Multiple transcription factor binding sites mediate adenovirus E1A transactivation

Interleukin-10 Induced Activating Transcription Factor 3 Transcriptional Suppression of Matrix Metalloproteinase-2 Gene Expression in Human Prostate CPTX-1532 Cells

Aberrant expression of the 72-kDa type IV collagenase [matrix metalloproteinase (MMP)-2] is implicated in the invasion and angiogenesis process of malignant tumors. We investigated the effects of interleukin (IL)-10 on MMP-2 expression in CPTX-1532 human prostate tumor cells. Our results demonstrate that IL-10 significantly inhibited MMP-2 transcription and protein expression induced by a phorbol ester, phorbol 12-myristate 13-acetate. The inhibitory effects of IL-10 on MMP-2 expression correlated with the suppression of MMP-2 promoter activity. To determine the mechanism of IL-10 action, we examined IL-10–dependent promoter activity with luciferase constructs from a 2-kbp promoter region of the human MMP-2 gene. We functionally characterized the promoter fragments by transient transfection experiments with CPTX-1532 cells. The experiments revealed that a cAMP responsive element binding protein (CREB) consensus domain was identified upstream of the 5′ transcriptional start site, which was highly responsive to IL-10–dependent down-regulation of promoter luciferase activity. Electrophoretic mobility shift assays combined with antibody “supershift assays” confirmed the data from the luciferase assays. Immunoblot assays of activating transcription factor (ATF) 3 immunoprecipitates with tyrosine specific antibodies revealed that IL-10 stimulated tyrosine phosphorylation of ATF3 to activate binding to the CREB domain and suppress MMP-2 expression. Studies with stable, IL-10 transfected CPTX-1532 subclones further showed that IL-10 failed to suppress MMP-2 expression in ATF3-deficient CPTX-1532 cells, where the ATF3 mRNA was destroyed with a DNAzyme oligonucleotide targeting the 5′ region of the mRNA. Finally, reconstitution of ATF3 successfully restored the inhibitory effects of IL-10 on MMP-2 gene expression. Taken together, these data demonstrate the critical role of tyrosine phosphorylated ATF3 and the CREB consensus domain in IL-10 suppression of MMP-2 gene expression in primary human prostate tumor cells.

The multifunctional role of E1A in the transcriptional regulation of CREB/CBP-dependent target genes

Oncoproteins encoded by the early region 1A (E1A) of adenoviruses (Ads) have been shown to be powerful tools to study gene regulatory mechanisms. As E1A proteins lack a sequence-specific DNA-binding activity, they modulate viral and cellular gene expression by interacting directly with a diverse array of cellular factors, among them sequence-specific transcription factors, proteins of the general transcription machinery, co-activators and chromatin-modifying enzymes. By making use of these factors, E1A affects major cellular events such as cell cycle control, differentiation, apoptosis, and oncogenic transformation. In this review we will focus on the interaction of E1A with cellular components involved in the cAMP/PKA signal transduction pathway and we will discuss the consequences of these interactions in respect to the activation of CREB/CBP-dependent target genes.

Identification of a region on the adenovirus E1A gene responsible for induction by phorbol ester tumor promoter

12-O-Tetradecanoylphorbol-13-acetate (TPA) treatment induces human adenovirus (Ad) early region 1A (E1A) messenger ribonucleic acid expression in infected or Ad-transformed cells. Here, we report that deletion analysis has identified a TPA-responsive element (TRE) in the E1A enhancer region. Deletion analysis indicates that the TRE is located upstream of the E1A cap site between nucleotides -237 and -47. Incubation of extracts from TPA-treated cells with radioactively labeled deoxyribonucleic acid (DNA) fragments containing the TRE (-237 to -47) form specific DNA-protein complexes as demonstrated by gel shift analysis and Southwestern blotting. These experiments provide evidence that novel protein-DNA complexes are formed on a region of the E1A promoter required for TPA-enhanced expression. We speculate that these DNA-binding proteins may interact with the TRE and play a critical role in the mechanism through which TPA upregulates transcription from the Ad E1A gene.

