Sungchul Hohng

• The Seoul Institute

Bacterial and bacteriophage RNA polymerases (RNAPs) have divergently evolved and share the RNA hairpin-dependent intrinsic termination of transcription. Here, we examined phage T7, T3 and SP6 RNAP terminations utilizing the single-molecule fluorescence assays we had developed for bacterial terminations. We discovered the phage termination mode or o...

Transcription termination has evolved to proceed through diverse mechanisms. For several classes of terminators, multiple models have been debatably proposed. Recent single-molecule studies on bacterial terminators have resolved several long-standing controversies. First, termination mode or outcome is twofold rather than single. RNA is released al...

Although Argonaute (AGO) proteins have been the focus of microRNA (miRNA) studies, we observed AGO-free mature miRNAs directly interacting with RNA-binding proteins, implying the sophisticated nature of fine-tuning gene regulation by miRNAs. To investigate microRNA-binding proteins (miRBPs) globally, we analyzed PAR-CLIP data sets to identify RBP q...

The R-loops forming around DNA double-strand breaks (DSBs) within actively transcribed genes play a critical role in the DSB repair process. However, the mechanisms underlying R-loop formation at DSBs remain poorly understood, with diverse proposed models involving protein factors associated with RNA polymerase (RNAP) loading, pausing/backtracking...

Although Argonaute (AGO) proteins have been the focus of microRNA (miRNA) studies, we observed AGO-free mature miRNAs directly interacting with RNA-binding proteins, implying the sophisticated nature of fine-tuning gene regulation by miRNAs. To investigate microRNA-binding proteins (miRBPs) globally, we analyzed PAR-CLIP data sets to identify RBP q...

Transcriptional pause is essential for all types of termination. In this single-molecule study on bacterial Rho factor-dependent terminators, we confirm that the three Rho-dependent termination routes operate compatibly together in a single terminator, and discover that their termination efficiencies depend on the terminational pauses in unexpected...

The determination of the oligomeric state of functional enzymes is essential for the mechanistic understanding of their catalytic activities. RecQ helicases have diverse biochemical activities, but it is still unclear how their activities are related to their oligomeric states. We use single-molecule multi-color fluorescence imaging to determine th...

MicroRNAs (miRNAs) are short regulatory RNAs that control gene expression at the post-transcriptional level. Various miRNAs playing important roles in cancer development are emerging as promising diagnostic biomarkers for early cancer detection. Accurate miRNA detection, however, remains challenging because they are small and highly homologous. Rec...

Rho is a general transcription termination factor in bacteria, but many aspects of its mechanism of action are unclear. Diverse models have been proposed for the initial interaction between the RNA polymerase (RNAP) and Rho (catch-up and stand-by pre-terminational models); for the terminational release of the RNA transcript (RNA shearing, RNAP hype...

G-quadruplexes (GQ) and R-loops are non-canonical nucleic acid structures related to gene regulation and genome instability that can be formed during transcription; however, their formation mechanisms remain elusive. To address this question, we developed a single-molecule fluorescence technique to monitor the formation of G-quadruplex and R-loop s...

Chromodomain-Helicase DNA binding protein 7 (CHD7) is an ATP dependent chromatin remodeler involved in maintaining open chromatin structure. Mutations of CHD7 gene causes multiple developmental disorders, notably CHARGE syndrome. However, there is not much known about the molecular mechanism by which CHD7 remodels nucleosomes. Here, we performed bi...

Single-molecule FRET (smFRET) has become a mainstream technique for studying biomolecular structural dynamics. The rapid and wide adoption of smFRET experiments by an ever-increasing number of groups has generated significant progress in sample preparation, measurement procedures, data analysis, algorithms and documentation. Several labs that emplo...

Two different molecular mechanisms, sliding and hopping, are employed by DNA-binding proteins for their one-dimensional facilitated diffusion on nonspecific DNA regions until reaching their specific target sequences. While it has been controversial whether RNA polymerases (RNAPs) use one-dimensional diffusion in targeting their promoters for transc...

