University of California, Santa Cruz

Department of Chemistry and Biochemistry



Structural Basis of Template Boundary Definition in Tetrahymena Telomerase
Jansson L.I., Akiyama B.M., Ooms A., Lu C., Rubin S.M., Stone M.D. Nat Struct Mol Bio. 2015.

Telomerase is required to maintain repetitive G-rich telomeric DNA sequences at chromosome ends. To do so, the telomerase reverse transcriptase (TERT) subunit reiteratively uses a small region of the integral telomerase RNA (TER) as a template. An essential feature of telomerase catalysis is the strict definition of the template boundary to determine the precise TER nucleotides to be reverse transcribed by TERT. We report the 3A crystal structure of the Tetrahymena TERT RNA-binding domain (tTRBD) bound to the template boundary element (TBE) of TER. tTRBD is wedged into the base of the TBE RNA stem-loop, and each of the flanking RNA strands wraps around opposite sides of the protein domain. The structure illustrates how the tTRBD establishes the template boundary by positioning the TBE at the correct distance from the TERT active site to prohibit copying of non-template nucleotides. 

The telomerase essential N-terminal domain promotes DNA synthesis by stabilizing short RNA-DNA hybrids.
Akiyama BM, Parks JW, Stone MD. Nucleic Acids Res. 2015

Telomerase is an enzyme that adds repetitive DNA sequences to the ends of chromosomes and consists of two main subunits: the telomerase reverse transcriptase (TERT) protein and an associated telomerase RNA (TER). The telomerase essential N-terminal (TEN) domain is a conserved region of TERT proposed to mediate DNA substrate interactions. Here, we have employed single molecule telomerase binding assays to investigate the function of the TEN domain. Our results reveal telomeric DNA substrates bound to telomerase exhibit a dynamic equilibrium between two states: a docked conformation and an alternative conformation. The relative stabilities of the docked and alternative states correlate with the number of basepairs that can be formed between the DNA substrate and the RNA template, with more basepairing favoring the docked state. The docked state is further buttressed by the TEN domain and mutations within the TEN domain substantially alter the DNA substrate structural equilibrium. We propose a model in which the TEN domain stabilizes short RNA-DNA duplexes in the active site of the enzyme, promoting the docked state to augment telomerase processivity.


Coordinated DNA dynamics during the human telomerase catalytic cycle
Parks J.W. and Stone M.D. Nat Commun. 2014.

The human telomerase reverse transcriptase (hTERT) utilizes a template within the integral RNA subunit (hTR) to direct extension of telomeres. Telomerase exhibits repeat addition processivity (RAP) and must therefore translocate the nascent DNA product into a new RNA:DNA hybrid register to prime each round of telomere repeat synthesis. Here, we use single-molecule FRET and nuclease protection assays to monitor telomere DNA structure and dynamics during the telomerase catalytic cycle. DNA translocation during RAP proceeds through a previously uncharacterized kinetic substep during which the 3' end of the DNA substrate base pairs downstream within the hTR template. The rate constant for DNA primer realignment reveals this step is not rate limiting for RAP, suggesting a second slow conformational change repositions the RNA:DNA hybrid into the telomerase active site and drives the extrusion of the 5' end of the DNA primer out of the enzyme complex.

Kinetic partitioning modulates human telomere DNA G-quadruplex structural polymorphism
Long, X. and Stone M.D. PLoS ONE 2014.