The structure of the site on adenovirus early region 1A responsible for binding to TATA-binding protein determined by NMR spectroscopy

Previous detailed mutational analysis has shown that the binding site on adenovirus (Ad) early region 1A (E1A) for TATA-binding protein (TBP) is located toward the N terminus of conserved region 3 (CR3). Here we demonstrate that synthetic peptides of between 15 and 22 amino acids, identical to amino acid sequences of CR3 present in the larger Ad5 E1A (13 S product) and in both the Ad12 E1A (13 and 12 S products) proteins that lie N-terminal to the zinc finger motif, can disrupt binding of E1A to TBP. These findings suggest that the peptides are biologically active in terms of interacting with TBP and must therefore comprise some, if not all, of the TBP binding site on E1A. The interaction between Ad12 E1A and TBP was confirmed by direct co-precipitation experiments. In 1H NMR studies of CR3 peptides, regular patterns of NOEs were observed from which their conformational preferences in aqueous solution were determined. Both Ad5 and Ad12 peptides were shown to contain regions of helical backbone structure in 50% trifluoroethanol. In each case, the type and intensities of NOE cross-peaks observed correlated best to alpha-helical turns. These helices are more extensive in larger peptides and extend from Glu141 to Val147 and from Arg144 to Pro152 in the full-length Ad5 and Ad12 13S E1A proteins, respectively. The structure of a 19-residue Ad5 CR3 peptide carrying the V147L mutation in the full-length protein that abolishes TBP binding was examined. No significant differences between the substituted and wild type peptides were observed, suggesting that this substitution in the intact protein may cause disruption of global rather than local structures.

Adeno-associated virus Rep78 protein interacts with protein kinase A and its homolog PRKX and inhibits CREB-dependent transcriptional activation

Adeno-associated virus (AAV) is a human parvovirus of the genus Dependovirus. AAV replication is largely restricted to cells which are coinfected with a helper virus. In the absence of a helper virus, the AAV genome can integrate into a specific chromosomal site where it remains latent until reactivated by superinfection of the host cell with an appropriate helper virus. Replication functions of AAV have been mapped to the Rep68 and Rep78 gene products. Rep proteins demonstrate DNA binding, endonuclease, and helicase activities and are involved in regulation of transcription from both AAV and heterologous promoters. AAV has been associated with suppression of oncogenicity in a range of viral and nonviral tumors. In this study we sought to identify and study cellular protein targets of AAV Rep, in order to develop a better understanding of the various activities of Rep. We used the yeast two-hybrid system to identify HeLa cell proteins that interact with AAV type 2 Rep78. We isolated several strongly interacting clones which were subsequently identified as PRKX (previously named PKX1), a recently described homolog of the protein kinase A (PKA) catalytic subunit (PKAc). The interaction was confirmed in vitro by using pMal-Rep pull-down assays. The region of Rep78 which interacts was mapped to a C-terminal zinc finger-like domain; Rep68, which lacks this domain, did not interact with PRKX. PRKX demonstrated autophosphorylation and kinase activity towards histone H1 and a PKA oligopeptide target. Autophosphorylation was inhibited by interaction with Rep78. In transfection assays, a PRKX expression vector was shown to be capable of activating CREB-dependent transcription. This activation was suppressed by Rep78 but not by Rep68. Since PRKX is a close homolog of PKAc, we investigated whether Rep78 could interact directly with PKAc. pMal-Rep78 was found to associate with purified PKAc and inhibited its kinase activity. Cotransfection experiments demonstrated that Rep78 could block the activation of CREB by a PKAc expression vector. These experiments suggest that AAV may perturb normal cyclic AMP response pathways in infected cells.

Structural determinants present in the C-terminal binding protein binding site of adenovirus early region 1A proteins

