Structures of Class 1 CRISPR-Cas surveillance complexes




Schwartz, Evan Andrew

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One of the lines of defense for bacteria against bacteriophages is CRISPR-Cas, an adaptive immune system that serves as a bacterial genetic memory of previous phage infection. CRISPR-Cas immune defense involves the use of Cas proteins assembled along a CRISPR RNA guide to target and destroy invading nucleic acids. Class 2 single peptide CRISPR-Cas effectors, such as Cas9, have been highly studied for their repurposing as a genome editing tool. However, the Class 1 multi-subunit CRISPR systems are relatively understudied compared to their Class 2 counterparts. Class 1 CRISPR systems can be further classified into type I, type III, and type IV systems. Despite receiving less focus, type I and type III systems together make up about 90% of all CRISPR systems in nature. Type Is are characterized by their ability to degrade double-stranded DNA using Cas3, a trans-acting helicase-nuclease, while type IIIs have been shown to cut single-stranded RNA with Cas7. Here, my collaborators and I investigate the chimeric type I-D complex and describe its identity as a hybrid between type I and type III systems, as well as the type III-D complex, noted by its use of subunit fusions, to help unfold evolution of Class 1 CRISPR-Cas systems.



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