The VP16 paradox: herpes simplex virus VP16 contains a long-range activation domain but within the natural multiprotein complex activates only from promoter-proximal positions

Synchronization of gene expression across eukaryotic communities through chemical rhythms

The synchronization is a recurring phenomenon in neuroscience, ecology, human sciences, and biology. However, controlling synchronization in complex eukaryotic consortia on extended spatial-temporal scales remains a major challenge. Here, to address this issue we construct a minimal synthetic system that directly converts chemical signals into a coherent gene expression synchronized among eukaryotic communities through rate-dependent hysteresis. Guided by chemical rhythms, isolated colonies of yeast Saccharomyces cerevisiae oscillate in near-perfect synchrony despite the absence of intercellular coupling or intrinsic oscillations. Increased speed of chemical rhythms and incorporation of feedback in the system architecture can tune synchronization and precision of the cell responses in a growing cell collectives. This synchronization mechanism remain robust under stress in the two-strain consortia composed of toxin-sensitive and toxin-producing strains. The sensitive cells can maintain the spatial-temporal synchronization for extended periods under the rhythmic toxin dosages produced by killer cells. Our study provides a simple molecular framework for generating global coordination of eukaryotic gene expression through dynamic environment.

An Orthologue of the Retinoic Acid Receptor (RAR) Is Present in the Ecdysozoa Phylum Priapulida

Signalling molecules and their cognate receptors are central components of the Metazoa endocrine system. Defining their presence or absence in extant animal lineages is critical to accurately devise evolutionary patterns, physiological shifts and the impact of endocrine disrupting chemicals. Here, we address the evolution of retinoic acid (RA) signalling in the Priapulida worm, Priapulus caudatus Lamarck, 1816, an Ecdysozoa. RA signalling has been shown to be central to chordate endocrine homeostasis, participating in multiple developmental and physiological processes. Priapulids, with their slow rate of molecular evolution and phylogenetic position, represent a key taxon to investigate the early phases of Ecdysozoa evolution. By exploring a draft genome assembly, we show, by means of phylogenetics and functional assays, that an orthologue of the nuclear receptor retinoic acid receptor (RAR) subfamily, a central mediator of RA signalling, is present in Ecdysozoa, contrary to previous perception. We further demonstrate that the Priapulida RAR displays low-affinity for retinoids (similar to annelids), and is not responsive to common endocrine disruptors acting via RAR. Our findings provide a timeline for RA signalling evolution in the Bilateria and give support to the hypothesis that the increase in RA affinity towards RAR is a late acquisition in the evolution of the Metazoa.

The retinoic acid receptor (RAR) in molluscs: Function, evolution and endocrine disruption insights

Retinoid acid receptor (RAR)-dependent signalling pathways are essential for the regulation and maintenance of essential biological functions and are recognized targets of disruptive anthropogenic compounds. Recent studies put forward the inability of mollusc RARs to bind and respond to the canonical vertebrate ligand, retinoic acid: a feature that seems to have been lost during evolution. Yet, these studies were carried out in a limited number of molluscs. Therefore, using an in vitro transactivation assay, the present work aimed to characterize phylogenetically relevant mollusc RARs, as monomers or as functional units with RXR, not only in the presence of vertebrate bone fine ligands but also known endocrine disruptors, described to modulate retinoid-dependent pathways. In general, none of the tested mollusc RARs were able to activate reporter gene transcription when exposed to retinoic acid isomers, suggesting that the ability to respond to retinoic acid was lost across molluscs. Similarly, the analysed mollusc RAR were unresponsive towards organochloride pesticides. In contrast, transcriptional repressions were observed with the RAR/RXR unit upon exposure to retinoids or RXR-specific ligands. Loss-of-function and gain-of-function mutations further corroborate the obtained results and suggest that the repressive behaviour, observed with mollusc and human RAR/RXR heterodimers, is possibly mediated by ligand biding to RXR.

