The CRISPRs, they are a-changin': how prokaryotes generate adaptive immunity

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.

Aptamer inhibitor selection of SpyCas9 through CE-SELEX

CRISPR/Cas9 is a natural immune system of archaea and bacteria, which has been widely used in gene editing. In order to better control and improve the accuracy and safety of the system, inhibitors for SpyCas9 as "switches" have been selected for several years. The available inhibitors currently are all natural polypeptides inhibitors derived from phages, except one small molecule inhibitor. These natural inhibitors are challenging to obtain and are available in limited quantities, and the small molecule inhibitor is cytotoxic. Herein, we discover aptamers against the SpyCas9 protein, by coupling CE-SELEX within one-round pressure controllable selection strategy. One of the identified aptamers, Apt2, shows high affinity at the nanomolar level and leads for effective SpyCas9 enzymatic inhibition in vitro. It is predicted that Apt2 interacts with the HNH and RuvC domains of SpyCas9, competitively inhibiting the binding of substrate DNA to SpyCas9. The proposed aptamer inhibitor is the oligonucleotide inhibitor of SpyCas9, which has the potential in construction of the universal, simple and precise CRISPR-Cas9 system activity control strategy. Meanwhile, these aptamers could also be valuable tools for study of the functions of CRISPR/Cas9 and the related functional mechanisms.

CRISPR genotyping methods: Tracing the evolution from spoligotyping to machine learning

CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated) systems provide prokaryotes with adaptive immunity defenses against foreign genetic invaders. The identification of CRISPR-Cas function is among the most impactful discoveries of recent decades that have shaped the development of genome editing in various organisms paving the way for a plethora of promising applications in biotechnology and health. Even before the discovery of CRISPR-Cas biological role, the particular structure of CRISPR loci has been explored for epidemiological genotyping of bacterial pathogens. CRISPR-Cas loci are arranged in CRISPR arrays of mostly identical direct repeats intercalated with invader-derived spacers and an operon of cas genes encoding the Cas protein components. Each small CRISPR RNA (crRNA) encoded within the CRISPR array constitutes a key functional unit of this RNA-based CRISPR-Cas defense system guiding the Cas effector proteins toward the foreign nucleic acids for their destruction. The information acquired from prior invader encounters and stored within CRISPR arrays turns out to be extremely valuable in tracing the microevolution and epidemiology of major bacterial pathogens. We review here the history of CRISPR-based typing strategies highlighting the first PCR-based methods that have set the stage for recent developments of high-throughput sequencing and machine learning-based approaches. A great amount of whole genome sequencing and metagenomic data accumulated in recent years opens up new avenues for combining experimental and computational approaches of high-resolution CRISPR-based typing.

Expression and Functional Analysis of the Compact Thermophilic Anoxybacillus flavithermus Cas9 Nuclease

Research on Cas9 nucleases from different organisms holds great promise for advancing genome engineering and gene therapy tools, as it could provide novel structural insights into CRISPR editing mechanisms, expanding its application area in biology and medicine. The subclass of thermophilic Cas9 nucleases is actively expanding due to the advances in genome sequencing allowing for the meticulous examination of various microorganisms' genomes in search of the novel CRISPR systems. The most prominent thermophilic Cas9 effectors known to date are GeoCas9, ThermoCas9, IgnaviCas9, AceCas9, and others. These nucleases are characterized by a varying temperature range of the activity and stringent PAM preferences; thus, further diversification of the naturally occurring thermophilic Cas9 subclass presents an intriguing task. This study focuses on generating a construct to express a compact Cas9 nuclease (AnoCas9) from the thermophilic microorganism Anoxybacillus flavithermus displaying the nuclease activity in the 37-60 °C range and the PAM preference of 5'-NNNNCDAA-3' in vitro. Here, we highlight the close relation of AnoCas9 to the GeoCas9 family of compact thermophilic Cas9 effectors. AnoCas9, beyond broadening the repertoire of Cas9 nucleases, suggests application in areas requiring the presence of thermostable CRISPR/Cas systems in vitro, such as sequencing libraries' enrichment, allele-specific isothermal PCR, and others.

