Far upstream sequences are required for efficient transcription from the adenovirus-2 E1A transcription unit

The MLL3/4 H3K4 methyltransferase complex in establishing an active enhancer landscape

Enhancers are cis-regulatory elements that play essential roles in tissue-specific gene expression during development. Enhancer function in the expression of developmental genes requires precise regulation, while deregulation of enhancer function could be the main cause of tissue-specific cancer development. MLL3/KMT2C and MLL4/KMT2D are two paralogous histone modifiers that belong to the SET1/MLL (also named COMPASS) family of lysine methyltransferases and play critical roles in enhancer-regulated gene activation. Importantly, large-scale DNA sequencing studies have revealed that they are amongst the most frequently mutated genes associated with human cancers. MLL3 and MLL4 form identical multi-protein complexes for modifying mono-methylation of histone H3 lysine 4 (H3K4) at enhancers, which together with the p300/CBP-mediated H3K27 acetylation can generate an active enhancer landscape for long-range target gene activation. Recent studies have provided a better understanding of the possible mechanisms underlying the roles of MLL3/MLL4 complexes in enhancer regulation. Moreover, accumulating studies offer new insights into our knowledge of the potential role of MLL3/MLL4 in cancer development. In this review, we summarize recent evidence on the molecular mechanisms of MLL3/MLL4 in the regulation of active enhancer landscape and long-range gene expression, and discuss their clinical implications in human cancers.

Cellular genetic tools to control oncolytic adenoviruses for virotherapy of cancer

Key challenges facing cancer therapy are the development of tumor-specific drugs and the implementation of potent multimodal treatment regimens. Oncolytic adenoviruses, featuring cancer-selective viral cell lysis and spread, constitute a particularly interesting drug platform towards both goals. First, as complex biological agents, adenoviruses allow for rational drug development by genetic incorporation of targeting mechanisms that exert their function at different stages of the viral replication cycle. Secondly, therapeutic genes implementing diverse cancer cell-killing activities can be inserted into the oncolytic adenovirus genome without loss of replication potential, thus deriving a "one-agent combination therapy". This article reviews an intriguing approach to derive oncolytic adenoviruses, which is to insert cellular genetic regulatory elements into adenovirus genomes for control of virus replication and therapeutic gene expression. This approach has been thoroughly investigated and optimized during the last decade for transcriptional targeting of adenovirus replication and gene expression to a wide panel of tumor types. More recently, further cellular regulatory mechanisms, such as mRNA stability and translation regulation, have been reported as tools for virus control. Consequently, oncolytic adenoviruses with a remarkable specificity profile for prostate cancer, gastrointestinal cancers, liver cancer, breast cancer, lung cancer, melanoma, and other cancers were derived. Such specificity profiles allow for the engineering of new generations of oncolytic adenoviruses with improved potency by enhancing viral cell binding and entry or by expressing therapeutic genes. Clearly, genetic engineering of viruses has great potential for the development of innovative antitumor drugs--towards targeted and multimodal cancer therapy.

In vitro evaluation of cancer-specific NF-kappaB-CEA enhancer-promoter system for 5-fluorouracil prodrug gene therapy in colon cancer cell lines

