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

Regulation of the MIE Locus During HCMV Latency and Reactivation

Human cytomegalovirus (HCMV) is a ubiquitous herpesviral pathogen that results in life-long infection. HCMV maintains a latent or quiescent infection in hematopoietic cells, which is broadly defined by transcriptional silencing and the absence of de novo virion production. However, upon cell differentiation coupled with immune dysfunction, the virus can reactivate, which leads to lytic replication in a variety of cell and tissue types. One of the mechanisms controlling the balance between latency and reactivation/lytic replication is the regulation of the major immediate-early (MIE) locus. This enhancer/promoter region is complex, and it is regulated by chromatinization and associated factors, as well as a variety of transcription factors. Herein, we discuss these factors and how they influence the MIE locus, which ultimately impacts the phase of HCMV infection.

Antisense antibacterial compounds

Extensive antibiotic use combined with poor historical drug stewardship practices have created a medical crisis in which once treatable bacterial infections are now increasingly unmanageable. To combat this, new antibiotics will need to be developed and safeguarded. An emerging class of antibiotics based upon nuclease-stable antisense technologies has proven valuable in preclinical testing against a variety of bacterial pathogens. This review describes the current state of development of antisense-based antibiotics, the mechanisms thus far employed by these compounds, and possible future avenues of research.

Alternative splicing of the human cytomegalovirus major immediate-early genes affects infectious-virus replication and control of cellular cyclin-dependent kinase

The major immediate-early (MIE) gene locus of human cytomegalovirus (HCMV) is the master switch that determines the outcomes of both lytic and latent infections. Here, we provide evidence that alteration in the splicing of HCMV (Towne strain) MIE genes affects infectious-virus replication, movement through the cell cycle, and cyclin-dependent kinase activity. Mutation of a conserved 24-nucleotide region in MIE exon 4 increased the abundance of IE1-p38 mRNA and decreased the abundance of IE1-p72 and IE2-p86 mRNAs. An increase in IE1-p38 protein was accompanied by a slight decrease in IE1-p72 protein and a significant decrease in IE2-p86 protein. The mutant virus had growth defects, which could not be complemented by wild-type IE1-p72 protein in trans. The phenotype of the mutant virus could not be explained by an increase in IE1-p38 protein, but prevention of the alternate splice returned the recombinant virus to the wild-type phenotype. The lower levels of IE1-p72 and IE2-p86 proteins correlated with a delay in early and late viral gene expression and movement into the S phase of the cell cycle. Mutant virus-infected cells had significantly higher levels of cdk-1 expression and enzymatic activity than cells infected with wild-type virus. The mutant virus induced a round-cell phenotype that accumulated in the G(2)/M compartment of the cell cycle with condensation and fragmentation of the chromatin. An inhibitor of viral DNA synthesis increased the round-cell phenotype. The round cells were characteristic of an abortive viral infection.

NF-kappaB-mediated activation of the chemokine CCL22 by the product of the human cytomegalovirus gene UL144 escapes regulation by viral IE86

The product of the human cytomegalovirus (HCMV) gene UL144, expressed at early times postinfection, is located in the UL/b' region of the viral genome and is related to members of the tumor necrosis factor receptor superfamily, but it does not bind tumor necrosis factor superfamily ligands. However, UL144 does activate NF-kappaB, resulting in NF-kappaB-mediated activation of the cellular chemokine CCL22. Consistent with this finding, isolates of HCMV lacking the UL/b' region show no such activation of CCL22. Recently, it has been suggested that activation of NF-kappaB is repressed by the product of the viral gene IE86: IE86 appears to block NF-kappaB binding to DNA but not nuclear translocation of NF-kappaB. Intriguingly, IE86 is detectable throughout an infection with the virus, so how UL144 is able to activate NF-kappaB in the presence of continued IE86 expression is unclear. Here we show that although IE86 does repress the UL144-mediated activation of a synthetic NF-kappaB promoter, it is unable to block UL144-mediated activation of the CCL22 promoter, and this lack of responsiveness to IE86 appears to be regulated by binding of the CREB transcription factor.

Exon 3 of the human cytomegalovirus major immediate-early region is required for efficient viral gene expression and for cellular cyclin modulation

The human cytomegalovirus (HCMV) major immediate-early (IE) proteins share an 85-amino-acid N-terminal domain specified by exons 2 and 3 of the major IE region, UL122-123. We have constructed IE Delta30-77, a recombinant virus that lacks the majority of IE exon 3 and consequently expresses smaller forms of both IE1 72- and IE2 86-kDa proteins. The mutant virus is viable but growth impaired at both high and low multiplicities of infection and exhibits a kinetic defect that is not rescued by growth in fibroblasts expressing IE1 72-kDa protein. The kinetics of mutant IE2 protein accumulation in IE Delta30-77 virus-infected cells are approximately normal compared to wild-type virus-infected cells, but the IE Delta30-77 virus is delayed in expression of early viral genes, including UL112-113 and UL44, and does not sustain expression of mutant IE1 protein as the infection progresses. Additionally, cells infected with IE Delta30-77 exhibit altered expression of cellular proteins compared to wild-type HCMV-infected cells. PML is not dispersed but is retained at ND10 sites following infection with IE Delta30-77 mutant virus. While the deletion mutant retains the ability to mediate the stabilization of cyclin B1, cdc6, and geminin in infected cells, its capacity to upregulate the expression of cyclin E has been reduced. These data indicate that the activity of one or both of the HCMV major IE proteins is required in vivo for the modulation of cell cycle proteins observed in cells infected with wild-type HCMV.

Analysis of splice variants of the immediate-early 1 region of human cytomegalovirus

The major immediate-early (MIE) gene of human cytomegalovirus (HCMV) produces multiple mRNAs through differential splicing and polyadenylation. Reverse transcriptase PCR was used to characterize transcripts from exons 1, 2, 3, and 4 (immediate-early 1 [IE1]). The expected IE72 and IE19 mRNAs were detected, as well as two heretofore-uncharacterized transcripts designated IE17.5 and IE9. The IE72, IE19, and IE17.5 transcripts utilized the same 5'-splice site in exon 3. IE9 utilized a cryptic 5'-splice site within exon 3. The IE19, IE17.5, and IE9 transcripts all used different 3'-splice sites within exon 4. These spliced species occur in infected human foreskin fibroblast (HFF) cells, with accumulation kinetics similar to those of IE72 mRNA. IE19 and IE9 RNAs were much more abundant than IE17.5 RNA. Transfection of CV-1 cells with cDNAs resulted in IE19 and IE17.5 proteins detectable by antibodies to either N-terminal or C-terminal epitopes. No IE9 protein product has been detected. We have not been able to detect IE19, IE17.5, or IE9 proteins during infection of HFF, HEL, or U373MG cells. Failure to detect IE19 protein contrasts with a previous report (M. Shirakata, M. Terauchi, M. Ablikin, K. Imadome, K. Hirai, T. Aso, and Y. Yamanashi, J. Virol. 76:3158-3167, 2002) of IE19 protein expression in HCMV-infected HEL cells. Our analysis suggests that an N-terminal breakdown product of IE72 may be mistaken for IE19. Expression of IE19 or IE17.5 from its respective cDNA results in repression of viral gene expression in infected cells. We speculate that expression of these proteins during infection may be restricted to specific conditions or cell types.