The C-terminal binding protein (CtBP) has previously been shown to bind to a highly conserved six-amino acid motif very close to the C terminus of adenovirus early region 1A (Ad E1A) proteins. We have developed an enzyme-linked immunosorbent assay that has facilitated the screening of synthetic peptides identical or similar to the binding site on Ad E1A for their ability to bind CtBP and thus inhibit its interaction with Ad12 E1A. It has been shown that amino acids both C-terminal and N-terminal to the original proposed binding site contribute to the interaction of peptides with CtBP. Single amino acid substitutions across the binding site appreciably alter the Kd of the peptide for CtBP, indicative of a marked reduction in the affinity of the peptide for CtBP. The solution structures of synthetic peptides equivalent to the C termini of both Ad5 and Ad12 E1A and two substituted forms of these have been determined by proton NMR spectroscopy. Both the Ad12 and Ad5 peptides dissolved in trifluoroethanol/water mixtures were found to adopt regular secondary structural conformations seen as a series of beta-turns. An Ad12 peptide bearing a substitution that resulted in only very weak binding to CtBP (Ad12 L258G) was found to be random coil in solution. However, a second mutant (Ad12 V256K), which bound to CtBP rather more strongly (although not as well as the wild type), adopted a conformation similar to that of the wild type. We conclude that secondary structure (beta-turns) and an appropriate series of amino acid side chains are necessary for recognition by CtBP.

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.

Adenovirus E4orf4 protein reduces phosphorylation of c-Fos and E1A proteins while simultaneously reducing the level of AP-1

Adenovirus E1A protein and cyclic AMP cooperate to induce transcription factor AP-1 and viral gene expression in mouse S49 cells. We report that a protein encoded within the viral E4 gene region acts to counterbalance the induction of AP-1 DNA-binding activity by E1A and cyclic AMP. Studies with mutant adenoviruses demonstrated that in the absence of E4orf4 protein, AP-1 DNA-binding activity is induced to substantially higher levels than in wild-type virus-infected cells. The induction is the result of increased production of JunB and c-Fos proteins. Hyperphosphorylated forms of c-Fos and E1A proteins accumulate in the absence of functional E4orf4 protein. We propose that the E4orf4 protein acts to inhibit the activity of a cellular kinase that phosphorylates both the E1A and c-Fos proteins. Phosphorylation-dependent alterations in the activity of c-Fos, E1A, or some unidentified protein might, then, lead to decreased synthesis of AP-1 components. This E4 function likely plays an important role in natural infections, since a mutant virus unable to express the E4orf4 protein is considerably more cytotoxic than the wild-type virus.

Upstream box/TATA box order is the major determinant of the direction of transcription

Mammalian gene promoters for transcription by RNA polymerase II are typically organized in the following order: upstream sequence motif(s)/TATA box/initiation site. Here we report studies in which the order, orientation and DNA sequences of these three elements are varied to determine how these affect polarity of transcription. We have constructed promoters with an 'octamer' upstream sequence ATTTGCAT (or its complement ATGCAAAT) in combination with several different TATA boxes and initiation (cap) sites, and tested these promoters in transfection experiments with cultured cells. TATA boxes derived from the adenovirus major late promoter (TATAAAA), immunoglobulin kappa light chain (TTATATA) and heavy chain (TAAATATA) promoter functioned equally well or even better when inverted. Only the beta-globin TATA box (CATAAAA) was poorly active when inverted. In addition, a symmetrical TATA box (TATATATA) derived from a casein gene was very active. Our results suggest that the asymmetry of most TATA boxes (consensus TATAAAA) is not a primary determinant of the polarity of transcription. We also found that the initiation (cap) site, which usually consists of an adenine embedded in a pyrimidine-rich region (PyPyCAPyPyPyPyPy), was permissive towards sequence alterations; even a randomly composed sequence worked well. However, an inverted, hence purine-rich, cap site reduced transcript levels to 1/7th, as did an oligo G sequence. Irrespective of the presence of a cap site, the configuration: 'TATA box/octamer' yielded a strong leftward, rather than rightward transcription. From this, we conclude that the polarity of transcription is primarily determined by the linear order of an upstream sequence relative to a TATA box, rather than by the individual orientations of either of these two elements.

Conversion of the E1A Cys4 zinc finger to a nonfunctional His2,Cys2 zinc finger by a single point mutation

Trans-activation by the adenovirus E1A 289R protein requires a zinc finger defined by Cys-154, Cys-157, Cys-171, and Cys-174. Whereas individually replacing the four cysteine residues with serines resulted in a loss of transactivation, only three of the Cys----Ser mutants (C157S, C171S, and C174S) lost the ability to bind Zn(II). X-ray absorption fine structure analysis revealed that, in the wild-type protein, Zn(II) is coordinated by four cysteine residues whereas in the C154S mutant, Zn(II) is coordinated by two histidines and two cysteines. The mutant protein probably retains, as ligands, two cysteines on the right side of the zinc finger (Cys-171 and Cys-174) and recruits two of the four histidines on the left side (His-149, His-152, His-158, and His-160), despite the presence of Cys-157. This finding may shed light on the general structural requirements of zinc fingers.

