CRISPR type II-A subgroups exhibit phylogenetically distinct mechanisms for prespacer insertion

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• CRISPR-Cas systems provide prokaryotes adaptive immunity against foreign nucleic acids.
• Immunity is achieved by integrating foreign DNA fragments (prespacers) into the CRISPR array.
• Type II-A CRISPR systems, involving Cas1 and Cas2 proteins, insert prespacers at the leader-repeat junction.

Purpose of the Study:

• To elucidate the mechanisms of prespacer insertion by Cas1 and Cas2 proteins in different groups of type II-A CRISPR systems.
• To understand how conserved DNA motifs at the leader-repeat junction interact with Cas1 and Cas2 proteins.
• To investigate the sequence-specific and morphology-dependent requirements for prespacer integration.

Main Methods:

• In vitro integration assays were performed using purified Cas1 and Cas2 proteins.
• Integration site recognition and prespacer morphology requirements were analyzed for different groups (G1, G2, G3).
• DNA sequence conservation at the 3' end of the leader was examined in relation to integration mechanisms.

Main Results:

• Type II-A systems G1 and G2 exhibit sequence-specific insertion, while G3 shows no sequence specificity.
• G1 recognizes a 12-bp leader-side sequence for initial insertion, followed by spacer-side insertion.
• G2 recognizes the full repeat sequence, allowing independent but faster leader-side insertions.
• Prespacer morphology requirements differ, with G1 needing a 5-nucleotide 3' overhang and G2 accommodating various forms.

Conclusions:

• Cas1-Cas2 interactions with DNA targets are intricate and vary significantly across type II-A CRISPR system groups.
• Mechanistic insights into prespacer insertion reveal distinct strategies employed by G1 and G2.
• Understanding these interactions can refine the Cas1-Cas2 toolset for targeted DNA insertion applications.