Chromodomain-helicase-DNA-binding protein 1 (CHD1) remodels chromatin by translocating nucleosomes along DNA, but its mechanism remains poorly understood. We use single-molecule fluorescence experiments to clarify the mechanism by which yeast CHD1 (Chd1p) remodels nucleosomes. We find that binding of ATP to Chd1p induces transient unwrapping of the...

MicroRNAs (miRNAs) are short (19–24 nt) non-coding RNAs that suppress the expression of protein coding genes at the post-transcriptional level. Differential expression profiles of miRNAs across a range of diseases have emerged as powerful biomarkers, making a reliable yet rapid profiling technique for miRNAs potentially essential in clinics. Here,...

Chromodomain-Helicase DNA binding protein 7 (CHD7) is an ATP dependent chromatin remodeler involved in maintaining open chromatin structure. Mutations of CHD7 gene causes multiple developmental disorders, notably CHARGE syndrome. However, there is not much known about the molecular mechanism by which CHD7 remodels nucleosomes. Here, we performed in...

G-quadruplex (GQ) is formed at various regions of DNA, including telomeres of chromosomes and regulatory regions of oncogenes. Since GQ is important in both gene regulation and genome instability, the biological and medical implications of this abnormal DNA structure have been intensively studied. Its formation mechanisms, however, are not clearly...

Despite extensive studies on transcription mechanisms, it is unknown how termination complexes are disassembled, especially in what order the essential components dissociate. Our single-molecule fluorescence study unveils that RNA transcript release precedes RNA polymerase (RNAP) dissociation from the DNA template much more often than their concurr...

Cotranscriptional RNA folding plays important roles in gene regulation steps such as splicing, transcription termination, and translation initiation. Progression of our understanding of cotranscriptional RNA folding mechanisms is still retarded by the lacking of experimental tools to study the kinetics of cotranscriptional RNA folding properly. In...

Maintaining stability of replication forks is important for genomic integrity. However, it is not clear how replisome proteins contribute to fork stability under replication stress. Here, we report that ATAD5, a PCNA unloader, plays multiple functions at stalled forks including promoting its restart. ATAD5 depletion increases genomic instability up...

(Abstract) Despite extensive studies on transcription mechanisms, it is unknown how termination complexes are disassembled, especially in what order the components dissociate. Our single-molecule fluorescence study unveils that RNA transcript release precedes RNA polymerase (RNAP) dissociation from DNA template in bacterial intrinsic termination of...

DOT1L is a histone H3 Lys79 methyltransferase whose activity is stimulated by histone H2B Lys120 ubiquitination, suggesting cross-talk between histone H3 methylation and H2B ubiquitination. Here, we present cryo-EM structures of DOT1L complexes with unmodified or H2B ubiquitinated nucleosomes, showing that DOT1L recognizes H2B ubiquitin and the H2A...

DOT1L is a histone H3 Lys79 methyltransferase whose activity is stimulated by histone H2B Lys120 ubiquitination, suggesting cross-talk between histone H3 methylation and H2B-ubiquitination. Here, we present cryo-EM structures of DOT1L complex with unmodified and H2B-ubiquitinated nucleosomes, showing that DOT1L recognizes H2B-ubiquitin and the H2A/...

Three-dimensional (3D) reconstruction of thick samples using superresolution fluorescence microscopy remains challenging due to high level of background noise and fast photobleaching of fluorescence probes. We develop superresolution fluorescence microscopy that can reconstruct 3D structures of thick samples with both high localization accuracy and...

Recent development of FRET-PAINT microscopy significantly improved the imaging speed of DNA-PAINT, the previously reported super-resolution fluorescence microscopy with no photobleaching problem. Here we try to achieve the ultimate speed limit of FRET-PAINT by optimizing the camera speed, dissociation rate of DNA probes, and bleed-through of the do...