Nuclear factor-kappa B (NF-κB) is a transcription factor with high transcriptional activity in cancer cells. In this study, we developed a novel enhancer–promoter system, κB4-CEA205, in which the basal carcinoembryonic antigen (CEA) promoter sequence (CEA205) was placed downstream of the four tandem-linked NF-κB DNA-binding sites (κB4). In combination with a κB4 enhancer, the transcriptional activity of the CEA promoter was significantly enhanced (three- to eight-fold) in cancer cell lines but not in normal cells. In cancer cell lines, the transcriptional activity of κB4-CEA205 was comparable with that of the SV40 promoter. We also constructed vectors in which the thymidine phosphorylase (TP) cDNA was under the control of CEA205, κB4, κB4-CEA205 and CMV promoters, respectively. TP protein and enzyme activity were detected at comparable levels in κB4-CEA205- and CMV-driven TP cDNA-transfected cancer cell lines (H630 and RKO). The κB4-TP and CEA205-TP-transfected cell lines, respectively, only demonstrated negligible and low levels of TP protein and enzyme activity. Both CMV- and κB4-CEA205-driven TP cDNA transiently transfected cells were 8- to 10-fold sensitised to 5-fluorouracil (5-FU) prodrug, 5′-deoxy-5-fluorouradine (5′-DFUR), in contrast to only 1.5- to 2-fold sensitised by the κB4- and CEA205-driven TP cDNA-transfected cells. The bystander killing effect of CMV- and κB4-CEA205-driven TP cDNA-transfected cells was comparable. This is the first report that indicates that the NF-κB DNA-binding site could be used as a novel cancer-specific enhancer to improve cancer-specific promoter activity in gene-directed enzyme prodrug therapy.

Mechanism of action of a distal NF-kappaB-dependent enhancer

The monocyte chemoattractant protein 1 gene (MCP-1) is regulated by TNF through an NF-kappaB-dependent distal enhancer and an Sp1-dependent promoter-proximal regulatory region. In the silent state, only the distal regulatory region is accessible to transcription factors. Upon activation by tumor necrosis factor, NF-kappaB binds to the distal regulatory region and recruits CBP and p300. CBP and p300 recruitment led to specific histone modifications that ultimately enabled the binding of Sp1 to the proximal regulatory region. During this process, a direct interaction between the distal and proximal regulatory regions occurred. Sp1, NF-kappaB, CBP, and p300 were required for this interaction. CBP/p300-mediated histone modifications enhanced the binding of the coactivator CARM1 to the distal regulatory region. CARM1, which is necessary for MCP-1 expression, was not required for distal-proximal region interactions, suggesting that it plays a later downstream activation event. The results describe a model in which the separation of the distal enhancer from the promoter-proximal region allows for two independent chromatin states to exist, preventing inappropriate gene activation at the promoter while at the same time allowing rapid induction through the distal regulatory region.

Potential limitations of transcription terminators used as transgene insulators in adenoviral vectors

The presence of adenoviral cis-elements interfering with the activity of tissue-specific promoters has seriously impaired the use of transcriptional targeting adenoviruses for gene therapy purposes. As an approach to overcome this limitation, transcription terminators were previously employed in cultured cells to insulate a transgene promoter from viral activation. To extend these studies in vivo, we have injected into heart and skeletal muscle, adenoviruses containing the human growth hormone terminator and the cardiac-specific alpha-myosin heavy chain promoter (alphaMyHC) driving the chloramphenicol acetyltransferase (CAT) reporter gene. Promoterless CAT constructs were also tested to study interfering viral transcription and terminator activity. Here we demonstrate that the presence of a terminator can produce undesirable effects on the activity of heterologous promoters. Our analysis shows that in particular conditions, a terminator can reduce the tissue specificity of the transgene promoter. By RNAse protection assay performed on cardiac myocytes, we also show that adenoviral elements can direct high levels of autonomous transcription within the E1A enhancer region. This finding supports the model that passive readthrough of the transgene promoter is responsible for loss of selective expression.

Insulation from viral transcriptional regulatory elements improves inducible transgene expression from adenovirus vectors in vitro and in vivo

