The viral erbA oncogene protein, a constitutive repressor in animal cells, is a hormone-regulated activator in yeast

Transmembrane signaling in Saccharomyces cerevisiae as a model for signaling in metazoans: state of the art after 25 years

In the very first article that appeared in Cellular Signalling, published in its inaugural issue in October 1989, we reviewed signal transduction pathways in Saccharomyces cerevisiae. Although this yeast was already a powerful model organism for the study of cellular processes, it was not yet a valuable instrument for the investigation of signaling cascades. In 1989, therefore, we discussed only two pathways, the Ras/cAMP and the mating (Fus3) signaling cascades. The pivotal findings concerning those pathways undoubtedly contributed to the realization that yeast is a relevant model for understanding signal transduction in higher eukaryotes. Consequently, the last 25 years have witnessed the discovery of many signal transduction pathways in S. cerevisiae, including the high osmotic glycerol (Hog1), Stl2/Mpk1 and Smk1 mitogen-activated protein (MAP) kinase pathways, the TOR, AMPK/Snf1, SPS, PLC1 and Pkr/Gcn2 cascades, and systems that sense and respond to various types of stress. For many cascades, orthologous pathways were identified in mammals following their discovery in yeast. Here we review advances in the understanding of signaling in S. cerevisiae over the last 25 years. When all pathways are analyzed together, some prominent themes emerge. First, wiring of signaling cascades may not be identical in all S. cerevisiae strains, but is probably specific to each genetic background. This situation complicates attempts to decipher and generalize these webs of reactions. Secondly, the Ras/cAMP and the TOR cascades are pivotal pathways that affect all processes of the life of the yeast cell, whereas the yeast MAP kinase pathways are not essential. Yeast cells deficient in all MAP kinases proliferate normally. Another theme is the existence of central molecular hubs, either as single proteins (e.g., Msn2/4, Flo11) or as multisubunit complexes (e.g., TORC1/2), which are controlled by numerous pathways and in turn determine the fate of the cell. It is also apparent that lipid signaling is less developed in yeast than in higher eukaryotes. Finally, feedback regulatory mechanisms seem to be at least as important and powerful as the pathways themselves. In the final chapter of this essay we dare to imagine the essence of our next review on signaling in yeast, to be published on the 50th anniversary of Cellular Signalling in 2039.

Heterodimers of retinoic acid receptors and thyroid hormone receptors display unique combinatorial regulatory properties

Nuclear receptors are ligand-regulated transcription factors that regulate key aspects of metazoan development, differentiation, and homeostasis. Nuclear receptors recognize target genes by binding to specific DNA recognition sequences, denoted hormone response elements (HREs). Many nuclear receptors can recognize HREs as either homodimers or heterodimers. Retinoid X receptors (RXRs), in particular, serve as important heterodimer partners for many other nuclear receptors, including thyroid hormone receptors (TRs), and RXR/TR heterodimers have been proposed to be the primary mediators of target gene regulation by T3 hormone. Here, we report that the retinoic acid receptors (RARs), a distinct class of nuclear receptors, are also efficient heterodimer partners for TRs. These RAR/TR heterodimers form with similar affinities as RXR/TR heterodimers on an assortment of consensus and natural HREs, and preferentially assemble with the RAR partner 5' of the TR moiety. The corepressor and coactivator recruitment properties of these RAR/TR heterodimers and their transcriptional activities in vivo are distinct from those observed with the corresponding RXR heterodimers. Our studies indicate that RXRs are not unique in their ability to partner with TRs, and that RARs can also serve as robust heterodimer partners and combinatorial regulators of T3-modulated gene expression.

Molecular cloning, overexpression, and characterization of steroid-inducible 3alpha-hydroxysteroid dehydrogenase/carbonyl reductase from Comamonas testosteroni. A novel member of the short-chain dehydrogenase/reductase superfamily

3alpha-Hydroxysteroid dehydrogenase/carbonyl reductase (3alpha-HSD/CR) from Comamonas testosteroni, a bacterium that is able to grow on steroids as the sole carbon source, catalyzes the oxidoreduction at position 3 of a variety of C19-27 steroids and the carbonyl reduction of a variety of nonsteroidal aldehydes and ketones. The gene of this steroid-inducible 3alpha-HSD/CR was cloned by screening a C. testosteroni gene bank with a homologous DNA probe that was obtained by polymerase chain reaction with two degenerative primers based on the N-terminal sequence of the purified enzyme. The 3alpha-HSD/CR gene is 774 base pairs long, and the deduced amino acid sequence comprises 258 residues with a calculated molecular mass of 26.4 kDa. A homology search revealed that amino acid sequences highly conserved in the short-chain dehydrogenase/reductase (SDR) superfamily are present in 3alpha-HSD/CR. Two consensus sequences of the SDR superfamily were found, an N-terminal Gly-X-X-X-Gly-X-Gly cofactor-binding motif and a Tyr-X-X-X-Lys segment (residues 155-159 in the 3alpha-HSD/CR sequence) essential for catalytic activity of SDR proteins. 3alpha-HSD/CR was overexpressed and purified to homogeneity, and its activity was determined for steroid and nonsteroidal carbonyl substrates. These results suggest that inducible 3alpha-HSD/CR from C. testosteroni is a novel member of the SDR superfamily.

Components of the SMRT corepressor complex exhibit distinctive interactions with the POZ domain oncoproteins PLZF, PLZF-RARalpha, and BCL-6

Many transcription factors function by repressing gene transcription. For a variety of these transcription factors the ability to physically recruit auxiliary proteins, denoted corepressors, is crucial for the ability to silence gene expression. We and others have previously implicated the SMRT corepressor in the actions of the PLZF transcription factor and in the function of its oncogenic derivative, PLZF-retinoic acid receptor (RARalpha), in promyelocytic leukemia. We report here that PLZF, and a structurally similar transcriptional repressor, BCL-6, can interact with a variety of corepressor proteins in addition to SMRT, including the mSin3A protein and (for PLZF) histone deacetylase-1. Unexpectedly, these additional interactions with corepressor components are nonequivalent for these otherwise similar oncoproteins, suggesting that transcriptional repression by BCL-6 and by PLZF may differ in mechanism. Furthermore, we demonstrate that the oncogenic PLZF-RARalpha chimera lacks several important corepressor interaction sites that are present in the native PLZF protein. Thus the t(11;17) translocation that creates the PLZF-RARalpha chimera generates an oncoprotein with potentially novel regulatory properties distinct from those of either parental protein. Our results demonstrate that otherwise similar transcription factors can differ notably in their interactions with the corepressor machinery.

