Doctor of Philosophy, Stanford University, Applied Physics (2015)
Master of Science, Stanford University, Applied Physics (2010)
Bachelor of Arts, Amherst College, Physics, Chemistry, Biology (2008)
Howard Chang, Postdoctoral Faculty Sponsor
RNA structure maps across mammalian cellular compartments.
Nature structural & molecular biology
RNA structure is intimately connected to each step of gene expression. Recent advances have enabled transcriptome-wide maps of RNA secondary structure, called 'RNA structuromes'. However, previous whole-cell analyses lacked the resolution to unravel the landscape and also the regulatory mechanisms of RNA structural changes across subcellular compartments. Here we reveal the RNA structuromes in three compartments, chromatin, nucleoplasm and cytoplasm, in human and mouse cells. The cytotopic structuromes substantially expand RNA structural information and enable detailed investigation of the central role of RNA structure in linking transcription, translation and RNA decay. We develop a resource with which to visualize the interplay of RNA-protein interactions, RNA modifications and RNA structure and predict both direct and indirect reader proteins of RNA modifications. We also validate a novel role for the RNA-binding protein LIN28A as an N6-methyladenosine modification 'anti-reader'. Our results highlight the dynamic nature of RNA structures and its functional importance in gene regulation.
View details for PubMedID 30886404
Atlas of Subcellular RNA Localization Revealed by APEX-Seq.
We introduce APEX-seq, a method for RNA sequencing based on direct proximity labeling of RNA using the peroxidase enzyme APEX2. APEX-seq in nine distinct subcellular locales produced a nanometer-resolution spatial map of the human transcriptome as a resource, revealing extensive patterns of localization for diverse RNA classes and transcript isoforms. We uncover a radial organization of the nuclear transcriptome, which is gated at the inner surface of the nuclear pore for cytoplasmic export of processed transcripts. We identify two distinct pathways of messenger RNA localization to mitochondria, each associated with specific sets of transcripts for building complementary macromolecular machines within the organelle. APEX-seq should be widely applicable to many systems, enabling comprehensive investigations of the spatial transcriptome.
View details for DOI 10.1016/j.cell.2019.05.027
View details for PubMedID 31230715
Real-time observation of polymerase-promoter contact remodeling during transcription initiation.
2017; 8 (1): 1178
Critical contacts made between the RNA polymerase (RNAP) holoenzyme and promoter DNA modulate not only the strength of promoter binding, but also the frequency and timing of promoter escape during transcription. Here, we describe a single-molecule optical-trapping assay to study transcription initiation in real time, and use it to map contacts formed between σ70 RNAP holoenzyme from E. coli and the T7A1 promoter, as well as to observe the remodeling of those contacts during the transition to the elongation phase. The strong binding contacts identified in certain well-known promoter regions, such as the -35 and -10 elements, do not necessarily coincide with the most highly conserved portions of these sequences. Strong contacts formed within the spacer region (-10 to -35) and with the -10 element are essential for initiation and promoter escape, respectively, and the holoenzyme releases contacts with promoter elements in a non-sequential fashion during escape.
View details for PubMedID 29079833
View details for PubMedCentralID PMC5660091
Direct observation of processive exoribonuclease motion using optical tweezers
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2015; 112 (49): 15101-15106
Bacterial RNases catalyze the turnover of RNA and are essential for gene expression and quality surveillance of transcripts. In Escherichia coli, the exoribonucleases RNase R and polynucleotide phosphorylase (PNPase) play critical roles in degrading RNA. Here, we developed an optical-trapping assay to monitor the translocation of individual enzymes along RNA-based substrates. Single-molecule records of motion reveal RNase R to be highly processive: one molecule can unwind over 500 bp of a structured substrate. However, enzyme progress is interrupted by pausing and stalling events that can slow degradation in a sequence-dependent fashion. We found that the distance traveled by PNPase through structured RNA is dependent on the A+U content of the substrate and that removal of its KH and S1 RNA-binding domains can reduce enzyme processivity without affecting the velocity. By a periodogram analysis of single-molecule records, we establish that PNPase takes discrete steps of six or seven nucleotides. These findings, in combination with previous structural and biochemical data, support an asymmetric inchworm mechanism for PNPase motion. The assay developed here for RNase R and PNPase is well suited to studies of other exonucleases and helicases.
View details for DOI 10.1073/pnas.1514028112
View details for Web of Science ID 000365989800041
View details for PubMedID 26598710
Real-time observation of the initiation of RNA polymerase II transcription.