trans-dominant mutants of E1A provide genetic evidence that the zinc finger of the trans-activating domain binds a transcription factor

The 289R E1A protein of adenovirus stimulates transcription of early viral and certain cellular genes. trans-Activation requires residues 140 to 188, which encompass a zinc finger. Several studies have indicated that trans-activation by E1A is mediated through cellular transcription factors. In particular, the ability of the trans-dominant E1A point mutant hr5 (Ser-185 to Asn) to inhibit wild-type E1A trans-activation was proposed to result from the sequestration of a cellular factor. Using site-directed mutagenesis, we individually replaced every residue within and flanking the trans-activating domain with a conservative amino acid, revealing 16 critical residues. Six of the individual substitutions lying in a contiguous stretch C terminal to the zinc finger (carboxyl region183-188) imparted a trans-dominant phenotype. trans-Dominance was even produced by deletion of the entire carboxyl region183-188. Conversely, an intact finger region147-177 was absolutely required for trans-dominance, since second-site substitution of every critical residue in this region abrogated the trans-dominant phenotype of the hr5 protein. These data indicate that the finger region147-177 bind a limiting cellular transcription factor and that the carboxyl region183-188 provides a separate and essential function. In addition, we show that four negatively charged residues within the trans-activating domain do not comprise a distinct acidic activating region. We present a model in which the trans-activating domain of E1A binds to two different cellular protein targets through the finger and carboxyl regions.

trans-dominant mutants of E1A provide genetic evidence that the zinc finger of the trans-activating domain binds a transcription factor

The 289R E1A protein of adenovirus stimulates transcription of early viral and certain cellular genes. trans-Activation requires residues 140 to 188, which encompass a zinc finger. Several studies have indicated that trans-activation by E1A is mediated through cellular transcription factors. In particular, the ability of the trans-dominant E1A point mutant hr5 (Ser-185 to Asn) to inhibit wild-type E1A trans-activation was proposed to result from the sequestration of a cellular factor. Using site-directed mutagenesis, we individually replaced every residue within and flanking the trans-activating domain with a conservative amino acid, revealing 16 critical residues. Six of the individual substitutions lying in a contiguous stretch C terminal to the zinc finger (carboxyl region183-188) imparted a trans-dominant phenotype. trans-Dominance was even produced by deletion of the entire carboxyl region183-188. Conversely, an intact finger region147-177 was absolutely required for trans-dominance, since second-site substitution of every critical residue in this region abrogated the trans-dominant phenotype of the hr5 protein. These data indicate that the finger region147-177 bind a limiting cellular transcription factor and that the carboxyl region183-188 provides a separate and essential function. In addition, we show that four negatively charged residues within the trans-activating domain do not comprise a distinct acidic activating region. We present a model in which the trans-activating domain of E1A binds to two different cellular protein targets through the finger and carboxyl regions.

E1A represses wild-type and F9-selected polyomavirus DNA replication by a mechanism not requiring depression of large tumor antigen transcription

Polyomavirus (Py) DNA replication may be regulated to a low-level replication state in specific target cells in mice as well as in certain undifferentiated murine cell lines, such as embryocarcinoma (EC) cells. To investigate possible mechanisms by which such control may occur, we have examined the effects of E1A on Py DNA replication. Adenovirus E1A proteins repress transcriptional activation of various enhancers, including those of Py, and can stimulate DNA replication in quiescent cells, but E1A effects on Py DNA replication were unknown. We found that constitutive E1A expression in NIH 3T3 cells depressed Py DNA replication very strongly. Two F9 EC cell-selected Py enhancer variants, PyF441 and PyF101, were also examined because undifferentiated EC cells are hypothesized to have an E1A-like activity responsible for the Py restriction, and these variants activate Py DNA replication in cis in undifferentiated F9 cells. Both variants were repressed by E1A, indicating that E1A activity in 3T3 cells is not equivalent to undifferentiated F9 cell E1A-like activity. We also examined transient inducible E1A expression in cells supplying Py large tumor antigen (T-Ag). Py DNA replication was again repressed, and the inhibition increased with E1A induction. Analysis of T-Ag mRNA levels indicated that E1A repression of Py DNA replication was not an indirect result of depression of T-Ag transcription. This suggests that E1A may repress Py DNA replication by a more direct mechanism, possibly by blocking enhancer activation of DNA replication in a manner uncoupled with enhancer transcriptional control.