The PPARα-dependent rodent liver tumor response is not relevant to humans: addressing misconceptions

A number of industrial chemicals and therapeutic agents cause liver tumors in rats and mice by activating the nuclear receptor peroxisome proliferator-activated receptor α (PPARα). The molecular and cellular events by which PPARα activators induce rodent hepatocarcinogenesis have been extensively studied elucidating a number of consistent mechanistic changes linked to the increased incidence of liver neoplasms. The weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis is summarized here. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators. The key events (KE) identified in the MOA are PPARα activation (KE1), alteration in cell growth pathways (KE2), perturbation of hepatocyte growth and survival (KE3), and selective clonal expansion of preneoplastic foci cells (KE4), which leads to the apical event-increases in hepatocellular adenomas and carcinomas (KE5). In addition, a number of concurrent molecular and cellular events have been classified as modulating factors, because they potentially alter the ability of PPARα activators to increase rodent liver cancer while not being key events themselves. These modulating factors include increases in oxidative stress and activation of NF-kB. PPARα activators are unlikely to induce liver tumors in humans due to biological differences in the response of KEs downstream of PPARα activation. This conclusion is based on minimal or no effects observed on cell growth pathways and hepatocellular proliferation in human primary hepatocytes and absence of alteration in growth pathways, hepatocyte proliferation, and tumors in the livers of species (hamsters, guinea pigs and cynomolgus monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Despite this overwhelming body of evidence and almost universal acceptance of the PPARα MOA and lack of human relevance, several reviews have selectively focused on specific studies that, as discussed, contradict the consensus opinion and suggest uncertainty. In the present review, we systematically address these most germane suggested weaknesses of the PPARα MOA.

Human relevance of rodent liver tumors: Key insights from a Toxicology Forum workshop on nongenotoxic modes of action

The Toxicology Forum sponsored a workshop in October 2016, on the human relevance of rodent liver tumors occurring via nongenotoxic modes of action (MOAs). The workshop focused on two nuclear receptor-mediated MOAs (Constitutive Androstane Receptor (CAR) and Peroxisome Proliferator Activated Receptor-alpha (PPARα), and on cytotoxicity. The goal of the meeting was to review the state of the science to (1) identify areas of consensus and differences, data gaps and research needs; (2) identify reasons for inconsistencies in current regulatory positions; and (3) consider what data are needed to demonstrate a specific MOA, and when additional research is needed to rule out alternative possibilities. Implications for quantitative risk assessment approaches were discussed, as were implications of not considering MOA and dose in hazard characterization and labeling schemes. Most, but not all, participants considered the CAR and PPARα MOAs as not relevant to humans based on quantitative and qualitative differences. In contrast, cytotoxicity is clearly relevant to humans, but a threshold applies. Questions remain for all three MOAs concerning what data are necessary to determine the MOA and to what extent it is necessary to exclude other MOAs.

Evaluation of sgRNA target sites for CRISPR-mediated repression of TP53

The CRISPR (clustered regularly interspaced short palindromic repeats) platform has been developed as a general method to direct proteins of interest to gene targets. While the native CRISPR system delivers a nuclease that cleaves and potentially mutates target genes, researchers have recently employed catalytically inactive CRISPR-associated 9 nuclease (dCas9) in order to target and repress genes without DNA cleavage or mutagenesis. With the intent of improving repression efficiency in mammalian cells, researchers have also fused dCas9 with a KRAB repressor domain. Here, we evaluated different genomic sgRNA targeting sites for repression of TP53. The sites spanned a 200-kb distance, which included the promoter, transcript sequence, and regions flanking the endogenous human TP53 gene. We showed that repression up to 86% can be achieved with dCas9 alone (i.e., without use of the KRAB domain) by targeting the complex to sites near the TP53 transcriptional start site. This work demonstrates that efficient transcriptional repression of endogenous human genes can be achieved by the targeted delivery of dCas9. Yet, the efficiency of repression strongly depends on the choice of the sgRNA target site.

