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Dimude JU, Midgley-Smith SL, Stein M, Rudolph CJ. Replication Termination: Containing Fork Fusion-Mediated Pathologies in Escherichia coli. Genes (Basel) 2016; 7:genes7080040. [PMID: 27463728 PMCID: PMC4999828 DOI: 10.3390/genes7080040] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2016] [Revised: 07/12/2016] [Accepted: 07/19/2016] [Indexed: 01/18/2023] Open
Abstract
Duplication of bacterial chromosomes is initiated via the assembly of two replication forks at a single defined origin. Forks proceed bi-directionally until they fuse in a specialised termination area opposite the origin. This area is flanked by polar replication fork pause sites that allow forks to enter but not to leave. The precise function of this replication fork trap has remained enigmatic, as no obvious phenotypes have been associated with its inactivation. However, the fork trap becomes a serious problem to cells if the second fork is stalled at an impediment, as replication cannot be completed, suggesting that a significant evolutionary advantage for maintaining this chromosomal arrangement must exist. Recently, we demonstrated that head-on fusion of replication forks can trigger over-replication of the chromosome. This over-replication is normally prevented by a number of proteins including RecG helicase and 3’ exonucleases. However, even in the absence of these proteins it can be safely contained within the replication fork trap, highlighting that multiple systems might be involved in coordinating replication fork fusions. Here, we discuss whether considering the problems associated with head-on replication fork fusion events helps us to better understand the important role of the replication fork trap in cellular metabolism.
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Affiliation(s)
- Juachi U Dimude
- Division of Biosciences, College of Health and Life Sciences, Brunel University London, Uxbridge UB8 3PH, UK.
| | - Sarah L Midgley-Smith
- Division of Biosciences, College of Health and Life Sciences, Brunel University London, Uxbridge UB8 3PH, UK.
| | - Monja Stein
- Division of Biosciences, College of Health and Life Sciences, Brunel University London, Uxbridge UB8 3PH, UK.
| | - Christian J Rudolph
- Division of Biosciences, College of Health and Life Sciences, Brunel University London, Uxbridge UB8 3PH, UK.
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2
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Pallejà A, Guzman E, Garcia-Vallvé S, Romeu A. In silico prediction of the origin of replication among bacteria: a case study of Bacteroides thetaiotaomicron. OMICS-A JOURNAL OF INTEGRATIVE BIOLOGY 2008; 12:201-10. [PMID: 18582175 DOI: 10.1089/omi.2008.0004] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The initiation of chromosomal replication occurs only once during the prokaryote cell cycle. Some origins of replication have been experimentally determined and have led to the development of in silico approaches to find the origin of replication among other prokaryotes. DNA base composition asymmetry is the basis of numerous in silico methods used to detect the origin and terminus of replication in prokaryotes. However, the composition asymmetry does not allow us to locate precisely the positions of the origin and terminus. Since DNA replication is a key step in the cell cycle it is important to determine properly the origin and terminus regions. Therefore, we have reviewed here the methods, tools, and databases for predicting the origins and terminuses of replication, and we have proposed some complementary analyses to reinforce these predictions. These analyses include finding the dnaA gene and its binding sites; making BLAST analyses of the intergenic sequences compared to related species; studying the gene order around the origin sequence; and studying the distribution of the genes encoded in the leading versus the lagging strand.
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Affiliation(s)
- Albert Pallejà
- Department of Biochemistry and Biotechnology, Evolutionary Genomics Group, Rovira i Virgili University, Tarragona, Catalunya, Spain.
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3
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Duggin IG, Wake RG, Bell SD, Hill TM. The replication fork trap and termination of chromosome replication. Mol Microbiol 2008; 70:1323-33. [PMID: 19019156 DOI: 10.1111/j.1365-2958.2008.06500.x] [Citation(s) in RCA: 82] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Bacteria that have a circular chromosome with a bidirectional DNA replication origin are thought to utilize a 'replication fork trap' to control termination of replication. The fork trap is an arrangement of replication pause sites that ensures that the two replication forks fuse within the terminus region of the chromosome, approximately opposite the origin on the circular map. However, the biological significance of the replication fork trap has been mysterious, as its inactivation has no obvious consequence. Here we review the research that led to the replication fork trap theory, and we aim to integrate several recent findings that contribute towards an understanding of the physiological roles of the replication fork trap. Likely roles include the prevention of over-replication, and the optimization of post-replicative mechanisms of chromosome segregation, such as that involving FtsK in Escherichia coli.
