The transcriptional silencer protein NRF: a repressor of NF-kappa B enhancers

Effect of Feed Supplementation with Bacillus coagulans on Nrf Gene Family Expression in Common Carp (Cyprinus carpio) under Long-Term Exposure to Cd2+

Nuclear factor-E2-related factor (Nrf) belongs to the Cap ‘n’ collar basic leucine zipper (CNC-bZIP) family, which plays an important role in the resistance to oxidative stress in the body. In this study, 12 Nrf genes were identified in the common carp genome database. Comparative genomic analysis showed that the Nrf genes of common carp had significant amplification, confirming that the common carp had experienced four genome-wide replication events. Phylogenetic analysis showed that all common carp Nrf clustered with scleractinian fish Nrf, indicating that they were highly conserved during evolution. In addition, tissue distribution results showed that most Nrf genes had a broad tissue distribution but exhibited tissue-specific expression patterns, demonstrating functional differences after WGD events. At 30 and 60 days of Cd2+ stress, most of the Nrf genes showed an increase in expression compared with the control group, indicating that they played a key role in the organism’s response to oxidative stress. To find a suitable concentration of Bacillus coagulans to activate the Nrf genes, we added three different concentrations (2.0 × 107 CFU/g, 2.0 × 108 CFU/g, and 2.0 × 109 CFU/g) of B. coagulans into the feed and defined them as L1, L2, and L3 groups, respectively. We investigated the effect of different concentrations of B. coagulans in the feed on the expression level of Nrf genes in the intestine of common carp under Cd2+ stress at 30 and 60 days. The results showed that, compared with the control/stress group, the expression of different Nrf genes was improved to varying degrees at three concentrations, and the effect of the L2 group (2.0 × 108 CFU/g) was the best. This suggests that the L2 group is the optimum concentration for activating Nrf gene expression when subjected to heavy metal Cd2+ stress and may act as an activation switch with a prominent role in the body’s resistance to oxidative stress and immune response.

Identification of new candidate genes for spina bifida through exome sequencing

PURPOSE

Neural tube defects are a group of birth defects caused by failure of neural tube closure during development. The etiology of NTD, requiring a complex interaction between environmental and genetic factors, is not well understood.

METHODS

We performed whole-exome sequencing (WES) in six trios, with a single affected proband with spina bifida, to identify rare/novel variants as potential causes of the NTD.

RESULTS

Our analysis identified four de novo and ten X-linked recessive variants in four of the six probands, all of them in genes previously never implicated in NTD. Among the 14 variants, we ruled out six of them, based on different criteria and pursued the evaluation of eight potential candidates in the following genes: RXRγ, DTX1, COL15A1, ARHGAP36, TKTL1, AMOT, GPR50, and NKRF. The de novo variants where located in the RXRγ, DTX1, and COL15A1 genes while ARHGAP36, TKTL1, AMOT, GPR50, and NKRF carry X-linked recessive variants. This analysis also revealed that four patients presented multiple variants, while we were unable to identify any significant variant in two patients.

CONCLUSIONS

Our preliminary conclusion support a major role for the de novo variants with respect to the X-linked recessive variants where the X-linked could represent a contribution to the phenotype in an oligogenic model.

Of Keeping and Tipping the Balance: Host Regulation and Viral Modulation of IRF3-Dependent IFNB1 Expression

The type I interferon (IFN) response is a principal component of our immune system that allows to counter a viral attack immediately upon viral entry into host cells. Upon engagement of aberrantly localised nucleic acids, germline-encoded pattern recognition receptors convey their find via a signalling cascade to prompt kinase-mediated activation of a specific set of five transcription factors. Within the nucleus, the coordinated interaction of these dimeric transcription factors with coactivators and the basal RNA transcription machinery is required to access the gene encoding the type I IFN IFNβ (IFNB1). Virus-induced release of IFNβ then induces the antiviral state of the system and mediates further mechanisms for defence. Due to its key role during the induction of the initial IFN response, the activity of the transcription factor interferon regulatory factor 3 (IRF3) is tightly regulated by the host and fiercely targeted by viral proteins at all conceivable levels. In this review, we will revisit the steps enabling the trans-activating potential of IRF3 after its activation and the subsequent assembly of the multi-protein complex at the IFNβ enhancer that controls gene expression. Further, we will inspect the regulatory mechanisms of these steps imposed by the host cell and present the manifold strategies viruses have evolved to intervene with IFNβ transcription downstream of IRF3 activation in order to secure establishment of a productive infection.

