1
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Kirsanova OV, Subach FV, Loiko AG, Eritja RI, Gromova ES. EcoRII Restriction Endonuclease Forms Specific Contacts to the Bases of Its Target Sequence Flipped from DNA in a Transition Complex with Photoactivatable Substrates. RUSSIAN JOURNAL OF BIOORGANIC CHEMISTRY 2021. [DOI: 10.1134/s106816202102014x] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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2
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Tutkus M, Sasnauskas G, Rutkauskas D. Probing the dynamics of restriction endonuclease NgoMIV-DNA interaction by single-molecule FRET. Biopolymers 2017; 107. [DOI: 10.1002/bip.23075] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2017] [Revised: 09/22/2017] [Accepted: 10/04/2017] [Indexed: 11/09/2022]
Affiliation(s)
- Marijonas Tutkus
- Institute of Physics, Center for Physical Sciences and Technology, Savanoriu 231; Vilnius 02300 Lithuania
| | - Giedrius Sasnauskas
- Institute of Biotechnology, Vilnius University, Sauletekio av. 7; Vilnius 10257 Lithuania
| | - Danielis Rutkauskas
- Institute of Physics, Center for Physical Sciences and Technology, Savanoriu 231; Vilnius 02300 Lithuania
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3
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Sasnauskas G, Tamulaitienė G, Tamulaitis G, Čalyševa J, Laime M, Rimšelienė R, Lubys A, Siksnys V. UbaLAI is a monomeric Type IIE restriction enzyme. Nucleic Acids Res 2017; 45:9583-9594. [PMID: 28934493 PMCID: PMC5766183 DOI: 10.1093/nar/gkx634] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/08/2017] [Revised: 07/08/2017] [Accepted: 07/11/2017] [Indexed: 01/11/2023] Open
Abstract
Type II restriction endonucleases (REases) form a large and highly diverse group of enzymes. Even REases specific for a common recognition site often vary in their oligomeric structure, domain organization and DNA cleavage mechanisms. Here we report biochemical and structural characterization of the monomeric restriction endonuclease UbaLAI, specific for the pseudosymmetric DNA sequence 5'-CC/WGG-3' (where W = A/T, and '/' marks the cleavage position). We present a 1.6 Å co-crystal structure of UbaLAI N-terminal domain (UbaLAI-N) and show that it resembles the B3-family domain of EcoRII specific for the 5'-CCWGG-3' sequence. We also find that UbaLAI C-terminal domain (UbaLAI-C) is closely related to the monomeric REase MvaI, another enzyme specific for the 5'-CCWGG-3' sequence. Kinetic studies of UbaLAI revealed that it requires two recognition sites for optimal activity, and, like other type IIE enzymes, uses one copy of a recognition site to stimulate cleavage of a second copy. We propose that during the reaction UbaLAI-N acts as a handle that tethers the monomeric UbaLAI-C domain to the DNA, thereby helping UbaLAI-C to perform two sequential DNA nicking reactions on the second recognition site during a single DNA-binding event. A similar reaction mechanism may be characteristic to other monomeric two-domain REases.
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Affiliation(s)
- Giedrius Sasnauskas
- Institute of Biotechnology, Vilnius University, Sauletekio av. 7, LT-10257 Vilnius, Lithuania
| | - Giedrė Tamulaitienė
- Institute of Biotechnology, Vilnius University, Sauletekio av. 7, LT-10257 Vilnius, Lithuania
| | - Gintautas Tamulaitis
- Institute of Biotechnology, Vilnius University, Sauletekio av. 7, LT-10257 Vilnius, Lithuania
| | - Jelena Čalyševa
- Institute of Biotechnology, Vilnius University, Sauletekio av. 7, LT-10257 Vilnius, Lithuania
| | - Miglė Laime
- Thermo Fisher Scientific Baltics, V. A. Graiciuno str. 8, LT-02241, Vilnius, Lithuania
| | - Renata Rimšelienė
- Thermo Fisher Scientific Baltics, V. A. Graiciuno str. 8, LT-02241, Vilnius, Lithuania
| | - Arvydas Lubys
- Thermo Fisher Scientific Baltics, V. A. Graiciuno str. 8, LT-02241, Vilnius, Lithuania
| | - Virginijus Siksnys
- Institute of Biotechnology, Vilnius University, Sauletekio av. 7, LT-10257 Vilnius, Lithuania
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Zaremba M, Siksnys V. An Engineered SS Bridge Blocks the Conformational Change Required for the Nuclease Activity of BfiI. Biochemistry 2015; 54:5340-7. [PMID: 26261897 DOI: 10.1021/acs.biochem.5b00437] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The type IIS restriction endonuclease BfiI is a homodimer, and each monomer is composed of the N-terminal catalytic and C-terminal DNA recognition domains connected by a 28-residue linker segment. In the crystal in the absence of cognate DNA, BfiI exists in a "closed" conformation, in which an interdomain linker occludes a putative DNA binding surface at the catalytic domain and sterically hinders access to the active site. Cognate DNA binding presumably triggers a conformational change from the inactive "closed" state to the catalytically competent "open" state. Here we show that the disulfide SS bridge engineered at the domain interface locks the enzyme in the "closed" state. In the "closed" SS-linked state, BfiI binds cognate DNA with the same affinity as the wild-type enzyme but does not cut it, indicating that cross-linking introduces a restraint on the conformational transition, which couples DNA recognition and cleavage. Disruption of the interdomain SS bridge by the reducing agent restores the DNA cleavage ability of BfiI.