Strain variations in single amino acids of the 86-kilodalton human cytomegalovirus major immediate-early protein (IE2) affect its functional and biochemical properties: implications of dynamic protein conformation

The 86-kDa major immediate-early protein, IEP86 (IE2, IE2(579aa), or ppUL122a), from the Towne and AD169 strains of human cytomegalovirus show four amino acid variations, namely, R68Q, K455E, T541A, and seven consecutive serines beginning at position 258 in Towne and eight serines in AD169. A commonly utilized IEP86 cDNA expression clone (herein called the original cDNA) (E. Baracchini, E. Glezer, K. Fish, R. M. Stenberg, J. A. Nelson, and P. Ghazal, Virology 188:518-529, 1992) shows the Towne R68 and seven serines but contains the AD169 E455 and A541 plus two amino acid mutations, M242I and A463T. In transcriptional activation analyses using several promoters, the IEP86 produced by the original cDNA was 40 to 60% less active than wild-type (WT) Towne IEP86, whereas AD169 IEP86 was two to three times more active than WT Towne IEP86. To determine which amino acid variations or mutations accounted for the differences in transcriptional activation, they were individually tested in the WT Towne IEP86 background. K455E, M242I, and the eighth serine had little effect on transcriptional activation or sumoylation when inserted into the Towne background. T541A significantly increased transcriptional activation on all promoters tested and showed increased sumoylation; T541A is the primary reason that WT AD169 IEP86 has increased activity over WT Towne IEP86. The increased sumoylation seen with T541A was quantitatively reduced to WT Towne levels when the K455E alteration was present, suggesting that K455 may be a sumoylation site or that E455 may cause alterations in the IEP86 structure which affect overall sumoylation. A463T was very deleterious to transcriptional activation and caused reduced sumoylation. The A436T mutation in the original cDNA is partially compensated by the presence of the T541A variation. Phosphopeptide mapping suggests that a threonine at 463 or 541 does not introduce a phosphorylation site. However, the A463T mutation does affect phosphorylation at a distant site, suggesting that it alters the conformation of the protein. Promoter-specific effects were noted with some of the amino acid variations, particularly T541A. Structural modeling is presented which suggests how A463T and T541A alter the functional structure of WT Towne IEP86. A hydrophobic core containing A463 is predicted to be responsible for the functional integrity of the carboxy-terminal region of IEP86 between amino acids 344 and 579.

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.

Human cytomegalovirus major immediate-early proteins and simian virus 40 large T antigen can inhibit apoptosis through activation of the phosphatidylinositide 3'-OH kinase pathway and the cellular kinase Akt

The temperature-sensitive cell line ts13 is mutated in CCG1, the gene encoding TAF(II)250, the largest of the TATA-binding protein-associated factors (TAFs) in TFIID. At the nonpermissive temperature, the temperature-sensitive phenotypes are (i) transcription defects, (ii) cell cycle arrest in G(1), and (iii) apoptosis. We previously demonstrated that the human cytomegalovirus (HCMV) major immediate-early proteins (MIEPs) can rescue the transcription defects and inhibit apoptosis at the nonpermissive temperature. In the work presented, we show that activation of the cellular kinase Akt alone can inhibit apoptosis in ts13 cells grown at the nonpermissive temperature. More significantly, we show that the HCMV MIEPs can activate Akt, resulting in the inhibition of apoptosis. In parallel experiments, we found that simian virus 40 (SV40) large T antigen can mediate the same function. These experiments were done by transfecting the HCMV major immediate-early gene or a cDNA encoding T antigen into ts13 cells, and thus neither viral attachment to receptors, viral tegument proteins, nor any other viral protein is required for Akt activation. Akt is activated by the phosphatidylinositide 3'-OH (PI3) kinase pathway. Using a specific inhibitor of PI3 kinase, we show that the ability of the MIEPs and T antigen to activate Akt and inhibit apoptosis is eliminated, suggesting that the viral proteins utilize the PI3 kinase pathway for Akt activation. Transfection of plasmids which express the individual 86-kDa (IEP86; IE2(579aa)) and 72-kDa (IEP72; IE1(491aa)) MIEPs indicate that each MIEP could inhibit apoptosis via activation of the PI3 kinase pathway.

Viable human cytomegalovirus recombinant virus with an internal deletion of the IE2 86 gene affects late stages of viral replication

Using bacterial artificial chromosome (BAC) technology, we have constructed and characterized a human cytomegalovirus recombinant virus with a mutation in the exon specific for the major immediate-early region 2 (IE2) gene product. The resulting IE2 86-kDa protein (IE2 86) has an internal deletion of amino acids 136 to 290 and is fused at the carboxy terminus to enhanced green fluorescent protein (EGFP). The deletion also removes the promoter and initiator methionine for the p40 form of IE2 and initiator methionine for the p60 form of the protein, and therefore, these late gene products are not produced. The mutant virus IE2 86 Delta SX-EGFP is viable but exhibits altered growth characteristics in tissue culture compared with a full-length wild-type (wt) IE2 86-EGFP virus or a revertant virus. When cells are infected with the mutant virus at a low multiplicity of infection (MOI), there is a marked delay in the production of infectious virus. This is associated with slower cell-to-cell spread of the virus. By immunofluorescence and Western blot analyses, we show that the early steps in the replication of the mutant virus are comparable to those for the wt. Although there is significantly less IE2 protein in the cells infected with the mutant, there is only a modest lag in the initial accumulation of IE1 72 and viral early proteins, and viral DNA replication proceeds normally. The mutation also has only a small effect on the synthesis of the viral major capsid protein. The most notable molecular defect in the mutant virus infection is that the steady-state levels of the pp65 (UL83) and pp28 (UL99) matrix proteins are greatly reduced. In the case of UL83, but not UL99, there is also a corresponding decrease in the amount of mRNA present in cells infected with the mutant virus.

