Cas9 function and host genome sampling in Type II-A CRISPR-Cas adaptation.

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Prokaryotic CRISPR-Cas systems provide adaptive immunity against foreign genetic elements like viruses and plasmids.
• Adaptation involves capturing foreign DNA fragments into the host CRISPR locus for future defense.
• Type II-A CRISPR-Cas systems, like the one in Streptococcus thermophilus, are crucial for microbial defense.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying the adaptation process in a Type II-A CRISPR-Cas system.
• To identify the specific Cas proteins required for DNA acquisition during adaptation.
• To investigate the role of Cas9, a known nuclease, in the adaptation phase.

Main Methods:

• Genetic manipulation of Streptococcus thermophilus strains to assess the function of individual Cas proteins (Cas1, Cas2, Csn2, Cas9).
• Analysis of CRISPR locus content to determine DNA acquisition patterns.
• Assessment of Cas9 nuclease activity to differentiate its roles in adaptation versus cleavage.

Main Results:

• The adaptation process in Streptococcus thermophilus Type II-A CRISPR-Cas systems requires the Cas1, Cas2, and Csn2 proteins.
• Cas9 is unexpectedly essential for the adaptation process, in addition to its known role in target destruction.
• Cas9's nuclease activity is not required for adaptation, suggesting a non-catalytic function.
• Extensive and unbiased acquisition of self-targeting host genome sequences occurs, which is normally suppressed by active target destruction.

Conclusions:

• The adaptation phase of CRISPR-Cas immunity is a complex process involving multiple Cas proteins, including Cas9.
• Cas9 plays a dual role in CRISPR-Cas immunity: essential for adaptation and for target DNA cleavage.
• The CRISPR-Cas system can acquire self-DNA, a process masked by active defense mechanisms.