Synthetic TAL effectors for targeted enhancement of transgene expression in plants

Transcription activator-like effectors (TALEs), secreted by the pathogenic bacteria Xanthomonas, specifically activate expression of targeted genes in plants. Here, we designed synthetic TALEs that bind to the flanking regions of the TATA-box motif on the CaMV 35S promoter for the purpose of understanding the engineerable 'hot-spots' for increasing transgene expression. We demonstrated that transient expression of de novo-engineered TALEs using agroinfiltration could significantly increase reporter gene expression in stable transgenic tobacco expressing the orange fluorescent protein reporter gene pporRFP under the control of synthetic inducible, minimal or full-length 35S promoters. Moreover, the additive effects of a combination of two different synthetic TALEs could significantly enhance the activation effects of TALEs on reporter gene expression more than when each TALE was used individually. We also studied the effects of the C-terminal domain and the activation domain of synthetic TALEs, as well as the best 'hot-spots' on the 35S promoter on targeted transgene activation. Furthermore, TALE activation of the Arabidopsis MYB transcription factor AtPAP1 (PRODUCTION OF ANTHOCYANIN PIGMENT 1) in stable transgenic tobacco gave rise to a dark purple colour on infiltrated leaves when driven by four copies of cis-regulatory elements of pathogenesis-related gene (PR1) with enhancer motifs B and A1 from the 35S promoter. These results provide novel insights into the potential applications of synthetic TALEs for targeted gene activation of transgenes in plants.

Direct activation of human and mouse Oct4 genes using engineered TALE and Cas9 transcription factors

The newly developed transcription activator-like effector protein (TALE) and clustered regularly interspaced short palindromic repeats/Cas9 transcription factors (TF) offered a powerful and precise approach for modulating gene expression. In this article, we systematically investigated the potential of these new tools in activating the stringently silenced pluripotency gene Oct4 (Pou5f1) in mouse and human somatic cells. First, with a number of TALEs and sgRNAs targeting various regions in the mouse and human Oct4 promoters, we found that the most efficient TALE-VP64s bound around -120 to -80 bp, while highly effective sgRNAs targeted from -147 to -89-bp upstream of the transcription start sites to induce high activity of luciferase reporters. In addition, we observed significant transcriptional synergy when multiple TFs were applied simultaneously. Although individual TFs exhibited marginal activity to up-regulate endogenous gene expression, optimized combinations of TALE-VP64s could enhance endogenous Oct4 transcription up to 30-fold in mouse NIH3T3 cells and 20-fold in human HEK293T cells. More importantly, the enhancement of OCT4 transcription ultimately generated OCT4 proteins. Furthermore, examination of different epigenetic modifiers showed that histone acetyltransferase p300 could enhance both TALE-VP64 and sgRNA/dCas9-VP64 induced transcription of endogenous OCT4. Taken together, our study suggested that engineered TALE-TF and dCas9-TF are useful tools for modulating gene expression in mammalian cells.

Complexity of Interferon-γ Interactions with HSV-1

The intricacies involving the role of interferon-gamma (IFN-γ) in herpesvirus infection and persistence are complex. Herpes simplex virus type 1 (HSV-1) uses a variety of receptors to enter cells and is transported to and from the host cell nucleus over the microtubule railroad via retrograde and anterograde transport. IFN-γ exerts dual but conflicting effects on microtubule organization. IFN-γ stimulates production of suppressors of cytokine signaling 1 and 3 (SOCS1 and SOCS3), which are involved in microtubule stability and are negative regulators of IFN-γ signaling when overexpressed. IFN-γ also interferes with the correct assembly of microtubules causing them to undergo severe bundling, contributing to its anti-viral effect. Factors leading to the decision for a replicative virus lytic cycle or latency in the trigeminal ganglion (TG) occur on histone 3 (H3), involve IFN-γ produced by natural killer cells and non-cytolytic CD8⁺T cells, SOCS1, SOCS3, and M2 anti-inflammatory microglia/macrophages maintained by inhibitory interleukin 10 (IL-10). Both M2 microglia and CD4⁺CD25⁺Foxp3⁺ Treg cells produce IL-10. Histone deacetylases (HDACs) are epigenetic regulators maintaining chromatin in an inactive state necessary for transcription of IFN-γ-activated genes and their anti-viral effect. Following inhibition of HDACs by stressors such as ultraviolet light, SOCS1 and SOCS3 are acetylated, and chromatin is relaxed and available for virus replication. SOCS1 prevents expression of MHC class 1 molecules on neuronal cells and SOCS3 attenuates cytokine-induced inflammation in the area. A model is presented to unify the effects of IFN-γ, SOCS1, SOCS3, and HSV-1 on H3 and chromatin structure in virus latency or reactivation. HSV-1 latency in the TG is viewed as an active ongoing process involving maintenance of microglia in an M2 anti-inflammatory state by IL-10. IL-10 is produced in an autocrine manner by the M2 microglia/macrophages and by virus-specific CD4⁺Foxp3⁺ Treg cells interacting with virus-specific non-cytolytic CD8⁺ T cells.

