Juhong Liu

The FBP Interacting Repressor Targets TFIIH to Inhibit Activated Transcription

FUSE-binding protein (FBP) binds the single-stranded far upstream element of active c-myc genes, possesses potent transcription activation and repression domains, and is necessary for c-myc expression. A novel 60 kDa protein, the FBP interacting repressor (FIR), blocked activator-dependent, but not basal, transcription through TFIIH. Recruited through FBPs nucleic acid-binding domain, FIR formed a ternary complex with FBP and FUSE. FIR repressed a c-myc reporter via the FUSE. The amino terminus of FIR contained an activator-selective repression domain capable of acting in cis or even in trans in vivo and in vitro. The repression domain of FIR targeted only TFIIHs p89/XPB helicase, required at several stages in transcription, but not factors required for promoter selection. Thus, FIR locks TFIIH in an activation-resistant configuration that still supports basal transcription.

Transcription-dependent dynamic supercoiling is a short-range genomic force

Transcription has the capacity to mechanically modify DNA topology, DNA structure and nucleosome arrangement. Resulting from ongoing transcription, these modifications in turn may provide instant feedback to the transcription machinery. To substantiate the connection between transcription and DNA dynamics, we charted an ENCODE map of transcription-dependent dynamic supercoiling in human Burkitts lymphoma cells by using psoralen photobinding to probe DNA topology in vivo. Dynamic supercoils spread ~1.5 kilobases upstream of the start sites of active genes. Low- and high-output promoters handled this torsional stress differently, as shown by using inhibitors of transcription and topoisomerases and by chromatin immunoprecipation of RNA polymerase and topoisomerases I and II. Whereas lower outputs are managed adequately by topoisomerase I, high-output promoters additionally require topoisomerase II. The genome-wide coupling between transcription and DNA topology emphasizes the importance of dynamic supercoiling for gene regulation.

Loss of FBP function arrests cellular proliferation and extinguishes c‐ myc expression

The c‐myc regulatory region includes binding sites for a large set of transcription factors. The present studies demonstrate that in the absence of FBP [far upstream element (FUSE)‐binding protein], which binds to the single‐stranded FUSE, the remainder of the set fails to sustain endogenous c‐myc expression. A dominant‐negative FBP DNA‐binding domain lacking effector activity or an antisense FBP RNA, expressed via replication‐defective adenovirus vectors, arrested cellular proliferation and extinguished native c‐myc transcription from the P1 and P2 promoters. The dominant‐negative FBP initially augmented the single‐stranded character of FUSE; however, once c‐myc expression was abolished, melting at FUSE could no longer be supported. In contrast, with antisense FBP RNA, the single‐stranded character of FUSE decreased monotonically as the transcription of endogenous c‐myc declined. Because transcription is the major source of super‐coiling in vivo, we propose that by binding torsionally strained DNA, FBP measures promoter activity directly. We also show that FUSE is predicted to behave as a torsion‐regulated switch poised to regulate c‐myc and to confer a higher order regulation on a large repertoire of factors.

The FUSE/FBP/FIR/TFIIH system is a molecular machine programming a pulse of c-myc expression

FarUpStream Element (FUSE) Binding Protein (FBP) binds the human c‐myc FUSE in vitro only in single‐stranded or supercoiled DNA. Because transcriptionally generated torsion melts FUSE in vitro even in linear DNA, and FBP/FBP Interacting Repressor (FIR) regulates transcription through TFIIH, these components have been speculated to be the mechanosensor (FUSE) and effectors (FBP/FIR) of a real‐time mechanism controlling c‐myc transcription. To ascertain whether the FUSE/FBP/FIR system operates according to this hypothesis in vivo, the flux of activators, repressors and chromatin remodeling complexes on the c‐myc promoter was monitored throughout the serum‐induced pulse of transcription. After transcription was switched on by conventional factors and chromatin regulators, FBP and FIR were recruited and established a dynamically remodeled loop with TFIIH at the P2 promoter. In XPB cells carrying mutant TFIIH, loop formation failed and the serum response was abnormal; RNAi depletion of FIR similarly disabled c‐myc regulation. Engineering FUSE into episomal vectors predictably re‐programmed metallothionein‐promoter‐driven reporter expression. The in vitro recruitment of FBP and FIR to dynamically stressed c‐myc DNA paralleled the in vivo process.

Defective Interplay of Activators and Repressors with TFIIH in Xeroderma Pigmentosum

Inherited mutations of the TFIIH helicase subunits xeroderma pigmentosum (XP) B or XPD yield overlapping DNA repair and transcription syndromes. The high risk of cancer in these patients is not fully explained by the repair defect. The transcription defect is subtle and has proven more difficult to evaluate. Here, XPB and XPD mutations are shown to block transcription activation by the FUSE Binding Protein (FBP), a regulator of c-myc expression, and repression by the FBP Interacting Repressor (FIR). Through TFIIH, FBP facilitates transcription until promoter escape, whereas after initiation, FIR uses TFIIH to delay promoter escape. Mutations in TFIIH that impair regulation by FBP and FIR affect proper regulation of c-myc expression and have implications in the development of malignancy.

