Crystal Structure of Streptococcus pyogenes Cas1 and Its Interaction with Csn2 in the Type II CRISPR-Cas System

A game of resistance: War between bacteria and phages and how phage cocktails can be the solution

While phages hold promise as an antibiotic alternative, they encounter significant challenges in combating bacterial infections, primarily due to the emergence of phage-resistant bacteria. Bacterial defence mechanisms like superinfection exclusion, CRISPR, and restriction-modification systems can hinder phage effectiveness. Innovative strategies, such as combining different phages into cocktails, have been explored to address these challenges. This review delves into these defence mechanisms and their impact at each stage of the infection cycle, their challenges, and the strategies phages have developed to counteract them. Additionally, we examine the role of phage cocktails in the evolving landscape of antibacterial treatments and discuss recent studies that highlight the effectiveness of diverse phage cocktails in targeting essential bacterial receptors and combating resistant strains.

Highly active CRISPR-adaptation proteins revealed by a robust enrichment technology

Natural prokaryotic defense via the CRISPR-Cas system requires spacer integration into the CRISPR array in a process called adaptation. To search for adaptation proteins with enhanced capabilities, we established a robust perpetual DNA packaging and transfer (PeDPaT) system that uses a strain of T7 phage to package plasmids and transfer them without killing the host, and then uses a different strain of T7 phage to repeat the cycle. We used PeDPaT to identify better adaptation proteins-Cas1 and Cas2-by enriching mutants that provide higher adaptation efficiency. We identified two mutant Cas1 proteins that show up to 10-fold enhanced adaptation in vivo. In vitro, one mutant has higher integration and DNA binding activities, and another has a higher disintegration activity compared to the wild-type Cas1. Lastly, we showed that their specificity for selecting a protospacer adjacent motif is decreased. The PeDPaT technology may be used for many robust screens requiring efficient and effortless DNA transduction.

The core Cas1 protein of CRISPR-Cas I-B in Leptospira shows metal-tunable nuclease activity

Leptospira interrogans serovar Copenhageni strain Fiocruz L1-130 is the causative agent of leptospirosis in animals and humans. This organism carries a functional cas1 gene classified under CRISPR-Cas I-B. In this study, using various nuclease assays and bioinformatics analysis, we report that the recombinant Cas1 (LinCas1) possesses metal-ion dependent DNase activity, which is inhibited upon substitution or chelation of metal-ion and/or interaction with recombinant Cas2 (LinCas2) of L. interrogans. Model of LinCas1 structure shows a shorter N-terminal domain unlike other Cas1 orthologs reported to date. The C-terminal domain of LinCas1 contains conserved divalent-metal binding residues (Glu108, His176, and Glu191) and the mutation of these residues leads to abolition in DNase activity. Immunoassay using anti-LinCas2 demonstrates that LinCas1 interacts with LinCas2 and attains a saturation point. Moreover, the nuclease activity of the LinCas1-Cas2 mixture on ds-DNA displayed a reduction in activity compared to the pure core LinCas proteins under in vitro condition. The DNase activity for LinCas1 is consistent with a role for this protein in the recognition/cleavage of foreign DNA and integration of foreign DNA as spacer into the CRISPR array.

Disintegration promotes protospacer integration by the Cas1-Cas2 complex

‘Disintegration’—the reversal of transposon DNA integration at a target site—is regarded as an abortive off-pathway reaction. Here, we challenge this view with a biochemical investigation of the mechanism of protospacer insertion, which is mechanistically analogous to DNA transposition, by the Streptococcus pyogenes Cas1-Cas2 complex. In supercoiled target sites, the predominant outcome is the disintegration of one-ended insertions that fail to complete the second integration event. In linear target sites, one-ended insertions far outnumber complete protospacer insertions. The second insertion event is most often accompanied by the disintegration of the first, mediated either by the 3′-hydroxyl exposed during integration or by water. One-ended integration intermediates may mature into complete spacer insertions via DNA repair pathways that are also involved in transposon mobility. We propose that disintegration-promoted integration is functionally important in the adaptive phase of CRISPR-mediated bacterial immunity, and perhaps in other analogous transposition reactions.

