displacement protein (CDP or CUX1) were identified within the MIE promoter/regulatory region. Binding assays demonstrated binding of CUX1 to MIE-region oligonucleotides containing the CUX1 core binding sequence ATCGAT and mutagenesis of this sequence abrogated CUX1 binding. Furthermore, CUX1 repressed expression of a luciferase reporter construct controlled by the MIE promoter, and mutation of CUX1 binding sites within the promoter diminished this repressive function of CUX1. In the context of virus infection of HEK293 cells transfected with the CUX1 expression vector, CUX1 showed evidence of association with the HCMV MIE regulatory region and inhibited the capacity of the virus to express ie1 and ie2 transcripts, suggesting that this cellular factor regulates MIE gene expression following virus entry. These data identify a role for CUX1 in repressing HCMV gene expression essential for initiation of the replicative cycle.

Mouse cytomegalovirus microRNAs dominate the cellular small RNA profile during lytic infection and show features of posttranscriptional regulation

MicroRNAs (miRNAs) are small, noncoding RNA molecules that regulate gene expression at the posttranscriptional level. Originally identified in a variety of organisms ranging from plants to mammals, miRNAs have recently been identified in several viruses. Viral miRNAs may play a role in modulating both viral and host gene expression. Here, we report on the identification and characterization of 18 viral miRNAs from mouse fibroblasts lytically infected with the murine cytomegalovirus (MCMV). The MCMV miRNAs are expressed at early times of infection and are scattered in small clusters throughout the genome with up to four distinct miRNAs processed from a single transcript. No significant homologies to human CMV-encoded miRNAs were found. Remarkably, as soon as 24 h after infection, MCMV miRNAs constituted about 35% of the total miRNA pool, and at 72 h postinfection, this proportion was increased to more than 60%. However, despite the abundance of viral miRNAs during the early phase of infection, the expression of some MCMV miRNAs appeared to be regulated. Hence, for three miRNAs we observed polyuridylation of their 3' end, coupled to subsequent degradation. Individual knockout mutants of two of the most abundant MCMV miRNAs, miR-m01-4 and miR-M44-1, or a double knockout mutant of miR-m21-1 and miR-M23-2, incurred no or only a very mild growth deficit in murine embryonic fibroblasts in vitro.

Identification and characterization of human cytomegalovirus-encoded microRNAs

MicroRNAs (miRNAs) are an extensive class of noncoding genes that regulate gene expression through posttranscriptional repression. Given the potential for large viral genomes to encode these transcripts, we examined the human cytomegalovirus AD169 genome for miRNAs using a bioinformatics approach. We identified 406 potential stem-loops, of which 110 were conserved between chimpanzee cytomegalovirus and several strains of human cytomegalovirus. Of these conserved stem-loops, 13 exhibited a significant score using the MiRscan algorithm. Examination of total RNA from human cytomegalovirus-infected cells demonstrated that 5 of the 13 predicted miRNAs were expressed during infection. These studies demonstrate that human cytomegalovirus encodes multiple conserved miRNAs and suggest that human cytomegalovirus may utilize an miRNA strategy to regulate cellular and viral gene function.

Novel immediate-early protein IE19 of human cytomegalovirus activates the origin recognition complex I promoter in a cooperative manner with IE72

The major immediate-early (MIE) gene of human cytomegalovirus (HCMV) expresses IE86, IE72, IE55, and IE18 mRNA by differential splicing. Reverse transcription-PCR with IE72-specific primers generated an 0.65-kb cDNA from HCMV-infected fibroblast RNA, which does not correspond to any known MIE cDNA. Nucleotide sequencing revealed that the 0.65-kb cDNA is from exons 1, 2, and 3 and part of exon 4, indicating that it is derived from a novel alternatively spliced mRNA of the MIE gene. The cDNA encodes a 172-amino-acid polypeptide, termed IE19, which corresponds to an IE72 variant with an internal deletion from Val(86) to Pro(404) and appears as a band at 38 kDa on a sodium dodecyl sulfate-polyacrylamide gel. IE19 mRNA was expressed at a low level in the immediate-early, early, and late period of viral infection. IE19 was localized in nuclei, and a transient-expression assay revealed that IE19 enhances IE72-dependent activation of the HsOrc1 promoter, which is identified here as an IE72 target promoter. Another MIE protein, IE86, activated the same promoter but only weakly compared to IE72, and coexpression of IE19 did not alter the IE86-mediated transcriptional activation. In addition, IE19 did not enhance the IE72-dependent activation of the HCMV UL54 promoter. These results suggest that IE19 is a transcriptional coactivator that works with IE72.

Dominance of virus over host factors in cross-species activation of human cytomegalovirus early gene expression

Human cytomegalovirus (HCMV) exhibits a highly restricted host range. In this study, we sought to examine the relative significance of host and viral factors in activating early gene expression of the HCMV UL54 (DNA polymerase) promoter in murine cells. Appropriate activation of the UL54 promoter at early times is essential for viral DNA replication. To study how the HCMV UL54 promoter is activated in murine cells, a transgenesis system based on yeast artificial chromosomes (YACs) was established for HCMV. A 178-kb YAC, containing a subgenomic fragment of HCMV encompassing the majority of the unique long (UL) region, was constructed by homologous recombination in yeast. This HCMV YAC backbone is defective for viral growth and lacks the major immediate-early (IE) gene region, thus permitting the analysis of essential cis-acting sequences when complemented in trans. To quantitatively measure the level of gene expression, we generated HCMV YACs containing a luciferase reporter gene inserted downstream of either the UL54 promoter or, as a control for late gene expression, the UL86 promoter, which directs expression of the major capsid protein. To determine the early gene activation pathway, point mutations were introduced into the inverted repeat 1 (IR1) element of the UL54 promoter of the HCMV YAC. In the transgenesis experiments, HCMV YACs and derivatives generated in yeast were introduced into NIH 3T3 murine cells by polyethylene glycol-mediated fusion. We found that infection of YAC, but not plasmid, transgenic lines with HCMV was sufficient to fully recapitulate the UL54 expression program at early times of infection, indicating the importance of remote regulatory elements in influencing regulation of the UL54 promoter. Moreover, YACs containing a mutant IR1 in the UL54 promoter led to reduced ( approximately 30-fold) reporter gene expression levels, indicating that HCMV major IE gene activation of the UL54 promoter is fully permissive in murine cells. In comparison with HCMV, infection of YAC transgenic NIH 3T3 lines with murine cytomegalovirus (MCMV) resulted in lower (more than one order of magnitude) efficiency in activating UL54 early gene expression. MCMV is therefore not able to fully activate HCMV early gene expression, indicating the significance of virus over host determinants in the cross-species activation of key early gene promoters. Finally, these studies show that YAC transgenesis can be a useful tool in functional analysis of viral proteins and control of gene expression for large viral genomes.