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Affiliation(s)
- Iain G Duggin
- Sir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, UK.
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4
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Hendrickson H, Lawrence JG. Mutational bias suggests that replication termination occurs near the dif site, not at Ter sites. Mol Microbiol 2007; 64:42-56. [PMID: 17376071 DOI: 10.1111/j.1365-2958.2007.05596.x] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
In bacteria, Ter sites bound to Tus/Rtp proteins halt replication forks moving only in one direction, providing a convenient mechanism to terminate them once the chromosome had been replicated. Considering the importance of replication termination and its position as a checkpoint in cell division, the accumulated knowledge on these systems has not dispelled fundamental questions regarding its role in cell biology: why are there so many copies of Ter, why are they distributed over such a large portion of the chromosome, why is the tus gene not conserved among bacteria, and why do tus mutants lack measurable phenotypes? Here we examine bacterial genomes using bioinformatics techniques to identify the region(s) where DNA polymerase III-mediated replication has historically been terminated. We find that in both Escherichia coli and Bacillus subtilis, changes in mutational bias patterns indicate that replication termination most likely occurs at or near the dif site. More importantly, there is no evidence from mutational bias signatures that replication forks originating at oriC have terminated at Ter sites. We propose that Ter sites participate in halting replication forks originating from DNA repair events, and not those originating at the chromosomal origin of replication.
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Affiliation(s)
- Heather Hendrickson
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260, USA
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5
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Neylon C, Kralicek AV, Hill TM, Dixon NE. Replication termination in Escherichia coli: structure and antihelicase activity of the Tus-Ter complex. Microbiol Mol Biol Rev 2005; 69:501-26. [PMID: 16148308 PMCID: PMC1197808 DOI: 10.1128/mmbr.69.3.501-526.2005] [Citation(s) in RCA: 105] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The arrest of DNA replication in Escherichia coli is triggered by the encounter of a replisome with a Tus protein-Ter DNA complex. A replication fork can pass through a Tus-Ter complex when traveling in one direction but not the other, and the chromosomal Ter sites are oriented so replication forks can enter, but not exit, the terminus region. The Tus-Ter complex acts by blocking the action of the replicative DnaB helicase, but details of the mechanism are uncertain. One proposed mechanism involves a specific interaction between Tus-Ter and the helicase that prevents further DNA unwinding, while another is that the Tus-Ter complex itself is sufficient to block the helicase in a polar manner, without the need for specific protein-protein interactions. This review integrates three decades of experimental information on the action of the Tus-Ter complex with information available from the Tus-TerA crystal structure. We conclude that while it is possible to explain polar fork arrest by a mechanism involving only the Tus-Ter interaction, there are also strong indications of a role for specific Tus-DnaB interactions. The evidence suggests, therefore, that the termination system is more subtle and complex than may have been assumed. We describe some further experiments and insights that may assist in unraveling the details of this fascinating process.
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Affiliation(s)
- Cameron Neylon
- School of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom.
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6
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Maisnier-Patin S, Nordström K, Dasgupta S. RecA-mediated rescue of Escherichia coli strains with replication forks arrested at the terminus. J Bacteriol 2001; 183:6065-73. [PMID: 11567007 PMCID: PMC99686 DOI: 10.1128/jb.183.20.6065-6073.2001] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2001] [Accepted: 07/20/2001] [Indexed: 11/20/2022] Open
Abstract
The recombinational rescue of chromosome replication was investigated in Escherichia coli strains with the unidirectional origin oriR1, from the plasmid R1, integrated within oriC in clockwise (intR1(CW)) or counterclockwise (intR1(CC)) orientations. Only the intR1(CC) strain, with replication forks arrested at the terminus, required RecA for survival. Unlike the strains with RecA-dependent replication known so far, the intR1(CC) strain did not require RecBCD, RecF, RecG, RecJ, RuvAB, or SOS activation for viability. The overall levels of degradation of replicating chromosomes caused by inactivation of RecA were similar in oriC and intR1(CC) strains. In the intR1(CC) strain, RecA was also needed to maintain the integrity of the chromosome when the unidirectional replication forks were blocked at the terminus. This was consistent with suppression of the RecA dependence of the intR1(CC) strain by inactivating Tus, the protein needed to block replication forks at Ter sites. Thus, RecA is essential during asymmetric chromosome replication for the stable maintenance of the forks arrested at the terminus and for their eventual passage across the termination barrier(s) independently of the SOS and some of the major recombination pathways.
