1
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Kohiyama M, Herrick J, Norris V. Open Questions about the Roles of DnaA, Related Proteins, and Hyperstructure Dynamics in the Cell Cycle. Life (Basel) 2023; 13:1890. [PMID: 37763294 PMCID: PMC10532879 DOI: 10.3390/life13091890] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2023] [Revised: 08/29/2023] [Accepted: 09/06/2023] [Indexed: 09/29/2023] Open
Abstract
The DnaA protein has long been considered to play the key role in the initiation of chromosome replication in modern bacteria. Many questions about this role, however, remain unanswered. Here, we raise these questions within a framework based on the dynamics of hyperstructures, alias large assemblies of molecules and macromolecules that perform a function. In these dynamics, hyperstructures can (1) emit and receive signals or (2) fuse and separate from one another. We ask whether the DnaA-based initiation hyperstructure acts as a logic gate receiving information from the membrane, the chromosome, and metabolism to trigger replication; we try to phrase some of these questions in terms of DNA supercoiling, strand opening, glycolytic enzymes, SeqA, ribonucleotide reductase, the macromolecular synthesis operon, post-translational modifications, and metabolic pools. Finally, we ask whether, underpinning the regulation of the cell cycle, there is a physico-chemical clock inherited from the first protocells, and whether this clock emits a single signal that triggers both chromosome replication and cell division.
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Affiliation(s)
- Masamichi Kohiyama
- Institut Jacques Monod, Université Paris Cité, CNRS, 75013 Paris, France;
| | - John Herrick
- Independent Researcher, 3 rue des Jeûneurs, 75002 Paris, France;
| | - Vic Norris
- CBSA UR 4312, University of Rouen Normandy, University of Caen Normandy, Normandy University, 76000 Rouen, France
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2
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Guo MS, Kawamura R, Littlehale ML, Marko JF, Laub MT. High-resolution, genome-wide mapping of positive supercoiling in chromosomes. eLife 2021; 10:e67236. [PMID: 34279217 PMCID: PMC8360656 DOI: 10.7554/elife.67236] [Citation(s) in RCA: 28] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2021] [Accepted: 07/16/2021] [Indexed: 12/15/2022] Open
Abstract
Supercoiling impacts DNA replication, transcription, protein binding to DNA, and the three-dimensional organization of chromosomes. However, there are currently no methods to directly interrogate or map positive supercoils, so their distribution in genomes remains unknown. Here, we describe a method, GapR-seq, based on the chromatin immunoprecipitation of GapR, a bacterial protein that preferentially recognizes overtwisted DNA, for generating high-resolution maps of positive supercoiling. Applying this method to Escherichia coli and Saccharomyces cerevisiae, we find that positive supercoiling is widespread, associated with transcription, and particularly enriched between convergently oriented genes, consistent with the 'twin-domain' model of supercoiling. In yeast, we also find positive supercoils associated with centromeres, cohesin-binding sites, autonomously replicating sites, and the borders of R-loops (DNA-RNA hybrids). Our results suggest that GapR-seq is a powerful approach, likely applicable in any organism, to investigate aspects of chromosome structure and organization not accessible by Hi-C or other existing methods.
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Affiliation(s)
- Monica S Guo
- Department of Biology, Massachusetts Institute of TechnologyCambridgeUnited States
| | - Ryo Kawamura
- Department of Molecular Biosciences, Northwestern UniversityEvanstonUnited States
| | - Megan L Littlehale
- Department of Biology, Massachusetts Institute of TechnologyCambridgeUnited States
| | - John F Marko
- Department of Molecular Biosciences, Northwestern UniversityEvanstonUnited States
- Department of Physics and Astronomy, Northwestern UniversityEvanstonUnited States
| | - Michael T Laub
- Department of Biology, Massachusetts Institute of TechnologyCambridgeUnited States
- Howard Hughes Medical Institute, Massachusetts Institute of TechnologyCambridgeUnited States
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3
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Chatterjee S, Jha JK, Ciaccia P, Venkova T, Chattoraj DK. Interactions of replication initiator RctB with single- and double-stranded DNA in origin opening of Vibrio cholerae chromosome 2. Nucleic Acids Res 2020; 48:11016-11029. [PMID: 33035310 DOI: 10.1093/nar/gkaa826] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2020] [Revised: 09/14/2020] [Accepted: 09/17/2020] [Indexed: 12/16/2022] Open
Abstract
Studies of bacterial chromosomes and plasmids indicate that their replication initiator proteins bind to origins of replication at many double-stranded sites and also at AT-rich regions where single-stranded DNA is exposed during origin opening. Single-strand binding apparently promotes origin opening by stabilizing an open structure, but how the initiator participates in this process and the contributions of the several binding sites remain unclear. Here, we show that the initiator protein of Vibrio cholerae specific to chromosome 2 (Chr2) also has single-strand binding activity in the AT-rich region of its origin. Binding is strand specific, depends on repeats of the sequence 5'ATCA and is greatly stabilized in vitro by specific double-stranded sites of the origin. The stability derives from the formation of ternary complexes of the initiator with the single- and double-stranded sites. An IHF site lies between these two kinds of sites in the Chr2 origin and an IHF-induced looping out of the intervening DNA mediates their interaction. Simultaneous binding to two kinds of sites in the origin appears to be a common mechanism by which bacterial replication initiators stabilize an open origin.
