Transcriptional activation by simian virus 40 large T antigen: interactions with multiple components of the transcription complex

Association Between Simian Virus 40 and Human Tumors

Simian virus 40 (SV40) is a small DNA tumor virus of monkey origin. This polyomavirus was administered to human populations mainly through contaminated polio vaccines, which were produced in naturally infected SV40 monkey cells. Previous molecular biology and recent immunological assays have indicated that SV40 is spreading in human populations, independently from earlier SV40-contaminated vaccines. SV40 DNA sequences have been detected at a higher prevalence in specific human cancer specimens, such as the brain and bone tumors, malignant pleural mesotheliomas, and lymphoproliferative disorders, compared to the corresponding normal tissues/specimens. However, other investigations, which reported negative data, did not confirm an association between SV40 and human tumors. To circumvent the controversies, which have arisen because of these molecular biology studies, immunological researches with newly developed indirect ELISA tests were carried out in serum samples from patients affected by the same kind of tumors as mentioned above. These innovative indirect ELISAs employ synthetic peptides as mimotopes/specific SV40 antigens. SV40 mimotopes do not cross-react with the homologous human polyomaviruses, BKPyV, and JCPyV. Immunological data obtained from indirect ELISAs, using SV40 mimotopes, employed to analyze serum samples from oncological patients, have indicated that these sera had a higher prevalence of antibodies against SV40 compared to healthy subjects. The main data on (i) the biology and genetics of SV40; (ii) the epidemiology of SV40 in the general population, (iii) the mechanisms of SV40 transformation; (iv) the putative role of SV40 in the onset/progression of specific human tumors, and (v) its association with other human diseases are reported in this review.

Transcription factor Spi-B binds unique sequences present in the tandem repeat promoter/enhancer of JC virus and supports viral activity

Progressive multifocal leukoencephalopathy (PML) is an often fatal demyelinating disease caused by lytic infection of oligodendrocytes with JC virus (JCV). The development of PML in non-immunosuppressed individuals is a growing concern with reports of mortality in patients treated with mAb therapies. JCV can persist in the kidneys, lymphoid tissue and bone marrow. JCV gene expression is restricted by non-coding viral regulatory region sequence variation and cellular transcription factors. Because JCV latency has been associated with cells undergoing haematopoietic development, transcription factors previously reported as lymphoid specific may regulate JCV gene expression. This study demonstrates that one such transcription factor, Spi-B, binds to sequences present in the JCV promoter/enhancer and may affect early virus gene expression in cells obtained from human brain tissue. We identified four potential Spi-B-binding sites present in the promoter/enhancer elements of JCV sequences from PML variants and the non-pathogenic archetype. Spi-B sites present in the promoter/enhancers of PML variants alone bound protein expressed in JCV susceptible brain and lymphoid-derived cell lines by electromobility shift assays. Expression of exogenous Spi-B in semi- and non-permissive cells increased early viral gene expression. Strikingly, mutation of the Spi-B core in a binding site unique to the Mad-4 variant was sufficient to abrogate viral activity in progenitor-derived astrocytes. These results suggest that Spi-B could regulate JCV gene expression in susceptible cells, and may play an important role in JCV activity in the immune and nervous systems.

Simian virus 40 activates ATR-Delta p53 signaling to override cell cycle and DNA replication control

During infection, simian virus 40 (SV40) attempts to take hold of the cell, while the host responds with various defense systems, including the ataxia-telangiectasia mutated/ATM-Rad3 related (ATM/ATR)-mediated DNA damage response pathways. Here we show that upon viral infection, ATR directly activates the p53 isoform Δp53, leading to upregulation of the Cdk inhibitor p21 and downregulation of cyclin A-Cdk2/1 (AK) activity, which force the host to stay in the replicative S phase. Moreover, downregulation of AK activity is a prerequisite for the generation of hypophosphorylated, origin-competent DNA polymerase α-primase (hypo-Polα), which is, unlike AK-phosphorylated Polα (P-Polα), recruited by SV40 large T antigen (T-Ag) to initiate viral DNA replication. Prevention of the downregulation of AK activity by inactivation of ATR-Δp53-p21 signaling significantly reduced the T-Ag-interacting hypo-Polα population and, accordingly, SV40 replication efficiency. Moreover, the ATR-Δp53 pathway facilitates the proteasomal degradation of the 180-kDa catalytic subunit of the non-T-Ag-interacting P-Polα, giving rise to T-Ag-interacting hypo-Polα. Thus, the purpose of activating the ATR-Δp53-p21-mediated intra-S checkpoint is to maintain the host in S phase, an optimal environment for SV40 replication, and to modulate the host DNA replicase, which is indispensable for viral amplification.

Enhancement of enteric adenovirus cultivation by viral transactivator proteins

Human enteric adenoviruses (HAdVs; serotypes 40 and 41) are important waterborne and food-borne pathogens. However, HAdVs are fastidious, are difficult to cultivate, and do not produce a clear cytopathic effect during cell culture within a reasonable time. Thus, we examined whether the viral transactivator proteins cytomegalovirus (CMV) IE1 and hepatitis B virus (HBV) X promoted the multiplication of HAdVs. Additionally, we constructed a new 293 cell line expressing CMV IE1 protein for cultivation assays. We analyzed the nucleic acid sequences of the promoter regions of both E1A and hexon genes, which are considered to be the most important regions for HAdV replication. Expression of either HBV X or CMV IE1 protein significantly increased the promoter activities of E1A and hexon genes of HAdVs by as much as 14-fold during cell cultivation. The promotion of HAdV expression was confirmed by increased levels of both adenoviral DNA and mRNA expression. Finally, the newly developed 293 cell line expressing CMV IE1 protein showed an increase in viral DNA ranging from 574% to 619% compared with the conventional 293 cell line. These results suggest that the newly constructed cell line could be useful for efficient cultivation and research of fastidious HAdVs.

Cell-type specific regulation of gene expression by simian virus 40 T antigens

SV40 transforms cells through the action of two oncoproteins, large T antigen and small t antigen. Small t antigen targets phosphatase PP2A, while large T antigen stimulates cell proliferation and survival by action on multiple proteins, including the tumor suppressors Rb and p53. Large T antigen also binds components of the transcription initiation complex and several transcription factors. We examined global gene expression in SV40-transformed mouse embryo fibroblasts, and in enterocytes obtained from transgenic mice. SV40 transformation alters the expression of approximately 800 cellular genes in both systems. Much of this regulation is observed in both MEFs and enterocytes and is consistent with T antigen action on the Rb-E2F pathway. However, the regulation of many genes is cell-type specific, suggesting that unique signaling pathways are activated in different cell types upon transformation, and that the consequences of SV40 transformation depends on the type of cell targeted.

Simian virus 40 large T overcomes p300 repression of c-Myc

We previously showed that in quiescent cells p300/CBP negatively regulates the cell cycle G1-S transition by keeping c-Myc in a repressed state and that adenovirus E1A induces c-Myc by binding to p300/CBP. Studies have shown that p300/CBP binding to simian virus 40 large T is indirect and mediated by p53. By using a series of large T mutants that fail to bind to various cellular proteins including p53 as well as cells where p300 is overexpressed or p53 is knocked down, we show that the association of large T with p300 contributes to the induction of c-Myc and the cell cycle. The induction of c-Myc by this mechanism is likely to be important in large T mediated cell cycle induction and cell transformation.

