Direct interaction between Sp1 and the BPV enhancer E2 protein mediates synergistic activation of transcription

Pdha-2: a model for studying transcriptional regulation in early spermatocytes

Precise temporal and tissue-specific expression of genes during spermatocyte differentiation is crucial for the formation of functional spermatozoa. However, the mechanisms that regulate gene expression during spermatogenesis are poorly understood. One testis-specific gene, Pdha-2, is beginning to emerge as a potentially important model for the study of these events. This review focuses on our current understanding of the expression and regulation of Pdha-2 during spermatogenesis.

trans activation by the full-length E2 proteins of human papillomavirus type 16 and bovine papillomavirus type 1 in vitro and in vivo: cooperation with activation domains of cellular transcription factors

Papillomaviral E2 genes encode proteins that regulate viral transcription. While the full-length bovine papillomavirus type 1 (BPV-1) E2 peptide is a strong trans activator, the homologous full-length E2 product of human papillomavirus type 16 (HPV-16) appeared to vary in function in previous studies. Here we show that when expressed from comparable constructs, the full-length E2 products of HPV-16 and BPV-1 trans activate a simple E2- and Sp1-dependent promoter up to approximately 100-fold in human keratinocytes and other epithelial cells as well as human and animal fibroblasts. Vaccinia virus-expressed, purified full-length HPV-16 and BPV-1 E2 proteins bound a consensus E2 site with high specific affinities (Kd = approximately 10(-9) M) and stimulated in vitro transcription up to six- to eightfold. In vivo and in vitro trans activation by either E2 protein required cooperation with another activator, such as Sp1, or other factors that interact with papillomavirus promoters, such as AP-1, Oct-1, nuclear factor 1/CTF, transcriptional enhancer factor 1, or USF. The glutamine-rich domain B of Sp1 or the mutually unrelated activation domains of other transcription factors were necessary and sufficient for cooperation with either E2 factor. We conclude that like BPV-1 E2, the HPV-16 E2 protein has the potential to function as a strong activator of viral gene expression in cooperation with cellular transcription factors.

The human papillomavirus type 16 E2 transcription factor binds with low cooperativity to two flanking sites and represses the E6 promoter through displacement of Sp1 and TFIID

The E6 promoters of all genital human papillomaviruses have a characteristic alignment of transcription factor binding sites. Activation of the basic transcription complex at the TATA box depends upon a sequence-aberrant Sp1 site. Repression of E6 promoters is achieved by two binding sites for the viral E2 protein positioned between the Sp1 site and the TATA box. We have purified the human papillomavirus type 16 E2 protein after expression in Escherichia coli and studied its binding and repression properties with oligonucleotides representing the homologous promoter sequences. A Kd value of 3 x 10(-10) M indicated binding properties expected for a native protein. We found low cooperativity in the binding of two E2 dimers to flanking sites, both when these sites were separated by 3 nucleotides, as in the natural promoter, and when they were further apart. E2 protein, bound close to the distal Sp1 site, displaced the Sp1 factor even when the aberrant sequence was replaced by a typical Sp1 core recognition site. The high affinity of E2 protein for its binding site even led to Sp1 displacement at concentrations of E2 protein nearly 2 orders of magnitude lower than those of Sp1. Functional analyses of mutated E6 promoter sequences showed repression by this distal E2 binding site in the complete absence of binding to the proximal E2 binding site. From our findings and observations published by others, we conclude that each of the E2 binding sites in the E6 promoter of genital human papillomaviruses plays a separate role by displacing the transcription factors Sp1 and TFIID.

A critical role for chromatin in mounting a synergistic transcriptional response to GAL4-VP16

The role of chromatin in mounting a synergistic transcriptional response to GAL4-VP16 was investigated. Strong synergy was observed when chromatin templates were used in vitro. The synergy was severely reduced when naked DNA templates were transcribed. In vivo synergy was strong when nonreplicating templates were used. However, the use of replicating templates, which involved transient disruptions of chromatin, led to strong reductions in synergy. In both of these low-synergy responses, transcription levels were high. We infer that strong synergy has a requirement for chromatin that may be understood in terms of the competition between multiple activator molecules and histone cores for promoter DNA.

A critical role for chromatin in mounting a synergistic transcriptional response to GAL4-VP16

The role of chromatin in mounting a synergistic transcriptional response to GAL4-VP16 was investigated. Strong synergy was observed when chromatin templates were used in vitro. The synergy was severely reduced when naked DNA templates were transcribed. In vivo synergy was strong when nonreplicating templates were used. However, the use of replicating templates, which involved transient disruptions of chromatin, led to strong reductions in synergy. In both of these low-synergy responses, transcription levels were high. We infer that strong synergy has a requirement for chromatin that may be understood in terms of the competition between multiple activator molecules and histone cores for promoter DNA.

The retinoblastoma gene product RB stimulates Sp1-mediated transcription by liberating Sp1 from a negative regulator

Studies have demonstrated that the retinoblastoma susceptibility gene product, RB, can either positively or negatively regulate expression of several genes through cis-acting elements in a cell-type-dependent manner. The nucleotide sequence of the retinoblastoma control element (RCE) motif, GCCACC or CCACCC, and the Sp1 consensus binding sequence, CCGCCC, can confer equal responsiveness to RB. Here, we report that RB activates transcription of the c-jun gene through the Sp1-binding site within the c-jun promoter. Preincubation of crude nuclear extracts with monoclonal antibodies to RB results in reduction of Sp1 complexes in a mobility shift assay, while addition of recombinant RB in mobility shift assay mixtures with CCL64 cell extracts leads to an enhancement of DNA-binding activity of SP1. These results suggest that RB is directly or indirectly involved in Sp1-DNA binding activity. A mechanism by which RB regulates transactivation is indicated by our detection of a heat-labile and protease-sensitive Sp1 negative regulator(s) (Sp1-I) that specifically inhibits Sp1 binding to a c-jun Sp1 site. This inhibition is reversed by addition of recombinant RB proteins, suggesting that RB stimulates Sp1-mediated transactivation by liberating Sp1 from Sp1-I. Additional evidence for Sp1-I involvement in Sp1-mediated transactivation was demonstrated by cotransfection of RB, GAL4-Sp1, and a GAL4-responsive template into CV-1 cells. Finally, we have identified Sp1-I, a approximately 20-kDa protein(s) that inhibits the Sp1 complexes from binding to DNA and that is also an RB-associated protein. These findings provide evidence for a functional link between two distinct classes of oncoproteins, RB and c-Jun, that are involved in the control of cell growth, and also define a novel mechanism for the regulation of c-jun expression.

