The unstructured C-terminus of the τ subunit of Escherichia coli DNA polymerase III holoenzyme is the site of interaction with the α subunit

The τ subunit of Escherichia coli DNA polymerase III holoenzyme interacts with the α subunit through its C-terminal Domain V, τC16. We show that the extreme C-terminal region of τC16 constitutes the site of interaction with α. The τC16 domain, but not a derivative of it with a C-terminal deletion of seven residues (τC16Δ7), forms an isolable complex with α. Surface plasmon resonance measurements were used to determine the dissociation constant (KD) of the α−τC16 complex to be ∼260 pM. Competition with immobilized τC16 by τC16 derivatives for binding to α gave values of KD of 7 μM for the α−τC16Δ7 complex. Low-level expression of the genes encoding τC16 and τC16▵7, but not τC16Δ11, is lethal to E. coli. Suppression of this lethal phenotype enabled selection of mutations in the 3′ end of the τC16 gene, that led to defects in α binding. The data suggest that the unstructured C-terminus of τ becomes folded into a helix–loop–helix in its complex with α. An N-terminally extended construct, τC24, was found to bind DNA in a salt-sensitive manner while no binding was observed for τC16, suggesting that the processivity switch of the replisome functionally involves Domain IV of τ

Specific binding of a hexanucleotide to HIV-1 reverse transcriptase: a novel class of bioactive molecules

Short oligonucleotides below 8–10 nt in length adopt relatively simple structures. Accordingly, they represent interesting and so far unexplored lead compounds as molecular tools and, potentially, for drug development as a rational improvement of efficacy seem to be less complex than for other classes of longer oligomeric nucleic acid. As a ‘proof of concept’, we describe the highly specific binding of the hexanucleotide UCGUGU (Hex-S3) to human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) as a model target. Ultraviolet (UV) cross-linking studies and competition experiments with primer/template substrates and a RT-directed aptamer suggest site-specific binding of Hex-S3 to the large subunit (p66) of the viral enzyme. The affinity of 5.3 μM is related to hexanucleotide-specific suppression of HIV-1 replication in human cells by up to three orders of magnitude indicating that Hex-S3 exerts specific and biologically relevant activity. Experimental evidence described here further suggests a systematic hexamer array-based search for new tools for molecular biology and novel lead compounds in nucleic acid-based drug development

Single-molecule experiments in biological physics: methods and applications

I review single-molecule experiments (SME) in biological physics. Recent technological developments have provided the tools to design and build scientific instruments of high enough sensitivity and precision to manipulate and visualize individual molecules and measure microscopic forces. Using SME it is possible to: manipulate molecules one at a time and measure distributions describing molecular properties; characterize the kinetics of biomolecular reactions and; detect molecular intermediates. SME provide the additional information about thermodynamics and kinetics of biomolecular processes. This complements information obtained in traditional bulk assays. In SME it is also possible to measure small energies and detect large Brownian deviations in biomolecular reactions, thereby offering new methods and systems to scrutinize the basic foundations of statistical mechanics. This review is written at a very introductory level emphasizing the importance of SME to scientists interested in knowing the common playground of ideas and the interdisciplinary topics accessible by these techniques. The review discusses SME from an experimental perspective, first exposing the most common experimental methodologies and later presenting various molecular systems where such techniques have been applied. I briefly discuss experimental techniques such as atomic-force microscopy (AFM), laser optical tweezers (LOT), magnetic tweezers (MT), biomembrane force probe (BFP) and single-molecule fluorescence (SMF). I then present several applications of SME to the study of nucleic acids (DNA, RNA and DNA condensation), proteins (protein-protein interactions, protein folding and molecular motors). Finally, I discuss applications of SME to the study of the nonequilibrium thermodynamics of small systems and the experimental verification of fluctuation theorems. I conclude with a discussion of open questions and future perspectives.Comment: Latex, 60 pages, 12 figures, Topical Review for J. Phys. C (Cond. Matt

Comparative modeling of DNA and RNA polymerases from Moniliophthora perniciosa mitochondrial plasmid

<p>Abstract</p> <p>Background</p> <p>The filamentous fungus <it>Moniliophthora perniciosa </it>(Stahel) Aime & Phillips-Mora is a hemibiotrophic Basidiomycota that causes witches' broom disease of cocoa (<it>Theobroma cacao </it>L.). This disease has resulted in a severe decrease in Brazilian cocoa production, which changed the position of Brazil in the market from the second largest cocoa exporter to a cocoa importer. Fungal mitochondrial plasmids are usually invertrons encoding DNA and RNA polymerases. Plasmid insertions into host mitochondrial genomes are probably associated with modifications in host generation time, which can be involved in fungal aging. This association suggests activity of polymerases, and these can be used as new targets for drugs against mitochondrial activity of fungi, more specifically against witches' broom disease. Sequencing and modeling: DNA and RNA polymerases of <it>M. perniciosa </it>mitochondrial plasmid were completely sequenced and their models were carried out by Comparative Homology approach. The sequences of DNA and RNA polymerase showed 25% of identity to 1XHX and 1ARO (pdb code) using BLASTp, which were used as templates. The models were constructed using Swiss PDB-Viewer and refined with a set of Molecular Mechanics (MM) and Molecular Dynamics (MD) in water carried out with AMBER 8.0, both working under the ff99 force fields, respectively. Ramachandran plots were generated by Procheck 3.0 and exhibited models with 97% and 98% for DNA and RNA polymerases, respectively. MD simulations in water showed models with thermodynamic stability after 2000 ps and 300 K of simulation.</p> <p>Conclusion</p> <p>This work contributes to the development of new alternatives for controlling the fungal agent of witches' broom disease.</p