SV40 large T antigen targets multiple cellular pathways to elicit cellular transformation

DNA tumor viruses such as simian virus 40 (SV40) express dominant acting oncoproteins that exert their effects by associating with key cellular targets and altering the signaling pathways they govern. Thus, tumor viruses have proved to be invaluable aids in identifying proteins that participate in tumorigenesis, and in understanding the molecular basis for the transformed phenotype. The roles played by the SV40-encoded 708 amino-acid large T antigen (T antigen), and 174 amino acid small T antigen (t antigen), in transformation have been examined extensively. These studies have firmly established that large T antigen's inhibition of the p53 and Rb-family of tumor suppressors and small T antigen's action on the pp2A phosphatase, are important for SV40-induced transformation. It is not yet clear if the Rb, p53 and pp2A proteins are the only targets through which SV40 transforms cells, or whether additional targets await discovery. Finally, expression of SV40 oncoproteins in transgenic mice results in effects ranging from hyperplasia to invasive carcinoma accompanied by metastasis, depending on the tissue in which they are expressed. Thus, the consequences of SV40 action on these targets depend on the cell type being studied. The identification of additional cellular targets important for transformation, and understanding the molecular basis for the cell type-specific action of the viral T antigens are two important areas through which SV40 will continue to contribute to our understanding of cancer.

Activation of CREB/ATF sites by polyomavirus large T antigen

Polyomavirus large T antigen (LT) has a direct role in viral replication and a profound effect on cell phenotype. It promotes cell cycle progression, immortalizes primary cells, blocks differentiation, and causes apoptosis. While much of large T function is related to its effects on tumor suppressors of the retinoblastoma susceptibility (Rb) gene family, we have previously shown that activation of the cyclin A promoter can occur through a non-Rb-dependent mechanism. Here we show that activation occurs via an ATF/CREB site. Investigation of the mechanism indicates that large T can synergize with CREB family members to activate transcription. Experiments with Gal4-CREB constructs show that synergy is independent of CREB phosphorylation by protein kinase A. Examination of synergy with Gal4-CREB deletion constructs indicates that large T acts on the constitutive activation domain of CREB. Large T can bind to CREB in vivo. Genetic analysis shows that the DNA-binding domain (residues 264 to 420) is sufficient to activate transcription when it is localized to the nucleus. Further analysis of the DNA-binding domain shows that while site-specific DNA binding is not required, non-site-specific DNA binding is important for the activation. Thus, CREB binding and DNA binding are both important for large T activation of CREB/ATF sites. In contrast to previous models where large T transactivation occurred indirectly, these results also suggest that large T can act directly at promoters to activate transcription.

Cell type-specific replication of simian virus 40 conferred by hormone response elements in the late promoter

The late genes of SV40 are not expressed at significant levels until after the onset of viral DNA replication. We previously identified two hormone response elements (HREs) in the late promoter that contribute to this delay. Mutants defective in these HREs overexpress late RNA at early, but not late, times after transfection of CV-1PD cells. Overexpression of nuclear receptors (NRs) that recognize these HREs leads to repression of the late promoter in a sequence-specific and titratable manner, resulting in a delay in late gene expression. These observations led to a model in which the late promoter is repressed at early times after infection by NRs, with this repression being relieved by titration of these repressors through simian virus 40 (SV40) genome replication to high copy number. Here, we tested this model in the context of the viral life cycle. SV40 genomes containing mutations in either or both HREs that significantly reduce NR binding without altering the coding of any proteins were constructed. Competition for replication between mutant and wild-type viruses in low-multiplicity coinfections indicated that the +1 HRE offered a significant selective advantage to the virus within a few cycles of infection in African green monkey kidney cell lines CV-1, CV-1P, TC-7, MA-134, and Vero but not in CV-1PD' cells. Interestingly, the +55 HRE offered a selective disadvantage in MA-134 cells but had no effect in CV-1, CV-1P, TC-7, Vero, and CV-1PD' cells. Thus, we conclude that these HREs are biologically important to the virus, but in a cell type-specific manner.

Simian virus 40 large T-antigen, but not small T-antigen, trans-activates the human cytomegalovirus major immediate early promoter

Cytomegalovirus infection is a major cause of morbidity in immunocompromised patients. The major immediate early promoter/enhancer (MIEP) of the human cytomegalovirus controls the expression of the immediate early genes 1 and 2 which play a central role both in primary and reactivated human cytomegalovirus (HCMV)-infections. Our previous studies have shown that co-infection of A549 cells with human cytomegalovirus and human polyomavirus BK resulted in enhanced expression of the immediate early genes 1 and 2 and that the early gene products of BK virus trans-activated the MIEP. However, neither the MIEP sequences required for mediating this trans-activation, nor the contribution of the individual BK virus early gene products were examined. The MIEP contains multiple binding sites for the transcription factors CREB, AP1, Sp1 and NFkappaB, which may mediate polyomavirus large T- or small t-antigens-induced promoter activation. Transient transfection studies in A549 cells demonstrated that SV40 large T-antigen, but not small t-antigen, trans-activated MIEP activity approximately 9-fold. Mutations in individual binding motifs in the context of the complete MIEP did not impair traits-activation by large T-antigen. The level of induction of a truncated MIEP consisting of a single set of CRE/AP1, NFkappaB, and Sp1 binding motifs by large T-antigen was reduced 2-fold compared to the full length MIEP. Extended truncations diminished trans-activation by large T-antigen. To determine the contribution of a single binding motif in the trans-activation by large T-antigen, a CRE/AP1, an NFkappaB, an Sp1, or a non-consensus Sp1-motif, respectively, was linked to the MIEP TATA-sequence respecting the natural spacing between the two transcription regulatory elements. Only the MIEP TATA-box with the correctly spaced non-consensus Sp1 binding site (GT-motif) was stimulated by large T-antigen. These results suggest that an isolated non-consensus Sp1-motif is important for trans-activation of the MIEP by large T-antigen, but that other cis-acting elements can compensate for this element in the context of the whole MIEP.

A geminivirus replication protein interacts with the retinoblastoma protein through a novel domain to determine symptoms and tissue specificity of infection in plants

Geminiviruses replicate in nuclei of mature plant cells after inducing the accumulation of host DNA replication machinery. Earlier studies showed that the viral replication factor, AL1, is sufficient for host induction and interacts with the cell cycle regulator, retinoblastoma (pRb). Unlike other DNA virus proteins, AL1 does not contain the pRb binding consensus, LXCXE, and interacts with plant pRb homo logues (pRBR) through a novel amino acid sequence. We mapped the pRBR binding domain of AL1 between amino acids 101 and 180 and identified two mutants that are differentially impacted for AL1-pRBR interactions. Plants infected with the E-N140 mutant, which is wild-type for pRBR binding, developed wild-type symptoms and accumulated viral DNA and AL1 protein in epidermal, mesophyll and vascular cells of mature leaves. Plants inoculated with the KEE146 mutant, which retains 16% pRBR binding activity, only developed chlorosis along the veins, and viral DNA, AL1 protein and the host DNA synthesis factor, proliferating cell nuclear antigen, were localized to vascular tissue. These results established the importance of AL1-pRBR interactions during geminivirus infection of plants.

