Type III-A CRISPR-Cas Csm Complexes: Assembly, Periodic RNA Cleavage, DNase Activity Regulation, and Autoimmunity

CRISPR-Cas Systems: A Functional Perspective and Innovations

Adaptation is a fundamental tenet of evolutionary biology and is essential for the survival of all organisms, including prokaryotes. The evolution of clustered regularity exemplifies this principle of interspaced short palindromic repeats (CRISPR) and associated proteins (Cas), an adaptive immune system that confers resistance to viral infections. By integrating short segments of viral genomes into their own, bacteria and archaea develop a molecular memory that enables them to mount a rapid and targeted response upon subsequent viral challenges. The fortuitous discovery of this immune mechanism prompted many studies and introduced researchers to novel tools that could potentially be developed from CRISPR-Cas and become clinically relevant as biotechnology rapidly advances in this area. Thus, a deeper understanding of the underpinnings of CRISPR-Cas and its possible therapeutic applications is required. This review analyses the mechanism of action of the CRISPR-Cas systems in detail and summarises the advances in developing biotechnological tools based on CRISPR, opening the field for further research.

CRISPR RNA binding drives structural ordering that primes Cas7-11 for target cleavage

Type III-E CRISPR-Cas effectors, referred to as Cas7-11 or giant Repeat-Associated Mysterious Protein, are single proteins that cleave target RNAs (tgRNAs) without nonspecific collateral cleavage, opening new possibilities for RNA editing. Here, biochemical assays combined with amide hydrogen-deuterium exchange mass spectrometry (HDX-MS) experiments reveal the dynamics of apo Cas7-11. The HDX-MS results suggest a mechanism by which CRISPR RNA (crRNA) stabilizes the folded state of the protein and subsequent tgRNA binding remodels it to the active form. HDX-MS shows that the four Cas7 RNA recognition motif (RRM) folds are well-folded, but insertion sequences, including disordered catalytic loops and β-hairpins of the Cas7.2/Cas7.3 active sites, fold upon binding crRNA leading to stronger interactions at domain-domain interfaces, and folding of the Cas7.1 processing site. TgRNA binding causes conformational changes around the catalytic loops of Cas7.2 and Cas7.3. We show that Cas7-11 cannot independently process the CRISPR array and that binding of partially processed crRNA induces multiple states in Cas7-11 and reduces tgRNA cleavage. The insertion domain interacts most stably with mature crRNA. Finally, we show a crRNA-induced conformational change in one of the tetratricopeptide repeat fused with Cas/HEF1-associated signal transducer (TPR-CHAT) binding sites providing an explanation for why crRNA binding facilitates TPR-CHAT binding.

Characterization of Five CRISPR Systems in Microcystis aeruginosa FACHB-524 with Focus on the In Vitro Antiviral Activity of One CRISPR System

Microcystis aeruginosa is an important species causing cyanobacterial blooms, which can be effectively infected and lysed by cyanophages. Several strategies have been developed by M. aeruginosa to resist cyanophage infections, including the CRISPR-Cas systems. However, detailed information on the CRISPR-Cas systems in M. aeruginosa is rare. In the present study, the CRISPR-Cas systems of M. aeruginosa FACHB-524 were analyzed by genome re-sequencing, which showed that there are two type I (Cluster 1, I-B1; Cluster 2, I-D) and three type III-B (Cluster 3/4/5) CRISPR-Cas systems in the cyanobacteria. Further comparison revealed that spacer sequences of two type III-B systems targeted several genes of the cyanophage MaMV (M. aeruginosa myovirus) strains. One of the type III systems (Cluster 4) was then cloned and expressed in Escherichia coli BL21 (DE3). Protein purification and mass spectrometry identification revealed that a Cmr-crRNA effector complex formed in the E. coli. Subsequently, T4 phage (T4) was used to infect the E. coli, expressing the Cmr-crRNA complex with or without accessory proteins. The results showed that the Cmr-crRNA effector complex exhibited anti-phage activity and the accessory protein Csx1 enhanced the immune activity of the complex. Collectively, our results comprehensively demonstrate the CRISPR systems encoded by a strain of M. aeruginosa, and for the first time, one of the CRISPR systems was constructed into E. coli, providing a foundation for further in-depth analysis of cyanobacterial CRISPR systems.

Nucleic acid recognition during prokaryotic immunity

Parasitic elements often spread to hosts through the delivery of their nucleic acids to the recipient. This is particularly true for the primary parasites of bacteria, bacteriophages (phages) and plasmids. Although bacterial immune systems can sense a diverse set of infection signals, such as a protein unique to the invader or the disruption of natural host processes, phage and plasmid nucleic acids represent some of the most common molecules that are recognized as foreign to initiate defense. In this review, we will discuss the various elements of invader nucleic acids that can be distinguished by bacterial host immune systems as "non-self" and how this signal is relayed to activate an immune response.

Mechanistic determinants and dynamics of cA6 synthesis in type III CRISPR-Cas effector complexes

Type III clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) systems (type III CRISPR-Cas systems) use guide RNAs to recognize RNA transcripts of foreign genetic elements, which triggers the generation of cyclic oligoadenylate (cOA) second messengers by the Cas10 subunit of the type III effector complex. In turn, cOAs bind and activate ancillary effector proteins to reinforce the host immune response. Type III systems utilize distinct cOAs, including cyclic tri- (cA3), tetra- (cA4) and hexa-adenylates (cA6). However, the molecular mechanisms dictating cOA product identity are poorly understood. Here we used cryoelectron microscopy to visualize the mechanism of cA6 biosynthesis by the Csm effector complex from Enterococcus italicus (EiCsm). We show that EiCsm synthesizes oligoadenylate nucleotides in 3'-5' direction using a set of conserved binding sites in the Cas10 Palm domains to determine the size of the nascent oligoadenylate chain. Our data also reveal that conformational dynamics induced by target RNA binding results in allosteric activation of Cas10 to trigger oligoadenylate synthesis. Mutations of a key structural element in Cas10 perturb cOA synthesis to favor cA3 and cA4 formation. Together, these results provide comprehensive insights into the dynamics of cOA synthesis in type III CRISPR-Cas systems and reveal key determinants of second messenger product selectivity, thereby illuminating potential avenues for their engineering.

Selective degradation of phage RNAs by the Csm6 ribonuclease provides robust type III CRISPR immunity in Streptococcus thermophilus

Type III CRISPR immune systems bind viral or plasmid RNA transcripts and activate Csm3/Cmr4 and Cas10 nucleases to uniquely cleave both invader RNA and DNA, respectively. Additionally, type III effector complexes generate cyclic oligoadenylate (cOA) signaling molecules to activate trans-acting, auxiliary Csm6/Csx1 ribonucleases, previously proposed to be non-specific in their in vivo RNA cleavage preference. Despite extensive in vitro studies, the nuclease requirements of type III systems in their native contexts remain poorly understood. Here we systematically investigated the in vivo roles for immunity of each of the three Streptococcus thermophilus (Sth) type III-A Cas nucleases and cOA signaling by challenging nuclease defective mutant strains with plasmid and phage infections. Our results reveal that RNA cleavage by Csm6 is both sufficient and essential for maintaining wild-type levels of immunity. Importantly, Csm6 RNase activity leads to immunity against even high levels of phage challenge without causing host cell dormancy or death. Transcriptomic analyses during phage infection indicated Csm6-mediated and crRNA-directed preferential cleavage of phage transcripts. Our findings highlight the critical role of Csm6 RNase activity in type III immunity and demonstrate specificity for invader RNA transcripts by Csm6 to ensure host cell survival upon phage infection.