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Affiliation(s)
- S Maisnier-Patin
- Department of Cell and Molecular Biology, Biomedical Centre, Uppsala University, S-751 24 Uppsala, Sweden
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7
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Peters JE, Craig NL. Tn7 transposes proximal to DNA double-strand breaks and into regions where chromosomal DNA replication terminates. Mol Cell 2000; 6:573-82. [PMID: 11030337 DOI: 10.1016/s1097-2765(00)00056-3] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
We report that the bacterial transposon Tn7 can preferentially transpose into regions where chromosomal DNA replication terminates. DNA double-strand breaks are associated with the termination of chromosomal replication; therefore, we directly tested the effect of DNA breaks on Tn7 transposition. When DNA double-strand breaks are induced at specific sites in the chromosome, Tn7 transposition is stimulated and insertions are directed proximal to the induced break. The targeting preference for the terminus of replication and DNA double-strand breaks is dependent on the Tn7-encoded protein TnsE. The results presented in this study could also explain the previous observation that Tn7 is attracted to events associated with conjugal DNA replication during plasmid DNA transfer.
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Affiliation(s)
- J E Peters
- Howard Hughes Medical Institute, Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
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8
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Abstract
This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.
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Affiliation(s)
- M K Berlyn
- Department of Biology and School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06520-8104, USA.
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9
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Mohanty BK, Sahoo T, Bastia D. The relationship between sequence-specific termination of DNA replication and transcription. EMBO J 1996; 15:2530-9. [PMID: 8665860 PMCID: PMC450185] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
In Escherichia coli and Bacillus subtilis replication fork arrest occurs in the terminus at sequence-specific sites by the binding of replication terminator proteins to the fork arrest sites. The protein-DNA complex causes polar arrest of the replication forks by inhibiting the activity of the replicative helicases in only one orientation of the terminus with respect to the replication origin. This activity has been named as polar contrahelicase. In this paper we report on a second novel activity of the terminator proteins of E.coli and B.subtilis, namely the ability of the proteins to block RNA chain elongation by several prokaryotic RNA polymerases in a polar mode. The replication terminator proteins ter and RTP of E.coli and B.subtilis respectively, impeded RNA chain elongation catalyzed by T7, SP6 and E.coli RNA polymerases in a polar mode at the replication arrest sites. The RNA chain anti-elongation and the contrahelicase activities were isopolar. Whereas one monomer of ter was necessary and sufficient to block RNA chain elongation, two interacting dimers of RTP were needed to effect the same blockage. The biological significance of the RNA chain anti-elongation activity is manifested in the functional inactivation of a replication arrest site by invasion of RNA chains from outside, and the consequent need to preserve replication arrest activity by restricting the passage of transcription through the terminus-terminator protein complex.
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Affiliation(s)
- B K Mohanty
- Department of Microbiology, Duke University Medical Centre, Durham, NC 27710, USA
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10
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Sharma B, Hill TM. TerF, the sixth identified replication arrest site in Escherichia coli, is located within the rcsC gene. J Bacteriol 1992; 174:7854-8. [PMID: 1447156 PMCID: PMC207506 DOI: 10.1128/jb.174.23.7854-7858.1992] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
We report the existence of a sixth replication arrest site, TerF, that is located within the coding sequences of the rcsC gene, a negative regulator of capsule biosynthesis. The TerF site is oriented to allow transcription of the rcsC gene but prevent DNA replication in the terminus-to-origin direction. Our results demonstrate that the TerF site is functional in both chromosomal and plasmid environments and that the stability of the Tus-TerF protein-DNA complex more closely resembles the plasmid R6K Ter sites than the chromosomal TerB site.