Telomeres are specialized chromatin structures found at the end of chromosomes and are crucial to the maintenance of eukaryotic genome stability. Human telomere DNA is comprised of the repeating sequence (T2AG3)n, which is predominantly double-stranded but terminates with a 3' single-stranded tail. The guanine-rich tail can fold into secondary structures known as a G-quadruplexes (GQs) that may exist as a polymorphic mixture of anti-parallel, parallel, and several hybrid topological isomers. Using single-molecule Förster resonance energy transfer (smFRET), we have reconstructed distributions of telomere DNA GQ conformations generated by an in situ refolding protocol commonly employed in single-molecule studies of GQ structure, or using a slow cooling DNA annealing protocol typically used in the preparation of GQ samples for ensemble biophysical analyses. We find the choice of GQ folding protocol has a marked impact on the observed distributions of DNA conformations under otherwise identical buffer conditions. A detailed analysis of the kinetics of GQ folding over timescales ranging from minutes to hours revealed the distribution of GQ structures generated by in situ refolding gradually equilibrates to resemble the distribution generated by the slow cooling DNA annealing protocol. Interestingly, conditions of low ionic strength, which promote transient GQ unfolding, permit the fraction of folded DNA molecules to partition into a distribution that more closely approximates the thermodynamic folding equilibrium. Our results are consistent with a model in which kinetic partitioning occurs during in situ folding at room temperature in the presence of K(+) ions, producing a long-lived non-equilibrium distribution of GQ structures in which the parallel conformation predominates on the timescale of minutes. These results suggest that telomere DNA GQ folding kinetics, and not just thermodynamic stability, likely contributes to the physiological ensemble GQ structures.


A conserved motif in Tetrahymena thermophila telomerase reverse transcriptase is proximal to the RNA template and is essential for boundary definition
Akiyama B.M., Gomez A., Stone M.D. JBC. 2013.

The ends of linear chromosomes are extended by telomerase, a ribonucleoprotein complex minimally consisting of a protein subunit called telomerase reverse transcriptase (TERT) and the telomerase RNA (TER). TERT functions by reverse transcribing a short template region of TER into telomeric DNA. Proper assembly of TERT and TER is essential for telomerase activity; however, a detailed understanding of how TERT interacts with TER is lacking. Previous studies have identified an RNA binding domain (RBD) within TERT, which includes three evolutionarily conserved sequence motifs: CP2, CP, and T. Here, we used site-directed hydroxyl radical probing to directly identify sites of interaction between the TERT RBD and TER, revealing that the CP2 motif is in close proximity to a conserved region of TER known as the template boundary element (TBE). Gel shift assays on CP2 mutants confirmed that the CP2 motif is an RNA binding determinant. Our results explain previous work that established that mutations to the CP2 motif of TERT and to the TBE of TER both permit misincorporation of nucleotides into the growing DNA strand beyond the canonical template. Taken together, these results suggest a model in which the CP2 motif binds the TBE to strictly define which TER nucleotides can be reverse transcribed.

Mechanical unfolding of human telomere G-quadruplex DNA probed by integrated fluorescence and magnetic tweezers spectroscopy
Long, X., Parks, J.W., Bagshaw, C., Stone M.D. NAR. 2013.

Single-molecule techniques facilitate analysis of mechanical transitions within nucleic acids and proteins. Here, we describe an integrated fluorescence and magnetic tweezers instrument that permits detection of nanometer-scale DNA structural rearrangements together with the application of a wide range of stretching forces to individual DNA molecules. We have analyzed the force-dependent equilibrium and rate constants for telomere DNA G-quadruplex (GQ) folding and unfolding, and have determined the location of the transition state barrier along the well-defined DNA-stretching reaction coordinate. Our results reveal the mechanical unfolding pathway of the telomere DNA GQ is characterized by a short distance (<1 nm) to the transition state for the unfolding reaction. This mechanical unfolding response reflects a critical contribution of long-range interactions to the global stability of the GQ fold, and suggests that telomere-associated proteins need only disrupt a few base pairs to destabilize GQ structures. Comparison of the GQ unfolded state with a single-stranded polyT DNA revealed the unfolded GQ exhibits a compacted non-native conformation reminiscent of the protein molten globule. We expect the capacity to interrogate macromolecular structural transitions with high spatial resolution under conditions of low forces will have broad application in analyses of nucleic acid and protein folding.


Single-Molecule FRET Reveals the Folding Dynamics of the Human Telomerase RNA Pseudoknot Domain
Hengesbach, M., Kim, N., Feigon, J., Stone, M.D. Angewandte Chemie. 2012.

Single-molecule FRET was used to analyze the folding of human telomerase RNA and allowed discrimination between the correctly folded and misfolded structures. Analyzing the effects of various mutations showed that base pairing, triplex formation, and Mg2+ ion binding act synergistically in the folding of the pseudoknot structure in a functional hTR construct.