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Affiliation(s)
- Soniya Chatterjee
- Basic Research Laboratory, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892-4260, USA
| | - Jyoti K Jha
- Basic Research Laboratory, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892-4260, USA
| | - Peter Ciaccia
- Basic Research Laboratory, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892-4260, USA
| | - Tatiana Venkova
- Basic Research Laboratory, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892-4260, USA
| | - Dhruba K Chattoraj
- Basic Research Laboratory, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892-4260, USA
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4
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Oliveira Paiva AM, van Eijk E, Friggen AH, Weigel C, Smits WK. Identification of the Unwinding Region in the Clostridioides difficile Chromosomal Origin of Replication. Front Microbiol 2020; 11:581401. [PMID: 33133049 PMCID: PMC7561715 DOI: 10.3389/fmicb.2020.581401] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2020] [Accepted: 08/31/2020] [Indexed: 12/14/2022] Open
Abstract
Faithful DNA replication is crucial for viability of cells across all kingdoms. Targeting DNA replication is a viable strategy for inhibition of bacterial pathogens. Clostridioides difficile is an important enteropathogen that causes potentially fatal intestinal inflammation. Knowledge about DNA replication in this organism is limited and no data is available on the very first steps of DNA replication. Here, we use a combination of in silico predictions and in vitro experiments to demonstrate that C. difficile employs a bipartite origin of replication that shows DnaA-dependent melting at oriC2, located in the dnaA-dnaN intergenic region. Analysis of putative origins of replication in different clostridia suggests that the main features of the origin architecture are conserved. This study is the first to characterize aspects of the origin region of C. difficile and contributes to our understanding of the initiation of DNA replication in clostridia.
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Affiliation(s)
- Ana M Oliveira Paiva
- Department of Medical Microbiology, Section Experimental Bacteriology, Leiden University Medical Center, Leiden, Netherlands.,Centre for Microbial Cell Biology, Leiden, Netherlands
| | - Erika van Eijk
- Department of Medical Microbiology, Section Experimental Bacteriology, Leiden University Medical Center, Leiden, Netherlands
| | - Annemieke H Friggen
- Department of Medical Microbiology, Section Experimental Bacteriology, Leiden University Medical Center, Leiden, Netherlands
| | - Christoph Weigel
- Institute of Biotechnology, Technische Universität Berlin, Berlin, Germany
| | - Wiep Klaas Smits
- Department of Medical Microbiology, Section Experimental Bacteriology, Leiden University Medical Center, Leiden, Netherlands.,Centre for Microbial Cell Biology, Leiden, Netherlands
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5
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Carrasco C, Pastrana CL, Aicart-Ramos C, Leuba SH, Khan S, Moreno-Herrero F. Dynamics of DNA nicking and unwinding by the RepC-PcrA complex. Nucleic Acids Res 2020; 48:2013-2025. [PMID: 31930301 PMCID: PMC7038956 DOI: 10.1093/nar/gkz1200] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2019] [Revised: 12/10/2019] [Accepted: 12/18/2019] [Indexed: 01/22/2023] Open
Abstract
The rolling-circle replication is the most common mechanism for the replication of small plasmids carrying antibiotic resistance genes in Gram-positive bacteria. It is initiated by the binding and nicking of double-stranded origin of replication by a replication initiator protein (Rep). Duplex unwinding is then performed by the PcrA helicase, whose processivity is critically promoted by its interaction with Rep. How Rep and PcrA proteins interact to nick and unwind the duplex is not fully understood. Here, we have used magnetic tweezers to monitor PcrA helicase unwinding and its relationship with the nicking activity of Staphylococcus aureus plasmid pT181 initiator RepC. Our results indicate that PcrA is a highly processive helicase prone to stochastic pausing, resulting in average translocation rates of 30 bp s-1, while a typical velocity of 50 bp s-1 is found in the absence of pausing. Single-strand DNA binding protein did not affect PcrA translocation velocity but slightly increased its processivity. Analysis of the degree of DNA supercoiling required for RepC nicking, and the time between RepC nicking and DNA unwinding, suggests that RepC and PcrA form a protein complex on the DNA binding site before nicking. A comprehensive model that rationalizes these findings is presented.
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Affiliation(s)
- Carolina Carrasco
- Department of Macromolecular Structures, Centro Nacional de Biotecnología, CSIC, Darwin 3, 28049 Cantoblanco, Madrid, Spain
| | - Cesar L Pastrana
- Department of Macromolecular Structures, Centro Nacional de Biotecnología, CSIC, Darwin 3, 28049 Cantoblanco, Madrid, Spain
| | - Clara Aicart-Ramos
- Department of Macromolecular Structures, Centro Nacional de Biotecnología, CSIC, Darwin 3, 28049 Cantoblanco, Madrid, Spain
| | - Sanford H Leuba
- Departments of Cell Biology and Bioengineering, UPMC Hillman Cancer Center, University of Pittsburgh School of Medicine, 5117 Centre Avenue, Pittsburgh, PA 15213, USA
| | - Saleem A Khan
- Department of Microbiology and Molecular Genetics, University of Pittsburgh School of Medicine, 450 Technology Drive, Pittsburgh, PA 15219, USA
| | - Fernando Moreno-Herrero
- Department of Macromolecular Structures, Centro Nacional de Biotecnología, CSIC, Darwin 3, 28049 Cantoblanco, Madrid, Spain
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6
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Abstract
In all kingdoms of life, DNA is used to encode hereditary information. Propagation of the genetic material between generations requires timely and accurate duplication of DNA by semiconservative replication prior to cell division to ensure each daughter cell receives the full complement of chromosomes. DNA synthesis of daughter strands starts at discrete sites, termed replication origins, and proceeds in a bidirectional manner until all genomic DNA is replicated. Despite the fundamental nature of these events, organisms have evolved surprisingly divergent strategies that control replication onset. Here, we discuss commonalities and differences in replication origin organization and recognition in the three domains of life.