J domain-independent regulation of the Rb family by polyomavirus large T antigen

The ability of polyomavirus large T antigen (LT) to promote cell cycling, to immortalize primary cells, and to block differentiation has been linked to its effects on tumor suppressors of the retinoblastoma susceptibility (Rb) gene family. Our previous studies have shown that LT requires an intact N-terminal DnaJ domain, in addition to an Rb binding site, for activation of simple E2F-containing promoters and stimulation of cell cycle progression. Here we show that some LT effects dependent on interaction with the Rb family are largely DnaJ independent. In differentiating C2C12 myoblasts, overexpression of LT caused apoptosis. Although this activity of LT completely depended on Rb binding, LTs with mutations in the J domain remained able to kill. Comparisons of Rb(-) and J(-) LTs revealed additional differences. Wild-type but not Rb(-) LT activated the cyclin A promoter under serum starvation conditions. Genetic analysis of the promoter linked the Rb requirement to an E2F site in the promoter. LTs with mutations in the J domain were still able to activate the promoter. Finally, J mutant LTs caused changes in phosphorylation of both pRb and p130. In the case of p130, Thr-986 was shown to be a site that is regulated by J mutant LT. Taken together, these observations reveal that LT regulation of Rb function can be separated into both DnaJ-dependent and DnaJ-independent pathways.

Physical and functional interaction between the Y-box binding protein YB-1 and human polyomavirus JC virus large T antigen

Y-box binding protein YB-1 is a member of a family of DNA and RNA binding proteins which have been shown to affect gene expression at both the transcriptional and translational levels. We have previously shown that YB-1 modulates transcription from the promoters of the ubiquitous human polyomavirus JC virus (JCV). Here we investigate the physical and functional interplay between YB-1 and the viral regulatory protein large T antigen (T-antigen), using JCV as a model system. Results of mobility band shift assays demonstrated that the efficiency of binding of YB-1 to a 23-bp single-stranded viral target sequence was significantly increased when T-antigen was included in the binding reaction mixture. Affinity chromatography and coimmunoprecipitation assays demonstrated that YB-1 and T-antigen physically interact with each other. Additionally, results of transcription studies demonstrated that these two proteins interact functionally on the JCV early and late gene promoters. Whereas ectopic expression of YB-1 and T-antigen results in synergistic transactivation of the viral late promoter, YB-1 alleviates T-antigen-mediated transcriptional suppression of the viral early promoter activity. Furthermore, we have localized, through the use of a series of deletion mutants, the sequences of these proteins which are important for their interaction. The T-antigen-interacting region of YB-1 is located in the cold shock domain of YB-1 and its immediate flanking sequences, and the YB-1-interacting domain of T-antigen maps to the carboxy-terminal half of T-antigen. Results of transient transfection assays with various YB-1 mutants and T-antigen expression constructs confirm the specificity of the functional interaction between YB-1 and T-antigen. Taken together, these data demonstrate that the cellular factor YB-1 and the viral regulatory protein T-antigen interact both physically and functionally and that this interaction modulates transcription from the JCV promoters.

New insights into the mechanism of inhibition of p53 by simian virus 40 large T antigen

Simian virus 40 (SV40) large tumor antigen (T antigen) has been shown to inhibit p53-dependent transcription by preventing p53 from binding to its cognate cis element. Data presented in this report provide the first direct functional evidence that T antigen, under certain conditions, may also repress p53-dependent transcription by a mechanism in which the transactivation domain of p53 is abrogated while DNA binding is unaffected. Specifically, p53 purified as a complex with T antigen from mouse cells was found to bind DNA as a transcriptionally inactive intact complex, while that purified from human cells was found to bind DNA independently of T antigen and could activate p53-dependent transcription. This difference in activity may be dependent on a different interaction of T antigen with mouse and human p53 and, in addition, on the presence of super T, which is found only in transformed rodent cells. These results suggest that subtle yet important differences exist between the inhibition of p53 by T antigen in mouse and human cells. The implications of this finding with respect to SV40-associated malignancies are discussed.

The J domain of papovaviral large tumor antigen is required for synergistic interaction with the POU-domain protein Tst-1/Oct6/SCIP

Large T antigens from polyomaviruses are multifunctional proteins with roles in transcriptional regulation, viral DNA replication, and cellular transformation. They have been shown to enhance the activity of various cellular transcription factors. In the case of the POU protein Tst-1/Oct6/SCIP, this enhancement involves a direct physical interaction between the POU domain of the transcription factor and the amino-terminal region of large T antigen. Here we have analyzed the structural requirements for synergistic interaction between the two proteins in greater detail. Tst-1/Oct6/SCIP and the related POU protein Brn-1 were both capable of direct physical interaction with large T antigen. Nevertheless, only Tst-1/Oct6/SCIP functioned synergistically with large T antigen. This differential behavior was due to differences in the amino-terminal regions of the proteins, as evident from chimeras between Tst-1/Oct6/SCIP and Brn-1. Synergy was specifically observed for constructs containing the amino-terminal region of Tst-1/Oct6/SCIP. Large T antigen, on the other hand, functioned synergistically with Tst-1/Oct6/SCIP only when the integrity of its J-domain-containing amino terminus was maintained. Mutations that disrupted the J domain concomitantly abolished the ability to enhance the function of Tst-1/Oct6/SCIP. The J domain of T antigen was also responsible for the physical interaction with Tst-1/Oct6/SCIP and could be replaced in this property by other J domains. Intriguingly, a heterologous J domain from a human DnaJ protein partially substituted for the amino terminus of T antigen even with regard to the synergistic enhancement of Tst-1/Oct6/SCIP function. Given the general role of J domains, we propose chaperone activity as the underlying mechanism for synergy between Tst-1/Oct6/SCIP and large T antigens.

Chromatin structure of the simian virus 40 late promoter: a deletional analysis

The goal of this study was to determine the minimal sequence within the simian virus 40 (SV40) late promoter region, nucleotides (nt) 255 to 424, capable of phasing nucleosomes as measured by its ability to confer the greatest endonuclease sensitivity on adjacent DNA sequences. To identify the minimal sequence, a deletional analysis of the late region was performed by utilizing a SV40 recombinant reporter system. The reporter system consisted of a series of unique restriction sites introduced into SV40 at nt 2666. The unique restriction sites allowed the insertion of test sequences as well as measurement of conferred endonuclease sensitivity. The results of the deletional analysis demonstrated that constructs capable of conferring the greatest nuclease sensitivities consistently included nt 255 to 280. The activator protein 4 (AP-4) and GTIIC transcription factor binding sequences lie within this region and were analyzed individually. Their abilities to confer nuclease sensitivity upon the reporter nearly matched that of the entire late domain. These results suggest that transcription factors AP-4 and transcription-enhancing factor which binds the GTIIC sequence are able to confer significant levels of nuclease sensitivity and are likely involved in the formation of the SV40 nucleosome-free region.

