The Oct-1 homoeodomain directs formation of a multiprotein-DNA complex with the HSV transactivator VP16

Ectopic expression and function of the Antp and Scr homeotic genes: the N terminus of the homeodomain is critical to functional specificity

The transcription factors encoded by homeotic genes determine cell fates during development. Each homeotic protein causes cells to follow a distinct pathway, presumably by differentially regulating downstream ‘target’ genes. The homeodomain, the DNA-binding part of homeotic proteins, is necessary for conferring the specificity of each homeotic protein's action. The two Drosophila homeotic proteins encoded by Antennapedia and Sex combs reduced determine cell fates in the epidermis and internal tissues of the posterior head and thorax. Genes encoding chimeric Antp/Scr proteins were introduced into flies and their effects on morphology and target gene regulation observed. We find that the N terminus of the homeodomain is critical for determining the specific effects of these homeotic proteins in vivo, but other parts of the proteins have some influence as well. The N-terminal part of the homeodomain has been observed, in crystal structures and in NMR studies in solution, to contact the minor groove of the DNA. The different effects of Antennapedia and Sex combs reduced proteins in vivo may depend on differences in DNA binding, protein-protein interactions, or both.

Varicella-zoster virus open reading frame 10 protein, the herpes simplex virus VP16 homolog, transactivates herpesvirus immediate-early gene promoters

The varicella-zoster virus (VZV) open reading frame 10 (ORF10) protein is the homolog of the herpes simplex virus type 1 (HSV-1) protein VP16. These are two virion tegument proteins that have extensive amino acid sequence identity in their amino-terminal and middle domains. ORF10, however, lacks the acidic carboxy terminus which is critical for transactivation by VP16. Earlier studies showed that VZV ORF10 does not form a tertiary complex with the TAATGARAT regulatory element (where R is a purine) with which HSV-1 VP16 interacts, suggesting that ORF10 may not have transactivating ability. Using transient-expression assays, we show that VZV ORF10 is able to transactivate VZV immediate-early (IE) gene (ORF62) and HSV-1 IE gene (ICP4 and ICP0) promoters. Furthermore, cell lines stably expressing ORF10 complement the HSV-1 mutant in1814, which lacks the transactivating function of VP16, and enhance the de novo synthesis of infectious virus following transfection of HSV-1 virion DNA. These results indicate that ORF10, like its HSV-1 homolog VP16, is a transactivating protein despite the absence of sequences similar to the VP16 carboxy-terminal domain. The transactivating function of the VZV ORF10 tegument protein may be critical for efficient initiation of viral infection.

The segment identity functions of Ultrabithorax are contained within its homeo domain and carboxy-terminal sequences

Using an in vivo assay for segment identity, the structural differences that distinguish two Drosophila homeotic selector proteins, Ultrabithorax (Ubx) and Antennapedia (Antp), have been investigated. There are at least two independent parts of Ubx and Antp that contribute to their functional specificities: (1) their homeo domains and (2) residues carboxy-terminal to their homeo domains (C-tails). In the absence of any C-tail, differences in 5 homeo domain amino acids are sufficient to distinguish between the functions of Ubx and Antp. Two of these are at the amino terminus of the homeo domain and could contact DNA directly. A three dimensional model suggests that the other 3 homeo domain residues and the C-tails are unlikely to contact DNA. In addition, we demonstrate that the assay used to measure the segment identity functions of Ubx and Antp is independent of any homeotic selector gene normally active in thoracic and abdominal segments. Therefore, it is likely that this assay measures the coordinate regulation of many downstream target genes. This expectation is confirmed for at least one Ubx target gene, Distal-less.

cDNA cloning of transcription factor E4TF1 subunits with Ets and notch motifs

E4TF1 was originally identified as one of the transcription factors responsible for adenovirus E4 gene transcription. It is composed of two subunits, a DNA binding protein with a molecular mass of 60 kDa and a 53-kDa transcription-activating protein. Heterodimerization of these two subunits is essential for the protein to function as a transcription factor. In this study, we identified a new E4TF1 subunit, designated E4TF1-47, which has no DNA binding activity but can associate with E4TF1-60. We then cloned the cDNAs for each of the E4TF1 subunits. E4TF1 was purified, and the partial amino acid sequence of each subunit was determined. The predicted amino acid sequence of each cDNA clone revealed that E4TF1-60 had an ETS domain, which is a DNA binding domain common to ets-related transcription factors. E4TF1-53 had four tandemly repeated notch-ankyrin motifs. The putative cDNA of E4TF1-47 coded almost the same amino acid sequences as E4TF1-53. Three hundred and thirty-two amino acids of the N termini of E4TF1-47 and -53 were identical except for one amino acid insertion in E4TF1-53, and they differ from each other at the C terminus. These three recombinant cDNA clones were expressed in Escherichia coli, and the proteins behaved in the same manner as purified proteins in a gel retardation assay. Nucleotide and predicted amino acid sequences were highly homologous to GABP-alpha and -beta, which is further supported by the observation that GABP-specific antibody can recognize human E4TF1.

Regulation of the HNF-1 homeodomain proteins by DCoH

The pattern of expression of homeodomain proteins often exceeds their apparent domain of activity. Tissue-specific proteins that modulate the in vivo activity of homeodomain proteins have been proposed to account for this functional restriction. The first identified example of such an accessory protein is DCoH, which confers transcriptional activity to the hepatocyte nuclear factor 1 and provides a model of how other accessory factors might modulate the function of homeodomain proteins.

Rough genes with Deformed homeobox substitutions exhibit rough regulatory specificity during Drosophila eye development

In certain cases, homeobox genes with different in vivo roles encode proteins with similar in vitro DNA binding specificities. To test the role of the homeobox in the regulatory specificity of such genes, rough homeobox sequences were changed in part or entirely to those of the Deformed gene, and the modified rough genes tested for their ability to rescue the rough mutant phenotype. Surprisingly, the chimaeric genes retained levels of rough regulatory specificity but acquired no novel functions. These results suggest that factors other than the DNA binding specificity of the homeodomain play crucial roles in determining the target, and thus the regulatory specificity, of such proteins.