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Affiliation(s)
- Babatunde Ekundayo
- Quantitative Biology, Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
| | - Franziska Bleichert
- Quantitative Biology, Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland
- * E-mail:
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7
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Leonard AC, Rao P, Kadam RP, Grimwade JE. Changing Perspectives on the Role of DnaA-ATP in Orisome Function and Timing Regulation. Front Microbiol 2019; 10:2009. [PMID: 31555240 PMCID: PMC6727663 DOI: 10.3389/fmicb.2019.02009] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2019] [Accepted: 08/16/2019] [Indexed: 01/20/2023] Open
Abstract
Bacteria, like all cells, must precisely duplicate their genomes before they divide. Regulation of this critical process focuses on forming a pre-replicative nucleoprotein complex, termed the orisome. Orisomes perform two essential mechanical tasks that configure the unique chromosomal replication origin, oriC to start a new round of chromosome replication: (1) unwinding origin DNA and (2) assisting with loading of the replicative DNA helicase on exposed single strands. In Escherichia coli, a necessary orisome component is the ATP-bound form of the bacterial initiator protein, DnaA. DnaA-ATP differs from DnaA-ADP in its ability to oligomerize into helical filaments, and in its ability to access a subset of low affinity recognition sites in the E. coli replication origin. The helical filaments have been proposed to play a role in both of the key mechanical tasks, but recent studies raise new questions about whether they are mandatory for orisome activity. It was recently shown that a version of E. coli oriC (oriCallADP), whose multiple low affinity DnaA recognition sites bind DnaA-ATP and DnaA-ADP similarly, was fully occupied and unwound by DnaA-ADP in vitro, and in vivo suppressed the lethality of DnaA mutants defective in ATP binding and ATP-specific oligomerization. However, despite their functional equivalency, orisomes assembled on oriCallADP were unable to trigger chromosome replication at the correct cell cycle time and displayed a hyper-initiation phenotype. Here we present a new perspective on DnaA-ATP, and suggest that in E. coli, DnaA-ATP is not required for mechanical functions, but rather is needed for site recognition and occupation, so that initiation timing is coupled to DnaA-ATP levels. We also discuss how other bacterial types may utilize DnaA-ATP and DnaA-ADP, and whether the high diversity of replication origins in the bacterial world reflects different regulatory strategies for how DnaA-ATP is used to control orisome assembly.
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Affiliation(s)
- Alan C Leonard
- Laboratory of Microbial Genetics, Department of Biomedical and Chemical Engineering and Science, Florida Institute of Technology, Melbourne, FL, United States
| | - Prassanna Rao
- Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, TN, United States
| | - Rohit P Kadam
- Laboratory of Microbial Genetics, Department of Biomedical and Chemical Engineering and Science, Florida Institute of Technology, Melbourne, FL, United States
| | - Julia E Grimwade
- Laboratory of Microbial Genetics, Department of Biomedical and Chemical Engineering and Science, Florida Institute of Technology, Melbourne, FL, United States
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8
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Richardson TT, Stevens D, Pelliciari S, Harran O, Sperlea T, Murray H. Identification of a basal system for unwinding a bacterial chromosome origin. EMBO J 2019; 38:e101649. [PMID: 31267560 PMCID: PMC6669920 DOI: 10.15252/embj.2019101649] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2019] [Revised: 06/06/2019] [Accepted: 06/07/2019] [Indexed: 01/03/2023] Open
Abstract
Genome duplication is essential for cell proliferation, and DNA synthesis is generally initiated by dedicated replication proteins at specific loci termed origins. In bacteria, the master initiator DnaA binds the chromosome origin (oriC) and unwinds the DNA duplex to permit helicase loading. However, despite decades of research it remained unclear how the information encoded within oriC guides DnaA-dependent strand separation. To address this fundamental question, we took a systematic genetic approach in vivo and identified the core set of essential sequence elements within the Bacillus subtilis chromosome origin unwinding region. Using this information, we then show in vitro that the minimal replication origin sequence elements are necessary and sufficient to promote the mechanical functions of DNA duplex unwinding by DnaA. Because the basal DNA unwinding system characterized here appears to be conserved throughout the bacterial domain, this discovery provides a framework for understanding oriC architecture, activity, regulation and diversity.