2015; 525 (7568): 274-277
Biochemical and structural studies have shown that the initiation of RNA polymerase II transcription proceeds in the following stages: assembly of the polymerase with general transcription factors and promoter DNA in a 'closed' preinitiation complex (PIC); unwinding of about 15 base pairs of the promoter DNA to form an 'open' complex; scanning downstream to a transcription start site; synthesis of a short transcript, thought to be about 10 nucleotides long; and promoter escape. Here we have assembled a 32-protein, 1.5-megadalton PIC derived from Saccharomyces cerevisiae, and observe subsequent initiation processes in real time with optical tweezers. Contrary to expectation, scanning driven by the transcription factor IIH involved the rapid opening of an extended transcription bubble, averaging 85 base pairs, accompanied by the synthesis of a transcript up to the entire length of the extended bubble, followed by promoter escape. PICs that failed to achieve promoter escape nevertheless formed open complexes and extended bubbles, which collapsed back to closed or open complexes, resulting in repeated futile scanning.
View details for DOI 10.1038/nature14882
View details for PubMedID 26331540
lncRNA Structure: Message to the Heart.
2016; 64 (1): 1-2
In this issue, Xue et al. (2016) describe the secondary structure of the heart-specific long non-coding RNA Braveheart, leading to the discovery of a short, asymmetric G-rich loop that controls cardiac lineage commitment by interacting with the transcription factor CNBP.
View details for DOI 10.1016/j.molcel.2016.09.030
View details for PubMedID 27716479
Binding and Translocation of Termination Factor Rho Studied at the Single-Molecule Level
JOURNAL OF MOLECULAR BIOLOGY
2012; 423 (5): 664-676
Rho termination factor is an essential hexameric helicase responsible for terminating 20-50% of all mRNA synthesis in Escherichia coli. We used single-molecule force spectroscopy to investigate Rho-RNA binding interactions at the Rho utilization site of the λtR1 terminator. Our results are consistent with Rho complexes adopting two states: one that binds 57 ± 2nt of RNA across all six of the Rho primary binding sites, and another that binds 85 ± 2nt at the six primary sites plus a single secondary site situated at the center of the hexamer. The single-molecule data serve to establish that Rho translocates 5'→3' toward RNA polymerase (RNAP) by a tethered-tracking mechanism, looping out the intervening RNA between the Rho utilization site and RNAP. These findings lead to a general model for Rho binding and translocation and establish a novel experimental approach that should facilitate additional single-molecule studies of RNA-binding proteins.
View details for DOI 10.1016/j.jmb.2012.07.027
View details for Web of Science ID 000310666400002
View details for PubMedID 22885804
View details for PubMedCentralID PMC3472157
Optical tweezers study life under tension
2011; 5 (6): 318-321
Optical tweezers have become one of the primary weapons in the arsenal of biophysicists, and have revolutionized the new field of single-molecule biophysics. Today's techniques allow high-resolution experiments on biological macromolecules that were mere pipe dreams only a decade ago.
View details for Web of Science ID 000291089000003
View details for PubMedCentralID PMC3229214
SPECTRAL ENERGY DISTRIBUTIONS OF HIGH-MASS PROTOSTELLAR OBJECTS: EVIDENCE OF HIGH ACCRETION RATES
ASTROPHYSICAL JOURNAL LETTERS
2008; 688 (1): L41-L44
View details for Web of Science ID 000262732700011
Circular differential double diffraction in chiral media
2007; 32 (13): 1836-1838
In an optically active liquid the diffraction angle depends on the circular polarization state of the incident light beam. We report the observation of circular differential diffraction in an isotropic chiral medium, and we demonstrate that double diffraction is an alternate means to determine the handedness (enantiomeric excess) of a solution.
View details for Web of Science ID 000248348300024
View details for PubMedID 17603586
Glucose-specific poly(allylamine) hydrogels- A reassessment
BIOORGANIC & MEDICINAL CHEMISTRY LETTERS
2007; 17 (1): 235-238
Polymer hydrogels synthesized by crosslinking poly(allylamine hydrochloride) with (+/-)-epichlorohydrin in the presence of d-glucose-6-phosphate monobarium salt do not show imprinting on the molecular level. A series of hydrogels was prepared using the following five templates: d-glucose-6-phosphate monobarium salt, d-glucose, l-glucose, barium hydrogen phosphate (BaHPO(4)), and d-gluconamide; a hydrogel was also prepared in the absence of a template. For all six hydrogels, batch binding studies were conducted with d-glucose, l-glucose, d-fructose, and d-gluconamide. The extent of analyte sugar binding was determined using (1)H NMR. Each hydrogel shows approximately the same relative binding affinity for the different sugar derivatives, and none displays selectivity for either glucose enantiomer. The results of the binding studies correlate with the octanol-water partition coefficients of the sugars, indicative that differential solubilities in the bulk polymer account for the binding affinities observed. Thus, in contrast to templated hydrogels prepared using methacrylate- or acrylamide-based reagents, true imprinting does not occur in this novel, crosslinked-poly(allylamine hydrochloride) system.
View details for DOI 10.1016/j.bmcl.2006.09.054
View details for Web of Science ID 000243630500046
View details for PubMedID 17035016