The dynamic response of upstream DNA to transcription-generated torsional stress

The torsional stress caused by counter-rotation of the transcription machinery and template generates supercoils in a closed topological domain, but has been presumed to be too short-lived to be significant in an open domain. This report shows that transcribing RNA polymerases dynamically sustain sufficient torsion to perturb DNA structure even on linear templates. Assays to capture and measure transcriptionally generated torque and to trap short-lived perturbations in DNA structure and conformation showed that the transient forces upstream of active promoters are large enough to drive the supercoil-sensitive far upstream element (FUSE) of the human c-myc into single-stranded DNA. An alternative non-B conformation of FUSE found in stably supercoiled DNA is not accessible dynamically. These results demonstrate that dynamic disturbance of DNA structure provides a real-time measure of ongoing genetic activity.

JTV1 co‐activates FBP to induce USP29 transcription and stabilize p53 in response to oxidative stress

c‐myc and p53 networks control proliferation, differentiation, and apoptosis and are responsive to, and cross‐regulate a variety of stresses and metabolic and biosynthetic processes. At c‐myc, the far upstream element binding protein (FBP) and FBP‐interacting repressor (FIR) program transcription by looping to RNA polymerase II complexes engaged at the promoter. Another FBP partner, JTV1/AIMP2, a structural subunit of a multi‐aminoacyl‐tRNA synthetase (ARS) complex, has also been reported to stabilize p53 via an apparently independent mechanism. Here, we show that in response to oxidative stress, JTV1 dissociates from the ARS complex, translocates to the nucleus, associates with FBP and co‐activates the transcription of a new FBP target, ubiquitin‐specific peptidase 29 (USP29). A previously uncharacterized deubiquitinating enzyme, USP29 binds to, cleaves poly‐ubiquitin chains from, and stabilizes p53. The accumulated p53 quickly induces apoptosis. Thus, FBP and JTV1 help to coordinate the molecular and cellular response to oxidative stress.

FBPs Are Calibrated Molecular Tools To Adjust Gene Expression

ABSTRACT The three far-upstream element (FUSE) binding protein (FBP) family members have been ascribed different functions in gene regulation. They were therefore examined with various biochemical, molecular biological, and cell biological tests to evaluate whether their sequence differences reflect functional customization or neutral changes at unselected residues. Each FBP displayed a characteristic profile of intrinsic transcription activation and repression, binding with protein partners, and subcellular trafficking. Although some differences, such as weakened FBP3 nuclear localization, were predictable from primary sequence differences, the unexpected failure of FBP3 to bind the FBP-interacting repressor (FIR) was traced to seemingly conservative substitutions within a small patch of an N-terminal α-helix. The transactivation strength and the FIR-binding strength of the FBPs were in the opposite order. Despite their distinguishing features and differential activities, the FBPs traffic to shared subnuclear sites and regulate many common target genes, including c-myc. Though a variety of functions have been attributed to the FBPs, based upon their panel of shared and unique features, we propose that they constitute a molecular regulatory kit that tunes the expression of shared targets through a common mechanism.

TFIIH Operates through an Expanded Proximal Promoter To Fine-Tune c-myc Expression

ABSTRACT A continuous stream of activating and repressing signals is processed by the transcription complex paused at the promoter of the c-myc proto-oncogene. The general transcription factor IIH (TFIIH) is held at promoters prior to promoter escape and so is well situated to channel the input of activators and repressors to modulate c-myc expression. We have compared cells expressing only a mutated p89 (xeroderma pigmentosum complementation group B [XPB]), the largest TFIIH subunit, with the same cells functionally complemented with the wild-type protein (XPB/wt-p89). Here, we show structural, compositional, and functional differences in transcription complexes between XPB and XPB/wt-89 cells at the native c-myc promoter. Remarkably, although the mean levels of c-Myc are only modestly elevated in XPB compared to those in XPB/wt-p89 cells, the range of expression and the cell-to-cell variation of c-Myc are markedly increased. Our modeling indicates that the data can be explained if TFIIH integrates inputs from multiple signals, regulating transcription at multiple kinetically equivalent steps between initiation and promoter escape. This helps to suppress the intrinsic noise of transcription and to ensure the steady transcriptional output of c-myc necessary for cellular homeostasis.

Hierarchical mechanisms build the DNA-binding specificity of FUSE binding protein

The far upstream element (FUSE) binding protein (FBP), a single-stranded nucleic acid binding protein, is recruited to the c-myc promoter after melting of FUSE by transcriptionally generated dynamic supercoils. Via interactions with TFIIH and FBP-interacting repressor (FIR), FBP modulates c-myc transcription. Here, we investigate the contributions of FBPs 4 K Homology (KH) domains to sequence selectivity. EMSA and missing contact point analysis revealed that FBP contacts 4 separate patches spanning a large segment of FUSE. A SELEX procedure using paired KH-domains defined the preferred subsequences for each KH domain. Unexpectedly, there was also a strong selection for the noncontacted residues between these subsequences, showing that the contact points must be optimally presented in a backbone that minimizes secondary structure. Strategic mutation of contact points defined in this study disabled FUSE activity in vivo. Because the biological specificity of FBP is tuned at several layers: (i) accessibility of the site; (ii) supercoil-driven melting; (iii) presentation of unhindered bases for recognition; and (iv) modular interaction of KH-domains with cognate bases, the FBP-FIR system and sequence-specific, single-strand DNA binding proteins in general are likely to prove versatile tools for adjusting gene expression.