Achillefs N. Kapanidis

Initial transcription by RNA polymerase proceeds through a DNA-scrunching mechanism.

Using fluorescence resonance energy transfer to monitor distances within single molecules of abortively initiating transcription initiation complexes, we show that initial transcription proceeds through a “scrunching” mechanism, in which RNA polymerase (RNAP) remains fixed on promoter DNA and pulls downstream DNA into itself and past its active center. We show further that putative alternative mechanisms for RNAP active-center translocation in initial transcription, involving “transient excursions” of RNAP relative to DNA or “inchworming” of RNAP relative to DNA, do not occur. The results support a model in which a stressed intermediate, with DNA-unwinding stress and DNA-compaction stress, is formed during initial transcription, and in which accumulated stress is used to drive breakage of interactions between RNAP and promoter DNA and between RNAP and initiation factors during promoter escape.

DAOSTORM: an algorithm for high- density super-resolution microscopy

We first investigated the qualitative performance of each algorithm for images of Alexa Fluor 647–immunolabeled microtubules in fixed COS-7 cells. We recorded data at high imaging density using total internal reflection fluorescence microscopy and direct (d)STORM photoswitching conditions5 (100 ms integration time, ~4,000 photons fluorophore–1 frame–1). We plotted localizations on raw images, illustrating the characteristic performance of each algorithm (Fig. 1a). SA1 only localized isolated molecules, which were fitted with small localization error. SA2 localized a larger fraction of the molecules but yielded large localization errors for overlapping molecules. DAOSTORM outperformed both sparse algorithms, identifying almost all molecules with small localization error. We quantified the performance of each algorithm by analyzing simulations of randomly distributed surface-immobilized fluorophores6. We compared observed localizations to simulated positions, calculating the recall5 and localization error at different imaging densities. Recall is the percentage of simulated fluorophores detected. Localization error is the root-mean-square distance between a localization and the simulated position. We also measured the precision5 and redundancy (Supplementary Methods), which did not vary substantially. DAOSTORM substantially outperformed the sparse algorithms in simulations at high signal-to-noise ratio typical of STORM data (bright organic fluorophores, 5,000 photons molecule–1 frame–1; Fig. 1b-c). SA1 showed poor recall at high density, with imaging density at half-maximum recall, ρHM, of 1.2 molecule μm –2. However, SA1 yielded small localization errors even at high imaging density because most overlapping molecules were rejected. SA2 had better recall performance (ρHM = 3.4 molecules μm –2) but gave large localization errors even at low imaging density (>0.1 molecules μm–2). In contrast, DAOSTORM gave small localization errors similar to the other ‘precise’ algorithm, SA1, together with a sixfold improvement in recall performance (ρHM = 7.5 molecules μm –2). For simulations at low signal-to-noise ratio typical of photoactivated localization microscopy data (fluorescent proteins, 200 photons molecule–1 frame–1; DAOSTORM: an algorithm for highdensity super-resolution microscopy

Reconfigurable, braced, three- dimensional DNA nanostructures

DNA nanotechnology makes use of the exquisite self-recognition of DNA in order to build on a molecular scale. Although static structures may find applications in structural biology and computer science, many applications in nanomedicine and nanorobotics require the additional capacity for controlled three-dimensional movement. DNA architectures can span three dimensions and DNA devices are capable of movement, but active control of well-defined three-dimensional structures has not been achieved. We demonstrate the operation of reconfigurable DNA tetrahedra whose shapes change precisely and reversibly in response to specific molecular signals. Shape changes are confirmed by gel electrophoresis and by bulk and single-molecule Förster resonance energy transfer measurements. DNA tetrahedra are natural building blocks for three-dimensional construction; they may be synthesized rapidly with high yield of a single stereoisomer, and their triangulated architecture conveys structural stability. The introduction of shape-changing structural modules opens new avenues for the manipulation of matter on the nanometre scale.

Conformational transitions in DNA polymerase I revealed by single-molecule FRET

The remarkable fidelity of most DNA polymerases depends on a series of early steps in the reaction pathway which allow the selection of the correct nucleotide substrate, while excluding all incorrect ones, before the enzyme is committed to the chemical step of nucleotide incorporation. The conformational transitions that are involved in these early steps are detectable with a variety of fluorescence assays and include the fingers-closing transition that has been characterized in structural studies. Using DNA polymerase I (Klenow fragment) labeled with both donor and acceptor fluorophores, we have employed single-molecule fluorescence resonance energy transfer to study the polymerase conformational transitions that precede nucleotide addition. Our experiments clearly distinguish the open and closed conformations that predominate in Pol-DNA and Pol-DNA-dNTP complexes, respectively. By contrast, the unliganded polymerase shows a broad distribution of FRET values, indicating a high degree of conformational flexibility in the protein in the absence of its substrates; such flexibility was not anticipated on the basis of the available crystallographic structures. Real-time observation of conformational dynamics showed that most of the unliganded polymerase molecules sample the open and closed conformations in the millisecond timescale. Ternary complexes formed in the presence of mismatched dNTPs or complementary ribonucleotides show unique FRET species, which we suggest are relevant to kinetic checkpoints that discriminate against these incorrect substrates.