circ_0000638 inhibits neodymium oxide-induced bronchial epithelial cell inflammation through the miR-498-5p/NF-κB axis

Inhalation of neodymium oxide, a type of rare earth metal, can induce various respiratory diseases, such as lung tissue inflammation, but the associated mechanisms underlying this induction are still unclear. In this study, we explored the role and mechanisms of circular RNA in neodymium oxide-induced airway inflammation. Our study showed that treating human bronchial epithelial (16HBE) cells with neodymium oxide caused an inflammatory response by upregulating the expression of interleukin-8 (IL-8) and interleukin-1 beta (IL-1β). Quantitative real-time polymerase chain reaction (qRT-PCR) analyses revealed significant downregulation of circRNA_0000638 in neodymium oxide-treated 16HBE cells. Data from functional analyses further showed that circ_0000638 inhibited inflammation induced by neodymium oxide in 16HBE cells. circ_0000638 targeted miR-498-5p and further increased the expression of NKRF (NF-κB repressing factor), which inhibited the activation of NF-κB (nuclear factor κB). Moreover, circ_0000638 reduced the expression of IL-8 and IL-1β by inhibiting NF-κB activation in neodymium oxide-treated 16HBE cells. These results suggest that circ_0000638 can inhibit NF-κB activation by competitively binding to miR-498-5p, further downregulating the expression of IL-8 and IL-1β in neodymium oxide-treated 16HBE cells.

Full-length NF-κB repressing factor contains an XRN2 binding domain

NF-κB repressing factor (NKRF) was recently identified as an RNA binding protein that together with its associated proteins, the 5′–3′ exonuclease XRN2 and the helicase DHX15, is required to process the precursor ribosomal RNA. XRN2 is a multi-functional ribonuclease that is also involved in processing mRNAs, tRNAs and lncRNAs. The activity and stability of XRN2 are controlled by its binding partners, PAXT-1, CDKN2AIP and CDKN2AIPNL. In each case, these proteins interact with XRN2 via an XRN2 binding domain (XTBD), the structure and mode of action of which is highly conserved. Rather surprisingly, although NKRF interacts directly with XRN2, it was not predicted to contain such a domain, and NKRF's interaction with XRN2 was therefore unexplained. We have identified an alternative upstream AUG start codon within the transcript that encodes NKRF and demonstrate that the full-length form of NKRF contains an XTBD that is conserved across species. Our data suggest that NKRF is tethered in the nucleolus by binding directly to rRNA and that the XTBD in the N-terminal extension of NKRF is essential for the retention of XRN2 in this sub-organelle. Thus, we propose NKRF regulates the early steps of pre-rRNA processing during ribosome biogenesis by controlling the spatial distribution of XRN2 and our data provide further support for the XTBD as an XRN2 interacting motif.

Inhibition of microRNA‑124‑3p protects against acute myocardial infarction by suppressing the apoptosis of cardiomyocytes

The aims of the present study were to investigate the roles and underlying mechanisms of microRNA‑124‑3p (miR‑124‑3p) in the progression of acute myocardial infarction (AMI). The expression of miR‑124‑3p was determined via reverse transcription‑quantitative polymerase chain reaction (RT‑qPCR). TargetScan analysis and a luciferase reporter assay were conducted to reveal the association between miR‑124‑3p and nuclear factor κ‑light‑chain‑enhancer of activated B cells (NF‑κB) repressing factor (NKRF). To investigate the role of miR‑124‑3p in AMI, a cell model of myocardial hypoxic/ischemic injury was established by subjecting H9c2 cardiac cells to hypoxia for 48 h. The viability of cells was determined using an MTT assay, and cell apoptosis was analyzed by flow cytometry. Additionally, the expression levels of inflammatory factors [tumor necrosis factor‑α (TNF‑α), interleukin (IL)‑1β and IL‑6] were measured via ELISA. Furthermore, gene and protein expression levels were determined by performing RT‑qPCR and western blot analyses, respectively. It was revealed that the expression of miR‑124‑3p was significantly increased in the blood of patients with AMI and hypoxia‑treated H9c2 cells. Additionally, it was demonstrated that NKRF was a direct target of miR‑124‑3p. The hypoxia‑induced decrease in the viability of H9c2 cells and increase in cell apoptosis were eliminated by the downregulation of miR‑124‑3p. Furthermore, hypoxia significantly increased the levels of TNF‑α, IL‑1β and IL‑6, whereas miR‑124‑3p downregulation eliminated these effects. Downregulated expression of B‑cell lymphoma 2, pro‑caspase 3 and pro‑caspase 9 protein, and upregulated expression of cleaved caspases 3 and 9 was observed in hypoxic H9c2 cells; the altered expression of these proteins was suppressed by miR‑124‑3p inhibitor. Additionally, miR‑124‑3p inhibitor suppressed the hypoxia‑induced activation of the NF‑κB signaling pathway in H9c2 cells. Furthermore, it was demonstrated that the various effects of miR‑124‑3p inhibitor on H9c2 cells were eliminated by the small interfering RNA‑mediated downregulation of NKRF. In conclusion, the results of the present study indicated that miR‑124‑3p downregulation protected against AMI via inhibition of inflammatory responses and the apoptosis of cardiomyocytes by regulating the NKRF/NF‑κB pathway.