The human cytomegalovirus 86-kilodalton major immediate-early protein interacts physically and functionally with histone acetyltransferase P/CAF

The major immediate-early proteins of human cytomegalovirus (HCMV) play a pivotal role in controlling viral and cellular gene expression during productive infection. As well as negatively autoregulating its own promoter, the HCMV 86-kDa major immediate early protein (IE86) activates viral early gene expression and is known to be a promiscuous transcriptional regulator of cellular genes. IE86 appears to act as a multimodal transcription factor. It is able to bind directly to target promoters to activate transcription but is also able to bridge between upstream binding factors such as CREB/ATF and the basal transcription complex as well as interacting directly with general transcription factors such as TATA-binding protein and TFIIB. We now show that IE86 is also able to interact directly with histone acetyltransferases during infection. At least one of these factors is the histone acetyltransferase CBP-associated factor (P/CAF). Furthermore, we show that this interaction results in synergistic transactivation by IE86 of IE86-responsive promoters. Recruitment of such chromatin-remodeling factors to target promoters by IE86 may help explain the ability of this viral protein to act as a promiscuous transactivator of cellular genes.

Analysis of transcripts expressed from the UL47 gene of human cytomegalovirus

The UL47 gene (b 60390-b 60338) located in the unique long region of the human cytomegalovirus (HCMV) AD169 strain genome was analyzed by RNA mapping. Northern blot analysis showed that the UL47 gene was expressed at late times after infection (72 h postinfection). The 9.7-kb transcript was expressed in the infected cells but not in phosphonoformate-treated cells at 72 hpi, indicating that the UL47 gene was only expressed at late times after infection. To map the 5'-end and 3'-end of UL47 transcripts, primer extension and RNase protection analysis were performed. Primer extension analysis revealed that the transcription initiation site of UL47 was located in 27 bp downstream (b 60323) of the TATA box motif. The sizes of UL47 ORF (approximately 2.9-kb) and UL48 ORF (approximately 6.7-kb) deduced from computer sequence analysis suggest that the expressed 9.7-kb transcript of UL47 uses the 3'-end polyadenylation signal of UL48. The result of RNase protection determined that the 3'-end of UL47 RNA utilized the 3'-end polyadenylation signal of UL48, which is located in HCMV genome b 70082.

DNA microarrays of the complex human cytomegalovirus genome: profiling kinetic class with drug sensitivity of viral gene expression

We describe, for the first time, the generation of a viral DNA chip for simultaneous expression measurements of nearly all known open reading frames (ORFs) in the largest member of the herpesvirus family, human cytomegalovirus (HCMV). In this study, an HCMV chip was fabricated and used to characterize the temporal class of viral gene expression. The viral chip is composed of microarrays of viral DNA prepared by robotic deposition of oligonucleotides on glass for ORFs in the HCMV genome. Viral gene expression was monitored by hybridization to the oligonucleotide microarrays with fluorescently labelled cDNAs prepared from mock-infected or infected human foreskin fibroblast cells. By using cycloheximide and ganciclovir to block de novo viral protein synthesis and viral DNA replication, respectively, the kinetic classes of array elements were classified. The expression profiles of known ORFs and many previously uncharacterized ORFs provided a temporal map of immediate-early (alpha), early (beta), early-late (gamma1), and late (gamma2) genes in the entire genome of HCMV. Sequence compositional analysis of the 5' noncoding DNA sequences of the temporal classes, performed by using algorithms that automatically search for defined and recurring motifs in unaligned sequences, indicated the presence of potential regulatory motifs for beta, gamma1, and gamma2 genes. In summary, these fabricated microarrays of viral DNA allow rapid and parallel analysis of gene expression at the whole viral genome level. The viral chip approach coupled with global biochemical and genetic strategies should greatly speed the functional analysis of established as well as newly discovered large viral genomes.

Growth of human cytomegalovirus in primary macrophages

Human cytomegalovirus (HCMV) is a major human pathogen that causes considerable disease among immunocompromised individuals. A primary infection results in life-long persistence of the virus in a latent form. HCMV is known to be transferred by blood products, bone marrow, and solid organs, but the cell type that carries the latent infection has been difficult to identify. We have recently demonstrated reactivation of latent HCMV in allogeneically stimulated monocyte-derived macrophages (Allo-MDM). Reactivation occurred only in macrophages produced by allogeneic but not mitogenic stimulation. The presence of dendritic cell markers on some Allo-MDM cells suggested that these macrophages were related to dendritic cells. However, dendritic cells obtained by stimulation of monocytes with interleukin-4 (IL-4) and granulocyte-macrophage colony stimulating factor (GM-CSF) were not permissive for HCMV infection. The cellular and cytokine components which are essential for HCMV replication and reactivation of virus were also examined in Allo-MDM. The importance of both CD4- or CD8-positive T cells in the generation of HCMV permissive Allo-MDM was demonstrated by negative selection or blocking experiments using antibodies directed against both HLA class I and HLA class II molecules. Examination of the cytokines essential for the generation of HCMV permissive Allo-MDM identified gamma-interferon (IFN-gamma, but not IL-1, IL-2, tumor necrosis factor alpha, or GM-CSF as critical components in the generation of these macrophages. However, addition of IFN-gamma to unstimulated macrophage cultures was insufficient to reactivate virus. These results indicate the importance of a specific moncyte stimulus in the generation of a unique HCMV permissive macrophage phenotype as well as why virus is commonly reactivated in transplant patients.