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Affiliation(s)
- Tomas T Richardson
- Centre for Bacterial Cell BiologyInstitute for Cell and Molecular BiosciencesNewcastle UniversityNewcastle Upon TyneUK
| | - Daniel Stevens
- Centre for Bacterial Cell BiologyInstitute for Cell and Molecular BiosciencesNewcastle UniversityNewcastle Upon TyneUK
| | - Simone Pelliciari
- Centre for Bacterial Cell BiologyInstitute for Cell and Molecular BiosciencesNewcastle UniversityNewcastle Upon TyneUK
| | - Omar Harran
- Centre for Bacterial Cell BiologyInstitute for Cell and Molecular BiosciencesNewcastle UniversityNewcastle Upon TyneUK
| | - Theodor Sperlea
- Chromosome Biology GroupLOEWE Center for Synthetic MicrobiologySYNMIKROPhilipps‐Universität MarburgMarburgGermany
| | - Heath Murray
- Centre for Bacterial Cell BiologyInstitute for Cell and Molecular BiosciencesNewcastle UniversityNewcastle Upon TyneUK
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9
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Grimwade JE, Rozgaja TA, Gupta R, Dyson K, Rao P, Leonard AC. Origin recognition is the predominant role for DnaA-ATP in initiation of chromosome replication. Nucleic Acids Res 2019; 46:6140-6151. [PMID: 29800247 PMCID: PMC6158602 DOI: 10.1093/nar/gky457] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/03/2017] [Accepted: 05/11/2018] [Indexed: 01/06/2023] Open
Abstract
In all cells, initiation of chromosome replication depends on the activity of AAA+ initiator proteins that form complexes with replication origin DNA. In bacteria, the conserved, adenosine triphosphate (ATP)-regulated initiator protein, DnaA, forms a complex with the origin, oriC, that mediates DNA strand separation and recruitment of replication machinery. Complex assembly and origin activation requires DnaA-ATP, which differs from DnaA-ADP in its ability to cooperatively bind specific low affinity sites and also to oligomerize into helical filaments. The degree to which each of these activities contributes to the DnaA-ATP requirement for initiation is not known. In this study, we compared the DnaA-ATP dependence of initiation from wild-type Escherichia coli oriC and a synthetic origin (oriCallADP), whose multiple low affinity DnaA sites bind DnaA-ATP and DnaA-ADP similarly. OriCallADP was fully occupied and unwound by DnaA-ADP in vitro, and, in vivo, oriCallADP suppressed lethality of DnaA mutants defective in ATP binding and ATP-specific oligomerization. However, loss of preferential DnaA-ATP binding caused over-initiation and increased sensitivity to replicative stress. The findings indicate both DnaA-ATP and DnaA-ADP can perform most of the mechanical functions needed for origin activation, and suggest that a key reason for ATP-regulation of DnaA is to control replication initiation frequency.
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Affiliation(s)
- Julia E Grimwade
- Department of Biological Sciences, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901, USA
| | - Tania A Rozgaja
- AREVA Inc North America, 6100 Southwest Blvd #400, Benbrook, TX 76109, USA
| | - Rajat Gupta
- Department of Biological Sciences, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901, USA
| | - Kyle Dyson
- University of Florida College of Medicine, P.O. Box 100215, Gainesville, FL 32610, USA
| | - Prassanna Rao
- Department of Biological Sciences, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901, USA
| | - Alan C Leonard
- Department of Biological Sciences, Florida Institute of Technology, 150 W. University Blvd., Melbourne, FL 32901, USA
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10
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Li X, Zhang Y, Zhou X, Hu X, Zhou Y, Liu D, Maxwell A, Mi K. The plasmid-borne quinolone resistance protein QnrB, a novel DnaA-binding protein, increases the bacterial mutation rate by triggering DNA replication stress. Mol Microbiol 2019; 111:1529-1543. [PMID: 30838726 PMCID: PMC6617969 DOI: 10.1111/mmi.14235] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 03/01/2019] [Indexed: 02/02/2023]
Abstract
Bacterial antibiotic resistance, a global health threat, is caused by plasmid transfer or genetic mutations. Quinolones are important antibiotics, partially because they are fully synthetic and resistance genes are unlikely to exist in nature; nonetheless, quinolone resistance proteins have been identified. The mechanism by which plasmid-borne quinolone resistance proteins promotes the selection of quinolone-resistant mutants is unclear. Here, we show that QnrB increases the bacterial mutation rate. Transcriptomic and genome sequencing analyses showed that QnrB promoted gene abundance near the origin of replication (oriC). In addition, the QnrB expression level correlated with the replication origin to terminus (oriC/ter) ratio, indicating QnrB-induced DNA replication stress. Our results also show that QnrB is a DnaA-binding protein that may act as an activator of DNA replication initiation. Interaction of QnrB with DnaA promoted the formation of the DnaA-oriC open complex, which leads to DNA replication over-initiation. Our data indicate that plasmid-borne QnrB increases bacterial mutation rates and that genetic changes can alleviate the fitness cost imposed by transmitted plasmids. Derivative mutations may impair antibiotic efficacy and threaten the value of antibiotic treatments. Enhanced understanding of how bacteria adapt to the antibiotic environment will lead to new therapeutic strategies for antibiotic-resistant infections.
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Affiliation(s)
- Xiaojing Li
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China
| | - Yujiao Zhang
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China.,Savaid Medical School, University of Chinese Academy of Sciences, Beijing, 101408, China
| | - Xintong Zhou
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China
| | - Xinling Hu
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China
| | - Yixuan Zhou
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China.,Savaid Medical School, University of Chinese Academy of Sciences, Beijing, 101408, China
| | - Di Liu
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China.,Savaid Medical School, University of Chinese Academy of Sciences, Beijing, 101408, China
| | - Anthony Maxwell
- Department of Biological Chemistry, John Innes Centre, Norwich Research Park, Norwich, NR4 7UH, UK
| | - Kaixia Mi
- CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences, Beijing, 100101, China.,Savaid Medical School, University of Chinese Academy of Sciences, Beijing, 101408, China
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11
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Jameson KH, Wilkinson AJ. Control of Initiation of DNA Replication in Bacillus subtilis and Escherichia coli. Genes (Basel) 2017; 8:E22. [PMID: 28075389 PMCID: PMC5295017 DOI: 10.3390/genes8010022] [Citation(s) in RCA: 36] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2016] [Revised: 12/16/2016] [Accepted: 12/20/2016] [Indexed: 01/21/2023] Open
Abstract
Initiation of DNA Replication is tightly regulated in all cells since imbalances in chromosomal copy number are deleterious and often lethal. In bacteria such as Bacillus subtilis and Escherichia coli, at the point of cytokinesis, there must be two complete copies of the chromosome to partition into the daughter cells following division at mid-cell during vegetative growth. Under conditions of rapid growth, when the time taken to replicate the chromosome exceeds the doubling time of the cells, there will be multiple initiations per cell cycle and daughter cells will inherit chromosomes that are already undergoing replication. In contrast, cells entering the sporulation pathway in B. subtilis can do so only during a short interval in the cell cycle when there are two, and only two, chromosomes per cell, one destined for the spore and one for the mother cell. Here, we briefly describe the overall process of DNA replication in bacteria before reviewing initiation of DNA replication in detail. The review covers DnaA-directed assembly of the replisome at oriC and the multitude of mechanisms of regulation of initiation, with a focus on the similarities and differences between E. coli and B. subtilis.