The rise and future of CRISPR-based approaches for high-throughput genomics

Clustered regularly interspaced short palindromic repeats (CRISPR) has revolutionized the field of genome editing. To circumvent the permanent modifications made by traditional CRISPR techniques and facilitate the study of both essential and nonessential genes, CRISPR interference (CRISPRi) was developed. This gene-silencing technique employs a deactivated Cas effector protein and a guide RNA to block transcription initiation or elongation. Continuous improvements and a better understanding of the mechanism of CRISPRi have expanded its scope, facilitating genome-wide high-throughput screens to investigate the genetic basis of phenotypes. Additionally, emerging CRISPR-based alternatives have further expanded the possibilities for genetic screening. This review delves into the mechanism of CRISPRi, compares it with other high-throughput gene-perturbation techniques, and highlights its superior capacities for studying complex microbial traits. We also explore the evolution of CRISPRi, emphasizing enhancements that have increased its capabilities, including multiplexing, inducibility, titratability, predictable knockdown efficacy, and adaptability to nonmodel microorganisms. Beyond CRISPRi, we discuss CRISPR activation, RNA-targeting CRISPR systems, and single-nucleotide resolution perturbation techniques for their potential in genome-wide high-throughput screens in microorganisms. Collectively, this review gives a comprehensive overview of the general workflow of a genome-wide CRISPRi screen, with an extensive discussion of strengths and weaknesses, future directions, and potential alternatives.

CRISPR RNA binding drives structural ordering that primes Cas7-11 for target cleavage

Type III-E CRISPR-Cas effectors, of which Cas7-11 is the first, are single proteins that cleave target RNAs without nonspecific collateral cleavage, opening new possibilities for RNA editing. Biochemical experiments combined with amide hydrogen-deuterium exchange (HDX-MS) experiments provide a first glimpse of the conformational dynamics of apo Cas7-11. HDX-MS revealed the backbone comprised of the four Cas7 zinc-binding RRM folds are well-folded but insertion sequences are highly dynamic and fold upon binding crRNA. The crRNA causes folding of disordered catalytic loops and β-hairpins, stronger interactions at domain-domain interfaces, and folding of the Cas7.1 processing site. Target RNA binding causes only minor ordering around the catalytic loops of Cas7.2 and Cas7.3. We show that Cas7-11 cannot fully process the CRISPR array and that binding of partially processed crRNA induces multiple states in Cas7-11 and reduces target RNA cleavage. The insertion domain shows the most ordering upon binding of mature crRNA. Finally, we show a crRNA-induced conformational change in one of the TPR-CHAT binding sites providing an explanation for why crRNA binding facilitates TPR-CHAT binding. The results provide the first glimpse of the apo state of Cas7-11 and reveal how its structure and function are regulated by crRNA binding.

Structural basis for the activity of the type VII CRISPR-Cas system

The newly identified type VII CRISPR-Cas candidate system uses a CRISPR RNA-guided ribonucleoprotein complex formed by Cas5 and Cas7 proteins to target RNA1. However, the RNA cleavage is executed by a dedicated Cas14 nuclease, which is distinct from the effector nucleases of the other CRISPR-Cas systems. Here we report seven cryo-electron microscopy structures of the Cas14-bound interference complex at different functional states. Cas14, a tetrameric protein in solution, is recruited to the Cas5-Cas7 complex in a target RNA-dependent manner. The N-terminal catalytic domain of Cas14 binds a stretch of the substrate RNA for cleavage, whereas the C-terminal domain is primarily responsible for tethering Cas14 to the Cas5-Cas7 complex. The biochemical cleavage assays corroborate the captured functional conformations, revealing that Cas14 binds to different sites on the Cas5-Cas7 complex to execute individual cleavage events. Notably, a plugged-in arginine of Cas7 sandwiched by a C-shaped clamp of C-terminal domain precisely modulates Cas14 binding. More interestingly, target RNA cleavage is altered by a complementary protospacer flanking sequence at the 5' end, but not at the 3' end. Altogether, our study elucidates critical molecular details underlying the assembly of the interference complex and substrate cleavage in the type VII CRISPR-Cas system, which may help rational engineering of the type VII CRISPR-Cas system for biotechnological applications.

Characterization of Ligilactobacillus salivarius CRISPR-Cas systems

Ligilactobacillus is a diverse genus among lactobacilli with phenotypes that reflect adaptation to various hosts. CRISPR-Cas systems are highly prevalent within lactobacilli, and Ligilactobacillus salivarius, the most abundant species of Ligilactobacillus, possesses both DNA- and RNA-targeting CRISPR-Cas systems. In this study, we explore the presence and functional properties of I-B, I-C, I-E, II-A, and III-A CRISPR-Cas systems in over 500 Ligilactobacillus genomes, emphasizing systems found in L. salivarius. We examined the I-E, II-A, and III-A CRISPR-Cas systems of two L. salivarius strains and observed occurrences of split cas genes and differences in CRISPR RNA maturation in native hosts. This prompted testing of the single Cas9 and multiprotein Cascade and Csm CRISPR-Cas effector complexes in a cell-free context to demonstrate the functionality of these systems. We also predicted self-targeting spacers within L. salivarius CRISPR-Cas systems and found that nearly a third of L. salivarius genomes possess unique self-targeting spacers that generally target elements other than prophages. With these two L. salivarius strains, we performed prophage induction coupled with RNA sequencing and discovered that the prophages residing within these strains are inducible and likely active elements, despite targeting by CRISPR-Cas systems. These findings deepen our comprehension of CRISPR-Cas systems in L. salivarius, further elucidating their relationship with associated prophages and providing a functional basis for the repurposing of these Cas effectors for bacterial manipulation.

IMPORTANCE

Ligilactobacillus salivarius is a diverse bacterial species widely used in the food and dietary supplement industries. In this study, we investigate the occurrence and diversity of their adaptive immune systems, CRISPR-Cas, in over 500 genomes. We establish their function and provide insights into their role in the interplay between the bacterial host and the predatory phages that infect them. Such findings expand our knowledge about these important CRISPR-Cas immune systems widespread across the bacterial tree of life and also provide a technical basis for the repurposing of these molecular machines for the development of molecular biology tools and the manipulation and engineering of bacteria and other life forms.

Unity among the diverse RNA-guided CRISPR-Cas interference mechanisms

CRISPR-Cas (clustered regularly interspaced short palindromic repeats-CRISPR-associated) systems are adaptive immune systems that protect bacteria and archaea from invading mobile genetic elements (MGEs). The Cas protein-CRISPR RNA (crRNA) complex uses complementarity of the crRNA "guide" region to specifically recognize the invader genome. CRISPR effectors that perform targeted destruction of the foreign genome have emerged independently as multi-subunit protein complexes (Class 1 systems) and as single multi-domain proteins (Class 2). These different CRISPR-Cas systems can cleave RNA, DNA, and protein in an RNA-guided manner to eliminate the invader, and in some cases, they initiate programmed cell death/dormancy. The versatile mechanisms of the different CRISPR-Cas systems to target and destroy nucleic acids have been adapted to develop various programmable-RNA-guided tools and have revolutionized the development of fast, accurate, and accessible genomic applications. In this review, we present the structure and interference mechanisms of different CRISPR-Cas systems and an analysis of their unified features. The three types of Class 1 systems (I, III, and IV) have a conserved right-handed helical filamentous structure that provides a backbone for sequence-specific targeting while using unique proteins with distinct mechanisms to destroy the invader. Similarly, all three Class 2 types (II, V, and VI) have a bilobed architecture that binds the RNA-DNA/RNA hybrid and uses different nuclease domains to cleave invading MGEs. Additionally, we highlight the mechanistic similarities of CRISPR-Cas enzymes with other RNA-cleaving enzymes and briefly present the evolutionary routes of the different CRISPR-Cas systems.