Late temporal gene expression from the human cytomegalovirus pp28US (UL99) promoter when integrated into the host cell chromosome

Toward understanding the temporal regulation of human cytomegalovirus (HCMV) late genes, we studied the regulation of the late gene promoter (pp28US, UL99) when outside the context of the viral genome and its response to the immediate early (IE) proteins. Expression of the luciferase reporter gene, regulated by the pp28US promoter, was synchronous with that of the endogenous viral pp28 gene, independently of whether the reporter was episomal or integrated into the glioblastoma cell line U373MG. Cotransfection of the reporter with expression vectors for each of the three major IE genes, IE72, IE86 and IE55, indicated that only IE86 transactivated the pp28US promoter. However, the magnitude of the promoter activation upon HCMV infection suggested that additional factors are also required for higher promoter activity. The promoter activation was specific to HCMV, as herpes simplex virus type 1 infection did not induce luciferase expression.

Human cytomegalovirus with IE-2 (UL122) deleted fails to express early lytic genes

Much evidence suggests that the major immediate-early (IE) transactivator of human cytomegalovirus (HCMV), IE-2, is likely to be critical for efficient viral replication; however, the lack of an IE-2 mutant HCMV has precluded an experimental test of this hypothesis. As an initial step toward characterizing an IE-2 mutant, we first cloned the HCMV Towne genome as a bacterial artificial chromosome (BAC) and analyzed the ability of transfected Towne-BAC DNA (T-BACwt) to produce plaques following introduction into permissive human fibroblasts. Like Towne viral DNA, transfected T-BACwt DNA was infectious in permissive cells, and the resulting virus stocks were indistinguishable from Towne virus. We then used homologous recombination in Escherichia coli to delete the majority of UL122, the open reading frame encoding the unique portion of IE-2, from T-BACwt. From this deleted BAC, a third BAC clone in which the deletion was repaired with wild-type UL122 was created. In numerous transfections of permissive human foreskin fibroblast cells with these three BAC DNA clones, the rescued BAC and T-BACwt consistently yielded plaques, while the UL122 mutant BAC never generated plaques, even after 4 weeks. Protein and mRNA of other IE genes were readily detected from transfected UL122 mutant BAC DNA; however, reverse transcription-PCR failed to detect mRNA expression from any of five early genes examined. The generalized failure of this mutant to express early genes is consistent with expectations from in vitro assays which have demonstrated that IE-2 transactivates most HCMV promoters. These experiments provide the first direct demonstration that IE-2 is required for successful HCMV infection and indicate that virus lacking IE-2 arrests early in the replication cycle.

Cloning and identification of regulatory gene UL76 of human cytomegalovirus

The major immediate-early promoter/enhancer (MIEP, -1139 to +52) of human cytomegalovirus (HCMV) is regulated by cell type-specific transcriptional factors, its own MIE proteins (IE2p40, IE1p55, IE1p72 and IE2p86) as well as viral proteins pUL69, pUL82 and pUL84. To investigate the hypothesis that the regulation of HCMV MIEP is modulated by additional viral genes, HCMV (AD169) genomic sublibraries were constructed and in vitro transient co-transfection assays were performed to assess the ability of these sublibraries to modulate MIEP expression. In this study, enhancement of MIEP expression was exhibited by a number of sublibraries, from one of which a genomic clone was selected for augmentation of expression. Subcloning the insert fragment led to the identification of the responsible locus, UL76. To generate a UL76-specific antibody for immunodetection, the UL76 ORF was constructed as a histidine-tagged fusion protein that was produced in prokaryotic cells. A polyclonal antibody raised against the UL76 fusion protein immunoreacts with a protein of 38 kDa (pUL76) in UL76 ORF-transfected cells. Additionally, pUL76 is present in HCMV-infected cells at the immediate-early to late stages of the reproductive cycle. Characterized by its highly basic composition (predicted pI 11.6), a free form of pUL76 tagged with green fluorescent protein was found to localize exclusively to the nucleus. In this report, pUL76 is defined as a novel regulatory protein that modulates both activation and repression of gene expression, depending on the promoter context and the ratio of transfected effector DNA.

Effects of human cytomegalovirus major immediate-early proteins in controlling the cell cycle and inhibiting apoptosis: studies with ts13 cells

The major immediate-early (MIE) gene of human cytomegalovirus (HCMV) encodes several MIE proteins (MIEPs) produced as a result of alternative splicing and polyadenylation of the primary transcript. Previously we demonstrated that the HCMV MIEPs expressed from the entire MIE gene could rescue the temperature-sensitive (ts) transcriptional defect in the ts13 cell line. This defect is caused by a ts mutation in TAFII250, the 250-kDa TATA binding protein-associated factor (TAF). These and other data suggested that the MIEPs perform a TAF-like function in complex with the basal transcription factor TFIID. In addition to the transcriptional defect, the ts mutation in ts13 cells results in a defect in cell cycle progression which ultimately leads to apoptosis. Since all of these defects can be rescued by wild-type TAFII250, we asked whether the MIEPs could rescue the cell cycle defect and/or affect the progression to apoptosis. We have found that the MIEPs, expressed from the entire MIE gene, do not rescue the cell cycle block in ts13 cells grown at the nonpermissive temperature. However, despite the maintenance of the cell cycle block, the ts13 cells which express the MIEPs are resistant to apoptosis. MIEP mutants, which have previously been shown to be defective in rescuing the ts transcriptional defect, maintained the ability to inhibit apoptosis. Hence, the MIEP functions which affect transcription appear to be separable from the functions which inhibit apoptosis. We discuss these data in the light of the HCMV life cycle and the possibility that the MIEPs promote cellular transformation by a "hit-and-run" mechanism.

Phosphorylation of the human cytomegalovirus 86-kilodalton immediate-early protein IE2

We have investigated the phosphorylation state of the human cytomegalovirus 86-kDa immediate-early (IE) protein IEP86 from transfected and infected cells. We show that multiple domains of IEP86 are phosphorylated by cellular kinases, both in vitro and in vivo. Our data suggest that serum-inducible kinases play a significant role in cell-mediated IE protein phosphorylation and that a member of the mitogen-activated protein (MAP) kinase (MAPK) family, extracellular regulated kinase 2 (ERK2), phosphorylates several domains of IEP86 in vitro. Alanine substitution mutagenesis was performed on specific serines or threonines (T27, S144, T233/S234, and T555) found in consensus MAP kinase motifs. Analysis of these mutations showed that T27 and T233/S234 are the major sites for serum-inducible kinases and are the major ERK2 sites in vitro. S144 appeared to be phosphorylated in a serum-independent manner in vitro. All of the mutations except T555 eliminated specific phosphorylation in vivo. In transient transfection analyses, IEP86 isoforms containing mutations in S144 and, especially, T233/S234 displayed increased transcriptional activation relative to the wild type, suggesting that phosphorylation at these sites in wild-type IEP86 may result in reduction of its transcriptional activation ability.