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Affiliation(s)
- Mindaugas Zaremba
- Institute of Biotechnology, Vilnius University , Graiciuno 8, Vilnius LT-02241, Lithuania
| | - Virginijus Siksnys
- Institute of Biotechnology, Vilnius University , Graiciuno 8, Vilnius LT-02241, Lithuania
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5
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Tamulaitis G, Rutkauskas M, Zaremba M, Grazulis S, Tamulaitiene G, Siksnys V. Functional significance of protein assemblies predicted by the crystal structure of the restriction endonuclease BsaWI. Nucleic Acids Res 2015; 43:8100-10. [PMID: 26240380 PMCID: PMC4652773 DOI: 10.1093/nar/gkv768] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2015] [Accepted: 07/17/2015] [Indexed: 02/03/2023] Open
Abstract
Type II restriction endonuclease BsaWI recognizes a degenerated sequence 5′-W/CCGGW-3′ (W stands for A or T, ‘/’ denotes the cleavage site). It belongs to a large family of restriction enzymes that contain a conserved CCGG tetranucleotide in their target sites. These enzymes are arranged as dimers or tetramers, and require binding of one, two or three DNA targets for their optimal catalytic activity. Here, we present a crystal structure and biochemical characterization of the restriction endonuclease BsaWI. BsaWI is arranged as an ‘open’ configuration dimer and binds a single DNA copy through a minor groove contacts. In the crystal primary BsaWI dimers form an indefinite linear chain via the C-terminal domain contacts implying possible higher order aggregates. We show that in solution BsaWI protein exists in a dimer-tetramer-oligomer equilibrium, but in the presence of specific DNA forms a tetramer bound to two target sites. Site-directed mutagenesis and kinetic experiments show that BsaWI is active as a tetramer and requires two target sites for optimal activity. We propose BsaWI mechanism that shares common features both with dimeric Ecl18kI/SgrAI and bona fide tetrameric NgoMIV/SfiI enzymes.
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Affiliation(s)
- Gintautas Tamulaitis
- Department of Protein-DNA Interactions, Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241 Vilnius, Lithuania
| | - Marius Rutkauskas
- Department of Protein-DNA Interactions, Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241 Vilnius, Lithuania
| | - Mindaugas Zaremba
- Department of Protein-DNA Interactions, Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241 Vilnius, Lithuania
| | - Saulius Grazulis
- Department of Protein-DNA Interactions, Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241 Vilnius, Lithuania
| | - Giedre Tamulaitiene
- Department of Protein-DNA Interactions, Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241 Vilnius, Lithuania
| | - Virginijus Siksnys
- Department of Protein-DNA Interactions, Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241 Vilnius, Lithuania
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6
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Rutkauskas D, Petkelyte M, Naujalis P, Sasnauskas G, Tamulaitis G, Zaremba M, Siksnys V. Restriction Enzyme Ecl18kI-Induced DNA Looping Dynamics by Single-Molecule FRET. J Phys Chem B 2014; 118:8575-82. [DOI: 10.1021/jp504546v] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
Affiliation(s)
- Danielis Rutkauskas
- Institute
of Physics, Center for Physical Sciences and Technology, Savanoriu
231, LT-02300, Vilnius, Lithuania
| | - Milda Petkelyte
- Institute
of Physics, Center for Physical Sciences and Technology, Savanoriu
231, LT-02300, Vilnius, Lithuania
| | - Paulius Naujalis
- Institute
of Physics, Center for Physical Sciences and Technology, Savanoriu
231, LT-02300, Vilnius, Lithuania
| | - Giedrius Sasnauskas
- Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241, Vilnius, Lithuania
| | - Gintautas Tamulaitis
- Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241, Vilnius, Lithuania
| | - Mindaugas Zaremba
- Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241, Vilnius, Lithuania
| | - Virginijus Siksnys
- Institute of Biotechnology, Vilnius University, Graiciuno 8, LT-02241, Vilnius, Lithuania
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7
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Pingoud A, Wilson GG, Wende W. Type II restriction endonucleases--a historical perspective and more. Nucleic Acids Res 2014; 42:7489-527. [PMID: 24878924 PMCID: PMC4081073 DOI: 10.1093/nar/gku447] [Citation(s) in RCA: 169] [Impact Index Per Article: 16.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2014] [Revised: 05/02/2014] [Accepted: 05/07/2014] [Indexed: 12/17/2022] Open
Abstract
This article continues the series of Surveys and Summaries on restriction endonucleases (REases) begun this year in Nucleic Acids Research. Here we discuss 'Type II' REases, the kind used for DNA analysis and cloning. We focus on their biochemistry: what they are, what they do, and how they do it. Type II REases are produced by prokaryotes to combat bacteriophages. With extreme accuracy, each recognizes a particular sequence in double-stranded DNA and cleaves at a fixed position within or nearby. The discoveries of these enzymes in the 1970s, and of the uses to which they could be put, have since impacted every corner of the life sciences. They became the enabling tools of molecular biology, genetics and biotechnology, and made analysis at the most fundamental levels routine. Hundreds of different REases have been discovered and are available commercially. Their genes have been cloned, sequenced and overexpressed. Most have been characterized to some extent, but few have been studied in depth. Here, we describe the original discoveries in this field, and the properties of the first Type II REases investigated. We discuss the mechanisms of sequence recognition and catalysis, and the varied oligomeric modes in which Type II REases act. We describe the surprising heterogeneity revealed by comparisons of their sequences and structures.