Multiple DNA elements are required for the growth regulation of the mouse E2F1 promoter

To prepare for the DNA synthesis (S) phase of the cell cycle, transcription of many genes required for nucleotide biosynthesis increases. The promoters of several of these genes contain binding sites for the E2F family of transcription factors, and, in many cases, mutation of these sites abolishes growth-regulated transcription. The RNA levels of one family member, E2F1, increase about 15-fold at the G1/S-phase boundary and expression of E2F1 in quiescent cells activates transcription from some G1/S-phase-specific promoters, suggesting that E2F1 plays a critical role in preparing cells to enter S phase. To elucidate the signal transduction pathway leading to the activation of genes required for DNA synthesis, we are investigating the mechanism by which expression of E2F1 is regulated. To determine whether levels of E2F1 mRNA are controlled by changes in promoter activity, we have cloned and characterized the mouse E2F1 promoter. Sequence analysis revealed two sets of overlapping E2F-binding sites located between -12 and -40 relative to the transcription initiation site. We show that these sites bind cellular E2F and that an E2F1 promoter fragment can be activated up to 100-fold by coexpression of E2F proteins. We also show that the activity of this E2F1 promoter fragment increases approximately 80-fold at the G1/S-phase boundary and that this activation is, in part, regulated by G0-specific repression via the E2F sites. However, the E2F sites are not sufficient to mediate growth-regulated transcriptional activity; our results indicate that multiple DNA elements are required for transcription regulation of the E2F1 promoter at the G1/S-phase boundary.

The retinoblastoma gene product RB stimulates Sp1-mediated transcription by liberating Sp1 from a negative regulator

Studies have demonstrated that the retinoblastoma susceptibility gene product, RB, can either positively or negatively regulate expression of several genes through cis-acting elements in a cell-type-dependent manner. The nucleotide sequence of the retinoblastoma control element (RCE) motif, GCCACC or CCACCC, and the Sp1 consensus binding sequence, CCGCCC, can confer equal responsiveness to RB. Here, we report that RB activates transcription of the c-jun gene through the Sp1-binding site within the c-jun promoter. Preincubation of crude nuclear extracts with monoclonal antibodies to RB results in reduction of Sp1 complexes in a mobility shift assay, while addition of recombinant RB in mobility shift assay mixtures with CCL64 cell extracts leads to an enhancement of DNA-binding activity of SP1. These results suggest that RB is directly or indirectly involved in Sp1-DNA binding activity. A mechanism by which RB regulates transactivation is indicated by our detection of a heat-labile and protease-sensitive Sp1 negative regulator(s) (Sp1-I) that specifically inhibits Sp1 binding to a c-jun Sp1 site. This inhibition is reversed by addition of recombinant RB proteins, suggesting that RB stimulates Sp1-mediated transactivation by liberating Sp1 from Sp1-I. Additional evidence for Sp1-I involvement in Sp1-mediated transactivation was demonstrated by cotransfection of RB, GAL4-Sp1, and a GAL4-responsive template into CV-1 cells. Finally, we have identified Sp1-I, a approximately 20-kDa protein(s) that inhibits the Sp1 complexes from binding to DNA and that is also an RB-associated protein. These findings provide evidence for a functional link between two distinct classes of oncoproteins, RB and c-Jun, that are involved in the control of cell growth, and also define a novel mechanism for the regulation of c-jun expression.

Regulation of DNA synthesis in division-arrested mouse C127 cells permissive for bovine papillomavirus DNA amplification

Spontaneous amplification of bovine papillomavirus type 1 DNA occurs following a prolonged period of serum starvation of wild-type virus-transformed C127 cell lines and is associated with abundant viral E2 protein synthesis and a concomitant induction of viral oncogene (E5 and E6) expression. We show here that a subpopulation of the permissive cells incorporate bromo-deoxyuridine under conditions of cell growth arrest (serum starvation), whereas DNA synthesis is suppressed in the resting population of nonpermissive cells. Flow cytometric measurements of the cellular DNA content of the permissive cell population indicated that it contained predominantly a 4n DNA content, suggesting that these cells were blocked in the G2 phase of the cell cycle. In keeping with the hypothesis that viral DNA amplification is associated with the induction of a cellular S phase, we observed a specific induction of expression of two cell proliferation-related cellular antigens (PCNA and Ki67) in a subpopulation of permissive cells. C127 cell lines transformed by an E5-minus bovine papillomavirus type 1 mutant, which was competent for autonomous plasmid replication in mitotic cells, were completely defective for the induction of DNA synthesis and mutant viral DNA amplification under conditions of serum starvation. Moreover, the E5 protein is shown by immunofluorescence analysis to be expressed at a high level specifically in the permissive cell population. These results imply a dual role for the viral E5 protein in the C127 model system, both as a transforming protein and as a factor required for the induction of viral DNA amplification in postmitotic cells. We suggest that E5 acts at an early step in the induction of this process in C127 cells and may be required to turn on host cell DNA synthesis as a prerequisite for viral DNA amplification.

Specificity determinants for the interaction of lambda repressor and P22 repressor dimers

The related phage lambda and phage P22 repressors each bind cooperatively to adjacent and separated operator sites, an interaction that involves a pair of repressor dimers. The specificities of these interactions differ: Each dimer interacts with its own type but not with dimers of the heterologous repressor. The two repressors exhibit significant amino acid sequence homology in their carboxy-terminal domains, which are responsible for both dimer formation and the dimer-dimer interaction. Here, we identify a collection of amino acid substitutions that disrupt the protein-protein interaction of DNA-bound lambda repressor dimers and show that several of these substitutions have the same effect when introduced at the corresponding positions of P22 repressor. We use this information to construct a variant of the lambda repressor bearing only six non-wild-type amino acids that has a switched specificity; that is, it binds cooperatively with P22 repressor, but not with wild-type lambda repressor. These results identify a series of residues that determine the specificities of the two interactions.