TAF-like functions of human cytomegalovirus immediate-early proteins

The human cytomegalovirus (HCMV) major immediate-early (IE) proteins IEP86 (IE2(579aa)) and IEP72 (IE1(491aa)) can transcriptionally activate a variety of simple promoters containing a TATA element and one upstream transcription factor binding site. In our previous studies, transcriptional activation was shown to correlate with IEP86 binding to both the TATA-box binding protein (TBP) and the transcription factor bound upstream. IEP72 often synergistically affects the activation by IEP86, although it has not previously been shown to directly interact in vitro with IEP86, TBP, or transcription factors (e.g., Sp1 and Tef-1) bound by IEP86. We report biochemical and genetic evidence suggesting that the major IE proteins may perform a function similar to that of the TBP-associated factors (TAFs) which make up TFIID. Consistent with this model, we found that the major IE proteins interact with a number of TAFs. In vitro, IEP86 bound with drosophila TAF(II)110 (dTAF(II)110) and human TAF(II)130 (hTAF(II)130), while IEP72 bound dTAF(II)40, dTAF(II)110, and hTAF(II)130. Regions on major IE proteins which mediate binding have been defined. In addition, our data indicate that both IEP72 and IEP86 can bind simultaneously to hTAF(II)130, suggesting that this TAF may provide bridging interactions between the two proteins for transcriptional activation and synergy. In agreement, a transcriptional activation mutant of IEP72 is unable to participate in bridging. Confirmation that these in vitro interactions were relevant was provided by data showing that both IEP72 and IEP86 copurify with TFIID and coimmunoprecipitate with purified TFIID derived from infected cell nuclei. To further support a TAF-like function of the IE proteins, we have found that the IE proteins expressed from the intact major IE gene, and to a lesser extent IEP86 alone, can rescue the temperature-sensitive (ts) transcriptional defect in TAF(II)250 in the BHK-21 cell line ts13. Analyses of mutations in the major IE region show that IEP86 is essential for rescue and that IEP72 augments its effect, and that mutations which affect TAF interactions are debilitated in rescue. Our data, showing that the IE proteins can bind with TFIID and rescue a ts transcriptional defect in TAF(II)250, support the model that the IE proteins perform a TAF-like function as components of TFIID.

Human cytomegalovirus IE2 86-kilodalton protein binds p53 but does not abrogate G1 checkpoint function

Physical interactions between human cytomegalovirus (HCMV) immediate-early (IE) proteins and key cell cycle regulatory proteins have been suggested as a mechanism whereby this herpesvirus modifies cellular control of proliferation. Observed similarities to interactions of other DNA virus proteins (human papillomavirus type 16 E6 and E7, simian virus 40 large T antigen, and adenovirus type 5 E1A and E1B) with cell cycle modulatory proteins such as p53 and Rb have suggested that HCMV IE proteins may likewise alter the G1-to-S phase transition. The IE2 region gene product IE86 has been shown to specifically bind p53, potentially modifying p53 G1 checkpoint function. To examine this possibility, p53-mediated G1 arrest in the presence of IE86 was assessed. Retroviral constructs were created to facilitate the stable expression of IE86 and IE72, another IE protein implicated in HCMV-mediated alteration of cell cycle progression. Western analysis and immunoprecipitation confirmed IE protein expression and binding of IE86 to p53, respectively. Chloramphenicol acetyltransferase assays examining the ability of IE86 to repress activity from the HCMV major IE promoter or activate the HCMV early promoter for the 2.2-kb class of RNAs demonstrated the functional integrity of the IE86 protein. Induction of DNA damage in normal, uninfected fibroblasts (FB) or FB expressing IE86 by actinomycin D (Act D) resulted in increased p53 levels, a predominance of the hypophosphorylated form of Rb, and increased expression of both p21(CIP1/WAF1) and mdm-2. Fluorescence-activated cell sorting revealed that both uninfected and IE86-expressing FB experienced dramatic G1 arrest following exposure to Act D. The clear demonstration of these p53-dependent responses in the presence of IE86 indicates that binding to this viral protein does not compromise the ability of p53 to elicit growth arrest following DNA damage.

Induction of the transcription factor Sp1 during human cytomegalovirus infection mediates upregulation of the p65 and p105/p50 NF-kappaB promoters

During human cytomegalovirus (HCMV) infection, the promoters for the classical NF-kappaB subunits (p65 and p105/p50) are transactivated. Previously, we demonstrated that the viral immediate-early (IE) proteins (IE1-72, IE2-55, and IE2-86) were involved in this upregulation. These viral factors alone, however, could not account for the entirety of the increased levels of transcription. Because one of the hallmarks of HCMV infection is the induction of cellular transcription factors, we hypothesized that one or more of these induced factors was also critical to the regulation of NF-kappaB during infection. Sp1 was one such factor that might be involved because p65 promoter activity was upregulated by Sp1 and both of the NF-kappaB subunit promoters are GC rich and contain Sp1 binding sites. Therefore, to detail the role that Sp1 plays in the regulation of NF-kappaB during infection, we initially examined Sp1 levels for changes during infection. HCMV infection resulted in increased Sp1 mRNA expression, protein levels, and DNA binding activity. Because both promoters were transactivated by Sp1, we reasoned that the upregulation of Sp1 played a role in p65 and p105/p50 promoter activity during infection. To address the specific role of Sp1 in p65 and p105/p50 promoter transactivation by HCMV, we mutated both promoters. These results demonstrated that the Sp1-specific DNA binding sites were involved in the virus-mediated transactivation. Last, to further dissect the role of HCMV in the Sp1-mediated induction of NF-kappaB, we examined the role that the viral IE genes played in Sp1 regulation. The IE gene products (IE1-72, IE2-55, and IE2-86) cooperated with Sp1 to increase promoter transactivation and physically interacted with Sp1. In addition, the IE2-86 product increased Sp1 DNA binding by possibly freeing up inactive Sp1. These data supported our hypothesis that Sp1 was involved in the upregulation of NF-kappaB during HCMV infection through the Sp1 binding sites in the p65 and p105/p50 promoters and additionally demonstrated a potential viral mechanism that might be responsible for the upregulation of Sp1 activity.

TAR and Sp1-independent transactivation of HIV long terminal repeat by the Tat protein in the presence of human cytomegalovirus IE1/IE2

OBJECTIVE

The HIV Tat protein is a transcriptional transactivator of the HIV-1 long terminal repeat (LTR) promoter element. Its activity depends on its direct interaction with the trans-activation response (TAR) element, although TAR-independent activation by Tat has been demonstrated in different cells. Herpesviruses in general and human cytomegalovirus (HCMV) in particular are often isolated from HIV-1-infected patients and could play a role in the activation of latent HIV and in a subsequent increase in HIV replication. HCMV immediate early gene products (IE1 and IE2) are nuclear phosphoproteins that play a pivotal role in HCMV replication and have been shown to transregulate both viral and cellular gene expression. It has repeatedly been shown that HCMV IE1/IE2 can independently transactivate HIV-1 LTR. The aim of this study was to investigate IE1/IE2 transactivation of HIV-1 LTR in a CD4+ T-cell line in the absence and presence of HIV-1 Tat to establish whether IE1/IE2 can synergize with Tat.