Genetic and biochemical analysis of p23 and ansamycin antibiotics in the function of Hsp90-dependent signaling proteins

The ubiquitous molecular chaperone Hsp90 acts in concert with a cohort of associated proteins to facilitate the functional maturation of a number of cellular signaling proteins, such as steroid hormone receptors and oncogene tyrosine kinases. The Hsp90-associated protein p23 is required for the assembly of functional steroid aporeceptor complexes in cell lysates, and Hsp90-binding ansamycin antibiotics disrupt the activity of Hsp90-dependent signaling proteins in cultured mammalian cells and prevent the association of p23 with Hsp90-receptor heterocomplexes; these observations have led to the hypotheses that p23 is required for the maturation of Hsp90 target proteins and that ansamycin antibiotics abrogate the activity of such proteins by disrupting the interaction of p23 with Hsp90. In this study, I demonstrate that ansamycin antibiotics disrupt the function of Hsp90 target proteins expressed in yeast cells; prevent the assembly of Sba1, a yeast p23-like protein, into steroid receptor-Hsp90 complexes; and result in the assembly of receptor-Hsp90 complexes that are defective for ligand binding. To assess the role of p23 in Hsp90 target protein function, I show that the activity of Hsp90 target proteins is unaffected by deletion of SBA1. Interestingly, steroid receptor activity in cells lacking Sba1 displays increased sensitivity to ansamycin antibiotics, and this phenotype is rescued by the expression of human p23 in yeast cells. These findings indicate that Hsp90-dependent signaling proteins can achieve a functional conformation in vivo in the absence of p23. Furthermore, while the presence of p23 decreases the sensitivity of Hsp90-dependent processes to ansamycin treatment, ansamycin antibiotics disrupt signaling through some mechanism other than altering the Hsp90-p23 interaction.

Characterization of receptor interaction and transcriptional repression by the corepressor SMRT

SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (nuclear receptor corepressor) are two related transcriptional corepressors that contain separable domains capable of interacting with unliganded nuclear receptors and repressing basal transcription. To decipher the mechanisms of receptor interaction and transcriptional repression by SMRT/N-CoR, we have characterized protein-protein interacting surfaces between SMRT and nuclear receptors and defined transcriptional repression domains of both SMRT and N-CoR. Deletional analysis reveals two individual nuclear receptor domains necessary for stable association with SMRT and a C-terminal helix essential for corepressor dissociation. Coordinately, two SMRT domains are found to interact independently with the receptors. Functional analysis reveals that SMRT contains two distinct repression domains, and the corresponding regions in N-CoR also repress basal transcription. Both repression domains in SMRT and N-CoR interact weakly with mSin3A, which in turn associates with a histone deacetylase HDAC1 in a mammalian two-hybrid assay. Far-Western analysis demonstrates a direct protein-protein interaction between two N-CoR repression domains with mSin3A. Finally we demonstrate that overexpression of full-length SMRT further represses basal transcription from natural promoters. Together, these results support a role of SMRT/N-CoR in corepression through the utilization of multiple mechanisms for receptor interactions and transcriptional repression.

Conserved interaction of the papillomavirus E2 transcriptional activator proteins with human and yeast TFIIB proteins

Papillomavirus early gene expression is regulated by the virus gene-encoded E2 proteins. The best-characterized E2 protein, encoded by bovine papillomavirus type 1 (BPV-1), has been shown to interact with basal transcription factor IIB (TFIIB) and the TATA binding protein basal transcription factor (N. M. Rank and P. F. Lambert, J. Virol. 69:6323-6334, 1995). We demonstrate that the potent E2 transcriptional activator protein encoded by a gene of human PV type 16 also interacts with TFIIB in vitro. Moreover, a direct comparison of domains within human TFIIB (hTFIIB) required for VP16 and BPV-1 E2 indicates that these acidic activators interact with hTFIIB in a qualitatively similar manner. Our mapping experiments identify hTFIIB interaction domains within the amino-terminal activation domain of BPV-1 E2. Finally, we demonstrate in vitro interaction between Saccharomyces cerevisiae TFIIB and BPV-1 E2, an observation that is consistent with the importance of the E2-TFIIB interaction for BPV-1 E2 transactivation in both systems.

Identification and characterization of the AF-1 transactivation domain of thyroid hormone receptor beta1

Physiological responses to thyroid hormones are regulated by a set of nuclear receptors (TRs) related to the steroid receptor superfamily of ligand-dependent transcription factors. Although TR isoforms are highly conserved in their DNA binding, ligand binding, and carboxyl-terminal transactivation domains, their amino-terminal regions are completely divergent. We examined the contribution of these amino-terminal sequences to TRbeta1 function. An amino-terminally truncated version of rat TRbeta1 lacking amino acids 4-89 was impaired in hormone-dependent activation in both yeast and mammalian cells. This defect was not due to impairment of DNA binding, because the truncated receptor displayed enhanced homodimer binding on several different TREs, indicating that residues in the amino-terminal domain of TRbeta1 interfere with homodimerization of the receptor. The presence of an autonomous transactivation domain in the amino-terminal region was demonstrated by its ability to activate transcription in a constitutive manner when fused to the GAL4 DNA binding domain. Deletional analyses localized the residues comprising the amino-terminal transactivation region of TRbeta1 to 19 amino acids residing between residues 69 and 89. Thus, the amino-terminal region of TRbeta1 contains an activation domain (AF-1) that can modulate the function of the receptor and may allow for the fine-tuning of receptor activity in various target tissues.

RAC3, a steroid/nuclear receptor-associated coactivator that is related to SRC-1 and TIF2

Steroids, thyroid hormones, vitamin D3, and retinoids are lipophilic small molecules that regulate diverse biological effects such as cell differentiation, development, and homeostasis. The actions of these hormones are mediated by steroid/nuclear receptors which function as ligand-dependent transcriptional regulators. Transcriptional activation by ligand-bound receptors is a complex process requiring dissociation and recruitment of several additional cofactors. We report here the cloning and characterization of receptor-associated coactivator 3 (RAC3), a human transcriptional coactivator for steroid/nuclear receptors. RAC3 interacts with several liganded receptors through a mechanism which requires their respective ligand-dependent activation domains. RAC3 can activate transcription when tethered to a heterologous DNA-binding domain. Overexpression of RAC3 enhances the ligand-dependent transcriptional activation by the receptors in mammalian cells. Sequence analysis reveals that RAC3 is related to steroid receptor coactivator 1 (SRC-1) and transcriptional intermediate factor 2 (TIF2), two of the most potent coactivators for steroid/nuclear receptors. Thus, RAC3 is a member of a growing coactivator network that should be useful as a tool for understanding hormone action and as a target for developing new therapeutic agents that can block hormone-dependent neoplasia.