Recent development of FRET-PAINT microscopy significantly improved the imaging speed of DNA-PAINT, the previously reported super-resolution fluorescence microscopy with no photobleaching problem. Here we try to achieve the ultimate speed limit of FRET-PAINT by optimizing the camera speed, dissociation rate of DNA probes, and bleed-through of the do...

Chromodomain-helicase-DNA-binding protein 1 (CHD1) remodels chromatin by translocating nucleosomes along DNA, but its mechanism remains poorly understood. Here, we employ a single-molecule fluorescence approach to characterize nucleosome remodeling by yeast CHD1 (Chd1p). We show that Chd1p translocates nucleosomes in steps of multiple base pairs pe...

Replication fork reversal is one of the major pathways for reactivating stalled DNA replication. Many enzymes with replication fork reversal activity have DNA-unwinding activity as well, but none of the fork reversal enzymes in the SWI/SNF family shows a separate DNA-unwinding activity, raising the question of how they initiate the remodeling proce...

RPA is known to stimulate the helicase activity of Werner syndrome protein (WRN), but the exact stimulation mechanism is not understood. We use single-molecule FRET and magnetic tweezers to investigate the helicase activity of WRN and its stimulation by RPA. We show that WRN alone is a weak helicase which repetitively unwind just a few tens of base...

Fluorescence resonance energy transfer (FRET) between organic fluorophores is conventionally used to monitor binding/dissociation and conformational change of macromolecules. Here we use FRET between a cyanine dye and single transition metal ions (tmFRET) to monitor the binding/dissociation of transition metal ions to a synthetic polypeptide, and s...

Prokaryotic Argonaute facilitates the target recognition process by the guide strand via a still unknown mechanism. Using single-molecule fluorescence resonance energy transfer and systematic mutagenesis of Thermus thermophilus Argonaute and guide-target base pairing, we study the kinetic roles of various structural features of guide strand in the...

Significance RNA molecules fold into functional structures as they are being synthesized by transcription. Here, we developed a single-molecule fluorescence resonance energy transfer (FRET) assay that permits real-time monitoring of the cotranscriptional folding of RNA molecules. This single-molecule FRET assay makes it possible to carry out contro...

Real-time optical imaging combined with single-molecule manipulation broadens the horizons for acquiring information about the spatiotemporal localization and the mechanical details of target molecules. To obtain an optical signal outside the focal plane without unintended interruption of the force signal in single-molecule optical imaging-force sp...

Argonaute proteins are key components of the microRNA-induced silencing complexes (miRISCs) that mediate the posttranscriptional gene silencing of microRNAs and small interfering RNA (siRNAs). The complex reaction mechanism of miRISC is expected to be characterized by tracing the reaction pathways of miRISC at the single-molecule level in real time...

Protein kinase M zeta (PKMζ), a constitutively active, atypical protein kinase C isoform, maintains a high level of expression in the brain after the induction of learning and long-term potentiation (LTP). Further, its overexpression enhances long-term memory and LTP. Thus, multiple lines of evidence suggest a significant role for persistently elev...

Super-resolution fluorescence microscopy in the current form is hard to be used to image the neural connectivity of thick tissue samples due to problems such as slow imaging speed, severe photobleaching of fluorescent probes, and high background noise. Recently developed DNA-PAINT solved the photobleaching problem, but its imaging speed is extremel...

Riboswitches regulate gene expression by coupling ligand binding to a structural transition of the riboswitch, but the coupling mechanism is still controversial. We addressed this issue by characterizing both the ligand-free state of the Escherichia coli thiamine pyrophosphate (TPP) riboswitch aptamer and its structural transition upon ligand bindi...

Application of BALM (binding activated localization microcopy) was shown to allow facile imaging of amyloid fibrils with a typical diameter of ∼14 nm FWHM. We also observed a twisted ribbon-like substructure of mutant amyloid fibrils and even what appear to be toxic amyloid oligomers with their characteristic morphological features consistent with...