Recombinant adenoviruses (Ad) are attractive vectors for gene transfer in vitro and in vivo. However, the widely used E1-deleted vectors as well as newer generation vectors contain viral sequences, including transcriptional elements for viral gene expression. These viral regulatory elements can interfere with heterologous promoters used to drive transgene expression and may impair tissue-specific or inducible transgene expression. This study demonstrates that the activity of a metal-inducible promoter is affected by Ad sequences both upstream and downstream of the transgene cassette in both orientations. Interference with expression from the heterologous promoter was particularly strong by viral regulatory elements located within Ad sequences nucleotides 1-341. This region is present in all recombinant Ad vectors, including helper-dependent vectors. An insulator element derived from the chicken gamma-globin locus (HS-4) was employed to shield the inducible promoter from viral enhancers as tested after gene transfer with first-generation Ad vectors in vitro and in vivo. Optimal shielding was obtained when the transgene expression cassette was flanked on both sides by HS-4 elements, except for when the HS-4 element was placed in 3'-->5' orientation in front of the promoter. The insulators reduced basal expression to barely detectable levels in the non-induced stage, and allowed for induction factors of approximately 40 and approximately 230 in vitro and in vivo, respectively. Induction ratios from Ad vectors without insulators were approximately 40-fold lower in vitro and approximately 15-fold lower in vivo. This study proves the potential of insulators to improve inducible or tissue-specific gene expression from adenovirus vectors, which is important for studying gene functions as well as for gene therapy approaches. Furthermore, our data show that insulators exert enhancer-blocking effects in episomal DNA.

The NFIII/OCT-1 binding site stimulates adenovirus DNA replication in vivo and is functionally redundant with adjacent sequences

The inverted terminal repeat (ITR) of adenovirus type 5 (Ad5) is 103 bp in length and contains the origin of DNA replication. Cellular transcription factors NFI/CTF and NFIII/OCT-1 bind to sites within the ITR and participate in the initiation of viral DNA replication in vitro. The ITR also contains multiple copies of two conserved sequence motifs that bind the cellular transcription factors SP1 and ATF. We have analyzed a series of viruses that carry deletions at the left terminus of Ad5. A virus carrying a deletion of the NFIII/OCT-1, SP1, and ATF sites within the ITR (mutant dl309-44/107) was wild type for virus growth. However, the deletion of these elements in addition to sequences immediately flanking the ITR (mutant dl309-44/195) resulted in a virus that grew poorly. The analysis of growth parameters of these and other mutants demonstrate that the NFIII/OCT-1 and adjacent SP1 sites augment the accumulation of viral DNA following infection. The function of these elements was most evident in coinfections with a wild-type virus, suggesting that these sites enhance the ability of a limiting trans-acting factor(s), that stimulates viral DNA replication, to interact with the ITR. The results of these analyses indicate functional redundancy between different transcription elements at the left terminus of the Ad5 genome and demonstrate that the NFIII/OCT-1 site and adjacent SP1 site, previously thought to be nonessential for adenovirus growth, play a role in viral DNA replication in vivo.

Redundant elements in the adenovirus type 5 inverted terminal repeat promote bidirectional transcription in vitro and are important for virus growth in vivo

The adenovirus inverted terminal repeat (ITR) contains a number of cis-acting elements that are involved in the initiation of viral DNA replication, as well as multiple binding motifs for the cellular transcription factors SP1 and ATF. In this study, we utilized a Hela cell transcription extract to demonstrate that the adenovirus type 5 ITR promotes bidirectional transcription in vitro. Primer extension analyses demonstrated that the ITR directed transcription at initiation sites both within the terminal repeat and at fixed distances outside of the ITR. The ITR also strongly stimulated transcription at the early region 1A (E1A) initiation site when it was situated immediately upstream of the E1A TATA box region. Deletion and point mutational analyses demonstrated that two distinct cis-acting elements were involved in these ITR-dependent transcriptional activities in vitro. Cellular transcription factors SP1 and ATF were previously shown to bind to these two regions. Analysis of viral mutants in vivo demonstrated that the NFIII/OCT-1 binding site and a conserved ATF motif were important for efficient viral growth. Regulatory elements in the ITR flanking region were found to functionally substitute for these sites.