Thyroid hormone-mediated enhancement of heterodimer formation between thyroid hormone receptor beta and retinoid X receptor

A subset of nuclear receptors, including those for thyroid hormone (TR), retinoic acid, vitamin D3, and eicosanoids, can form heterodimers with the retinoid X receptor (RXR) on DNA regulatory elements in the absence of their cognate ligands. In a mammalian two-hybrid assay, we have found that recruitment of a VP16-RXR chimera by a Gal4-TRbeta ligand-binding domain fusion is enhanced up to 50-fold by thyroid hormone (T3). This was also observed with a mutant fusion, Gal4-TR(L454A), lacking ligand-inducible activation function (AF-2) and unable to interact with putative coactivators, suggesting that the AF-2 activity of TR or intermediary cofactors is not involved in this effect. The wild-type and mutant Gal4-TR fusions also exhibited hormone-dependent recruitment of RXR in yeast. Hormone-dependent recruitment of RXR was also evident with another Gal4-TR mutant, AHTm, which does not interact with the nuclear receptor corepressor N-CoR, suggesting that ligand-enhanced dimerization is not a result of T3-induced corepressor release. Finally, we have shown that the interaction between RXR and TR is augmented by T3 in vitro, arguing against altered expression of either partner in vivo mediating this effect. We propose that ligand-dependent heterodimerization of TR and RXR in solution may provide a further level of control in nuclear receptor signaling.

Thyroid hormone resistance syndrome manifests as an aberrant interaction between mutant T3 receptors and transcriptional corepressors

Nuclear hormone receptors are hormone-regulated transcription factors that play critical roles in chordate development and homeostasis. Aberrant nuclear hormone receptors have been implicated as causal agents in a number of endocrine and neoplastic diseases. The syndrome of Resistance to Thyroid Hormone (RTH) is a human genetic disease characterized by an impaired physiological response to thyroid hormone. RTH is associated with diverse mutations in the thyroid hormone receptor beta-gene. The resulting mutant receptors function as dominant negatives, interfering with the actions of normal thyroid hormone receptors coexpressed in the same cells. We report here that RTH receptors interact aberrantly with a newly recognized family of transcriptional corepressors variously denoted as nuclear receptor corepressor (N-CoR), retinoid X receptor interacting protein-13 (RIP-13), silencing mediator for retinoid and thyroid hormone receptors (SMRT), and thyroid hormone receptor-associating cofactor (TRAC). All RTH receptors tested exhibit an impaired ability to dissociate from corepressors in the presence of thyroid hormone. Two of the RTH mutations uncouple corepressor dissociation from hormone binding; two additional RTH mutants exhibit an unusually strong interaction with corepressor under all hormone conditions tested. Finally, artificial mutants that abolish corepressor binding abrogate the dominant negative activity of RTH mutants. We suggest that an altered corepressor interaction is likely to play a critical role in the dominant negative potency of RTH mutants and may contribute to the variable phenotype in this disorder.

Activation and repression by nuclear hormone receptors: hormone modulates an equilibrium between active and repressive states

Transactivation-defective retinoid X and thyroid hormone receptors have been used to examine mechanisms of hormonal activation. Activation and repression of transcription by retinoid X and thyroid hormone receptors are shown to be mediated by physically distinct and functionally independent regions of the hormone binding domain. Nevertheless, the ability of receptors to respond to hormone requires communication between both functional domains. Deletion of the hormone-dependent transactivation function of the retinoid X receptor, the common subunit of heterodimeric nuclear receptors, significantly impairs hormone-dependent transcription by retinoic acid, thyroid hormone, and vitamin D receptors. The results indicate that receptors do not exist in static off and on conformations but that hormone alters an equilibrium between inactive and active states.

Steroid hormone signalling system and fungi

Three components of the steroid hormone signalling system, 17 beta-hydroxysteroid dehydrogenase, androgen binding proteins and steroid hormone signalling molecule testosterone were determined in the filamentous fungus Cochliobolus lunatus for the first time in a fungus. Their possible role in C. lunatus is discussed in comparison with their role in mammalian steroid hormone signalling system. The results are in accordance with the hypothesis, that the elements of primordial signal transduction system should exist in present day eukaryotic microorganisms.

A role for Hsp90 in retinoid receptor signal transduction

The ubiquitous heat shock protein Hsp90 appears to participate directly in the function of a broad range of cellular signal transduction components, including steroid hormone receptors; however, an evolutionarily related subclass of intracellular receptors, exemplified by the retinoid receptors RAR and RXR, had been inferred from biochemical studies to function independently of Hsp90. To examine this issue genetically, we measured mammalian and avian retinoid receptor activity in a Saccharomyces cerevisiae strain in which the expression of the yeast Hsp90 homologue could be conditionally repressed approximately 20-fold relative to wild type. We tested transcriptional activation by RAR or RXR-RAR, from two types of retinoic acid response elements, triggered by three different agonist ligands. In every condition, we found that activation was severely compromised under conditions of low Hsp90 expression. We showed that the defect was in signal transduction rather than transcription activation per se, and that high affinity hormone binding was abolished in extracts of cells producing low levels of Hsp90. We suggest that Hsp90 may function in at least one step of signal transduction by all members of the intracellular receptor superfamily.

Human dioxin receptor chimera transactivation in a yeast model system and studies on receptor agonists and antagonists

A yeast dioxin receptor chimera model has been developed to study ligand binding and transactivation properties of the human dioxin receptor. Using this new yeast model, the human dioxin receptor chimera was found to possess a constitutive transactivity on a LacZ reporter gene, however, the transactivation by the chimera was enhanced by the addition of several polycyclic aromatic hydrocarbons to the culture medium. The order of best polycyclic aromatic hydrocarbon inducer to worst correlated well with the known in vitro dioxin receptor binding affinities for these polycyclic aromatic hydrocarbons. 7,8-Benzoflavone, a weak dioxin receptor agonist and strong antagonist of the mammalian dioxin receptor also behaved as a weak agonist and strong antagonist of the human dioxin receptor chimera expressed in yeast. The implications for these findings as well as the utility of this new yeast human dioxin receptor chimera model are discussed.