Interaction of the transcription factor TFIID with simian virus 40 (SV40) large T antigen interferes with replication of SV40 DNA in vitro

Simian virus 40 (SV40) large tumor (T) antigen is the major regulatory protein that directs the course of viral infection, primarily by interacting with host cell proteins and modulating their functions. Initiation of viral DNA replication requires specific interactions of T antigen bound to the viral origin of DNA replication with cellular replication proteins. Transcription factors are thought to stimulate initiation of viral DNA replication, but the mechanism of stimulation is poorly understood. Since the transcription factor TATA-binding protein (TBP) binds to sequences within the origin of replication and interacts specifically with T antigen, we examined whether TBP complexes stimulate SV40 DNA replication in vitro. On the contrary, we found that depletion of TBP complexes from human cell extracts increased their ability to support viral DNA replication, and readdition of TBP complexes to the depleted extracts diminished their activity. We have mapped the sites of interaction between the proteins to residues 181 to 205 of T antigen and 184 to 220 of TBP. Titration of fusion proteins containing either of these peptides into undepleted cell extracts stimulated their replication activity, suggesting that they prevented the T antigen-TBP interaction that interfered with replication activity. TBP complexes also interfered with origin DNA unwinding by purified T antigen, and addition of either the T antigen or the TBP fusion peptide relieved the inhibition. These results suggest that TBP complexes associate with a T-antigen surface that is also required for origin DNA unwinding and viral DNA replication. We speculate that competition among cellular proteins for T antigen may play a role in regulating the course of viral infection.

Transactivation of a ribosomal gene by simian virus 40 large-T antigen requires at least three activities of the protein

Simian virus 40 large-T antigen transactivates the ribosomal genes which are transcribed by RNA polymerase (pol I), as well as genes that are dependent on either pol II or pol III. This report identifies regions and activities of T antigen that are required to transactivate a pol I-dependent rat ribosomal gene promoter. By using the rat ribosomal gene (rDNA) promoter linked to a chloramphenicol acetyltransferase gene, we show that at least three separable T-antigen regions are necessary to achieve wild-type levels of transactivation of rDNA in transiently transfected monkey cells. One activity depends on the region of T antigen shared with small-t antigen (T/t common region). A second activity maps to amino acids 109 to 626 and is highly sensitive to mutational inactivation. Complementation analyses suggest that at least one activity in this region is independent of and must be in cis with the activity within the T/t common region. In addition, a functional nuclear localization signal is required for maximal T-antigen-mediated transactivation of rat rDNA. The three activities work in concert to override cellular species-specific controls and transactivate the rat ribosomal gene promoter. Finally, we provide evidence that although the tumor suppressor protein Rb has been shown to repress a pol I-dependent promoter, transactivation of the rat rDNA promoter does not depend on T antigen's ability to bind the tumor suppressor product Rb.

The replication protein A binding site in simian virus 40 (SV40) T antigen and its role in the initial steps of SV40 DNA replication

Physical interactions of simian virus 40 (SV40) large tumor (T) antigen with cellular DNA polymerase alpha-primase (Pol/Prim) and replication protein A (RPA) appear to be responsible for multiple functional interactions among these proteins that are required for initiation of viral DNA replication at the origin, as well as during lagging-strand synthesis. In this study, we mapped an RPA binding site in T antigen (residues 164 to 249) that is embedded within the DNA binding domain of T antigen. Two monoclonal antibodies whose epitopes map within this region specifically interfered with RPA binding to T antigen but did not affect T-antigen binding to origin DNA or Pol/Prim, ATPase, or DNA helicase activity and had only a modest effect on origin DNA unwinding, suggesting that they could be used to test the functional importance of this RPA binding site in the initiation of viral DNA replication. To rule out a possible effect of these antibodies on origin DNA unwinding, we used a two-step initiation reaction in which an underwound template was first generated in the absence of primer synthesis. In the second step, primer synthesis was monitored with or without the antibodies. Alternatively, an underwound primed template was formed in the first step, and primer elongation was tested with or without antibodies in the second step. The results show that the antibodies specifically inhibited both primer synthesis and primer elongation, demonstrating that this RPA binding site in T antigen plays an essential role in both events.

Simian virus 40 large T antigen stabilizes the TATA-binding protein-TFIIA complex on the TATA element

Large T antigen (T antigen), the early gene product of simian virus 40 (SV40), is a potent transcriptional activator of both cellular and viral genes. Recently we have shown that T antigen is tightly associated with TFIID and, in this position, performs a TATA-binding protein (TBP)-associated factor (TAF)-like function. Based on this observation, we asked whether T antigen affected steps in preinitiation complex assembly. Using purified components in in vitro complex assembly assays, we found that T antigen specifically enhances the formation of the TBP-TFIIA complex on the TATA element. T antigen accomplishes this by increasing the rate of formation of the TBP-TFIIA complex on the TATA element and by stabilizing the complexes after they are formed on the promoter. In addition, DNA immunoprecipitation experiments indicate that T antigen is associated with the stabilized TBP-TFIIA complexes bound to the DNA. In this regard, it has previously been shown that T antigen interacts with TBP; in the present study, we show that T antigen also interacts with TFIIA in vitro. In testing the ability of T antigen to stabilize the TBP-TFIIA complex, we found that stabilization is highly sensitive to the specific sequence context of the TATA element. Previous studies showed that T antigen could activate simple promoters containing the TATA elements from the hsp70 and c-fos gene promoters but failed to significantly activate similar promoters containing the TATA elements from the promoters of the SV40 early and adenovirus E2a genes. We find that the ability to stabilize the TBP-TFIIA complex on the hsp70 and c-fos TATA elements, and not on the SV40 early and E2A TATA elements, correlates with the ability or inability to activate promoters containing these TATA elements.

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.

Simian virus 40 large T antigen interacts with human TFIIB-related factor and small nuclear RNA-activating protein complex for transcriptional activation of TATA-containing polymerase III promoters

The TATA-binding protein (TBP) is common to the basal transcription factors of all three RNA polymerases, being associated with polymerase-specific TBP-associated factors (TAFs). Simian virus 40 large T antigen has previously been shown to interact with the TBP-TAFII complexes, TFIID (B. Damania and J. C. Alwine, Genes Dev. 10:1369-1381, 1996), and the TBP-TAFI complex, SL1 (W. Zhai, J. Tuan, and L. Comai, Genes Dev. 11: 1605-1617, 1997), and in both cases these interactions are critical for transcriptional activation. We show a similar mechanism for activation of the class 3 polymerase III (pol III) promoter for the U6 RNA gene. Large T antigen can activate this promoter, which contains a TATA box and an upstream proximal sequence element but cannot activate the TATA-less, intragenic VAI promoter (a class 2, pol III promoter). Mutants of large T antigen that cannot activate pol II promoters also fail to activate the U6 promoter. We provide evidence that large T antigen can interact with the TBP-containing pol III transcription factor human TFIIB-related factor (hBRF), as well as with at least two of the three TAFs in the pol III-specific small nuclear RNA-activating protein complex (SNAPc). In addition, we demonstrate that large T antigen can cofractionate and coimmunoprecipitate with the hBRF-containing complex TFIIIB derived from HeLa cells infected with a recombinant adenovirus which expresses large T antigen. Hence, similar to its function with pol I and pol II promoters, large T antigen interacts with TBP-containing, basal pol III transcription factors and appears to perform a TAF-like function.