METHODS

HIV-1 LTR transactivation by HCMV IE1/IE2 in the presence and absence of HIV-1 Tat was determined by transient transfection experiments of J-Jhan lymphoblastoid cells with a series of different expression vectors.

RESULTS

We found a strong synergistic transactivation between HIV Tat and the IE1-IE2 complex on HIV LTR activity using vectors driven either by wild-type LTR or by the nuclear factor NF-kappa(B) response element-mutated HIV LTR. IE1/IE2 synergism with HIV Tat was also observed in Sp1 binding site-mutated for TAR-deleted LTR, which cannot be activated by Tat alone. This cooperation is abolished when the region in IE2 that binds the TATA box binding protein is deleted.

CONCLUSIONS

The results obtained indicate that Sp1-binding and TAR sequences are not strictly required for Tat responsiveness when Tat is directed to the HIV promoter by HCMV IE1-IE2. This synergistic effect is mediated by the IE2 and TATA-binding region, and could play a major role in HIV activation when cells are infected by both viruses, a feature often observed in AIDS patients.

The human cytomegalovirus IE1-72 protein interacts with the cellular p107 protein and relieves p107-mediated transcriptional repression of an E2F-responsive promoter

The Rb-related p107 protein has been implicated as an important control element in proper cell cycle progression. The p107 protein is thought to restrict cellular proliferation in part through its interaction with the E2F family of transcription factors and is, therefore, a specific target for regulation by several DNA viruses. Here, we demonstrate that p107 protein levels are induced in a biphasic manner in human fibroblasts during productive infection by the human cytomegalovirus (HCMV). Expression patterns of p107 protein levels during HCMV infection of human embryonic lung cells (HELs) demonstrate a sustained induction from early to late times of infection. We also demonstrate that the HCMV immediate-early protein IE1-72 complexes in vivo with the p107 protein and that this interaction can be reconstituted in an in vitro system by using reticulocyte-translated protein. Our data demonstrate that the interaction between p107 and the IE1-72 protein occurs at times of infection that temporally match the second tier of p107 protein induction and the phosphorylation pattern of the IE1-72 protein. Furthermore, we show here that the ability of p107 to transcriptionally repress E2F-responsive promoters can be overcome by expression of the IE1-72 protein. This effect appears to be specific, since the IE1-72 protein is not capable of relieving Rb-mediated repression of an E2F-responsive promoter. Finally, our data demonstrate that HCMV infection can induce cellular proliferation in quiescent cells and that IE1-72 expression alone can, to a degree, drive a similar progression through the cell cycle. These data suggest that IE1-72-mediated transactivation of E2F-responsive promoters through alleviation of p107 transcriptional repression may play a key role in the cell cycle progression stimulated by HCMV infection.

A deletion mutant in the human cytomegalovirus gene encoding IE1(491aa) is replication defective due to a failure in autoregulation

Human cytomegalovirus (CMV) replication begins with the expression of two regulatory proteins, IE1(491aa) and IE2(579aa), produced from differentially spliced transcripts under control of the ie1/ie2 promoter-enhancer. A deletion mutation removing all 406 IE1(491aa)-specific amino acids was engineered into the viral genome and this mutant (RC303 delta Acc) was propagated on an IE1(491aa)-expressing human fibroblast cell line (ihfie1.3). RC303 delta Acc failed to replicate on normal human fibroblasts at low multiplicities of infection (mois). At mois > 3 plaque-forming units per cell, virus replication and production of progeny were comparable to wild type. However, at mois between 0.01 and 1, mutant virus replicated slowly on normal fibroblasts, a pattern that suggested initiation of productive infection required multiple hits. Replication of RC303 delta Acc correlated with the ability to express IE2(579aa), consistent with a role for IE1(491aa) in positive autoregulation of the ie1/ie2 promoter-enhancer and with data suggesting that virion transactivators compensate for the lack of IE1(491aa) under high moi conditions. ie1-deficient CMV should be completely avirulent, suggesting its utility as a gene therapy vector for hematopoietic progenitors that are normal sites of CMV latency.

The IE2 regulatory protein of human cytomegalovirus induces expression of the human transforming growth factor beta1 gene through an Egr-1 binding site

Increases in transforming growth factor beta1 (TGF-beta1) mRNA and biological activity in the early phase of human cytomegalovirus (CMV) infection in fibroblasts are paralleled by increased TGF-beta1-chloramphenicol acetyltransferase (CAT) reporter gene activity. To determine how CMV infection transactivates the TGF-beta1 promoter, we examined the effects of the cotransfected IE2 regulatory protein of human CMV on 5'-deleted TGF-beta1 promoter-CAT reporter genes in transient DNA transfection assays. Two upstream TGF-beta1 promoter regions each containing an Egr-1 consensus site were shown to be important for IE2-induced transactivation in a cell type that displayed greatly reduced nonspecific activity. Furthermore, transfer of an Egr-l site from between positions -125 and -98, but not point mutant versions of this site, to a heterologous promoter also conveyed IE2 responsiveness. Addition of an IE2 expression vector or use of the U373 A45 astrocytoma cell line expressing IE2 also produced synergistic stimulation of GAL4-Egr-l-mediated activation of a target promoter containing GAL4 binding sites. The 80-kDa IE2 protein present in A45 cells proved to selectively bind to glutathione S-transferase (GST)-Egr-1 beads. The results of in vitro protein binding assays also revealed that an intact in vitro-translated IE2 protein bound directly to the GST-Egr-1 fusion protein through the zinc finger domain of the Egr-1 protein and that this binding activity was abolished by deletion of parts of the zinc finger DNA-binding domain. Similarly, the Egr-1 protein was found to associate preferentially with a small region within the C-terminal half of the IE2 protein adjacent to the DNA-binding and dimerization domains that are important for both transactivation and downregulation. We conclude from these observations that IE2 may regulate transcription of the TGF-beta1 gene as well as other potential cellular targets by virtue of its ability to interact with the Egr-1 DNA-binding protein.