General repression of enhanson activity by the adenovirus-2 E1A proteins

It has been shown previously that the adenovirus 2 (Ad2) E1A proteins repress activation of transcription by the SV40, polyomavirus and immunoglobulin gene enhancers. Here, we demonstrate that the repression of the SV40 enhancer is not specifically mediated by one of its constituent enhansons and/or proto-enhancers, but that each is subject to repression individually. This inhibitory effect of the E1A proteins is also observed with the AP-1 factor-binding enhansons from the polyomavirus and human metallothionein enhancers, and the MHC class I gene H-2Kb enhanson, which binds the KBF1/H2TF1/TC-IIB protein. Repression by the E1A gene products may, in fact, extend to all enhancer trans-activators, because the transcriptional activities of nuclear receptors (e.g., the estrogen and glucocorticoid receptors), of the yeast enhancer factor GAL4 expressed in HeLa cells, and of chimeric trans-activators (such as GAL-VP16) are all similarly inhibited. The E1A protein domains 2 and 3, including the acidic amino acid stretch that has been shown previously to be necessary for E1A-mediated trans-activation, are not required for repression. These results indicate that the amino-terminal region of the protein, which contains domain 1, plays a crucial role in repression, possibly by interfering in the transcriptional activation process at a step common to all trans-acting enhancer factors.

Cis- and trans-acting factors for transcription of the adenovirus 12 E1A gene

Cis- and trans-acting factors were analyzed for transcription of the adenovirus 12 E1A gene possessing two sites for transcription initiation. These sites are located at nucleotide positions 306 and 445 with respect to the left end of the viral genome as position 1. The template activity of DNAs with various deletions at the 5'-upstream region of the E1A gene was examined in a cell-free system using a nuclear extract of Ehrlich ascites tumor cells. A DNA region specifically stimulating transcription initiated at the site distal to the E1A coding sequence was found located between positions 1 and 166. No DNA sequence affecting transcription from a proximal start-site appeared to be present in the region between positions 1 and 378. DNaseI-footprinting indicated that factors present in the extract bind to two distinct DNA segments, both of which are located within a region stimulating distal transcription. Two footprints were observed, one between positions 19 and 55 and the other between 77 and 94. The former footprint was inhibited by synthetic oligonucleotides containing a sequence recognized by nuclear factor I and the latter contained a sequence similar to one present in the B-enhancer of polyoma virus. Competition of in vitro transcription with synthetic oligonucleotides indicated (a) nuclear factor(s) bound to the region between positions 19 and 55 to be responsible for stimulating distal transcription of the adenovirus 12 E1A gene.

c-myc products trans-activate the adenovirus E4 promoter in EC stem cells by using the same target sequence as E1A products

Using a short-term transfection assay, we show that the E4 early adenovirus promoter is expressed to a certain extent in undifferentiated F9 and PCC4 cells, which are known to possess cellular E1A-like activity. We have also observed that c-myc products trans-activate the E4 promoter in EC stem cells and HeLa cells. Using 5' deletion mutants of the E4 promoter, we show that the same target sequence is used by c-myc and E1A. This sequence is located between positions -179 and -158 upstream of the cap site and is known to contain an activating transcription factor (ATF)-binding site. Moreover, the basal of level of activity of the deletion mutants. is related to the number of ATF binding sites. We therefore suggest that c-myc is a functional cellular homolog of the viral E1A gene and that it might correspond to one of the cellular E1A-like activities previously described for EC stem cells. We have also observed that only a c-myc plasmid coding for both p67 and 64 proteins, in contrast to one coding for p64 only, is able to trans-activate the E4 and E2A adenovirus promoter, suggesting that the p67 protein plays an essential part in activation.