Synergistic activation of transcription is mediated by the N-terminal domain of Drosophila fushi tarazu homeoprotein and can occur without DNA binding by the protein

Synergistic activation of transcription by Drosophila segmentation genes in tissue culture cells provides a model with which to study combinatorial regulation. We examined the synergistic activation of an engrailed-derived promoter by the pair-rule proteins paired (PRD) and fushi tarazu (FTZ). Synergistic activation by PRD requires regions of the homeodomain or adjacent sequences, and that by FTZ requires the first 171 residues. Surprisingly, deletion of the FTZ homeodomain does not reduce the capacity of the protein for synergistic activation, although this mutation abolishes any detectable DNA-binding activity. This finding suggests that FTZ can function through protein-protein interactions with PRD or other components of the homeoprotein transcription complex, adding a new layer of mechanisms that could underlie the functional specificities and combinatorial regulation of homeoproteins.

Synergistic activation of transcription is mediated by the N-terminal domain of Drosophila fushi tarazu homeoprotein and can occur without DNA binding by the protein

Synergistic activation of transcription by Drosophila segmentation genes in tissue culture cells provides a model with which to study combinatorial regulation. We examined the synergistic activation of an engrailed-derived promoter by the pair-rule proteins paired (PRD) and fushi tarazu (FTZ). Synergistic activation by PRD requires regions of the homeodomain or adjacent sequences, and that by FTZ requires the first 171 residues. Surprisingly, deletion of the FTZ homeodomain does not reduce the capacity of the protein for synergistic activation, although this mutation abolishes any detectable DNA-binding activity. This finding suggests that FTZ can function through protein-protein interactions with PRD or other components of the homeoprotein transcription complex, adding a new layer of mechanisms that could underlie the functional specificities and combinatorial regulation of homeoproteins.

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.

cDNA cloning of transcription factor E4TF1 subunits with Ets and notch motifs

E4TF1 was originally identified as one of the transcription factors responsible for adenovirus E4 gene transcription. It is composed of two subunits, a DNA binding protein with a molecular mass of 60 kDa and a 53-kDa transcription-activating protein. Heterodimerization of these two subunits is essential for the protein to function as a transcription factor. In this study, we identified a new E4TF1 subunit, designated E4TF1-47, which has no DNA binding activity but can associate with E4TF1-60. We then cloned the cDNAs for each of the E4TF1 subunits. E4TF1 was purified, and the partial amino acid sequence of each subunit was determined. The predicted amino acid sequence of each cDNA clone revealed that E4TF1-60 had an ETS domain, which is a DNA binding domain common to ets-related transcription factors. E4TF1-53 had four tandemly repeated notch-ankyrin motifs. The putative cDNA of E4TF1-47 coded almost the same amino acid sequences as E4TF1-53. Three hundred and thirty-two amino acids of the N termini of E4TF1-47 and -53 were identical except for one amino acid insertion in E4TF1-53, and they differ from each other at the C terminus. These three recombinant cDNA clones were expressed in Escherichia coli, and the proteins behaved in the same manner as purified proteins in a gel retardation assay. Nucleotide and predicted amino acid sequences were highly homologous to GABP-alpha and -beta, which is further supported by the observation that GABP-specific antibody can recognize human E4TF1.

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

Simian virus 40 (SV40) large T antigen is a potent transcriptional activator of both viral and cellular promoters. Within the SV40 late promoter, a specific upstream element necessary for T-antigen transcriptional activation is the binding site for transcription-enhancing factor 1 (TEF-1). The promoter structure necessary for T-antigen-mediated transcriptional activation appears to be simple. For example, a promoter consisting of upstream TEF-1 binding sites (or other factor-binding sites) and a downstream TATA or initiator element is efficiently activated. It has been demonstrated that transcriptional activation by T antigen does not require direct binding to the DNA; thus, the most direct effect that T antigen could have on these simple promoters would be through protein-protein interactions with either upstream-bound transcription factors, the basal transcription complex, or both. To determine whether such interactions occur, full-length T antigen or segments of it was fused to the glutathione-binding site (GST fusions) or to the Gal4 DNA-binding domain (amino acids 1 to 147) (Gal4 fusions). With the GST fusions, it was found that TEF-1 and the TATA-binding protein (TBP) bound different regions of T antigen. A GST fusion containing amino acids 5 to 172 (region T1) efficiently bound TBP. TEF-1 bound neither region T1 nor a region between amino acids 168 and 373 (region T2); however, it bound efficiently to the combined region (T5) containing amino acids 5 to 383.(ABSTRACT TRUNCATED AT 250 WORDS)

B-myc inhibits neoplastic transformation and transcriptional activation by c-myc

B-myc is a recently described myc gene whose product has not been functionally characterized. The predicted product of B-myc is a 168-amino-acid protein with extensive homology to the c-Myc amino-terminal region, previously shown to contain a transcriptional activation domain. We hypothesized that B-Myc might also function in transcriptional regulation, although its role in regulating gene expression is predicted to be unique, because B-Myc lacks the specific DNA-binding motif found in other Myc proteins. To determine whether B-Myc could interact with the transcriptional machinery, we studied the transcriptional activation properties of a chimeric protein containing B-Myc sequences fused to the DNA-binding domain of the yeast transcriptional activator GAL4 (GAL4-B-Myc). We found that GAL4-B-Myc strongly activated expression of a GAL4-regulated reporter gene in mammalian cells. In addition, full-length B-Myc was able to inhibit or squelch reporter gene activation by a GAL4 chimeric protein containing the c-Myc transcriptional activation domain. We also observed that B-Myc dramatically inhibited the neoplastic cotransforming activity of c-Myc and activated Ras in rat embryo cells. Because B-Myc inhibits both neoplastic transformation and transcriptional activation by c-Myc, we suggest that the transforming activity of c-Myc is related to its ability to regulate transcription. Whether B-Myc functions biologically to squelch transcription and/or to regulate transcription through a specific DNA-binding protein remains unestablished.

