Lack of hepcidin gene expression and severe tissue iron overload in upstream stimulatory factor 2 (USF2) knockout mice

We previously reported the disruption of the murine gene encoding the transcription factor USF2 and its consequences on glucose-dependent gene regulation in the liver. We report here a peculiar phenotype of Usf2(-/-) mice that progressively develop multivisceral iron overload; plasma iron overcomes transferrin binding capacity, and nontransferrin-bound iron accumulates in various tissues including pancreas and heart. In contrast, the splenic iron content is strikingly lower in knockout animals than in controls. To identify genes that may account for the abnormalities of iron homeostasis in Usf2(-/-) mice, we used suppressive subtractive hybridization between livers from Usf2(-/-) and wild-type mice. We isolated a cDNA encoding a peptide, hepcidin (also referred to as LEAP-1, for liver-expressed antimicrobial peptide), that was very recently purified from human blood ultrafiltrate and from urine as a disulfide-bonded peptide exhibiting antimicrobial activity. Accumulation of iron in the liver has been recently reported to up-regulate hepcidin expression, whereas our data clearly show that a complete defect in hepcidin expression is responsible for progressive tissue iron overload. The striking similarity of the alterations in iron metabolism between HFE knockout mice, a murine model of hereditary hemochromatosis, and the Usf2(-/-) hepcidin-deficient mice suggests that hepcidin may function in the same regulatory pathway as HFE. We propose that hepcidin acts as a signaling molecule that is required in conjunction with HFE to regulate both intestinal iron absorption and iron storage in macrophages.

Severe iron deficiency anemia in transgenic mice expressing liver hepcidin

We recently reported the hemochromatosis-like phenotype observed in our Usf2 knockout mice. In these mice, as in murine models of hemochromatosis and patients with hereditary hemochromatosis, iron accumulates in parenchymal cells (in particular, liver and pancreas), whereas the reticuloendothelial system is spared from this iron loading. We suggested that this phenotypic trait could be attributed to the absence, in the Usf2 knockout mice, of a secreted liver-specific peptide, hepcidin. We conjectured that the reverse situation, namely overexpression of hepcidin, might result in phenotypic traits of iron deficiency. This question was addressed by generating transgenic mice expressing hepcidin under the control of the liver-specific transthyretin promoter. We found that the majority of the transgenic mice were born with a pale skin and died within a few hours after birth. These transgenic animals had decreased body iron levels and presented severe microcytic hypochromic anemia. So far, three mosaic transgenic animals have survived. They were unequivocally identified by physical features, including reduced body size, pallor, hairless and crumpled skin. These pleiotropic effects were found to be associated with erythrocyte abnormalities, with marked anisocytosis, poikylocytosis and hypochromia, which are features characteristic of iron-deficiency anemia. These results strongly support the proposed role of hepcidin as a putative iron-regulatory hormone. The animal models devoid of hepcidin (the Usf2 knockout mice) or overexpressing the peptide (the transgenic mice presented in this paper) represent valuable tools for investigating iron homeostasis in vivo and for deciphering the molecular mechanisms of hepcidin action.