Transcription factor Sp1 mediates cell-specific trans-activation of the human cytomegalovirus DNA polymerase gene promoter by immediate-early protein IE86 in glioblastoma U373MG cells

Human cytomegalovirus (HCMV) gene expression is highly cell and tissue specific. Cell factor-mediated regulatory interactions are involved in regulating the restricted expression of the HCMV major immediate-early (IE) gene (J. F. Baskar, P. P. Smith, G. Nilaver, R. A. Jupp, S. Hoffmann, N. J. Peffer, D. J. Tenney, A. M. Colberg-Poley, P. Ghazal, and J. A. Nelson, 70:3207-3213, 1996). To gain an understanding of HCMV early gene activation, we studied the effect of each of the three major IE proteins, IE72, IE86, and IE55, on the HCMV DNA polymerase gene (pol; UL54) promoter. In transient-expression assays, the IE86 protein alone was able to transactivate the pol promoter, but IE72 and IE55 were not, in permissive U373MG cells. However, we were unable to detect IE86-mediated transactivation in nonpermissive HeLa or C33-A cells. Using electrophoretic mobility shift assays (EMSAs), we found that expression of the IE86 protein in U373MG cells resulted in specific binding of a DNA complex to an inverted-repeat element, IR1, of the pol promoter. Antibody supershifting and EMSA-Western blotting experiments further showed that IE86 and the cellular transcription factor Sp1 were components of the IR1 DNA-binding complex. Furthermore, we found that binding of DNA by Sp1 was dramatically increased in the presence of IE86. Interestingly, this IE86-induced DNA-binding activity of Sp1 was inhibited by a repressor activity presented in HeLa cells. In summary, our study suggests that a viral regulatory protein can modulate the DNA binding activity of a cellular transcription factor, resulting in cell-specific transactivation of viral genes.

Binding of SP1 to the immediate-early protein-responsive element of the human cytomegalovirus DNA polymerase promoter

Human cytomegalovirus (HCMV), a member of the herpesvirus family of DNA viruses, encodes two major immediate-early (IE) transcription factors, IE72 and IE86, that are important for regulated expression of the viral genome. The purpose of this study was to identify the host cellular components required for regulation of the HCMV DNA polymerase promoter (UL54) by HCMV IE proteins. Extensive mutagenesis defined a DNA element located between -54 and -43 relative to the transcription start site that was required for both basal transcriptional activity and transactivation by viral IE proteins. A single copy of the UL54 -54/-43 sequence enhanced the responsiveness of a heterologous minimal promoter to HCMV IE proteins. Fractionation of extracts prepared from uninfected cells led to the isolation of two cellular proteins with apparent molecular masses of 95 and 105 kDa that bound specifically to the UL54 -54/-43 element. Biochemical and immunochemical analyses identified this protein as the transcription factor SP1. Although initial inspection of the UL54 -54/-43 sequence did not predict an SP1 binding site, subsequent analyses indicated that it is indeed a nonconsensus GC box. We propose that SP1 is required to direct basal levels of promoter activity and that SP1-regulated transcription complexes allow the entry of HCMV IE proteins into the transcription cycle.

Cytomegalovirus selectively blocks antigen processing and presentation of its immediate-early gene product

Recognition of virus-infected cells by CD8+ cytotoxic T lymphocytes requires that the viral proteins be processed into peptides, the derived peptides transported into the endoplasmic reticulum and inserted into the binding groove of a major histocompatibility complex class I molecule, and the antigenic complex exported to the cell surface. However, viral pathogens can disrupt this process and interfere with immune recognition. These mechanisms may be vital to large viruses such as human cytomegalovirus (CMV), which causes persistent infection despite producing over 200 potentially antigenic proteins during the sequential immediate-early, early and late phases of viral gene expression. Products of CMV early-phase gene expression can globally block class I presentation and prevent recognition of infected cells by cytotoxic T lymphocytes, but an essential viral transcription factor, the 72K principal immediate-early protein, is abundantly expressed before this blockade. However, only a few host CD8+ cytotoxic T lymphocytes specific for immediate-early protein are present in seropositive individuals, and these lyse CMV-infected cells poorly. Here we demonstrate selective abrogation of immediate-early peptide presentation by a CMV matrix protein with associated kinase activity and suggest that modification of a viral protein can result in limiting access to the processing machinery and evasion of cytotoxic-T-cell recognition.

The enhancer domain of the human cytomegalovirus major immediate-early promoter determines cell type-specific expression in transgenic mice

The human cytomegalovirus (HCMV) major immediate-early promoter (MIEP) is one of the first promoters to activate upon infection. To examine HCMV MIEP tissue-specific expression, transgenic mice were established containing the lacZ gene regulated by the MIEP (nucleotides -670 to +54). In the transgenic mice, lacZ expression was demonstrated in 19 of 29 tissues tested by histochemical and immunochemical analyses. These tissues included brain, eye, spinal cord, esophagus, stomach, pancreas, kidney, bladder, testis, ovary, spleen, salivary gland, thymus, bone marrow, skin, cartilage, and cardiac, striated and smooth muscles. Although expression was observed in multiple organs, promoter activity was restricted to specific cell types. The cell types which demonstrated HCMV MIEP expression included retinal cells of the eye, ductile cells of the salivary gland, exocrine cells of the pancreas, mucosal cells of the stomach and intestine, neuronal cells of the brain, muscle fibers, thecal cells of the corpus luteum, and Leydig and sperm cells of the testis. These observations indicate that the HCMV MIEP is not a pan-specific promoter and that the majority of expressing tissues correlate with tissues naturally infected by the virus in the human host.

Regulation of a human cytomegalovirus immediate-early gene (US3) by a silencer-enhancer combination

The US3 open reading frame of human cytomegalovirus (HCMV) is transcribed at immediate-early (IE) times after infection. Upstream of the US3 promoter, between -84 and -259 bp relative to the transcription start site, there are five copies of an 18-bp repeat, referred to as 5R2. Between -340 and -560 bp there are seven copies of a 10-bp dyad repeat, referred to as 7R1. We investigated the roles of these repeats in transcription from the US3 promoter in human foreskin fibroblast or HeLa cells. In transient transfection assays, the region containing 5R2 up-regulated transcription and was responsive to the p65 subunit of NF-kappa B. The DNA region containing 7R1 down-regulated transcription from either the US3 promoter or a heterologous promoter in a position- and orientation-independent manner. Mutational analysis and transient transfections indicated that DNA containing the 10-bp dyad or one-half of the dyad was sufficient to cause repression of downstream gene expression. DNA probes containing one or more copies of the pentanucleotide sequence TGTCG specifically bound cellular proteins, as demonstrated by electrophoretic mobility shift assays and cold-competition electrophoretic mobility shift assays. Two different DNA-protein complexes were detected with DNA probes containing one or two copies of the pentanucleotide. In HCMV-infected cell nuclear extracts, one of the DNA-protein complexes was present in amounts inversely proportional to the amount of US3 transcription. Its formation was affected by dephosphorylation of the DNA-binding protein(s). Transient dephosphorylation of the cellular repressor protein may occur during HCMV infection. Repression of US3 transcription may relate to the number of pentanucleotides and the cellular proteins that bind to it. Twenty-one copies of a TRTCG motif (R = purine) were found clustered upstream of the US3 gene and also in the modulator upstream of the HCMV IE1 and IE2 genes.