Zebrafish pou[c]: a divergent POU family gene ubiquitously expressed during embryogenesis

We report the isolation and characterization of cDNA for a novel zebrafish (Brachyodanio rerio) POU domain gene, pou[c], which is ubiquitously expressed during embryonic development. This gene encodes a 610 amino acids long protein with a 149 amino acid POU domain ending only 8 residues before the C terminus. The 453 amino acids long region N-terminal to the POU domain contains several features typical of transcriptional activation domains such as an acidic region with a putative amphipathic alpha-helix, a glutamine-rich region, and short threonine- and/or serine-rich regions. Comparison of the POU domain of pou[c] to other known POU sequences clearly show that pou[c] has the most divergent POU domain sequence reported to date. Thus, we suggest that pou[c] should be placed as the presently sole member of a new, sixth class of POU proteins. DNA-binding studies revealed that pou[c] is not an octamer-binding transcription factor like the Oct proteins described from mammals, chicken and Xenopus. Rather, pou[c] binds with high affinity to the TAATGARAT motif found in the promoters of the herpes simplex virus immediate early genes and to degenerate octamer-TAATGA motifs. Circular permutation analyses also show that pou[c] induces DNA bending upon sequence-specific binding.

B-myc inhibits neoplastic transformation and transcriptional activation by c-myc

B-myc is a recently described myc gene whose product has not been functionally characterized. The predicted product of B-myc is a 168-amino-acid protein with extensive homology to the c-Myc amino-terminal region, previously shown to contain a transcriptional activation domain. We hypothesized that B-Myc might also function in transcriptional regulation, although its role in regulating gene expression is predicted to be unique, because B-Myc lacks the specific DNA-binding motif found in other Myc proteins. To determine whether B-Myc could interact with the transcriptional machinery, we studied the transcriptional activation properties of a chimeric protein containing B-Myc sequences fused to the DNA-binding domain of the yeast transcriptional activator GAL4 (GAL4-B-Myc). We found that GAL4-B-Myc strongly activated expression of a GAL4-regulated reporter gene in mammalian cells. In addition, full-length B-Myc was able to inhibit or squelch reporter gene activation by a GAL4 chimeric protein containing the c-Myc transcriptional activation domain. We also observed that B-Myc dramatically inhibited the neoplastic cotransforming activity of c-Myc and activated Ras in rat embryo cells. Because B-Myc inhibits both neoplastic transformation and transcriptional activation by c-Myc, we suggest that the transforming activity of c-Myc is related to its ability to regulate transcription. Whether B-Myc functions biologically to squelch transcription and/or to regulate transcription through a specific DNA-binding protein remains unestablished.

Mapping of a major surface-exposed site in herpes simplex virus protein Vmw65 to a region of direct interaction in a transcription complex assembly

The cellular factor Oct-1 is selectively recruited, together with at least one other cellular protein (CFF), into a multicomponent transcription complex whose assembly is directed by the herpes simplex virus regulatory protein Vmw65 (VP16). The acidic carboxy terminus of Vmw65 is not involved in assembly of the complex but is absolutely required for subsequent transcriptional activation. Elucidation of the mechanism of action of Vmw65 is important for an understanding not only of combinatorial control of gene expression by POU- and homeodomain proteins but also of the interaction(s) between activation domains of regulatory proteins and components of the basal transcriptional apparatus. We used a combination of limited proteolysis with a number of site-specific proteases and immunological detection to demonstrate the presence of two main surface-exposed regions in Vmw65. We mapped these sites to within a few amino acids at positions 365-370 408/409. The site at 408/409 is indicative of a flexible exposed linker region between the acidic carboxy-terminal activation domain (residues 430-480) and an N-terminal domain involved in complex formation with the two cellular factors. The site around residues 365-370 is precisely within a region that results from this and other laboratories have shown to be critical for complex formation. Furthermore, we show that this site is selectively protected from proteolysis after complex assembly. Finally, using a series of overlapping peptide encompassing this region, we show that the eight amino acids, R-E-H-A-Y-S-R-A, from positions 360 through 367 are sufficient to inhibit complex formation by intact Vmw65. We propose that these residues contain sufficient information to selectively bind one of the cellular partners involved in complex assembly and that these residues are located in a physical surface-exposed domain of the protein.

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

Simian virus 40 (SV40) large T antigen is a potent transcriptional activator of both viral and cellular promoters. Within the SV40 late promoter, a specific upstream element necessary for T-antigen transcriptional activation is the binding site for transcription-enhancing factor 1 (TEF-1). The promoter structure necessary for T-antigen-mediated transcriptional activation appears to be simple. For example, a promoter consisting of upstream TEF-1 binding sites (or other factor-binding sites) and a downstream TATA or initiator element is efficiently activated. It has been demonstrated that transcriptional activation by T antigen does not require direct binding to the DNA; thus, the most direct effect that T antigen could have on these simple promoters would be through protein-protein interactions with either upstream-bound transcription factors, the basal transcription complex, or both. To determine whether such interactions occur, full-length T antigen or segments of it was fused to the glutathione-binding site (GST fusions) or to the Gal4 DNA-binding domain (amino acids 1 to 147) (Gal4 fusions). With the GST fusions, it was found that TEF-1 and the TATA-binding protein (TBP) bound different regions of T antigen. A GST fusion containing amino acids 5 to 172 (region T1) efficiently bound TBP. TEF-1 bound neither region T1 nor a region between amino acids 168 and 373 (region T2); however, it bound efficiently to the combined region (T5) containing amino acids 5 to 383.(ABSTRACT TRUNCATED AT 250 WORDS)