Mode of action framework analysis for receptor-mediated toxicity: The peroxisome proliferator-activated receptor alpha (PPARα) as a case study

Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Targeted transcriptional activation of silent oct4 pluripotency gene by combining designer TALEs and inhibition of epigenetic modifiers

Specific control of gene activity is a valuable tool to study and engineer cellular functions. Recent studies uncovered the potential of transcription activator-like effector (TALE) proteins that can be tailored to activate user-defined target genes. It remains however unclear whether and how epigenetic modifications interfere with TALE-mediated transcriptional activation. We studied the activity of five designer TALEs (dTALEs) targeting the oct4 pluripotency gene. In vitro assays showed that the five dTALEs that target distinct sites in the oct4 promoter had the expected DNA specificity and comparable affinities to their corresponding DNA targets. In contrast to their similar in vitro properties, transcriptional activation of oct4 by these distinct dTALEs varied up to 25-fold. While dTALEs efficiently upregulated transcription of the active oct4 promoter in embryonic stem cells (ESCs) they failed to activate the silenced oct4 promoter in ESC-derived neural stem cells (NSCs), indicating that as for endogenous transcription factors also dTALE activity is limited by repressive epigenetic mechanisms. We therefore targeted the activity of epigenetic modulators and found that chemical inhibition of histone deacetylases by valproic acid or DNA methyltransferases by 5-aza-2′-deoxycytidine facilitated dTALE-mediated activation of the epigenetically silenced oct4 promoter in NSCs. Notably, demethylation of the oct4 promoter occurred only if chemical inhibitors and dTALEs were applied together but not upon treatment with inhibitors or dTALEs only. These results show that dTALEs in combination with chemical manipulation of epigenetic modifiers facilitate targeted transcriptional activation of epigenetically silenced target genes.

Noninvasive imaging of therapeutic gene expression using a bidirectional transcriptional amplification strategy

Promoters that limit transgene expression to tumors play a vital role in cancer gene therapy. Although tumor specific, the human Survivin promoter (pSurv) elicits low levels of transcription. A bidirectional two-step transcriptional amplification (TSTA) system was designed to enhance expression of the therapeutic gene (TG) tumor necrosis factor-alpha-related apoptosis-inducing ligand (TRAIL or TR) and the reporter gene firefly luciferase (FL) from pSurv. An adenoviral vector carrying the enhanced targeting apparatus (Ad-pSurv-TR-G8-FL) was tested for efficiency and specificity of gene expression in cells and in living animals. Compared to the one-step systems (Ad-pSurv-FL or Ad-pSurv-TR), the bidirectional TSTA system showed tenfold higher expression of both the therapeutic and the reporter gene and their expression correlated in cells (R(2) = 0.99) and in animals (R(2) = 0.67). Noninvasive quantitative monitoring of magnitude and time variation of TRAIL gene expression was feasible by bioluminescence imaging of the transcriptionally linked FL gene in xenograft tumors following intratumoral adenoviral injection. Moreover, the TSTA adenovirus maintained promoter specificity in nontarget tissues following tail vein administration. These studies demonstrate the potential of the bidirectional TSTA system to achieve high levels of gene expression from a weak promoter, while preserving specificity and the ability to image expression of the TG noninvasively.