Cockayne syndrome protein B (CSB) belongs to the SWI2/SNF2 ATP-dependent chromatin remodeler family, and CSB is the only ATP-dependent chromatin remodeler essential for transcription-coupled nucleotide excision DNA repair. CSB alone remodels nucleosomes ?10-fold slower than the ACF remodeling complex. Strikingly, NAP1-like histone chaperones intera...

Eukaryotic mRNA decay is tightly modulated by RNA-binding proteins (RBPs) and microRNAs (miRNAs). RBP AU-binding factor 1 (AUF1) has four isoforms resulting from alternative splicing and is critical for miRNA-mediated gene silencing with a distinct preference of target miRNAs. Previously, we have shown that AUF1 facilitates miRNA loading to Argonau...

Over the last 2 decades, single-molecule Forster resonance energy transfer (FRET) has been widely used to address important questions in molecular biology. However, a conventional approach based on a single donor–acceptor pair is not powerful enough to study complex biological systems. To address this challenge, single-molecule multicolor FRET tech...

The reactivation of stalled DNA replication via fork regression invokes Holliday junction formation, branch migration, and the recovery of the replication fork after DNA repair or error-free DNA synthesis. The coordination mechanism for these DNA structural transitions by molecular motors, however, remains unclear. Here we perform single-molecule f...

MicroRNA maturation is initiated by RNase III DROSHA that cleaves the stem loop of primary microRNA. DROSHA functions together with its cofactor DGCR8 in a heterotrimeric complex known as Microprocessor. Here, we report the X-ray structure of DROSHA in complex with the C-terminal helix of DGCR8. We find that DROSHA contains two DGCR8-binding sites,...

In eukaryotes, small RNAs play important roles in both gene regulation and resistance to viral infection. Argonaute proteins have been identified as a key component of the effector complexes of various RNA-silencing pathways, but the mechanistic roles of Argonaute proteins in these pathways are not clearly understood. To address this question, we p...

Eukaryotic gene expression is tightly regulated post-transcriptionally by RNA-binding proteins (RBPs) and microRNAs. The RBP AU-rich-binding factor 1 (AUF1) isoform p37 was found to have high affinity for the microRNA let-7b in vitro (Kd = ∼6 nM) in cells. Ribonucleoprotein immunoprecipitation, in vitro association, and single-molecule-binding anal...

Argonaute is a key enzyme of various RNA silencing pathways. We use single-molecule fluorescence measurements to characterize the reaction mechanisms of the core-RISC (RNA-induced silencing complex) composed of human Argonaute 2 and a small RNA. We found that target binding of core-RISC starts at the seed region, resulting in four distinct reaction...

MicroRNA (miRNA) maturation is initiated by Microprocessor composed of RNase III DROSHA and its cofactor DGCR8, whose fidelity is critical for generation of functional miRNAs. To understand how Microprocessor recognizes pri-miRNAs, we here reconstitute human Microprocessor with purified recombinant proteins. We find that Microprocessor is an ∼364 k...

A library of Trp-containing amphiphilic peptides was synthesized and screened for the ability to bind to pre-miRNA targets. Two members of this family, peptides Ac-WKKLLKWLKKLLKLAG-NH2 (2 b) and Ac-WKKLLKWLKKLLDabLAG-NH2 (4 b) were found to have nanomolar binding affinities to pre-let7a-1. Peptides 2 b and 4 b caused an increase in the in vitro Dic...

Interactions between a protein and a ligand are essential to all biological processes. Binding and dissociation are the two fundamental steps of ligand-protein interactions, and determine the binding affinity. Intrinsic conformational dynamics of proteins have been suggested to play crucial roles in ligand binding and dissociation. Here, we demonst...