Mouse Oct-1 contains a composite homeodomain of human Oct-1 and Oct-2

Members of the Oct family of transcription factors specifically interact with the octamer motif, ATGC-AAAT, a regulatory element important for tissue- and cell-specific transcription as well as for the expression of housekeeping genes. Except for Oct-1, all Oct factors are expressed in a temporally and spatially restricted mode during murine development and their number varies in a given cell type. Despite its ubiquitous expression pattern Oct-1 may play a role in murine development. As a first step towards elucidating the role of Oct-1 we report the complementary DNA cloning of the mouse Oct-1 gene. Two large transcripts of 5 and 14 kb are derived from a single gene. The expression patterns of three splicing products of Oct-1 are similar in a number of cells and tissues. In the POU region murine Oct-1 differs in four amino acids from the human homologue and these differences are restricted to helices 1 and 2. Interestingly, two of the four variant amino acids are identical to those in human and mouse Oct-2 and thus the murine Oct-1 homeodomain is intermediary in sequence between human Oct-1 and Oct-2. These two amino acids together with a third one have been shown to be relevant for the interaction between human Oct-1 and herpes simplex virus transactivator VP16. Nevertheless, VP16 interacts albeit weakly with murine Oct-1. We speculate that the differences in the human and mouse Oct-1 homeodomains reflect host-specific differences in protein-protein interactions.

Altering the boundaries of Hox3.1 expression: evidence for antipodal gene regulation

To investigate the function of region-specific patterns of mouse homeobox gene expression during embryogenesis, we programmed a minimal change in the distribution of Hox3.1 transcripts along the anteroposterior body axis in transgenic mice. Regulatory sequences from Hox1.4, a gene normally expressed more anteriorly than Hox3.1, were chosen to direct expression of a Hox3.1 transgene. Offspring of independent transgenic lines expressed the transgene more anteriorly than the Hox3.1 gene. Rather than predicted posterior transformations, we observed anterior transformations of vertebrae in newborn mice. Transgenic mice also developed profound gastrointestinal tissue malformations, which may provide a molecular explanation for human developmental disorders often involving these same two regions. Paradoxically, vertebral transformations in the transgenic mice were strikingly similar to those reported in mice homozygous for a null mutation of the Hox3.1 gene. This observation suggests that Hox genes may be regulated antipodally, with over- or underexpression resulting in similar phenotypes.

Cloning, chromosomal localization and expression pattern of the POU domain gene Oct-11

POU domain genes encode a family of highly conserved transacting factors that influence the transcriptional activity of several cell type-specific and ubiquitous genes. We have cloned and sequenced cDNAs encoding a novel mouse POU domain protein, Oct-11, that is closely related within the POU domain to the POU class II proteins, Oct-1 and Oct-2. Recombinant Oct-11 protein binds specifically to an octamer sequence in vitro. The Oct-11 gene is expressed during mouse embryogenesis and in the adult thymus and testis. In addition, it is abundant in the myeloma cell line P3/NS-1/1-Ag4.1. We describe the structure of Oct-11 and its chromosomal localization, and discuss the evidence that the POU class II gene family has evolved by duplication and divergence of a common ancestral gene.

Differential positive control by Oct-1 and Oct-2: activation of a transcriptionally silent motif through Oct-1 and VP16 corecruitment

Transcriptional regulation by the ubiquitous human POU homeo domain protein Oct-1 and the related B-cell protein Oct-2 is a model for understanding how proteins that recognize the same regulatory site elicit different programs of gene transcription. Here, we describe a mechanism for differential promoter activation whereby only Oct-1, through selective corecruitment with the herpesvirus trans-activator VP16, acquires the ability to stimulate transcription from a TAATGARAT-containing site that responds to neither Oct-1 nor Oct-2 alone. To measure differential in vivo activation by human Oct-1 and Oct-2 in response to VP16, we have developed a transient assay in murine NIH-3T3 cells. Surprisingly, murine Oct-1 associates with VP16 much less effectively than its human counterpart, most likely because the murine Oct-1 homeo domain differs at four positions from the human Oct-1 homeo domain. The murine cell transient assay shows directly that human Oct-1, but not human Oct-2, can respond to VP16 in vivo. The Oct-1 DNA-binding POU domain is sufficient and the Oct-1 homeo domain is critical for this response, because an Oct-1 POU domain containing the Oct-2 homeo domain fails to respond to the VP16-induced positive control of transcription. Thus, by selective homeo domain interaction and corecruitment to an otherwise silent regulatory element, VP16 expands the repertoire of sites responsive to Oct-1 without affecting the activity of its close relative Oct-2.

A single amino acid exchange transfers VP16-induced positive control from the Oct-1 to the Oct-2 homeo domain

The selective association of the herpesvirus trans-activator VP16 with the human Oct-1 homeo domain is a model for differential positive transcriptional control by homeo domains. VP16 discriminates between the closely related homeo domains of Oct-1 and Oct-2 by distinguishing among their seven amino-acid differences; these differences lie on the surface that is thought to be accessible when the homeo domain is bound to DNA. Only two of these seven differences are recognized by VP16, one in each of the first two alpha-helices of the tri-alpha-helical homeo domain. The major determinant for selective association with VP16 in vitro and VP16-induced positive control in vivo is a single glutamic acid residue at position 22 in the first alpha-helix of the Oct-1 homeo domain, but the acidic properties of this residue are not critical for association with VP16 in vitro or in vivo, because it can be replaced by glutamine with little or no deleterious effect. Mere replacement of the single corresponding alanine residue in the Oct-2 homeo domain with the key glutamic acid residue is sufficient to confer on the Oct-2 homeo domain the ability to associate with VP16 in vitro and respond to VP16-induced positive control in vivo. Thus, the specificity of homeo domain positive control can be conferred by a single amino acid difference.