Promoter Crosstalk Effects on Gene Expression

Closely spaced transcription signals formally assigned to different, neighboring genes can functionally interact both in their authentic genomic context as well as in engineered transgene constellations. To describe these promoter crosstalk effects quantitatively and qualitatively, we used various combinations of inducible and constitutive expression signals linked in cis. Our results demonstrate that such interactions can be bidirectional, making it difficult to unambiguously assign a particular promoter element exclusively to a specific gene. We show that especially for inducible promoters, crosstalk effects can cause a substantial distortion in the expression of proximal genes, challenging established strategies in generating transgenic animal models and tissue culture systems. Furthermore, these findings provide guidelines for the design of transgenic transcription units that, while refractory to interactions with their chromosomal environment, leave the expression programs of neighboring genes largely untouched.

Effect of promoter strength on protein expression and immunogenicity of an HSV-1 amplicon vector encoding HIV-1 Gag

Helper-free herpes simplex virus type-1 (HSV-1) amplicon vectors elicit robust immune responses to encoded proteins, including human immunodeficiency virus type-1 (HIV-1) antigens. To improve this vaccine delivery system, seven amplicon vectors were constructed, each encoding HIV-1 Gag under the control of a different promoter. Gag expression levels were analyzed in murine and human cell lines, as well as in biopsied tissue samples from injected mice; these data were then compared with Gag-specific T cell responses in BALB/c mice. The magnitude of the amplicon-induced immune response was found to correlate strongly with the level of Gag production both in vitro and in vivo. Interestingly, the best correlation of the strength of the amplicon-induced immune response was with antigen expression in cultured DC rather than expression at the tissue site of injection or in cultured cell lines. These findings may have implications for the generation of improved HSV-1 amplicon vectors for HIV-1 vaccine delivery.

An activation domain in the C-terminal subunit of HCF-1 is important for transactivation by VP16 and LZIP

In herpes simplex virus, lytic replication is initiated by the viral transactivator VP16 acting with cellular cofactors Oct-1 and HCF-1. Although this activator complex has been studied in detail, the role of HCF-1 remains elusive. Here, we show that HCF-1 contains an activation domain (HCF-1(AD)) required for maximal transactivation by VP16 and its cellular counterpart LZIP. Expression of the VP16 cofactor p300 augments HCF-1(AD) activity, suggesting a mechanism of synergy. Infection of cells lacking the HCF-1(AD) leads to reduced viral immediate-early gene expression and lowered viral titers. These findings underscore the importance of HCF-1 to herpes simplex virus replication and VP16 transactivation.

Proximal promoter-independent activation of the far-upstream FGF-inducible response element of syndecan-1 gene

Far upstream enhancers are predicted to act by looping and activating general transcription factors on core promoters and to require proximal promoter sequences for appropriate gene activation in time and space. We have previously described an FGF-inducible response element (FiRE) located far upstream on the syndecan-1 gene. The FiRE is activated specifically by members of the fibroblast growth factor (FGF) family in NIH3T3 cells. Here we describe the requirements of syndecan-1 proximal promoter for the activation of FiRE by FGF-2. Transient and stable transfections revealed that neither proximal promoter SP1 sites nor TATA-box are required for the FGF-2 induced activation of FiRE. Notably, the enhancer is activated in both orientations by FGF-2 even in the absence of proximal promoter. Importantly, removal of the promoter did not affect the growth factor specificity of FiRE. Proximal promoter independent activation of syndecan-1 gene by FGF-2 might be required during development when syndecan-1 proximal promoter needs to be largely attenuated but simultaneous transient and rapid FGF-2 induced transcription is required.

Differences in determinants required for complex formation and transactivation in related VP16 proteins