Mechanisms of transcriptional regulation of cellular genes by SV40 large T- and small T-antigens

During the past decade a number of virus-encoded transcriptional trans-activators that regulate the expression of viral genes have been reported. These trans-activators may also affect the expression or activity of several cellular genes or gene products to create an optimal cellular environment that favors viral replication. Among the better-studied viral trans-activating proteins are the Simian virus 40 large T- and small t-antigens. During the last few years, mechanisms by which these two viral proteins influence cellular gene expression start to emerge. They are grouped provisionally and reflect the methods used to determine the effects of large T-antigen. Large T-antigen may influence cellular gene expression by: i. altering mRNA levels of cellular transcription factors; ii. interacting with and regulating the DNA-binding or transcriptional activity of specific transcription factors; iii. functionally substitution of eukaryotic transcription factors; iv. direct binding to DNA; or v. regulating components of signaling transduction pathways. Small t-ag seems to exert its effect mainly through inhibiting a cellular phosphatase, protein phosphatase 2A, thereby modulating components of signal transduction pathways and preventing dephosphorylation of several transcription factors. However, small t-ag may also control cellular gene expression by regulating mRNA levels of transcription factors or by interacting with other transcription factors.

DNA-dependent transregulation by IE1 of Autographa californica nuclear polyhedrosis virus: IE1 domains required for transactivation and DNA binding

IE1 is the principal early transregulator of Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV). The 582-residue protein stimulates viral transcription and binds as a dimer to 28-bp palindromic repeats (28-mers) comprising the AcMNPV homologous region (hr) transcription enhancers. To define IE1 domains responsible for hr-dependent transactivation, we first constructed a series of IE1 fusions to the DNA binding domain of the yeast GAL4 transactivator. In transfection assays, GAL4-IE1 fusions stimulated transcription from a TATA-containing AcMNPV promoter only upon cis linkage to GAL4 DNA binding sites. IE1 N-terminal residues 8 to 118 were sufficient for GAL4-binding-site-dependent transactivation. To identify IE1 residues required for hr interaction, we tested a series of IE1 mutations for 28-mer binding by using electrophoretic mobility shift assays. Deletion of IE1 residues other than the N-terminal transactivation domain eliminated 28-mer binding. Of 14 insertion mutations, only IE1(I425) and IE1(I553) failed to bind the 28-mer either as homodimers or as heterodimers with functional IE1. In contrast to insertion IE1(I425), IE1(I553) also failed to compete with wild-type IE1 for DNA binding and suggested a defect in oligomerization. Consistent with loss of oligomerization, substitutions within a hydrophobic repeat (residues 543 to 568) at the IE1 C terminus abolished 28-mer binding and demonstrated that this helix-loop-helix-like domain is required for DNA interaction. These data confirm that IE1 contains separable domains for transactivation and oligomerization-dependent DNA binding. Furthermore, they support a model wherein hr-mediated transactivation by IE1 involves sequence-specific DNA binding that contributes to transcriptional stimulation by interaction with components of the basal transcription complex.

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.

Tiny T antigen: an autonomous polyomavirus T antigen amino-terminal domain

Three mRNAs from the murine polyomavirus early region encode the three well-characterized tumor antigens. We report the existence of a fourth alternatively spliced mRNA which encodes a fourth tumor antigen, tiny T antigen, which comprises the amino-terminal domain common to all of the T antigens but is extended by six unique amino acid residues. The amount of tiny T antigen in infected cells is small because of its short half-life. Tiny T antigen stimulates the ATPase activity of Hsc70, most likely because of its DnaJ-like motif. The common amino-terminal domain may interface with chaperone complexes to assist the T antigens in carrying out their diverse functions of replication, transcription, and transformation in the appropriate cellular compartments.

Transcription enhancer factor 1 interacts with a basic helix-loop-helix zipper protein, Max, for positive regulation of cardiac alpha-myosin heavy-chain gene expression

The M-CAT binding factor transcription enhancer factor 1 (TEF-1) has been implicated in the regulation of several cardiac and skeletal muscle genes. Previously, we identified an E-box-M-CAT hybrid (EM) motif that is responsible for the basal and cyclic AMP-inducible expression of the rat cardiac alpha-myosin heavy chain (alpha-MHC) gene in cardiac myocytes. In this study, we report that two factors, TEF-1 and a basic helix-loop-helix leucine zipper protein, Max, bind to the alpha-MHC EM motif. We also found that Max was a part of the cardiac troponin T M-CAT-TEF-1 complex even when the DNA template did not contain an apparent E-box binding site. In the protein-protein interaction assay, a stable association of Max with TEF-1 was observed when glutathione S-transferase (GST)-TEF-1 or GST-Max was used to pull down in vitro-translated Max or TEF-1, respectively. In addition, Max was coimmunoprecipitated with TEF-1, thus documenting an in vivo TEF-1-Max interaction. In the transient transcription assay, overexpression of either Max or TEF-1 resulted a mild activation of the alpha-MHC-chloramphenicol acetyltransferase (CAT) reporter gene at lower concentrations and repression of this gene at higher concentrations. However, when Max and TEF-1 expression plasmids were transfected together, the repression mediated by a single expression plasmid was alleviated and a three- to fourfold transactivation of the alpha-MHC-CAT reporter gene was observed. This effect was abolished once the EM motif in the promoter-reporter construct was mutated, thus suggesting that the synergistic transactivation function of the TEF-1-Max heterotypic complex is mediated through binding of the complex to the EM motif. These results demonstrate a novel association between Max and TEF-1 and indicate a positive cooperation between these two factors in alpha-MHC gene regulation.

Identification of a minimal promoter element of the mouse epidermal growth factor gene

We have previously generated a transgenic mouse line (EGF/Tag) in which simian virus 40 (SV40) T-antigen expression is directed by the mouse epidermal growth factor (EGF) gene promoter. In these mice, cellular hyperproliferation is observed in the submaxillary gland associated with SV40 T-antigen expression. In addition, SV40 T-antigen-expressing tumours of prostatic origin are seen. We have now derived immortalized cell lines from these tissues and have used the cells to perform a functional analysis of the EGF gene promoter. Cells were transfected with EGF promoter/reporter constructs, and an element located between 51 and 35 bases upstream of the EGF mRNA start site required for basal activity of the promoter was identified. Electrophoretic mobility-shift analysis suggests that three proteins bind to this region, one of which is either Sp1 or a closely related protein.

Purification of the simian virus 40 (SV40) T antigen DNA-binding domain and characterization of its interactions with the SV40 origin

To better define protein-DNA interactions at a eukaryotic origin, the domain of simian virus 40 (SV40) large T antigen that specifically interacts with the SV40 origin has been purified and its binding to DNA has been characterized. Evidence is presented that the affinity of the purified T antigen DNA-binding domain for the SV40 origin is comparable to that of the full-length T antigen. Furthermore, stable binding of the T antigen DNA-binding domain to the SV40 origin requires pairs of pentanucleotide recognition sites separated by approximately one turn of a DNA double helix and positioned in a head-to-head orientation. Although two pairs of pentanucleotides are present in the SV40 origin, footprinting and band shift experiments indicate that binding is limited to dimer formation on a single pair of pentanucleotides. Finally, it is demonstrated that the T antigen DNA-binding domain interacts poorly with single-stranded DNA.