Genetic dissection of thyroid hormone receptor beta: identification of mutations that separate hormone binding and transcriptional activation

The thyroid hormone receptors (TR) are members of the nuclear receptor family of ligand-mediated transcription factors. The large region of TR that lies C-terminal to its DNA-binding domain subserves functions of ligand binding, dimerization, and transactivation. Little is known regarding the structural or functional determinants of these processes. We have utilized genetic screening in the yeast Saccharomyces cerevisiae to identify residues involved in these functions. Random mutations of the rat TR beta 1 isoform between amino acid residues 179 and 456 were screened, and mutants with reduced hormone-dependent activation of reporter gene activity were isolated. In this paper we describe the characterization of a class of mutants that exhibit a dissociation between hormone binding and transcriptional activation. These mutants retained hormone binding (> 15% of the wild-type level) yet failed to transactivate a reporter gene. A number of these mutations occurred within the D region, which links the DNA-binding and ligand-binding domains of the receptor. One subset of these mutations abrogated DNA binding, supporting a role of the D region in this process. The remainder retain DNA binding and thus highlight residues critical for receptor activation. In addition, an unexpected group of "superactivator" mutations that led to enhanced hormone-dependent activation in S. cerevisiae were found. These mutations localized to the carboxy-terminal portion of the receptor in a region which contains elements conserved across the superfamily of nuclear receptors. The hormone-dependent phenotype of these superactivator mutations suggests an important role of this segment in ligand-mediated transcriptional activation.

Phytochrome assembly in living cells of the yeast Saccharomyces cerevisiae

The biological activity of the plant photoreceptor phytochrome requires the specific association of a linear tetrapyrrole prosthetic group with a large apoprotein. As an initial step to develop an in vivo assay system for structure-function analysis of the phytochrome photoreceptor, we undertook experiments to reconstitute holophytochrome in the yeast Saccharomyces cerevisiae. Here we show that yeast cells expressing recombinant oat apophytochrome A can take up exogenous linear tetrapyrroles, and, in a time-dependent manner, these pigments combine with the apoprotein to form photoactive holophytochrome in situ. Cell viability measurements indicate that holophytochrome assembly occurs in living cells. Unlike phytochrome A in higher plant tissue, which is rapidly degraded upon photoactivation, the reconstituted photoreceptor appears to be light stable in yeast. Reconstitution of photoactive phytochrome in yeast cells should enable us to exploit the power of yeast genetics for structure-function dissection of this important plant photoreceptor.

A highly conserved region in the hormone-binding domain of the human estrogen receptor functions as an efficient transactivation domain in yeast

Human estrogen receptor (hER) mutants which activate transcription in the absence of hormone were isolated by random mutagenesis and genetic selection in the yeast Saccharomyces cerevisiae. Twenty constitutive hER mutants defining ten different alleles were selected. All sequence changes resulted in truncations of the receptor within a 123-amino-acid (aa) segment (aa 270 to 393) spanning the D region and the N-terminal part of region E which contains the hormone-binding domain (HBD). Transactivation assays using both the constitutive hER mutants and a series of deleted receptor derivatives generated in vitro revealed that the N-terminal part of region E, between aa 302 and 339, contains an efficient transcriptional activation function which is constitutively active in yeast. The location of this transactivation function in hER is similar to that of the tau 2 activation function of the glucocorticoid receptor and corresponds to a sequence which is highly conserved among the steroid hormone receptors. Thus, a conserved region exists in the HBD of the hER which can function as an autonomous transactivation domain.

Ligand modulates the conversion of DNA-bound vitamin D3 receptor (VDR) homodimers into VDR-retinoid X receptor heterodimers

Protein dimerization facilitates cooperative, high-affinity interactions with DNA. Nuclear hormone receptors, for example, bind either as homodimers or as heterodimers with retinoid X receptors (RXR) to half-site repeats that are stabilized by protein-protein interactions mediated by residues within both the DNA- and ligand-binding domains. In vivo, ligand binding among the subfamily of steroid receptors unmasks the nuclear localization and DNA-binding domains from a complex with auxiliary factors such as the heat shock proteins. However, the role of ligand is less clear among nuclear receptors, since they are constitutively localized to the nucleus and are presumably associated with DNA in the absence of ligand. In this study, we have begun to explore the role of the ligand in vitamin D3 receptor (VDR) function by examining its effect on receptor homodimer and heterodimer formation. Our results demonstrate that VDR is a monomer in solution; VDR binding to a specific DNA element leads to the formation of a homodimeric complex through a monomeric intermediate. We find that 1,25-dihydroxyvitamin D3, the ligand for VDR, decreases the amount of the DNA-bound VDR homodimer complex. It does so by significantly decreasing the rate of conversion of DNA-bound monomer to homodimer and at the same time enhancing the dissociation of the dimeric complex. This effectively stabilizes the bound monomeric species, which in turn serves to favor the formation of a VDR-RXR heterodimer. The ligand for RXR, 9-cis retinoic acid, has the opposite effect of destabilizing the heterodimeric-DNA complex. These results may explain how a nuclear receptor can bind DNA constitutively but still act to regulate transcription in a fully hormone-dependent manner.

Ligand modulates the conversion of DNA-bound vitamin D3 receptor (VDR) homodimers into VDR-retinoid X receptor heterodimers

Protein dimerization facilitates cooperative, high-affinity interactions with DNA. Nuclear hormone receptors, for example, bind either as homodimers or as heterodimers with retinoid X receptors (RXR) to half-site repeats that are stabilized by protein-protein interactions mediated by residues within both the DNA- and ligand-binding domains. In vivo, ligand binding among the subfamily of steroid receptors unmasks the nuclear localization and DNA-binding domains from a complex with auxiliary factors such as the heat shock proteins. However, the role of ligand is less clear among nuclear receptors, since they are constitutively localized to the nucleus and are presumably associated with DNA in the absence of ligand. In this study, we have begun to explore the role of the ligand in vitamin D3 receptor (VDR) function by examining its effect on receptor homodimer and heterodimer formation. Our results demonstrate that VDR is a monomer in solution; VDR binding to a specific DNA element leads to the formation of a homodimeric complex through a monomeric intermediate. We find that 1,25-dihydroxyvitamin D3, the ligand for VDR, decreases the amount of the DNA-bound VDR homodimer complex. It does so by significantly decreasing the rate of conversion of DNA-bound monomer to homodimer and at the same time enhancing the dissociation of the dimeric complex. This effectively stabilizes the bound monomeric species, which in turn serves to favor the formation of a VDR-RXR heterodimer. The ligand for RXR, 9-cis retinoic acid, has the opposite effect of destabilizing the heterodimeric-DNA complex. These results may explain how a nuclear receptor can bind DNA constitutively but still act to regulate transcription in a fully hormone-dependent manner.