POU domain transcription factors from different subclasses stimulate adenovirus DNA replication

POU domain proteins constitute a family of eukaryotic transcription factors that exert critical functions during development. They contain a conserved 160 amino acids DNA binding domain, the POU domain. Genetic data have demonstrated that some POU domain proteins are essential for the proliferation of specific cell types, suggesting a possible role in DNA replication. In addition, the ubiquitous POU transcription factor Oct-1 or its isolated POU domain enhances adenovirus DNA replication. Here we compared the DNA binding specificities of POU domain proteins from different subclasses. They exhibit overlapping, yet distinct binding site preferences. Furthermore, purified Pit-1, Oct-1, Oct-2, Oct-6, Oct-4 and zebrafish POU[C] could all stimulate adenovirus DNA replication in a reconstituted in vitro system. Thus, activation appears to depend on a property common to most POU domain proteins. Adenovirus DNA replication is also stimulated by the transcription factor NFI/CTF. In contrast to NFI, the POU domain did not enhance binding of precursor terminal protein-DNA polymerase to the origin nor did it stabilize the preinitiation complex. These results suggest that the POU domain acts on a rate limiting step after formation of the preinitiation complex.

Functional interference between the ubiquitous and constitutive octamer transcription factor 1 (OTF-1) and the glucocorticoid receptor by direct protein-protein interaction involving the homeo subdomain of OTF-1

The ubiquitous and constitutive octamer transcription factor OTF-1 (Oct 1) is the target of positive regulation by the potent herpes simplex virus trans-activator VP16, which forms a complex with the homeodomain of OTF-1. Here we present evidence that the glucocorticoid receptor can negatively regulate OTF-1 function by a mechanism that is independent of DNA binding. In vivo-expressed glucocorticoid receptor inhibited in a hormone-dependent manner activation of a minimal promoter construct carrying a functional octamer site. Moreover, expression of the receptor in vivo resulted in hormone-dependent repression of OTF-1-dependent DNA-binding activity in nuclear extract. In vitro, the DNA-binding activity of partially purified OTF-1 was repressed following incubation with purified glucocorticoid receptor. Cross-linking and immunoprecipitation experiments indicated that the functional interference may be due to a strong association between these two proteins in solution. Finally, preliminary evidence indicates that the homeo subdomain of OTF-1 that directs formation of a complex with VP16 may also be critical for interaction with the glucocorticoid receptor. Thus, OTF-1 is a target for both positive and negative regulation by protein-protein interaction. Moreover, the functional interference between OTF-1 and the glucocorticoid receptor represents a novel regulatory mechanism in the cross-coupling of signal transduction pathways of nuclear receptors and constitutive transcription factors.

The herpes simplex virus type 1 ICP6 gene is regulated by a 'leaky' early promoter

Expression from the promoter for the large subunit (ICP6) of the ribonucleotide reductase encoded by herpes simplex virus type 1 (HSV-1) has been examined. Using the lacZ reporter gene fused in-frame with ICP6 regulatory sequences to assay expression quantitatively, we showed that the ICP6 promoter responded very weakly to the alpha-transinducing factor (TIF) in the absence of all other viral gene products, but much more strongly to immediate early proteins. Similar patterns of regulation were observed when the reporter gene construct was located at two different positions within the the viral genome or in a stably transfected Vero cell line. Infection of the stably transfected cells with various HSV-1 mutants identified ICP0 as the major transactivator of the ICP6 promoter.

Functional interference between the ubiquitous and constitutive octamer transcription factor 1 (OTF-1) and the glucocorticoid receptor by direct protein-protein interaction involving the homeo subdomain of OTF-1

The ubiquitous and constitutive octamer transcription factor OTF-1 (Oct 1) is the target of positive regulation by the potent herpes simplex virus trans-activator VP16, which forms a complex with the homeodomain of OTF-1. Here we present evidence that the glucocorticoid receptor can negatively regulate OTF-1 function by a mechanism that is independent of DNA binding. In vivo-expressed glucocorticoid receptor inhibited in a hormone-dependent manner activation of a minimal promoter construct carrying a functional octamer site. Moreover, expression of the receptor in vivo resulted in hormone-dependent repression of OTF-1-dependent DNA-binding activity in nuclear extract. In vitro, the DNA-binding activity of partially purified OTF-1 was repressed following incubation with purified glucocorticoid receptor. Cross-linking and immunoprecipitation experiments indicated that the functional interference may be due to a strong association between these two proteins in solution. Finally, preliminary evidence indicates that the homeo subdomain of OTF-1 that directs formation of a complex with VP16 may also be critical for interaction with the glucocorticoid receptor. Thus, OTF-1 is a target for both positive and negative regulation by protein-protein interaction. Moreover, the functional interference between OTF-1 and the glucocorticoid receptor represents a novel regulatory mechanism in the cross-coupling of signal transduction pathways of nuclear receptors and constitutive transcription factors.

Recognition of the surface of a homeo domain protein

Homeo domain proteins exhibit distinct biological functions with specificities that cannot be predicted by their sequence specificities for binding DNA. Recognition of the surface of the Oct-1 POU homeo domain provides a general model for the contribution of selective protein-protein interactions to the functional specificity of the homeo domain family of factors. The assembly of Oct-1 into a multiprotein complex on the herpes simplex virus alpha/IE enhancer is specified by the interactions of its homeo domain with ancillary factors. This complex (C1 complex) is composed of the viral alpha TIF protein (VP16), Oct-1, and one additional cellular component, the C1 factor. Variants of the Oct-1 POU homeo domain were generated by site-directed mutagenesis, which altered the residues predicted to form the exposed surface of the domain-DNA complex. Proteins with single amino acid substitutions on the surface of either helix 1 or 2 of the Oct-1 POU homeo domain had decreased abilities to form the C1 complex. The behavior of these mutants in a cooperative DNA-binding assay with alpha TIF suggested that the Oct-1 POU homeo domain is principally recognized by alpha TIF in the C1 complex. The preferential recognition of Oct-1 over the closely related Oct-2 protein is critically influenced by a single residue on the surface of helix 1 because the introduction of this residue into the Oct-2 POU homeo domain significantly enhanced its ability to form a C1 complex.