Retention of the RNA ends provides the molecular memory for maintaining the activation of the Csm complex

The type III CRISPR-Cas effector complex Csm functions as a molecular Swiss army knife that provides multilevel defense against foreign nucleic acids. The coordinated action of three catalytic activities of the Csm complex enables simultaneous degradation of the invader's RNA transcripts, destruction of the template DNA and synthesis of signaling molecules (cyclic oligoadenylates cAn) that activate auxiliary proteins to reinforce CRISPR-Cas defense. Here, we employed single-molecule techniques to connect the kinetics of RNA binding, dissociation, and DNA hydrolysis by the Csm complex from Streptococcus thermophilus. Although single-stranded RNA is cleaved rapidly (within seconds), dual-color FCS experiments and single-molecule TIRF microscopy revealed that Csm remains bound to terminal RNA cleavage products with a half-life of over 1 hour while releasing the internal RNA fragments quickly. Using a continuous fluorescent DNA degradation assay, we observed that RNA-regulated single-stranded DNase activity decreases on a similar timescale. These findings suggest that after fast target RNA cleavage the terminal RNA cleavage products stay bound within the Csm complex, keeping the Cas10 subunit activated for DNA destruction. Additionally, we demonstrate that during Cas10 activation, the complex remains capable of RNA turnover, i.e. of ongoing degradation of target RNA.

RNA targeting and cleavage by the type III-Dv CRISPR effector complex

CRISPR-Cas are adaptive immune systems in bacteria and archaea that utilize CRISPR RNA-guided surveillance complexes to target complementary RNA or DNA for destruction1-5. Target RNA cleavage at regular intervals is characteristic of type III effector complexes6-8. Here, we determine the structures of the Synechocystis type III-Dv complex, an apparent evolutionary intermediate from multi-protein to single-protein type III effectors9,10, in pre- and post-cleavage states. The structures show how multi-subunit fusion proteins in the effector are tethered together in an unusual arrangement to assemble into an active and programmable RNA endonuclease and how the effector utilizes a distinct mechanism for target RNA seeding from other type III effectors. Using structural, biochemical, and quantum/classical molecular dynamics simulation, we study the structure and dynamics of the three catalytic sites, where a 2'-OH of the ribose on the target RNA acts as a nucleophile for in line self-cleavage of the upstream scissile phosphate. Strikingly, the arrangement at the catalytic residues of most type III complexes resembles the active site of ribozymes, including the hammerhead, pistol, and Varkud satellite ribozymes. Our work provides detailed molecular insight into the mechanisms of RNA targeting and cleavage by an important intermediate in the evolution of type III effector complexes.

Type-III-A structure of mycobacteria CRISPR-Csm complexes involving atypical crRNAs

Tuberculosis (TB), a leading cause of mortality globally, is a chronic infectious disease caused by Mycobacterium tuberculosis that primarily infiltrates the lung. The mature crRNAs in M. tuberculosis transcribed from the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) locus exhibit an atypical structure featured with 5' and 3' repeat tags at both ends of the intact crRNA, in contrast to typical Type-III-A crRNAs that possess 5' repeat tags and partial crRNA sequences. However, this structural peculiarity particularly concerning the specific binding characteristics of the 3' repeat end within the mature crRNA within the Csm complex, has not been comprehensively elucidated. Here, our Mycobacteria CRISPR-Csm complexes structure represents the largest Csm complex reported to date. It incorporates an atypical Type-III-A CRISPR RNA (crRNA) (46 nt) with 5' 8-nt and 3' 4-nt repeat sequences in the stoichiometry of Mycobacteria Csm1125364151. The PAM-independent single-stranded RNAs (ssRNAs) are the most suitable substrate for the Csm complex. The 3'-repeat end trimming of mature crRNA was not necessary for its cleavage activity in Type-III-A Csm complex. Our work broadens our understanding of the Type-III-A Csm complex and identifies another mature crRNA processing mechanism in the Type-III-A CRISPR-Cas system based on structural biology.

Progress of CRISPR-based programmable RNA manipulation and detection

Prokaryotic clustered regularly interspaced short palindromic repeats and CRISPR associated (CRISPR-Cas) systems provide adaptive immunity by using RNA-guided endonucleases to recognize and eliminate invading foreign nucleic acids. Type II Cas9, type V Cas12, type VI Cas13, and type III Csm/Cmr complexes have been well characterized and developed as programmable platforms for selectively targeting and manipulating RNA molecules of interest in prokaryotic and eukaryotic cells. These Cas effectors exhibit remarkable diversity of ribonucleoprotein (RNP) composition, target recognition and cleavage mechanisms, and self discrimination mechanisms, which are leveraged for various RNA targeting applications. Here, we summarize the current understanding of mechanistic and functional characteristics of these Cas effectors, give an overview on RNA detection and manipulation toolbox established so far including knockdown, editing, imaging, modification, and mapping RNA-protein interactions, and discuss the future directions for CRISPR-based RNA targeting tools. This article is categorized under: RNA Methods > RNA Analyses in Cells RNA Processing > RNA Editing and Modification RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Precise transcript targeting by CRISPR-Csm complexes

Robust and precise transcript targeting in mammalian cells remains a difficult challenge using existing approaches due to inefficiency, imprecision and subcellular compartmentalization. Here we show that the clustered regularly interspaced short palindromic repeats (CRISPR)-Csm complex, a multiprotein effector from type III CRISPR immune systems in prokaryotes, provides surgical RNA ablation of both nuclear and cytoplasmic transcripts. As part of the most widely occurring CRISPR adaptive immune pathway, CRISPR-Csm uses a programmable RNA-guided mechanism to find and degrade target RNA molecules without inducing indiscriminate trans-cleavage of cellular RNAs, giving it an important advantage over the CRISPR-Cas13 family of enzymes. Using single-vector delivery of the Streptococcus thermophilus Csm complex, we observe high-efficiency RNA knockdown (90-99%) and minimal off-target effects in human cells, outperforming existing technologies including short hairpin RNA- and Cas13-mediated knockdown. We also find that catalytically inactivated Csm achieves specific and durable RNA binding, a property we harness for live-cell RNA imaging. These results establish the feasibility and efficacy of multiprotein CRISPR-Cas effector complexes as RNA-targeting tools in eukaryotes.

The structure of a Type III-A CRISPR-Cas effector complex reveals conserved and idiosyncratic contacts to target RNA and crRNA among Type III-A systems

Type III CRISPR-Cas systems employ multiprotein effector complexes bound to small CRISPR RNAs (crRNAs) to detect foreign RNA transcripts and elicit a complex immune response that leads to the destruction of invading RNA and DNA. Type III systems are among the most widespread in nature, and emerging interest in harnessing these systems for biotechnology applications highlights the need for detailed structural analyses of representatives from diverse organisms. We performed cryo-EM reconstructions of the Type III-A Cas10-Csm effector complex from S. epidermidis bound to an intact, cognate target RNA and identified two oligomeric states, a 276 kDa complex and a 318 kDa complex. 3.1 Å density for the well-ordered 276 kDa complex allowed construction of atomic models for the Csm2, Csm3, Csm4 and Csm5 subunits within the complex along with the crRNA and target RNA. We also collected small-angle X-ray scattering data which was consistent with the 276 kDa Cas10-Csm architecture we identified. Detailed comparisons between the S. epidermidis Cas10-Csm structure and the well-resolved bacterial (S. thermophilus) and archaeal (T. onnurineus) Cas10-Csm structures reveal differences in how the complexes interact with target RNA and crRNA which are likely to have functional ramifications. These structural comparisons shed light on the unique features of Type III-A systems from diverse organisms and will assist in improving biotechnologies derived from Type III-A effector complexes.