The T/t common exon of simian virus 40, JC, and BK polyomavirus T antigens can functionally replace the J-domain of the Escherichia coli DnaJ molecular chaperone

The N-terminal 70 residue "J-domain" of the Escherichia coli DnaJ molecular chaperone is the defining and highly conserved feature of a large protein family. Based upon limited, yet significant, amino acid sequence homology to the J-domain, the DNA encoding the T/t common exon of the simian virus 40 (SV40), JC, or BK polyoma virus T antigen oncoproteins was used to construct J-domain replacement chimeras of the E. coli DnaJ chaperone. The virally encoded J-domains successfully substituted for the bacterial counterpart in vivo as shown by (i) complementation for viability at low and high temperature of a hypersensitive bacterial reporter strain, and (ii) the restoration of bacteriophage lambda plaque forming ability in the same strain. The amino acid change, H42Q, in the SV40 T/t and the JC virus T/t exon, which is positionally equivalent to the canonical dnaJ259 H33Q mutation within the E. coli J-domain, entirely abolished complementing activity. These results strongly suggest that the heretofore functionally undefined viral T/t common exon represents a bona fide J-domain that preserves critical features of the characteristic domain fold essential for J-domain interaction with the ATPase domain of the Hsp70 family. This finding has implications for the regulation of DNA tumor virus T antigens by molecular chaperones.

Transcriptional activation of the human epidermal growth factor receptor promoter by human p53

The human epidermal growth factor receptor (EGFR) promoter is activated by both wild-type and tumor-derived mutant p53. In this communication, we demonstrate that EGFR promoter sequence requirements for transactivation by wild-type and mutant p53 are different. Transient-expression assays with EGFR promoter deletions identified a wild-type human p53 response element, 5'-AGCTAGACGTCCGGGCAGCCCCCGGCG -3', from positions --265 to --239. Electrophoretic mobility shift analysis and DNase I footprinting assays indicated that wild-type p53 binds sequence specifically to the response element. Using circularly permuted DNA fragments containing the p53-binding site, we show that wild-type p53 binding induces DNA bending at this site. We further show that the EGFR promoter is also activated by tumor-derived p53 mutants p53-143A, p53-175H, p53-248W, p53-273H, and p53-281G. However, the transactivation by mutant p53 does not require the wild-type p53-binding site. The minimal EGFR promoter from positions --104 to --20 which does not contain the wild-type p53-binding site is transactivated by the p53 mutants but not by the wild-type protein, showing a difference in the mechanism of transactivation by wild-type and mutant p53. Transactivation of the EGFR promoter by p53 may represent a novel mechanism of cell growth regulation.

Chromatin structure and factor site occupancies in an in vivo-assembled transcription elongation complex

The chromatin structure specific to the SV40 late transcription elongation complex as well as the occupancy of several sites that bind transcription factors have been examined. These features have been determined by assessing blockage to restriction enzyme digestion. Cleavage specific to the elongation complex has been quantified using ternary complex analysis. This method involves radioactively labeling the complex by in vitro transcription followed by determining the extent of linearization by electrophoresis in an agarose gel. It was found that not only is the origin region devoid of nucleosomes, but there is also no stable factor occupancy at the BglI, SphI, KpnI and MspI restriction enzyme sites within this region. Thus these sites were cleaved to a high degree, meaning that the binding sites for a number of transcription factors, including OBP/TEF-1, TBP, DAP, as well as a proposed positioned nucleosome, are unoccupied in the native viral transcription elongation complex. The absence of these trans-acting factors from their respective binding sites in the elongation complex indicates that they bind only transiently, possibly cycling on and off during the transcription cycle. This finding implies that various forms of transcription complex are assembled and disassembled during transcription and thus supports a 'hit-and-run' model of factor function.

Activation of the human thymidine kinase (TK) promoter by simian virus 40 large T antigen requires both the T antigen pRb family-binding domain and TK promoter sequences resembling E2F-binding sites

Infection of quiescent cells with the DNA tumor virus simian virus 40 induces expression of the cellular thymidine kinase (TK) gene a minimum of 10- to 20-fold, and this induction depends upon the viral protein large T antigen (T-Ag). To define both human TK promoter elements and T-Ag functional domains required for transcriptional induction, we have established a system in which stable Rat-1 transfectants harboring TK promoter-luciferase hybrid genes are infected with recombinant adenoviruses expressing either wild-type or mutant forms of T-Ag and luciferase expression is measured as an indicator of promoter activity. The results show that (i) a 135-bp TK promoter fragment is activated 10- to 15-fold by viral infection; (ii) this activation is the result of both T-Ag-dependent and -independent mechanisms; (iii) the T-Ag pRb family-binding domain, but not the p53-binding, helicase, or ATPase domain, is required for activation; and (iv) activation is severely diminished with a TK promoter fragment in which E2F-like-binding sites have been removed. These data demonstrate a requirement for both an E2F-related factor and a pRb family member in activation of the TK promoter by T-Ag. This contrasts with the promiscuous activation of many cellular and viral genes by T-Ag, which is independent of its ability to bind pRb.

Interaction of human immunodeficiency virus type 1 Tat with a unique site of TFIID inhibits negative cofactor Dr1 and stabilizes the TFIID-TFIIA complex

We have previously reported the direct physical interaction between the human immunodeficiency virus (HIV) type I Tat protein and the basal transcription factor TBP/TFIID. Affinity chromatography demonstrated that wild-type Tat, but not a transactivation mutant of Tat, was capable of depleting TBP/TFIID from cell extracts. These experiments represented the first demonstration of a basal transcription factor that binds, in an activation-dependent manner, to Tat. We now report that the Tat-TBP interaction can be detected in HIV type 1-infected cells. The domain of TBP interacting with Tat has been mapped from amino acids 163 to 196 by using deletion and site-specific mutants of TBP. This domain of TBP, which includes the HI and S2 domains, is distinct from the H2 binding site for other activator proteins, such as E1A. The interaction of Tat with TFIID regulates the binding of accessory proteins to TFIID. Tat stabilizes the interaction of TFIID with TFIIA in a gel shift assay. In addition, Tat competes for Dr1 interaction with TBP. Our results suggest that the basal transcription factor TBP/TFIID represents an important regulatory molecule in HIV transcription.

Association of p300 and CBP with simian virus 40 large T antigen

p300 and the CREB-binding protein CBP are two large nuclear phosphoproteins that are structurally highly related. Both function, in part, as transcriptional adapters and are targeted by the adenovirus E1A oncoprotein. We show here that p300 and CBP interact with another transforming protein, the simian virus 40 large T antigen (T). This interaction depends on the integrity of a region of T which is critical for its transforming and mitogenic properties and includes its LXCXE Rb-binding motif. T interferes with normal p300 and CBP function on at least two different levels. The presence of T alters the phosphorylation states of both proteins and inhibits their transcriptional activities on certain promoters. Although E1A and T show little sequence similarity, they interact with the same domain of p300 and CBP, suggesting that this region exhibits considerable flexibility in accommodating diverse protein ligands.