Inhibition of human cytomegalovirus immediate-early gene expression by an antisense oligonucleotide complementary to immediate-early RNA

ISIS 2922 is a phosphorothioate oligonucleotide that is complementary to human cytomegalovirus (CMV) immediate-early (IE) RNA and that exhibits potent and specific antiviral activity against CMV in cell culture assays. Specific assay systems were developed to separately characterize the antisense and nonantisense components of the antiviral activity mediated by ISIS 2922. In U373 cells transformed with cDNA encoding the CMV IE 55-kDa (IE55) protein, expression was inhibited at nanomolar concentrations comparable to effective concentrations in antiviral assays. The specificity of inhibition was demonstrated by using control oligonucleotides incorporating progressive base changes to destabilize oligonucleotide-RNA base pairing and by showing a lack of inhibition of the CMV IE72 product expressed from the same promoter. Inhibition of IE55 protein expression correlated with a reduction in mRNA levels consistent with an RNase H-mediated termination event. Studies with virus-infected cells demonstrated that antisense and nonantisense mechanisms contribute to the antiviral activity of ISIS 2922. Base complementarity to target RNA was important for optimal activity in antiviral assays, but base changes affecting parameters other than hybridization affinity also influenced antiviral activity. Sequence-independent inhibition of virus adsorption to host cells by phosphorothioate oligonucleotides was also observed at high concentrations. Therefore, at least three different mechanisms may contribute to the antiviral activity of ISIS 2922 in cell culture: antisense-mediated inhibition of target gene expression; nonantisense, sequence-dependent inhibition of virus replication; and sequence-independent inhibition of virus adsorption to host cells.

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.

Human cytomegalovirus upregulates NF-kappa B activity by transactivating the NF-kappa B p105/p50 and p65 promoters

During human cytomegalovirus (HCMV) infection, a series of regulated events take place following virus binding and entry into the cell, including the upregulation of cellular transcription factors, such as NF-kappa B, which play an essential role in the viral life cycle. We show here that NF-kappa B message is induced during HCMV infection and that the induction is biphasic, suggesting an initial induction at immediate-early (IE) times and a second round of induction at early times. This hypothesis is supported by experiments using cyclohexamide, which showed that the first tier of induction was drug insensitive, while the second tier was drug sensitive. We then show that virus binding alone is sufficient to stimulate NF-kappa DNA binding activity, supporting its role in the initial induction of NF-kappa B. To begin to elucidate the mechanism(s) for the second tier of NF-kappa B regulation, we examined promoter constructs from the NF-kappa B subunits (p105/p50 and p65) for responsiveness following HCMV infection. HCMV infection transactivated the p105/p50 and p65 promoters. The viral IE proteins (IE1-72, IE2-55, and IE2-86) are expressed during the time we see NF-kappa B induction, so we examined their role in NF-kappa B induction. The IE1-72, IE2-55, and IE2-86 proteins transactivated the p65 promoter, while only the IE2-55 protein transactivated the p105/p50 promoter. The p105/p50 promoter has NF-kappa B sites; therefore, upregulation could also be caused by an autoregulatory mechanism. The p65 promoter, however, has been demonstrated to contain only Sp1 sites. To investigate the potential role of SP1, we examined nuclear extracts from HCMV-infected cells. Here, we show that there is a biphasic increase in SP1 activity during viral infection and that there is apparently an absolute requirement for SP1 in the transactivation of the p65 promoter. In conclusion, we suggest a model in which the initial induction of NF-kappa B occurs through viral modulation of cellular factors and the sustained levels of NF-kappa B induction are regulated by a combination of cellular and viral factors.

Retinoid activation of retinoic acid receptors but not of retinoid X receptors promotes cellular differentiation and replication of human cytomegalovirus in embryonal cells

The susceptibility of human embryonal cell line NT-2/D1 to replicate human cytomegalovirus (hCMV) is dependent on retinoic acid (RA) stimulation. Physiological responses to retinoic acid involve two distinct subfamilies of nuclear receptors, the RA receptors (RARs) and retinoid X receptors (RXRs), which function by activating transcription as heterodimeric or RXR homodimeric complexes from cis-acting DNA response elements. At present, it is not clear whether the association between these two classes of receptors can lead to multiple distinct induction pathways by signalling one or both receptor partners. Here we have determined, by selectively activating endogenous receptors with novel synthetic ligands specific for either RARs or RXRs, what ligand interaction is physiological in the retinoid receptor pathways necessary for inducing replication of hCMV in differentiated embryonal cells. We show that ligand binding to RAR alone is sufficient and that exclusive ligand activation of RXR is insufficient for inducing replication of hCMV. We also find that differentiation and inhibition of NT-2/D1 cell growth are promoted by compounds that signal the RAR pathway. These results provide direct evidence that RAR ligand-mediated physiological responses are separable and distinct from RXR ligand activation functions. Moreover, our results provide insight into a hormone response pathway for cellular differentiation that might be coopted by hCMV in the host.

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.

Structural Organization of the Spliced Immediate-Early Gene Complex that Encodes the Major Acidic Nuclear (IE1) and Transactivator (IE2) Proteins of African Green Monkey Cytomegalovirus

Total immediate-early (IE) RNA synthesized after infection with African green monkey cytomegalovirus (SCMV) in the presence of cycloheximide contained a major 2.3-kb mRNA species that acted as template for in vitro synthesis of a single 94-kD nuclear protein. The same IE RNA hybridized predominantly to a 1.8-kb subregion of the 220-kb genome which mapped 1.5 kb to the left of the in vitro transcription start site and TATATAA motif previously associated with the powerful MIE (IE94) enhancer region. However, DNA sequence and S1-mapping analysis of a 5-kb region downstream from the promoter revealed the existence of a far upstream noncoding first exon and four additional spliced exons capable of encoding two alternative protein products with shared N-terminal domains. This region is similar in structure to that of the MIE gene complex of human cytomegalovirus (HCMV), including being highly CpG suppressed. Exons 2, 3, and 4 encode an acidic protein equivalent to the 68-kD IE1 protein (UL123) of HCMV and exons 2, 3, and 5 encode a protein equivalent to the 80-kD IE2 (UL122) DNA-binding protein of HCMV. Transcripts from across the IE2 region were detected within the cycloheximide RNA, but they were present at 10- to 20-fold lower abundance than IE1 transcripts. The proposed 547-codon IE1 (IE94) acidic phosphoprotein of SCMV displays minimal residual homology with the IE1 protein of HCMV, but both associate with metaphase chromosomes and have large C-terminal glutamic-acid-rich domains. In contrast, the proposed 583-codon IE2 protein of SCMV displays extensive amino acid similarity to the HCMV IE2 transcriptional regulatory protein especially within C-terminal domains that are known to play a major role in promoter targeting for both transactivation and negative autoregulation functions. Copyright 1995 S. Karger AG, Basel

Transcriptional activation by the human cytomegalovirus immediate-early proteins: requirements for simple promoter structures and interactions with multiple components of the transcription complex