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Affiliation(s)
- Katie H Jameson
- Institute of Integrative Biology, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK.
| | - Anthony J Wilkinson
- Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, UK.
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12
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Jha JK, Ramachandran R, Chattoraj DK. Opening the Strands of Replication Origins-Still an Open Question. Front Mol Biosci 2016; 3:62. [PMID: 27747216 PMCID: PMC5043065 DOI: 10.3389/fmolb.2016.00062] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/30/2016] [Accepted: 09/16/2016] [Indexed: 11/22/2022] Open
Abstract
The local separation of duplex DNA strands (strand opening) is necessary for initiating basic transactions on DNA such as transcription, replication, and homologous recombination. Strand opening is commonly a stage at which these processes are regulated. Many different mechanisms are used to open the DNA duplex, the details of which are of great current interest. In this review, we focus on a few well-studied cases of DNA replication origin opening in bacteria. In particular, we discuss the opening of origins that support the theta (θ) mode of replication, which is used by all chromosomal origins and many extra-chromosomal elements such as plasmids and phages. Although the details of opening can vary among different origins, a common theme is binding of the initiator to multiple sites at the origin, causing stress that opens an adjacent and intrinsically unstable A+T rich region. The initiator stabilizes the opening by capturing one of the open strands. How the initiator binding energy is harnessed for strand opening remains to be understood.
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Affiliation(s)
- Jyoti K Jha
- Laboratory of Biochemistry and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health Bethesda, MD, USA
| | - Revathy Ramachandran
- Laboratory of Biochemistry and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health Bethesda, MD, USA
| | - Dhruba K Chattoraj
- Laboratory of Biochemistry and Molecular Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health Bethesda, MD, USA
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13
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Noguchi Y, Sakiyama Y, Kawakami H, Katayama T. The Arg Fingers of Key DnaA Protomers Are Oriented Inward within the Replication Origin oriC and Stimulate DnaA Subcomplexes in the Initiation Complex. J Biol Chem 2015; 290:20295-312. [PMID: 26126826 DOI: 10.1074/jbc.m115.662601] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2015] [Indexed: 01/01/2023] Open
Abstract
ATP-DnaA binds to multiple DnaA boxes in the Escherichia coli replication origin (oriC) and forms left-half and right-half subcomplexes that promote DNA unwinding and DnaB helicase loading. DnaA forms homo-oligomers in a head-to-tail manner via interactions between the bound ATP and Arg-285 of the adjacent protomer. DnaA boxes R1 and R4 reside at the outer edges of the DnaA-binding region and have opposite orientations. In this study, roles for the protomers bound at R1 and R4 were elucidated using chimeric DnaA molecules that had alternative DNA binding sequence specificity and chimeric oriC molecules bearing the alternative DnaA binding sequence at R1 or R4. In vitro, protomers at R1 and R4 promoted initiation regardless of whether the bound nucleotide was ADP or ATP. Arg-285 was shown to play an important role in the formation of subcomplexes that were active in oriC unwinding and DnaB loading. The results of in vivo analysis using the chimeric molecules were consistent with the in vitro data. Taken together, the data suggest a model in which DnaA subcomplexes form in symmetrically opposed orientations and in which the Arg-285 fingers face inward to mediate interactions with adjacent protomers. This mode is consistent with initiation regulation by ATP-DnaA and bidirectional loading of DnaB helicases.
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Affiliation(s)
- Yasunori Noguchi
- From the Department of Molecular Biology, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
| | - Yukari Sakiyama
- From the Department of Molecular Biology, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
| | - Hironori Kawakami
- From the Department of Molecular Biology, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
| | - Tsutomu Katayama
- From the Department of Molecular Biology, Graduate School of Pharmaceutical Sciences, Kyushu University, Fukuoka 812-8582, Japan
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14
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Leonard AC, Grimwade JE. The orisome: structure and function. Front Microbiol 2015; 6:545. [PMID: 26082765 PMCID: PMC4451416 DOI: 10.3389/fmicb.2015.00545] [Citation(s) in RCA: 69] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2015] [Accepted: 05/18/2015] [Indexed: 11/15/2022] Open
Abstract
During the cell division cycle of all bacteria, DNA-protein complexes termed orisomes trigger the onset of chromosome duplication. Orisome assembly is both staged and stringently regulated to ensure that DNA synthesis begins at a precise time and only once at each origin per cycle. Orisomes comprise multiple copies of the initiator protein DnaA, which oligomerizes after interacting with specifically positioned recognition sites in the unique chromosomal replication origin, oriC. Since DnaA is highly conserved, it is logical to expect that all bacterial orisomes will share fundamental attributes. Indeed, although mechanistic details remain to be determined, all bacterial orisomes are capable of unwinding oriC DNA and assisting with loading of DNA helicase onto the single-strands. However, comparative analysis of oriCs reveals that the arrangement and number of DnaA recognition sites is surprisingly variable among bacterial types, suggesting there are many paths to produce functional orisome complexes. Fundamental questions exist about why these different paths exist and which features of orisomes must be shared among diverse bacterial types. In this review we present the current understanding of orisome assembly and function in Escherichia coli and compare the replication origins among the related members of the Gammaproteobacteria. From this information we propose that the diversity in orisome assembly reflects both the requirement to regulate the conformation of origin DNA as well as to provide an appropriate cell cycle timing mechanism that reflects the lifestyle of the bacteria. We suggest that identification of shared steps in orisome assembly may reveal particularly good targets for new antibiotics.
