Domgaard, H. | Search bioRxiv

bioRxiv2022

CasDinG is an ATP-dependent 5'-3' DNA helicase with accessory domains essential for type IV CRISPR immunity

CRISPR-associated DinG protein (CasDinG) is essential to type IV-A CRISPR function. However, the enzymatic activities of CasDinG are unknown. Here we demonstrate that CasDinG from Pseudomonas aeruginosa strain 83 is an ATP- and metal-dependent 5-3 DNA helicase. The crystal structure of CasDinG reveals a helicase core of two RecA-like domains with three accessory domains (N-terminal, arch, and vestigial FeS). To examine the in vivo function of these CasDinG domains, we first identified the preferred PAM sequence (5-GNAWN-3 on the 5-side of the target) with a plasmid library containing all combinations of the five nucleotides upstream of the target sequence. Plasmid clearance assays (using a 5-GGAAA-3 PAM) with CasDinG domain mutants demonstrated the vFeS and arch accessory domains are both essential for type IV immunity. These results provide a needed structural and biochemical framework for understanding the type IV-A CRISPR system.

biochemistry↗

bioRxiv2022

Cas12a2 elicits abortive infection via RNA-triggered destruction of double-stranded DNA

Bacterial abortive infection systems limit the spread of foreign invaders by shutting down or killing infected cells before the invaders can replicate1, 2. Several RNA-targeting CRISPR-Cas systems (e.g., types III and VI) cause Abi phenotypes by activating indiscriminate RNases3-5. However, a CRISPR-mediated abortive mechanism that relies on indiscriminate DNase activity has yet to be observed. Here we report that RNA targeting by the type V Cas12a2 nuclease drives abortive infection through non-specific cleavage of double-stranded (ds)DNA. Upon recognition of an RNA target with an activating protospacer-flanking sequence, Cas12a2 efficiently degrades single-stranded (ss)RNA, ssDNA, and dsDNA. Within cells, the dsDNase activity induces an SOS response and impairs growth, stemming the infection. Finally, we harnessed the collateral activity of Cas12a2 for direct RNA detection, demonstrating that Cas12a2 can be repurposed as an RNA-guided, RNA-targeting tool. These findings expand the known defensive capabilities of CRISPR-Cas systems and create additional opportunities for CRISPR technologies.

microbiology↗