Utility of CRISPR/Cas mediated electrochemical biosensors

Electrochemical biosensors represent a class of sensors that employ biological materials as sensitive elements, electrodes as conversion elements, and potential or current as detection signals. The integration of CRISPR/Cas systems into electrochemical biosensors holds immense potential, offering enhanced versatility, heightened sensitivity and specificity, reduced recovery time, and the ability to capture and identify analytes at low concentrations. In this review, we provided a succinct summary of the fundamental principles underlying electrochemical biosensors and CRISPR/Cas systems, and new progress of electrochemical biosensors based on CRISPR/Cas systems in virus, bacteria, and cancer detections. Besides, we discussed its pros and cons, present gaps, potential problem-solvers, and future prospects. To sum up, CRISPR/Cas mediated electrochemical biosensors will surely benefit us a lot in the detection of cells and microorganisms, and of course in other promising fields.

Unveil the Secret of the Bacteria and Phage Arms Race

Bacteria have developed different mechanisms to defend against phages, such as preventing phages from being adsorbed on the surface of host bacteria; through the superinfection exclusion (Sie) block of phage's nucleic acid injection; by restricting modification (R-M) systems, CRISPR-Cas, aborting infection (Abi) and other defense systems to interfere with the replication of phage genes in the host; through the quorum sensing (QS) enhancement of phage's resistant effect. At the same time, phages have also evolved a variety of counter-defense strategies, such as degrading extracellular polymeric substances (EPS) that mask receptors or recognize new receptors, thereby regaining the ability to adsorb host cells; modifying its own genes to prevent the R-M systems from recognizing phage genes or evolving proteins that can inhibit the R-M complex; through the gene mutation itself, building nucleus-like compartments or evolving anti-CRISPR (Acr) proteins to resist CRISPR-Cas systems; and by producing antirepressors or blocking the combination of autoinducers (AIs) and its receptors to suppress the QS. The arms race between bacteria and phages is conducive to the coevolution between bacteria and phages. This review details bacterial anti-phage strategies and anti-defense strategies of phages and will provide basic theoretical support for phage therapy while deeply understanding the interaction mechanism between bacteria and phages.

Regulation of CRISPR-Associated Genes by Rv1776c (CasR) in Mycobacterium tuberculosis

The CRISPR-Cas system is an adaptive immune system for many bacteria and archaea to defend against foreign nucleic acid invasion, and this system is conserved in the genome of M. tuberculosis (Mtb). Although the CRISPR-Cas system-mediated immune defense mechanism has been revealed in Mtb, the regulation of cas gene expression is poorly understood. In this study, we identified a transcription factor, CasR (CRISPR-associated protein repressor, encoded by Rv1776c), and it could bind to the upstream DNA sequence of the CRISPR-Cas gene cluster and regulate the expression of cas genes. EMSA and ChIP assays confirmed that CasR could interact with the upstream sequence of the csm6 promoter, both in vivo and in vitro. Furthermore, DNA footprinting assay revealed that CasR recognized a 20 bp palindromic sequence motif and negatively regulated the expression of csm6. In conclusion, our research elucidates the regulatory effect of CasR on the expression of CRISPR-associated genes in mycobacteria, thus providing insight into gene expression regulation of the CRISPR-Cas system.

Recent Advances in Genome-Engineering Strategies

In October 2020, the chemistry Nobel Prize was awarded to Emmanuelle Charpentier and Jennifer A. Doudna for the discovery of a new promising genome-editing tool: the genetic scissors of CRISPR-Cas9. The identification of CRISPR arrays and the subsequent identification of cas genes, which together represent an adaptive immunological system that exists not only in bacteria but also in archaea, led to the development of diverse strategies used for precise DNA editing, providing new insights in basic research and in clinical practice. Due to their advantageous features, the CRISPR-Cas systems are already employed in several biological and medical research fields as the most suitable technique for genome engineering. In this review, we aim to describe the CRISPR-Cas systems that have been identified among prokaryotic organisms and engineered for genome manipulation studies. Furthermore, a comprehensive comparison between the innovative CRISPR-Cas methodology and the previously utilized ZFN and TALEN editing nucleases is also discussed. Ultimately, we highlight the contribution of CRISPR-Cas methodology in modern biomedicine and the current plethora of available applications for gene KO, repression and/or overexpression, as well as their potential implementation in therapeutical strategies that aim to improve patients' quality of life.