Identification and characterization of a human herpesvirus 6 gene segment capable of transactivating the human immunodeficiency virus type 1 long terminal repeat in an Sp1 binding site-dependent manner

The human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) is transactivated by various extracellular signals and viral cofactors that include human herpesviruses. These transactivators are capable of transactivating the HIV-1 LTR through the transactivation response element, NF-kappa B, or other regulatory binding elements. Human herpesvirus 6 (HHV-6) is a potential cofactor of HIV-1. Here, we report that an HHV-6 gene segment, ZVH14, which can neoplastically transform NIH 3T3 and human keratinocytes, is capable of transactivating HIV-1 LTR chloramphenicol acetyltransferase constructs in an Sp1 binding site-dependent manner. Transactivation increased synergistically in the presence of multiple Sp1 sites and was dramatically reduced by cotransfection with oligomers designed to form triplex structures with HIV-1 LTR Sp1 binding sites. HIV-1 LTR NF-kappa B sites were not essential for ZVH14-mediated transactivation. A putative open reading frame in ZVH14, B115, which may encode a highly basic peptide consisting of 115 amino acid residues, showed transactivation capacity similar to that of ZVH14. This open reading frame also transactivated the HIV-1 LTR in an Sp1 site-dependent fashion in African green monkey kidney cells and human T cells. These data suggest that HHV-6 may stimulate HIV-1 replication via transactivation of Sp1 binding sites present in the HIV-1 promoter.

Human papillomavirus type 11 E2 proteins repress the homologous E6 promoter by interfering with the binding of host transcription factors to adjacent elements

The E6 promoter of human papillomaviruses (HPVs) trophic for epithelia for the lower genital tract and the upper respiratory tract is regulated in vitro by homologous and heterologous papillomaviral E2 proteins that bind to a consensus responsive sequence (E2-RS) ACCN6GGT. When HPV type 11 (HPV-11) expression is examined in epithelial cell lines, the HPV-11 E2-C protein, which lacks the amino-terminal transactivating domain of the full-length E2 protein, invariably represses the homologous viral E6 promoter. In contrast, when the novel constitutive enhancer (CE) CE II is deleted, not only is the basal promoter activity much reduced, it is further repressed by the intact HPV-11 E2 protein (M. T. Chin, T. R. Broker, and L. T. Chow, J. Virol. 63:2967-2976, 1989). Here, we demonstrated that, when expressed from a stronger surrogate promoter, the HPV-11 E2 protein represses the E6 promoter effectively, regardless of CE II. By performing systematic mutational analyses of the four highly conserved copies of the HPV-11 E2-RS and of the adjacent enhancer-promoter elements, we show that the furthest upstream, promoter-distal E2-RS copy 1 plays no apparent role in E6 promoter regulation. Repression by the homologous HPV-11 E2 proteins is mediated through each of the three promoter-proximal copies of the E2-RS, but the presence of CE II abrogates the full-length E2 protein repression exerted at E2-RS copy 2. Repression is alleviated when the two (for E2) or three (for E2-C) promoter-proximal copies of E2-RS are mutated. We specifically demonstrate that repression exerted at E2-RS 3 is due to preclusion of binding of the host transcription factor Sp1 or Sp1-like proteins to a nonconsensus sequence AGGAGG located 1 bp upstream of the tandem E2 protein binding sites 3 and 4. A 3-bp insertion between the adjacent Sp1 and E2-RS 3 sites permits both Sp1 and E2 proteins to bind, with a concomitant relief of E2-RS 3-mediated repression. Similar mutational analyses show that proteins that bind to the GT-1 motif near the upstream E2-RS 2 help abrogate repression by the E2 protein in the presence of CE II. The implications of these results with respect to the viral infectious cycle and during viral oncogenesis are discussed.

The bovine papillomavirus type 1 (BPV1) replication protein E1 modulates transcriptional activation by interacting with BPV1 E2

The study of bovine papillomavirus type 1 (BPV1) DNA replication has shown that E1 and E2 are the only viral proteins required for this process. Both E1 and E2 interact with the viral origin of replication (ori). The BPV1 E2 protein is also a well-characterized transcriptional regulator. We show in this report that E1 can modulate transcription by interactions with E2. At low concentrations, E1 enhanced the E2-mediated transactivation of heterologous promoters containing the BPV1 ori by promoting cooperative binding of both E1 and E2 to the DNA. In contrast, in the presence of excess E1, transactivation by E2 is repressed. This last process, however, does not require cooperative DNA binding of the two proteins. These results imply that the balance between these two distinct types of interaction is crucial both for control of replication and for early viral transcription.

Transcriptional regulation of multidrug resistance in breast cancer

The development of cross-resistance to many natural product anticancer drugs, termed multidrug resistance (MDR), is one of the major reasons why cancer chemotherapy ultimately fails. This type of MDR is often associated with over-expression of the MDR1 gene product, P-glycoprotein (Pgp), a multifunctional drug transporter. The expression of MDR in breast tumors is related to their origination from a tissue that constitutively expresses Pgp as well as to the development of resistance during successive courses of chemotherapy. Therefore, understanding the mechanisms that regulate the transcriptional activation of MDR1 may afford a means of reducing or eliminating MDR. We have found that MDR1 expression can be modulated by type I cAMP-dependent protein kinase (PKA), opening up the possibility of modulating MDR by selectively down-regulating the activity of PKA-dependent transcription factors which upregulate MDR1 expression. High levels of type I PKA occurs in primary breast carcinomas and patients exhibiting this phenotype show decreased survival. The selective type I cAMP-dependent protein kinase (PKA) inhibitors, 8-Cl-cAMP and Rp8-Cl-cAMP[S] may be particularly useful for downregulating PKA-dependent MDR-associated transcription factors, and we have found these compounds to downregulate transient expression of a reporter gene under the control of several MDR1 promoter elements. Thus, investigations of this nature should not only lead to a greater understanding of the mechanisms governing the expression of MDR, but also provide a focus for pharmacologic intervention by a new class of inhibitors.

