Mg2+-dependent conformational rearrangements of CRISPR-Cas12a R-loop complex are mandatory for complete double-stranded DNA cleavage

CRISPR-Cas12a, an RNA-guided DNA targeting endonuclease, has been widely used for genome editing and nucleic acid detection. As part of the essential processes for both of these applications, the two strands of double-stranded DNA are sequentially cleaved by a single catalytic site of Cas12a, but the mechanistic details that govern the generation of complete breaks in double-stranded DNA remain to be elucidated. Here, using single-molecule fluorescence resonance energy transfer assay, we identified two conformational intermediates that form consecutively following the initial cleavage of the nontarget strand. Specifically, these two intermediates are the result of further unwinding of the target DNA in the protospacer-adjacent motif (PAM)–distal region and the subsequent binding of the target strand to the catalytic site. Notably, the PAM-distal DNA unwound conformation was stabilized by Mg2+ ions, thereby significantly promoting the binding and cleavage of the target strand. These findings enabled us to propose a Mg2+-dependent kinetic model for the mechanism whereby Cas12a achieves cleavage of the target DNA, highlighting the presence of conformational rearrangements for the complete cleavage of the double-stranded DNA target.

Mechanism of R-loop formation and conformational activation of Cas9

Cas9 is a CRISPR-associated endonuclease capable of RNA-guided, site-specific DNA cleavage. The programmable activity of Cas9 has been widely utilized for genome editing applications. Despite extensive studies, the precise mechanism of target DNA binding and on-/off-target discrimination remains incompletely understood. Here we report cryo-EM structures of intermediate binding states of Streptococcus pyogenes Cas9 that reveal domain rearrangements induced by R-loop propagation and PAM-distal duplex positioning. At early stages of binding, the Cas9 REC2 and REC3 domains form a positively charged cleft that accommodates the PAM-distal duplex of the DNA substrate. Target hybridisation past the seed region positions the guide-target heteroduplex into the central binding channel and results in a conformational rearrangement of the REC lobe. Extension of the R-loop to 16 base pairs triggers the relocation of the HNH domain towards the target DNA strand in a catalytically incompetent conformation. The structures indicate that incomplete target strand pairing fails to induce the conformational displacements necessary for nuclease domain activation. Our results establish a structural basis for target DNA-dependent activation of Cas9 that advances our understanding of its off-target activity and will facilitate the development of novel Cas9 variants and guide RNA designs with enhanced specificity and activity.

Structural basis for mismatch surveillance by CRISPR/Cas9

The widespread use of CRISPR/Cas9 as a programmable genome editing tool has been hindered by off-target DNA cleavage (Cong et al., 2013; Doudna, 2020; Fu et al., 2013; Jinek et al., 2013). While analysis of such off-target editing events have enabled the development of Cas9 variants with greater discrimination against mismatches (Chen et al., 2017; Kleinstiver et al., 2016; Slaymaker et al., 2016), the underlying molecular mechanisms by which Cas9 rejects or accepts mismatches are poorly understood (Kim et al., 2019; Liu et al., 2020; Slaymaker and Gaudelli, 2021). Here, we used kinetic analysis to guide cryo-EM structure determination of Cas9 at different stages of mismatch surveillance. We observe a distinct, previously undescribed linear conformation of the duplex formed between the guide RNA (gRNA) and DNA target strand (TS), that occurs in the presence of PAM-distal mismatches, preventing Cas9 activation. The canonical kinked gRNA:TS duplex is a prerequisite for Cas9 activation, acting as a structural scaffold to facilitate Cas9 conformational rearrangements necessary for DNA cleavage. We observe that highly tolerated PAM- distal mismatches achieve this kinked conformation through stabilization of a distorted duplex conformation via a flexible loop in the RuvC domain. Our results provide molecular insights into the underlying structural mechanisms that may facilitate off- target cleavage by Cas9 and provides a molecular blueprint for the design of next- generation high fidelity Cas9 variants that selectively reduce off-target DNA cleavage while retaining efficient cleavage of on-target DNA.