Homoharringtonine deregulates MYC transcriptional expression by directly binding NF-κB repressing factor

Homoharringtonine (HHT), a known protein synthesis inhibitor, has an anti-myeloid leukemia effect and potentiates the therapeutic efficacy of anthracycline/cytarabine induction regimens for acute myelogenous leukemia (AML) with favorable and intermediate prognoses, especially in the t(8;21) subtype. Here we provide evidence showing that HHT inhibits the activity of leukemia-initiating cells (Lin^-/Sca-1^-/c-kit⁺; LICs) in a t(8;21) murine leukemia model and exerts a down-regulating effect on MYC pathway genes in human t(8;21) leukemia cells (Kasumi-1). We discovered that NF-κB repressing factor (NKRF) is bound directly by HHT via the second double-strand RNA-binding motif (DSRM2) domain, which is the nuclear localization signal of NKRF. A series of deletion and mutagenesis experiments mapped HHT direct binding sites to K479 and C480 amino acids in the DSRM2 domain. HHT treatment shifts NKRF from the nucleus (including nucleoli) to the cytoplasm by occupying the DSRM2 domain, strengthens the p65-NKRF interaction, and interferes with p65-p50 complex formation, thereby attenuating the transactivation activity of p65 on the MYC gene. Moreover, HHT significantly decreases the expression of KIT, a frequently mutated and/or highly expressed gene in t(8;21) AML, in concert with MYC down-regulation. Our work thus identifies a mechanism of action of HHT that is different from, but acts in concert with, the known mode of action of this compound. These results justify further clinical testing of HHT in AML.

Modelling the interactions between TLR4 and IFNβ pathways

Bacterial lipopolysaccharide (LPS) association with their connate receptor TLR4 triggers Type I interferon signaling cascade through its MyD88 independent downstream. Compared to plethora of reported empirical data on both TLR4 and Type I interferon pathways, there is no known model to decipher crosstalk mechanisms between these two crucial innate immune pathogen activated pathways regulating vital transcriptional factors such as nuclear factor-κB (NFκB), IFNβ, the interferon-stimulated gene factor-3 (ISGF3) and an important cancer drug target protein kinase-R (PKR). Innate immune system is based on a sensitive balance of intricate interactions. In elucidating these interactions, in silico integration of pathways has great potential. Attempts confined to single pathway may not be effective in truly addressing source of real systems behavior. This is the first report combining toll-like receptor-4 (TLR4) and interferon beta (IFNβ) pathways in a single in silico model, analyzing their interactions, pinpointing the source of delay in PKR late phase activity and limiting the transcription of IFN and PKR by using a method including an statistical physics technique in reaction equations. The model quite successfully recapitulates published interferon regulatory factor-3 (IRF3) and IFNβ data from mouse macrophages and PKR data from mouse embryonic fibroblast cell lines. The simulations end up with an estimate of IRF3, IFNβ, ISGF3 dose dependent profiles mimicking nonlinear dose response characteristic of the system. Involvement of concomitant PKR downstream can unravel elusive mechanisms in specific profiles like NFκB regulation.

