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Mechanisms of Theta Plasmid Replication

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  • Authors: Joshua Lilly1, Manel Camps2
  • Editors: Marcelo Tolmasky3, Juan Carlos Alonso4
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    Affiliations: 1: Department of Microbiology and Environmental Toxicology, University of California Santa Cruz, Santa Cruz, CA 95064; 2: Department of Microbiology and Environmental Toxicology, University of California Santa Cruz, Santa Cruz, CA 95064; 3: California State University, Fullerton, CA; 4: Centro Nacional de Biotecnología, Cantoblanco, Madrid, Spain
    • Source: microbiolspec January 2015 vol. 3 no. 1 doi:10.1128/microbiolspec.PLAS-0029-2014
  • Received 11 November 2014 Accepted 11 November 2014 Published 16 January 2015
  • Manel Camps, [email protected]
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  • Abstract:

    Plasmids are autonomously replicating pieces of DNA. This article discusses theta plasmid replication, which is a class of circular plasmid replication that includes ColE1-like origins of replication popular with expression vectors. All modalities of theta plasmid replication initiate synthesis with the leading strand at a predetermined site and complete replication through recruitment of the host's replisome, which extends the leading strand continuously while synthesizing the lagging strand discontinuously. There are clear differences between different modalities of theta plasmid replication in mechanisms of DNA duplex melting and in priming of leading- and lagging-strand synthesis. In some replicons duplex melting depends on transcription, while other replicons rely on plasmid-encoded -acting proteins (Reps); primers for leading-strand synthesis can be generated through processing of a transcript or in other replicons by the action of host- or plasmid-encoded primases. None of these processes require DNA breaks. The frequency of replication initiation is tightly regulated to facilitate establishment in permissive hosts and to achieve a steady state. The last section of the article reviews how plasmid copy number is sensed and how this feedback modulates the frequency of replication.

  • Citation: Lilly J, Camps M. 2015. Mechanisms of Theta Plasmid Replication. Microbiol Spectrum 3(1):PLAS-0029-2014. doi:10.1128/microbiolspec.PLAS-0029-2014.

References

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2015-01-16
2020-07-12

Abstract:

Plasmids are autonomously replicating pieces of DNA. This article discusses theta plasmid replication, which is a class of circular plasmid replication that includes ColE1-like origins of replication popular with expression vectors. All modalities of theta plasmid replication initiate synthesis with the leading strand at a predetermined site and complete replication through recruitment of the host's replisome, which extends the leading strand continuously while synthesizing the lagging strand discontinuously. There are clear differences between different modalities of theta plasmid replication in mechanisms of DNA duplex melting and in priming of leading- and lagging-strand synthesis. In some replicons duplex melting depends on transcription, while other replicons rely on plasmid-encoded -acting proteins (Reps); primers for leading-strand synthesis can be generated through processing of a transcript or in other replicons by the action of host- or plasmid-encoded primases. None of these processes require DNA breaks. The frequency of replication initiation is tightly regulated to facilitate establishment in permissive hosts and to achieve a steady state. The last section of the article reviews how plasmid copy number is sensed and how this feedback modulates the frequency of replication.

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Mechanisms of Theta Plasmid Replication
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Citation: Lilly J, Camps M. 2015. Mechanisms of theta plasmid replication. microbiolspec 3(1): doi:10.1128/microbiolspec.PLAS-0029-2014
/content/microbiolspec/10.1128/microbiolspec.PLAS-0029-2014.fig1
FIGURE 1
Model of plasmid replication by the strand-displacement mechanism. (I) Parental DNA duplex (solid black lines) depicting the two single-stranded replication initiation sites, (light gray box) and (dark gray box). Vertical lines show hybridization between DNA strands. (II) The DNA duplex is melted through binding of RepC (possibly in concert with the RepA helicase), allowing the two sites to form hairpins (ball and stick). (III) The base of the hairpin is recognized by RepB′, which initiates the synthesis of an RNA primer (light gray dashed line). Extension of the free 3′-OH of the primer by Pol III (assisted by the RepA helicase) is shown as dashed black arrows. Two D-loops are formed, one for each direction of synthesis, as parental strands are displaced and dissociate from each other, leaving ssDNA intermediates. This is shown as areas where one of the strands has no hydrogen bonding. (IV) Synthesis continues in both directions, extending the area of D-loop formation. (V) Elongation is completed and termination of replication occurs on both strands at the sites in which replication began. At this point, the sites on the newly synthesized daughter strands are restored. (VI) Segregation: the two daughter strands are ligated, resulting in two DNA duplexes, each containing a parental strand (solid black line) and daughter strand (dashed black line).
microbiolspec/3/1/PLAS-0029-2014-fig1_thmb.gif
microbiolspec/3/1/PLAS-0029-2014-fig1.gif
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FIGURE 1

Model of plasmid replication by the strand-displacement mechanism. (I) Parental DNA duplex (solid black lines) depicting the two single-stranded replication initiation sites, (light gray box) and (dark gray box). Vertical lines show hybridization between DNA strands. (II) The DNA duplex is melted through binding of RepC (possibly in concert with the RepA helicase), allowing the two sites to form hairpins (ball and stick). (III) The base of the hairpin is recognized by RepB′, which initiates the synthesis of an RNA primer (light gray dashed line). Extension of the free 3′-OH of the primer by Pol III (assisted by the RepA helicase) is shown as dashed black arrows. Two D-loops are formed, one for each direction of synthesis, as parental strands are displaced and dissociate from each other, leaving ssDNA intermediates. This is shown as areas where one of the strands has no hydrogen bonding. (IV) Synthesis continues in both directions, extending the area of D-loop formation. (V) Elongation is completed and termination of replication occurs on both strands at the sites in which replication began. At this point, the sites on the newly synthesized daughter strands are restored. (VI) Segregation: the two daughter strands are ligated, resulting in two DNA duplexes, each containing a parental strand (solid black line) and daughter strand (dashed black line).

Source: microbiolspec January 2015 vol. 3 no. 1 doi:10.1128/microbiolspec.PLAS-0029-2014
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Mechanisms of Theta Plasmid Replication
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Citation: Lilly J, Camps M. 2015. Mechanisms of theta plasmid replication. microbiolspec 3(1): doi:10.1128/microbiolspec.PLAS-0029-2014
/content/microbiolspec/10.1128/microbiolspec.PLAS-0029-2014.T1
TABLE 1
Comparison of the three basic modes of plasmid replication initiation in circular plasmids
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TABLE 1

Comparison of the three basic modes of plasmid replication initiation in circular plasmids

Source: microbiolspec January 2015 vol. 3 no. 1 doi:10.1128/microbiolspec.PLAS-0029-2014

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