Type IV-A CRISPR-Csf complex: Assembly, dsDNA targeting, and CasDinG recruitment

Type IV CRISPR-Cas systems, which are primarily found on plasmids and exhibit a strong plasmid-targeting preference, are the only one of the six known CRISPR-Cas types for which the mechanistic details of their function remain unknown. Here, we provide high-resolution functional snapshots of type IV-A Csf complexes before and after target dsDNA binding, either in the absence or presence of CasDinG, revealing the mechanisms underlying Csf^crRNA complex assembly, "DWN" PAM-dependent dsDNA targeting, R-loop formation, and CasDinG recruitment. Furthermore, we establish that CasDinG, a signature DinG family helicase, harbors ssDNA-stimulated ATPase activity and ATP-dependent 5'-3' DNA helicase activity. In addition, we show that CasDinG unwinds the non-target strand (NTS) and target strand (TS) of target dsDNA from the Csf^crRNA complex. These molecular details advance our mechanistic understanding of type IV-A CRISPR-Csf function and should enable Csf complexes to be harnessed as genome-engineering tools for biotechnological applications.

CRISPR-Based Genome Editing Tools: An Accelerator in Crop Breeding for a Changing Future

Genome editing is an important strategy to maintain global food security and achieve sustainable agricultural development. Among all genome editing tools, CRISPR-Cas is currently the most prevalent and offers the most promise. In this review, we summarize the development of CRISPR-Cas systems, outline their classification and distinctive features, delineate their natural mechanisms in plant genome editing and exemplify the applications in plant research. Both classical and recently discovered CRISPR-Cas systems are included, detailing the class, type, structures and functions of each. We conclude by highlighting the challenges that come with CRISPR-Cas and offer suggestions on how to tackle them. We believe the gene editing toolbox will be greatly enriched, providing new avenues for a more efficient and precise breeding of climate-resilient crops.

Cryo-EM structure and protease activity of the type III-E CRISPR-Cas effector

The recently discovered type III-E CRISPR-Cas effector Cas7-11 shows promise when used as an RNA manipulation tool, but its structure and the mechanisms underlying its function remain unclear. Here we present four cryo-EM structures of Desulfonema ishimotonii Cas7-11-crRNA complex in pre-target and target RNA-bound states, and the cryo-EM structure of DiCas7-11-crRNA bound to its accessory protein DiCsx29. These data reveal structural elements for pre-crRNA processing, target RNA cleavage and regulation. Moreover, a 3' seed region of crRNA is involved in regulating RNA cleavage activity of DiCas7-11-crRNA-Csx29. Our analysis also shows that both the minimal mismatch of 4 nt to the 5' handle of crRNA and the minimal matching of the first 12 nt of the spacer by the target RNA are essential for triggering the protease activity of DiCas7-11-crRNA-Csx29 towards DiCsx30. Taken together, we propose that target RNA recognition and cleavage regulate and fine-tune the protease activity of DiCas7-11-crRNA-Csx29, thus preventing auto-immune responses.

Structure of the Saccharolobus solfataricus type III-D CRISPR effector

CRISPR-Cas is a prokaryotic adaptive immune system, classified into six different types, each characterised by a signature protein. Type III systems, classified based on the presence of a Cas10 subunit, are rather diverse multi-subunit assemblies with a range of enzymatic activities and downstream ancillary effectors. The broad array of current biotechnological CRISPR applications is mainly based on proteins classified as Type II, however recent developments established the feasibility and efficacy of multi-protein Type III CRISPR-Cas effector complexes as RNA-targeting tools in eukaryotes. The crenarchaeon Saccharolobus solfataricus has two type III system subtypes (III-B and III-D). Here, we report the cryo-EM structure of the Csm Type III-D complex from S. solfataricus (SsoCsm), which uses CRISPR RNA to bind target RNA molecules, activating the Cas10 subunit for antiviral defence. The structure reveals the complex organisation, subunit/subunit connectivity and protein/guide RNA interactions of the SsoCsm complex, one of the largest CRISPR effectors known.

RNA-targeting CRISPR-Cas systems

CRISPR-Cas is a widespread adaptive immune system in bacteria and archaea that protects against viral infection by targeting specific invading nucleic acid sequences. Whereas some CRISPR-Cas systems sense and cleave viral DNA, type III and type VI CRISPR-Cas systems sense RNA that results from viral transcription and perhaps invasion by RNA viruses. The sequence-specific detection of viral RNA evokes a cell-wide response that typically involves global damage to halt the infection. How can one make sense of an immune strategy that encompasses broad, collateral effects rather than specific, targeted destruction? In this Review, we summarize the current understanding of RNA-targeting CRISPR-Cas systems. We detail the composition and properties of type III and type VI systems, outline the cellular defence processes that are instigated upon viral RNA sensing and describe the biological rationale behind the broad RNA-activated immune responses as an effective strategy to combat viral infection.

Bacterial origins of cyclic nucleotide-activated antiviral immune signaling

Second-messenger-mediated signaling by cyclic oligonucleotides (cOs) composed of distinct base, ring size, and 3'-5'/2'-5' linkage combinations constitutes the initial trigger resulting in activation of signaling pathways that have an impact on immune-mediated antiviral defense against invading viruses and phages. Bacteria and archaea have evolved CRISPR, CBASS, Pycsar, and Thoeris surveillance complexes that involve cO-mediated activation of effectors resulting in antiviral defense through either targeted nuclease activity, effector oligomerization-mediated depletion of essential cellular metabolites or disruption of host cell membrane functions. Notably, antiviral defense capitalizes on an abortive infection mechanism, whereby infected cells die prior to completion of the phage replication cycle. In turn, phages have evolved small proteins that target and degrade/sequester cOs, thereby suppressing host immunity. This review presents a structure-based mechanistic perspective of recent advances in the field of cO-mediated antiviral defense, in particular highlighting the ancient evolutionary adaptation by metazoans of bacterial cell-autonomous innate immune mechanisms.

Target RNA-guided protease activity in type III-E CRISPR-Cas system

The type III-E CRISPR-Cas systems are newly identified adaptive immune systems in prokaryotes that use a single Cas7-11 protein to specifically cleave target RNA. Cas7-11 could associate with Csx29, a putative caspase-like protein encoded by the gene frequently found in the type III-E loci, suggesting a functional linkage between the RNase and protease activities in type III-E systems. Here, we demonstrated that target RNA recognition would stimulate the proteolytic activity of Csx29, and protein Csx30 is the endogenous substrate. More interestingly, while the cognate target RNA recognition would activate Csx29, non-cognate target RNA with the complementary 3' anti-tag sequence inhibits the enzymatic activity. Csx30 could bind to the sigma factor RpoE, which may initiate the stress response after proteolytic cleavage. Combined with biochemical and structural studies, we have elucidated the mechanisms underlying the target RNA-guided proteolytic activity of Csx29. Our work will guide further developments leveraging this simple RNA targeting system for RNA and protein-related applications.