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Affiliation(s)
- Alan C Leonard
- Department of Biological Sciences, Florida Institute of Technology, Melbourne FL, USA
| | - Julia E Grimwade
- Department of Biological Sciences, Florida Institute of Technology, Melbourne FL, USA
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15
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Wolański M, Donczew R, Zawilak-Pawlik A, Zakrzewska-Czerwińska J. oriC-encoded instructions for the initiation of bacterial chromosome replication. Front Microbiol 2015; 5:735. [PMID: 25610430 PMCID: PMC4285127 DOI: 10.3389/fmicb.2014.00735] [Citation(s) in RCA: 57] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2014] [Accepted: 12/05/2014] [Indexed: 01/09/2023] Open
Abstract
Replication of the bacterial chromosome initiates at a single origin of replication that is called oriC. This occurs via the concerted action of numerous proteins, including DnaA, which acts as an initiator. The origin sequences vary across species, but all bacterial oriCs contain the information necessary to guide assembly of the DnaA protein complex at oriC, triggering the unwinding of DNA and the beginning of replication. The requisite information is encoded in the unique arrangement of specific sequences called DnaA boxes, which form a framework for DnaA binding and assembly. Other crucial sequences of bacterial origin include DNA unwinding element (DUE, which designates the site at which oriC melts under the influence of DnaA) and binding sites for additional proteins that positively or negatively regulate the initiation process. In this review, we summarize our current knowledge and understanding of the information encoded in bacterial origins of chromosomal replication, particularly in the context of replication initiation and its regulation. We show that oriC encoded instructions allow not only for initiation but also for precise regulation of replication initiation and coordination of chromosomal replication with the cell cycle (also in response to environmental signals). We focus on Escherichia coli, and then expand our discussion to include several other microorganisms in which additional regulatory proteins have been recently shown to be involved in coordinating replication initiation to other cellular processes (e.g., Bacillus, Caulobacter, Helicobacter, Mycobacterium, and Streptomyces). We discuss diversity of bacterial oriC regions with the main focus on roles of individual DNA recognition sequences at oriC in binding the initiator and regulatory proteins as well as the overall impact of these proteins on the formation of initiation complex.
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Affiliation(s)
- Marcin Wolański
- Department of Molecular Microbiology, Faculty of Biotechnology, University of Wrocław Wrocław, Poland
| | - Rafał Donczew
- Department of Microbiology, Ludwik Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences Wrocław, Poland
| | - Anna Zawilak-Pawlik
- Department of Microbiology, Ludwik Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences Wrocław, Poland
| | - Jolanta Zakrzewska-Czerwińska
- Department of Molecular Microbiology, Faculty of Biotechnology, University of Wrocław Wrocław, Poland ; Department of Microbiology, Ludwik Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences Wrocław, Poland
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16
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Cheng HM, Gröger P, Hartmann A, Schlierf M. Bacterial initiators form dynamic filaments on single-stranded DNA monomer by monomer. Nucleic Acids Res 2014; 43:396-405. [PMID: 25477384 PMCID: PMC4288177 DOI: 10.1093/nar/gku1284] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
DNA replication initiation is mediated across all domains of life by initiator proteins oligomerizing at replication origins. Recently, it was shown that initiators can directly bind single-stranded DNA (ssDNA) and thus might enhance origin melting. In this study, we used single-molecule fluorescence assays to probe the ssDNA binding mechanism of the replication initiator DnaA. Our experiments revealed that DnaA forms a dynamic filament on ssDNA in 3′ to 5′ directionality in the presence of ATP and analogs. After nucleation with a three-monomer seed, monomers dynamically assemble and disassemble one monomer at a time at the 5′ end, each monomer binding three nucleotides of ssDNA. The addition of adjacent double-stranded DnaA binding sites stabilized the DnaA filament on ssDNA. Our results extend the current models of origin melting via DnaA ssDNA interaction.