Histone H2B gene transcription during Xenopus early development requires functional cooperation between proteins bound to the CCAAT and octamer motifs

The ubiquitously expressed transcription factor Oct-1 and several other members of the POU domain protein family bind to a site, termed the octamer motif, that functions in the promoter and enhancer regions of a variety of genes expressed under diverse conditions. An octamer motif present in a conserved histone H2B-specific promoter element is required for S-phase-specific transcription of mammalian histone H2B genes in cultured cells. We have previously shown that the octamer motif in a Xenopus histone H2B gene promoter was inactive in nondividing frog oocytes. Here we show that the octamer motif, in addition to regulatory elements (TATAA, CCAAT, and ATF motifs) that are active in oocytes, is required for maximal H2B gene transcription in developing frog embryos. Factors binding to each of the H2B upstream promoter elements are present in oocytes and increase slightly in abundance during early development. The activity of the H2B octamer motif in embryos is not specifically associated with increased binding by Oct-1 or the appearance of novel octamer-binding proteins but requires the presence of an intact CCAAT motif. Our results indicate that synergistic interactions among promoter-bound factors are important for octamer-dependent H2B transcription. We suggest that the activity of the H2B promoter is regulated primarily by changes in the interactions between proteins already bound to the promoter rather than by alterations in their intrinsic abilities to bind DNA.

A novel B cell-derived coactivator potentiates the activation of immunoglobulin promoters by octamer-binding transcription factors

A novel B cell-restricted activity, required for high levels of octamer/Oct-dependent transcription from an immunoglobulin heavy chain (IgH) promoter, was detected in an in vitro system consisting of HeLa cell-derived extracts complemented with fractionated B cell nuclear proteins. The factor responsible for this activity was designated Oct coactivator from B cells (OCA-B). OCA-B stimulates the transcription from an IgH promoter in conjunction with either Oct-1 or Oct-2 but shows no significant effect on the octamer/Oct-dependent transcription of the ubiquitously expressed histone H2B promoter and the transcription of USF- and Sp1-regulated promoters. Taken together, our results suggest that OCA-B is a tissue-, promoter-, and factor-specific coactivator and that OCA-B may be a major determinant for B cell-specific activation of immunoglobulin promoters. In light of the evidence showing physical and functional interactions between Oct factors and OCA-B, we propose a mechanism of action for OCA-B and discuss the implications of OCA-B for the transcriptional regulation of other tissue-specific promoters.

Histone H2B gene transcription during Xenopus early development requires functional cooperation between proteins bound to the CCAAT and octamer motifs

The ubiquitously expressed transcription factor Oct-1 and several other members of the POU domain protein family bind to a site, termed the octamer motif, that functions in the promoter and enhancer regions of a variety of genes expressed under diverse conditions. An octamer motif present in a conserved histone H2B-specific promoter element is required for S-phase-specific transcription of mammalian histone H2B genes in cultured cells. We have previously shown that the octamer motif in a Xenopus histone H2B gene promoter was inactive in nondividing frog oocytes. Here we show that the octamer motif, in addition to regulatory elements (TATAA, CCAAT, and ATF motifs) that are active in oocytes, is required for maximal H2B gene transcription in developing frog embryos. Factors binding to each of the H2B upstream promoter elements are present in oocytes and increase slightly in abundance during early development. The activity of the H2B octamer motif in embryos is not specifically associated with increased binding by Oct-1 or the appearance of novel octamer-binding proteins but requires the presence of an intact CCAAT motif. Our results indicate that synergistic interactions among promoter-bound factors are important for octamer-dependent H2B transcription. We suggest that the activity of the H2B promoter is regulated primarily by changes in the interactions between proteins already bound to the promoter rather than by alterations in their intrinsic abilities to bind DNA.

Human Oct3 gene family: cDNA sequences, alternative splicing, gene organization, chromosomal location, and expression at low levels in adult tissues

Transcription factors containing the POU-domain have been shown to be important regulators of tissue-specific gene expression in the pituitary and lymphoid cells. Using a polymerase chain reaction (PCR)-based strategy, we have searched for similar factors that may be expressed in adult human pancreatic islets. This approach resulted in the amplification of sequences encoding the octamer binding proteins Oct1 and Oct3 (also called Oct4). The isolation of cDNAs encoding Oct3 revealed the expression of two isoforms of this transcription factor termed Oct3A and Oct3B that are generated by alternative splicing. Human Oct3A and Oct3B are composed of 360 and 265 amino acids, respectively, of which the 225 amino acids at the COOH-termini are identical. The sequence of human Oct3A shows 87% amino acid identity with mouse Oct3. Reverse-transcriptase PCR showed low levels of expression of both Oct3A and Oct3B mRNA in all adult human tissues examined. We also isolated and characterized the human Oct3 gene (OTF3) and a related gene, OTF3C. The human Oct3 gene, localized to human chromosome 6 in the region of the MHC complex, spans about 7 kb and consists of five exons. The Oct3-related gene, OTF3C, is a retroposon and has been localized to human chromosome 8. Southern blotting and PCR amplification of human DNA indicated the presence of other OTF3-related genes as has been previously noted in the mouse. Two polymorphisms which can be typed using PCR were identified in OTF3 which will facilitate genetic studies of this gene.

Human and Drosophila homeodomain proteins that enhance the DNA-binding activity of serum response factor

Cells with distinct developmental histories can respond differentially to identical signals, suggesting that signals are interpreted in a fashion that reflects a cell's identity. How this might occur is suggested by the observation that proteins of the homeodomain family, including a newly identified human protein, enhance the DNA-binding activity of serum response factor, a protein required for the induction of genes by growth and differentiation factors. Interaction with proteins of the serum response factor family may allow homeodomain proteins to specify the transcriptional response to inductive signals. Moreover, because the ability to enhance the binding of serum response factor to DNA residues within the homeodomain but is independent of homeodomain DNA-binding activity, this additional activity of the homeodomain may account for some of specificity of action of homeodomain proteins in development.

Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity

Previous cotransfection experiments had demonstrated that ectopic expression of the lymphocyte-specific transcription factor Oct2 could efficiently activate a promoter containing an octamer motif. Oct2 expression was unable to stimulate a multimerized octamer enhancer element in HeLa cells, however. We have tested a variety of Oct2 isoforms generated by alternative splicing for the capability to activate an octamer enhancer in nonlymphoid cells and a B-cell line. Our analyses show that several Oct2 isoforms can stimulate from a remote position but that this stimulation is restricted to B cells. This result indicates the involvement of either a B-cell-specific cofactor or a specific modification of a cofactor or the Oct2 protein in Oct2-mediated enhancer activation. Mutational analyses indicate that the carboxy-terminal domain of Oct2 is critical for enhancer activation. Moreover, this domain conferred enhancing activity when fused to the Oct1 protein, which by itself was unable to stimulate from a remote position. The glutamine-rich activation domain present in the amino-terminal portion of Oct2 and the POU domain contribute only marginally to the transactivation function from a distal position.

Oct-1 and Oct-2 potentiate functional interactions of a transcription factor with the proximal sequence element of small nuclear RNA genes

The promoters of both RNA polymerase II- and RNA polymerase III-transcribed small nuclear RNA (snRNA) genes contain an essential and highly conserved proximal sequence element (PSE) approximately 55 bp upstream from the transcription start site. In addition, the upstream enhancers of all snRNA genes contain binding sites for octamer-binding transcription factors (Octs), and functional studies have indicated that the PSE and octamer elements work cooperatively. The present study has identified and characterized a novel transcription factor (designated PTF) which specifically binds to the PSE sequence of both RNA polymerase II- and RNA polymerase III-transcribed snRNA genes. PTF binding is markedly potentiated by Oct binding to an adjacent octamer site. This potentiation is effected by Oct-1, Oct-2, or the conserved POU domain of these factors. In agreement with these results and despite the independent binding of Octs to the promoter, PTF and Oct-1 enhance transcription from the 7SK promoter in an interdependent manner. Moreover, the POU domain of Oct-1 is sufficient for significant in vitro activity in the presence of PTF. These results suggest that essential activation domains reside in PTF and that the potentiation of PTF binding by Octs plays a key role in the function of octamer-containing snRNA gene enhancers.

Ethidium bromide provides a simple tool for identifying genuine DNA-independent protein associations

DNA-dependent and DNA-independent associations of DNA-binding proteins are important in transcriptional regulation. The analysis of DNA-independent associations frequently relies on assaying protein interaction in the absence of target DNA sequences. We have found that contaminating DNA in protein preparations can stabilize DNA-dependent associations that may appear DNA-independent. Three cellular proteins of 70, 85, and 110 kDa coimmunoprecipitated with the octamer motif-binding protein Oct-2 because of the presence of contaminating DNA in the cell extracts. In addition, heterodimer formation between Oct-1 (or Oct-2) and Pit-1 during protein-affinity chromatography was stabilized by the contaminating DNA. In both instances, these DNA-dependent protein associations were selectively inhibited by ethidium bromide in the precipitation reaction without any evident effect on DNA-independent protein associations. Thus, ethidium bromide may serve as a simple and general indicator of DNA-dependent and DNA-independent protein associations.

Mapping functional specificity in the Dfd and Ubx homeo domains

To define homeo domain subregions that are important for embryonic targeting specificity of homeotic proteins, we generated a series of Deformed/Ultrabithorax chimeric genes in which parts of the Deformed homeo box region were substituted with Ultrabithorax sequences. Chimeric coding regions were attached to heat shock promoters and introduced into the Drosophila genome by P-element transformation. After heat-induced ectopic expression in embryos, we examined the cuticular phenotypes induced by the resulting chimeric proteins. We also tested the ability of the chimeric proteins to regulate transcription units that are normal targets of Deformed and Ultrabithorax. Our results indicate that specific amino acid residues at the amino end of the Ultrabithorax homeo domain are required to specifically regulate Antennapedia transcription; and in the context of a Deformed protein, these amino-end residues are sufficient to switch from Deformed- to Ultrabithorax-like targeting specificity. Although residues in the amino end of the homeo domain are also important in determining a Deformed-like targeting specificity, other regions of the Deformed homeo domain are also required for full activity.

Oct-1 and Oct-2 potentiate functional interactions of a transcription factor with the proximal sequence element of small nuclear RNA genes

The promoters of both RNA polymerase II- and RNA polymerase III-transcribed small nuclear RNA (snRNA) genes contain an essential and highly conserved proximal sequence element (PSE) approximately 55 bp upstream from the transcription start site. In addition, the upstream enhancers of all snRNA genes contain binding sites for octamer-binding transcription factors (Octs), and functional studies have indicated that the PSE and octamer elements work cooperatively. The present study has identified and characterized a novel transcription factor (designated PTF) which specifically binds to the PSE sequence of both RNA polymerase II- and RNA polymerase III-transcribed snRNA genes. PTF binding is markedly potentiated by Oct binding to an adjacent octamer site. This potentiation is effected by Oct-1, Oct-2, or the conserved POU domain of these factors. In agreement with these results and despite the independent binding of Octs to the promoter, PTF and Oct-1 enhance transcription from the 7SK promoter in an interdependent manner. Moreover, the POU domain of Oct-1 is sufficient for significant in vitro activity in the presence of PTF. These results suggest that essential activation domains reside in PTF and that the potentiation of PTF binding by Octs plays a key role in the function of octamer-containing snRNA gene enhancers.

Oct2 transactivation from a remote enhancer position requires a B-cell-restricted activity

Previous cotransfection experiments had demonstrated that ectopic expression of the lymphocyte-specific transcription factor Oct2 could efficiently activate a promoter containing an octamer motif. Oct2 expression was unable to stimulate a multimerized octamer enhancer element in HeLa cells, however. We have tested a variety of Oct2 isoforms generated by alternative splicing for the capability to activate an octamer enhancer in nonlymphoid cells and a B-cell line. Our analyses show that several Oct2 isoforms can stimulate from a remote position but that this stimulation is restricted to B cells. This result indicates the involvement of either a B-cell-specific cofactor or a specific modification of a cofactor or the Oct2 protein in Oct2-mediated enhancer activation. Mutational analyses indicate that the carboxy-terminal domain of Oct2 is critical for enhancer activation. Moreover, this domain conferred enhancing activity when fused to the Oct1 protein, which by itself was unable to stimulate from a remote position. The glutamine-rich activation domain present in the amino-terminal portion of Oct2 and the POU domain contribute only marginally to the transactivation function from a distal position.