Structural basis for the non-self RNA-activated protease activity of the type III-E CRISPR nuclease-protease Craspase

The RNA-targeting type III-E CRISPR-gRAMP effector interacts with a caspase-like protease TPR-CHAT to form the CRISPR-guided caspase complex (Craspase), but their functional mechanism is unknown. Here, we report cryo-EM structures of the type III-E gRAMP^crRNA and gRAMP^crRNA-TPR-CHAT complexes, before and after either self or non-self RNA target binding, and elucidate the mechanisms underlying RNA-targeting and non-self RNA-induced protease activation. The associated TPR-CHAT adopted a distinct conformation upon self versus non-self RNA target binding, with nucleotides at positions -1 and -2 of the CRISPR-derived RNA (crRNA) serving as a sensor. Only binding of the non-self RNA target activated the TPR-CHAT protease, leading to cleavage of Csx30 protein. Furthermore, TPR-CHAT structurally resembled eukaryotic separase, but with a distinct mechanism for protease regulation. Our findings should facilitate the development of gRAMP-based RNA manipulation tools, and advance our understanding of the virus-host discrimination process governed by a nuclease-protease Craspase during type III-E CRISPR-Cas immunity.

High-efficiency genome editing of an extreme thermophile Thermus thermophilus using endogenous type I and type III CRISPR-Cas systems

Thermus thermophilus is an attractive species in the bioindustry due to its valuable natural products, abundant thermophilic enzymes, and promising fermentation capacities. However, efficient and versatile genome editing tools are not available for this species. In this study, we developed an efficient genome editing tool for T. thermophilus HB27 based on its endogenous type I-B, I-C, and III-A/B CRISPR-Cas systems. First, we systematically characterized the DNA interference capabilities of the different types of the native CRISPR-Cas systems in T. thermophilus HB27. We found that genomic manipulations such as gene deletion, mutation, and in situ tagging could be easily implemented by a series of genome-editing plasmids carrying an artificial self-targeting mini-CRISPR and a donor DNA responsible for the recombinant recovery. We also compared the genome editing efficiency of different CRISPR-Cas systems and the editing plasmids with donor DNAs of different lengths. Additionally, we developed a reporter gene system for T. thermophilus based on a heat-stable β-galactosidase gene TTP0042, and constructed an engineered strain with a high production capacity of superoxide dismutases by genome modification.

Target RNA activates the protease activity of Craspase to confer antiviral defense

In the type III-E CRISPR-Cas system, a Cas effector (gRAMP) is associated with a TPR-CHAT to form Craspase (CRISPR-guided caspase). However, both the structural features of gRAMP and the immunity mechanism remain unknown for this system. Here, we report structures of gRAMP-crRNA and gRAMP:cRNA:target RNA as well as structures of Craspase and Craspase complexed with cognate target RNA (CTR) or non-cognate target RNA (NTR). Importantly, the 3' anti-tag region of NTR and CTR binds at two distinct channels in Craspase, and CTR with a non-complementary 3' anti-tag induces a marked conformational change of the TPR-CHAT, which allosterically activates its protease activity to cleave an ancillary protein Csx30. This cleavage then triggers an abortive infection as the antiviral strategy of the type III-E system. Together, our study provides crucial insights into both the catalytic mechanism of the gRAMP and the immunity mechanism of the type III-E system.

Structure and function of a bacterial type III-E CRISPR-Cas7-11 complex

The type III-E CRISPR-Cas system uses a single multidomain effector called Cas7-11 (also named gRAMP) to cleave RNA and associate with a caspase-like protease Csx29, showing promising potential for RNA-targeting applications. The structural and molecular mechanisms of the type III-E CRISPR-Cas system remain poorly understood. Here we report four cryo-electron microscopy structures of Cas7-11 at different functional states. Cas7-11 has four Cas7-like domains, which assemble into a helical filament to accommodate CRISPR RNA (crRNA), and a Cas11-like domain facilitating crRNA-target RNA duplex formation. The Cas7.1 domain is critical for crRNA maturation, whereas Cas7.2 and Cas7.3 are responsible for target RNA cleavage. Target RNA binding induces the structural arrangements of Csx29, potentially exposing the catalytic site of Csx29. These results delineate the molecular mechanisms underlying pre-crRNA processing, target RNA recognition and cleavage for Cas7-11, and provide a structural framework to understand the role of Csx29 in type III-E CRISPR system.

RNA-triggered protein cleavage and cell growth arrest by the type III-E CRISPR nuclease-protease

The type III-E CRISPR-Cas7-11 effector binds a CRISPR RNA (crRNA) and the putative protease Csx29 and catalyzes crRNA-guided RNA cleavage. We report cryo-electron microscopy structures of the Cas7-11-crRNA-Csx29 complex with and without target RNA (tgRNA), and demonstrate that tgRNA binding induces conformational changes in Csx29. Biochemical experiments revealed tgRNA-dependent cleavage of the accessory protein Csx30 by Csx29. Reconstitution of the system in bacteria showed that Csx30 cleavage yields toxic protein fragments that cause growth arrest, which is regulated by Csx31. Csx30 binds Csx31 and the associated sigma factor RpoE (RNA polymerase, extracytoplasmic E), suggesting that Csx30-mediated RpoE inhibition modulates the cellular response to infection. We engineered the Cas7-11-Csx29-Csx30 system for programmable RNA sensing in mammalian cells. Overall, the Cas7-11-Csx29 effector is an RNA-dependent nuclease-protease.

Molecular mechanism of active Cas7-11 in processing CRISPR RNA and interfering target RNA

Cas7-11 is a Type III-E CRISPR Cas effector that confers programmable RNA cleavage and has potential applications in RNA interference. Cas7-11 encodes a single polypeptide containing four Cas7- and one Cas11-like segments that obscures the distinction between the multi-subunit Class 1 and the single-subunit Class-2 CRISPR Cas systems. We report a cryo-EM (cryo-electron microscopy) structure of the active Cas7-11 from Desulfonema ishimotonii (DiCas7-11) that reveals the molecular basis for RNA processing and interference activities. DiCas7-11 arranges its Cas7- and Cas11-like domains in an extended form that resembles the backbone made up by four Cas7 and one Cas11 subunits in the multi-subunit enzymes. Unlike the multi-subunit enzymes, however, the backbone of DiCas7-11 contains evolutionarily different Cas7 and Cas11 domains, giving rise to their unique functionality. The first Cas7-like domain nearly engulfs the last 15 direct repeat nucleotides in processing and recognition of the CRISPR RNA, and its free-standing fragment retains most of the activity. Both the second and the third Cas7-like domains mediate target RNA cleavage in a metal-dependent manner. The structure and mutational data indicate that the long variable insertion to the fourth Cas7 domain has little impact on RNA processing or targeting, suggesting the possibility for engineering a compact and programmable RNA interference tool.

Characterization of the self-targeting Type IV CRISPR interference system in Pseudomonas oleovorans

Bacterial Type IV CRISPR-Cas systems are thought to rely on multi-subunit ribonucleoprotein complexes to interfere with mobile genetic elements, but the substrate requirements and potential DNA nuclease activities for many systems within this type are uncharacterized. Here we show that the native Pseudomonas oleovorans Type IV-A CRISPR-Cas system targets DNA in a PAM-dependent manner and elicits interference without showing DNA nuclease activity. We found that the first crRNA of P. oleovorans contains a perfect match in the host gene coding for the Type IV pilus biogenesis protein PilN. Deletion of the native Type IV CRISPR array resulted in upregulation of pilN operon transcription in the absence of genome cleavage, indicating that Type IV-A CRISPR-Cas systems can function in host gene regulation. These systems resemble CRISPR interference (CRISPRi) methodology but represent a natural CRISPRi-like system that is found in many Pseudomonas and Klebsiella species and allows for gene silencing using engineered crRNAs.

