CRISPR-Cas9, which imparts adaptive immunity against foreign genomic invaders in certain prokaryotes, has been repurposed for genome-engineering applications. More recently, another RNA-guided CRISPR endonuclease called Cpf1 (also known as Cas12a) was identified and is also being repurposed. Little is known about the kinetics and mechanism of Cpf1 DNA interaction and how sequence mismatches between the DNA target and guide-RNA influence this interaction. We used single-molecule fluorescence analysis and biochemical assays to characterize DNA interrogation, cleavage, and product release by three Cpf1 orthologs. Our Cpf1 data are consistent with the DNA interrogation mechanism proposed for Cas9. They both bind any DNA in search of protospacer-adjacent motif (PAM) sequences, verify the target sequence directionally from the PAM-proximal end, and rapidly reject any targets that lack a PAM or that are poorly matched with the guide-RNA. Unlike Cas9, which requires 9 bp for stable binding and ∼16 bp for cleavage, Cpf1 requires an ∼17-bp sequence match for both stable binding and cleavage. Unlike Cas9, which does not release the DNA cleavage products, Cpf1 rapidly releases the PAM-distal cleavage product, but not the PAM-proximal product. Solution pH, reducing conditions, and 5' guanine in guide-RNA differentially affected different Cpf1 orthologs. Our findings have important implications on Cpf1-based genome engineering and manipulation applications.
Cas9 (from Streptococcus pyogenes) in complex with a guide RNA targets complementary DNA for cleavage. Here, we developed a single-molecule FRET analysis to study the mechanisms of specificity enhancement of two engineered Cas9s (eCas9 and Cas9-HF1). A DNA-unwinding assay showed that mismatches affect cleavage reactions through rebalancing the unwinding-rewinding equilibrium. Increasing PAM-distal mismatches facilitates rewinding, and the associated cleavage impairment shows that cleavage proceeds from the unwound state. Engineered Cas9s depopulate the unwound state more readily with mismatches. The intrinsic cleavage rate is much lower for engineered Cas9s, preventing cleavage from transiently unwound off-targets. Engineered Cas9s require approximately one additional base pair match for stable binding, freeing them from sites that would otherwise sequester them. Therefore, engineered Cas9s achieve their improved specificity by inhibiting stable DNA binding to partially matching sequences, making DNA unwinding more sensitive to mismatches and slowing down the intrinsic cleavage reaction.
The Escherichia coli type I-E CRISPR-Cas system Cascade effector is a multisubunit complex that binds CRISPR RNA (crRNA). Through its 32-nucleotide spacer sequence, Cascade-bound crRNA recognizes protospacers in foreign DNA, causing its destruction during CRISPR interference or acquisition of additional spacers in CRISPR array during primed CRISPR adaptation. Within Cascade, the crRNA spacer interacts with a hexamer of Cas7 subunits. We show that crRNAs with a spacer length reduced to 14 nucleotides cause primed adaptation, while crRNAs with spacer lengths of more than 20 nucleotides cause both primed adaptation and target interference in vivo. Shortened crRNAs assemble into altered-stoichiometry Cascade effector complexes containing less than the normal amount of Cas7 subunits. The results show that Cascade assembly is driven by crRNA and suggest that multisubunit type I CRISPR effectors may have evolved from much simpler ancestral complexes.
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