Identification of the 5' regulatory elements of avian lipoprotein lipase gene: synergistic effect of multiple factors

The organization of cis-acting regulatory elements of the chicken lipoprotein lipase gene was investigated in 5.4 kb of 5' flanking sequences. Various lengths of 5' flanking sequence were linked to the bacterial chloramphenicol acyltransferase (CAT) gene and transfected into primary cultures of chicken adipocytes by DEAE-dextran transfection method. Negative elements are present between -1947 and -139 of the 5' flanking sequence. Removal of these sequences revealed the presence of positive elements located within 138 bp upstream of the major transcription start site. Sequence analysis showed that the region from the major transcription start site to -138 contains an inverted GC box (ACCACGCCCC), a CCAAT element and two direct repeats of the octamer motif, ATTTGCAT. DNase I footprinting assays using a probe extending from -175 to +191, identified three sites protected by nuclear factors. Site I (-126 to -123), a C-rich sequence, GCCC, was identified only on the coding strand. Site II covered the sequence from -95 to -68 and includes the GC box. Site III, from -53 to -26, contained two octamer repeats. Site I is the 5' portion of a 10 bp sequence (CCCTCCCCCC; -126/-116) which is perfectly conserved in the avian and the human promoter. Single or multiple copies of a 37 bp DNA fragment (-138/-102) containing the 10 bp conserved sequence were cloned into LPLCAT-51, upstream or downstream of the major transcription start site and in both orientations; transfection and CAT activity assays with these constructs indicate that the -138/-102 fragment has an enhancer like activity. Additional 5' and internal deletions of LPLCAT-138 suggest that the factors binding to the C-rich element, the GC box and the two octamer repeats have a synergistic effect on promoter activity.

In vitro and in vivo binding of human immunodeficiency virus type 1 Tat protein and Sp1 transcription factor

Recent genetic experiments have suggested that tat transactivation of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat requires functional upstream enhancer sequences--Sp1 sites, in particular. In these experiments, HeLa cell nuclear extracts were passed over affinity matrices containing chemically synthesized or bacterially expressed HIV-1 Tat. Assay of material that bound to and eluted from the Tat matrices revealed the presence of the Sp1 transcription factor. Other transcription factors (Oct and NF-kappa B) also bound to Tat matrices but with less efficiency--in parallel with the lower capacities of these binding motifs to confer Tat responsiveness on a basal HIV-1 promoter compared with Sp1 sites. Passage of nuclear extracts over matrices containing other neutral proteins, including bovine serum albumin, ovalbumin, and lysozyme, revealed no or reduced binding. Cross-linking experiments indicated that the purified Sp1 and Tat proteins can form multimeric complexes in the absence of other proteins. The region of Tat responsible for Sp1 binding was localized to a region encompassing residues 30 to 62. Immunoprecipitation experiments with HIV-1-infected T lymphocytes indicated coimmunoprecipitation of Tat and Sp1. These experiments extend previous genetic experiments and suggest a direct interaction between Tat and Sp1 during transactivation.

Importance of the bovine papillomavirus P2443 promoter in the regulation of E2 and E5 expression

The full-length bovine papillomavirus E2 gene product (E2TA), which has a direct role in DNA replication and functions as a transcriptional activator, can be expressed from an unspliced mRNA transcribed from the P2443 promoter or from spliced mRNAs transcribed from other upstream promoters. The regulation of E2 expression from these promoters is still in question. In the background of wild-type protein coding sequences, this study identified the P2443 promoter as the major source of E2TA as well as E5 expression in C127 cells.

Interaction between transcription factors Sp1 and YY1

A basal level of transcription is usually observed when all but a small region of DNA has been deleted from a eukaryotic gene promoter. These promoter elements, which are necessary and sufficient for specific transcription initiation, are referred to as minimal or core promoter elements. One element that is commonly present in a core promoter is the initiator. It has been demonstrated that the presence of Sp1 binding sites can greatly enhance the level of transcription initiation at initiator elements. A binding site for the YY1 transcription factor, located at the initiation site of the adeno-associated virus P5 promoter, functions as an initiator element; a synergistic enhancement of its activity is observed in vitro when upstream Sp1 binding sites are present. Here we report that this synergistic activation probably occurs through protein-protein interactions.

Purification of the human NF-E2 complex: cDNA cloning of the hematopoietic cell-specific subunit and evidence for an associated partner

The human globin locus control region-binding protein, NF-E2, was purified by DNA affinity chromatography. Its tissue-specific component, p45 NF-E2, was cloned by use of a low-stringency library screen with murine p45 NF-E2 cDNA (N. C. Andrews, H. Erdjument-Bromage, M. B. Davidson, P. Tempst, and S. H. Orkin, Nature [London] 362:722-728, 1993). The human p45 NF-E2 gene was localized to chromosome 12q13 by fluorescent in situ hybridization. Human p45 NF-E2 and murine p45 NF-E2 are highly homologous basic region-leucine zipper (bZIP) proteins with identical DNA-binding domains. Immunoprecipitation experiments demonstrated that p45 NF-E2 is associated in vivo with an 18-kDa protein (p18). Because bZIP proteins bind DNA as dimers, we infer that native NF-E2 must be a heterodimer of 45- and 18-kDa subunits. Although AP-1 and CREB copurified with NF-E2, no evidence was found for heterodimer formation between p45 NF-E2 and proteins other than p18. Thus, p18 appears to be the sole specific partner of p45 NF-E2 in erythroid cells. Cloning of human p45 NF-E2 should permit studies of the role of NF-E2 in globin gene regulation and erythroid differentiation.

Differentiation-specific expression from the bovine papillomavirus type 1 P2443 and late promoters

The papillomavirus life cycle is tightly linked with keratinocyte differentiation in squamous epithelia. Vegetative viral DNA replication begins in the spinous layer, while synthesis of capsid proteins and virus maturation is restricted to the most differentiated or granular layer of the epithelium. In this study, in situ hybridization of bovine fibropapillomas was used to demonstrate that the activity of two promoters of bovine papillomavirus type 1 (BPV-1) is regulated in a differentiation-specific manner. In situ hybridization with a late promoter (PL)-specific oligonucleotide probe suggested that PL is dramatically upregulated in the granular layer of the fibropapilloma. Northern (RNA) blot analysis of RNA from BPV-1-infected fibropapillomas indicated that the three major BPV-1 late-region mRNAs were transcribed from PL. These RNAs include the previously described L1 (major capsid) mRNA as well as two larger mRNAs. The two larger mRNAs were characterized and shown to contain the L2 (minor capsid protein) open reading frame as well as the L1 open reading frame. In contrast to PL, the P2443 promoter was maximally active in basal keratinocytes and the fibroma. The major mRNA transcribed from P2443 is the putative E5 oncoprotein mRNA which is spliced between nucleotides 2505 and 3225. No signal was detected above the basal layer with use of a probe specific for this mRNA. The E5 oncoprotein has previously been localized by immunoperoxidase staining to the granular cell layer as well as the basal cell layer of the fibropapilloma (S. Burnett, N. Jareborg, and D. DiMaio, Proc. Natl. Acad. Sci. USA 89:5665-5669, 1992). These data suggest that E5 proteins in the basal cell and granular cell layers are not translated from the same mRNA.