Characterization of Human DNA Polymerase Delta and Its Subassemblies Reconstituted by Expression in the Multibac System

Mammalian DNA polymerase δ (Pol δ), a four-subunit enzyme, plays a crucial and versatile role in DNA replication and DNA repair processes. We have reconstituted human Pol δ complexes in insect cells infected with a single baculovirus into which one or more subunits were assembled. This system allowed for the efficient expression of the tetrameric Pol δ holoenzyme, the p125/p50 core dimer, the core+p68 trimer and the core+p12 trimer, as well as the p125 catalytic subunit. These were isolated in milligram amounts with reproducible purity and specific activities by a highly standardized protocol. We have systematically compared their activities in order to gain insights into the roles of the p12 and p68 subunits, as well as their responses to PCNA. The relative specific activities (apparent kcat) of the Pol δ holoenzyme, core+p68, core+p12 and p125/p50 core were 100, 109, 40, and 29. The corresponding apparent Kd's for PCNA were 7.1, 8.7, 9.3 and 73 nM. Our results support the hypothesis that Pol δ interacts with PCNA through multiple interactions, and that there may be a redundancy in binding interactions that may permit Pol δ to adopt flexible configurations with PCNA. The abilities of the Pol δ complexes to fully extend singly primed M13 DNA were examined. All the subassemblies except the core+p68 were defective in their abilities to completely extend the primer, showing that the p68 subunit has an important function in synthesis of long stretches of DNA in this assay. The core+p68 trimer could be reconstituted by addition of p12

Integrating Ion Mobility Mass Spectrometry with Molecular Modelling to Determine the Architecture of Multiprotein Complexes

Current challenges in the field of structural genomics point to the need for new tools and technologies for obtaining structures of macromolecular protein complexes. Here, we present an integrative computational method that uses molecular modelling, ion mobility-mass spectrometry (IM-MS) and incomplete atomic structures, usually from X-ray crystallography, to generate models of the subunit architecture of protein complexes. We begin by analyzing protein complexes using IM-MS, and by taking measurements of both intact complexes and sub-complexes that are generated in solution. We then examine available high resolution structural data and use a suite of computational methods to account for missing residues at the subunit and/or domain level. High-order complexes and sub-complexes are then constructed that conform to distance and connectivity constraints imposed by IM-MS data. We illustrate our method by applying it to multimeric protein complexes within the Escherichia coli replisome: the sliding clamp, (β2), the γ complex (γ3δδ′), the DnaB helicase (DnaB6) and the Single-Stranded Binding Protein (SSB4)

Clamp loader ATPases and the evolution of DNA replication machinery

Clamp loaders are pentameric ATPases of the AAA+ family that operate to ensure processive DNA replication. They do so by loading onto DNA the ring-shaped sliding clamps that tether the polymerase to the DNA. Structural and biochemical analysis of clamp loaders has shown how, despite differences in composition across different branches of life, all clamp loaders undergo the same concerted conformational transformations, which generate a binding surface for the open clamp and an internal spiral chamber into which the DNA at the replication fork can slide, triggering ATP hydrolysis, release of the clamp loader, and closure of the clamp round the DNA. We review here the current understanding of the clamp loader mechanism and discuss the implications of the differences between clamp loaders from the different branches of life

Routine single particle CryoEM sample and grid characterization by tomography

Single particle cryo-electron microscopy (cryoEM) is often performed under the assumption that particles are not adsorbed to the air-water interfaces and in thin, vitreous ice. In this study, we performed fiducial-less tomography on over 50 different cryoEM grid/sample preparations to determine the particle distribution within the ice and the overall geometry of the ice in grid holes. Surprisingly, by studying particles in holes in 3D from over 1000 tomograms, we have determined that the vast majority of particles (approximately 90%) are adsorbed to an air-water interface. The implications of this observation are wide-ranging, with potential ramifications regarding protein denaturation, conformational change, and preferred orientation. We also show that fiducial-less cryo-electron tomography on single particle grids may be used to determine ice thickness, optimal single particle collection areas and strategies, particle heterogeneity, and de novo models for template picking and single particle alignment

Structure of T7 RNA polymerase complexed to the transcriptional inhibitor T7 lysozyme.

The T7 RNA polymerase-T7 lysozyme complex regulates phage gene expression during infection of Escherichia coli. The 2.8 A crystal structure of the complex reveals that lysozyme binds at a site remote from the polymerase active site, suggesting an indirect mechanism of inhibition. Comparison of the T7 RNA polymerase structure with that of the homologous pol I family of DNA polymerases reveals identities in the catalytic site but also differences specific to RNA polymerase function. The structure of T7 RNA polymerase presented here differs significantly from a previously published structure. Sequence similarities between phage RNA polymerases and those from mitochondria and chloroplasts, when interpreted in the context of our revised model of T7 RNA polymerase, suggest a conserved fold