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CRISPR stands for Clustered Regularly Interspaced Short Palindromic Repeats is a adaptive immune system found in bacteria and archaea that protects against viral infections. This system enables prokaryotic cells to identify, remember, and neutralize foreign genetic elements, primarily bacteriophages, by storing fragments of the invader’s DNA as a genetic memory.The CRISPR immune response begins during an initial infection. Cas (CRISPR-associated) proteins play a central role in this...
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END nucleases: Antiphage defense systems targeting multiple hypermodified phage genomes.

Wearn-Xin Yee1, Yan-Jiun Lee2, Timothy A Klein1

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Researchers identified a novel bacterial defense system, END nucleases, that targets phages with modified DNA. Inhibitors of this system are crucial for certain phages to infect clinical bacterial strains.

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Area of Science:

  • Bacteriology and Virology
  • Molecular Biology
  • Genetics and Genomics

Background:

  • Prokaryotic defense islands contain diverse phage defense systems.
  • Identifying specific systems limiting phage reproduction in clinical strains is crucial.
  • CRISPR-based systems offer tools for genetic manipulation of defense islands.

Purpose of the Study:

  • To identify specific defense systems within bacterial defense islands that inhibit lytic phage reproduction.
  • To characterize a novel Type IIS restriction endonuclease-like protein (END^PaCF1) from *Pseudomonas aeruginosa*.
  • To investigate the mechanism of action and phage-encoded inhibitors of this novel defense system.

Main Methods:

  • Utilized CRISPR-Cas3 system to delete defense islands in a *Pseudomonas aeruginosa* clinical isolate.
  • Assessed bacterial sensitization to lytic phages after defense island deletion.
  • Characterized the END^PaCF1 protein, its DNA modification sensing, and interaction with phage inhibitors.

Main Results:

  • Deletion of a specific defense island sensitized the clinical isolate to *Pbunavirus* phages.
  • Identified a novel Type IIS restriction endonuclease-like protein (END^PaCF1) lacking a methyltransferase, protecting against phages with hypermodified DNA.
  • Discovered that END nucleases possess modular sensing (iEndoIII domain) and cleavage domains, and are inhibited by phage-encoded proteins.

Conclusions:

  • The END nuclease system, featuring a fused iEndoIII domain, provides broad protection against hypermodified phage DNA.
  • Phages encode inhibitors targeting the iEndoIII domain of END nucleases, essential for successful infection.
  • This study reveals a novel bacterial defense mechanism and its counter-defense strategies by phages, impacting phage therapy development.