Cyclic AMP induction of early adenovirus promoters involves sequences required for E1A trans-activation

Early in adenovirus infection, the E1A (early region 1A) oncogene products trans-activate the other early viral transcription units, as well as some cellular promoters. The mechanism by which E1A elicits its activity is still unknown. In this report, I show that the adenovirus E2a and E3 promoters are cAMP inducible in rat pheochromocytoma PC12 cells and that this activation requires the presence of the cAMP-dependent protein kinase II. Using deletion mutants of the E2a promoter, it was found that the sequence TACGTCAT located between positions -70 and -77 is involved in both the cAMP response and the E1A trans-activation. Also, in the mutant PC12 cell line A126-2B, which lacks the cAMP-dependent protein kinase II, E1A is still able to activate E2a and E3 promoters. This suggests that E1A products may circumvent the lack of the kinase by activating an alternative signal transduction pathway, which could mimic the effect of agonists of adenylate cyclase. I propose that E1A is capable of modifying by phosphorylation, either directly or indirectly, the transcription factor that binds the ACGTCA motif. Such a factor, termed ATF (adenovirus transcription factor), has already been characterized and appears to have strong similarities to the transcriptional factor CREB (cAMP responsive element binding protein), which binds homologous sequences in cAMP responsive genes, such as somatostatin and c-fos.

Organization of the transcriptional control region of the E1b gene of adenovirus type 5

Genetic analysis of the transcriptional control sequences of the E1b gene of adenovirus type 5 identified two regions that stimulated specific transcription by whole cell extracts from uninfected cells. The first region, located within 50 nucleotides (position -50) 5' to the transcription initiation (cap) site, contains a G+C-rich consensus-binding site (GC box) for the transcription factor Sp1 and a TATA box. Unambiguous stimulatory activity of the second region, between positions -358 and -127, was observed only in the absence of the GC box. DNase I protection experiments (footprinting) with crude nuclear extracts from uninfected cells revealed multiple DNA-protein interactions at the control region. Proximal to the initiation site, both the GC box and the cap site were protected; however, protection of the TATA box was not observed. In the distal region, four protein-binding sites, designated I through IV, were located between positions -250 and -120. Three of the four mapped in protein-coding sequences of the adjacent E1a gene. Sites I and II were 5' to position -218 whereas sites III and IV were 3' to position -218. This finding was consistent with results of the transcriptional analysis indicating that subsets of the distal region were sufficient for stimulation of transcription in vitro in the absence of the GC box. Within the boundaries of site I, a 10-base-pair protected sequence was similar to one located 5' to the adenovirus E1a, E2a, E3, E4, E2 late, and polypeptide IX transcription initiation sites. Sequences within the boundaries of the other three sites were similar to those within other viral and cellular enhancers.

Promoter trans-activation of protooncogenes c-fos and c-myc, but not c-Ha-ras, by products of adenovirus early region 1A

The E1A (early region 1A) oncogene products of adenovirus type 2 trans-activate the other early viral transcription units, as well as some cellular promoters. Using a short-term cotransfection assay in murine NIH 3T3 fibroblasts, we show that c-fos and c-myc promoter activities are stimulated by the E1A proteins, whereas c-Ha-ras transcription is not affected. The product of E1A 13S mRNA is responsible for the trans-activation, whereas the 12S mRNA product has no effect. Analysis of the c-fos promoter sequences required for the E1A stimulation shows that responsive sequences are located between positions -402 and -240 upstream of the transcription initiation site. This same region also contains the c-fos serum-responsive element. Furthermore, transcription of the endogenous c-fos gene in HeLa cells is increased after E1A transfection.

Mutational analysis of the adenovirus E1a gene: the role of transcriptional regulation in transformation

To determine whether the transcription regulatory activities of the adenoviral E1a gene play a role in its ability to transform primary cells we have constructed an extensive series of mutations within the E1a gene. The mutants have been characterized for their ability to transactivate the adenoviral early promoters, repress the transcriptional stimulation of the polyoma virus enhancer, establish primary baby rat kidney cells in culture and cooperate with the activated Ha-ras oncogene in morphologically transforming these cells. The mutant phenotypes reveal that: (i) the two transcription regulatory activities of E1a are separable since essential protein domains map within different regions of the protein; (ii) transactivation is unlikely to contribute significantly to E1a-mediated transformation since several isolated mutants lost the ability to transactivate but were nevertheless efficient at transformation; and (iii) both establishment and oncogene cooperation are linked to enhancer repression suggesting that E1a transforms cells by the repression of a cellular enhancer.