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Affiliation(s)
- Alfred Pingoud
- Institute of Biochemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
| | - Geoffrey G Wilson
- New England Biolabs Inc., 240 County Road, Ipswich, MA 01938-2723, USA
| | - Wolfgang Wende
- Institute of Biochemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, D-35392 Giessen, Germany
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Horton JR, Borgaro JG, Griggs RM, Quimby A, Guan S, Zhang X, Wilson GG, Zheng Y, Zhu Z, Cheng X. Structure of 5-hydroxymethylcytosine-specific restriction enzyme, AbaSI, in complex with DNA. Nucleic Acids Res 2014; 42:7947-59. [PMID: 24895434 PMCID: PMC4081097 DOI: 10.1093/nar/gku497] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023] Open
Abstract
AbaSI, a member of the PvuRts1I-family of modification-dependent restriction endonucleases, cleaves deoxyribonucleic acid (DNA) containing 5-hydroxymethylctosine (5hmC) and glucosylated 5hmC (g5hmC), but not DNA containing unmodified cytosine. AbaSI has been used as a tool for mapping the genomic locations of 5hmC, an important epigenetic modification in the DNA of higher organisms. Here we report the crystal structures of AbaSI in the presence and absence of DNA. These structures provide considerable, although incomplete, insight into how this enzyme acts. AbaSI appears to be mainly a homodimer in solution, but interacts with DNA in our structures as a homotetramer. Each AbaSI subunit comprises an N-terminal, Vsr-like, cleavage domain containing a single catalytic site, and a C-terminal, SRA-like, 5hmC-binding domain. Two N-terminal helices mediate most of the homodimer interface. Dimerization brings together the two catalytic sites required for double-strand cleavage, and separates the 5hmC binding-domains by ∼70 Å, consistent with the known activity of AbaSI which cleaves DNA optimally between symmetrically modified cytosines ∼22 bp apart. The eukaryotic SET and RING-associated (SRA) domains bind to DNA containing 5-methylcytosine (5mC) in the hemi-methylated CpG sequence. They make contacts in both the major and minor DNA grooves, and flip the modified cytosine out of the helix into a conserved binding pocket. In contrast, the SRA-like domain of AbaSI, which has no sequence specificity, contacts only the minor DNA groove, and in our current structures the 5hmC remains intra-helical. A conserved, binding pocket is nevertheless present in this domain, suitable for accommodating 5hmC and g5hmC. We consider it likely, therefore, that base-flipping is part of the recognition and cleavage mechanism of AbaSI, but that our structures represent an earlier, pre-flipped stage, prior to actual recognition.
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Affiliation(s)
- John R Horton
- Department of Biochemistry, Emory University School of Medicine, 1510 Clifton Road, Atlanta, Georgia 30322, USA
| | - Janine G Borgaro
- New England Biolabs, Inc., 240 County Road, Ipswich, MA 01938, USA
| | - Rose M Griggs
- Department of Biochemistry, Emory University School of Medicine, 1510 Clifton Road, Atlanta, Georgia 30322, USA
| | - Aine Quimby
- New England Biolabs, Inc., 240 County Road, Ipswich, MA 01938, USA
| | - Shengxi Guan
- New England Biolabs, Inc., 240 County Road, Ipswich, MA 01938, USA
| | - Xing Zhang
- Department of Biochemistry, Emory University School of Medicine, 1510 Clifton Road, Atlanta, Georgia 30322, USA
| | | | - Yu Zheng
- New England Biolabs, Inc., 240 County Road, Ipswich, MA 01938, USA
| | - Zhenyu Zhu
- New England Biolabs, Inc., 240 County Road, Ipswich, MA 01938, USA
| | - Xiaodong Cheng
- Department of Biochemistry, Emory University School of Medicine, 1510 Clifton Road, Atlanta, Georgia 30322, USA
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9
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Rajendran A, Endo M, Sugiyama H. State-of-the-Art High-Speed Atomic Force Microscopy for Investigation of Single-Molecular Dynamics of Proteins. Chem Rev 2013; 114:1493-520. [DOI: 10.1021/cr300253x] [Citation(s) in RCA: 70] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Affiliation(s)
- Arivazhagan Rajendran
- Department
of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-oiwakecho
Sakyo-ku, Kyoto 606-8502, Japan
| | - Masayuki Endo
- Institute
for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida-ushinomiyacho,
Sakyo-ku, Kyoto 606-8501, Japan
- CREST, Japan Science and Technology Corporation (JST), Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan
| | - Hiroshi Sugiyama
- Department
of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa-oiwakecho
Sakyo-ku, Kyoto 606-8502, Japan
- Institute
for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Yoshida-ushinomiyacho,
Sakyo-ku, Kyoto 606-8501, Japan
- CREST, Japan Science and Technology Corporation (JST), Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan
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Vasu K, Nagaraja V. Diverse functions of restriction-modification systems in addition to cellular defense. Microbiol Mol Biol Rev 2013; 77:53-72. [PMID: 23471617 PMCID: PMC3591985 DOI: 10.1128/mmbr.00044-12] [Citation(s) in RCA: 383] [Impact Index Per Article: 34.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Restriction-modification (R-M) systems are ubiquitous and are often considered primitive immune systems in bacteria. Their diversity and prevalence across the prokaryotic kingdom are an indication of their success as a defense mechanism against invading genomes. However, their cellular defense function does not adequately explain the basis for their immaculate specificity in sequence recognition and nonuniform distribution, ranging from none to too many, in diverse species. The present review deals with new developments which provide insights into the roles of these enzymes in other aspects of cellular function. In this review, emphasis is placed on novel hypotheses and various findings that have not yet been dealt with in a critical review. Emerging studies indicate their role in various cellular processes other than host defense, virulence, and even controlling the rate of evolution of the organism. We also discuss how R-M systems could have successfully evolved and be involved in additional cellular portfolios, thereby increasing the relative fitness of their hosts in the population.