Double-stranded ribonucleic acid-activated protein kinase (PKR) downregulates translation as a defense mechanism against viral infection. In fish species, PKZ, a PKR-like protein kinase containing left-handed deoxyribonucleic acid (Z-DNA) binding domains, performs a similar role in the antiviral response. To understand the role of PKZ in Z-DNA reco...

Sub-diffraction limit imaging of SnO2 nanowires (NWs) interfacing cells is studied using stochastic optical reconstruction microscopy. This method allows the estimation of the dimension of the NWs with a rather high resolution compared with previous imaging methods. In addition, since this strategy is compatible with biological environments, the NW...

We developed a hybrid technique combining optical tweezers and single-molecule three-color fluorescence resonance energy transfer (FRET). In demonstrative experiments, we observed the force-sensitive correlated motion of three helical arms of a Holliday junction and identified the independent unfolding/folding dynamics of two DNA hairpins of the sa...

Argonaute (Ago) is the catalytic core of small RNA-based gene regulation. Despite plenty of mechanistic studies on Ago, the dynamical aspects and the mechanistic determinants of target mRNA binding and dissociation of Ago-guide strand remain unclear. Here, by using single-molecule fluorescence resonance energy transfer (FRET) assays and Thermus the...

The manner in which Z-DNA is stabilized at high salt concentrations remains unclear. Here, we systematically examine the Z-DNA-stabilizing capabilities of different salts. The strong correlation between the double-stranded DNA denaturation and B-to-Z transition efficiencies indicates that Z-DNA is mainly stabilized by the Hofmeister effect.

Since its first demonstration about twenty years ago, single-molecule fluorescence resonance energy transfer (FRET) has undergone remarkable technical advances. In this tutorial review, we will discuss two technical advances that increase the information content of the single-molecule FRET measurements: single-molecule multi-color FRET and single-m...

The specificity of proteases for the residues in and length of substrates is key to understanding their regulatory mechanism, but little is known about length selectivity. Crystal structure analyses of the bacterial aminopeptidase PepS, combined with functional and single-molecule FRET assays, have elucidated a molecular basis for length selectivit...

Protein dynamics have been suggested to have a crucial role in biomolecular recognition, but the precise molecular mechanisms remain unclear. Herein, we performed single-molecule fluorescence resonance energy transfer measurements for wild-type maltose-binding protein (MBP) and its variants to demonstrate the interplay of conformational dynamics an...

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Guidelines for submitting commentsPolicy: Comments that contribute to the discussion of the article will be posted within approximately three business days. We do not accept anonymous comments. Please include your email address; the address will not be displayed in the posted comment. Cell Press Editors will screen the comments to ensure that they...

Single-molecule fluorescence imaging has greatly contributed to our understanding of many bio-molecular systems. While reactions occurring in the range of several minutes can be readily studied using conventional single-molecule fluorescence microscopes, data acquisition for longer time scales is hindered by the focal drift of high numerical apertu...

The viral sensor MDA5 distinguishes between cellular and viral dsRNAs by length-dependent recognition in the range of ∼0.5-7 kb. The ability to discriminate dsRNA length at this scale sets MDA5 apart from other dsRNA receptors of the immune system. We have shown previously that MDA5 forms filaments along dsRNA that disassemble upon ATP hydrolysis....

A plausible consequence of rugged energy landscapes of biomolecules is that functionally competent folded states may not be unique, as is generally assumed. Indeed, molecule-to-molecule variations in the dynamics of enzymes and ribozymes under folding conditions have recently been identified in single molecule experiments. However, systematic quant...

A plausible consequence of the rugged folding energy landscapes inherent to biomolecules is that there may be more than one functionally competent folded state. Indeed, molecule-to-molecule variations in the folding dynamics of enzymes and ribozymes have recently been identified in single-molecule experiments, but without systematic quantification...

There is no confocal microscope optimized for single-molecule imaging in live cells and superresolution fluorescence imaging. By combining the swiftness of the line-scanning method and the high sensitivity of wide-field detection, we have developed a, to our knowledge, novel confocal fluorescence microscope with a good optical-sectioning capability...