Cell types involved in replication and distribution of human cytomegalovirus

As the number of patients suffering from severe HCMV infections has steadily increased, there is a growing need to understand the molecular mechanisms by which the virus causes disease. The factors that control infection at one time and the events leading to virus multiplication at another time are only beginning to be understood. The interaction of HCMV with different host cells is one key for elucidating these processes. Through modern techniques, much has been learned about the biology of HCMV infections in culture systems. In addition to endothelial cells, epithelial cells, and smooth muscle cells, fibroblasts are one cell population preferentially infected in solid tissues in vivo. From these sites of multiplication, the virus may be carried by peripheral monocytes and circulating endothelial cells to reach distant sites of the body. This would explain the multiorgan involvement in acute HCMV infection and the modes of viral transmission. From what has been learned mainly from human fibroblast culture systems, future studies will focus on how HCMV regulates the expression of its putative 200 genes in different host cells at different stages of cell differentiation and activation to result in viral latency and pathogenesis.

Binding of cellular repressor protein or the IE2 protein to a cis-acting negative regulatory element upstream of a human cytomegalovirus early promoter

We have previously shown that the human cytomegalovirus early UL4 promoter has upstream negative and positive cis-acting regulatory elements. In the absence of the upstream negative regulatory region, the positive element confers strong transcriptional activity. The positive element contains a CCAAT box dyad symmetry and binds the cellular transcription factor NF-Y. The effect of the negative regulatory element is negated by the viral IE2 protein (L. Huang, C.L. Malone, and M.F. Stinski, J. Virol. 68:2108, 1994). We investigated the binding of cellular or viral IE2 protein to the negative regulatory region. The major cis-acting negative regulatory element was located between -168 and -134 bp relative to the transcription start site. This element could be transferred to a heterologous promoter, and it functioned in either orientation. Mutational analysis demonstrated that a core DNA sequence in the cis-acting negative regulatory element, 5'-GTTTGGAATCGTT-3', was required for the binding of either a cellular repressor protein(s) or the viral IE2 protein. The cellular DNA binding activity was present in both nonpermissive HeLa and permissive human fibroblast cells but more abundant in HeLa cells. Binding of the cellular repressor protein to the upstream cis-acting negative regulatory element correlates with repression of transcription from the early UL4 promoter. Binding of the viral IE2 protein correlates with negation of the repressive effect.

The human cytomegalovirus UL98 gene transcription unit overlaps with the pp28 true late gene (UL99) and encodes a 58-kilodalton early protein

A murine monoclonal antibody (I2) reacts strongly with the nucleus of human cytomegalovirus (HCMV)-infected human fibroblasts. Western blot (immunoblot) analysis using I2 demonstrated that a protein with an apparent molecular mass of 58-kDa (E58) was expressed at 5 h after infection, and levels increased through 72 h. Immunoblot screening of an early cDNA expression library resulted in a positive clone which hybridized to the right end of the XbaI C fragment of the HCMV Towne strain. Further analysis demonstrated that the E58-specific clone was homologous to the putative UL98 open reading frame, which has been proposed to encode the viral alkaline exonuclease homolog. RNA analysis demonstrated a 3.0-kb RNA which is expressed at early times after infection, as well as in the absence of viral DNA replication, and which is 3' coterminal with the pp28 (UL99) gene region. Insertion of the UL98 genomic sequence into a eucaryotic expression vector and subsequent Western blot analysis using I2 demonstrated that the expressed protein comigrated with E58 from infected cells. E58 also reacts specifically with a previously described antibody, anti-P2-1, which was proposed to recognize a putative late 58-kDa protein. E58 comigrates with the putative late 58-kDa protein, indicating that these two proteins are likely the same. Analysis of the UL98 promoter revealed a TATATAA sequence located at nucleotide 142525. Insertion of the putative promoter 5' to a reporter gene demonstrated that the UL98 promoter was activated in cotransfection experiments with IE1 and IE2 proteins. These studies demonstrate that UL98 is a bona fide early gene, which is consistent with its probable role as the viral alkaline exonuclease gene.

Growth kinetics of human cytomegalovirus are altered in monocyte-derived macrophages

Stimulation of monocytes/macrophages with activated nonadherent cells allows productive nonlytic growth of human cytomegalovirus (HCMV), but the viral replication cycle is delayed relative to replication of HCMV in human fibroblasts. Analysis of infected monocyte-derived macrophage (MDM) mRNA for major immediate-early (MIE 86, 72, and 55) and late (pp65 and gB) gene expression by reverse transcription PCR indicates that transcription peaks at 3 and 7 days postinfection (dpi), respectively. In contrast, in human fibroblast controls, mRNA for MIE and late gene expression peaked at 5 and 48 h postinfection, respectively. Consistent with reverse transcription PCR experiments, double-label antibody experiments first detected MIE antigen expression at 12 h postinfection, peaking at 3 dpi, and late (pp65 or gB) antigen expression at 5 dpi, peaking at 7 dpi. MIE antigen was not detected between 3 and 7 dpi but reappeared and was coexpressed with pp65 in enlarged MDM nuclei at 7 dpi. After 7 dpi, macrophages with numerous vacuoles containing large amounts of pp65 and gB were observed in culture. These vacuoles were frequently seen at cellular contact points, suggesting that cell-to-cell transfer of virus was the major mode of viral transmission. Consistent with this observation, infectious virus was recovered from MDM cellular lysates but not culture supernatant. The delayed growth and compartmentalization of HCMV in macrophages may allow the cell to accommodate the viral replication cycle without cell lysis. In addition, the macrophage may function as a vehicle for cell-to-cell transmission of HCMV.

Isolation and characterization of a low-abundance splice variant from the human cytomegalovirus major immediate-early gene region

The major immediate-early (IE) gene region of human cytomegalovirus (HCMV) encodes several proteins as a result of differential RNA splicing events. By expression vector cloning of HCMV IE mRNA, we isolated and characterized a cDNA for a novel splice variant from the major IE gene region. The RNA product is a derivative of the IE55 mRNA and contains an additional splice from nucleotides 170,635 to 170,307 in the IE2 gene region (UL122), resulting in a 1.4-kb mRNA. The predicted open reading frame codes for a 164-amino-acid protein with a calculated molecular mass of 18 kDa (IE18). Mung bean nuclease analysis and PCR were used to characterize expression of IE18 mRNA in HCMV-infected cells. While the 1.4-kb mRNA was detected in infected human fibroblasts in the presence of a protein synthesis inhibitor, it was not detectable during a normal infection. However, the 1.4-kb mRNA was readily detected in infected human monocyte-derived macrophages at IE times. These results suggest that the novel IE18 mRNA exhibits cell type-specific expression indicating differential regulation of the major IE gene region in different permissive cell types.