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Affiliation(s)
- Kommireddy Vasu
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore
| | - Valakunja Nagaraja
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore
- Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India
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11
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A sequence-specific nicking endonuclease from streptomyces: purification, physical and catalytic properties. ISRN BIOCHEMISTRY 2013; 2013:287158. [PMID: 25937959 PMCID: PMC4392989 DOI: 10.1155/2013/287158] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/12/2013] [Accepted: 07/17/2013] [Indexed: 11/18/2022]
Abstract
A sequence-specific nicking endonuclease from Streptomyces designated as DC13 was purified to near homogeneity. Starting with 30 grams of wet cells, the enzyme was purified by ammonium sulfate fractionation, DEAE cellulose, and phenyl-Sepharose chromatography. The purified protein had a specific activity 1000 units/mg and migrated on SDS-PAGE gel with an estimated molecular weight of 71 kDa. Determination of subunit composition by gel filtration chromatography indicated that the native enzyme is a monomer. When incubated with different DNA substrates including pBluescript II KS, pUC118, pET-15b, and pET-26b, the enzyme converted these supercoiled plasmids to a mixture of open circular and linear DNA products, with the open circular DNA as the major cleavage product. Analysis of the kinetic of DNA cleavage showed that the enzyme appeared to cleave super-coiled plasmid in two distinct steps: a rapid cleavage of super-coiled plasmid to an open circular DNA followed a much slower step to linear DNA. The DNA cleavage reaction of the enzyme required Mg(2+) as a cofactor. Based on the monomeric nature of the enzyme, the kinetics of DNA cleavage exhibited by the enzyme, and cofactor requirement, it is suggested here that the purified enzyme is a sequence-specific nicking endonuclease that is similar to type IIS restriction endonuclease.
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Structural and Functional Analysis of Proteins by High-Speed Atomic Force Microscopy. STRUCTURAL AND MECHANISTIC ENZYMOLOGY - BRINGING TOGETHER EXPERIMENTS AND COMPUTING 2012; 87:5-55. [DOI: 10.1016/b978-0-12-398312-1.00002-0] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
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13
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Zaremba M, Owsicka A, Tamulaitis G, Sasnauskas G, Shlyakhtenko LS, Lushnikov AY, Lyubchenko YL, Laurens N, van den Broek B, Wuite GJL, Siksnys V. DNA synapsis through transient tetramerization triggers cleavage by Ecl18kI restriction enzyme. Nucleic Acids Res 2010; 38:7142-54. [PMID: 20571089 PMCID: PMC2978343 DOI: 10.1093/nar/gkq560] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
To cut DNA at their target sites, restriction enzymes assemble into different oligomeric structures. The Ecl18kI endonuclease in the crystal is arranged as a tetramer made of two dimers each bound to a DNA copy. However, free in solution Ecl18kI is a dimer. To find out whether the Ecl18kI dimer or tetramer represents the functionally important assembly, we generated mutants aimed at disrupting the putative dimer–dimer interface and analysed the functional properties of Ecl18kI and mutant variants. We show by atomic force microscopy that on two-site DNA, Ecl18kI loops out an intervening DNA fragment and forms a tetramer. Using the tethered particle motion technique, we demonstrate that in solution DNA looping is highly dynamic and involves a transient interaction between the two DNA-bound dimers. Furthermore, we show that Ecl18kI cleaves DNA in the synaptic complex much faster than when acting on a single recognition site. Contrary to Ecl18kI, the tetramerization interface mutant R174A binds DNA as a dimer, shows no DNA looping and is virtually inactive. We conclude that Ecl18kI follows the association model for the synaptic complex assembly in which it binds to the target site as a dimer and then associates into a transient tetrameric form to accomplish the cleavage reaction.
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Affiliation(s)
- Mindaugas Zaremba
- Institute of Biotechnology, Graiciuno 8, LT-02241, Vilnius, Lithuania
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14
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Gilmore JL, Suzuki Y, Tamulaitis G, Siksnys V, Takeyasu K, Lyubchenko YL. Single-molecule dynamics of the DNA-EcoRII protein complexes revealed with high-speed atomic force microscopy. Biochemistry 2009; 48:10492-8. [PMID: 19788335 DOI: 10.1021/bi9010368] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The study of interactions of protein with DNA is important for gaining a fundamental understanding of how numerous biological processes occur, including recombination, transcription, repair, etc. In this study, we use the EcoRII restriction enzyme, which employs a three-site binding mechanism to catalyze cleavage of a single recognition site. Using high-speed atomic force microscopy (HS-AFM) to image single-molecule interactions in real time, we were able to observe binding, translocation, and dissociation mechanisms of the EcoRII protein. The results show that the protein can translocate along DNA to search for the specific binding site. Also, once specifically bound at a single site, the protein is capable of translocating along the DNA to locate the second specific binding site. Furthermore, two alternative modes of dissociation of the EcoRII protein from the loop structure were observed, which result in the protein stably bound as monomers to two sites or bound to a single site as a dimer. From these observations, we propose a model in which this pathway is involved in the formation and dynamics of a catalytically active three-site complex.