An N-terminal deletion mutant of simian virus 40 (SV40) large T antigen oligomerizes incorrectly on SV40 DNA but retains the ability to bind to DNA polymerase alpha and replicate SV40 DNA in vitro

A peptide encompassing the N-terminal 82 amino acids of simian virus 40 (SV40) large T antigen was previously shown to bind to the large subunit of DNA polymerase alpha-primase (I. Dornreiter, A. Höss, A. K. Arthur, and E. Fanning, EMBO J. 9:3329-3336, 1990). We report here that a mutant T antigen, T83-708, lacking residues 2 to 82 retained the ability to bind to DNA polymerase alpha-primase, implying that it carries a second binding site for DNA polymerase alpha-primase. The mutant protein also retained ATPase, helicase, and SV40 origin DNA-binding activity. However, its SV40 DNA replication activity in vitro was reduced compared with that of wild-type protein. The reduction in replication activity was accompanied by a lower DNA-binding affinity to SV40 origin sequences and aberrant oligomerization on viral origin DNA. Thus, the first 82 residues of SV40 T antigen are not strictly required for its interaction with DNA polymerase alpha-primase or for DNA replication function but may play a role in correct hexamer assembly and efficient DNA binding at the origin.

The major transcriptional transactivation domain of simian virus 40 large T antigen associates nonconcurrently with multiple components of the transcriptional preinitiation complex

Simian virus 40 (SV40) large T antigen (Tag) is a promiscuous transcriptional transactivator; however, its mechanism of transactivation remains unknown. Recent studies have suggested the possible involvement of protein-protein interactions with TBP, the TATA box-binding protein of TFIID, and TEF-1, an enhancer-binding factor. We show here that (i) the Tag domain containing amino acids 133 to 249 directly interacts with the general transcription factor TFIIB, the activator protein Sp1, and the 140-kDa subunit of RNA polymerase II, as well as with TBP and TEF-1; (ii) these interactions can also occur when these transcription factors are present in their functional states in cellular extracts; (iii) binding of Tag to TBP is eliminated by preincubation of TBP either at 48 degrees C or with the adenovirus 13S E1a protein; (iv) this domain of Tag cannot bind concurrently to more than one of these transcription factors; and (v) the substitution of Tag amino acid residues 173 and 174 inactivates the ability of this Tag domain both to associate with any of these transcription factors and to transactivate the SV40 late promoter. Thus, we conclude that SV40 Tag probably does not transactivate via the concurrent interaction with multiple components of the preinitiation complex. Rather, we hypothesize that transactivation by Tag may primarily occur by removing or preventing the binding of factors that inhibit the formation of preinitiation complexes.

The block of adipocyte differentiation by a C-terminally truncated, but not by full-length, simian virus 40 large tumor antigen is dependent on an intact retinoblastoma susceptibility protein family binding domain

Simian virus 40 (SV40) can promote cell transformation and suppress differentiation. It does this partly by targeting tumor suppressors such as p53 and members of the retinoblastoma susceptibility protein (Rb) family. This work concentrates on mechanisms by which SV40 large tumor antigen (SVLT) suppresses adipocyte differentiation. We created cell lines derived from murine 3T3-L1 preadipocytes expressing different versions of SV40 early-region sequences. SVLT-expressing cells failed to exhibit adipocyte morphology, to induce glycerophosphate dehydrogenase activity, and to induce differentiation-dependent mRNA for adipocyte P2. SVLT alone was sufficient, in the absence of SV40 small tumor antigen, to inhibit differentiation. A truncated SVLT containing only the N-terminal 121 amino acids (SVLT1-121) blocked differentiation, thus mapping at least one differentiation blocking function to the N-terminal region. K1 (Glu-107-->Lys) point mutants of SVLT, which are unable to bind to the Rb protein family or induce neoplastic transformation, are defective for blocking differentiation in the case of SVLT1-121 but retain the ability to block differentiation in the case of full-length SVLT. This finding demonstrates that Rb family proteins are important in regulating adipocyte differentiation but that other functions of full-length SVLT can block adipocyte differentiation independently of RB family binding and transformation.

Induction of transcription by a viral regulatory protein depends on the relative strengths of functional TATA boxes

The mechanisms by which viral regulatory proteins activate the cellular transcription apparatus without binding to specific DNA elements are not fully understood. Several lines of evidence suggest that activation by one such regulatory protein, herpes simplex virus ICP4, could be mediated, at least in part, by TFIID. To test this model, we replaced the TATA box of the ICP4-responsive viral thymidine kinase gene with functional TATA boxes that displayed different apparent affinities for TATA-box-binding protein as measured by DNase I footprinting. We measured the effects of these TATA boxes on ICP4 induction by constructing ICP4-deficient recombinant viruses containing the different TATA alleles and comparing their expression in cells lacking or expressing ICP4. Overall, ICP4 induced weak TATA boxes (those that displayed low apparent affinity for TATA-box-binding protein and low basal expression) the most (18- to 41-fold) and strong TATA boxes the least (7- to 10-fold). Therefore, ICP4 induction correlated inversely with TATA box strength. Using a reconstituted in vitro transcription assay, we determined that the relative levels of induction by ICP4 of the different TATA alleles were similar to those measured in vivo, suggesting that ICP4 was the only viral protein required for induction. These results fit a model in which ICP4 acts in part to enhance binding of TFIID to the TATA box. We compare and contrast these results with those observed with the viral regulatory proteins adenovirus E1a and simian virus 40 large T antigen and the cellular coactivator PC4.

A TEF-1-independent mechanism for activation of the simian virus 40 (SV40) late promoter by mutant SV40 large T antigens

Simian virus 40 (SV40) large tumor antigen (T antigen) stimulates the activity of the SV40 late promoter and a number of cellular and other viral promoters. We have characterized the ability of T antigens with mutations in the DNA-binding domain and within the N-terminal 85 residues to activate the SV40 late promoter. T antigens lacking both nonspecific and sequence-specific DNA-binding activities were able to induce the late promoter. Mutations within the N-terminal 85 residues of T antigen diminished activation by less than twofold. Activation by wild-type and most of the mutant T antigens required intact binding sites for the cellular transcription factor TEF-1 in the late promoter. Curiously, two mutants altered in the N-terminal region and an additional mutant altered in the DNA-binding domain activated a late promoter derivative lacking TEF-1 binding sites, indicating the existence of a TEF-1-independent pathway for activation of the late promoter. A consensus binding site for the TATA binding protein, TBP, was created in variants of late promoters either containing or lacking TEF-1 binding sites. Basal expression was increased by the consensus TBP binding site only when TEF-1 binding sites were present, leading to a reduction in the degree of activation by T antigen. However, activation by a mutant T antigen of the promoter lacking TEF-1 sites was unchanged or slightly enhanced by the consensus TBP binding site. These results suggest that some mutant T antigens can stabilize an interaction between TBP and additional factors bound to the late promoter.