Isolation and characterization of insertion mutants in E1A of adenovirus type 5

We have constructed a series of insertion mutations at 18 sites in the coding sequences of early region 1A (E1A) of human adenovirus type 5 (Ad5). At each site we have introduced three types of mutation: a 39-bp insertion specifying a 13-aa residue oligopeptide, a 39-bp insertion containing chain termination codons in all three reading frames, and a "collapsed" insert of 6-bp forming a conventional linker insertion mutation. All mutants were sequenced to determine the precise location, structure, and orientation of the inserts. The mutants were assayed for their abilities to trans-activate and to repress using transient expression assays in HeLa cells cotransfected with the E1A mutant plasmids and a reporter plasmid containing the bacterial beta-galactosidase (lac Z) gene under the control of Ad5 early promoters. The mutants were also tested for their ability to transform baby rat kidney cells in cooperation with either E1B or the ras oncogene. Each mutant was rescued into virus and infectivity was compared in HeLa and 293 cells. In addition, E1A protein synthesis was analyzed in cells infected with the mutant viruses and the insertions were found to have pronounced but unpredictable effects on electrophoretic mobility of E1A proteins in SDS-polyacrylamide gels. The results of functional assays indicated that only mutations mapping in, or deleting, the unique region of the 13 S mRNA product had any effect on ability to trans-activate and that a perfect correlation existed between ability of a mutant to trans-activate and to replicate efficiently in HeLa cells or to transform baby rat kidney cells in an E1A plus E1B mediated assay. In contrast, insertions near conserved region 2 of exon I and in the NH2-terminal portion of exon II significantly reduced repression activity but left transforming activity with E1B or with ras essentially unaffected suggesting that the repression function of E1A is separate from, or at least nonessential in, transformation.

Adenovirus E1A protein activates transcription of the E1A gene subsequent to transcription complex formation

The mechanism of transcriptional activation of the adenovirus E1A and E3 genes by E1A protein during infection was examined by using transcription-competition assays. Infection of HeLa cells with one virus led to inhibition of mRNA accumulation from a superinfecting virus. Synthesis of the E1A 289R protein by the first virus to infect reduced inhibition of transcription of the superinfecting virus, indicating that the E1A 289R protein was limiting for E1A-activated transcription. Infection with an E1A- virus, followed 6 h later by superinfection with a wild-type virus, led to preferential transcriptional activation of the E1A gene of the first virus, suggesting that a host transcription component(s) stably associated with the E1A promoter in the absence of E1A protein and that this complex was the substrate for transcriptional activation by E1A protein. The limiting host transcription component(s) bound to the E1A promoter to form a complex with a half-life greater than 24 h in the absence of E1A 289R protein, as demonstrated in a challenge assay with a large excess of superinfecting virus. In the presence of the E1A 289R protein, the E1A gene of the superinfecting virus was gradually activated with a reduction in E1A mRNA accumulation from the first virus. The kinetics of the activation suggest that this was due to an indirect effect rather than to destabilization of stable transcription complexes by the 289R protein.

Nucleotide sequence and regulation of the adenovirus type 3 E2A early promoter

The nucleotide sequence of the adenovirus serotype 3 E2A early promoter has been determined. In contrast to Ad2, the Ad3 E2A early promoter possessed only one TATA-like box and one nuclear transcription factor E2F binding site and lacked the silencer sequences; however, as in Ad2, the ATF binding site was present. Moreover, the Ad3 E2A promoter harbored a protein binding sequence recognized by the SP1 factor. By transient expression analysis in HeLa cells, we demonstrated that the E1A gene products of Ad3 and Ad2 stimulated Ad3 E2A transcription. In competition experiments, the Ad3 E2A promoter was used in preference to the Ad2 E2A promoter.