Cooperative DNA binding of the bovine papillomavirus E2 transcriptional activator is antagonized by truncated E2 polypeptides

Cooperative DNA binding of the bovine papillomavirus type 1 (BPV-1) E2 transcriptional activator (E2-TA) is thought to play a role in the transcriptional synergism of multiple E2-responsive DNA elements (J. Ham, N. Dostatni, J.-M. Gauthier, and M. Yaniv, Trends Biochem. Sci. 16:440-444, 1991). Binding-equilibrium considerations show that such involvement is unlikely, thereby suggesting that the E2-TA cooperative capacity may have evolved to play other, different roles. The role of cooperative interactions in the antagonistic activity of BPV-1-positive and BPV-1-negative E2 regulatory proteins was investigated by an in vitro quantitative gel shift assay. Viral repressor E2-TR, a truncated peptide encompassing the activator DNA-binding domain, possesses a small but measurable cooperative capacity. Furthermore, the minimal E2 DNA-binding domain interacts with the activator in a positive, heterocooperative manner. As a result, the in vitro competition of full-length and truncated E2 peptides appears to be (macroscopically) noncooperative. This heterocooperative effect is probably dominant in latently infected G0-G1 cells, in which repressor E2-TR is 10- to 20-fold more abundant than the activator. The data are discussed considering the possible role of homo- and heterocooperative DNA binding in E2-conditional gene expression.

Mutational analysis of sequences downstream of the TATA box of the herpes simplex virus type 1 major capsid protein (VP5/UL19) promoter

Transient expression assays with the herpes simplex virus type 1 (HSV-1) promoter/leader controlling the beta gamma (leaky-late) VP5 (UL19) mRNA encoding the major capsid protein showed that no more than 36 to 72 bases of VP5 leader are required for full-level expression. Constructs lacking the viral leader and the transcription initiation site expressed the reporter gene at about 20% of the maximum level. We confirmed this observation by using recombinant viruses in which VP5 promoter/leader deletions controlling the bacterial beta-galactosidase gene were inserted into the nonessential glycoprotein C (UL44) locus of the genome. Sequences within +36 are required for full-level expression, and removal of all leader sequences including the cap site resulted in a 10-fold decrease in reporter mRNA accumulation. The removal of the leader sequence had a measurable effect upon the kinetics of reporter mRNA accumulation, but insertion of the entire VP5 leader and cap site into a construct in which the reporter gene was controlled by the kinetically early (beta) dUTPase (UL50) promoter did not result in any significant change in the kinetics of dUTPase promoter expression. These results suggest that DNA sequences both 5' and 3' of the TATA box are important determinants of the beta gamma kinetics and levels of VP5 mRNA accumulation in the infected cell.

Repression of bovine papillomavirus type 1 transcription by the E1 replication protein

Bovine papillomavirus type 1 (BPV-1) is the prototype virus for the study of papillomavirus gene regulation. The functions of the BPV-1 E2 proteins in transcriptional regulation have been well characterized. The BPV-1 E1 protein is required for viral DNA replication and can bind to the origin of replication alone or in a complex with the E2 transactivator protein. In this study, we demonstrated that the BPV-1 E1 protein is also involved in transcriptional regulation. The E1 protein significantly repressed E2-transactivated transcription from the major early promoter P89. This activity is consistent with the elevated level of P89 transcription observed in BPV-1 E1 open reading frame mutants. Transcriptional repression by E1 correlated with the ability of an E1-E2 protein complex to bind the replication origin but was not dependent on viral DNA replication. These studies identify a new mechanism involved in the regulation of papillomavirus transcription which has implications regarding expression of the viral transforming functions.

Purification of the human NF-E2 complex: cDNA cloning of the hematopoietic cell-specific subunit and evidence for an associated partner

The human globin locus control region-binding protein, NF-E2, was purified by DNA affinity chromatography. Its tissue-specific component, p45 NF-E2, was cloned by use of a low-stringency library screen with murine p45 NF-E2 cDNA (N. C. Andrews, H. Erdjument-Bromage, M. B. Davidson, P. Tempst, and S. H. Orkin, Nature [London] 362:722-728, 1993). The human p45 NF-E2 gene was localized to chromosome 12q13 by fluorescent in situ hybridization. Human p45 NF-E2 and murine p45 NF-E2 are highly homologous basic region-leucine zipper (bZIP) proteins with identical DNA-binding domains. Immunoprecipitation experiments demonstrated that p45 NF-E2 is associated in vivo with an 18-kDa protein (p18). Because bZIP proteins bind DNA as dimers, we infer that native NF-E2 must be a heterodimer of 45- and 18-kDa subunits. Although AP-1 and CREB copurified with NF-E2, no evidence was found for heterodimer formation between p45 NF-E2 and proteins other than p18. Thus, p18 appears to be the sole specific partner of p45 NF-E2 in erythroid cells. Cloning of human p45 NF-E2 should permit studies of the role of NF-E2 in globin gene regulation and erythroid differentiation.

Mechanism of developmental regulation of alpha pi, the chicken embryonic alpha-globin gene

The chicken alpha pi-globin gene is expressed during development only in the primitive erythrocyte lineage and not in the definitive lineage. We show that stage-specific expression is maintained when plasmids containing the alpha pi promoter are transfected into primitive and definitive lineage primary erythroid cells and that the information contained in the promoter is sufficient to confer this specificity. Detailed analysis of binding sites in the promoter for trans-acting factors, together with studies of the effects of mutagenesis on expression, reveals that the factors critical to stage-specific expression are all present in both primitive and definitive lineages, but at various concentrations. We identify three proteins, an NF1 family member, a Y-box factor, and an Sp1-like factor, which interact to stimulate or inhibit transcription. We propose that the concentration-dependent action of these factors, together with the general erythroid factor GATA-1, is responsible for the stage-specific expression of the alpha pi-globin gene.