DNA dynamics and computation based on toehold-free strand displacement

We present a simple and effective scheme of a dynamic switch for DNA nanostructures. Under such a framework of toehold-free strand displacement, blocking strands at an excess amount are applied to displace the complementation of specific segments of paired duplexes. The functional mechanism of the scheme is illustrated by modelling the base pairing kinetics of competing strands on a target strand. Simulation reveals the unique properties of toehold-free strand displacement in equilibrium control, which can be leveraged for information processing. Based on the controllable dynamics in the binding of preformed DNA nanostructures, a multi-input-multi-output (MIMO) Boolean function is controlled by the presence of the blockers. In conclusion, we implement two MIMO Boolean functions (one with 4-bit input and 2-bit output, and the other with 16-bit input and 8-bit output) to showcase the controllable dynamics.

Probing the stability of the SpCas9-DNA complex after cleavage

CRISPR-Cas9 is a ribonucleoprotein complex that sequence-specifically binds and cleaves double-stranded DNA. Wildtype Cas9 as well as its nickase and cleavage-incompetent mutants have been used in various biological techniques due to their versatility and programmable specificity. Cas9 has been shown to bind very stably to DNA even after cleavage of the individual DNA strands, inhibiting further turnovers and considerably slowing down in-vivo repair processes. This poses an obstacle in genome editing applications. Here, we employed single-molecule magnetic tweezers to investigate the binding stability of different S. pyogenes Cas9 variants after cleavage by challenging them with supercoiling. We find that different release mechanisms occur depending on which DNA strand is cleaved. After non-target strand cleavage, supercoils are immediately but slowly released by swiveling of the non-target strand around the DNA with friction. Consequently, Cas9 and its non-target strand nicking mutant stay stably bound to the DNA for many hours even at elevated torsional stress. After target-strand cleavage, supercoils are only removed after the collapse of the R-loop. We identified several states with different stabilities of the R-loop. Most importantly, we find that the post-cleavage state of Cas9 exhibits a higher stability compared to the pre-cleavage state. This suggests that Cas9 has evolved to remain tightly bound to its cut target.

Dynamic mechanisms of CRISPR interference by Escherichia coli CRISPR-Cas3

Type I CRISPR-Cas3 uses an RNA-guided multi Cas-protein complex, Cascade, which detects and degrades foreign nucleic acids via the helicase-nuclease Cas3 protein. Despite many studies using cryoEM and smFRET, the precise mechanism of Cas3-mediated cleavage and degradation of target DNA remains elusive. Here we reconstitute the CRISPR-Cas3 system in vitro to show how the Escherichia coli Cas3 (EcoCas3) with EcoCascade exhibits collateral non-specific ssDNA cleavage and target specific DNA degradation. Partial binding of EcoCascade to target DNA with tolerated mismatches within the spacer sequence, but not the PAM, elicits collateral ssDNA cleavage activity of recruited EcoCas3. Conversely, stable binding with complete R-loop formation drives EcoCas3 to nick the non-target strand (NTS) in the bound DNA. Helicase-dependent unwinding then combines with trans ssDNA cleavage of the target strand and repetitive cis cleavage of the NTS to degrade the target dsDNA substrate. High-speed atomic force microscopy demonstrates that EcoCas3 bound to EcoCascade repeatedly reels and releases the target DNA, followed by target fragmentation. Together, these results provide a revised model for collateral ssDNA cleavage and target dsDNA degradation by CRISPR-Cas3, furthering understanding of type I CRISPR priming and interference and informing future genome editing tools.