The C-terminal domain of Tetrahymena thermophila telomerase holoenzyme protein p65 induces multiple structural changes in telomerase RNA.
Akiyama B.M., Loper J., Najarro K., Stone M.D. RNA. 2012.

The unique cellular activity of the telomerase reverse transcriptase ribonucleoprotein (RNP) requires proper assembly of protein and RNA components into a functional complex. In the ciliate model organism Tetrahymena thermophila, the La-domain protein p65 is required for in vivo assembly of telomerase. Single-molecule and biochemical studies have shown that p65 promotes efficient RNA assembly with the telomerase reverse transcriptase (TERT) protein, in part by inducing a bend in the conserved stem IV region of telomerase RNA (TER). The domain architecture of p65 consists of an N-terminal domain, a La-RRM motif, and a C-terminal domain (CTD). Using single-molecule Förster resonance energy transfer (smFRET), we demonstrate the p65(CTD) is necessary for the RNA remodeling activity of the protein and is sufficient to induce a substantial conformational change in stem IV of TER. Moreover, nuclease protection assays directly map the site of p65(CTD) interaction to stem IV and reveal that, in addition to bending stem IV, p65 binding reorganizes nucleotides that comprise the low-affinity TERT binding site within stem-loop IV.


The RNA accordion model for template positioning by telomerase RNA during telomeric DNA synthesis.
Berman, A. J., Akiyama, B. M., Stone, M. D., Cech, T. R. Nature: Struct. & Molec. Biol. 2011.

Telomerase is a ribonucleoprotein (RNP) enzyme that maintains the ends of linear eukaryotic chromosomes and whose activation is a hallmark of 90% of all cancers. This RNP minimally contains a reverse transcriptase protein subunit (TERT) that catalyzes telomeric DNA synthesis and an RNA subunit (TER) that has templating, architectural and protein-scaffolding roles. Telomerase is unique among polymerases in that it synthesizes multiple copies of the template on the 3′ end of a primer following a single binding event, a process known as repeat addition processivity (RAP). Using biochemical assays and single-molecule Förster resonance energy transfer (smFRET) experiments onTetrahymena thermophila telomerase, we now directly demonstrate that TER contributes to template positioning within the active site and to the template translocation required for RAP. We propose that the single-stranded RNA elements flanking the template act as a molecular accordion, undergoing reciprocal extension and compaction during telomerase translocation.

Single-molecule analysis of telomerase structure and function.
Hengesbach M, Akiyama BM, Stone M.D. Curr Opin Chem Biol. 2011.

The telomerase ribonucleoprotein is a specialized reverse transcriptase required to maintain protective chromosome end-capping structures called telomeres. In most cells, telomerase is not active and the natural shortening of telomeres with each round of DNA replication ultimately triggers cell growth arrest. In contrast, the presence of telomerase confers a high level of renewal capacity upon rapidly dividing cells. Telomerase is aberrantly activated in 90% of human cancers and thus represents an important target for anticancer therapeutics. However, the naturally low abundance of telomerase has hampered efforts to obtain high-resolution models for telomerase structure and function. To circumvent these challenges, single-molecule techniques have recently been employed to investigate telomerase assembly, structure, and catalysis.


Dynamics of nucleosome remodelling by individual ACF complexes.
Blosser, T.R., Yang, J.R., Stone, M.D., Narlikar, G.J., Zhuang, X. Nature. 2009.

The ATP-dependent chromatin assembly and remodelling factor (ACF) functions to generate regularly spaced nucleosomes, which are required for heritable gene silencing. The mechanism by which ACF mobilizes nucleosomes remains poorly understood. Here we report a single-molecule FRET study that monitors the remodelling of individual nucleosomes by ACF in real time, revealing previously unknown remodelling intermediates and dynamics. In the presence of ACF and ATP, the nucleosomes exhibit gradual translocation along DNA interrupted by well-defined kinetic pauses that occurred after approximately seven or three to four base pairs of translocation. The binding of ACF, translocation of DNA and exiting of translocation pauses are all ATP-dependent, revealing three distinct functional roles of ATP during remodelling. At equilibrium, a continuously bound ACF complex can move the nucleosome back-and-forth many times before dissociation, indicating that ACF is a highly processive and bidirectional nucleosome translocase.