A CRISPR-Cas9-integrase complex generates precise DNA fragments for genome integration

CRISPR-Cas9 is an RNA-guided DNA endonuclease involved in bacterial adaptive immunity and widely repurposed for genome editing in human cells, animals and plants. In bacteria, RNA molecules that guide Cas9's activity derive from foreign DNA fragments that are captured and integrated into the host CRISPR genomic locus by the Cas1-Cas2 CRISPR integrase. How cells generate the specific lengths of DNA required for integrase capture is a central unanswered question of type II-A CRISPR-based adaptive immunity. Here, we show that an integrase supercomplex comprising guide RNA and the proteins Cas1, Cas2, Csn2 and Cas9 generates precisely trimmed 30-base pair DNA molecules required for genome integration. The HNH active site of Cas9 catalyzes exonucleolytic DNA trimming by a mechanism that is independent of the guide RNA sequence. These results show that Cas9 possesses a distinct catalytic capacity for generating immunological memory in prokaryotes.

Deletion of csn2 gene affects acid tolerance and exopolysaccharide synthesis in Streptococcus mutans

Csn2 is an important protein of the CRISPR-Cas system. The physiological function of this protein and its regulatory role in Streptococcus mutans, as the primary causative agent of human dental caries, is still unclear. In this study, we investigated whether csn2 deletion would affect S. mutans physiology and virulence gene expression. We used microscopic imaging, acid killing assays, pH drop, biofilm formation, and exopolysaccharide (EPS) production tests to determine whether csn2 deletion influenced S. mutans colony morphology, acid tolerance/production, and glucan formation abilities. Comparisons were made between quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) data from the UA159 and csn2 deletion strain to determine the impact of csn2 knockout on S. mutans gene expression. The results showed that deletion of S. mutans csn2 changed its colony morphotype and made it more sensitive to acid. The expression levels of aciduricity genes, including leuA, leuB, leuC, and leuD, were significantly down-regulated. Acid adaptation restored the aciduricity of csn2 mutant and enhanced the ability to synthesize EPS. The expression levels of EPS synthesis-related genes, including gtfC and gtfD, were significantly up-regulated after acid adaptation. In summary, deletion of S. mutans csn2 exerted multiple effects on the virulence traits of this pathogen, including acid tolerance and EPS formation, and that these alterations could partially be attributed to changes in gene expression upon loss of csn2. Understanding the function of csn2 in S. mutans might lead to novel strategies to prevent or treat imbalances in oral microbiota that may favor diseases.

GalaxyRefine2: simultaneous refinement of inaccurate local regions and overall protein structure

The 3D structure of a protein can be predicted from its amino acid sequence with high accuracy for a large fraction of cases because of the availability of large quantities of experimental data and the advance of computational algorithms. Recently, deep learning methods exploiting the coevolution information obtained by comparing related protein sequences have been successfully used to generate highly accurate model structures even in the absence of template structure information. However, structures predicted based on either template structures or related sequences require further improvement in regions for which information is missing. Refining a predicted protein structure with insufficient information on certain regions is critical because these regions may be connected to functional specificity that is not conserved among related proteins. The GalaxyRefine2 web server, freely available via http://galaxy.seoklab.org/refine2, is an upgraded version of the GalaxyRefine protein structure refinement server and reflects recent developments successfully tested through CASP blind prediction experiments. This method adopts an iterative optimization approach involving various structure move sets to refine both local and global structures. The estimation of local error and hybridization of available homolog structures are also employed for effective conformation search.

Real-time observation of CRISPR spacer acquisition by Cas1-Cas2 integrase

Cas1 integrase associates with Cas2 to insert short DNA fragments into a CRISPR array, establishing nucleic acid memory in prokaryotes. Here we applied single-molecule FRET methods to the Enterococcus faecalis (Efa) Cas1–Cas2 system to establish a kinetic framework describing target-searching, integration, and post-synapsis events. EfaCas1–Cas2 on its own is not able to find the CRISPR repeat in the CRISPR array; it only does so after prespacer loading. The leader sequence adjacent to the repeat further stabilizes EfaCas1–Cas2 contacts, enabling leader-side integration and subsequent spacer-side integration. The resulting post-synaptic complex has a surprisingly short mean lifetime. Remarkably, transcription efficiently resolves the postsynaptic complex and we predict that this is a conserved mechanism that ensures efficient and directional spacer integration in many CRISPR systems. Overall, our study provides a complete model of spacer acquisition, which can be harnessed for DNA-based information storage and cell lineage tracing technologies.