Analysis and mapping of a family of 3'-coterminal transcripts containing coding sequences for human cytomegalovirus open reading frames UL93 through UL99

Human cytomegalovirus (HCMV) open reading frames (ORFs) UL93 through UL99 are contained within a region of viral genome that is well conserved in all herpesviruses. Previous reports detailing the expression of ORF UL99 (also referred to as the 28-kDa virion phosphoprotein or pp28) indicated that the pattern of transcription proximal to pp28 is extremely complex and involves a number of large overlapping transcripts, none of which have been characterized. We have used an RNA-mapping approach consisting of Northern (RNA) hybridization, RNase protection, and primer extensions to determine the coding capacity of several large-molecular-weight transcripts which overlap the 1.3- and 1.6-kb UL99-specific transcripts. Our results suggest that six differentially regulated transcripts with sizes of 2.6, 4.7, 5.6, 7.3, 9.1, and 10.5 kb, and derived from the same strand of the viral genome overlap, are 3'-coterminal with the smaller UL99-specific transcripts. On the basis of 5'-end mapping via primer extension and RNase protection, we have determined that the 2.6- to 10.5-kb messages initiate upstream of each of the potential ORFs in this region, UL98, UL97, UL96, UL95, UL94, and UL93. By using cycloheximide and ganciclovir [9-(1,3-dihydroxy-2-propoxymethyl)guanine] to block de novo viral protein synthesis and viral DNA replication, respectively, we have determined that the 2.6-, 4.7-, 5.6-, and 7.3-kb messages have characteristics of early or early-late transcripts, whereas the 9.1- and 10.5-kb messages appear to be true late transcripts. The evolutionary conservation of ORFs UL93 through UL99 and their transcriptional regulation in other herpesviruses are discussed.

CCAAT box-dependent activation of the TATA-less human DNA polymerase alpha promoter by the human cytomegalovirus 72-kilodalton major immediate-early protein

Human cytomegalovirus (HCMV) immediate-early (IE) proteins are known potent transregulators of viral and cellular gene expression upon HCMV infection. HCMV is known to activate a number of cellular genes intimately associated with the cell cycle and DNA replication by mechanisms involving the viral major IE 86-kDa protein (IE2). We have recently shown that IE2 mediates this activation in a TATA-dependent manner and interacts directly with the TATA-binding protein. However, a number of TATA-less cellular promoters, e.g., DNA polymerase alpha and dihydrofolate reductase, are also activated by HCMV infection. Consequently, we have asked how HCMV mediates this activation. We show that, consistent with its known TATA dependency, IE2 does not activate the DNA polymerase alpha promoter. In contrast, this promoter is strongly activated by the major IE 72-kDa protein (IE1). Whilst deletion of ATF or E2F sites within the DNA polymerase alpha promoter had little effect on IE1-mediated activation, removal of the CCAAT box appeared to abolish high levels of activation by IE1. Consistent with this observation, we also find that IE1 interacts directly with the CCAAT box binding factor CTF1 in vitro and massively augments CTF1-mediated activation of the DNA polymerase alpha promoter in transient transfection assays.

Protein-protein interactions between human cytomegalovirus IE2-580aa and pUL84 in lytically infected cells

The human cytomegalovirus immediate-early protein IE2-580aa (ppUL122a) activates transcription of viral and cellular genes and represses its own transcription through sequence-specific binding to the major immediate-early promoter. In lytically infected cells, IE2-580aa interacts with a 75-kDa viral protein (p75), an early protein that is also synthesized at late times after infection. Here we show that p75 is the product of the UL84 gene. Its association with IE2-580aa in infected cells suggests that pUL84 is involved in transcription control.

Identification of sequence elements in the human cytomegalovirus DNA polymerase gene promoter required for activation by viral gene products

To determine the mechanisms involved in the regulation of human cytomegalovirus early gene expression, we have examined the gene that encodes the viral DNA polymerase (UL54, pol). Our previous studies demonstrated that sequences required for activation of the pol promoter by immediate-early proteins are contained within a region from -128 to +20 and that cellular proteins can bind to this activation domain. In this study, we demonstrate by competition analysis that binding of cellular proteins to pol is associated with an 18-bp region containing a single copy of a novel inverted repeat, IR1. Time course analysis indicated that viral infection increased the level of protein binding to IR1, concurrent with the activation of the pol promoter. Mutation of the IR1 element abrogated binding of cellular factors to the pol promoter and reduced by threefold the activation by immediate-early proteins. Similarly, mutation of IR1 rendered the promoter poorly responsive to activation by viral infection. Mutation of additional sequence elements in the pol promoter had little effect, indicating that IR1 plays the major role in pol promoter regulation. These studies demonstrate that the interaction between cellular factors and IR1 is important for the regulation of expression of the polymerase gene by viral proteins.

A human cytomegalovirus early promoter with upstream negative and positive cis-acting elements: IE2 negates the effect of the negative element, and NF-Y binds to the positive element

The human cytomegalovirus early promoter for the UL4 gene, which codes for an early viral envelope glycoprotein designated gpUL4, requires immediate-early viral protein two (IE2) synthesis to be activated (C.-P. Chang, C. L. Malone, and M. F. Stinski, J. Virol. 63:281, 1989). We investigated the cis-acting and trans-acting factors that regulate transcription from this UL4 promoter. In transient transfection assays, the viral IE2 protein negated the effect of an upstream cis-acting negative element and enhanced downstream gene expression. A cis-acting positive element contributed to the activity of the viral promoter when an upstream cis-acting negative element was deleted or when the viral IE2 protein was present. The cellular protein(s) that binds to the cis-acting negative element requires further investigation. The cellular protein that binds to the cis-acting positive element was characterized. Two DNA sequence-specific protein complexes were detected with DNA probes spanning the region containing the cis-acting positive element and human cytomegalovirus-infected human fibroblast cell nuclear extracts. The more slowly migrating complex was labeled complex A, and the faster was labeled complex B. Only complex B was detected with mock-infected cell nuclear extracts. Competition experiments confirmed the specificity of the A and B complexes. The protein bound to the DNA in both the complexes contacts a CCAAT box imperfect dyad symmetry (5'CCAATCACTGG3'). Either CCAAT box within the dyad symmetry could compete for binding the nuclear factor. Mutation of the CCAAT box dyad symmetry resulted in a decrease of the transcriptional activity from the UL4 promoter. A cellular transcription factor, antigenically related to nuclear factor-Y (NF-Y), was found in both complexes A and B. Events associated with viral infection caused phosphorylation of protein complex A. Dephosphorylation of the DNA-binding protein converts complex A to complex B. The effect of phosphorylation of NF-Y is not known.

