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Bacteria and archaea are susceptible to viral infections just like eukaryotes; therefore, they have developed a unique adaptive immune system to protect themselves. Clustered regularly interspaced short palindromic repeats and CRISPR-associated proteins (CRISPR-Cas) are present in more than 45% of known bacteria and 90% of known archaea.
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Genome editing technologies allow scientists to modify an organism’s DNA via the addition, removal, or rearrangement of genetic material at specific genomic locations. These types of techniques could potentially be used to cure genetic disorders such as hemophilia and sickle cell anemia. One popular and widely used DNA-editing research tool that could lead to safe and effective cures for genetic disorders is the CRISPR-Cas9 system. CRISPR-Cas9 stands for Clustered Regularly Interspaced...
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Classification and evolution of type II CRISPR-Cas systems.

Krzysztof Chylinski1, Kira S Makarova2, Emmanuelle Charpentier3

  • 1The Laboratory for Molecular Infection Medicine Sweden (MIMS), Umeå Centre for Microbial Research (UCMR), Department of Molecular Biology, Umeå University, Umeå 90187, Sweden Max F. Perutz Laboratories, University of Vienna, Vienna 1030, Austria.

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CRISPR-Cas type II systems, utilizing the Cas9 protein for DNA targeting, are rare in bacteria and absent in archaea. Their evolution involved acquiring genes from mobile elements and other CRISPR-Cas systems.

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Area of Science:

  • Microbiology
  • Genetics
  • Molecular Biology

Background:

  • CRISPR-Cas systems provide adaptive immunity in prokaryotes.
  • Type II CRISPR-Cas systems are characterized by the Cas9 endonuclease and a dual-tracrRNA:crRNA guide.

Purpose of the Study:

  • To conduct an updated census of CRISPR-Cas systems in bacterial and archaeal genomes.
  • To investigate the evolutionary origins of type II CRISPR-Cas systems.

Main Methods:

  • Bioinformatic analysis of bacterial and archaeal genomes.
  • Phylogenomic analysis of CRISPR-Cas associated genes.
  • Comparative genomics to identify homologs of Cas9.

Main Results:

  • Type II CRISPR-Cas systems are rare (approx. 5% of bacterial genomes) and absent in archaea.
  • Type II systems are enriched in bacterial pathogens and commensals.
  • Phylogenomic analysis indicates horizontal gene transfer and recombination events in the evolution of type II systems.

Conclusions:

  • Type II CRISPR-Cas systems evolved through recombination with mobile genetic elements and other CRISPR-Cas loci.
  • The unique characteristics of type II systems, particularly Cas9, make them valuable for genomic engineering applications.