Recognition DNA sequences of interferon regulatory factor 1 (IRF-1) and IRF-2, regulators of cell growth and the interferon system

Interferon (IFN) regulatory factor 1 (IRF-1) and IRF-2 were originally identified as transcription factors involved in the regulation of the IFN system. IRF-1 functions as a transcriptional activator, while IRF-2 represses IRF-1 function. More recently, evidence has been provided that IRF-1 and IRF-2 manifest antioncogenic and oncogenic properties, respectively, and that loss of one or both of the IRF-1 alleles may be critical for the development of human hematopoietic neoplasms. Both factors show a high degree of structural similarity in their N-terminal DNA-binding domains, and previous studies suggested that IRF-1 and IRF-2 bind to similar or identical cis elements within type I IFN (IFN-alpha and -beta) and IFN-inducible genes. However, the exact recognition sequences of these two factors have not yet been determined; hence, the spectrum of the IRF-responsive genes remains unclear. In this study, we determined the DNA sequences recognized by IRF-1 and IRF-2, using a polymerase chain reaction-assisted DNA-binding site selection method. We report that sequences selected by this method and the affinities for each sequence were virtually indistinguishable between IRF-1 and IRF-2. We confirm the presence of two contiguous IRF recognition sequences within the promoter region of the IFN-beta gene and of at least one such sequence in all of the IFN-inducible genes examined. Furthermore, we report the presence of potential IRF sequences in the upstream region of several genes involved in cell growth control.

Formation of a cleavasome: enhancer DNA-2 stabilizes an active conformation of NaeI dimer

Cleavage of DNA by NaeI-type restriction enzymes is stimulated by a DNA element with affinity for the activator site of the enzyme: a cleavage-enhancer DNA element. Measurements of the mobility of NaeI activity in comparison with protein standards on gel permeation columns and glycerol gradients demonstrated that NaeI, without enhancer, can form a 70,000 MW dimer. The dimer, however, is inactive: it could not cleave the "resistant" NaeI site in M13mp18 DNA in the absence of enhancer. In cleavage assays, enhancer stimulated either DNA nicking or DNA cleavage, depending upon NaeI concentration, and reduced the NaeI concentration required for the transition from nicking to cleavage activity. A gel mobility-shift assay of the interaction of NaeI with enhancer showed the formation of two complexes. Results using different sized DNAs and different percentage acrylamide gels for gel mobility-shift analysis implied that the two complexes were caused by NaeI monomer and dimer structures rather than one and two DNA binding. Dimer formation increased with the affinity of enhancer for NaeI. UV cross-linking "captured" the NaeI-enhancer complex; electrophoretic analysis of the cross-linked products showed NaeI dimer bound to enhancer. These results imply a model for cleavage enhancement in which enhancer binding stabilizes an active NaeI dimer conformation ("cleavasome") that cleaves both DNA strands before dissociating.

Mechanism of developmental regulation of alpha pi, the chicken embryonic alpha-globin gene

The chicken alpha pi-globin gene is expressed during development only in the primitive erythrocyte lineage and not in the definitive lineage. We show that stage-specific expression is maintained when plasmids containing the alpha pi promoter are transfected into primitive and definitive lineage primary erythroid cells and that the information contained in the promoter is sufficient to confer this specificity. Detailed analysis of binding sites in the promoter for trans-acting factors, together with studies of the effects of mutagenesis on expression, reveals that the factors critical to stage-specific expression are all present in both primitive and definitive lineages, but at various concentrations. We identify three proteins, an NF1 family member, a Y-box factor, and an Sp1-like factor, which interact to stimulate or inhibit transcription. We propose that the concentration-dependent action of these factors, together with the general erythroid factor GATA-1, is responsible for the stage-specific expression of the alpha pi-globin gene.

Recognition DNA sequences of interferon regulatory factor 1 (IRF-1) and IRF-2, regulators of cell growth and the interferon system

Interferon (IFN) regulatory factor 1 (IRF-1) and IRF-2 were originally identified as transcription factors involved in the regulation of the IFN system. IRF-1 functions as a transcriptional activator, while IRF-2 represses IRF-1 function. More recently, evidence has been provided that IRF-1 and IRF-2 manifest antioncogenic and oncogenic properties, respectively, and that loss of one or both of the IRF-1 alleles may be critical for the development of human hematopoietic neoplasms. Both factors show a high degree of structural similarity in their N-terminal DNA-binding domains, and previous studies suggested that IRF-1 and IRF-2 bind to similar or identical cis elements within type I IFN (IFN-alpha and -beta) and IFN-inducible genes. However, the exact recognition sequences of these two factors have not yet been determined; hence, the spectrum of the IRF-responsive genes remains unclear. In this study, we determined the DNA sequences recognized by IRF-1 and IRF-2, using a polymerase chain reaction-assisted DNA-binding site selection method. We report that sequences selected by this method and the affinities for each sequence were virtually indistinguishable between IRF-1 and IRF-2. We confirm the presence of two contiguous IRF recognition sequences within the promoter region of the IFN-beta gene and of at least one such sequence in all of the IFN-inducible genes examined. Furthermore, we report the presence of potential IRF sequences in the upstream region of several genes involved in cell growth control.

CCAAT/enhancer binding protein alpha (C/EBP alpha) undifferentiated protein: a developmentally regulated nuclear protein that binds to the C/EBP alpha gene promoter

During differentiation of 3T3-L1 preadipocytes into adipocytes, transcription of the C/EBP alpha (CCA-AT/enhancer binding protein alpha) gene is activated. The promoter of the C/EBP alpha gene contains a bipartite cis element with binding sites for C/EBP alpha undifferentiated protein (CUP) and an Sp1-like GT box binding protein. Binding of CUP to this element is markedly enhanced by its interaction with the Sp1-like protein. CUP, purified approximately 100,000-fold from HeLa cell nuclear extracts, appears to be composed of at least two types of subunit. Evidence is presented that a CUP-containing protein complex bridges between the CUP/Sp1-like GT box element and a downstream cis element, which contains a C/EBP binding site. During differentiation of 3T3-L1 preadipocytes into adipocytes, CUP activity or expression decreases as expression of C/EBP alpha increases. It is suggested that bridging by the CUP-containing protein complex may play a role in transcriptional regulation of the C/EBP alpha gene.