Manipulation of the nuclear factor-kappaB pathway and the innate immune response by viruses

Viral and microbial constituents contain specific motifs or pathogen-associated molecular patterns (PAMPs) that are recognized by cell surface- and endosome-associated Toll-like receptors (TLRs). In addition, intracellular viral double-stranded RNA is detected by two recently characterized DExD/H box RNA helicases, RIG-I and Mda-5. Both TLR-dependent and -independent pathways engage the IkappaB kinase (IKK) complex and related kinases TBK-1 and IKKvarepsilon. Activation of the nuclear factor kappaB (NF-kappaB) and interferon regulatory factor (IRF) transcription factor pathways are essential immediate early steps of immune activation; as a result, both pathways represent prime candidates for viral interference. Many viruses have developed strategies to manipulate NF-kappaB signaling through the use of multifunctional viral proteins that target the host innate immune response pathways. This review discusses three rapidly evolving areas of research on viral pathogenesis: the recognition and signaling in response to virus infection through TLR-dependent and -independent mechanisms, the involvement of NF-kappaB in the host innate immune response and the multitude of strategies used by different viruses to short circuit the NF-kappaB pathway.

Innate immune responses in NF-kappaB-repressing factor-deficient mice

NF-kappaB-repressing factor (NRF) is a transcriptional silencer protein that specifically counteracts the basal activity of several NF-kappaB-dependent promoters by direct binding to specific neighboring DNA sequences. In cell culture experiments, the reduction of NRF mRNA leads to a derepression of beta interferon, interleukin-8, and inducible nitric oxide synthase transcription. The X chromosome-located single-copy NRF gene is ubiquitously expressed and encodes a protein of 690 amino acids. The N-terminal part contains a nuclear localization signal, the DNA-binding domain, and the NF-kappaB-repressing domain, while the C-terminal part is responsible for double-stranded RNA binding and nucleolar localization. To study the function of NRF in a systemic context, transgenic mice lacking the NRF gene were created. Against predictions from in vitro experiments, mice with a deletion of the NRF gene are viable and have a phenotype that is indistinguishable from wild-type mice, even after challenge with different pathogens. The data hint towards an unexpected functional redundancy of NRF.

Macrolide- and tetracycline-adjustable siRNA-mediated gene silencing in mammalian cells using polymerase II-dependent promoter derivatives

RNA interference has emerged as a powerful technology for downregulation of specific genes in cells and animals. We have pioneered macrolide- and tetracycline-adjustable short interfering RNA (siRNA) expression for conditional target gene translation fine-tuning in mammalian/human cell lines based on modified RNA polymerase II promoters. Established macrolide- and tetracycline-dependent transactivators/trans-silencers bound and activated modified target promoters tailored for optimal siRNA expression in response to clinical antibiotics' dosing regimes and modulated desired target genes in Chinese hamster ovary (CHO-K1) and human fibrosarcoma (HT-1080) cells with high precision. Further optimization of adjustable RNA polymerase II-based siRNA-specific promoters as well as their combination with various transmodulators enabled near-perfect regulation configurations in specific cell types. Devoid of major genetic constraints compared to basic RNA polymerase III-based siRNA-specific promoters, we expect RNA polymerase II counterparts to significantly advance siRNA-based molecular interventions in biopharmaceutical manufacturing and gene-function analysis as well as gene therapy and tissue engineering.

Mechanism for transcriptional synergy between interferon regulatory factor (IRF)-3 and IRF-7 in activation of the interferon-β gene promoter

The interferon-beta promoter has been studied extensively as a model system for combinatorial transcriptional regulation. In virus-infected cells the transcription factors ATF-2, c-Jun, interferon regulatory factor (IRF)-3, IRF-7 and NF-kappaB, and the coactivators p300/CBP play critical roles in the activation of this and other promoters. It remains unclear, however, why most other combinations of AP-1, IRF and Rel proteins fail to activate the interferon-beta gene. Here we have explored how different IRFs may cooperate with other factors to activate transcription. First we showed in undifferentiated embryonic carcinoma cells that ectopic expression of either IRF-3 or IRF-7, but not IRF-1, was sufficient to allow virus-dependent activation of the interferon-beta promoter. Moreover, the activity of IRF-3 and IRF-7 was strongly affected by promoter context, with IRF-7 preferentially being recruited to the natural interferon-beta promoter. We fully reconstituted activation of this promoter in insect cells. Maximal synergy required IRF-3 and IRF-7 but not IRF-1, and was strongly dependent on the presence of p300/CBP, even when these coactivators only modestly affected the activity of each factor by itself. These results suggest that specificity in activation of the interferon-beta gene depends on a unique promoter context and on the role played by coactivators as architectural factors.