CRISPR DNA elements controlling site-specific spacer integration and proper repeat length by a Type II CRISPR-Cas system

CRISPR–Cas systems provide heritable immunity against viruses by capturing short invader DNA sequences, termed spacers, and incorporating them into the CRISPR loci of the prokaryotic host genome. Here, we investigate DNA elements that control accurate spacer uptake in the type II-A CRISPR locus of Streptococcus thermophilus. We determined that purified Cas1 and Cas2 proteins catalyze spacer integration with high specificity for CRISPR repeat junctions. We show that 10 bp of the CRISPR leader sequence is critical for stimulating polarized integration preferentially at the repeat proximal to the leader. Spacer integration proceeds through a two-step transesterification reaction where the 3′ hydroxyl groups of the spacer target both repeat borders on opposite strands. The leader-proximal end of the repeat is preferentially targeted for the first site of integration through recognition of sequences spanning the leader-repeat junction. Subsequently, second-site integration at the leader-distal end of the repeat is specified by multiple determinants including a length-defining mechanism relying on a repeat element proximal to the second site of integration. Our results highlight the intrinsic ability of type II Cas1/Cas2 proteins to coordinate directional and site-specific spacer integration into the CRISPR locus to ensure precise duplication of the repeat required for CRISPR immunity.

Cas4 Facilitates PAM-Compatible Spacer Selection during CRISPR Adaptation

CRISPR-Cas systems adapt their immunological memory against their invaders by integrating short DNA fragments into clustered regularly interspaced short palindromic repeat (CRISPR) loci. While Cas1 and Cas2 make up the core machinery of the CRISPR integration process, various class I and II CRISPR-Cas systems encode Cas4 proteins for which the role is unknown. Here, we introduced the CRISPR adaptation genes cas1, cas2, and cas4 from the type I-D CRISPR-Cas system of Synechocystis sp. 6803 into Escherichia coli and observed that cas4 is strictly required for the selection of targets with protospacer adjacent motifs (PAMs) conferring I-D CRISPR interference in the native host Synechocystis. We propose a model in which Cas4 assists the CRISPR adaptation complex Cas1-2 by providing DNA substrates tailored for the correct PAM. Introducing functional spacers that target DNA sequences with the correct PAM is key to successful CRISPR interference, providing a better chance of surviving infection by mobile genetic elements.

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Cas4 facilitates the integration of PAM-compatible spacers

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Spacer length variation is dictated by Cas1-2

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Cas4 shortens spacer length

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Cas4-selected PAMs license type I-D CRISPR interference

Insights to the Assembly of a Functionally Active Leptospiral ClpP1P2 Protease Complex along with Its ATPase Chaperone ClpX

Leptospira interrogans genome is predicted to encode multiple isoforms of caseinolytic proteases (ClpP1 and ClpP2). The ClpP proteins with the aid of its ATPase chaperone are known to be involved in establishing cellular proteostasis and have emerged as a target for developing new antibiotics. We report the molecular characterization of recombinant ClpP1 (rClpP1) and rClpP2 of Leptospira along with its ATPase chaperone rClpX. The two isoforms of rClpPs when coupled together in an equivalent concentration exhibit optimum activity on small fluorogenic peptide substrates, whereas the pure rClpP isoforms are enzymatically inactive. Isothermal titration calorimetry analysis suggests that the two rClpP isoforms bind each other moderately in a 1:1 stoichiometry with a dissociation constant of 2.02 ± 0.1 μM at 37 °C and is thermodynamically favored. Size exclusion chromatography fractionates the majority of pure rClpP1 at ≥308 kDa (14-21-mer) and the pure rClpP2 at 308 kDa (tetradecamer), whereas the functionally active rClpP isoform mixture fractionates as a tetradecamer. The distinct and unprecedented oligomeric form of rClpP1 was also evident through native-gel and dynamic light scattering. Moreover, the rClpP isoform mixture formed after the site-directed mutation of either or both the isoforms at one of the catalytic triad residues (Ser 98/97 to Ala 98/97) resulted in the complete loss of protease activity. The rClpP isoform mixture gets stimulated to degrade the casein substrate in the presence of rClpX and in an energy-dependent manner. On the contrary, pure rClpP1 or the rClpP2 isoform in association with rClpX are incapable of forming operative protease. The reported finding suggests that in Leptospira, the enzymatic activity of the rClpP protease complex in the presence or absence of cochaperone is performed solely by the tetradecamer structure which is hypothesized to be composed of 2-stacked ClpP heptameric rings, wherein each ring is a homo-oligomer of ClpP1 and ClpP2 subunits. Understanding the activities and regulation principle of multi-isoforms of ClpP in pathogenic bacteria may aid in intervening disease outcomes particularly to the co-evolving antibiotic resistance strains.