Functional analysis of the human somatic angiotensin I-converting enzyme gene promoter

Angiotensin I-converting enzyme (ACE) is a key enzyme in the regulation of systemic blood pressure and plays a major role in the renin-angiotensin and bradykinin-kinin systems, at the luminal surface of the vascular endothelia. To identify the promoter region, the transcription regulatory elements and the cell specificity of the ACE gene, five successive DNA deletions of the 5' upstream region (-1214, -754, -472, -343, -132 bp relative to the start site of transcription) were isolated and fused in sense and antisense orientations to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene in the promoterless plasmid pBLCAT3. Promoter activities were measured in transient transfection assays using three different cell lines from rabbit endothelium (RE), human embryocarcinoma (Tera-1) and hepatocarcinoma cells (HepG2). All five fragments of the ACE promoter region directed expression of the CAT gene when transfected into the endothelial and the embryocarcinoma cells, which contain endogenous ACE mRNA and express ACE activity. In contrast only minimal levels of promoter activity were obtained on transfection into hepatocarcinoma cells in which endogenous ACE mRNA and ACE activity were not detected. Transfection of RE and Tera-1 cells demonstrated that promoter activity was defined by the length of the ACE promoter sequence inserted into the construct. The 132 bases located upstream from the transcription start site were sufficient to confer ACE promoter activity, whereas the sequences upstream from -472 bp and between -343 bp and -132 bp were responsible for a decrease of promoter activity. Furthermore, the minimal 132 bp of the ACE promoter contains elements which direct cell-specific CAT expression. In addition, the DNA transfection study in the presence of dexamethasone suggested that the potential glucocorticoid regulatory elements, located in the sequence of the ACE promoter, are not functional.

Interaction between transcription regulatory regions of prolactin chromatin

The regulation of transcription requires complex interactions between proteins bound to DNA sequences that are often separated by hundreds of base pairs. As demonstrated by a nuclear ligation assay, the distal enhancer and the proximal promoter regions of the rat prolactin gene were found to be juxtaposed. By acting through its receptor bound to the distal enhancer, estrogen stimulated the interaction between the distal and proximal regulatory regions two- to threefold compared to control values. Thus, the chromatin structure of the prolactin gene may facilitate the occurrence of protein-protein interactions between transcription factors bound to widely separated regulatory elements.

Identification and characterization of novel promoters in the genome of human papillomavirus type 18

Most studies on the regulation of gene expression in human papillomaviruses (HPV) have focused on the promoter for the early genes E6 and E7. This promoter is located at the junction between the long control region and the E6 open reading frame. RNA mapping studies have suggested that additional promoters may exist in other parts of the genome. In this study, we used a combination of transcription in vitro and an analysis of RNA produced in vivo in transfected cells to identify three novel promoters in the genome of human papillomavirus type 18. These promoters are located in front of the E2 gene (P2598), within the E2 coding sequences (P3036), and at the end of the L2 open reading frame (P5600). They were active in HeLa cells, as shown by a chloramphenicol acetyltransferase assay. The activity of the P3036 promoter was stimulated by the bovine papillomavirus type 1 E2 protein.

Evidence for physical interaction between the zinc-finger transcription factors YY1 and Sp1

Two promoter elements are important for basal-level transcription, the TATA motif typically located 30 nucleotides upstream of the transcription initiation site and the initiator (Inr) element encompassing the start site. The mechanism of how Inr elements work is poorly understood, partly because very few proteins that bind to Inr elements have been identified and isolated. The recently cloned YY1 is such an Inr-binding protein. YY1 is able to direct transcription upon binding to its recognition sequence in vitro. The ability of YY1 to initiate transcription is augmented by the presence of a TATA motif or binding sites for transcription factor Sp1. To study the mechanism underlying the apparent functional cooperation between YY1 and Sp1, we explored the possibility of protein-protein interactions between these two transcription factors. We found that YY1 and Sp1 can form a physical complex. In addition, we identified domains within YY1 and Sp1 that mediate their interactions with each other. The physical interaction between YY1 and Sp1 may thus form the basis for the functional interplay observed previously.

The octamer/mu E4 region of the immunoglobulin heavy-chain enhancer mediates gene repression in myeloma x T-lymphoma hybrids

We have shown previously that the immunoglobulin heavy-chain enhancer acts as a repressor of gene transcription in hybrids between immunoglobulin-producing myelomas and a T-lymphoma line. We have now mapped this repressive activity to a 51-bp enhancer subfragment which contains the octamer and mu E4 protein-binding motifs. Even a single copy of this subfragment will repress gene expression in hybrid cells. Mutational analyses of the repressor fragment suggest that in non-B cells, a strong transcriptional repressor(s) functions through the same motifs important for gene activation in B cells. Changes in chromatin structure that accompany reporter gene repression suggest a general mechanism for prohibiting immunoglobulin heavy-chain locus activation in inappropriate cell types.

The acidic transcriptional activation domains of VP16 and p53 bind the cellular replication protein A and stimulate in vitro BPV-1 DNA replication

For papillomavirus DNA replication, the E2 enhancer protein cooperatively assists in binding of the E1 helicase to the origin. We report that, at limiting E1 and E2 levels, the enhancer proteins GAL4-VP16 and GAL4-p53(1-73) stimulate BPV in vitro DNA replication. This cell-free system was used to ascertain whether the acidic activation domains have a cellular target important for replication. Cellular extracts were depleted of replication activity by passage through a VP16 affinity column. The protein depleted was the cellular factor replication protein A. The direct interaction between replication protein A and VP16, as well as the activation of replication by VP16, is dependent upon the C-terminus of the VP16 activation domain. E2 and the activation domain of p53 also interact with replication protein A. We suggest that a link between transcription and replication involves factors that help convert a closed DNA complex to an open complex.