Cellular transformation by E1 genes of enteric adenoviruses

The ability of Ad40 and Ad41 E1A plus E1B genes to transform BRK cells was considerably lower than that of Ad5 and Ad12 corresponding genes. However, as for Ad5, the E1A genes of enteric adenoviruses could cooperate with an activated ras oncogene for full cell transformation and the Ad41 E1B could be complemented by E1A gene of Ad5 or Ad12 for cell transformation. Complementation studies suggested that the conserved region 1 of Ad41 E1A was responsible for this inefficient transformation. The Ad40- and Ad41-transformed cell lines exhibited a low level of major histocompatibility complex (MHC) class I antigens correlated to the low level of Ad12-transformed cells. Class I MHC antigen amounts expressed at the surface of the cells transformed by the weakly oncogenic Ad3 were between the high level of Ad5- and the low level of Ad12-transformed cells.

E1A gene products stimulate in vitro transcription from the adenovirus early region 4 promoter by enhancing a stable preinitiation complex

A cell-free transcription system using nuclear extracts prepared from adenovirus type 5 (Ad5)-infected and mock-infected cells were utilized to study the E1A-mediated transactivation of transcription from the Ad5 early region 4 (E4) promoter. The transcription activity of Ad-infected cell extracts was several times higher than that of mock-infected cell extracts when the E4 wild-type (WT) promoter was used. The increased activity required expression of E1A proteins. However, the transcription activities of both cell extracts were not different when the mutant promoter containing only the E4 TATA box was used. The formation of a stable preinitiation complex at the E4 WT promoter was facilitated by expression of E1A, whereas it was not facilitated at the mutant promoter. The results suggested that the E1A proteins stimulated transcription from the E4 promoter by enhancing the formation of a stable preinitiation complex through the upstream elements, together with the TATA box.

E1a-dependent expression of adenovirus genes in OTF963 embryonal carcinoma cells: role of E1a-induced differentiation

Some undifferentiated F9 embryonal carcinoma cells allow adenovirus genes to be expressed independently of the E1a oncogene normally required for their activation; this has been attributed to a cellular equivalent of E1a in F9 cells. However, transcription of all early genes was low in undifferentiated OTF963 embryonic carcinoma cells during the first 48 hr after infection with adenovirus type 5 (Ad5). Transcription then increased to about the level seen 16 hr after infection of cells induced to differentiate by retinoic acid (RA) (referred to as RA-dF9 cells), but this increase did not occur in cells infected by the E1a deletion mutant dl312. Addition of E1a in trans, or of RA, had no immediate effect on viral transcription in OTF963 cells, but viral transcription increased about 48 hr after these additions. Ad5 induced transcription of several differentiation-specific genes in OTF963 cells with about the same kinetics as their induction by RA. These genes were superinduced in RA-dF9 cells by cAMP or infection by adenovirus. We suggest the small amount of E1a produced early in infection of OTF963 cells activates cellular genes, some of which are differentiation specific and required for efficient transcription of viral genes, so that E1a both induces and is induced by differentiation. The simple hypothesis of a cellular equivalent to E1a does not adequately explain the complex interactions between viral and cellular genes in OTF963 embryonic carcinoma cells.

Immediate early protein of pseudorabies virus is a general transactivator but stimulates only suboptimally utilized promoters. A clue to specificity?

Pseudorabies virus, a herpesvirus, encodes an immediate early (IE) protein that is known to be a general and strong transactivator of transcription. We have tested the activity of this IE protein with a set of well-defined promoters containing a TATA box and one type of upstream factor binding site (for Sp1, NF-kappa B, heavy metal responsive factors, octamer factors or glucocorticoid receptor). All promoters were strongly activated by IE protein, i.e. the IE protein did not preferentially activate transcription via a particular type of upstream element. Activation did not require a bona fide TATA box, since a promoter construct with three Sp1 sites but no TATA box was also activated. Our data are not compatible with a model in which IE protein would bypass the need for upstream factors. Rather, the properties of IE protein, especially a failure to induce strong transcription from a promoter with only a TATA box but no upstream sequences, mimic the action of a remotely placed, cis-active, enhancer DNA. The IE protein was found to have no effect on transcription units that are expressed to their maximal potential, irrespective of whether this was high or low. Such optimal transcription conditions are observed in the presence of a strong enhancer, or with multiple tandem copies of an upstream binding site and/or a high concentration of the corresponding factor. The property of stimulating only "suboptimally" utilized promoters may be exploited by pseudorabies virus to restrict the specificity of the IE protein to the viral early promoters and a subset of cellular promoters.