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Affiliation(s)
- Jamie L Gilmore
- Department of Pharmaceutical Sciences, University of Nebraska Medical Center, 986025 Nebraska Medical Center, Omaha, Nebraska 68198-6025, USA
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15
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Golovenko D, Manakova E, Tamulaitiene G, Grazulis S, Siksnys V. Structural mechanisms for the 5'-CCWGG sequence recognition by the N- and C-terminal domains of EcoRII. Nucleic Acids Res 2009; 37:6613-24. [PMID: 19729506 PMCID: PMC2770665 DOI: 10.1093/nar/gkp699] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
EcoRII restriction endonuclease is specific for the 5′-CCWGG sequence (W stands for A or T); however, it shows no activity on a single recognition site. To activate cleavage it requires binding of an additional target site as an allosteric effector. EcoRII dimer consists of three structural units: a central catalytic core, made from two copies of the C-terminal domain (EcoRII-C), and two N-terminal effector DNA binding domains (EcoRII-N). Here, we report DNA-bound EcoRII-N and EcoRII-C structures, which show that EcoRII combines two radically different structural mechanisms to interact with the effector and substrate DNA. The catalytic EcoRII-C dimer flips out the central T:A base pair and makes symmetric interactions with the CC:GG half-sites. The EcoRII-N effector domain monomer binds to the target site asymmetrically in a single defined orientation which is determined by specific hydrogen bonding and van der Waals interactions with the central T:A pair in the major groove. The EcoRII-N mode of the target site recognition is shared by the large class of higher plant transcription factors of the B3 superfamily.
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Affiliation(s)
- Dmitrij Golovenko
- Institute of Biotechnology, Graiciuno 8, LT-02241 Vilnius, Lithuania
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16
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Szczepek M, Mackeldanz P, Möncke-Buchner E, Alves J, Krüger DH, Reuter M. Molecular analysis of restriction endonuclease EcoRII from Escherichia coli reveals precise regulation of its enzymatic activity by autoinhibition. Mol Microbiol 2009; 72:1011-21. [PMID: 19400796 DOI: 10.1111/j.1365-2958.2009.06702.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Bacterial restriction endonuclease EcoRII requires two recognition sites to cleave DNA. Proteolysis of EcoRII revealed the existence of two stable domains, EcoRII-N and EcoRII-C. Reduction of the enzyme to its C-terminal domain, EcoRII-C, unleashed the enzyme activity; this truncated form no longer needed two recognition sites and cleaved DNA much more efficiently than EcoRII wild-type. The crystal structure of EcoRII showed that probably the N-terminal domain sterically occludes the catalytic site, thus apparently controlling the cleavage activity. Based on these data, EcoRII was the first restriction endonuclease for which an autoinhibition mechanism as regulatory strategy was proposed. In this study, we probed this assumption and searched for the inhibitory element that mediates autoinhibition. Here we show that repression of EcoRII-C is achieved by addition of the inhibitory domain EcoRII-N or by single soluble peptides thereof in trans. Moreover, we perturbed contacts between the N- and the C-terminal domain of EcoRII by site-directed mutagenesis and proved that beta-strand B1 and alpha-helix H2 are essential for autoinhibition; deletion of either secondary structural element completely relieved EcoRII autoinhibition. This potent regulation principle that keeps EcoRII enzyme activity controlled might protect bacteria against suicidal restriction of rare unmodified recognition sites in the cellular genome.
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Affiliation(s)
- Michal Szczepek
- Institute of Medical Virology, Helmut-Ruska-Haus, Charité-Universitätsmedizin Berlin, Charitéplatz 1, D-10117 Berlin, Germany
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17
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Xu CY, Yu F, Xu SJ, Ding Y, Sun LH, Tang L, Hu XJ, Zhang ZH, He JH. Crystal structure and function of C-terminal Sau3AI domain. BIOCHIMICA ET BIOPHYSICA ACTA-PROTEINS AND PROTEOMICS 2008; 1794:118-23. [PMID: 18930848 DOI: 10.1016/j.bbapap.2008.09.008] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/08/2008] [Revised: 08/25/2008] [Accepted: 09/11/2008] [Indexed: 11/16/2022]
Abstract
Sau3AI is a type II restriction enzyme that recognizes the 5'-GATC-3' sequence in double-strand DNA and cleaves at 5' to the G residue. The C-terminal domain of Sau3AI (Sau3AI-C), which contains amino acids from 233 to 489, was crystallized and its structure was solved by using the Multi-wavelength Anomalous Diffraction method. The Sau3AI-C structure at 1.9 A resolution is similar to the structure of MutH, a DNA mismatch repair protein that shares high sequence similarity with the N-terminal Sau3AI domain. The functional analysis shows that Sau3AI-C can bind DNA with one recognition sequence but has no cleavage activity. These results indicate that Sau3AI is a pseudo-dimer belonging to the type IIe restriction enzymes and the Sau3AI-C is the allosteric effector domain that assists DNA binding and cleavage.
