Pervasive generation of oppositely oriented spacers during CRISPR adaptation

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• Prokaryotic CRISPR adaptation involves integrating foreign DNA (protospacers) into CRISPR arrays as spacers.
• CRISPR systems, utilizing CRISPR ribonucleic acid (crRNA), provide adaptive immunity against viruses and plasmids.
• Protospacer adjacent motifs (PAMs) influence CRISPR adaptation and crRNA function.

Purpose of the Study:

• To investigate the orientation preferences of protospacer integration into CRISPR cassettes in Escherichia coli.
• To determine the role of the AAG motif in dictating spacer orientation during CRISPR adaptation.
• To understand how spacer orientation impacts the efficacy of CRISPR-mediated immunity.

Main Methods:

• Analysis of spacer sequences and their orientations in the CRISPR cassette of Escherichia coli.
• Comparison of protospacer integration patterns with and without the AAG motif.
• In silico prediction of crRNA functionality based on spacer orientation.

Main Results:

• A majority of plasmid-derived protospacers integrate into CRISPR cassettes in two opposing orientations, forming complementary spacer pairs.
• Protospacers containing the AAG motif exhibit a strong preference for an orientation that yields functional crRNA.
• Protospacers lacking the AAG motif are integrated in both orientations at similar frequencies.

Conclusions:

• Spacer orientation is not random and is influenced by specific motifs like AAG.
• The biased orientation preference, particularly with the AAG motif, enhances the diversity of spacers and the likelihood of effective CRISPR immunity.
• This mechanism optimizes the prokaryotic adaptive immune system against foreign genetic elements.