The human cytomegalovirus 2.7-kilobase RNA promoter contains a functional binding site for the adenovirus major late transcription factor

We have examined the factors which influence the expression of a major 2.7-kilobase (kb) early transcript encoded by the long repeat of the human cytomegalovirus (HCMV) strain AD169 genome. Previously, by deletion analysis, we determined that the promoter for this early RNA consisted of multiple cis-acting elements (Klucher et al., J. Virol. 63:5334-5343, 1989). Using extracts prepared from HeLa cells as well as from infected and uninfected foreskin fibroblasts, we also obtained evidence for the interaction of a cellular factor with one of these elements. In this study, we have further defined the specificity and functional importance of this binding. On the basis of DNase I footprinting and methylation interference assays, we localized the site of interaction to a region (nucleotides -113 to -106 relative to the mRNA start site) which contains homology to the binding site for the adenovirus major late transcription factor (MLTF), also referred to as the upstream stimulatory factor (USF). The contact points of binding between the cellular factor and the guanine residues within this segment were consistent with the pattern of binding for USF/MLTF. Additionally, by using oligonucleotides containing the binding sites for USF/MLTF from the adenovirus major late promoter and the HCMV 2.7-kb RNA promoter as competitors in gel retardation assays, we were able to show that USF/MLTF bound to the two promoters with similar affinity. Correlation of the binding activity with in vivo functional importance was provided by mutagenesis and transient-expression assays. A point mutation within the HCMV USF/MLTF site lowered the affinity of binding 5- to 10-fold and decreased the inducible activity of the HCMV 2.7-kb RNA promoter by approximately 50%. Furthermore, the addition of the HCMV USF/MLTF site to a minimal 2.7-kb RNA promoter containing only the TATA sequence resulted in an increase in HCMV inducible transcriptional activity of 6- to 20-fold. However, the HCMV USF/MLTF site could not functionally substitute for the TATA sequence. These studies further support the idea that for maximal response to the HCMV infection, the 2.7-kb RNA promoter requires multiple cis-acting sequences, two of which include the binding sites for USF/MLTF and TFIID.

The adenovirus EII early promoter has multiple EIA-sensitive elements, two of which function cooperatively in basal and virus-induced transcription

The mechanism by which the adenovirus-encoded nuclear oncogene EIA activates transcription of several viral and host promoters is an important issue in the regulation of eucaryotic gene expression and virus-host cell interactions. Identification of cis-acting elements of the promoters and the cognate host transcription factors that are targets for EIA action is crucial for our understanding of the EIA-mediated control of coordinately regulated genes. The adenovirus EII early promoter has a complex architecture and contains two overlapping promoters with start sites at +1 (major promoter) and -26 (minor promoter). The major promoter responds strongly to virus-encoded trans activators EIA and EIV and contains four elements: a TAGA motif analogous to the TATA box, two EIIF sites present in an inverted orientation, and an ATF/CREB site. To determine precisely the roles played by these cis-acting elements in both basal and virus-induced transcription when the promoter is situated in its natural context, we investigated the phenotype of a series of linker scan promoter substitution mutants inserted into the viral chromosome. Promoter constructs harboring linker scan mutations in each element were rebuilt into a novel EIA- adenovirus vector, and transcriptional activity was monitored in virus-infected cells. In the absence of virus-encoded trans activators, basal activity in vivo was dependent on all four cis-acting elements. Surprisingly, a promoter mutant with only one of the two EIIF sites intact could not promote transcription in vivo, suggesting that the two EIIF sites function cooperatively even in basal transcription. Promoters harboring mutations in either of these two EIIF sites also failed to bind to an infection-specific form of EIIF in gel shift assays and competed only very weakly for EIIF binding with the wild-type promoter fragment. The dramatic cooperativity shown by the two inverted EIIF sites of the EII promoter both in vivo and in vitro could reflect simultaneous contact of both sites by the transcription factor EIIF. Furthermore, promoter mutants with mutations in the TAGA motif, the two EIIF sites, and the single ATF site all failed to respond to virus-encoded trans activators. Whereas recent results demonstrate that EIIF activity can be modulated independently by EIV, leading to transactivation of this promoter, our results and those published previously strongly indicate that the three different transcription factors that bind to TAGA, EIIF, and ATF motifs of the EII early promoter are all targets for EIA regulation in vivo. Thus, strong transactivation of the EII early promoter through these multiple EIA-sensitive elements and independently by the recently discovered EIV pathway suggests that the EII early promoter is stringently regulated in virus-infected cells.(ABSTRACT TRUNCATED AT 400 WORDS)

