CasA mediates Cas3-catalyzed target degradation during CRISPR RNA-guided interference

Significance Bacteria use clustered regularly interspaced short palindromic repeats (CRISPRs) together with CRISPR-associated (Cas) proteins to defend themselves against viral infection. The CRISPR locus contains short segments acquired from viral genomes, and RNAs derived from these segments assemble with Cas proteins into programmable DNA-binding complexes that target DNA molecules complementary to the guide RNA for cleavage. In type I CRISPR-Cas systems, the CRISPR-associated complex for antiviral defense (Cascade) binds to target DNA sequences and then recruits the Cas3 enzyme to repeatedly cleave the bound DNA. In this study, we show how Cascade positions both the DNA and Cas3 to ensure DNA cleavage. In bacteria, the clustered regularly interspaced short palindromic repeats (CRISPR)–associated (Cas) DNA-targeting complex Cascade (CRISPR-associated complex for antiviral defense) uses CRISPR RNA (crRNA) guides to bind complementary DNA targets at sites adjacent to a trinucleotide signature sequence called the protospacer adjacent motif (PAM). The Cascade complex then recruits Cas3, a nuclease-helicase that catalyzes unwinding and cleavage of foreign double-stranded DNA (dsDNA) bearing a sequence matching that of the crRNA. Cascade comprises the CasA–E proteins and one crRNA, forming a structure that binds and unwinds dsDNA to form an R loop in which the target strand of the DNA base pairs with the 32-nt RNA guide sequence. Single-particle electron microscopy reconstructions of dsDNA-bound Cascade with and without Cas3 reveal that Cascade positions the PAM-proximal end of the DNA duplex at the CasA subunit and near the site of Cas3 association. The finding that the DNA target and Cas3 colocalize with CasA implicates this subunit in a key target-validation step during DNA interference. We show biochemically that base pairing of the PAM region is unnecessary for target binding but critical for Cas3-mediated degradation. In addition, the L1 loop of CasA, previously implicated in PAM recognition, is essential for Cas3 activation following target binding by Cascade. Together, these data show that the CasA subunit of Cascade functions as an essential partner of Cas3 by recognizing DNA target sites and positioning Cas3 adjacent to the PAM to ensure cleavage.