Mismatch type impacts interference and priming activities in the type I-E CRISPR-Cas system

Area of Science:

• Molecular Biology
• Genetics
• Immunology

Background:

• Type I-E CRISPR-Cas systems use Cascade and Cas3 to target foreign nucleic acids.
• Priming allows rapid adaptation by acquiring new spacers after DNA targeting.
• Mutations in the PAM or seed region can affect interference and priming.

Purpose of the Study:

• To systematically investigate the influence of crRNA seed sequences on mutational tolerance in Type I-E CRISPR-Cas systems.
• To clarify the role of crRNA sequence variations in CRISPR immunity outcomes.

Main Methods:

• Engineered four E. coli strains with distinct crRNA spacer sequences.
• Tested CRISPR interference and priming against a plasmid library for each strain.
• Utilized in vitro biochemistry to analyze Cascade conformation and Cas3 recruitment.

Main Results:

• Mutations to C or G in the seed region were often deleterious, particularly at positions 1, 2, and 4.
• The specific crRNA sequence significantly modulated the severity of defects caused by target mutations (e.g., rC-dC, rA/G-dG).
• Mismatch type at the seed's first position altered Cascade conformation, reducing target binding and Cas3 recruitment.

Conclusions:

• Nucleotide identity in target mutations is crucial for Type I-E CRISPR immunity.
• The crRNA sequence plays a critical role in determining immune outcomes when target DNA is mutated.
• This highlights a complex interplay between target and guide sequences in CRISPR-based defense.