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Affiliation(s)
- Chun-Yan Xu
- Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
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18
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Tamulaitis G, Zaremba M, Szczepanowski RH, Bochtler M, Siksnys V. How PspGI, catalytic domain of EcoRII and Ecl18kI acquire specificities for different DNA targets. Nucleic Acids Res 2008; 36:6101-8. [PMID: 18820295 PMCID: PMC2577355 DOI: 10.1093/nar/gkn621] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
Restriction endonucleases Ecl18kI and PspGI/catalytic domain of EcoRII recognize CCNGG and CCWGG sequences (W stands for A or T), respectively. The enzymes are structurally similar, interact identically with the palindromic CC:GG parts of their recognition sequences and flip the nucleotides at their centers. Specificity for the central nucleotides could be influenced by the strength/stability of the base pair to be disrupted and/or by direct interactions of the enzymes with the flipped bases. Here, we address the importance of these contributions. We demonstrate that wt Ecl18kI cleaves oligoduplexes containing canonical, mismatched and abasic sites in the central position of its target sequence CCNGG with equal efficiencies. In contrast, substitutions in the binding pocket for the extrahelical base alter the Ecl18kI preference for the target site: the W61Y mutant prefers only certain mismatched substrates, and the W61A variant cuts exclusively at abasic sites, suggesting that pocket interactions play a major role in base discrimination. PspGI and catalytic domain of EcoRII probe the stability of the central base pair and the identity of the flipped bases in the pockets. This ‘double check’ mechanism explains their extraordinary specificity for an A/T pair in the flipping position.
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19
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Subach F, Kirsanova O, Liquier J, Gromova ES. Resolution of the EcoRII restriction endonuclease-DNA complex structure in solution using fluorescence spectroscopy. Biophys Chem 2008; 138:107-14. [PMID: 18814946 DOI: 10.1016/j.bpc.2008.09.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2008] [Revised: 09/06/2008] [Accepted: 09/07/2008] [Indexed: 11/27/2022]
Abstract
The X-ray structure for the type IIE EcoRII restriction endonuclease has been resolved [X.E. Zhou, Y. Wang, M. Reuter, M. Mucke, D.H. Kruger, E.J. Meehan and L. Chen. Crystal structure of type IIE restriction endonuclease EcoRII reveals an autoinhibition mechanism by a novel effector-binding fold. J. Mol. Biol. 335 (2004) 307-319.], but the structure of the R.EcoRII-DNA complex is still unknown. The aim of this article was to examine the structure of the pre-reactive R.EcoRII-DNA complex in solution by fluorescence spectroscopy. The structure for the R.EcoRII-DNA complex was resolved by determining the fluorescence resonance energy transfer (FRET) between two fluorescent dyes, covalently attached near the EcoRII recognition sites, that were located at opposite ends of a lengthy two-site DNA molecule. Analysis of the FRET data from the two-site DNA revealed a likely model for the arrangement of the two EcoRII recognition sites relative to each other in the R.EcoRII-DNA complex in the presence of Ca(2+) ions. According to this model, the R.EcoRII binds the two-site DNA and forms a DNA loop in which the EcoRII recognition sites are 20+/-10 A distant to each other and situated at an angle of 70+/-10 degrees.
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Affiliation(s)
- Fedor Subach
- Department of Chemistry, Moscow State University, Moscow, 119991, Russia
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20
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Jakubauskas A, Sasnauskas G, Giedriene J, Janulaitis A. Domain organization and functional analysis of type IIS restriction endonuclease Eco31I. Biochemistry 2008; 47:8546-56. [PMID: 18642930 DOI: 10.1021/bi800660u] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Type IIS restriction endonuclease Eco31I harbors a single HNH active site and cleaves both DNA strands close to its recognition sequence, 5'-GGTCTC(1/5). A two-domain organization of Eco31I was determined by limited proteolysis. Analysis of proteolytic fragments revealed that the N-terminal domain of Eco31I is responsible for the specific DNA binding, while the C-terminal domain contains the HNH nuclease-like active site. Gel-shift and gel-filtration experiments revealed that a monomer of the N-terminal domain of Eco31I is able to bind a single copy of cognate DNA. However, in contrast to other studied type IIS enzymes, the isolated catalytic domain of Eco31I was inactive. Steady-state and transient kinetic analysis of Eco31I reactions was inconsistent with dimerization of Eco31I on DNA. Thus, we propose that Eco31I interacts with individual copies of its recognition sequence in its monomeric form and presumably remains a monomer as it cleaves both strands of double-stranded DNA. The domain organization and reaction mechanism established for Eco31I should be common for a group of evolutionary related type IIS restriction endonucleases Alw26I, BsaI, BsmAI, BsmBI and Esp3I that recognize DNA sequences bearing the common pentanucleotide 5'-GTCTC.
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21
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Subach FV, Liquier J, Gromova ES. Investigation of restriction endonuclease EcoRII complex with DNA in solution by FTIR spectroscopy. RUSS J GEN CHEM+ 2008. [DOI: 10.1134/s1070363208050435] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
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22
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Shlyakhtenko LS, Gilmore J, Portillo A, Tamulaitis G, Siksnys V, Lyubchenko YL. Direct visualization of the EcoRII-DNA triple synaptic complex by atomic force microscopy. Biochemistry 2007; 46:11128-36. [PMID: 17845057 DOI: 10.1021/bi701123u] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Interactions between distantly separated DNA regions mediated by specialized proteins lead to the formation of synaptic protein-DNA complexes. This is a ubiquitous phenomenon which is critical in various genetic processes. Although such interactions typically occur between two sites, interactions among three specific DNA regions have been identified, and a corresponding model has been proposed. Atomic force microscopy was used to test this model for the EcoRII restriction enzyme and provide direct visualization and characterization of synaptic protein-DNA complexes involving three DNA binding sites. The complex appeared in the images as a two-loop structure, and the length measurements proved the site specificity of the protein in the complex. The protein volume measurements showed that an EcoRII dimer is the core of the three-site synaptosome. Other complexes were identified and analyzed. The protein volume data showed that the dimeric form of the protein is responsible for the formation of other types of synaptic complexes as well. The applications of these results to the mechanisms of the protein-DNA interactions are discussed.