We have utilized a number of well-defined, simple, synthetic promoters (upstream factor binding sites and TATA elements) to analyze the activation mechanisms of the human cytomegalovirus immediate-early (IE) proteins. We found that the 86-kDa IE protein (known as IEP86, IE2(559aa), or ppUL122a) can recognize and activate a variety of simple promoters, in agreement with the observation that it is a promiscuous activator. However, in the comparison of otherwise identical promoters IEP86 does have preferences for specific TATA elements (hsp70 > adenovirus E2 > simian virus 40 early) and specific upstream transcription factor binding sites (CAAT > SP1 approximately Tef-1 > ATF; no activation with AP1 or OCT). In contrast, the 72-kDa IE protein (known as IEP72, IE1(491aa), or ppUL123) alone did not significantly activate the simple promoters under our experimental conditions. However, each promoter activated by IEP86 was synergistically affected by the addition of IEP72. In addition, the 55-kDa IE protein (IEP55, a splice variant form of IE2, IE2(425aa), or ppUL122b) repeatedly had a negative effect, downregulating the activation of promoters caused by IEP86 and the synergy of IEP86 and IEP72. We show that the ability of IEP86 to activate many simple promoters correlates not only with its previously described ability to interact with the TATA-binding protein (TBP) (B. A. Furnari, E. Poma, T. F. Kowalik, S.-M. Huong, and E.-S. Huang, J. Virol. 67:4981-4991, 1993; C. Hagemeier, S. Walker, R. Caswell, T. Kouzarides, and J. Sinclair, J. Virol. 66:4452-4456, 1992; R. Jupp, S. Hoffman, R. M. Stenberg, J. A. Nelson, and P. Ghazal, J. Virol. 67:7539-7546, 1993) but also with its ability to interact with the transcription factors which bind to the upstream element of promoters it activated (e.g., SP1 and Tef-1 but not Oct-1). This ability to have multiple interactions with the promoter complex may be crucial for transcriptional activation, since the IE proteins cannot activate promoters having only a TATA element or only an upstream transcription factor binding site. In addition, we show that proteins which bind IEP86 also bind to IEP55. Thus, the negative effect on transcription noted with IEP55 may be the result of competition with IEP86 for interaction with the promoter complex. The synergy caused by IEP72 appears to be mediated by a more indirect mechanism. This is suggested by our observation that IEP72 could not bind to any of the proteins tested (TBP, Tef-1, or Oct-1) or to IEP86.

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.

Human cytomegalovirus IE86 protein interacts with promoter-bound TATA-binding protein via a specific region distinct from the autorepression domain

The major immediate-early gene of human cytomegalovirus encodes several isoforms of an immediate-early protein which has distinct transcriptional regulatory properties. The IE86 isoform autorepresses the major immediate-early promoter by directly binding the cis repression signal element located between the TATA box and the mRNA cap site. In addition to this activity, IE86 stimulates other viral and cellular promoters. One mechanism by which eukaryotic regulatory proteins are thought to stimulate transcription is by contacting one or more general transcription factors. We show that the IE86 protein physically interacts with the DNA-binding subunit (TATA-binding protein) human transcription factor IID via the TATA-binding protein-contacting domain in the N terminus of IE86. In a mobility shift assay, IE86 was also observed to stabilize the binding of TATA-binding protein to promoter DNA. The domains within IE86 responsible for mediating transactivation and repression functioned independently. These experiments thus demonstrate the elegant ability of human cytomegalovirus to join different protein domains to produce distinct multifunctional proteins.

Site-specific inhibition of RNA polymerase II preinitiation complex assembly by human cytomegalovirus IE86 protein

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 (UL122, IE2), is a DNA-binding protein that represses the MIEP through its cognate recognition sequence (designated the cis repression signal [crs]) located between the TATA box and the initiation site of transcription. Purified recombinant IE86 protein was shown to repress MIEP transcription in vitro, in a cis-acting mediated pathway, with nuclear extracts from HeLa S3, U373-MG, and primary human foreskin fibroblast cells. Repression of the MIEP by IE86 was shown by two criteria to be dependent on the direct interaction of IE86 with the crs element. Core promoter constructs containing essentially the MIEP TATA box and crs element were also specifically repressed by IE86 but not by a mutant IE86 protein, indicating the general transcription machinery as the target for IE86 repression. Kinetic and template commitment experiments demonstrated that IE86 affects preinitiation complex formation but not the rate of reinitiation. Sarkosyl inhibition experiments further revealed that IE86 was unable to effect repression by either disassembling or preventing the elongation of a preexisting transcription complex. Further, the ability of IE86 to interact with the DNA-binding subunit of TFIID was shown not to be required for repression. These functional protein-DNA and protein-protein interaction experiments demonstrate that IE86 specifically interferes with the assembly of RNA polymerase II preinitiation complexes. The biological significance of these results and the precise mechanism by which IE86 represses transcription are discussed.

Identification and mapping of dimerization and DNA-binding domains in the C terminus of the IE2 regulatory protein of human cytomegalovirus

The 80-kDa IE2 nuclear phosphoprotein encoded by the human cytomegalovirus (HCMV) major immediate-early (MIE) gene behaves both as a nonspecific transactivator of heterologous reporter genes and as a specific repressor of its own promoter-enhancer region. To begin to examine the biochemical properties of the IE2 protein, we prepared panels of N-terminal and C-terminal truncation mutants by in vitro translation procedures. In cross-linking experiments, the C-terminal half of IE2 (which is sufficient for down-regulation) formed dimers but N-terminal segments did not do so. Cotranslated Oct2/IE2 fusion proteins containing the same IE2 C-terminal region from codons 266 to 579 also formed mixed-subunit DNA-bound oligomeric complexes in gel mobility shift assays. Furthermore, an IE2 domain bounded by codons 388 to 542 proved to immunoprecipitate as heterodimers with cotranslated subunits containing known epitopes for specific antibodies. Deletion up to codon 428 or truncation back to codon 504 prevented this interaction. In direct gel shift DNA-binding assays, a bacterial GST/IE2(346-579) fusion protein bound to a 30-mer oligonucleotide probe encompassing the major immediate-early gene negative cis-regulatory target DNA sequence but failed to bind to a single-base-pair insertion mutant probe (delta CRS). This specific DNA-binding activity was abolished by further deletion up to codon 388 on the N-terminal side or by truncation at codon 542 on the C-terminal side. Therefore, the minimal DNA-binding domain requires additional amino acid motifs on both sides of the dimerization domain. This segment of IE2 is functionally important for both transactivation and down-regulation and contains several highly conserved amino acid motifs that are shared amongst the equivalent HCMV, simian CMV, mouse CMV, rat CMV, and human herpesvirus 6 proteins from other betaherpesviruses.

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.