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Affiliation(s)
- Hsin-Mei Cheng
- B CUBE - Center for Molecular Bioengineering, Technische Universität Dresden, Arnoldstraße 18, 01307 Dresden, Germany
| | - Philip Gröger
- B CUBE - Center for Molecular Bioengineering, Technische Universität Dresden, Arnoldstraße 18, 01307 Dresden, Germany
| | - Andreas Hartmann
- B CUBE - Center for Molecular Bioengineering, Technische Universität Dresden, Arnoldstraße 18, 01307 Dresden, Germany
| | - Michael Schlierf
- B CUBE - Center for Molecular Bioengineering, Technische Universität Dresden, Arnoldstraße 18, 01307 Dresden, Germany
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17
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Donczew R, Mielke T, Jaworski P, Zakrzewska-Czerwińska J, Zawilak-Pawlik A. Assembly of Helicobacter pylori initiation complex is determined by sequence-specific and topology-sensitive DnaA-oriC interactions. J Mol Biol 2014; 426:2769-82. [PMID: 24862285 DOI: 10.1016/j.jmb.2014.05.018] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/19/2014] [Revised: 05/15/2014] [Accepted: 05/16/2014] [Indexed: 10/25/2022]
Abstract
In bacteria, chromosome replication is initiated by binding of the DnaA initiator protein to DnaA boxes located in the origin of chromosomal replication (oriC). This leads to DNA helix opening within the DNA-unwinding element. Helicobacter pylori oriC, the first bipartite origin identified in Gram-negative bacteria, contains two subregions, oriC1 and oriC2, flanking the dnaA gene. The DNA-unwinding element region is localized in the oriC2 subregion downstream of dnaA. Surprisingly, oriC2-DnaA interactions were shown to depend on DNA topology, which is unusual in bacteria but is similar to initiator-origin interactions observed in higher organisms. In this work, we identified three DnaA boxes in the oriC2 subregion, two of which were bound only as supercoiled DNA. We found that all three DnaA boxes play important roles in orisome assembly and subsequent DNA unwinding, but different functions can be assigned to individual boxes. This suggests that the H. pylori oriC may be functionally divided, similar to what was described recently for Escherichia coli oriC. On the basis of these results, we propose a model of initiation complex formation in H. pylori.
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Affiliation(s)
- Rafał Donczew
- Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Department of Microbiology, Weigla 12, 53-114 Wrocław, Poland.
| | - Thorsten Mielke
- Max Planck Institute for Molecular Genetics, Ihnestrasse 63-73, 14195 Berlin, Germany.
| | - Paweł Jaworski
- Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Department of Microbiology, Weigla 12, 53-114 Wrocław, Poland.
| | - Jolanta Zakrzewska-Czerwińska
- Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Department of Microbiology, Weigla 12, 53-114 Wrocław, Poland; University of Wrocław, Faculty of Biotechnology, Joliot-Curie 14a, 50-138 Wrocław, Poland.
| | - Anna Zawilak-Pawlik
- Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, Department of Microbiology, Weigla 12, 53-114 Wrocław, Poland.
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18
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Wegrzyn K, Fuentes-Perez ME, Bury K, Rajewska M, Moreno-Herrero F, Konieczny I. Sequence-specific interactions of Rep proteins with ssDNA in the AT-rich region of the plasmid replication origin. Nucleic Acids Res 2014; 42:7807-18. [PMID: 24838560 PMCID: PMC4081077 DOI: 10.1093/nar/gku453] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/02/2022] Open
Abstract
The DNA unwinding element (DUE) is a sequence rich in adenine and thymine residues present within the origin region of both prokaryotic and eukaryotic replicons. Recently, it has been shown that this is the site where bacterial DnaA proteins, the chromosomal replication initiators, form a specific nucleoprotein filament. DnaA proteins contain a DNA binding domain (DBD) and belong to the family of origin binding proteins (OBPs). To date there has been no data on whether OBPs structurally different from DnaA can form nucleoprotein complexes within the DUE. In this work we demonstrate that plasmid Rep proteins, composed of two Winged Helix domains, distinct from the DBD, specifically bind to one of the strands of ssDNA within the DUE. We observed nucleoprotein complexes formed by these Rep proteins, involving both dsDNA containing the Rep-binding sites (iterons) and the strand-specific ssDNA of the DUE. Formation of these complexes required the presence of all repeated sequence elements located within the DUE. Any changes in these repeated sequences resulted in the disturbance in Rep-ssDNA DUE complex formation and the lack of origin replication activity in vivo or in vitro.
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Affiliation(s)
- Katarzyna Wegrzyn
- Department of Molecular and Cellular Biology, Intercollegiate Faculty of Biotechnology, University of Gdansk, 24 Kladki, 80-822 Gdansk, Poland
| | - Maria Eugenia Fuentes-Perez
- Department of Macromolecular Structures, Centro Nacional de Biotecnologia, CSIC, Darwin 3, 28049 Cantoblanco, Madrid, Spain
| | - Katarzyna Bury
- Department of Molecular and Cellular Biology, Intercollegiate Faculty of Biotechnology, University of Gdansk, 24 Kladki, 80-822 Gdansk, Poland
| | - Magdalena Rajewska
- Department of Molecular and Cellular Biology, Intercollegiate Faculty of Biotechnology, University of Gdansk, 24 Kladki, 80-822 Gdansk, Poland
| | - Fernando Moreno-Herrero
- Department of Macromolecular Structures, Centro Nacional de Biotecnologia, CSIC, Darwin 3, 28049 Cantoblanco, Madrid, Spain
| | - Igor Konieczny
- Department of Molecular and Cellular Biology, Intercollegiate Faculty of Biotechnology, University of Gdansk, 24 Kladki, 80-822 Gdansk, Poland
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19
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Donczew R, Zakrzewska-Czerwińska J, Zawilak-Pawlik A. Beyond DnaA: the role of DNA topology and DNA methylation in bacterial replication initiation. J Mol Biol 2014; 426:2269-82. [PMID: 24747048 DOI: 10.1016/j.jmb.2014.04.009] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2014] [Revised: 04/10/2014] [Accepted: 04/11/2014] [Indexed: 12/31/2022]
Abstract
The replication of chromosomal DNA is a fundamental event in the life cycle of every cell. The first step of replication, initiation, is controlled by multiple factors to ensure only one round of replication per cell cycle. The process of initiation has been described most thoroughly for bacteria, especially Escherichia coli, and involves many regulatory proteins that vary considerably between different species. These proteins control the activity of the two key players of initiation in bacteria: the initiator protein DnaA and the origin of chromosome replication (oriC). Factors involved in the control of the availability, activity, or oligomerization of DnaA during initiation are generally regarded as the most important and thus have been thoroughly characterized. Other aspects of the initiation process, such as origin accessibility and susceptibility to unwinding, have been less explored. However, recent findings indicate that these factors have a significant role. This review focuses on DNA topology, conformation, and methylation as important factors that regulate the initiation process in bacteria. We present a comprehensive summary of the factors involved in the modulation of DNA topology, both locally at oriC and more globally at the level of the entire chromosome. We show clearly that the conformation of oriC dynamically changes, and control of this conformation constitutes another, important factor in the regulation of bacterial replication initiation. Furthermore, the process of initiation appears to be associated with the dynamics of the entire chromosome and this association is an important but largely unexplored phenomenon.