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Affiliation(s)
- Luda S Shlyakhtenko
- Department of Pharmaceutical Sciences, College of Pharmacy, University of Nebraska Medical Center, 986025 Nebraska Medical Center, Omaha, Nebraska 68198-6025, USA
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23
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Bellamy SRW, Milsom SE, Kovacheva YS, Sessions RB, Halford SE. A switch in the mechanism of communication between the two DNA-binding sites in the SfiI restriction endonuclease. J Mol Biol 2007; 373:1169-83. [PMID: 17870087 PMCID: PMC2082129 DOI: 10.1016/j.jmb.2007.08.030] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2007] [Revised: 08/10/2007] [Accepted: 08/14/2007] [Indexed: 11/29/2022]
Abstract
While many Type II restriction enzymes are dimers with a single DNA-binding cleft between the subunits, SfiI is a tetramer of identical subunits. Two of its subunits (a dimeric unit) create one DNA-binding cleft, and the other two create a second cleft on the opposite side of the protein. The two clefts bind specific DNA cooperatively to give a complex of SfiI with two recognition sites. This complex is responsible for essentially all of the DNA-cleavage reactions by SfiI: virtually none is due to the complex with one site. The communication between the DNA-binding clefts was examined by disrupting one of the very few polar interactions in the otherwise hydrophobic interface between the dimeric units: a tyrosine hydroxyl was removed by mutation to phenylalanine. The mutant protein remained tetrameric in solution and could bind two DNA sites. But instead of being activated by binding two sites, like wild-type SfiI, it showed maximal activity when bound to a single site and had a lower activity when bound to two sites. This interaction across the dimer interface thus enforces in wild-type SfiI a cooperative transition between inactive and active states in both dimers, but without this interaction as in the mutant protein, a single dimer can undergo the transition to give a stable intermediate with one inactive dimer and one active dimer.
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Affiliation(s)
- Stuart R W Bellamy
- The DNA-Protein Interactions Unit, Department of Biochemistry, School of Medical Sciences, University of Bristol, University Walk, Bristol BS8 1TD, UK.
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24
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Marshall JJ, Gowers DM, Halford SE. Restriction endonucleases that bridge and excise two recognition sites from DNA. J Mol Biol 2007; 367:419-31. [PMID: 17266985 PMCID: PMC1892151 DOI: 10.1016/j.jmb.2006.12.070] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2006] [Revised: 12/22/2006] [Accepted: 12/28/2006] [Indexed: 11/28/2022]
Abstract
Most restriction endonucleases bridge two target sites before cleaving DNA: examples include all of the translocating Type I and Type III systems, and many Type II nucleases acting at their sites. A subset of Type II enzymes, the IIB systems, recognise bipartite sequences, like Type I sites, but cut specified phosphodiester bonds near their sites, like Type IIS enzymes. However, they make two double-strand breaks, one either side of the site, to release the recognition sequence on a short DNA fragment; 34 bp long in the case of the archetype, BcgI. It has been suggested that BcgI needs to interact with two recognition sites to cleave DNA but whether this is a general requirement for Type IIB enzymes had yet to be established. Ten Type IIB nucleases were tested against DNA substrates with one or two copies of the requisite sequences. With one exception, they all bridged two sites before cutting the DNA, usually in concerted reactions at both sites. The sites were ideally positioned in cis rather than in trans and were bridged through 3-D space, like Type II enzymes, rather than along the 1-D contour of the DNA, as seen with Type I enzymes. The standard mode of action for the restriction enzymes that excise their recognition sites from DNA thus involves concurrent action at two DNA sites.
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25
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Tamulaitiene G, Jakubauskas A, Urbanke C, Huber R, Grazulis S, Siksnys V. The crystal structure of the rare-cutting restriction enzyme SdaI reveals unexpected domain architecture. Structure 2006; 14:1389-400. [PMID: 16962970 DOI: 10.1016/j.str.2006.07.002] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/24/2006] [Revised: 07/04/2006] [Accepted: 07/05/2006] [Indexed: 01/31/2023]
Abstract
Rare-cutting restriction enzymes are important tools in genome analysis. We report here the crystal structure of SdaI restriction endonuclease, which is specific for the 8 bp sequence CCTGCA/GG ("/" designates the cleavage site). Unlike orthodox Type IIP enzymes, which are single domain proteins, the SdaI monomer is composed of two structural domains. The N domain contains a classical winged helix-turn-helix (wHTH) DNA binding motif, while the C domain shows a typical restriction endonuclease fold. The active site of SdaI is located within the C domain and represents a variant of the canonical PD-(D/E)XK motif. SdaI determinants of sequence specificity are clustered on the recognition helix of the wHTH motif at the N domain. The modular architecture of SdaI, wherein one domain mediates DNA binding while the other domain is predicted to catalyze hydrolysis, distinguishes SdaI from previously characterized restriction enzymes interacting with symmetric recognition sequences.