Regulation of metallothionein gene expression by oxidative stress and metal ions

The metallothioneins (MT) are small, cysteine-rich heavy metal-binding proteins which participate in an array of protective stress responses. Although a single essential function of MT has not been demonstrated, MT of higher eukaryotes evolved as a mechanism to regulate zinc levels and distribution within cells and organisms. These proteins can also protect against some toxic metals and oxidative stress-inducing agents. In mice, among the four known MT genes, the MT-I and -II genes are most widely expressed. Transcription of these genes is rapidly and dramatically up-regulated in response to zinc and cadmium, as well as in response to agents which cause oxidative stress and/or inflammation. The six zinc-finger metal-responsive transcription factor MTF-1 plays a central role in transcriptional activation of the MT-I gene in response to metals and oxidative stress. Mutation of the MTF-1 gene abolishes these responses, and MTF-1 is induced to bind to the metal response elements in proximal MT promoter in cells treated with zinc or during oxidative stress. The exact molecular mechanisms of action of MTF-1 are not fully understood. Our studies suggest that the DNA-binding activity of MTF-1 in vivo and in vitro is reversibly activated by zinc interactions with the zinc-finger domain. This reflects heterogeneity in the structure and function of the six zinc fingers. We hypothesize that MTF-1 functions as a sensor of free zinc pools in the cell. Changes in free zinc may occur in response to chemically diverse inducers. MTF-1 also exerts effects on MT-I gene transcription which are independent of a large increase in MTF-1 DNA-binding activity. For example, cadmium, which has little effect on the DNA-binding activity of MTF-1 in vivo or in vitro, is a more potent inducer of MT gene expression than is zinc. The basic helix-loop-helix-leucine zipper protein, USF (upstream stimulatory factor family), also plays a role in regulating transcription of the mouse MT-I gene in response to cadmium or H2O2. Expression of dominant negative USF-1 or deletion of its binding site from the proximal promoter attenuates induction of the mouse MT-I gene. USF apparently functions in this context by interacting with as yet unidentified proteins which bind to an antioxidant response element which overlaps the USF-binding site (USF/ARE). Interestingly, this composite element does not participate in the induction of MT-I gene transcription by zinc or redox-cycling quinones. Thus, regulation of the mouse MT-I gene by metals and oxidative stress involves multiple signaling pathways which depend on the species of metal ion and the nature of the oxidative stress.

The adenovirus major late transcription factor USF is a member of the helix-loop-helix group of regulatory proteins and binds to DNA as a dimer

We isolated full-length cDNAs encoding the 43-kD form of human upstream stimulatory factor (USF), a cellular factor required for efficient transcription of the adenovirus major late (AdML) promoter in vitro. Sequence analysis showed USF to be a member of the c-myc-related family of DNA-binding proteins. Using proteins translated in vitro, we identified a DNA-binding domain near the carboxyl terminus, which includes both a helix-loop-helix motif and a leucine repeat. We show that USF interacts with its target DNA as a dimer. The leucine repeat is required for efficient DNA binding of the intact protein and for interactions between full-length and truncated USF proteins. Interestingly, it is not required for DNA binding of the isolated helix-loop-helix domain. The structure of different cDNA clones indicates that USF RNA is differentially spliced, and alternative exon usage may regulate the levels of functional USF protein.

Cooperative interaction of an initiator-binding transcription initiation factor and the helix-loop-helix activator USF

Transcription initiation by mammalian RNA polymerase II is effected by multiple common factors interacting through minimal promoter elements and regulated by gene-specific factors interacting with distal control elements. Minimal promoter elements that can function independently or together, depending on the specific promoter, include the upstream TATA box and a pyrimidine-rich initiator (Inr) overlapping the transcription start site. The binding of TFIID to the TATA element promotes the assembly of other factors into a preinitiation complex but factors which function at the Inr have not been defined. We show here that a novel factor (TFII-I) binds specifically to Inr elements, supports basal transcription from the adenovirus major late promoter and is immunologically related to the helix-loop-helix activator USF. We further show that TFII-I also binds to the upstream high-affinity USF site (E box), that USF also binds to the Inr, and that TFII-I and USF interact cooperatively at both Inr and E box sites. Thus, TFII-I represents a novel type of transcription initiation factor whose interactions at multiple promoter elements may aid novel communication mechanisms between upstream regulatory factors and the general transcriptional machinery.

Cloning of an intrinsic human TFIID subunit that interacts with multiple transcriptional activators

TFIID is a multisubunit protein complex comprised of the TATA-binding protein (TBP) and multiple TBP-associated factors (TAFs). The TAFs in TFIID are essential for activator-dependent transcription. The cloning of a complementary DNA encoding a human TFIID TAF, TAFII55, that has no known homolog in Drosophila TFIID is now described. TAFII55 is shown to interact with the largest subunit (TAFII230) of human TFIID through its central region and with multiple activators--including Sp1, YY1, USF, CTF, adenoviral E1A, and human immunodeficiency virus-type 1 Tat proteins--through a distinct amino-terminal domain. The TAFII55-interacting region of Sp1 was localized to its DNA-binding domain, which is distinct from the glutamine-rich activation domains previously shown to interact with Drosophila TAFII110. Thus, this human TFIID TAF may be a co-activator that mediates a response to multiple activators through a distinct mechanism.