Assembly of Complex RNAs by Splinted Ligation
Akiyama, B.M., Stone, M.D. (2009). Methods in Enzymology. Vol. 469, 27-46.

Mechanistic studies of RNA enzymes (ribozymes) and ribonucleoprotein (RNP) complexes such as the ribosome and telomerase, often seek to characterize RNA structural features, either dynamic or static, and relate these properties to specific catalytic functions. Many experimental techniques that probe RNA structure–function relationships rely upon site-specific incorporation of chemically modified ribonucleotides into the RNA of interest, often in the form of chemical cross-linkers to probe for sites of protein–RNA interaction or small organic fluorophores to measure dynamic structural properties of RNAs. The ability to arbitrarily modify any RNA molecule has been greatly enabled by modern RNA synthesis techniques; however, there remains a practical size limitation (~ 70 bases). Consequently, experimental approaches involving specific chemical modifications of larger RNAs require the use of RNA ligation methods. The aim of this chapter is to describe a general approach for covalently joining multiple site-specifically modified RNA fragments, drawing from our fluorescence-based structural studies of telomerase RNA as an example.

A single-molecule assay for telomerase structure-function analysis.
Wu, J.Y., Stone, M.D., Zhuang, X. (2009). Nucleic Acids Research Advance Access published on November 17, 2009

The activity of the telomerase ribonucleoprotein enzyme is essential for the maintenance of genome stability and normal cell development. Despite the biomedical importance of telomerase activity, detailed structural models for the enzyme remain to be established. Here we report a singlemolecule assay for direct structural analysis of catalytically active telomerase enzymes. In this assay, oligonucleotide hybridization was used to probe the primer-extension activity of individual telomerase enzymes with single nucleotide sensitivity, allowing precise discrimination between inactive, active and processive enzyme binding events. FRET signals from enzyme molecules during the active and processive binding events were then used to determine the global organization of telomerase RNA within catalytically active holoenzymes. Using this assay, we have identified an active conformation of telomerase among a heterogeneous population of enzymes with distinct structures.


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Stepwise protein-mediated RNA folding directs assembly of telomerase ribonucleoprotein.
Stone, M.D., Mihalusova, M., O'Connor C, M., Prathapam, R., Collins, K., and Zhuang, X. (2007). Nature 446, 458

Telomerase is an essential cellular ribonucleoprotein (RNP) that solves the end replication problem and maintains chromosome stability by adding telomeric DNA to the termini of linear chromosomes. Genetic mutations that abrogate the normal assembly of telomerase RNP cause human disease. It is therefore of fundamental and medical importance to decipher cellular strategies for telomerase biogenesis, which will require new insights into how specific interactions occur in a precise order along the RNP assembly pathway. Here we use a single-molecule approach to dissect the individual assembly steps of telomerase. Direct observation of complex formation in real time revealed two sequential steps of protein-induced RNA folding, establishing a hierarchical RNP assembly mechanism: interaction with the telomerase holoenzyme protein p65 induces structural rearrangement of telomerase RNA, which in turn directs the binding of the telomerase reverse transcriptase to form the functional ternary complex. This hierarchical assembly process is facilitated by an evolutionarily conserved structural motif within the RNA. These results identify the RNA folding pathway during telomerase biogenesis and define the mechanism of action for an essential telomerase holoenzyme protein.

E. coli DNA gyrase uses the energy of ATP hydrolysis to introduce essential negative supercoils into the genome, thereby working against the mechanical stresses that accumulate in supercoiled DNA. Using a magnetic-tweezers assay, we demonstrate that small changes in force and torque can switch gyrase among three distinct modes of activity. Under low mechanical stress, gyrase introduces negative supercoils by a mechanism that depends on DNA wrapping. Elevated tension or positive torque suppresses DNA wrapping, revealing a second mode of activity that resembles the activity of topoisomerase IV. This 'distal T-segment capture' mode results in active relaxation of left-handed braids and positive supercoils. A third mode is responsible for the ATP-independent relaxation of negative supercoils. We present a branched kinetic model that quantitatively accounts for all of our single-molecule results and agrees with existing biochemical data.