Functional analysis of mouse Hoxa-7 in Saccharomyces cerevisiae: sequences outside the homeodomain base contact zone influence binding and activation

The murine developmental control gene product, Hoxa-7, was shown to function as a DNA-binding transactivator in Saccharomyces cerevisiae. The importance of the ATTA core, the preference for antp class flanking nucleotides, the importance of Asn-51 of the homeodomain (HD), and the synergism of multiple binding sites all reflect properties that have previously been described for HOM or Hox proteins in tissue culture systems. A comparison of contact positions among genes of paralog groups and classes of mammalian HDs points to a lack of diversity in positions that make base contact, suggesting that besides the combination of HD amino acid-base pair contacts, another means of recognizing differences between targets must exist if Hox genes select different targets. The HD of antennapedia is identical to the Hoxa-7 HD. The interaction of Hoxa-7 with the exact sequence used in the nuclear magnetic resonance three-dimensional structural analysis on the antennapedia HD was studied. Hoxa-7 binding and transactivation was influenced by sequences outside of the known base contact zone of this site. We conclude that Hoxa-7 protein has a second means to interact with DNA or/and that the sequences flanking the base contact zone influence HD interactions by distorting DNA within the contact zone (base or backbone). This result is discussed in terms of DNA flexure and two modes of transcription used in S. cerevisiae.

Functional analysis of mouse Hoxa-7 in Saccharomyces cerevisiae: sequences outside the homeodomain base contact zone influence binding and activation

The murine developmental control gene product, Hoxa-7, was shown to function as a DNA-binding transactivator in Saccharomyces cerevisiae. The importance of the ATTA core, the preference for antp class flanking nucleotides, the importance of Asn-51 of the homeodomain (HD), and the synergism of multiple binding sites all reflect properties that have previously been described for HOM or Hox proteins in tissue culture systems. A comparison of contact positions among genes of paralog groups and classes of mammalian HDs points to a lack of diversity in positions that make base contact, suggesting that besides the combination of HD amino acid-base pair contacts, another means of recognizing differences between targets must exist if Hox genes select different targets. The HD of antennapedia is identical to the Hoxa-7 HD. The interaction of Hoxa-7 with the exact sequence used in the nuclear magnetic resonance three-dimensional structural analysis on the antennapedia HD was studied. Hoxa-7 binding and transactivation was influenced by sequences outside of the known base contact zone of this site. We conclude that Hoxa-7 protein has a second means to interact with DNA or/and that the sequences flanking the base contact zone influence HD interactions by distorting DNA within the contact zone (base or backbone). This result is discussed in terms of DNA flexure and two modes of transcription used in S. cerevisiae.

Reconstitution of retinoid X receptor function and combinatorial regulation of other nuclear hormone receptors in the yeast Saccharomyces cerevisiae

The nuclear hormone receptor family of transcription factors regulates gene expression via a complex combinatorial network of interactions. Of particular interest is the ability of retinoid X receptors (RXRs) to form heterodimers with retinoic acid receptors (RARs) and thyroid hormone receptors (TRs), thereby modifying their activities. We report here that RXR, RAR, and TR function can be reconstituted in the yeast Saccharomyces cerevisiae and demonstrate that the combinatorial regulation seen in vertebrate cells can be reproduced in the yeast background. Using this system, we have shown that RARs respond to a wide variety of retinoid ligands but that RXRs are specific for the 9-cis isomer of retinoic acid. RXR enhanced the activity of RARs and TRs on a variety of hormone response elements without demonstrably altering their DNA specificity. Interestingly, the ability of RXR to potentiate gene activation by RARs and by TRs varied for different receptor isoforms.

Differential transcriptional activation by v-myb and c-myb in animal cells and Saccharomyces cerevisiae

The v-myb oncogene and its cellular homolog c-myb encode sequence-specific DNA-binding proteins which regulate transcription from promoters containing Myb-binding sites in animal cells. We have developed a Saccharomyces cerevisiae system to assay transcriptional activation by v-Myb and c-Myb. In yeast strains containing integrated reporter genes, activation was strictly dependent upon both the Myb DNA-binding domain and the Myb recognition element. BAS1, an endogenous Myb-related yeast protein, was not required for transactivation by animal Myb proteins and by itself had no detectable effect on a Myb reporter gene. Deletion analyses demonstrated that a domain of v-Myb C terminal to the previously mapped Myb transcriptional activation domain was required for transactivation in animal cells but not in S. cerevisiae. The same domain is also required for the efficient transformation of myeloid cells by v-Myb. In contrast to results in animal cells, in S. cerevisiae the full-length c-Myb was a much stronger transactivator than a protein bearing the oncogenic N- and C-terminal truncations of v-Myb. These results imply that negative regulation of c-Myb by its own termini requires an additional animal cell protein or small molecule that is not present in S. cerevisiae.

Differential transcriptional activation by v-myb and c-myb in animal cells and Saccharomyces cerevisiae

The v-myb oncogene and its cellular homolog c-myb encode sequence-specific DNA-binding proteins which regulate transcription from promoters containing Myb-binding sites in animal cells. We have developed a Saccharomyces cerevisiae system to assay transcriptional activation by v-Myb and c-Myb. In yeast strains containing integrated reporter genes, activation was strictly dependent upon both the Myb DNA-binding domain and the Myb recognition element. BAS1, an endogenous Myb-related yeast protein, was not required for transactivation by animal Myb proteins and by itself had no detectable effect on a Myb reporter gene. Deletion analyses demonstrated that a domain of v-Myb C terminal to the previously mapped Myb transcriptional activation domain was required for transactivation in animal cells but not in S. cerevisiae. The same domain is also required for the efficient transformation of myeloid cells by v-Myb. In contrast to results in animal cells, in S. cerevisiae the full-length c-Myb was a much stronger transactivator than a protein bearing the oncogenic N- and C-terminal truncations of v-Myb. These results imply that negative regulation of c-Myb by its own termini requires an additional animal cell protein or small molecule that is not present in S. cerevisiae.