Craspase is a CRISPR RNA-guided, RNA-activated protease

The CRISPR-Cas type III-E RNA-targeting effector complex gRAMP/Cas7-11 is associated with a caspase-like protein (TPR-CHAT/Csx29) to form Craspase (CRISPR-guided caspase). Here, we use cryo-electron microscopy snapshots of Craspase to explain its target RNA cleavage and protease activation mechanisms. Target-guide pairing extending into the 5' region of the guide RNA displaces a gating loop in gRAMP, which triggers an extensive conformational relay that allosterically aligns the protease catalytic dyad and opens an amino acid side-chain-binding pocket. We further define Csx30 as the endogenous protein substrate that is site-specifically proteolyzed by RNA-activated Craspase. This protease activity is switched off by target RNA cleavage by gRAMP and is not activated by RNA targets containing a matching protospacer flanking sequence. We thus conclude that Craspase is a target RNA-activated protease with self-regulatory capacity.

Allosteric control of type I-A CRISPR-Cas3 complexes and establishment as effective nucleic acid detection and human genome editing tools

Type I CRISPR-Cas systems typically rely on a two-step process to degrade DNA. First, an RNA-guided complex named Cascade identifies the complementary DNA target. The helicase-nuclease fusion enzyme Cas3 is then recruited in trans for processive DNA degradation. Contrary to this model, here, we show that type I-A Cascade and Cas3 function as an integral effector complex. We provide four cryoelectron microscopy (cryo-EM) snapshots of the Pyrococcus furiosus (Pfu) type I-A effector complex in different stages of DNA recognition and degradation. The HD nuclease of Cas3 is autoinhibited inside the effector complex. It is only allosterically activated upon full R-loop formation, when the entire targeted region has been validated by the RNA guide. The mechanistic insights inspired us to convert Pfu Cascade-Cas3 into a high-sensitivity, low-background, and temperature-activated nucleic acid detection tool. Moreover, Pfu CRISPR-Cas3 shows robust bi-directional deletion-editing activity in human cells, which could find usage in allele-specific inactivation of disease-causing mutations.

Type III-A CRISPR systems as a versatile gene knockdown technology

CRISPR-Cas systems are functionally diverse prokaryotic antiviral defense systems, which encompass six distinct types (I-VI) that each encode different effector Cas nucleases with distinct nucleic acid cleavage specificities. By harnessing the unique attributes of the various CRISPR-Cas systems, a range of innovative CRISPR-based DNA and RNA targeting tools and technologies have been developed. Here, we exploit the ability of type III-A CRISPR-Cas systems to carry out RNA-guided and sequence-specific target RNA cleavage for establishment of research tools for post-transcriptional control of gene expression. Type III-A systems from three bacterial species (L. lactis, S. epidermidis, and S. thermophilus) were each expressed on a single plasmid in E. coli, and the efficiency and specificity of gene knockdown was assessed by northern blot and transcriptomic analysis. We show that engineered type III-A modules can be programmed using tailored CRISPR RNAs to efficiently knock down gene expression of both coding and noncoding RNAs in vivo. Moreover, simultaneous degradation of multiple cellular mRNA transcripts can be directed by utilizing a CRISPR array expressing corresponding gene-targeting crRNAs. Our results demonstrate the utility of distinct type III-A modules to serve as specific and effective gene knockdown platforms in heterologous cells. This transcriptome engineering technology has the potential to be further refined and exploited for key applications including gene discovery and gene pathway analyses in additional prokaryotic and perhaps eukaryotic cells and organisms.

Structure and engineering of the type III-E CRISPR-Cas7-11 effector complex

The type III-E CRISPR-Cas effector Cas7-11, with dual RNase activities for precursor CRISPR RNA (pre-crRNA) processing and crRNA-guided target RNA cleavage, is a new platform for bacterial and mammalian RNA targeting. We report the 2.5-Å resolution cryoelectron microscopy structure of Cas7-11 in complex with a crRNA and its target RNA. Cas7-11 adopts a modular architecture comprising seven domains (Cas7.1-Cas7.4, Cas11, INS, and CTE) and four interdomain linkers. The crRNA 5' tag is recognized and processed by Cas7.1, whereas the crRNA spacer hybridizes with the target RNA. Consistent with our biochemical data, the catalytic residues for programmable cleavage in Cas7.2 and Cas7.3 neighbor the scissile phosphates before the flipped-out fourth and tenth nucleotides in the target RNA, respectively. Using structural insights, we rationally engineered a compact Cas7-11 variant (Cas7-11S) for single-vector AAV packaging for transcript knockdown in human cells, enabling in vivo Cas7-11 applications.

Structures of an active type III-A CRISPR effector complex

Clustered regularly interspaced short palindromic repeats (CRISPR) and their CRISPR-associated proteins (Cas) provide many prokaryotes with an adaptive immune system against invading genetic material. Type III CRISPR systems are unique in that they can degrade both RNA and DNA. In response to invading nucleic acids, they produce cyclic oligoadenylates that act as secondary messengers, activating cellular nucleases that aid in the immune response. Here, we present seven single-particle cryo-EM structures of the type III-A Staphylococcus epidermidis CRISPR effector complex. The structures reveal the intact S. epidermidis effector complex in an apo, ATP-bound, cognate target RNA-bound, and non-cognate target RNA-bound states and illustrate how the effector complex binds and presents crRNA. The complexes bound to target RNA capture the type III-A effector complex in a post-RNA cleavage state. The ATP-bound structures give details about how ATP binds to Cas10 to facilitate cyclic oligoadenylate production.

Structural rearrangements allow nucleic acid discrimination by type I-D Cascade

CRISPR-Cas systems are adaptive immune systems that protect prokaryotes from foreign nucleic acids, such as bacteriophages. Two of the most prevalent CRISPR-Cas systems include type I and type III. Interestingly, the type I-D interference proteins contain characteristic features of both type I and type III systems. Here, we present the structures of type I-D Cascade bound to both a double-stranded (ds)DNA and a single-stranded (ss)RNA target at 2.9 and 3.1 Å, respectively. We show that type I-D Cascade is capable of specifically binding ssRNA and reveal how PAM recognition of dsDNA targets initiates long-range structural rearrangements that likely primes Cas10d for Cas3′ binding and subsequent non-target strand DNA cleavage. These structures allow us to model how binding of the anti-CRISPR protein AcrID1 likely blocks target dsDNA binding via competitive inhibition of the DNA substrate engagement with the Cas10d active site. This work elucidates the unique mechanisms used by type I-D Cascade for discrimination of single-stranded and double stranded targets. Thus, our data supports a model for the hybrid nature of this complex with features of type III and type I systems.

I-D CRISPR-Cascade can target both single-stranded and double-stranded nucleic acids. Here, Schwartz et. al determine these structures and reveal large-scale rearrangements that allow for target discrimination and destruction.