Simian virus 40 large T antigen contains two independent activities that cooperate with a ras oncogene to transform rat embryo fibroblasts

The simian virus 40 large T antigen immortalizes growing primary cells in culture. In addition, this viral oncoprotein cooperates with an activated ras protein to produce dense foci on monolayers of rat embryo fibroblasts (REF). The relationship between independent immortalization and cooperative transformation with ras has not been defined. Previously, two regions of T antigen were shown to contain immortalization activities. An N-terminal fragment consisting of amino acids 1 to 147 immortalizes rodent cells (L. Sompayrac and K. J. Danna, Virology 181:412-415, 1991). Loss-of-function analysis indicated that immortalization depended on integrity of the T-antigen segments containing amino acids 351 to 450 and 533 to 626 (T. D. Kierstead and M. J. Tevethia, J. Virol. 67:1817-1829, 1993). The experiments described here were directed toward determining whether these same T-antigen regions were sufficient for cooperation with ras. Initially, constructs that produce T antigens containing amino acids 176 to 708 (T176-708) or 1 to 147 were tested in a ras cooperation assay. Both polypeptides cooperated with ras to produce dense foci on monolayers of primary REF. These results showed that T antigen contains two separate ras cooperation activities. In order to determine the N-terminal limit of the ras cooperation activity contained within the T176-708 polypeptide, a series of constructs designed to produce fusion proteins containing T-antigen segments beginning at residues 251, 301, 337, 351, 371, 401, 451, 501, 551, 601, and 651 was generated. Each of these constructs was tested for the capacity to cooperate with ras to produce dense foci on REF monolayers. The results indicated that a polypeptide containing T-antigen amino acids 251 to 708 (T251-708) was sufficient to cooperate with ras, whereas the more extensively truncated products were not. The abilities of the N-terminally truncated T antigens to bind p53 were examined in p53-deficient cells infected with a recombinant vaccinia virus expressing a phenotypically wild-type mouse p53. The results showed that polypeptides containing T-antigen amino acids 251 to 708, 301 to 708, 337 to 708, or 351 to 708 retained p53-binding capacity. The introduction into the T251-708 polypeptide of deletions that either prevented p53 binding (dl434-444) or did not prevent p53 binding (dl400) abrogated ras cooperation. These results indicated that although p53 binding may be necessary for ras cooperation, an additional, as-yet-undefined activity contained within the T251-708 polypeptide is needed.

The cellular transcription factor USF cooperates with varicella-zoster virus immediate-early protein 62 to symmetrically activate a bidirectional viral promoter

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediate-early protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.

The cellular transcription factor USF cooperates with varicella-zoster virus immediate-early protein 62 to symmetrically activate a bidirectional viral promoter

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediate-early protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.

Transrepression of RNA polymerase II promoters by the simian virus 40 small t antigen

Simian virus 40 (SV40) small t antigen (t) can activate transcription from certain RNA polymerase II and III promoters (M. Loeken, I. Bikel, D. M. Livingston, and J. Brady, Cell 55:1171-1177, 1988). Here we report a new function of t, its ability to repress human c-fos promoter and AP-1 transcriptional activity in CV-1P cells. This function is the product of a discrete N-terminal domain of t, because the large T antigen (T)/t-common polypeptide, which contains only the first 82 amino acids common to both T and t of SV40, was, like the intact protein, an active repressor. The data further suggest that the t- and T/t-common-mediated repression of c-fos expression was most likely manifest at the level of transcription. In keeping with the possibility that t affects the expression of the genomic c-fos promoter, it also led to repression of AP-1 formation. Thus, SV40 is both an activator and a repressor of transcription. Its ability to inhibit c-fos expression should be considered in light of the natural history of SV40 in its natural host.

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.

A cell-specific factor represses stimulation of transcription in vitro by transcriptional enhancer factor 1

Transcription in HeLa cell extracts in vitro was stimulated 8- to 10-fold by a recombinant chimera, GAL-TEF-1, consisting of the DNA-binding domain of GAL4 and the activation function of the HeLa cell activator TEF-1. In contrast, only a 2- to 3-fold stimulation was obtained with GAL-TEF-1 in extracts from BJA-B lymphoid cells. Stimulation by GAL-TEF-1 in BJA-B extracts was dramatically increased by the addition of immunopurified HeLa cell TFIID, suggesting that BJA-B TFIID lacks or contains lower quantities of a TATA-binding-protein-associated factor(s) required for the activity of the TEF-1 activation function. However, chromatography, immunopurification, and transcriptional reconstitution experiments indicated that BJA-B extracts did not lack the previously identified TATA-binding-protein-associated factors required for TEF-1 activity but rather contained a negatively acting factor(s) which inhibited transactivation by GAL-TEF-1. These results indicate that the relative lack of activity of the TEF-1 activation function in vitro in BJA-B cell extracts does not result from the absence of positively acting factors from the presence of a cell-specific negatively acting factor(s).

A cell-specific factor represses stimulation of transcription in vitro by transcriptional enhancer factor 1

Transcription in HeLa cell extracts in vitro was stimulated 8- to 10-fold by a recombinant chimera, GAL-TEF-1, consisting of the DNA-binding domain of GAL4 and the activation function of the HeLa cell activator TEF-1. In contrast, only a 2- to 3-fold stimulation was obtained with GAL-TEF-1 in extracts from BJA-B lymphoid cells. Stimulation by GAL-TEF-1 in BJA-B extracts was dramatically increased by the addition of immunopurified HeLa cell TFIID, suggesting that BJA-B TFIID lacks or contains lower quantities of a TATA-binding-protein-associated factor(s) required for the activity of the TEF-1 activation function. However, chromatography, immunopurification, and transcriptional reconstitution experiments indicated that BJA-B extracts did not lack the previously identified TATA-binding-protein-associated factors required for TEF-1 activity but rather contained a negatively acting factor(s) which inhibited transactivation by GAL-TEF-1. These results indicate that the relative lack of activity of the TEF-1 activation function in vitro in BJA-B cell extracts does not result from the absence of positively acting factors from the presence of a cell-specific negatively acting factor(s).

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.

The POU domain protein Tst-1 and papovaviral large tumor antigen function synergistically to stimulate glia-specific gene expression of JC virus

Synergism between transcriptional activators is a powerful way of potentiating their function. Here we show that the glial POU domain protein Tst-1 (also known as Oct-6 and SCIP) and large tumor antigen (T antigen) synergistically increased transcription from both the early and the late promoters of papovavirus JC in glial cells. Synergism between both proteins did not require T-antigen-mediated DNA replication or direct binding of T antigen to the promoter. The ability of T antigen to functionally cooperate with Tst-1 was contained within its N-terminal region, shown by the fact that small tumor antigen (t antigen) could substitute for T antigen in transfection experiments. In addition to this functional synergism, a direct interaction between Tst-1 and T antigen was observed in vitro. Using deletion mutants of Tst-1 and T antigen, the POU domain of Tst-1 and the N-terminal region of T antigen were found to participate in this interaction. Because of the low levels of Tst-1 present in oligodendrocytes, synergism between Tst-1 and T antigen could be an important factor in establishing the lytic infection of oligodendrocytes by JC virus during the course of the fatal demyelinating disease progressive multifocal leukoencephalopathy.

The zinc finger region of the adenovirus E1A transactivating domain complexes with the TATA box binding protein

The 289R E1A protein of adenovirus transactivates a variety of viral and cellular promoters through protein-protein interactions. In earlier studies, mutational analyses of the E1A transactivating domain identified residues that are critical for transactivation and implied that the zinc finger region of the transactivating domain binds a transcription factor. Also, the E1A activation domain was found to bind to the TATA box binding protein (TBP) in vitro. Here, we tested the significance of the E1A-TBP interaction for E1A transactivation by analyzing the effects of conservative substitutions at each of the 49 residues of the E1A activation domain. Seven of the substitutions significantly diminished TBP binding in vitro. All of these were in the zinc finger region and were defective for transactivation in vivo. The perfect correlation between reduced TBP binding and transactivation argues strongly that a direct interaction between the E1A activation domain and TBP is critical to the mechanism of E1A activation. This genetic analysis leads us to further suggest that another factor, which is limiting, is also necessary for E1A-mediated transactivation.