Molecular organization of the type II-A CRISPR adaptation module and its interaction with Cas9 via Csn2

Clustered regularly interspaced short palindromic repeats (CRISPRs) and CRISPR-associated (Cas) proteins provide microbial adaptive immunity against invading foreign nucleic acids. In type II-A CRISPR–Cas systems, the Cas1–Cas2 integrase complex and the subtype-specific Csn2 comprise the CRISPR adaptation module, which cooperates with the Cas9 nuclease effector for spacer selection. Here, we report the molecular organization of the Streptococcus pyogenes type II-A CRISPR adaptation module and its interaction with Cas9 via Csn2. We determined the crystal structure of S. pyogenes type II-A Cas2. Chromatographic and calorimetric analyses revealed the stoichiometry and topology of the type II-A adaptation module composed of Cas1, Cas2 and Csn2. We also demonstrated that Cas9 interacts with Csn2 in a direct and stoichiometric manner. Our results reveal a network of molecular interactions among type II-A Cas proteins and highlight the role of Csn2 in coordinating Cas components involved in the adaptation and interference stages of CRISPR-mediated immunity.

CRISPR-Cas adaptive immunity and the three Rs

In this summary, we focus on fundamental biology of Clustered Regularly Interspersed Short Palindromic Repeats (CRISPR)-Cas (CRISPR-associated proteins) adaptive immunity in bacteria. Emphasis is placed on emerging information about functional interplay between Cas proteins and proteins that remodel DNA during homologous recombination (HR), DNA replication or DNA repair. We highlight how replication forks may act as ‘trigger points’ for CRISPR adaptation events, and the potential for cascade-interference complexes to act as precise roadblocks in DNA replication by an invader MGE (mobile genetic element), without the need for DNA double-strand breaks.

CRISPR-Cas: Adapting to change

Bacteria and archaea are engaged in a constant arms race to defend against the ever-present threats of viruses and invasion by mobile genetic elements. The most flexible weapons in the prokaryotic defense arsenal are the CRISPR-Cas adaptive immune systems. These systems are capable of selective identification and neutralization of foreign DNA and/or RNA. CRISPR-Cas systems rely on stored genetic memories to facilitate target recognition. Thus, to keep pace with a changing pool of hostile invaders, the CRISPR memory banks must be regularly updated with new information through a process termed CRISPR adaptation. In this Review, we outline the recent advances in our understanding of the molecular mechanisms governing CRISPR adaptation. Specifically, the conserved protein machinery Cas1-Cas2 is the cornerstone of adaptive immunity in a range of diverse CRISPR-Cas systems.

Protecting genome integrity during CRISPR immune adaptation

Bacterial CRISPR-Cas systems include genomic arrays of short repeats flanking foreign DNA sequences and provide adaptive immunity against viruses. Integration of foreign DNA must occur specifically to avoid damaging the genome or the CRISPR array, but surprisingly promiscuous activity occurs in vitro. Here we reconstituted full-site DNA integration and show that the Streptococcus pyogenes type II-A Cas1-Cas2 integrase maintains specificity in part through limitations on the second integration step. At non-CRISPR sites, integration stalls at the half-site intermediate, thereby enabling reaction reversal. S. pyogenes Cas1-Cas2 is highly specific for the leader-proximal repeat and recognizes the repeat's palindromic ends, thus fitting a model of independent recognition by distal Cas1 active sites. These findings suggest that DNA-insertion sites are less common than suggested by previous work, thereby preventing toxicity during CRISPR immune adaptation and maintaining host genome integrity.