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26
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Zaremba M, Sasnauskas G, Urbanke C, Siksnys V. Allosteric communication network in the tetrameric restriction endonuclease Bse634I. J Mol Biol 2006; 363:800-12. [PMID: 16987525 DOI: 10.1016/j.jmb.2006.08.050] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2006] [Revised: 08/03/2006] [Accepted: 08/17/2006] [Indexed: 11/20/2022]
Abstract
Restriction endonuclease Bse634I is a homotetramer arranged as a dimer of two primary dimers. Bse634I displays its maximum catalytic efficiency upon binding of two copies of cognate DNA, one per each primary dimer. The catalytic activity of Bse634I on a single DNA copy is down-regulated due to the cross-talking interactions between the primary dimers. The mechanism of signal propagation between the individual active sites of Bse634I remains unclear. To identify communication pathways involved in the catalytic activity regulation of Bse634I tetramer we mutated a selected set of amino acid residues at the dimer-dimer interface and analysed the oligomeric state and catalytic properties of the mutant proteins. We demonstrate that alanine replacement of N262 and V263 residues located in the loop at the tetramerisation interface did not inhibit tetramer assembly but dramatically altered the catalytic properties of Bse634I despite of the distal location from the active site. Kinetic analysis using cognate hairpin oligonucleotide and one and two-site plasmids as substrates allowed us to identify two types of communication signals propagated through the dimer-dimer interface in the Bse634I tetramer: the inhibitory, or "stopper" and the activating, or "sync" signal. We suggest that the interplay between the two signals determines the catalytic and regulatory properties of the Bse634I and mutant proteins.
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Affiliation(s)
- Mindaugas Zaremba
- Institute of Biotechnology, Graiciuno 8, Vilnius, LT-02241, Lithuania
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27
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Gemmen GJ, Millin R, Smith DE. Tension-dependent DNA cleavage by restriction endonucleases: two-site enzymes are "switched off" at low force. Proc Natl Acad Sci U S A 2006; 103:11555-60. [PMID: 16868081 PMCID: PMC1520314 DOI: 10.1073/pnas.0604463103] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
DNA looping occurs in many important protein-DNA interactions, including those regulating replication, transcription, and recombination. Recent theoretical studies predict that tension of only a few piconewtons acting on DNA would almost completely inhibit DNA looping. Here, we study restriction endonucleases that require interaction at two separated sites for efficient cleavage. Using optical tweezers we measured the dependence of cleavage activity on DNA tension with 15 known or suspected two-site enzymes (BfiI, BpmI, BsgI, BspMI, Cfr9I, Cfr10I, Eco57I, EcoRII, FokI, HpaII, MboII, NarI, SacII, Sau3AI, and SgrAI) and six one-site enzymes (BamHI, EcoRI, EcoRV, HaeIII, HindIII, and DNaseI). All of the one-site enzymes were virtually unaffected by 5 pN of tension, whereas all of the two-site enzymes were completely inhibited. These enzymes thus constitute a remarkable example of a tension sensing "molecular switch." A detailed study of one enzyme, Sau3AI, indicated that the activity decreased exponentially with tension and the decrease was approximately 10-fold at 0.7 pN. At higher forces (approximately 20-40 pN) cleavage by the one-site enzymes EcoRV and HaeIII was partly inhibited and cleavage by HindIII was enhanced, whereas BamHI, EcoRI, and DNaseI were largely unaffected. These findings correlate with structural data showing that EcoRV bends DNA sharply, whereas BamHI, EcoRI, and DNaseI do not. Thus, DNA-directed enzyme activity involving either DNA looping or bending can be modulated by tension, a mechanism that could facilitate mechanosensory transduction in vivo.
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Affiliation(s)
- Gregory J. Gemmen
- Department of Physics, University of California at San Diego, Mail Code 0379, 9500 Gilman Drive, La Jolla, CA 92093
| | - Rachel Millin
- Department of Physics, University of California at San Diego, Mail Code 0379, 9500 Gilman Drive, La Jolla, CA 92093
| | - Douglas E. Smith
- Department of Physics, University of California at San Diego, Mail Code 0379, 9500 Gilman Drive, La Jolla, CA 92093
- To whom correspondence should be addressed. E-mail:
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28
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Gemmen GJ, Millin R, Smith DE. DNA looping by two-site restriction endonucleases: heterogeneous probability distributions for loop size and unbinding force. Nucleic Acids Res 2006; 34:2864-77. [PMID: 16723432 PMCID: PMC1474071 DOI: 10.1093/nar/gkl382] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Proteins interacting at multiple sites on DNA via looping play an important role in many fundamental biochemical processes. Restriction endonucleases that must bind at two recognition sites for efficient activity are a useful model system for studying such interactions. Here we used single DNA manipulation to study sixteen known or suspected two-site endonucleases. In eleven cases (BpmI, BsgI, BspMI, Cfr10I, Eco57I, EcoRII, FokI, HpaII, NarI, Sau3AI and SgrAI) we found that substitution of Ca2+ for Mg2+ blocked cleavage and enabled us to observe stable DNA looping. Forced disruption of these loops allowed us to measure the frequency of looping and probability distributions for loop size and unbinding force for each enzyme. In four cases we observed bimodal unbinding force distributions, indicating conformational heterogeneity and/or complex binding energy landscapes. Measured unlooping events ranged in size from 7 to 7500 bp and the most probable size ranged from less than 75 bp to nearly 500 bp, depending on the enzyme. In most cases the size distributions were in much closer agreement with theoretical models that postulate sharp DNA kinking than with classical models of DNA elasticity. Our findings indicate that DNA looping is highly variable depending on the specific protein and does not depend solely on the mechanical properties of DNA.
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Affiliation(s)
| | | | - Douglas E. Smith
- To whom correspondence should be addressed. Tel: +1 858 534 5241;
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