Nucleolar localization and mobility analysis of the NF-κB repressing factor NRF

NF-κB plays a central role in mediating pathogen and cytokine-stimulated gene transcription. NF-κB repressing factor (NRF) has been shown to interact with specific negative regulatory DNA elements (NRE) to mediate transcriptional repression by inhibition of the NF-κB activity at certain promoters. mRNA ablation experiments demonstrated that the trans-acting NRF protein is involved in constitutive but not post-stimulated silencing of IFN-β, IL-8 and iNOS genes by binding to cis-acting NRE elements in their promoters.

We have examined the subcellular localization and mobility of the NRF protein. Since neither tagging nor overexpression perturbs NRF localization the GFP-tagged protein was used for detailed localization and mobility studies. Owing to an N-terminal nuclear localization sequence, all NRF fragments that contain this signal show a constitutive nuclear accumulation. C-terminal NRF fragments also localize to the nucleus although no canonical NLS motifs were detected. Full-length NRF is highly enriched in nucleoli and only a small fraction of NRF is found in the nucleoplasm and cytoplasm. This relationship was found to be independent of the protein expression rate. FRAP analysis proved to be a sensitive method to determine protein mobility and made it possible to differentiate between the NRF protein fragments. Nucleolar localization correlated inversely with mobility. The data demonstrate that a series of neighboring fragments in a large central domain of the protein contribute to the strong nucleolar affinity. These properties were not altered by viral infection or LPS treatment. Several sequence motifs for RNA binding were predicted by computer-mediated databank searches. We found that NRF binds to double stranded RNA (dsRNA). This property mapped to several NRF fragments which correlate with the nucleolar affinity domain. Since treatment with actinomycin D releases NRF from nucleoli the identified RNA binding motifs might act as nucleolar localization signals.

Novel promoter/transactivator configurations for macrolide- and streptogramin-responsive transgene expression in mammalian cells

BACKGROUND

The recently developed heterologous macrolide- (E.REX system) and streptogramin- (PIP system) responsive gene regulation systems show significant differences in their regulation performance in diverse cell lines.

METHODS

In order to provide optimal regulation modalities for a wide variety of mammalian cell lines, we have performed a detailed analysis of E.REX and PIP systems modified in (i) the transactivation domains of the antibiotic-dependent transactivators, (ii) the type of minimal promoter used, and (iii) the spacing between the operator module and the minimal promoter.

RESULTS

These novel E.REX and PIP regulation components showed not only dramatically improved regulation performance in some cell types, but also enabled their use in cell lines which had previously been inaccessible to regulated transgene expression.

CONCLUSIONS

Due to their modular set-up the novel E.REX and PIP regulation systems presented here are most versatile and ready for future upgrades using different cell-specific key regulation components.

Identification of a negative response element in the human inducible nitric-oxide synthase (hiNOS) promoter: The role of NF-kappa B-repressing factor (NRF) in basal repression of the hiNOS gene

Although nuclear factor (NF)-kappaB plays a central role in mediating cytokine-stimulated human inducible nitric-oxide synthase (hiNOS) gene transcription, very little is known about the factors involved in silencing of the hiNOS promoter. NF-kappaB-repressing factor (NRF) interacts with a specific negative regulatory element (NRE) to mediate transcriptional repression of certain NF-kappaB responsive genes. By sequence comparison with the IFN-beta and IL-8 promoters, we identified an NRE in the hiNOS promoter located at -6.7 kb upstream. In A549 and HeLa human cells, constitutive NRF mRNA expression is detected by RT-PCR. Gel shift assay showed constitutive NRF binding to the hiNOS NRE. Mutation of the -6.7-kb NRE site in the hiNOS promoter resulted in loss of NRF binding and increased basal but not cytokine-stimulated hiNOS transcription in promoter transfection experiments. Interestingly, overexpression of NRF suppressed both basal and cytokine-induced hiNOS promoter activity that depended on an intact cis-acting NRE motif. By using stably transformed HeLa cells with the tetracycline on/off expression system, reduction of cellular NRF by expressing antisense NRF increased basal iNOS promoter activity and resulted in constitutive iNOS mRNA expression. These data demonstrate that the transacting NRF protein is involved in constitutive silencing of the hiNOS gene by binding to a cis-acting NRE upstream in the hiNOS promoter.

