Shared CRISPR arrays underpin type I-A/I-B coexistence

Area of Science:

• Microbiology
• Molecular Biology
• Genomics

Background:

• CRISPR-Cas systems offer adaptive immunity in prokaryotes.
• The coexistence and coordination of multiple CRISPR-Cas subtypes within a single genome are not well understood.

Purpose of the Study:

• To investigate the mechanism of coexistence and coordination between Type I-A and Type I-B CRISPR-Cas systems.
• To elucidate how a shared CRISPR array is processed and utilized by different subtypes.

Main Methods:

• Comparative genomic analysis of CRISPR-Cas systems.
• Biochemical characterization of Cas6a and Cas6b enzymes from Thermococcus siculi RG-20 (Tsi).
• Plasmid interference assays and structural modeling.

Main Results:

• Nearly one-third of Type I-A CRISPR-Cas systems are adjacent to Type I-B systems, often sharing a single CRISPR array.
• TsiCas6a and TsiCas6b independently process a shared pre-crRNA into mature crRNAs with similar efficiency.
• CrRNAs produced by either Cas6a or Cas6b can guide both Type I-A and Type I-B interference complexes, demonstrating interchangeability.
• Structural modeling revealed distinct yet complementary RNA recognition strategies for Cas6a and Cas6b.

Conclusions:

• A shared-array logic allows a single pre-crRNA to be processed and utilized by multiple Type I CRISPR-Cas effectors.
• This mechanism provides a potential explanation for CRISPR-Cas subtype co-existence and the presence of array-less interference modules.
• This strategy may represent an evolutionary bet-hedging approach against anti-CRISPR proteins (Acrs).