A conserved C-terminal sequence that is deleted in v-ErbA is essential for the biological activities of c-ErbA (the thyroid hormone receptor)

The thyroid hormone (T3) receptor type alpha, the c-ErbA alpha proto-oncoprotein, stimulates transcription of T3-dependent promoters, interferes with AP-1 activity, and induces erythroid differentiation in a ligand-dependent manner. The v-ErbA oncoprotein does not bind hormone and has lost all of these activities. Using c-ErbA/v-ErbA chimeras, we found that a deletion of 9 amino acids, conserved among many members of the nuclear receptor superfamily, which are located at the extreme carboxy terminus of c-ErbA alpha is responsible for loss of both transactivation and transcriptional interference activities. Single, double, and triple amino acid substitutions within this region completely abolished T3-dependent transcriptional activation, interference with AP-1 activity, and decreased T3 binding by c-ErbA alpha. However, the lower T3 binding by these mutants does not fully account for the loss of transactivation and transcriptional interference, since a c-ErbA/v-ErbA chimera which was similarly reduced in T3 binding activity has retained both of these functions. Deletion of homologous residues in the retinoic acid receptor alpha (RAR alpha) resulted in a similar loss of transactivation and transcriptional interference activities. The ability of c-ErbA alpha to induce differentiation of transformed erythroblasts is also impaired by all of the mutations introduced into the conserved carboxy-terminal sequence. We conclude that this 9-amino-acid conserved region is essential for normal biological function of c-ErbA alpha and RAR alpha and possibly other T3 and RA receptors.

A conserved C-terminal sequence that is deleted in v-ErbA is essential for the biological activities of c-ErbA (the thyroid hormone receptor)

The thyroid hormone (T3) receptor type alpha, the c-ErbA alpha proto-oncoprotein, stimulates transcription of T3-dependent promoters, interferes with AP-1 activity, and induces erythroid differentiation in a ligand-dependent manner. The v-ErbA oncoprotein does not bind hormone and has lost all of these activities. Using c-ErbA/v-ErbA chimeras, we found that a deletion of 9 amino acids, conserved among many members of the nuclear receptor superfamily, which are located at the extreme carboxy terminus of c-ErbA alpha is responsible for loss of both transactivation and transcriptional interference activities. Single, double, and triple amino acid substitutions within this region completely abolished T3-dependent transcriptional activation, interference with AP-1 activity, and decreased T3 binding by c-ErbA alpha. However, the lower T3 binding by these mutants does not fully account for the loss of transactivation and transcriptional interference, since a c-ErbA/v-ErbA chimera which was similarly reduced in T3 binding activity has retained both of these functions. Deletion of homologous residues in the retinoic acid receptor alpha (RAR alpha) resulted in a similar loss of transactivation and transcriptional interference activities. The ability of c-ErbA alpha to induce differentiation of transformed erythroblasts is also impaired by all of the mutations introduced into the conserved carboxy-terminal sequence. We conclude that this 9-amino-acid conserved region is essential for normal biological function of c-ErbA alpha and RAR alpha and possibly other T3 and RA receptors.

Efficient transactivation by retinoic acid receptors in yeast requires retinoid X receptors

All-trans and 9-cis retinoic acids are natural derivatives of vitamin A that modulate gene expression as a consequence of binding to nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). RXRs form heterodimers with RARs in vitro, and such complexes display enhanced binding affinities for cognate DNA response elements. As yeast is devoid of endogenous RARs and RXRs, we used this organism to investigate whether transactivation in vivo requires RAR/RXR heterodimers. Using a domain-swapping approach, we demonstrate that chimeric RAR alpha 1 and RXR alpha containing the DNA-binding domain of the estrogen receptor activate transcription of a cognate reporter gene in yeast, independently of each other. These activities result from an inducible transcription activation function located in the ligand-binding domains of RAR alpha 1 and RXR alpha and a constitutive activation function located in the A/B region of RAR alpha 1. The inducible activation function of RXR alpha is induced exclusively by 9-cis-retinoic acid in this system. Transactivation of a reporter gene containing a retinoic acid response element by RAR alpha was considerably increased by RXR alpha, even in the absence of ligand. Optimal induction was achieved with 9-cis-retinoic acid, which stimulates the activity of both receptors. This study illustrates the utility of yeast to investigate signal transduction by retinoids in the absence of endogenous RARs, RXRs, and detectable retinoic acid isomerization.

Functional erythroid promoters created by interaction of the transcription factor GATA-1 with CACCC and AP-1/NFE-2 elements

We have investigated interactions between the erythroid transcription factor GATA-1 and factors binding two cis-acting elements commonly linked to GATA sites in erythroid control elements. GATA-1 is present at all stages of erythroid differentiation, is necessary for erythropoiesis, and binds sites in all erythroid control elements. However, minimal promoters containing GATA-1 sites are inactive when tested in erythroid cells. Based on this observation, two erythroid cis elements, here termed CACCC and AP-1/NFE-2, were linked to GATA sites in minimal promoters. None of the elements linked only to a TATA box created an active promoter, but GATA sites linked to either CACCC or AP-1/NFE-2 elements formed strong erythroid promoters. A mutation of T to C at position -175 in the gamma-globin promoter GATA site, associated with hereditary persistence of fetal hemoglobin (HPFH), increased expression of these promoters in both fetal and adult cells. A construct bearing the beta-globin CACCC element was more active in adult and less active in fetal erythroid cells, when compared with the gamma-globin CACCC element. These studies suggest that erythroid control elements are formed by the interactions of at least three transcription factors, none of which functions alone.

Genetic dissection of the signaling domain of a mammalian steroid receptor in yeast

The mechanism of signal transduction by steroid receptor proteins is complex and not yet understood. We describe here a facile genetic strategy for dissection of the rat glucocorticoid receptor "signaling domain," a region of the protein that binds and transduces the hormonal signal. We found that the characteristics of signal transduction by the receptor expressed in yeast were similar to those of endogenous receptors in mammalian cells. Interestingly, the rank order of particular ligands differed between species with respect to receptor binding and biological efficacy. This suggests that factors in addition to the receptor alone must determine or influence ligand efficacy in vivo. To obtain a collection of receptors with distinct defects in signal transduction, we screened in yeast an extensive series of random point mutations introduced in that region in vitro. Three phenotypic classes were obtained: one group failed to bind hormone, a second displayed altered ligand specificity, and a third bound hormone but lacked regulatory activity. Our results demonstrate that analysis of glucocorticoid receptor action in yeast provides a general approach for analyzing the mechanism of signaling by the nuclear receptor family and may facilitate identification of non-receptor factors that participate in this process.