Identification of a chlamydial protease-like activity factor responsible for the degradation of host transcription factors

Microbial pathogens have been selected for the capacity to evade or manipulate host responses in order to survive after infection. Chlamydia, an obligate intracellular pathogen and the causative agent for many human diseases, can escape T lymphocyte immune recognition by degrading host transcription factors required for major histocompatibility complex (MHC) antigen expression. We have now identified a chlamydial protease- or proteasome-like activity factor (CPAF) that is secreted into the host cell cytosol and that is both necessary and sufficient for the degradation of host transcription factors RFX5 and upstream stimulation factor 1 (USF-1). The CPAF gene is highly conserved among chlamydial strains, but has no significant overall homology with other known genes. Thus, CPAF represents a unique secreted protein produced by an obligate intracellular bacterial pathogen to interfere with effective host adaptive immunity.

Activation of the MHC class II transactivator CIITA by interferon-gamma requires cooperative interaction between Stat1 and USF-1

CIITA is the mediator of MHC class II gene induction by interferon-gamma (IFNgamma). The CIITA gene is itself selectively activated via one of its four promoters (PIV). We show here that three cis-acting elements, the GAS, the E box, and the IRF-1-binding site, as well as the transacting factors Stat1 and IRF-1, are essential for activation of CIITA promoter IV by IFNgamma. Stat1 binds to the GAS site only in the presence of the ubiquitous factor USF-1, which binds to the adjacent E box. Indeed, Stat1 and USF-1 bind to the GAS/E box motif in a cooperative manner. The specificity for CIITA activation by IFNgamma is thus dictated by the GAS/E box motif and by the selective interaction of IFNgamma-activated Stat1 and USF-1. This clarifies the missing link in the overall pathway of IFNgamma activation of MHC-II expression.

Ubiquitous expression of the 43- and 44-kDa forms of transcription factor USF in mammalian cells

USF is a helix-loop-helix transcription factor that, like Myc, recognizes the DNA binding motif CACGTG. Two different forms of USF, characterized by apparent molecular weights of 43,000 and 44,000, were originally identified in HeLa cells by biochemical analysis. Clones for the 43-kDa USF were first characterized, but only partial clones for the human 44-kDa USF (USF2, or FIP) have been reported. Here we describe a complete cDNA for the 44-kDa USF from murine cells. Analysis of this clone has revealed that the various USF family members are quite divergent in their N-terminal amino acid sequences, while a high degree of conservation characterizes their dimerization and DNA-binding domains. Interestingly, the 3' noncoding region of the 44-kDa USF cDNAs displayed an unusual degree of conservation between human and mouse. In vitro transcription/translation experiments indicated a possible role for this region in translation regulation. Alternative splicing forms of the 44-kDa USF messages exist in both mouse and human. Examination of the tissue and cell-type distribution of USF by Northern blot and gel retardation assays revealed that while expression of both the 43- and 44-kDa USF species is ubiquitous, different ratios of USF homo- and heterodimers are found in different cells.

Structure and function of the b/HLH/Z domain of USF

The basic/helix-loop-helix/leucine zipper (b/HLH/Z) transcription factor upstream stimulatory factor (USF) and its isolated DNA binding domain undergo a random coil to alpha-helix folding transition on recognizing their cognate DNA. The USF b/HLH cocrystal structure resembles the structure of the b/HLH/Z domain of the homologous protein Max and reveals (i) that the truncated, b/HLH DNA binding domain homodimerizes, forming a parallel, left-handed four-helix bundle, and (ii) that the basic region becomes alpha-helical on binding to the major groove of the DNA sequence CACGTG. Hydrodynamic measurements show that the b/HLH/Z DNA binding domain of USF exists as a bivalent homotetramer. This tetramer forms at the USF physiological intranuclear concentration, and depends on the integrity of the leucine zipper motif. The ability to bind simultaneously to two independent sites suggests a role in DNA looping for the b/HLH/Z and Myc-related families of eukaryotic transcription factors.