The human cytomegalovirus 86-kilodalton immediate-early 2 protein: synthesis as a precursor polypeptide and interaction with a 75-kilodalton protein of probable viral origin

The immediate-early 2 (IE2) 86-kDa polypeptide, a major immediate-early gene product of human cytomegalovirus, regulates transcription both positively and negatively. We report two new properties of the IE2 86-kDa polypeptide in infected cells. Immunoprecipitation of infected cell proteins from human embryonic lung cells by antipeptide or monoclonal antibodies specific for IE2 epitopes revealed three closely migrating polypeptide species. The slowest, p86, behaved as expected for the mature 86-kDa IE2 polypeptide. The middle species, p80, was immunoprecipitated from denatured as well as native samples and labeled to steady state rapidly. Pulse-chase analysis demonstrated directly that p80 was a metabolic precursor to p86. The fastest-migrating species, p75, was not detected by probing blots of the immunoprecipitated proteins with IE2-specific antisera; p75 was not precipitated from denatured protein samples; and the products of partial proteolysis of p75 were distinct from those of p86. These properties established p75 as an unrelated coprecipitated polypeptide complexed with p86. The p75 proteins coprecipitated from cells infected with two different strains of human cytomegalovirus, AD169 and Towne, had different mobilities. p75 was detected as early as 6 h and as late as 72 h after infection, but it was not synthesized in cells released from a cycloheximide block. Thus, it is likely that p75 is an early viral protein.

Characterization of S-adenosylmethionine decarboxylase induced by human cytomegalovirus infection

Infection of human diploid embryonic lung (MRC5) cells by human cytomegalovirus (HCMV), strain AD169, increased the activity of a key enzyme in the synthesis of polyamines: S-adenosylmethionine decarboxylase (E.C. 4.1.1.50). The initial peak of S-adenosylmethionine decarboxylase activity occurred about 15 h postinfection. S-Adenosylmethionine decarboxylase was purified using a highly specific affinity chromatography step from HCMV-infected and control uninfected MRC5 cells. No difference was found between the two enzymes in their stability to heat or effect of pH on activity. Both enzymes were activated only by putrescine. The appKm for S-adenosylmethionine for the virus-induced enzyme was 1.7 times higher than the appKm for the control enzyme. The most dramatic difference observed was in the effect of high salt concentration on enzyme activity. S-Adenosylmethionine decarboxylase from HCMV-infected cells was unaffected by 0.8 M NaCl, whereas the enzyme from uninfected cells was inhibited by 50% at 0.45 M NaCl and was significantly inhibited at a concentration of 0.8 M NaCl. Thus, different forms of S-adenosylmethionine decarboxylase probably exist in infected and uninfected MRC5 cells.

Direct interaction of the human cytomegalovirus IE86 protein with the cis repression signal does not preclude TBP from binding to the TATA box

The human cytomegalovirus major immediate-early gene encodes several protein isoforms which autoregulate the major immediate-early promoter (MIEP). One of these isoforms, the IE86 protein, represses the MIEP through a DNA sequence located between the TATA box and the transcription initiation site, designated the cis repression signal (crs). Through mutational analysis, amino acid domains within IE86 responsible for binding the crs element were located at the C terminus. Mutation of the putative zinc finger domain, which precluded IE86 from binding DNA, converted the protein from a repressor of MIEP transcription into an activator. DNase I protection analysis demonstrated that the IE86 footprint overlapped the sequence protected by the TATA-binding protein (TBP). Investigation of whether IE86 was able to displace TBP from DNA revealed that both proteins could bind DNA simultaneously. However, higher concentrations of IE86 were required to obtain protection of the crs element in the presence of prebound TBP. Similarly, higher concentrations of TBP were required to obtain protection in the presence of prebound IE86. These observations indicate that steric hinderance impairs but does not prevent both proteins from binding DNA synchronously.

Human cytomegalovirus immediate-early gene 2 protein interacts with itself and with several novel cellular proteins

The human cytomegalovirus immediate-early gene product 2 (IE2) is able to transactivate homologous and heterologous promoters alone or augmented by immediate-early gene product 1 (IE1). IE2 has also been shown to autoregulate the major immediate-early promoter by directly binding to a cis repression signal located between the TATA box and the cap site. However, IE2 has not been shown to act directly through a specific DNA sequence in transactivating various promoters. To understand whether IE2 can be indirectly involved in DNA sequence-specific transactivation through interactions with other transcriptional factors, we performed a study of the interactions of IE2 with cellular proteins. In order to study these interactions, IE cDNAs were subcloned into a bacterial expression vector, pGEX2T, by polymerase chain reaction amplification to produce fusion proteins which were full-length as well as proteins which contained various functional domains. We were able to demonstrate IE2's ability to interact directly or indirectly with several cellular proteins ranging from > 200 to 14 kDa through glutathione S-transferase-fusion protein precipitation and far-Western analysis. These interactions have been mapped to domains within IE2 which are known to be necessary for either transactivation or both transactivation and autoregulation. All of the IE2-associated proteins are nuclear proteins, and a subset are phosphorylated. In vitro-synthesized 35S-IE2 protein and bacterially expressed IE2 fusion proteins were used to study IE2-IE2 interaction by binding assay and far-Western analysis. IE2-IE2 interactions were mapped to a domain containing a putative helix-turn-helix motif located near the C terminus of IE2, between amino acids 456 and 539. However, IE2 was unable to directly interact with either IE1, an alternatively spliced variant of IE2 (55 kDa), or IE2 deletion mutants that did not contain the multimerization domain.