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Affiliation(s)
- B Sharma
- Department of Bioscience and Biotechnology, Drexel University, Philadelphia, Pennsylvania 19104
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11
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Zhou HS, Byrd C, Meyer RJ. Probing the activation of the replicative origin of broad host-range plasmid R1162 with Tus, the E.coli anti-helicase protein. Nucleic Acids Res 1991; 19:5379-83. [PMID: 1923822 PMCID: PMC328902 DOI: 10.1093/nar/19.19.5379] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
The E.coli Tus protein is an anti-helicase involved in the termination of chromosome replication. The binding site for this protein, ter, was cloned into derivatives of the broad host-range plasmid R1162. The ter site caused the orientation-specific termination of plasmid replication fork movement in cell extracts containing Tus. Plasmids were constructed so that two sites for initiation of R1162 replication flanked the iteron-containing domain of the origin. In these plasmids, the site next to the AT-rich region within the iteron-containing domain was more active. In addition, when ter was placed between the more active site and the iterons, initiation of replication from this site was specifically inhibited. The data support a model for entry of the essential, plasmid-encoded helicase at one side of the direct repeats, and for its movement primarily in one direction away from these repeats to activate the initiation sites for DNA replication.
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Affiliation(s)
- H S Zhou
- Department of Microbiology, University of Texas, Austin 78712
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12
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Abstract
Recent progress in studies on the bacterial chromosome is summarized. Although the greatest amount of information comes from studies on Escherichia coli, reports on studies of many other bacteria are also included. A compilation of the sizes of chromosomal DNAs as determined by pulsed-field electrophoresis is given, as well as a discussion of factors that affect gene dosage, including redundancy of chromosomes on the one hand and inactivation of chromosomes on the other hand. The distinction between a large plasmid and a second chromosome is discussed. Recent information on repeated sequences and chromosomal rearrangements is presented. The growing understanding of limitations on the rearrangements that can be tolerated by bacteria and those that cannot is summarized, and the sensitive region flanking the terminator loci is described. Sources and types of genetic variation in bacteria are listed, from simple single nucleotide mutations to intragenic and intergenic recombinations. A model depicting the dynamics of the evolution and genetic activity of the bacterial chromosome is described which entails acquisition by recombination of clonal segments within the chromosome. The model is consistent with the existence of only a few genetic types of E. coli worldwide. Finally, there is a summary of recent reports on lateral genetic exchange across great taxonomic distances, yet another source of genetic variation and innovation.
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Affiliation(s)
- S Krawiec
- Department of Biology, Lehigh University, Bethlehem, Pennsylvania 18015
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13
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Lee EH, Kornberg A, Hidaka M, Kobayashi T, Horiuchi T. Escherichia coli replication termination protein impedes the action of helicases. Proc Natl Acad Sci U S A 1989; 86:9104-8. [PMID: 2556700 PMCID: PMC298442 DOI: 10.1073/pnas.86.23.9104] [Citation(s) in RCA: 122] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Identification of the consensus sequence for termination of replication (ter) in Escherichia coli and the isolation of the ter-binding protein (TBP) allowed us to test their effects on replication forks initiated at the unique origin of the E. coli chromosome (oriC) in a purified enzyme system. Replication was severely impeded by ter in a unique orientation when purified TBP was supplied to bind it. The target for blockage within the replication complex can now be ascribed to the inability of dnaB helicase to separate the duplex strands when it encounters ter bound by TBP. Other helicases, such as rep and uvrD proteins, that translocate on DNA and displace strands in the direction opposite to that of dnaB protein are also blocked, but only when the TBP-bound ter is oriented in the other direction. From these results, we infer that the orientation of ter confers a particular polarity on the TBP seated on it, such that a helicase is blocked when it confronts TBP from one side, but can act, presumably by displacing TBP, when facing its other side. Thus, the intrinsic nature of the oriented TBP-ter complex is responsible for impeding the helicases, rather than any protein-protein interactions.