The FIP3-Rab11 protein complex regulates recycling endosome targeting to the cleavage furrow during late cytokinesis

An integral part of cell division is the separation of daughter cells via cytokinesis. There is now good evidence that the completion of cytokinesis requires coordinated membrane trafficking to deliver new membrane to the tip of the furrow and to complete the abscission. Here we have examined membrane traffic in cytokinesis and describe several novel observations. First, we show that Rab11- and FIP3-containing recycling endosomes accumulate near the cleavage furrow and are required for successful completion of cytokinesis. Second, we demonstrate that the Rab11-FIP3 protein complex is intimately involved in the delivery of endosomes to the cleavage furrow. Significantly, although FIP3 recruitment to endosomes is Rab11 dependent, we find that the targeting of FIP3 to the midbody is independent of Rab11. Third, we show that the Rab11-FIP3 complex is required for a late stage of cytokinesis, possibly abscission. Finally, we demonstrate that localization of FIP3 is subject to substantial spatial and temporal regulation. These data provide the first detailed analysis of recycling endosomes in cell division and provide a new model for membrane traffic to the furrow. We propose that the dynamic Rab11-FIP3 interaction controls the delivery, targeting, and fusion of recycling endosomes with furrow during late cytokinesis and abscission.

Binding of disparate transcriptional activators to nucleosomal DNA is inherently cooperative

To investigate mechanisms by which multiple transcription factors access complex promoters and enhancers within cellular chromatin, we have analyzed the binding of disparate factors to nucleosome cores. We used a purified in vitro system to analyze binding of four activator proteins, two GAL4 derivatives, USF, and NF-kappa B (KBF1), to reconstituted nucleosome cores containing different combinations of binding sites. Here we show that binding of any two or all three of these factors to nucleosomal DNA is inherently cooperative. Thus, the binuclear Zn clusters of GAL4, the helix-loop-helix/basic domains of USF, and the rel domain of NF-kappa B all participated in cooperative nucleosome binding, illustrating that this effect is not restricted to a particular DNA-binding domain. Simultaneous binding by two factors increased the affinity of individual factors for nucleosomal DNA by up to 2 orders of magnitude. Importantly, cooperative binding resulted in efficient nucleosome binding by factors (USF and NF-kappa B) which independently possess little nucleosome-binding ability. The participation of GAL4 derivatives in cooperative nucleosome binding required only DNA-binding and dimerization domains, indicating that disruption of histone-DNA contacts by factor binding was responsible for the increased affinity of additional factors. Cooperative nucleosome binding required sequence-specific binding of all transcription factors, appeared to have spatial constraints, and was independent of the orientation of the binding sites on the nucleosome. These results indicate that cooperative nucleosome binding is a general mechanism that may play a significant role in loading complex enhancer and promoter elements with multiple diverse factors in chromatin and contribute to the generation of threshold responses and transcriptional synergy by multiple activator sites in vivo.

Inhibition of transcriptional regulator Yin-Yang-1 by association with c-Myc

Yin-Yang-1 (YY1) regulates the transcription of many genes, including the oncogenes c-fos and c-myc. Depending on the context, YY1 acts as a transcriptional repressor, a transcriptional activator, or a transcriptional initiator. The yeast two-hybrid system was used to screen a human complementary DNA (cDNA) library for proteins that associate with YY1, and a c-myc cDNA was isolated. Affinity chromatography confirmed that YY1 associates with c-Myc but not with Max. In cotransfections, c-Myc inhibits both the repressor and the activator functions of YY1, which suggests that one way c-Myc acts is by modulating the activity of YY1.

c-Myc represses transcription in vivo by a novel mechanism dependent on the initiator element and Myc box II

We show that c-Myc, in addition to activating transcription through E-box Myc binding sites (Ems), also represses transcription by a mechanism dependent on initiator (Inr) elements of the basal promoters of susceptible genes. Repression was first observed as a component of c-Myc biphasic regulation of the adenovirus-2 major late promoter (MLP), which contains both Inr and Ems sequences. Two differentiation-specific genes containing Inr, the C/EBP alpha and albumin genes, are repressed through their basal promoters by c-Myc, but are activated by the related B-HLH-LZ factor, USF. Repression requires both the B-HLH-LZ and Myc box II (MBII) domains. Significantly, a MBII deletion mutant which is deficient in repression, but transactivates normally, fails to cooperate with an activated ras gene to transform primary fibroblasts. Thus Myc-dependent transactivation is insufficient for Ras cooperation and the novel transcription repression function is implicated in Ras cooperation as well as the suppression of Inr-dependent genes.