The NF-kappa b repressing factor is involved in basal repression and interleukin (IL)-1-induced activation of IL-8 transcription by binding to a conserved NF-kappa b-flanking sequence element

Interleukin (IL)-8, a prototypic chemokine, is rapidly induced by the pro-inflammatory cytokine IL-1 but is barely detectable in noninduced cells. Although there is clear evidence that the transcription factor NF-kappaB plays a central role in inducible IL-8 transcription, very little is known about the cis-elements and trans-acting factors involved in silencing of the IL-8 promoter. By sequence comparison with the interferon-beta promoter, we found a negative regulatory element (NRE) in the IL-8 promoter overlapping partially with the NF-kappaB response element. Here we show that an NF-kappaB-repressing factor (NRF) binds to the IL-8 promoter NF-kappaB-NRE. Reduction of cellular NRF by expressing NRF antisense RNA results in spontaneous IL-8 gene expression. In contrast, IL-1-induced IL-8 secretion is strongly impaired by expressing NRF antisense RNA. Mutation of the NRE site results in loss of NRF binding and increased basal IL-8 transcription. On the other hand IL-1-induced IL-8 transcription is decreased by mutating the NRE. These data provide evidence for a dual role of the NRF in IL-8 transcription. Although in the absence of stimulation it is involved in transcriptional silencing, in IL-1-induced cells it is required for full induction of the IL-8 promoter.

Translation of NRF mRNA is mediated by highly efficient internal ribosome entry

The ubiquitous transcription factor NRF (NF-kappaB repressing factor) is a constitutive transcriptional silencer of the multifunctional cytokine interferon-beta. NRF mRNA contains a long 5' untranslated region (5'UTR) predicted to fold into a strong secondary structure. The presence of stable hairpins is known to be incompatible with efficient translation by ribosomal scanning. Using dicistronic reporter gene constructs, we show that the NRF 5'UTR acts as an internal ribosome entry site (IRES) which directs ribosomes to the downstream start codon by a cap-independent mechanism. The relative activity of this IRES in various cell lines is at least 30-fold higher than that of picornaviral IRESs. The NRF 5'UTR also functions as a translational enhancer in the context of monocistronic mRNAs. Our results indicate that the NRF 5'UTR contains a highly potent IRES, which may allow for an alternate mode of translation under physiological conditions in which cap-dependent translation is inhibited.

NFkappaB activation is required for interferon regulatory factor-1-mediated interferon beta induction

The interferon regulatory factor 1 (IRF-1) acts as a transcriptional inducer of the interferon beta (IFN-beta) gene and interferon-stimulated genes. Here we report that IRF-1-mediated IFN-beta induction depends on NFkappaB activity. IRF-1 by itself initiates NFkappaB activation by inducing a reduction in cellular MAD3/IkappaBalpha, an inhibitor of NFkappaB. After nuclear translocation, NFkappaB synergizes with IRF-1 on the cis-elements positive regulatory domain (PRD)II and PRDI/III to induce transcription of the IFN-beta gene. In contrast with IFN-beta transcription induced by dsRNA or virus, c-Jun/ATF-2 binding to PRDIV is not involved. Recombinant MAD3/IkappaBalpha is phosphorylated in vitro by extracts from IRF-1-expressing cells. IRF-1-dependent MAD3/IkappaBalpha degradation is not detectable in cells expressing a dominant negative mutant of the protein kinase PKR, suggesting that PKR mediates MAD3/IkappaBalpha degradation.

Transcriptional repression of type I IFN genes

Transcriptional regulation is a consequence of the combination of both activation and repression for establishing specific patterns of eukaryotic gene expression. The regulation of the expression of type I interferon (IFN-A and IFN-B) multigene family is controlled primarily at the transcriptional level and has been widely studied as a model for understanding the mechanisms of stable repression, transient virus induction and postinduction repression of the genes. The positive and negative regulatory elements required for this on/off switch have been defined within a complex 5' upstream region of their transcription start site. The differential expression pattern of type I IFN genes is thought to involve both substitutions in the virus responsive element (VRE) and presence or absence of negatively acting sequences surrounding the VRE. In this review we discuss several mechanisms of negative regulation due to the existence of common or specific elements in the IFN-B and IFN-A genes and we summarize recent studies on transcriptional repressors that bind to these promoters.