Proto-oncogene erbA expression and increased abundance of progesterone receptors in the mouse uterus after passive immunisation against progesterone before implantation

Passive immunisation with a monoclonal anti-progesterone antibody (DB3) prevents pregnancy in the mouse, and antibody is localised in the endometrium before the onset of implantation. BALB/c female mice were injected intraperitoneally with 9 nmol of DB3 (a dose known to cause 100% infertility) 32 h post coitum, and the uterus was removed at various times after injection. Using a monoclonal anti-progesterone receptor antibody (PR6), expression of progesterone receptors was found to be abundant in uterine tissue of DB3-treated mice; this was associated with substantial progesterone receptor mRNA levels and with maximum localisation of DB3 antibody as detected by anti-idiotype antibody. Control animals treated with an equal amount of the mouse myeloma protein P3 showed very low levels of progesterone receptor in the uterus. DB3 treatment also affected uterine expression of the proto-oncogene erbA product (which shows primary sequence homology with the progesterone receptor) as revealed by specific antiserum to the ERBA protein and by in situ hybridisation with a cDNA probe to v-erbA. Time-course studies indicated that the erbA gene was expressed at a high level before progesterone receptor expression increased, that its expression was dependent on the presence of the embryo and that erbA expression persisted longer in DB3-treated females. The observations suggest that anti-progesterone immunisation has a direct effect within the uterus, involving persistence of proto-oncogene erbA expression (which itself may represent an early maternal response to pregnancy) and increased progesterone receptor levels resulting from an unopposed oestrogen effect derived from local ligand withdrawal.

Accumulation of different c-erbA transcripts during rat brain development and in cortical neurons cultured in a synthetic medium

1. Accumulation of different c-erbA transcripts was studied, during rat brain maturation and in cortical neurons differentiating in a serum-free medium, by quantitative Northern blot hybridization. 2. The alpha and beta forms of c-erbA mRNAs exhibit different patterns of accumulation, with a precocious increase in the alpha forms compared with the beta forms both in vivo and in culture. 3. erbA alpha 2 mRNA (2.6 kb) is by far the predominant form, with a maximum at birth (PO). 4. The accumulation patterns of both alpha and beta forms show discrete differences in isolated neurons compared to brain cortices; in particular the pattern of alpha 2 mRNA accumulation in culture suggests its predominant localization to neurons. 5. The presence of T3 in the culture medium does not have significant effects on the level of any of erbA mRNAs. 6. Possible implications and relationships with neuronal terminal differentiation are discussed.

The mechanism of RU486 antagonism is dependent on the conformation of the carboxy-terminal tail of the human progesterone receptor

The human progesterone receptor form B (hPR-B) was expressed in Saccharomyces cerevisiae together with a specific reporter plasmid. To understand the mechanism underlying antagonist ligand activity, libraries of hormone binding domain (HBD)-mutated hPR-B molecules were prepared. A mutant receptor was identified that had lost the ability to bind either progesterone or R5020; it could still bind RU486 and, surprisingly, fully activated transcription in the presence of this "antagonist" and other antiprogestins. When this receptor mutant was assayed in mammalian cells, RU486 again demonstrated agonistic activity. Sequence analysis indicated that the mutant phenotype was due to truncation of the carboxy (C)-terminal 42 aa. We conclude that amino acids in the extreme C-terminal region are required for the receptor to bind progesterone, while antagonists bind to a site located more N-terminal of the HBD. Our results suggest that the extreme C-terminal region of the receptor contains an inhibitory function that silences receptor transactivation in the absence of agonist and in the presence of antagonist.

Retinoid and thyroid hormone receptors: ligand-regulated transcription factors as proto-oncogenes

The retroviral v-erb A locus is derived from a cellular gene, c-erb A, encoding a thyroid hormone receptor. The v-erb A and c-erb A proteins are, in turn, members of a larger family of structurally and functionally interrelated polypeptides that includes the steroid, retinoic acid, and vitamin D3 receptors. These nuclear hormone receptors act by binding to specific sites on the cell genome and, in response to cognate hormone, modulating the transcription of adjacent 'target' genes. The expression, properties, and mechanisms of action of the thyroid hormone receptors (c-erb A proteins) and the closely related retinoic acid receptors are discussed.

Modulation of activity of the promoter of the human MDR1 gene by Ras and p53

Drug resistance in human cancer is associated with overexpression of the multidrug resistance (MDR1) gene, which confers cross-resistance to hydrophobic natural product cytotoxic drugs. Expression of the MDR1 gene can occur de novo in human cancers in the absence of drug treatment. The promoter of the human MDR1 gene was shown to be a target for the c-Ha-Ras-1 oncogene and the p53 tumor suppressor gene products, both of which are associated with tumor progression. The stimulatory effect of c-Ha-Ras-1 was not specific for the MDR1 promoter alone, whereas a mutant p53 specifically stimulated the MDR1 promoter and wild-type p53 exerted specific repression. These results imply that the MDR1 gene could be activated during tumor progression associated with mutations in Ras and p53.

Evolution of regulation of steroid-mediated intercellular communication in vertebrates: insights from flavonoids, signals that mediate plant-rhizobia symbiosis

Various flavonoids, such as genistein, luteolin, and coumestrol, have actions in mammals that are mediated by binding either to classical estrogen receptors or to type II receptors, which also bind estrogen. These actions are of intense interest because they may be the basis for the protective actions of plants against certain cancers, such as breast cancer. The biological activity of flavonoids in mammals raises some questions. Is the hormonal action of flavonoids "an accident" derived from their phenolic groups and general hydrophobicity, which gives them some properties in common with estrogens? Or do flavonoids regulate gene transcription in other organisms? And, if so, is there a connection between their actions in these organisms and in mammals? Some answers to these questions are provided by the actions of plant-derived flavonoids in regulating gene transcription in rhizobia, bacteria that form nitrogen-fixing nodules in the roots of legumes, which has several interesting similarities with steroid-mediated actions in vertebrates. First, there is specificity in the actions of flavonoids in rhizobia; oxidation or reduction of the flavonoid or removal of a hydroxyl group can alter its biological activity. Moreover, some flavonoids are anti-inducers functioning like steroid antagonists to negate the actions of inducers. Also there are sequence similarities between various steroid metabolizing enzymes and proteins found in rhizobia, which indicates that these proteins are derived from a common ancestor. For example, 17 beta-hydroxysteroid dehydrogenase, which catalyzes the interconversion of the alcohol and ketone a C17 on estrogens and androgens, 11 beta-hydroxysteroid dehydrogenase, which catalyzes the interconversion of the alcohol and ketone at C11 of glucocorticoids, and 3 alpha,20 beta-hydroxysteroid dehydrogenase, which catalyzes the interconversion of the alcohol and ketone at C20 of corticosteroids, are homologs of proteins found in rhizobia. Thus, the binding of flavonoids to vertebrate proteins may represent an evolutionary linkage between the actions of steroids in mammals and communication between plants and rhizobia.