Familial combined hyperlipidemia is associated with upstream transcription factor 1 (USF1)

Familial combined hyperlipidemia (FCHL), characterized by elevated levels of serum total cholesterol, triglycerides or both, is observed in about 20% of individuals with premature coronary heart disease. We previously identified a locus linked to FCHL on 1q21-q23 in Finnish families with the disease. This region has also been linked to FCHL in families from other populations as well as to type 2 diabetes mellitus. These clinical entities have several overlapping phenotypic features, raising the possibility that the same gene may underlie the obtained linkage results. Here, we show that the human gene encoding thioredoxin interacting protein (TXNIP) on 1q, which underlies combined hyperlipidemia in mice, is not associated with FCHL. We show that FCHL is linked and associated with the gene encoding upstream transcription factor 1 (USF1) in 60 extended families with FCHL, including 721 genotyped individuals (P = 0.00002), especially in males with high triglycerides (P = 0.0000009). Expression profiles in fat biopsy samples from individuals with FCHL seemed to differ depending on their carrier status for the associated USF1 haplotype. USF1 encodes a transcription factor known to regulate several genes of glucose and lipid metabolism.

Recruitment of histone modifications by USF proteins at a vertebrate barrier element

The chicken beta-globin 5'HS4 insulator element acts as a barrier to the encroachment of chromosomal silencing. Endogenous 5'HS4 sequences are highly enriched with histone acetylation and H3K4 methylation regardless of neighboring gene expression. We report here that 5'HS4 elements recruit these histone modifications when protecting a reporter transgene from chromosomal silencing. Deletion studies identified a single protein binding site within 5'HS4, footprint IV, that is necessary for the recruitment of histone modifications and for barrier activity. We have determined that USF proteins bind to footprint IV. USF1 is present in complexes with histone modifying enzymes in cell extracts, and these enzymes specifically interact with the endogenous 5'HS4 element. Knockdown of USF1 expression leads to a loss of histone modification recruitment and subsequent encroachment of H3K9 methylation. We propose that barrier activity requires the constitutive recruitment of H3K4 methylation and histone acetylation at multiple residues to counteract the propagation of condensed chromatin structures.

Upstream stimulating factors: highly versatile stress-responsive transcription factors

Upstream stimulating factors (USF), USF-1 and USF-2, are members of the eucaryotic evolutionary conserved basic-Helix-Loop-Helix-Leucine Zipper transcription factor family. They interact with high affinity to cognate E-box regulatory elements (CANNTG), which are largely represented across the whole genome in eucaryotes. The ubiquitously expressed USF-transcription factors participate in distinct transcriptional processes, mediating recruitment of chromatin remodelling enzymes and interacting with co-activators and members of the transcription pre-initiation complex. Results obtained from both cell lines and knock-out mice indicates that USF factors are key regulators of a wide number of gene regulation networks, including the stress and immune responses, cell cycle and proliferation, lipid and glucid metabolism, and in melanocytes USF-1 has been implicated as a key UV-activated regulator of genes associated with pigmentation. This review will focus on general characteristics of the USF-transcription factors and their place in some regulatory networks.

An essential E box in the promoter of the gene encoding the mRNA cap-binding protein (eukaryotic initiation factor 4E) is a target for activation by c-myc

The mRNA cap-binding protein (eukaryotic initiation factor 4E [eIF4E]) binds the m7 GpppN cap on mRNA, thereby initiating translation. eIF4E is essential and rate limiting for protein synthesis. Overexpression of eIF4E transforms cells, and mutations in eIF4E arrest cells in G, in cdc33 mutants. In this work, we identified the promoter region of the gene encoding eIF4E, because we previously identified eIF4E as a potential myc-regulated gene. In support of our previous data, a minimal, functional, 403-nucleotide promoter region of eIF4E was found to contain CACGTG E box repeats, and this core eIF4E promoter was myc responsive in cotransfections with c-myc. A direct role for myc in activating the eIF4E promoter was demonstrated by cotransfections with two dominant negative mutants of c-myc (MycdeltaTAD and MycdeltaBR) which equally suppressed promoter function. Furthermore, electrophoretic mobility shift assays demonstrated quantitative binding to the E box motifs that correlated with myc levels in the electrophoretic mobility shift assay extracts; supershift assays demonstrated max and USF binding to the same motif. cis mutations in the core or flank of the eIF4E E box simultaneously altered myc-max and USF binding and inactivated the promoter. Indeed, mutations of this E box inactivated the promoter in all cells tested, suggesting it is essential for expression of eIF4E. Furthermore, the GGCCACGTG(A/T)C(C/G) sequence is shared with other in vivo targets for c-myc, but unlike other targets, it is located in the immediate promoter region. Its critical function in the eIF4E promoter coupled with the known functional significance of eIF4E in growth regulation makes it a particularly interesting target for c-myc regulation.