Structure of the mini-RNA-guided endonuclease CRISPR-CasΦ3

CasΦ is a novel family of miniaturized RNA-guided endonucleases from phages 1,2. These novel ribonucleoproteins (RNPs) provide a compact scaffold gathering all key activities of a genome editing tool2. Here, we provide the first structural insight into CasΦ singular DNA targeting and cleavage mechanism by determining the cryoEM structure of CasΦ3 with the triple strand R-loop generated after DNA cleavage. The structure reveals the unique machinery for target unwinding to form the crRNA-DNA hybrid and cleaving the target DNA. The protospacer adjacent motif (PAM) is recognised by the target strand (T-strand) and non-target strand (NT-strand) PAM interacting domains (TPID and NPID). Unwinding occurs after insertion of the conserved α1 helix disrupting the dsDNA, thus facilitating the crRNA-DNA hybrid formation. The NT-strand is funnelled towards the RuvC catalytic site, while a long helix of TPID separates the displaced NT-strand and the crRNA-DNA hybrid avoiding DNA re-annealing. The crRNA-DNA hybrid is directed to the stop (STP) domain that splits the hybrid guiding the T-strand towards the RuvC active site. The conserved RuvC insertion of the CasΦ family is extended along the hybrid, interacting with the phosphate backbone of the crRNA. A cluster of hydrophobic residues anchors the RuvC insertion in a cavity of the STP domain. The assembly of the hybrid promotes the shortening of the RuvC insertion, thus pulling the STP towards the RuvC active site to activate catalysis. These findings illustrate why CasΦ unleashes unspecific cleavage activity, degrading ssDNA molecules after activation. Site-directed mutagenesis in key residues support CasΦ3 target DNA and non-specific ssDNA cutting mechanism. Our analysis provides new avenues to redesign the compact CRISPR-CasΦ nucleases for genome editing.

A gate and clamp regulate sequential DNA strand cleavage by CRISPR-Cas12a

CRISPR-Cas12a has been widely used for genome editing and diagnostic applications, yet it is not fully understood how RNA-guided DNA recognition activates the sequential cleavage of the non-target strand (NTS) followed by the target strand (TS). Here we used singlemolecule magnetic tweezers microscopy, ensemble gel-based assays and nanopore sequencing to explore the coupling of DNA unwinding and cleavage. In addition to dynamic R-loop formation, we also directly observed transient dsDNA unwinding downstream of the 20 bp DNA:RNA hybrid and, following NTS cleavage and prior to TS cleavage, formation of a hyperstable "clamped" Cas12a-DNA intermediate resistant to DNA twisting. Alanine substitution of a conserved aromatic amino acid "gate" in the REC2 domain that normally caps the heteroduplex produced more frequent and extended downstream DNA breathing, a longer-lived twist-resistant state, and a 16-fold faster rate of TS cleavage. We suggest that both breathing and clamping events, regulated by the gate and by NTS cleavage, deliver the unwound TS to the RuvC nuclease and result from previously described REC2 and NUC domain motions.

A Hyperthermophilic Argonaute From Ferroglobus placidus With Specificity on Guide Binding Pattern

Argonaute proteins (Agos) from thermophilic archaea are involved in several important processes, such as host defense and DNA replication. The catalytic mechanism of Ago from different microbes with great diversity and genome editing potential is attracting increasing attention. Here, we describe an Argonaute from hyperthermophilic Ferroglobus placidus (FpAgo), with a typical DNA-guided DNA endonuclease activity but adopted with only a short guide 15–20 nt length rather than a broad guide selectivity for reported Agos. FpAgo performed the precise cleavage of phosphodiester bonds between 10 and 11 nt on the target strand (counting from the guide strand) guided strictly by 5′-phosphorylated DNA at temperatures ranging from 75 to 99°C. The cleavage activity was regulated by the divalent cations Mn2+, Mg2+, Co2+, and Ni2+. In addition, FpAgo possesses guide/target mismatch tolerance in the seed region but is sensitive to mismatches in the 3′-guide region. Notably, the EMSA assay revealed that the FpAgo-guide-target ternary complex exhibited a stronger binding affinity for short 15 and 16 nt guide DNAs than longer guides. Moreover, we performed structural modeling analyses that implied the unique PAZ domain of FpAgo for 3′-guide recognition and binding to affect guide length specificity. This study broadens our understanding of thermophilic Agos and paves the way for their use in DNA manipulation.