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The CRISPR-Cas Immune System and Genetic Transfers: Reaching an Equilibrium

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  • Authors: Julie E. Samson1, Alfonso H. Magadan2, Sylvain Moineau3
  • Editors: Marcelo Tolmasky4, Juan Carlos Alonso5
  • VIEW AFFILIATIONS HIDE AFFILIATIONS
    Affiliations: 1: Département de Biochimie, Microbiologie et Bio-Informatique, Faculté des Sciences et de Génie, Groupe de Recherche en Écologie Buccale, Félix d'Hérelle Reference Center for Bacterial Viruses, Faculté de Médecine Dentaire, Université Laval, Québec City, Québec, G1V 0A6, Canada; 2: Département de Biochimie, Microbiologie et Bio-Informatique, Faculté des Sciences et de Génie, Groupe de Recherche en Écologie Buccale, Félix d'Hérelle Reference Center for Bacterial Viruses, Faculté de Médecine Dentaire, Université Laval, Québec City, Québec, G1V 0A6, Canada; 3: Département de Biochimie, Microbiologie et Bio-Informatique, Faculté des Sciences et de Génie, Groupe de Recherche en Écologie Buccale, Félix d'Hérelle Reference Center for Bacterial Viruses, Faculté de Médecine Dentaire, Université Laval, Québec City, Québec, G1V 0A6, Canada; 4: California State University, Fullerton, CA; 5: Centro Nacional de Biotecnología, Cantoblanco, Madrid, Spain
  • Source: microbiolspec February 2015 vol. 3 no. 1 doi:10.1128/microbiolspec.PLAS-0034-2014
  • Received 25 November 2014 Accepted 25 November 2014 Published 20 February 2015
  • Sylvain Moineau, Sylvain.Moineau@bcm.ulaval.ca
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  • Abstract:

    Horizontal gene transfer drives the evolution of bacterial genomes, including the adaptation to changing environmental conditions. Exogenous DNA can enter a bacterial cell through transformation (free DNA or plasmids) or through the transfer of mobile genetic elements by conjugation (plasmids) and transduction (bacteriophages). Favorable genes can be acquired, but undesirable traits can also be inadvertently acquired through these processes. Bacteria have systems, such as clustered regularly interspaced short palindromic repeat CRISPR–associated genes (CRISPR-Cas), that can cleave foreign nucleic acid molecules. In this review, we discuss recent advances in understanding CRISPR-Cas system activity against mobile genetic element transfer through transformation and conjugation. We also highlight how CRISPR-Cas systems influence bacterial evolution and how CRISPR-Cas components affect plasmid replication.

  • Citation: Samson J, Magadan A, Moineau S. 2015. The CRISPR-Cas Immune System and Genetic Transfers: Reaching an Equilibrium. Microbiol Spectrum 3(1):PLAS-0034-2014. doi:10.1128/microbiolspec.PLAS-0034-2014.

Key Concept Ranking

Mobile Genetic Elements
1.1692212
Bacteria and Archaea
0.80849576
Conjugative Plasmids
0.49763384
1.1692212

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2015-02-20
2017-11-19

Abstract:

Horizontal gene transfer drives the evolution of bacterial genomes, including the adaptation to changing environmental conditions. Exogenous DNA can enter a bacterial cell through transformation (free DNA or plasmids) or through the transfer of mobile genetic elements by conjugation (plasmids) and transduction (bacteriophages). Favorable genes can be acquired, but undesirable traits can also be inadvertently acquired through these processes. Bacteria have systems, such as clustered regularly interspaced short palindromic repeat CRISPR–associated genes (CRISPR-Cas), that can cleave foreign nucleic acid molecules. In this review, we discuss recent advances in understanding CRISPR-Cas system activity against mobile genetic element transfer through transformation and conjugation. We also highlight how CRISPR-Cas systems influence bacterial evolution and how CRISPR-Cas components affect plasmid replication.

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FIGURE 1

Genetic organization of a type II CRISPR-Cas system and its general steps of action. The CRISPR locus is composed of repeats (black diamonds) interspaced with spacers (red and white rectangles) of similar length. In the vicinity of the CRISPR array, genes (colored arrows) are coding for proteins necessary for the immunity process. During the adaptation step, a repeat and, most importantly, a new spacer (red rectangle) is acquired in the CRISPR locus, usually at the 5′ region. Transcription of the CRISPR locus leads to pre-crRNAs that are processed, leading to short crRNAs. These crRNAs are assembled with Cas protein(s) in ribonucleoprotein complexes that act as surveillance guides looking for matching invading sequences. If the crRNA sequence matches a protospacer found on the foreign and invading nucleic acid molecule and if a PAM (gray box) is present next to the protospacer (for type I and II systems), this leads to the cleavage of the invading molecule (interference step). doi:10.1128/microbiolspec.PLAS-0034-2014.f1

Source: microbiolspec February 2015 vol. 3 no. 1 doi:10.1128/microbiolspec.PLAS-0034-2014
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Image of FIGURE 2

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FIGURE 2

Probable action of CRISPR-Cas systems against invading plasmids. (A) Plasmids entering a bacterial cell via natural transformation (free DNA), conjugation (pilus not represented), and transduction (phagemids). Of note, to date, no CRISPR-Cas system has been identified that cleaves ssDNA molecules . However, after their entry in the bacteria, ssDNA are usually transformed into double-stranded DNA molecules (dsDNA) and maintained as plasmids or integrated within the chromosome. If the new dsDNA molecule contains a protospacer matching a crRNA sequence, it will be cleaved by the CRISPR-Cas machinery in a sequence-specific manner. (B) During artificial transformation of the bacterium with heat treatments or electroporation, dsDNA directly enters the cells. Thus, the CRISPR-Cas ribonucleoprotein complexes can directly target these dsDNA molecules to eliminate them. (C) Some CRISPR-Cas systems (type III) cleave RNA molecules. After transcription of plasmid genes, these molecules are silenced by the CRISPR-Cas system, and after a few rounds of bacterial replication, these plasmids may be lost. doi:10.1128/microbiolspec.PLAS-0034-2014.f2

Source: microbiolspec February 2015 vol. 3 no. 1 doi:10.1128/microbiolspec.PLAS-0034-2014
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