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Affiliation(s)
- Rafał Donczew
- Institute of Immunology and Experimental Therapy, Department of Microbiology, Polish Academy of Sciences, Weigla 12, 53-114 Wrocław, Poland.
| | - Jolanta Zakrzewska-Czerwińska
- Institute of Immunology and Experimental Therapy, Department of Microbiology, Polish Academy of Sciences, Weigla 12, 53-114 Wrocław, Poland; Faculty of Biotechnology, University of Wrocław, Joliot-Curie 14a, 50-138 Wrocław, Poland.
| | - Anna Zawilak-Pawlik
- Institute of Immunology and Experimental Therapy, Department of Microbiology, Polish Academy of Sciences, Weigla 12, 53-114 Wrocław, Poland.
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20
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Galburt EA, Tomko EJ, Stump WT, Ruiz Manzano A. Force-dependent melting of supercoiled DNA at thermophilic temperatures. Biophys Chem 2014; 187-188:23-8. [PMID: 24486433 DOI: 10.1016/j.bpc.2014.01.001] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2013] [Revised: 01/07/2014] [Accepted: 01/07/2014] [Indexed: 11/28/2022]
Abstract
Local DNA opening plays an important role in DNA metabolism as the double-helix must be melted before the information contained within may be accessed. Cells finely tune the torsional state of their genomes to strike a balance between stability and accessibility. For example, while mesophilic life forms maintain negatively superhelical genomes, thermophilic life forms use unique mechanisms to maintain relaxed or even positively supercoiled genomes. Here, we use a single-molecule magnetic tweezers approach to quantify the force-dependent equilibrium between DNA melting and supercoiling at high temperatures populated by Thermophiles. We show that negatively supercoiled DNA denatures at 0.5 pN lower tension at thermophilic vs. mesophilic temperatures. This work demonstrates the ability to monitor DNA supercoiling at high temperature and opens the possibility to perform magnetic tweezers assays on thermophilic systems. The data allow for an estimation of the relative energies of base-pairing and DNA bending as a function of temperature and support speculation as to different general mechanisms of DNA opening in different environments. Lastly, our results imply that average in vivo DNA tensions range between 0.3 and 1.1 pN.
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Affiliation(s)
- E A Galburt
- Department of Biochemistry and Molecular Biophysics, Washington University in Saint Louis, 660 South Euclid Avenue, Saint Louis, MO, 63110, USA
| | - E J Tomko
- Department of Biochemistry and Molecular Biophysics, Washington University in Saint Louis, 660 South Euclid Avenue, Saint Louis, MO, 63110, USA
| | - W T Stump
- Department of Biochemistry and Molecular Biophysics, Washington University in Saint Louis, 660 South Euclid Avenue, Saint Louis, MO, 63110, USA
| | - A Ruiz Manzano
- Department of Biochemistry and Molecular Biophysics, Washington University in Saint Louis, 660 South Euclid Avenue, Saint Louis, MO, 63110, USA
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21
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Bacterial replication, transcription and translation: mechanistic insights from single-molecule biochemical studies. Nat Rev Microbiol 2013; 11:303-15. [DOI: 10.1038/nrmicro2994] [Citation(s) in RCA: 56] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
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22
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Abstract
The initiation of DNA replication represents a committing step to cell proliferation. Appropriate replication onset depends on multiprotein complexes that help properly distinguish origin regions, generate nascent replication bubbles, and promote replisome formation. This review describes initiation systems employed by bacteria, archaea, and eukaryotes, with a focus on comparing and contrasting molecular mechanisms among organisms. Although commonalities can be found in the functional domains and strategies used to carry out and regulate initiation, many key participants have markedly different activities and appear to have evolved convergently. Despite significant advances in the field, major questions still persist in understanding how initiation programs are executed at the molecular level.
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Affiliation(s)
- Alessandro Costa
- Clare Hall Laboratories, London Research Institute, Cancer Research UK, Hertfordshire, EN6 3LD United Kingdom
| | - Iris V. Hood
- Department of Molecular and Cell Biology, California Institute for Quantitative Biosciences, University of California, Berkeley, California 94720
| | - James M. Berger
- Department of Molecular and Cell Biology, California Institute for Quantitative Biosciences, University of California, Berkeley, California 94720
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