Antiviral activity of a phosphorothioate oligonucleotide complementary to RNA of the human cytomegalovirus major immediate-early region

Phosphorothioate oligonucleotides complementary to mRNA of the human cytomegalovirus (HCMV) DNA polymerase gene or to RNA transcripts of the major immediate-early regions 1 and 2 (IE1 and IE2) of HCMV were evaluated for antiviral activity in a 96-well immunoassay with primary human dermal fibroblasts as host cells. Oligonucleotides complementary to RNA of the IE2 region exhibited the most potent antiviral activity. One of these oligonucleotides, ISIS 2922, was at least 30-fold more potent than the nucleoside analog, ganciclovir, with a 50% effective concentration of 0.37 microM in the 96-well immunoassay. In an infectious virus yield reduction assay, ISIS 2922 and ganciclovir reduced production of infectious virus by 2 log units at concentrations of 2.2 and 36 microM, respectively. A control oligonucleotide showed no inhibition of virus production at concentrations as high as 3 microM. ISIS 2922 reduced IE protein synthesis in HCMV-infected cells in a dose-dependent manner which correlated with antiviral activity. The antiviral activity of ISIS 2922 was not due to oligonucleotide-induced cytotoxicity since effects on cell viability or proliferation were observed only at concentrations well in excess of effective antiviral concentrations. The specificity and potency of ISIS 2922 suggest that it may be useful for the treatment of cytomegalovirus disease in humans.

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.

Open reading frames UL44, IRS1/TRS1, and UL36-38 are required for transient complementation of human cytomegalovirus oriLyt-dependent DNA synthesis

Previous results showed that plasmids containing human cytomegalovirus (HCMV) oriLyt are replicated after transfection into permissive cells if essential trans-acting factors are supplied by HCMV infection (D. G. Anders, M. A. Kacica, G. S. Pari, and S. M. Punturieri, J. Virol. 66:3373-3384, 1992). We have now used oriLyt as a reporter of HCMV DNA replication in a transient complementation assay in which cotransfected cosmid clones, instead of HCMV infection, provided essential trans-acting factors. Complemented replication was oriLyt dependent and phosphonoformic acid sensitive and produced tandem arrays typical of HCMV lytic-phase DNA synthesis. Thus, this assay provides a valid genetic test to find previously unidentified genes that are essential for DNA synthesis and to corroborate functional predictions made by nucleotide sequence comparisons and biochemical analyses. Five cosmids were necessary and sufficient to produce origin-dependent DNA synthesis; all but one of these required cosmids contain at least one candidate homolog of herpes simplex virus type 1 replication genes. We further used the assay to define essential regions in two of the required cosmids, pCM1017 and pCM1052. Results presented show that UL44, proposed on the basis of biochemical evidence to be the HCMV DNA polymerase accessory protein, was required for complementation. In addition, three genomic regions encoding regulatory proteins also were needed to produce origin-dependent DNA synthesis in this assay: (i) IRS1/TRS1, which cooperates with the major immediate-early proteins to activate UL44 expression; (ii) UL36-38; and (iii) the major immediate-early region comprising IE1 and IE2. Combined, these results unequivocally establish the utility of this approach for mapping HCMV replication genes. Thus, it will now be possible to define the set of HCMV genes necessary and sufficient for initiating and performing lytic-phase DNA synthesis as well as to identify those virus genes needed for their expression in human fibroblasts.

In vivo and in vitro analysis of transcriptional activation mediated by the human cytomegalovirus major immediate-early proteins

To define mechanistically how the human cytomegalovirus (HCMV) major immediate-early (IE) proteins induce early-gene transcription, the IE1 72-kDa protein, the IE2 55-kDa protein, and the IE2 86-kDa protein were analyzed for their ability to activate transcription from an HCMV early promoter in vivo and in vitro. In transient-expression assays in U373MG astrocytoma/glioblastoma and HeLa cells, only the IE2 86-kDa protein was able to activate the HCMV early promoter to high levels. In HeLa cells, the IE1 72-kDa protein was able to activate the promoter to a low but detectable level, and the level of promoter activity observed in response to the IE2 86-kDa protein was increased synergistically following cotransfection of the constructs expressing both IE proteins. To examine the interaction of the HCMV IE proteins with the RNA polymerase II transcription machinery, we assayed the ability of Escherichia coli-synthesized proteins to activate the HCMV early promoter in nuclear extracts prepared from U373MG cells, HeLa cells, and Drosophila embryos. The results of the in vitro experiments correlated well with those obtained in vivo. The basal activity of the promoter was minimal in both the HeLa and U373MG extracts but was stimulated 6- to 10-fold by the IE2 86-kDa protein. With a histone H1-deficient extract from Drosophila embryos, the HCMV early promoter was quite active and was stimulated two- to fourfold by the IE2 86-kDa protein. Addition of histone H1 at 1 molecule per 40 to 50 bp of DNA template significantly repressed basal transcription from this promoter. However, the IE2 86-kDa protein, but none of the other IE proteins, was able to counteract the H1-mediated repression and stimulate transcription at least 10- to 20-fold. The promoter specificity of the activation was demonstrated by the inability of the IE2 86-kDa protein to activate the Drosophila Krüppel promoter in either the presence or absence of histone H1. These results suggest that one mechanism of transcription activation by the IE2 86-kDa protein involves antirepression.

In vivo and in vitro analysis of transcriptional activation mediated by the human cytomegalovirus major immediate-early proteins

To define mechanistically how the human cytomegalovirus (HCMV) major immediate-early (IE) proteins induce early-gene transcription, the IE1 72-kDa protein, the IE2 55-kDa protein, and the IE2 86-kDa protein were analyzed for their ability to activate transcription from an HCMV early promoter in vivo and in vitro. In transient-expression assays in U373MG astrocytoma/glioblastoma and HeLa cells, only the IE2 86-kDa protein was able to activate the HCMV early promoter to high levels. In HeLa cells, the IE1 72-kDa protein was able to activate the promoter to a low but detectable level, and the level of promoter activity observed in response to the IE2 86-kDa protein was increased synergistically following cotransfection of the constructs expressing both IE proteins. To examine the interaction of the HCMV IE proteins with the RNA polymerase II transcription machinery, we assayed the ability of Escherichia coli-synthesized proteins to activate the HCMV early promoter in nuclear extracts prepared from U373MG cells, HeLa cells, and Drosophila embryos. The results of the in vitro experiments correlated well with those obtained in vivo. The basal activity of the promoter was minimal in both the HeLa and U373MG extracts but was stimulated 6- to 10-fold by the IE2 86-kDa protein. With a histone H1-deficient extract from Drosophila embryos, the HCMV early promoter was quite active and was stimulated two- to fourfold by the IE2 86-kDa protein. Addition of histone H1 at 1 molecule per 40 to 50 bp of DNA template significantly repressed basal transcription from this promoter. However, the IE2 86-kDa protein, but none of the other IE proteins, was able to counteract the H1-mediated repression and stimulate transcription at least 10- to 20-fold. The promoter specificity of the activation was demonstrated by the inability of the IE2 86-kDa protein to activate the Drosophila Krüppel promoter in either the presence or absence of histone H1. These results suggest that one mechanism of transcription activation by the IE2 86-kDa protein involves antirepression.