Type III CRISPR-based RNA editing for programmable control of SARS-CoV-2 and human coronaviruses

Gene-editing technologies, including the widespread usage of CRISPR endonucleases, have the potential for clinical treatments of various human diseases. Due to the rapid mutations of SARS-CoV-2, specific and effective prevention and treatment by CRISPR toolkits for coronavirus disease 2019 (COVID-19) are urgently needed to control the current pandemic spread. Here, we designed Type III CRISPR endonuclease antivirals for coronaviruses (TEAR-CoV) as a therapeutic to combat SARS-CoV-2 infection. We provided a proof of principle demonstration that TEAR-CoV-based RNA engineering approach leads to RNA-guided transcript degradation both in vitro and in eukaryotic cells, which could be used to broadly target RNA viruses. We report that TEAR-CoV not only cleaves SARS-CoV-2 genome and mRNA transcripts, but also degrades live influenza A virus (IAV), impeding viral replication in cells and in mice. Moreover, bioinformatics screening of gRNAs along RNA sequences reveals that a group of five gRNAs (hCoV-gRNAs) could potentially target 99.98% of human coronaviruses. TEAR-CoV also exerted specific targeting and cleavage of common human coronaviruses. The fast design and broad targeting of TEAR-CoV may represent a versatile antiviral approach for SARS-CoV-2 or potentially other emerging human coronaviruses.

Assembly of Cas7 subunits of Leptospira on the mature crRNA of CRISPR-Cas I-B is modulated by divalent ions

The spirochete Leptospira interrogans serovar Copenhageni harbors the genetic elements of the CRISPR-Cas type I-B system in its genome. CRISPR-Cas is a CRISPR RNA (crRNA) mediated adaptive immune system in most prokaryotes against mobile genetic elements (MGEs). To eliminate the intruding MGEs, CRISPR-Cas type I systems utilize a Cascade (CRISPR-associated complex for antiviral defense) complex composed of Cas5, Cas6, Cas7, and Cas8 bound with a crRNA. The Cas7 is essentially known to constitute the major component of the Cascade complex. The present study reports the biochemical characterization of the Cas7 (LinCas7) from the CRISPR-Cas type I-B system of L. interrogans serovar Copenhageni. The pure recombinant LinCas7 (rLinCas7) exists as a monomer in the solution by size exclusion chromatography. The rLinCas7 demonstrates an endoDNase activity dependent upon divalent Mg²⁺ ions, monovalent ions, pH, temperature, and substrate size. Analysis of ribonucleoprotein composite (rLinCas7-crRNA) by electron microscopy and native-PAGE demonstrated that rLinCas7 could oligomerize on the mature CRISPR RNA (crRNA) framework in the presence of Mg²⁺ ions. The ribonucleoprotein composite attains a helical shape similar to the backbone of the Cascade complex. However, in the absence of Mg²⁺ ions, rLinCas7 acts as an RNase. The fluorescence spectroscopy disclosed a weak interaction (K_d = 26.81 mM) between rLinCas7 and Mg²⁺ ions, leading to an overall conformational change in rLinCas7 that modulates the rLinCas7's activity on DNA and RNA substrates. The nuclease activity of LinCas7 characterized in this study aids to the functional divergences among proteins of the Cas7 family from different CRISPR-Cas systems in various organisms.

Structural and biochemical characterization of in vivo assembled Lactococcus lactis CRISPR-Csm complex

The small RNA-mediated immunity in bacteria depends on foreign RNA-activated and self RNA-inhibited enzymatic activities. The multi-subunit Type III-A CRISPR-Cas effector complex (Csm) exemplifies this principle and is in addition regulated by cellular metabolites such as divalent metals and ATP. Recognition of the foreign or cognate target RNA (CTR) triggers its single-stranded deoxyribonuclease (DNase) and cyclic oligoadenylate (cOA) synthesis activities. The same activities remain dormant in the presence of the self or non-cognate target RNA (NTR) that differs from CTR only in its 3'-protospacer flanking sequence (3'-PFS). Here we employ electron cryomicroscopy (cryoEM), functional assays, and comparative cross-linking to study in vivo assembled mesophilic Lactococcus lactis Csm (LlCsm) at the three functional states: apo, the CTR- and the NTR-bound. Unlike previously studied Csm complexes, we observed binding of 3'-PFS to Csm in absence of bound ATP and analyzed the structures of the four RNA cleavage sites. Interestingly, comparative crosslinking results indicate a tightening of the Csm3-Csm4 interface as a result of CTR but not NTR binding, reflecting a possible role of protein dynamics change during activation.

Structural Principles of CRISPR-Cas Enzymes Used in Nucleic Acid Detection

Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)-based technology has revolutionized the field of biomedicine with broad applications in genome editing, therapeutics and diagnostics. While a majority of applications involve the RNA-guided site-specific DNA or RNA cleavage by CRISPR enzymes, recent successes in nucleic acid detection rely on their collateral and non-specific cleavage activated by viral DNA or RNA. Ranging in enzyme composition, the mechanism for distinguishing self- from foreign-nucleic acids, the usage of second messengers, and enzymology, the CRISPR enzymes provide a diverse set of diagnosis tools in further innovations. Structural biology plays an important role in elucidating the mechanisms of these CRISPR enzymes. Here we summarize and compare structures of three types of CRISPR enzymes used in nucleic acid detection captured in their respective functional forms and illustrate the current understanding of their activation mechanism.

Reprogramming CRISPR-Mediated RNA Interference for Silencing of Essential Genes in Sulfolobales

The manipulation of gene expression levels in vivo is often key to elucidating gene function and regulatory network interactions, especially when it comes to the investigation of essential genes that cannot be deleted from the model organism's genome. Several techniques have been developed for prokaryotes that allow to interfere with transcription initiation of specific genes by blocking or modifying promoter regions. However, a tool functionally similar to RNAi used in eukaryotes to efficiently degrade mRNA posttranscriptionally did not exist until recently. Type III CRISPR-Cas systems use small RNAs (crRNAs) that guide effector complexes (encoded by cas genes) which act as site-specific RNA endonuclease and can thus be harnessed for targeted posttranscriptional gene silencing. Guide RNAs complementary to the desired target mRNA that, in addition, exhibit complementarity to repeat sequences found in the CRISPR arrays, effectively suppress unspecific DNA and RNA activities of the CRISPR-Cas complexes. Here we describe the use of endogenous type III CRISPR-Cas systems in two model organisms of Crenarchaeota, Saccharolobus solfataricus and Sulfolobus acidocaldarius.

Specificity and sensitivity of an RNA targeting type III CRISPR complex coupled with a NucC endonuclease effector

Type III CRISPR systems detect invading RNA, resulting in the activation of the enzymatic Cas10 subunit. The Cas10 cyclase domain generates cyclic oligoadenylate (cOA) second messenger molecules, activating a variety of effector nucleases that degrade nucleic acids to provide immunity. The prophage-encoded Vibrio metoecus type III-B (VmeCmr) locus is uncharacterised, lacks the HD nuclease domain in Cas10 and encodes a NucC DNA nuclease effector that is also found associated with Cyclic-oligonucleotide-based anti-phage signalling systems (CBASS). Here we demonstrate that VmeCmr is activated by target RNA binding, generating cyclic-triadenylate (cA₃) to stimulate a robust NucC-mediated DNase activity. The specificity of VmeCmr is probed, revealing the importance of specific nucleotide positions in segment 1 of the RNA duplex and the protospacer flanking sequence (PFS). We harness this programmable system to demonstrate the potential for a highly specific and sensitive assay for detection of the SARS-CoV-2 virus RNA with a limit of detection (LoD) of 2 fM using a commercial plate reader without any extrinsic amplification step. The sensitivity is highly dependent on the guide RNA used, suggesting that target RNA secondary structure plays an important role that may also be relevant in vivo.