Transcriptional regulation of cyclooxygenase-2 in mouse skin carcinoma cells. Regulatory role of CCAAT/enhancer-binding proteins in the differential expression of cyclooxygenase-2 in normal and neoplastic tissues

Many studies have suggested that overexpression of cyclooxygenase-2 (COX-2) contributes to the development of tumors in several tissues. COX-2 expression tends to be up-regulated in various types of tumors and transformed cell lines, and the overexpression of COX-2 is caused by enhanced transcription of the gene. In an attempt to characterize the signaling pathway leading to the overexpression of COX-2 in the mouse skin carcinoma cell line JWF2, we investigated cis- and trans-acting factors required for COX-2 expression and demonstrated a molecular mechanism by which COX-2 is expressed differentially in normal and neoplastic tissues. Two regions of the COX-2 promoter containing an E-box and nuclear factor IL6 site were identified as the positive regulatory elements through transient transfections with luciferase reporter vectors containing the various 5'-flanking regions of the promoter. Moreover, electrophoretic mobility shift assays and cotransfection experiments showed that upstream stimulatory factors and CCAAT/enhancer-binding proteins (C/EBPs) bind to the E-box and nuclear factor IL6 site, respectively, and functionally transactivate the COX-2 promoter. We also found that C/EBP isoforms are expressed differentially during mouse skin carcinogenesis, suggesting that overexpression of COX-2 in tumors may be caused by a change in C/EBP expression levels.

Cloning of an inr- and E-box-binding protein, TFII-I, that interacts physically and functionally with USF1

The transcription factor TFII-I has been shown to bind independently to two distinct promoter elements, a pyrimidine-rich initiator (Inr) and a recognition site (E-box) for upstream stimulatory factor 1 (USF1), and to stimulate USF1 binding to both of these sites. Here we describe the isolation of a cDNA encoding TFII-I and demonstrate that the corresponding 120 kDa polypeptide, when expressed ectopically, is capable of binding to both Inr and E-box elements. The primary structure of TFII-I reveals novel features that include six directly repeated 90 residue motifs that each possess a potential helix-loop/span-helix homology. These unique structural features suggest that TFII-I may have the capacity for multiple protein-protein and, potentially, multiple protein-DNA interactions. Consistent with this hypothesis and with previous in vitro studies, we further demonstrate that ectopic TFII-I and USF1 can act synergistically, and in some cases independently, to activate transcription in vivo through both Inr and the E-box elements of the adenovirus major late promoter. We also describe domains of USF1 that are necessary for its independent and synergistic activation functions.

The Usf-1 transcription factor is a novel target for the stress-responsive p38 kinase and mediates UV-induced Tyrosinase expression

The stress-activated signalling cascade leading to phosphorylation of the p38 family of kinases plays a crucial role during development and in the cellular response to a wide variety of stress-inducing agents. Although alterations in gene expression characteristic of the stress response require the regulation of key transcription factors by the p38 family, few downstream targets for this signalling pathway have been identified. By examining the ability of pigment cells to respond to UV irradiation as part of the UV-induced tanning response, we show that while the microphthalmia-associated transcription factor Mitf regulates basal Tyrosinase expression, it is the ubiquitous basic helix-loop-helix-leucine zipper transcription factor Usf-1 that is required for the UV activation of the Tyrosinase promoter. Consistent with this we demonstrate that Usf-1 is phosphorylated and activated by the stress-responsive p38 kinase. The results suggest that activation of Usf-1 by p38 at a wide variety of viral and cellular promoters will provide a link between stimuli as diverse as UV irradiation, glucose, viral infection and pro-inflammatory cytokines, and the changes in gene expression associated with the stress response.