Mechanochemical analysis of DNA gyrase using rotor bead tracking.
Gore, J., Bryant, Z., Stone, M.D., Nollmann, M., Cozzarelli, N. R., and Bustamante, C. (2006). Nature 439, 100-104.

DNA gyrase is a molecular machine that uses the energy of ATP hydrolysis to introduce essential negative supercoils into DNA. The directionality of supercoiling is ensured by chiral wrapping of the DNA around a specialized domain of the enzyme before strand passage. Here we observe the activity of gyrase in real time by tracking the rotation of a submicrometre bead attached to theside of a stretched DNA molecule. In the presence of gyrase and ATP, we observe bursts of rotation corresponding to the processive, stepwise introduction of negative supercoils in strict multiples of two. Changes in DNA tension have no detectable effect on supercoiling velocity, but the enzyme becomes markedly less processive as tension is increased over a range of only a few tenths of piconewtons. This behaviour is quantitatively explained by a simple mechanochemical model in which processivity depends on a kinetic competition between dissociation and rapid, tension-sensitive DNA wrapping. In a high-resolution variant of our assay, we directly detect rotational pauses corresponding to two kinetic substeps: an ATP-independent step at the end of the reaction cycle, and an ATP-binding step in the middle of the cycle, subsequent to DNA wrapping.


Nature Abstract

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Structural transitions and elasticity from torque measurements on DNA.
Bryant, Z., Stone, M.D., Gore, J., Smith, S. B., Cozzarelli, N. R., and Bustamante, C. (2003). Nature 424, 338-341.

Knowledge of the elastic properties of DNA is required to understand the structural dynamics of cellular processes such as replication and transcription. Measurements of force and extension on single molecules of DNA have allowed direct determination of the molecule's mechanical properties, provided rigorous tests of theories of polymer elasticity, revealed unforeseen structural transitions induced by mechanical stresses, and established an experimental and conceptual framework for mechanical assays of enzymes that act on DNA. However, a complete description of DNA mechanics must also consider the effects of torque, a quantity that has hitherto not been directly measured in micromanipulation experiments. We have measured torque as a function of twist for stretched DNA—torsional strain in over- or underwound molecules was used to power the rotation of submicrometre beads serving as calibrated loads. Here we report tests of the linearity of DNA's twist elasticity, direct measurements of the torsional modulus (finding a value approx40% higher than generally accepted), characterization of torque-induced structural transitions, and the establishment of a framework for future assays of torque and twist generation by DNA-dependent enzymes. We also show that cooperative structural transitions in DNA can be exploited to construct constant-torque wind-up motors and force–torque converters.

Nature Abstract

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Chirality sensing by Escherichia coli topoisomerase IV and the mechanism of type II topoisomerases.
Stone, M.D., Bryant, Z., Crisona, N. J., Smith, S. B., Vologodskii, A., Bustamante, C., and Cozzarelli, N. R. (2003). Proc Natl Acad Sci U S A 100

Escherichia coli topoisomerase (Topo) IV is an essential type II Topo that removes DNA entanglements created during DNA replication. Topo IV relaxes (+) supercoils much faster than (–) supercoils, promoting replication while sparing the essential (–) supercoils. Here, we investigate the mechanism underlying this chiral preference. Using DNA binding assays and a single-molecule DNA braiding system, we show that Topo IV recognizes the chiral crossings imposedby the left-handed superhelix of a (+) supercoiled DNA, rather than global topology, twist deformation, or local writhe. Monte Carlo simulations of braid, supercoil, and catenane configurations demonstrate how a preference for a single-crossing geometry during strand passage can allow Topo IV to perform its physiological functions. Single-enzyme braid relaxation experiments also provide a direct measure of the processivity of the enzyme and offer insight into its mechanochemical cycle. 

PNAS Abstract

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