Characterization of the major locus of immediate-early genes of rat cytomegalovirus

A major locus of rat cytomegalovirus (RCMV) immediate-early (IE) RNA transcription was identified. A cDNA library from rat embryo fibroblasts infected with RCMV under IE conditions was constructed and screened by using appropriate RCMV DNA probes, revealing at least two IE genes (IE1 and IE2) transcribed from this locus by differential splicing. The first three exons (the first is noncoding) are spliced to exon 4 to form IE1 and to exon 5 to form IE2. The structural organization of the RCMV major IE region is therefore similar to that of human cytomegalovirus (HCMV) and murine cytomegalovirus (MCMV). When we compared the predicted amino acid sequences of the IE1 proteins of RCMV, HCMV, and MCMV, no areas of homology were found across all three proteins, while a few small areas of homology were found between RCMV IE1 and MCMV IE1. In contrast, large areas of homology were found across the carboxyl half of RCMV IE2, HCMV IE2, and MCMV ie3 proteins. In addition, similarities were found at the beginning of exon 5 of RCMV and MCMV. The possible significance of these conserved regions is discussed. Dinucleotide frequency analysis demonstrated a decrease in CpG frequency over the IE region. The IE gene products were able to transactivate heterologous promoters.

Selective interference with class I major histocompatibility complex presentation of the major immediate-early protein following infection with human cytomegalovirus

Responses of cytotoxic T-cells (Tc) to human cytomegalovirus (CMV) represent the predominant mechanism by which hosts resist CMV infection. The CMV major immediate-early protein (IE) is present throughout the virus replicative cycle. Studies were performed to determine whether Tc specific for IE effectively lyse CMV-infected targets and are thus capable of providing protective immunity against infection. After in vitro stimulation of peripheral blood mononuclear cells with CMV-infected autologous fibroblasts, Tc specific for IE were not readily detectable in CMV-reactive polyclonal Tc lines. However, after stimulation of peripheral blood mononuclear cells with cells selectively expressing IE, weak but detectable IE-specific Tc responses were observed. The frequency of IE-specific Tc clones derived from cultures stimulated with IE-expressing cells was 50 to 100 times lower than the frequency of Tc clones specific for other CMV proteins isolated from cultures stimulated with CMV-infected cells. All of the IE-specific Tc clones, which efficiently lysed targets selectively expressing IE, demonstrated minimal lysis of CMV-infected fibroblasts, despite abundant IE expression in these target cells. In contrast to these results with IE, other viral proteins were efficiently presented during all phases of CMV infection. These data suggest that CMV has evolved a unique mechanism for selectively limiting the presentation of the potentially immunogenic IE protein, which may preclude IE-specific Tc from providing protective immunity to CMV infection.

An isoform variant of the cytomegalovirus immediate-early auto repressor functions as a transcriptional activator

The major immediate-early promoter (MIEP) of human cytomegalovirus directs the expression of several differentially spliced and polyadenylated mRNAs. These mRNAs encode nuclear phosphorproteins (IE55, IE72, and IE86), which consist of common and unique amino acid sequences. To date, very little is known of the functional role of the 55-kDa (IE55) protein. Here we present evidence that the IE55 protein is a positive activator of the MIEP. In human fibroblast cells IE55 protein activated the MIEP between 10- and 30-fold. Fusion of IE55 to the GAL4 DNA binding domain resulted in a chimeric protein capable of trans-activating a reporter with GAL4 recognition sequences. These results strongly suggest that IE55 is a bona fide transcriptional activator protein. In addition, the IE55 protein was found not to act synergistically with the IE72 activator protein. The IE55 protein shares the same amino acid sequence as IE86 except for a 154-amino-acid deletion at the C-terminal end of the protein. These proteins were functionally antagonistic; IE55 relieved repression by IE86 and, conversely, IE86 negated IE55 activation. Mutagenesis of the MIEP revealed that the target sequence for activation by IE55 is different from the IE86 autorepressive response element. These experiments suggest that the mechanism of action of the IE55 and IE86 isoforms is distinct. Moreover, from these results it is apparent that the interplay of these factors might be critical in determining the level of HCMV replication in the host.

Functional analysis of the true late human cytomegalovirus pp28 upstream promoter: cis-acting elements and viral trans-acting proteins necessary for promoter activation

As a model for analyzing the regulation of human cytomegalovirus late genes, we investigated the 28-kDa phosphoprotein (pp28) gene region. Transcripts of 1.6 and 1.3 kb were expressed in wild-type human cytomegalovirus-infected cells but not in cells infected with a DNA-negative temperature-sensitive mutant (ts66), indicating that DNA replication is absolutely required for pp28 gene expression. Transient promoter activation studies revealed that the pp28 gene region upstream promoter (pp28US) functioned early when expressed independently of the viral genome. However, the promoter was not efficiently activated by immediate-early (IE) proteins but was activated equally well by both wild-type virus and ts66. Deletion analysis of the pp28US promoter indicated that sequences upstream of the CAP site between -107 and -32 were required for activation of the pp28 promoter. Within that region exist a 10-bp sequence at -90 (AGTGAT CGTG) and its inverted repeat at -32 which positively influence pp28 promoter function. Therefore, in the case of the pp28US promoter, viral proteins interact through discrete sequences to facilitate late gene expression.

Human cytomegalovirus UL36-38 and US3 immediate-early genes: temporally regulated expression of nuclear, cytoplasmic, and polysome-associated transcripts during infection

During permissive in vitro infection, the human cytomegalovirus (HCMV) UL36-38 and US3 immediate-early (IE) regions give rise to multiple distinct species of RNA in a temporally regulated manner. We have compared the temporally regulated expression of the UL36-38 and US3 regions with that of the well-characterized major IE (MIE) region. Northern (RNA) blot hybridizations with antisense RNA probes were used to examine RNA isolated from infected cells at IE, early, and late times after infection and from cells infected in the presence of anisomycin (used to block de novo viral protein synthesis) or in the presence of phosphonoformate (used to block HCMV DNA synthesis). Different US3 region transcripts were expressed in the cytoplasm during the IE and late phases of infection, with kinetics similar to those of the MIE region. In contrast, various cytoplasmic transcripts from the UL36-38 region were expressed during each of the IE, early, and late phases of infection, including some expressed from IE through late times. The levels of steady-state RNA from the US3 and MIE regions were increased dramatically by infection in the presence of anisomycin, predominantly because of an increase in multiply spliced transcripts. Two of the three UL36-38 IE transcripts were largely unaffected by anisomycin and were expressed abundantly throughout infection, but a third, multiply spliced UL36-38 IE transcript was abundant only during infection in the presence of anisomycin. Nuclear, cytoplasmic, and polysome-associated transcripts from the three IE regions were not significantly different qualitatively or quantitatively. These results suggest that posttranscriptional controls at the levels of nuclear retention or polysome exclusion of transcripts are not operative for the IE region genes. Overall, these results indicate common features of expression of US3, MIE, and UL36-38, in addition to distinctive expression of the UL36-38 region during all temporal phases of expression.