Characterization of a transcriptional promoter of human papillomavirus 18 and modulation of its expression by simian virus 40 and adenovirus early antigens

RNA present in cells derived from cervical carcinoma that contained human papillomavirus 18 genomes was initiated in the 1.053-kilobase BamHI fragment that covered the complete noncoding region of this virus. When cloned upstream of the chloramphenicol acetyltransferase gene, this viral fragment directed the expression of the bacterial enzyme only in the sense orientation. Initiation sites were mapped around the ATG of open reading frame E6. This promoter was active in some human and simian cell lines, and its expression was modulated positively by simian virus 40 large T antigen and negatively by adenovirus type 5 E1a antigen.

Two different cis-active elements transfer the transcriptional effects of both EGF and phorbol esters

Short cis-active sequences of the rat prolactin or Moloney murine leukemia virus genes transfer transcriptional regulation by both epidermal growth factor and phorbol esters to fusion genes. These sequences act in a position- and orientation-independent manner. Competitive binding analyses with nuclear extracts from stimulated and unstimulated cells suggest that different trans-acting factors associate with the regulatory sequence of each gene. A model is proposed suggesting that both epidermal growth factor and phorbol esters stimulate the transcription of responsive genes via discrete classes of hormone-dependent, enhancer-like elements that bind different trans-acting factors, even in the absence of hormone stimulation.

Sequence-independent autoregulation of the adenovirus type 5 E1A transcription unit

The adenovirus E1A gene is known to be autoregulated at the level of transcription. Autoregulation was found to be mediated by products of the E1A 13S mRNA, which induced a fivefold increase in E1A transcription rate. Deletion analysis suggested that the autoregulation did not require any specific sequence in the E1A transcriptional control region. This conclusion was reinforced by the demonstration that a cellular alpha-globin gene substituted for the E1A gene on the adenovirus chromosome was also positively regulated by E1A gene products.

The adenovirus type 5 E1A enhancer contains two functionally distinct domains: one is specific for E1A and the other modulates all early units in cis

The adenovirus type 5 genome contains two distinct enhancer elements located at the left end of the viral chromosome. The first element is repeated and specifically regulates region E1A transcription within infected cells. One copy of this element is sufficient to fully activate E1A transcription in vivo. The second element is located between these repeated sequences and regulates transcription in cis of all early regions on the chromosome. These enhancer elements function independently of each other, and neither element is required for efficient viral DNA replication. Since mutations within the two E1A enhancer components generate different physiological responses, transcriptional enhancement can be achieved through multiple mechanisms.

Negative regulatory sequences in the EIa-inducible enhancer of the adenovirus-2 early EIIa promoter

The adenovirus type 2 early EIIa (EIIaE) transcriptional control region exhibits an EIa-dependent enhancer activity (Imperiale et al., 1985, Proc. Natl. Acad. Sci. USA 82, 381-385). We have determined the sequence requirements for this enhancer activity by analysing the enhancing capacity of the entire EIIa promoter region, or portions of it, when inserted approximately 400 bp upstream of the rabbit beta-globin gene. Globin-specific transcription efficiency from the resulting recombinants was measured after transfection into HeLa cells, both in the presence and absence of the EIa products. It was found that the minimal EIIa element with bidirectional, EIa-dependent enhancer activity extends between -111 and -27 relative to the EIIaE major startsite (+1). Furthermore an extensive deletion analysis revealed, within this element, three functionally distinct regions: a central region between about -90 and -70, corresponding to an essential EIIaE upstream promoter element, and two flanking control elements (about 20 bp each) which, in the absence of the EIa products, exert a negative effect on the enhancer activity. Deletion of either one of these control elements renders the EIIaE enhancer activity constitutive, suggesting that the EIa products stimulate the EIIaE enhancer by relieving the negative control mediated by these sequences.