Digging into the lesser-known aspects of CRISPR biology

A long time has passed since regularly interspaced DNA repeats were discovered in prokaryotes. Today, those enigmatic repetitive elements termed clustered regularly interspaced short palindromic repeats (CRISPR) are acknowledged as an emblematic part of multicomponent CRISPR-Cas (CRISPR associated) systems. These systems are involved in a variety of roles in bacteria and archaea, notably, that of conferring protection against transmissible genetic elements through an adaptive immune-like response. This review summarises the present knowledge on the diversity, molecular mechanisms and biology of CRISPR-Cas. We pay special attention to the most recent findings related to the determinants and consequences of CRISPR-Cas activity. Research on the basic features of these systems illustrates how instrumental the study of prokaryotes is for understanding biology in general, ultimately providing valuable tools for diverse fields and fuelling research beyond the mainstream.

Correlation between type IIIA CRISPR-Cas system and SCCmec in Staphylococcus epidermidis

A subculture of S.epidermidis strain ATCC35984 that is amenable to genetically manipulate was occasionally found in our laboratory. This mutant exhibited susceptibility to methicillin in contrast to its parent strain. To unveil the underlying mechanism, whole-genome sequencing of the mutant was performed. A comparative analysis revealed that a large DNA fragment encompassing the CRISPR-Cas system, type I R-M system and the SCCmec element was deleted from the mutant. The large chromosomal deletion associated with CRISPR-Cas system was also observed to occur spontaneously in S. epidermidis in another independent laboratory, or artificially induced by introducing engineering crRNAs in other bacterial species. These findings imply the CRISPR-Cas systems can affect bacterial genome remodeling through deletion of the integrated MGEs (mobile genetic elements). Further bioinformatics analysis identified a higher carriage rate of SCCmec element in the S. epidermidis strains harboring the CRISPR-Cas system. MLST typing and phylogenetic analysis of those CRIPSR-Cas-positive S. epidermidis strains revealed multiple origins. In addition, distinct types of SCCmec carried in those strains suggested that acquisition of this MGE originated from multiple independent recombination events. Intriguingly, CRISPR-Cas systems are found to be always located in the vicinity of orfX gene among staphylococci. Allelic analysis of CRISPR loci flanking cas genes disclosed that the loci distal to the orfX gene are considerably stable and conserved, which probably serve as recombination hotspot between CRISPR-Cas system and phage or plasmid. Therefore, the findings generally support the notion that incomplete immune protection of CRISPR-Cas system can promote dissemination of its neighboring SCCmec element.

Virus detection via programmable Type III-A CRISPR-Cas systems

Among the currently available virus detection assays, those based on the programmable CRISPR-Cas enzymes have the advantage of rapid reporting and high sensitivity without the requirement of thermocyclers. Type III-A CRISPR-Cas system is a multi-component and multipronged immune effector, activated by viral RNA that previously has not been repurposed for disease detection owing in part to the complex enzyme reconstitution process and functionality. Here, we describe the construction and application of a virus detection method, based on an in vivo-reconstituted Type III-A CRISPR-Cas system. This system harnesses both RNA- and transcription-activated dual nucleic acid cleavage activities as well as internal signal amplification that allow virus detection with high sensitivity and at multiple settings. We demonstrate the use of the Type III-A system-based method in detection of SARS-CoV-2 that reached 2000 copies/μl sensitivity in amplification-free and 60 copies/μl sensitivity via isothermal amplification within 30 min and diagnosed SARS-CoV-2-infected patients in both settings. The high sensitivity, flexible reaction conditions, and the small molecular-driven amplification make the Type III-A system a potentially unique nucleic acid detection method with broad applications.

Structure-based functional mechanisms and biotechnology applications of anti-CRISPR proteins

CRISPR loci and Cas proteins provide adaptive immunity in prokaryotes against invading bacteriophages and plasmids. In response, bacteriophages have evolved a broad spectrum of anti-CRISPR proteins (anti-CRISPRs) to counteract and overcome this immunity pathway. Numerous anti-CRISPRs have been identified to date, which suppress single-subunit Cas effectors (in CRISPR class 2, type II, V and VI systems) and multisubunit Cascade effectors (in CRISPR class 1, type I and III systems). Crystallography and cryo-electron microscopy structural studies of anti-CRISPRs bound to effector complexes, complemented by functional experiments in vitro and in vivo, have identified four major CRISPR-Cas suppression mechanisms: inhibition of CRISPR-Cas complex assembly, blocking of target binding, prevention of target cleavage, and degradation of cyclic oligonucleotide signalling molecules. In this Review, we discuss novel mechanistic insights into anti-CRISPR function that have emerged from X-ray crystallography and cryo-electron microscopy studies, and how these structures in combination with function studies provide valuable tools for the ever-growing CRISPR-Cas biotechnology toolbox, to be used for precise and robust genome editing and other applications.

Cyclic oligoadenylate signalling and regulation by ring nucleases during type III CRISPR defence

In prokaryotes, CRISPR-Cas immune systems recognise and cleave foreign nucleic acids to defend against Mobile Genetic Elements (MGEs). Type III CRISPR-Cas complexes also synthesise cyclic oligoadenylate (cOA) second messengers, which activate CRISPR ancillary proteins involved in antiviral defence. In particular, cOA-stimulated nucleases degrade RNA and DNA non-specifically, which slows MGE replication but also impedes cell growth, necessitating mechanisms to eliminate cOA in order to mitigate collateral damage. Extant cOA is degraded by a new class of enzyme termed a 'ring nuclease', which cleaves cOA specifically and switches off CRISPR ancillary enzymes. Several ring nuclease families have been characterised to date, including a family used by MGEs to circumvent CRISPR immunity, and encompass diverse protein folds and distinct cOA cleavage mechanisms. In this review we outline cOA signalling, discuss how different ring nucleases regulate the cOA signalling pathway, and reflect on parallels between cyclic nucleotide-based immune systems to reveal new areas for exploration.

Structure of a type IV CRISPR-Cas ribonucleoprotein complex

We reveal the cryo-electron microscopy structure of a type IV-B CRISPR ribonucleoprotein (RNP) complex (Csf) at 3.9-Å resolution. The complex best resembles the type III-A CRISPR Csm effector complex, consisting of a Cas7-like (Csf2) filament intertwined with a small subunit (Cas11) filament, but the complex lacks subunits for RNA processing and target DNA cleavage. Surprisingly, instead of assembling around a CRISPR-derived RNA (crRNA), the complex assembles upon heterogeneous RNA of a regular length arranged in a pseudo-A-form configuration. These findings provide a high-resolution glimpse into the assembly and function of enigmatic type IV CRISPR systems, expanding our understanding of class I CRISPR-Cas system architecture, and suggesting a function for type IV-B RNPs that may be distinct from other class 1 CRISPR-associated systems.

Structural basis for self-cleavage prevention by tag:anti-tag pairing complementarity in type VI Cas13 CRISPR systems

Bacteria and archaea apply CRISPR-Cas surveillance complexes to defend against foreign invaders. These invading genetic elements are captured and integrated into the CRISPR array as spacer elements, guiding sequence-specific DNA/RNA targeting and cleavage. Recently, in vivo studies have shown that target RNAs with extended complementarity with repeat sequences flanking the target element (tag:anti-tag pairing) can dramatically reduce RNA cleavage by the type VI-A Cas13a system. Here, we report the cryo-EM structure of Leptotrichia shahii LshCas13acrRNA in complex with target RNA harboring tag:anti-tag pairing complementarity, with the observed conformational changes providing a molecular explanation for inactivation of the composite HEPN domain cleavage activity. These structural insights, together with in vitro biochemical and in vivo cell-based assays on key mutants, define the molecular principles underlying Cas13a's capacity to target and discriminate between self and non-self RNA targets. Our studies illuminate approaches to regulate Cas13a's cleavage activity, thereby influencing Cas13a-mediated biotechnological applications.