Structural variation of type I-F CRISPR RNA guided DNA surveillance

CRISPR-Cas systems are adaptive immune systems against invading nucleic acids. Interestingly, although the CRISPR-Cas machinery that extends the CRISPR array upon novel threads is highly conserved, the surveillance and effector modules diversified substantially during the course of evolution. The ongoing ‘bacteria-virus’ arms race is supposed to be the main driving force of the continuing co-evolution and structural diversification of bacterial defense and viral CRISPR evasion mechanisms. Our comparative structural analysis of the type I-F variant (I-Fv) CRISPR Cascade complex with the evolutionary closely related type I-F Cascade sheds light on the astonishingly high modularity of CRISPR-Cas systems that might have emerged in response to the ‘bacteria-virus’ arms race.