Evolution of enzymatic regulation of prostaglandin action: novel connections to regulation of human sex and adrenal function, antibiotic synthesis and nitrogen fixation

The recent determination of the amino acid sequences of enzymes that metabolize prostaglandins and steroids has revealed interesting connections between some of these enzymes. Human placental 15-hydroxyprostaglandin dehydrogenase, which catalyzes the oxidation of the C15 alcohol on prostaglandins E2 and F2 alpha, is homologous to 11 beta-hydroxysteroid, 17 beta-hydroxysteroid, and 3 alpha, 20 beta-hydroxysteroid dehydrogenases. That is, these four enzymes are derived from a common ancestor. Moreover, enzymes important in synthesis of antibiotics and proteins synthesized by soil bacteria that form nitrogen-fixing nodules in alfalfa and soybeans are homologous to 15-hydroxyprostaglandin dehydrogenase. These homologies provide important insights into the origins of intercellular communication that is mediated by prostaglandins, steroids, and fatty acids.

v-erbA overexpression is required to extinguish c-erbA function in erythroid cell differentiation and regulation of the erbA target gene CAII

The v-erbA oncoprotein represents a retrovirus-transduced oncogenic version of the thyroid hormone (T3/T4) receptor c-erbA (type alpha). It contributes to virus-induced erythroleukemia by efficiently arresting differentiation of red cell progenitors and by suppressing transcription of erythrocyte-specific genes. Here, we show that v-erbA and c-erbA bind directly to sequences within the promoter of the erythrocyte-specific carbonic anhydrase II (CAII), a gene whose transcription is efficiently suppressed by v-erbA. This erbA-binding site confers thyroid hormone responsiveness to a heterologous promoter in transient expression experiments and is a target for efficient down-regulation of CAII transcription by the v-erbA oncoprotein. In stably transformed erythroblasts coexpressing the v-erbA oncoprotein and the c-erbA/T3 receptor at an approximately equimolar ratio, c-erbA activity is dominant over v-erbA. T3 efficiently induced erythroid differentiation in these cells, thus overcoming the v-erbA-mediated differentiation arrest. Likewise, T3 activated CAII transcription as well as transient expression of a T3-responsive reporter gene containing the CAII-specific erbA-binding site. The c-erbA-dependent activation of this CAII reporter construct could only be suppressed by very high amounts of v-erbA. Our results suggest that overexpression of v-erbA is required for its function as an oncoprotein.

DNA sequences in chromosomes II and VII code for pyruvate carboxylase isoenzymes in Saccharomyces cerevisiae: analysis of pyruvate carboxylase-deficient strains

A gene encoding pyruvate carboxylase has previously been isolated from Saccharomyces cerevisiae. We have isolated a second gene, PYC2, from the same organism also encoding a pyruvate carboxylase. The gene PYC2 is situated on the right arm of chromosome II between the DUR 1, 2 markers and the telomere. We localized the previously isolated gene, which we designate PYC1, to chromosome VII. Disruption of either of the genes did not produce marked changes in the phenotype. However, simultaneous disruption of both genes resulted in inability to grow on glucose as sole carbon source, unless aspartate was added to the medium. This indicates that in wild-type yeast there is no bypass for the reaction catalysed by pyruvate carboxylase. The coding regions of both genes exhibit a homology of 90% at the amino acid level and 85% at the nucleotide level. No appreciable homology was found in the corresponding flanking regions. No differences in the Km values for ATP or pyruvate were observed between the enzymes obtained from strains carrying inactive, disrupted versions of one or other of the genes.

v-erbA oncogene function in neoplasia correlates with its ability to repress retinoic acid receptor action

The v-erbA oncoprotein of avian erythroblastosis virus is an aberrant version of a thyroid hormone receptor and functions in neoplasia by blocking erythroid differentiation and by modifying the growth properties of fibroblasts. v-erbA has been proposed to represent a novel dominant negative oncogene, acting in the cancer cell by interfering with the actions of its normal cell homologs, the thyroid hormone receptors. We report here that v-erbA can actually interfere with the actions of a variety of members of the steroid/retinoid receptor family and that the ability of v-erbA to act in neoplasia best correlates not with suppression of c-erbA action, but with interference with the retinoic acid receptor response. We suggest that v-erbA may act in neoplasia by promiscuously interfering with a retinoid-mediated differentiation process.

Rat liver c-erb A beta 1 thyroid hormone receptor is a constitutive activator in yeast (Saccharomyces cerevisiae): essential role of domains D,E and F in hormone-independent transcription

To assess thyroid hormone receptor (TR)-mediated activation of transcription in yeast in the presence or absence of thyroid hormone (T3), we developed a co-expression system using a TR-beta 1 expression vector and a reporter plasmid containing a 16 base pair palindromic thyroid hormone response element (TRE) upstream from a proximal CYC1 promoter that was fused to the beta-galactosidase lac Z gene of Escherichia coli. Although TR-beta 1 functions as a repressor in most mammalian systems, using our system we observed a unique thyroid hormone-independent transcriptional response indicating that wild TR-beta 1 acted as a constitutive activator in yeast; the addition of 1 microM T3 induced a moderate but significant (p less than 0.01) 25-40% further increase in transcriptional activity. Using a series of rat TR-beta 1 mutant constructs, we found that deletion of domain D and portions of E completely eliminated transcriptional activity, whereas truncations of domain F and E permitted a partial (20-40%) response compared to wild TR-beta 1 in the presence or absence of T3. These observations indicate that TR-beta 1 functions as an activator in yeast and that domains D,E and F play important interactive roles in its hormone-independent gene activation with the D domain likely being the most essential. Furthermore, our results suggest that the different transcriptional property of TR-beta 1 in yeast compared to mammalian cells i.e. activator vs repressor function, is likely determined by transcriptional factor differences which are dependent upon cellular origin.

Genealogy of regulation of human sex and adrenal function, prostaglandin action, snapdragon and petunia flower colors, antibiotics, and nitrogen fixation: functional diversity from two ancestral dehydrogenases

Metabolism of steroid hormones by dehydrogenases is an important mechanism for regulating steroid hormone action. Analysis of recently reported amino acid sequences of 11 beta-hydroxysteroid dehydrogenase, 17 beta-hydroxysteroid dehydrogenase, and 3 alpha, 20 beta-hydroxysteroid dehydrogenase reveals that they are descended from a common ancestor. Unexpectedly, this superfamily of dehydrogenases has other interesting relatives: 15-hydroxyprostaglandin dehydrogenase, proteins found in nitrogen-fixing bacteria, and enzymes important in the synthesis of antibiotics. The novel lineage of these proteins and the actions of flavonoids in regulating gene transcription in nitrogen-fixing bacteria and mammals provide new insights into the evolution of regulation of gene transcription by intercellular signals in multicellular animals.