VP16-H is an essential structural protein of herpes simplex virus type 1 (HSV-1) and is also a potent activator of virus immediate-early (IE) gene expression. Current models of functional determinants within VP16-H indicate that it consists of two domains, an N-terminal domain involved in recruiting VP16-H to a multicomponent DNA binding complex with two host proteins, Oct-1 and host cell factor (HCF), and an acidic C-terminal domain exclusively involved in transactivation. VP16-E, from equine herpesvirus 1 (EHV-1), exhibits strong conservation with the N-terminal domain of VP16-H but, with the exception of a short segment at the extreme C terminus, lacks almost the entire acidic C-terminal domain. Studies of key activation determinants within the C terminus of VP16-H would predict that VP16-E may activate poorly, if at all. However, VP16-E is a potent activator of both EHV-1 and HSV-1 IE gene transcription. We show that VP16-E does not follow the simple two-domain model of VP16-H. Thus, despite the conservation in the N-terminal domains, this region in VP16-E is not sufficient for assembly into the DNA binding complex with Oct-1 and HCF. The short conserved determinant close to the C terminus is completely dispensable in VP16-H but is absolutely required in VP16-E. In activation studies, the potency of intact VP16-E was not recapitulated in chimeric proteins in which it was fused with a GAL4 DNA binding domain. Furthermore, a chimeric protein consisting of the C-terminal region of VP16-E fused to the N-terminal domain of VP16-H, while able to promote complex formation, nevertheless exhibited very weak activation. These results indicate that the mode of recruitment of the activation domain, i.e., through complex formation with Oct-1 and HCF, may be crucial for activation and that key determinants required for activation in VP16-E, and possibly VP16-H, may involve interactions between regions of the C terminus and the N terminus rather than discrete domains with independent functions.

Lack of enhancer function in mammals is unique to oocytes and fertilized eggs

Previous studies have shown that the lack of novel coactivator activity in mouse oocytes and one-cell embryos (fertilized eggs) renders them incapable of utilizing Gal4:VP16-dependent enhancers (distal elements) but not promoters (proximal elements) in regulating transcription. This coactivator activity first appears in two- to four-cell embryos coincident with the major activation of zygotic gene expression. Here we show that whereas oocytes and fertilized eggs could utilize Sp1-dependent promoters, they could not utilize Sp1-dependent enhancers, although they showed promoter repression, which is a requirement for delineating enhancer function. In contrast, both Sp1-dependent promoters and enhancers were functional in two- to four-cell embryos. Furthermore, the same embryonic stem cell mRNA that provided the coactivator activity for Gal4:VP16-dependent enhancer function also provided Sp1-dependent enhancer function in oocytes. Therefore, the coactivator activity appears to be a requirement for general enhancer function. To determine whether the absence of enhancer function is a unique property of oocytes or a general property of other terminally differentiated cells, transcription was examined in terminally differentiated hNT neurons and their precursors, undifferentiated NT2 stem cells. The results showed that both cell types could utilize enhancers and promoters. Thus, in mammals, the lack of enhancer function appears to be unique to oocytes and fertilized eggs, suggesting that it provides a safeguard against premature activation of genes prior to zygotic gene expression during development.

The herpesvirus transactivator VP16 mimics a human basic domain leucine zipper protein, luman, in its interaction with HCF

In human cells infected with herpes simplex virus (HSV), viral gene expression is initiated by the virion protein VP16. VP16 does not bind DNA directly but forms a multiprotein complex on the viral immediate-early gene promoters with two cellular proteins: the POU domain protein Oct-1 and host cell factor (HCF; also called C1, VCAF, and CFF). Despite its apparent role in stabilizing the VP16-induced transcription complex, the natural biological role of HCF is unclear. Only recently HCF has been implicated in control of the cell cycle. To determine the role of HCF in cells and answer why HSV has evolved an HCF-dependent mechanism for the initiation of the lytic cycle, we identified the first human ligand for HCF (R. Lu et al., Mol. Cell. Biol. 17:5117-5126, 1997). This protein, Luman, is a member of the CREB/ATF family of transcription factors that can activate transcription from promoters containing cyclic AMP response elements (CRE). Here we provide evidence that Luman and VP16 share two important structural features: an acidic activation domain and a common mechanism for binding HCF. We found that Luman, its homolog in Drosophila, dCREB-A (also known as BBF-2), and VP16 bind to HCF by a motif, (D/E)HXY(S/A), present in all three proteins. In addition, a mutation (P134S) in HCF that prevents VP16 binding also abolishes its binding to Luman and dCREB-A. We also show that while interaction with HCF is not required for the ability of Luman to activate transcription when tethered to the GAL4 promoter, it appears to be essential for Luman to activate transcription through CRE sites. These data suggest that the HCF-Luman interaction may represent a conserved mechanism for transcriptional regulation in metazoans, and HSV mimics this interaction with HCF to monitor the physiological state of the host cell.