Herpes simplex virus immediate early gene expression in the absence of transinduction by Vmw65 varies during the cell cycle

The requirement for the herpes simplex virus type 1 (HSV-1) protein Vmw65 (VP16) for activation of immediate early (IE) gene expression was examined in synchronized HeLa cells. Analyses of IE RNA levels were conducted during infection with a viral Vmw65 mutant, in1814. The results revealed an increased requirement for Vmw65 when cultures reached G2 phase of the cell cycle. The levels of IE RNAs 1, 2, and 4 were reduced 5-10 times more in G2 than G1/S for in1814-infected cells when compared to cells infected with wild-type virus or 1814R (a rescued virus), and similar but smaller effects were observed on IE RNA 3 levels. The relative decrease at G2 was reversed by resynchronization of cells to G1/S. Mutant in1814 formed plaques less efficiently on cells at G2 than on cells synchronized at G1/S. The results show that, in the absence of functional Vmw65, HSV-1 IE gene expression and replication vary during the cell cycle.

Activation of simian virus 40 transcription in vitro by T antigen

Simian virus 40 is repressed when the viral early gene product large tumor antigen (TAg) binds to specific sites within the viral origin and DNA replication ensues. Late transcription is activated by TAg, even in the absence of viral DNA replication. We show here that TAg produced in human 293 cells can selectively activate Simian virus 40 transcription in a cell-free system. In the absence of DNA binding by TAg, early and late transcription are both activated, as they are in vivo, suggesting that the effect might be mediated by a cellular component(s) utilized by both the early and late promoters. When TAg binds to the viral origin of replication, early transcription is repressed but the late promoter activation is unaffected. Various preparations of TAg differed in their activities, with some able both to bind DNA and to activate transcription and others able to do only one or the other. Since these variations might be explained by variable amounts of associated factors that copurified with TAg, we asked whether a bacterially derived protein could regulate transcription. An NH2-terminal 272-amino-acid fragment of TAg, produced in Escherichia coli as a glutathione S-transferase fusion protein, retains the ability to activate transcription in vitro, similar to that of the full-length protein. Structural features of this region that might be important are discussed.

The hsp70 gene CCAAT-binding factor mediates transcriptional activation by the adenovirus E1a protein

Expression of the human hsp70 gene is cell cycle regulated and is inducible by both serum and the adenovirus E1a protein (K. Milarski and R. Morimoto, Proc. Natl. Acad. Sci. USA 83:9517-9521, 1986; M. C. Simon, K. Kitchener, H.-T. Kao, E. Hickey, L. Weber, R. Voellmy, N. Heintz, and J. R. Nevins, Mol. Cell. Biol. 7:2884-2890, 1987; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986; B. Wu and R. Morimoto, Proc. Natl. Acad. Sci. USA 82:6070-6074, 1985). This regulated expression is predominantly controlled by the CCAAT element at position -70 relative to the transcriptional initiation site (G. Williams, T. McClanahan, and R. Morimoto, Mol. Cell. Biol. 9:2574-2587, 1989; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986). A corresponding CCAAT-binding factor (CBF) of 999 amino acids has recently been cloned and shown to stimulate transcription selectively from the hsp70 promoter in a CCAAT element-dependent manner (L. Lum, L. Sultzman, R. Kaufman, D. Linzer, and B. Wu, Mol. Cell. Biol. 10:6709-6717, 1990). We report here that the first 192 residues of CBF, when fused to the DNA-binding domain of the heterologous activator GAL-4, are necessary and sufficient to mediate E1a-dependent transcriptional activation. E1a and CBF exhibit complex formation in vitro, suggesting that an in vivo interaction between these proteins may be relevant to the well-characterized E1a-induced transcriptional activation of the hsp70 promoter.

The hsp70 gene CCAAT-binding factor mediates transcriptional activation by the adenovirus E1a protein

Expression of the human hsp70 gene is cell cycle regulated and is inducible by both serum and the adenovirus E1a protein (K. Milarski and R. Morimoto, Proc. Natl. Acad. Sci. USA 83:9517-9521, 1986; M. C. Simon, K. Kitchener, H.-T. Kao, E. Hickey, L. Weber, R. Voellmy, N. Heintz, and J. R. Nevins, Mol. Cell. Biol. 7:2884-2890, 1987; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986; B. Wu and R. Morimoto, Proc. Natl. Acad. Sci. USA 82:6070-6074, 1985). This regulated expression is predominantly controlled by the CCAAT element at position -70 relative to the transcriptional initiation site (G. Williams, T. McClanahan, and R. Morimoto, Mol. Cell. Biol. 9:2574-2587, 1989; B. Wu, H. Hurst, N. Jones, and R. Morimoto, Mol. Cell. Biol. 6:2994-2999, 1986). A corresponding CCAAT-binding factor (CBF) of 999 amino acids has recently been cloned and shown to stimulate transcription selectively from the hsp70 promoter in a CCAAT element-dependent manner (L. Lum, L. Sultzman, R. Kaufman, D. Linzer, and B. Wu, Mol. Cell. Biol. 10:6709-6717, 1990). We report here that the first 192 residues of CBF, when fused to the DNA-binding domain of the heterologous activator GAL-4, are necessary and sufficient to mediate E1a-dependent transcriptional activation. E1a and CBF exhibit complex formation in vitro, suggesting that an in vivo interaction between these proteins may be relevant to the well-characterized E1a-induced transcriptional activation of the hsp70 promoter.