The octamer/mu E4 region of the immunoglobulin heavy-chain enhancer mediates gene repression in myeloma x T-lymphoma hybrids

We have shown previously that the immunoglobulin heavy-chain enhancer acts as a repressor of gene transcription in hybrids between immunoglobulin-producing myelomas and a T-lymphoma line. We have now mapped this repressive activity to a 51-bp enhancer subfragment which contains the octamer and mu E4 protein-binding motifs. Even a single copy of this subfragment will repress gene expression in hybrid cells. Mutational analyses of the repressor fragment suggest that in non-B cells, a strong transcriptional repressor(s) functions through the same motifs important for gene activation in B cells. Changes in chromatin structure that accompany reporter gene repression suggest a general mechanism for prohibiting immunoglobulin heavy-chain locus activation in inappropriate cell types.

Identification of a DNA structural motif that includes the binding sites for Sp1, p53 and GA-binding protein

We have analyzed predicted helical twist angles in the 21-bp repeat region of the SV40 genome, using a semi-empirical model previously shown to accurately predict backbone conformations. Unexpectedly, the pattern of twist angles characteristic of the six GC-boxes is repeated an additional five times at positions that are regularly interspersed with the six GC-box sequences. These patterns of helical twist angles are associated with a second, imperfectly-repeated sequence motif, the TR-box 5'-RRNTRGG. Unrelated DNA sequences that interact with trans-acting factors (p53 and GABP) exhibit similar twist angle patterns, due to elements of the general form 5'-RRRYRRR that occur as interspersed arrays with a spacing of 10-11 bp and an offset of 4-6 bp. Arrays of these elements, which we call pyrimidine sandwich elements (PSEs), may play an important role in the interaction of trans-acting factors with DNA control regions. In 13 human proto-oncogenes analyzed, we identified 31 PSE arrays, 11 of which were in the 5'-flanking regions of the genes. The most extensive array was found in the promoter region of the K-ras gene. Extending over 80 bp of DNA, it contained 16 PSEs that showed an average deviation from the SV40 criterion pattern of angles of only 1.2 degrees.

Current concepts in DRA gene regulation

The purpose of this review is to provide an interpretative view of work from our laboratory on the DRA gene, and incorporate it with work from other laboratories. Specially, we will deal with: (a) the functional roles of transcription factors in DRA gene regulation; (b) the mechanisms of DRA induction by cytokines; (c) the analysis of DRA gene control in primary untransformed cells, and (d) interactions among transcription factors critical for DRA gene expression.

The regulation of human globin gene expression

The haemopoietic system provides a well-characterized and accessible system for studying the mechanisms of developmental regulation and differentiation in higher eukaryotes. Our current understanding of the steps involved in the early stages of differentiation are poorly understood but a great deal is now known about the mechanisms by which globin expression is regulated in cells committed to the erythroid lineage. Many of the critical cis-acting sequences and some of the important trans-acting factors involved have been identified and current work is focusing on how these interact to produce high levels of tissue-specific and developmentally regulated expression of the human globin genes.

Enhancer-dependent transcription of the epsilon-globin promoter requires promoter-bound GATA-1 and enhancer-bound AP-1/NF-E2

We analyzed epsilon-globin transcription in erythroid cells and in erythroid extracts to determine the requirements for enhancer-dependent expression of this gene. Mutations that abolished GATA-1 binding at a single position in the promoter prevented interaction with enhancers, whereas elimination of a second more distal promoter GATA-1 site had no effect. Deletion or mutation of the GATA-1 sites in either the human beta-globin locus control region DNase-hypersensitive site II enhancer or the chicken beta A/epsilon-globin enhancer did not diminish the ability of the enhancers to interact with the promoter. In contrast, mutation of the AP-1/NF-E2 sites in these enhancers resulted in elimination of enhancement. In vitro transcription of these constructs was promoter dependent and was not sensitive to abolition of GATA-1 binding in the promoter, consistent with the role of GATA-1 solely as a mediator of the enhancer effect. Thus, GATA-1 regulates the response of the epsilon-globin gene to enhancers through a specific site in the promoter and requires enhancer AP-1/NF-E2 binding to transduce the enhancer effect on transcription.

Erythroid differentiation of mouse erythroleukemia cells results in reorganization of protein-DNA complexes in the mouse beta maj globin promoter but not its distal enhancer

Dimethyl sulfoxide (DMSO) induction of mouse erythroleukemia (MEL) cells represents a well-defined in vitro system of terminal erythroid differentiation. We have studied the molecular mechanisms of transcriptional activation of the mouse beta maj globin gene during MEL cell differentiation by analyzing nuclear factor-DNA interactions in vivo at the gene's upstream promoter and a distal enhancer, 5'HS-2. Genomic footprinting data indicate that three motifs, CAC, NF-E2/AP1, and GATA-1, of the 5'HS-2 enhancer are bound with nuclear factors in MEL cells both prior to and after DMSO induction. No obvious conformational change of these nuclear factor-DNA complexes could be detected upon terminal differentiation of MEL cells. On the other hand, DMSO induction of MEL cells leads to the formation of specific nuclear factor-DNA complexes at several transcriptional regulatory elements of the mouse beta maj globin upstream promoter. Our genomic footprinting data have interesting implications with respect to the molecular mechanisms of transcriptional regulation and chromatin change of the mouse beta maj globin gene during erythroid differentiation.

Erythroid differentiation of mouse erythroleukemia cells results in reorganization of protein-DNA complexes in the mouse beta maj globin promoter but not its distal enhancer

Dimethyl sulfoxide (DMSO) induction of mouse erythroleukemia (MEL) cells represents a well-defined in vitro system of terminal erythroid differentiation. We have studied the molecular mechanisms of transcriptional activation of the mouse beta maj globin gene during MEL cell differentiation by analyzing nuclear factor-DNA interactions in vivo at the gene's upstream promoter and a distal enhancer, 5'HS-2. Genomic footprinting data indicate that three motifs, CAC, NF-E2/AP1, and GATA-1, of the 5'HS-2 enhancer are bound with nuclear factors in MEL cells both prior to and after DMSO induction. No obvious conformational change of these nuclear factor-DNA complexes could be detected upon terminal differentiation of MEL cells. On the other hand, DMSO induction of MEL cells leads to the formation of specific nuclear factor-DNA complexes at several transcriptional regulatory elements of the mouse beta maj globin upstream promoter. Our genomic footprinting data have interesting implications with respect to the molecular mechanisms of transcriptional regulation and chromatin change of the mouse beta maj globin gene during erythroid differentiation.

Synergism in transcriptional activation: a kinetic view

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