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The Antiviral System of Bacteria and Archaea: CRISPR01:23

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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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Foreign DNA capture during CRISPR-Cas adaptive immunity.

James K Nuñez1, Lucas B Harrington1, Philip J Kranzusch1,2

  • 1Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720, USA.

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Summary

Bacteria and archaea use CRISPR immunity to defend against viruses by integrating foreign DNA. Researchers revealed how the Cas1-Cas2 enzyme complex captures DNA, acting as a molecular ruler for CRISPR locus construction.

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Area of Science:

  • Molecular Biology
  • Microbial Genetics
  • Structural Biology

Background:

  • Bacteria and archaea possess adaptive immune systems against phages and plasmids.
  • This immunity involves integrating foreign DNA fragments (spacers) into CRISPR loci.
  • The Cas1-Cas2 integrase complex is crucial for acquiring these spacers.

Purpose of the Study:

  • To elucidate the mechanism by which the Cas1-Cas2 complex selects foreign DNA substrates.
  • To understand the structural basis of spacer acquisition in CRISPR immunity.
  • To reveal how Cas1-Cas2 functions as a molecular ruler in CRISPR locus formation.

Main Methods:

  • X-ray crystallography was used to determine the structure of the Escherichia coli Cas1-Cas2 complex.
  • The study involved analyzing the complex bound to cognate 33-nucleotide protospacer DNA substrates.
  • Structural data was interpreted to understand protein-DNA interactions and substrate selection.

Main Results:

  • The Cas1-Cas2 complex forms a curved binding surface that spans the entire length of the protospacer DNA.
  • The complex splays the DNA ends, positioning the terminal 3'-OH groups for nucleophilic attack within Cas1 active sites.
  • Phosphodiester backbone interactions between the DNA and the protein complex explain the observed sequence-nonspecific substrate selection.

Conclusions:

  • The study reveals the structural basis for foreign DNA capture by the Cas1-Cas2 complex.
  • It uncovers the mechanism by which Cas1-Cas2 acts as a molecular ruler, dictating the size and architecture of CRISPR loci.
  • These findings provide critical insights into the fundamental process of adaptive immunity in prokaryotes.