Immunochemical characterization and transacting properties of upstream stimulatory factor isoforms

The ubiquitous upstream stimulatory factor (USF) transcription factors encoded by two distinct genes (USF1 and USF2) exist under the form of various dimers able to bind E-boxes. We report the molecular cloning and functional characterization of USF2 isoforms, corresponding to a 44-kDa subunit, USF2a, and a new 38-kDa subunit, USF2b, generated by differential splicing. Using specific anti-USF antibodies, we define the different binding complexes in various nuclear extracts. In vivo, the USF1/USF2a heterodimer represents over 66% of the USF binding activity whereas the USF1 and USF2a homodimers represent less than 10%, which strongly suggests an in vivo preferential association in heterodimers. In particular, an USF1/USF2b heterodimer accounted for almost 15% of the USF species in some cells. The preferential heterodimerization of USF subunits was reproduced ex vivo, while the in vitro association of cotranslated subunits, or recombinant USF proteins, appeared to be random. In transiently transfected HeLa or hepatoma cells, USF2a and USF1 homodimers transactivated a minimal promoter with similar efficiency, whereas USF2b, which lacks an internal 67-amino acid domain, was a poor transactivator. Additionally, USF2b was an efficient as USF1 and USF2a homodimers in transactivating the liver-specific pyruvate kinase gene promoter.

Chlamydia inhibits interferon gamma-inducible major histocompatibility complex class II expression by degradation of upstream stimulatory factor 1

We report that chlamydiae, which are obligate intracellular bacterial pathogens, can inhibit interferon (IFN)-gamma-inducible major histocompatibility complex (MHC) class II expression. However, the IFN-gamma-induced IFN regulatory factor-1 (IRF-1) and intercellular adhesion molecule 1 (ICAM-1) expression is not affected, suggesting that chlamydia may selectively target the IFN-gamma signaling pathways required for MHC class II expression. Chlamydial inhibition of MHC class II expression is correlated with degradation of upstream stimulatory factor (USF)-1, a constitutively and ubiquitously expressed transcription factor required for IFN-gamma induction of class II transactivator (CIITA) but not of IRF-1 and ICAM-1. CIITA is an obligate mediator of IFN-gamma-inducible MHC class II expression. Thus, diminished CIITA expression as a result of USF-1 degradation may account for the suppression of the IFN-gamma-inducible MHC class II in chlamydia-infected cells. These results reveal a novel immune evasion strategy used by the intracellular bacterial pathogen chlamydia that improves our understanding of the molecular basis of pathogenesis.

Targeting the microphthalmia basic helix-loop-helix-leucine zipper transcription factor to a subset of E-box elements in vitro and in vivo

The development of melanocytes, which are pigment-producing cells responsible for skin, hair, and eye color, is absolutely dependent on the action of the microphthalmia basic helix-loop-helix-leucine zipper (bHLH-LZ) transcription factor (Mi); mice lacking a functional Mi protein are entirely devoid of pigment cells. Mi has been shown to activate transcription of the tyrosinase, TRP-1, TRP-2, and QNR-71 genes through specific E-box elements, most notably the highly conserved M box. We investigated the mechanism which enables Mi to be recruited specifically to a restricted subset of E boxes in target promoters while being prevented from binding E-box elements in other promoters. We show both in vitro and in vivo that the presence of a T residue flanking a CATGTG E box is an essential determinant of the ability of Mi to bind DNA, and we successfully predict that the CATGTG E box from the P gene would not bind Mi. In contrast, no specific requirement for the sequences flanking a CACGTG E box was observed, and no binding to an atypical E box in the c-Kit promoter was detected. The relevance of these observations to the control of melanocyte-specific gene expression was highlighted by the fact that the E-box elements located in the tyrosinase, TRP-1, TRP-2, and QNR-71 promoters without exception possess a 5' flanking T residue which is entirely conserved between species as diverse as man and turtle. The ability of Mi to discriminate between different E-box motifs provides a mechanism to restrict the repertoire of genes which are likely to be regulated by Mi and provides insight into the ability of bHLH-LZ transcription factors to achieve the specificity required for the precise coordination of transcription during development.