Topoisomerase II resolves intrinsic topological problems of double-stranded DNA. As part of its essential cellular functions, the enzyme generates DNA breaks, but the regulation of this potentially dangerous process is not well understood. Here we report single-molecule fluorescence experiments that reveal a previously uncharacterized sequence of e...

Single-molecule techniques have been used for only a subset of biological problems because of difficulties in studying proteins that require cofactors or post-translational modifications. Here, we present a new method integrating single-molecule fluorescence microscopy and immunopurification to study protein complexes. We used this method to invest...

Z-DNA, a left-handed isoform of Watson and Crick’s B-DNA, is rarely formed without the help of high salt concentrations or negative supercoiling. However, Z-DNA-binding proteins can efficiently convert specific sequences of the B conformation into the Z conformation in relaxed DNA under physiological salt conditions. As in the case of many other sp...

Hfq is a key regulator involved in multiple aspects of stress tolerance and virulence of bacteria. There has been an intriguing question as to how this RNA chaperone achieves two completely opposite functions—annealing and unwinding—for different RNA substrates. To address this question, we studied the Hfq-mediated interaction of fragments of a non...

Six interfluorophore FRET efficiencies Eij (see scheme) can be determined in real time by a single-molecule four-color FRET technique both in confocal and in total-internal-reflection fluorescence microscopy. This technique was used to probe the correlated motion of the four arms of the Holliday junction, and to assess correlation of RecA-mediated...

Transition rates of the Holliday junction at 20 °C with 50 mM MgCl2. The dwell-time histograms of isoI (a), and isoII (b) were made from more than 30 molecules with all three dyes, and fitted to single-exponential functions, and their transition times were obtained as 0.22 s, and 0.24 s, respectively. These numbers are little bit larger than the nu...

The conformational dynamics of the Holliday junction with a different labeling scheme. (a) Dye labeling scheme. (b) Conformational dynamics between two isoforms. (c) Fluorescence intensity time traces of the Holliday junction upon 532-nm excitation (upper graphs), and 633-nm excitation (lower graphs). The experimental condition is the same as in Fi...

Photobleaching time of Alexa 488, Cy5 and Cy7. (a) Typical intensity time traces of the Holliday junction labeled with Alexa488 and Cy3. (b) Histogram of Alexa488 photobleaching time. The histogram was fitted to a single-exponential curve, and 43 s of photobleaching time was obtained. (c), and (e) Representative intensity time traces of the Hollida...

Full understanding of complex biological interactions frequently requires multi-color detection capability in doing single-molecule fluorescence resonance energy transfer (FRET) experiments. Existing single-molecule three-color FRET techniques, however, suffer from severe photobleaching of Alexa 488, or its alternative dyes, and have been limitedly...

Which comes first, the deformation or the binding?High specificity of protein-DNA interaction is often related with specific deformation of the binding site. B-Z transition is the most dramatic structural change induced by protein-DNA interaction, where some segment of DNA abruptly changes from the right-handed B-DNA to the left-handed Z-DNA by the...

Single-molecule three-color FRET is powerful in studying complex molecular interactions. Existing techniques, however, have been limitedly used for immobilized molecules, and never realized in TIRF microscopy. The main reason of this limited utilization is poor photostability of FRET probes selected for single-molecule three-color FRET.In this work...

Single-molecule fluorescence resonance energy transfer (smFRET) is one of the most general and adaptable single-molecule techniques. Despite the explosive growth in the application of smFRET to answer biological questions in the last decade, the technique has been practiced mostly by biophysicists. We provide a practical guide to using smFRET, focu...

Despite the recent advances in single-molecule manipulation techniques, purely mechanical approaches cannot detect subtle conformational changes in the biologically important regime of weak forces. We developed a hybrid scheme combining force and fluorescence that allowed us to examine the effect of subpiconewton forces on the nanometer scale motio...