A discrete cis element in the human immunodeficiency virus long terminal repeat mediates synergistic trans activation by cytomegalovirus immediate-early proteins

The major immediate-early (IE) promoter of human cytomegalovirus directs the expression of several differentially spliced and polyadenylated mRNAs that encode isoformic proteins with apparent molecular masses of 55, 72, and 86 kDa. All of these proteins are potent transcriptional regulatory proteins. We are interested in the collateral interactions between human cytomegalovirus and human immunodeficiency virus (HIV) in the context of dual infection of a cell. The roles of the specific IE protein isoforms and their respective response elements involved in trans activation of the HIV long terminal repeat (LTR) are not known. Here we present evidence that major IE proteins IE86, IE72, and IE55 are capable of trans-activating the HIV LTR in a T-cell line, HUT-78. The IE55 isoform noncooperatively stimulates the HIV LTR in the presence of either isoform IE72 or IE86. Interactions between isoforms IE72 and IE86, however, result in strong synergistic activation of the LTR. Our results suggest that a specific 155-amino-acid protein domain that is unique for the IE86 protein participates in this synergic interaction. Point mutational analysis of the LTR identified a distinct cis-acting target site, located between nucleotide positions -174 and -163, that mediates exclusively synergistic trans activation by the IE72 and IE86 proteins. Finally, this study underscores the role of a cellular intermediate(s) for communicating the synergic interactions between two IE trans activators.

An inducible promoter mediates abundant expression from the immediate-early 2 gene region of human cytomegalovirus at late times after infection

An abundant late transcript of 1.5 kb originates from the immediate-early 2 (IE-2) gene region of human cytomegalovirus (HCMV) at late times after infection. The transcriptional start of this RNA was precisely mapped, and the putative promoter region was cloned in front of the CAT gene as reporter. This region, which comprises 78 nucleotides of IE-2 sequence upstream of the determined cap site, was strongly activated by viral superinfection at late times in the replicative cycle. As shown by RNase protection analyses, the authentic transcription start is used. No activation of this late promoter was observed after cotransfection with an expression plasmid containing the HCMV IE-1 and -2 gene region. This result suggests that, compared with early and early late promoters of HCMV, different or additional viral functions are required for the activation of true late promoters.

Expression of the human cytomegalovirus UL36-38 immediate early region during permissive infection

We have characterized the expression of human cytomegalovirus immediate early (IE) genes encoding the UL36-38 open reading frames. Single-stranded RNA hybridization probes, transcribed from UL36-38 cDNAs or smaller exon-specific clones, were used in analyses of RNA isolated during various phases of infection in the presence and absence of inhibitors of protein and HCMV DNA synthesis. There are three IE transcripts that arise by the use of alternative promoters and splicing signals. Different patterns of expression are observed for each of the three IE RNAs during permissive infection. Comparison of nucleotide sequences of UL36-38 IE cDNAs with those of the genomic sequence verified the use of polyadenylation and splice signals. An additional abundant early class RNA, totally encoded within the UL36-38 region, was found to be expressed from a different promoter than the IE RNAs. In vitro transcription and translation of the IE cDNAs were used to express the UL36-38 IE proteins. These studies define the structure of the UL36-38 gene products and demonstrate that expression from the region is both qualitatively and temporally complex.

Human cytomegalovirus ie2 negatively regulates alpha gene expression via a short target sequence near the transcription start site

Repression of human cytomegalovirus alpha (immediate-early) gene expression is under the control of the viral ie2 gene. Here we show that ie2 negatively regulates gene expression directed by the strong cytomegalovirus enhancer via a specific 15-bp target sequence (which we term cis repression signal [crs]). This crs is located between -14 and +1 relative to the transcription start site and will function in an orientation-independent fashion, consistent with repression occurring at the transcriptional level. Repression is dominant over transactivation by ie1 gene products. The crs (5'-CGTTTAGTGAACCGT-3') does not contain previously recognized binding sites for cellular transcription factors, and a precise copy is not found elsewhere in the human cytomegalovirus genome. The position of the signal near the transcription start site appears to be important in function; addition of the crs near the transcription start site of a heterologous promoter, from the thymidine kinase gene of herpes simplex virus type 1, conferred cytomegalovirus ie2-dependent repression upon this promoter. Thus, we propose that an ie2 gene product or an induced cellular protein mediates repression by binding to crs. Negative regulation of alpha gene expression may be important during viral replication or latency.

Characterization of a human cytomegalovirus 1.6-kilobase late mRNA and identification of its putative protein product

In a previous study (J. Martinez, R. S. Lahijani, and S. C. St. Jeor, J. Virol. 63:233-241, 1989), we identified a late, unspliced 1.6-kb mRNA that maps to the HindIII R fragment of human cytomegalovirus (HCMV) AD169. In the present study, the direction of transcription of this mRNA was determined by Northern (RNA) analysis with strand-specific probes. Primer extension was used to precisely map the 5' end of the mRNA. An open reading frame (ORF) designated ORF 2-1, located 176 nucleotides downstream from the cap site of the 1.6-kb mRNA, was identified. A synthetic peptide was made representing a hydrophilic region in the amino terminus of ORF 2-1. Immunoprecipitation and Western immunoblot analysis of infected HEL cell lysates, using affinity-purified antibody to the peptide (anti-P2-1), detected a viral protein with an apparent molecular mass of 58 kDa late in infection. Further support for the presence of this protein in infected-cell lysates was obtained by an enzyme-linked immunosorbent assay. Expression of viral antigens in intact infected HEL cells was assessed by immunofluorescence. General cytoplasmic staining was observed at 62 h postinfection, in contrast to a localized staining observed in the nuclear and perinuclear region at 96 h postinfection.

Transcriptional analysis of the eight-kilobase mRNA encoding the major capsid protein of human cytomegalovirus

The 8-kilobase mRNA coding for the major capsid protein (MCP) of human cytomegalovirus was precisely mapped. Two 5' ends of the transcript were located within HindIII fragment a, 29 and 34 base pairs, respectively, downstream of the sequence TATTAGA. The 3' end was localized within HindIII fragment b of the viral genome. The MCP transcript was synthesized at late times after infection and was not detected before viral DNA replication. In addition, the MCP promoter region could be identified, which strongly responded to viral trans activation at early and late times after infection in a transient expression assay.