cis-dominant defect in activation of adenovirus type 5 E1b early RNA synthesis

A cis-dominant mutation in the adjacent E1a gene disrupted the accumulation of adenovirus type 5 E1b mRNA during the early phase of infection. Steady-state levels of cytoplasmic and nuclear E1b RNAs in cells infected with dl312, a strain that lacks the E1a TATA box, cap site, and much of the coding sequence, were reduced 5- to 10-fold even when the E1a activator was provided in trans. The strain was defective for early E1b RNA synthesis but not for E1b RNA made late or during prolonged incubation in the presence of an inhibitor of DNA replication. The defect in E1b RNA synthesis could not be attributed to the E1a promoter sequences missing in dl312 DNA. If the E1a protein-coding region contains cis-acting regulatory sequences, they are not part of the previously mapped E1b transcriptional control region and may represent additional regulatory elements that ensure prompt and efficient E1b expression during the early phase of infection.

Two distinct cellular proteins interact with the EIa-responsive element of an adenovirus early promoter

EIa-dependent transactivation of the adenovirus EIIa early (EIIaE) promoter is correlated with the activation of the cellular transcription factor E2F. In this study we identified a cellular protein, C alpha, that is distinct from E2F and that binds two sites in the EIIaE promoter, one of which overlaps with the proximal E2F binding site of the EIIaE promoter. The possible involvement of C alpha in the EIa responsiveness of this promoter is discussed.

Adenovirus early region 3 promoter regulation by E1A/E1B is independent of alterations in DNA binding and gene activation of CREB/ATF and AP1

Transcription of the adenovirus early region 3 promoter is strongly induced by the adenovirus E1A protein. Previous DNase I footprinting has indicated that four regions in this promoter serve as binding sites for HeLa nuclear proteins. These include binding sites for NF-1 (site IV), AP1 (site III), CREB/activating transcription factor (ATF) (site II), and TATA (site I). To determine the relative importance of these sites in both the in vivo and in vitro transcriptional regulation of the E3 promoter, oligonucleotide-directed mutagenesis of these sites was performed. Each of these constructs was assayed by transfection onto HeLa cells in the presence of either dl434, an E1A/E1B deletion mutant, or wild-type adenovirus. Mutations of either the ATF- or AP1-binding sites but not the TATA- and NF1-binding sites resulted in severe decreases in both basal and E1A/E1B-induced transcriptional levels. These constructs were also assayed in in vitro transcription assays with cellular extracts prepared from dl434-infected or wild-type-adenovirus-infected HeLa cells. The wild-type E3 promoter was transcribed approximately 30 times more efficiently in extracts containing the E1A/E1B proteins compared with extracts lacking these proteins. Mutations of either the TATA element, the ATF site, or the AP1-binding site decreased both basal and E1A/E1B-induced transcriptional levels. Gel retardation analysis using these extracts indicated that the binding to ATF, AP1, or NF1 oligonucleotides was not altered in the presence of the E1A/E1B proteins compared with extracts lacking these proteins. Northern (RNA) blot analysis of c-jun and CREB RNA prepared from wild-type adenovirus and dl434-infected cells indicated that the levels of these RNAs were not altered by the E1A/E1B proteins. Immunoprecipitation of AP1 and CREB from both dl434- and wild-type-adenovirus-infected cells indicated that the amounts of these proteins were not significantly altered. These results suggest that E1A/E1B-induced activation of the E3 promoter does not involve activation of transcription factor genes nor a change in the DNA binding activity of important promoter-binding components. Our results are consistent with a model in which the E1A/E1B proteins either directly or indirectly alter the interactions of factors that bind to the basal E3 promoter transcription complex, thereby inducing transcription.