Translocation of σ70 with RNA Polymerase during Transcription

Using fluorescence resonance energy transfer, we show that, in the majority of transcription complexes, sigma(70) is not released from RNA polymerase upon transition from initiation to elongation, but, instead, remains associated with RNA polymerase and translocates with RNA polymerase. The results argue against the presumption that there are necessary subunit-composition differences, and corresponding necessary mechanistic differences, in initiation and elongation. The methods of this report should be generalizable to monitor movement of any molecule relative to any nucleic acid.

DNA-Based Biosensors

Compared to advances in enzyme sensors, immunosensors, and microbial biosensors, relatively little work exists on DNA based biosensors. Here we review the DNA based biosensors that rely on nucleic acid hybridization. Major types DNA biosensors--electrochemical, optical, acoustic, and piezoelectric--are introduced and compared. The specificity and response characteristics of DNA biosensors are discussed. Overall, a promising future is foreseen for the DNA based sensor technology.

Fluorescent probes and bioconjugation chemistries for single-molecule fluorescence analysis of biomolecules

Fluorescence-based detection of single biomolecules in solution and at room temperature has opened new avenues for understanding biological mechanisms. Single-molecule fluorescence spectroscopy (SMFS) of biomolecules requires careful selection of fluorophores, sites of incorporation, and labeling chemistries. SMFS-compatible fluorophores should permit extended, uninterrupted observations of fluorescence with high signal-to-noise ratios; more stringent considerations apply for specific methodologies, such as fluorescence resonance energy transfer and fluorescence anisotropy. Strategies for site-specific in vitro labeling of small proteins exploit the reactivity of the amino acid cysteine (Cys), allowing incorporation of one or more fluorophores; labeling of closely spaced Cys residues using bis-functionalized fluorophores allows probing of the orientation of individual protein domains. For in vitro labeling of large proteins, the options include peptide ligation, intein-mediated labeling, puromycin-based l...

Biology, one molecule at a time

Single-molecule techniques have moved from being a fascinating curiosity to a highlight of life science research. The single-molecule approach to biology offers distinct advantages over the conventional approach of taking bulk measurements; this additional information content usually comes at the cost of the additional complexity. Popular single-molecule methods include optical and magnetic tweezers, atomic force microscopy, tethered particle motion and single-molecule fluorescence spectroscopy; the complement of these methods offers a wide range of spatial and temporal capabilities. These approaches have been instrumental in addressing important biological questions in diverse areas such as protein-DNA interactions, protein folding and the function(s) of membrane proteins.

Multiscale Spatial Organization of RNA Polymerase in Escherichia coli

Nucleic acid synthesis is spatially organized in many organisms. In bacteria, however, the spatial distribution of transcription remains obscure, owing largely to the diffraction limit of conventional light microscopy (200-300 nm). Here, we use photoactivated localization microscopy to localize individual molecules of RNA polymerase (RNAP) in Escherichia coli with a spatial resolution of ∼40 nm. In cells growing rapidly in nutrient-rich media, we find that RNAP is organized in 2-8 bands. The band number scaled directly with cell size (and so with the chromosome number), and bands often contained clusters of >70 tightly packed RNAPs (possibly engaged on one long ribosomal RNA operon of 6000 bp) and clusters of such clusters (perhaps reflecting a structure like the eukaryotic nucleolus where many different ribosomal RNA operons are transcribed). In nutrient-poor media, RNAPs were located in only 1-2 bands; within these bands, a disproportionate number of RNAPs were found in clusters containing ∼20-50 RNAPs. Apart from their importance for bacterial transcription, our studies pave the way for molecular-level analysis of several cellular processes at the nanometer scale.

Single-molecule DNA repair in live bacteria

Cellular DNA damage is reversed by balanced repair pathways that avoid accumulation of toxic intermediates. Despite their importance, the organization of DNA repair pathways and the function of repair enzymes in vivo have remained unclear because of the inability to directly observe individual reactions in living cells. Here, we used photoactivation, localization, and tracking in live Escherichia coli to directly visualize single fluorescent labeled DNA polymerase I (Pol) and ligase (Lig) molecules searching for DNA gaps and nicks, performing transient reactions, and releasing their products. Our general approach provides enzymatic rates and copy numbers, substrate-search times, diffusion characteristics, and the spatial distribution of reaction sites, at the single-cell level, all in one measurement. Single repair events last 2.1 s (Pol) and 2.5 s (Lig), respectively. Pol and Lig activities increased fivefold over the basal level within minutes of DNA methylation damage; their rates were limited by upstream base excision repair pathway steps. Pol and Lig spent >80% of their time searching for free substrates, thereby minimizing both the number and lifetime of toxic repair intermediates. We integrated these single-molecule observations to generate a quantitative, systems-level description of a model repair pathway in vivo.