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Affiliation(s)
- E H Lee
- Department of Biochemistry, Stanford University, CA 94305-5307
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14
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Affiliation(s)
- P L Kuempel
- Molecular, Cellular, and Developmental Biology Department, University of Colorado, Boulder 80309
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15
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François V, Louarn J, Louarn JM. The terminus of the Escherichia coli chromosome is flanked by several polar replication pause sites. Mol Microbiol 1989; 3:995-1002. [PMID: 2532703 DOI: 10.1111/j.1365-2958.1989.tb00250.x] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Replication of two small 'constrained' regions of the Escherichia coli chromosome, one bordered by replication terminator T1 and the other by T2, displays normal velocity in the normal direction whereas it is much slower in the opposite direction (de Massy et al., 1987). The presence of multiple polar terminators has been investigated, using a bacteriophage lambda derivative which provides a replication origin movable to predetermined loci and inducible on demand. The amount of DNA made from this induced origin was determined by in vivo labelling and hybridization to probes of the surrounding region. A redundancy of terminator-like sequences, or pause sites, has been disclosed. So far, two polar pause sites, in the same orientation and separated by 50 or 80 kb, have been localized on each side of the terminus region. The results are discussed in relation to previously observations indicating that these regions are refractory to genomic inversions.
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Affiliation(s)
- V François
- Centre de Biochimie et de Génétique Cellulaires du CNRS, Toulouse, France
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16
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Affiliation(s)
- M Masters
- Department of Molecular Biology, University of Edinburgh, UK
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17
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Hill TM, Tecklenburg ML, Pelletier AJ, Kuempel PL. tus, the trans-acting gene required for termination of DNA replication in Escherichia coli, encodes a DNA-binding protein. Proc Natl Acad Sci U S A 1989; 86:1593-7. [PMID: 2646639 PMCID: PMC286744 DOI: 10.1073/pnas.86.5.1593] [Citation(s) in RCA: 85] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
The components for termination of DNA replication in Escherichia coli include the terminator signals T1 and T2 and the trans-acting gene tus. We have shown previously that tus maps in a 4-kilobase region of the chromosomal terminus near T2. Through the use of deletion and insertion mutants, the location of the tus gene has now been precisely identified. We sequenced 2416 nucleotides in this region and identified a 927-base-pair open reading frame which encodes Tus. Insertion of a kanamycin-resistance gene in this open reading frame abolished tus activity. We also demonstrated that crude extracts of tus+ cells contain a protein which binds to the T2 terminator sequence.
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Affiliation(s)
- T M Hill
- Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder 80309-0347
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18
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Pelletier AJ, Hill TM, Kuempel PL. Termination sites T1 and T2 from the Escherichia coli chromosome inhibit DNA replication in ColE1-derived plasmids. J Bacteriol 1989; 171:1739-41. [PMID: 2646296 PMCID: PMC209807 DOI: 10.1128/jb.171.3.1739-1741.1989] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Inhibition sites T1 and T2 from the Escherichia coli terminus functioned with the same characteristics in ColE1-derived plasmids and in the chromosome. These characteristics included polarity and dependence on tus, a trans-acting factor required for inhibition. Inhibition in the terminus region of the R6K plasmid was also tus dependent.
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Affiliation(s)
- A J Pelletier
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder 80309-0347
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19
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Hill TM, Pelletier AJ, Tecklenburg ML, Kuempel PL. Identification of the DNA sequence from the E. coli terminus region that halts replication forks. Cell 1988; 55:459-66. [PMID: 2846183 DOI: 10.1016/0092-8674(88)90032-3] [Citation(s) in RCA: 111] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
The terminus region of the E. coli chromosome contains two loci, T1 and T2, that inhibit the progress of replication forks and require the trans-acting factor tus. We have identified a 23 bp terminator signal at T1 and T2 that is within 100 bp of the sites of replication arrest. When an oligodeoxyribonucleotide containing the terminator signal was inserted into a plasmid, replication was halted only in a tus+ strain and when the terminator signal was oriented properly. We also found this terminator sequence in the terminus region of the plasmid R6K and in the origin region of RepFIIA class plasmids. In addition, we found striking similarities between the E. coli terminator signal and the terminator sequence of B. subtilis.
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Affiliation(s)
- T M Hill
- Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder 80309
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