Sequence-independent autoregulation of the adenovirus type 5 E1A transcription unit

The adenovirus E1A gene is known to be autoregulated at the level of transcription. Autoregulation was found to be mediated by products of the E1A 13S mRNA, which induced a fivefold increase in E1A transcription rate. Deletion analysis suggested that the autoregulation did not require any specific sequence in the E1A transcriptional control region. This conclusion was reinforced by the demonstration that a cellular alpha-globin gene substituted for the E1A gene on the adenovirus chromosome was also positively regulated by E1A gene products.

Transcriptional analysis of the adenovirus-5 EIII promoter: absence of sequence specificity for stimulation by EIa gene products

To identify the adenovirus-5 EIII promoter sequences that are involved in basal level of transcription, a series of promoter deletion mutants were analyzed in vivo by transfection into HeLa cells and in vitro using a HeLa whole cell extract system. Three regions within the EIII promoter were shown to be important for efficient transcription: the TATA sequence, an upstream element centered at -55/-57, and an additional element located between -111 and -233. In vivo transcriptional analysis of EIII promoter deletions in the presence of adenovirus EIa gene products have demonstrated that the same three regions are required for EIa-stimulated transcription. We conclude that there is no sequence element in the EIII promoter between -15 and -233 that is uniquely required for the stimulation of EIII transcription by EIa gene products.

A trans-acting factor is responsible for the simian virus 40 enhancer activity in vitro

Stimulation of in vitro transcription by the simian virus 40 enhancer involves a rapid and stable binding of a trans-acting factor with both the 5'- and 3'-domains of the enhancer sequence. The enhancer factor, which differs from other types of transcriptional factors, can interact with other enhancer elements.

A human hepatitis B viral enhancer element

Fragments of the cloned hepatitis B virus (HBV) genome were assayed in vivo for the presence of a transcriptional enhancer element. We demonstrate that sequences positioned approximately 450 bp upstream from the HBcAg gene promoter are required for its efficient activity. These HBV stimulatory sequences activate transcription when inserted upstream to a heterologous SV40 early promoter. Like other known enhancer elements, this HBV sequence acts in an orientation-independent manner. Furthermore, the HBV enhancer element exhibits a preferred activity in a human hepatoma cell line.

Adenovirus-2 E1A products repress enhancer-induced stimulation of transcription

The adenovirus-2 early region 1A (E1A) products repress activation of transcription induced by the simian virus 40, polyoma virus and adenovirus-2 E1A enhancers. The repression probably involves an interaction between the enhancer elements and a trans-acting factor(s), possibly the E1A products themselves.

Short and long range activation by the SV40 enhancer

Activation of transcription by the SV40 enhancer decreases in an apparently biphasic manner when DNA sequences are interposed between the SV40 enhancer and either the homologous SV40 early or the heterologous conalbumin promoter elements. With increasing lengths of short DNA fragments (up to about 150 bp) activation of transcription decreases to less than 10% of the maximum. This short range effect is observed for both the SV40 early and conalbumin promoter elements and for either orientation of the SV40 enhancer. With the conalbumin promoter, increasing the length of the interposing DNA to 275 bp decreases activation to approximately 4%. Larger inserts, of 650 or 3737 bp, lead to an activation of 0.5%. However, in these recombinants, transcription is still activated at least 10 fold compared to an enhancerless recombinant. The implication of these results is discussed.

Individual products of the adenovirus 12S and 13S EIa mRNAs stimulate viral EIIa and EIII expression at the transcriptional level

Recombinant plasmids containing mutant or wild-type adenovirus serotype 2 EIa genes that produce the 12S mRNA alone, the 13S mRNA alone, or both mRNAs were cotransfected into HeLa cells with plasmids containing the viral EIIa or EIII transcription units. The amount of RNA produced from the EIIa and EIII promoters was increased by the products of both the 13S and the 12S RNAs. By measuring the level of specific transcription in nuclei isolated from transfected cells we directly demonstrate that the increased amount of EIIa RNA is due to stimulation of the rate of transcription.