Biology and engineering of integrative and conjugative elements: Construction and analyses of hybrid ICEs reveal element functions that affect species-specific efficiencies

Integrative and conjugative elements (ICEs) are mobile genetic elements that reside in a bacterial host chromosome and are prominent drivers of bacterial evolution. They are also powerful tools for genetic analyses and engineering. Transfer of an ICE to a new host involves many steps, including excision from the chromosome, DNA processing and replication, transfer across the envelope of the donor and recipient, processing of the DNA, and eventual integration into the chromosome of the new host (now a stable transconjugant). Interactions between an ICE and its host throughout the life cycle likely influence the efficiencies of acquisition by new hosts. Here, we investigated how different functional modules of two ICEs, Tn916 and ICEBs1, affect the transfer efficiencies into different host bacteria. We constructed hybrid elements that utilize the high-efficiency regulatory and excision modules of ICEBs1 and the conjugation genes of Tn916. These elements produced more transconjugants than Tn916, likely due to an increase in the number of cells expressing element genes and a corresponding increase in excision. We also found that several Tn916 and ICEBs1 components can substitute for one another. Using B. subtilis donors and three Enterococcus species as recipients, we found that different hybrid elements were more readily acquired by some species than others, demonstrating species-specific interactions in steps of the ICE life cycle. This work demonstrates that hybrid elements utilizing the efficient regulatory functions of ICEBs1 can be built to enable efficient transfer into and engineering of a variety of other species.

Horizontal gene transfer helps drive microbial evolution, enabling bacteria to rapidly acquire new genes and traits. Integrative and conjugative elements (ICEs) are mobile genetic elements that reside in a bacterial host chromosome and are prominent drivers of horizontal gene transfer. They are also powerful tools for genetic analyses and engineering. Some ICEs carry genes that confer obvious properties to host bacteria, including antibiotic resistances, symbiosis, and pathogenesis. When activated, an ICE-encoded machine is made that can transfer the element to other cells, where it then integrates into the chromosome of the new host. Specific ICEs transfer more effectively into some bacterial species compared to others, yet little is known about the determinants of the efficiencies and specificity of acquisition by different bacterial species. We made and utilized hybrid ICEs, composed of parts of two different elements, to investigate determinants of transfer efficiencies. Our findings demonstrate that there are species-specific interactions that help determine efficiencies of stable acquisition, and that this explains, in part, the efficiencies of different ICEs. These hybrid elements are also useful in genetic engineering and synthetic biology to move genes and pathways into different bacterial species with greater efficiencies than can be achieved with naturally occurring ICEs.

Genomic insights of Acinetobacter baumannii ST374 reveal wide and increasing resistome and virulome

WGS-based surveillance has significantly improved the ability to track global spread and emergence of multidrug-resistant clones of clinically relevant pathogens. In this study, we performed the genomic characterization and comparative analysis of an Acinetobacter baumannii (strain Ac56) belonging to the sequence type ST374, which was isolated for the first time in Brazil, in 1996. Genomic analysis of Ac56 predicted a total of 5373 genes, with 3012 being identical across nine genomes of A. baumannii isolates of ST374 from European, Asian, North and South American countries. GoeBURST analysis grouped ST374 lineages into clonal complex CC3 (international clone IC-III). Resistome analysis of ST374 clone predicted genes associated with resistance to heavy metals and clinically relevant beta-lactams and aminoglycosides antibiotics. In this regard, in two closely related A. baumannii strains, the intrinsic bla_ADC gene was linked to the insertion sequence ISAba1; including the Ac56 strain, where it has been possibly associated with intermediate susceptibility to meropenem. Other four carbapenem-resistant A. baumannii strains carried the ISAba1/bla_OXA-23 gene array, which was associated with the transposon Tn2008 or with Tn2006 in an AbaR4-type resistance island. While most virulence genes were shared for A. baumannii strains of ST374, three isolates from Thailand harbored KL49 capsular loci, previously identified in the hypervirulent A. baumannii LAC-4 strain. Analysis of thirty-four predicted plasmids showed eight major groups, of which GR-6 (LN-1) and GR-2 (LN-2) were prevalent. All strains, including the earliest isolate Ac56 harbored at least one complete prophage, whereas none CRISPR-associated (cas) gene was detected. In summary, genomic data of A. baumannii ST374 reveal a potential of this lineage to become a successful clone.

A Review: Computational Approaches to Design sgRNA of CRISPR-Cas9

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Clustered regularly interspaced short palindromic repeats along with CRISPR-associated protein mechanisms preserve the memory of previous experiences with DNA invaders, in particular spacers that are embedded in CRISPR arrays between coordinate repeats. There has been a fast progression in the comprehension of this immune system and its implementations; however, there are numerous points of view that anticipate explanations to make the field an energetic research zone. The efficiency of CRISPR-Cas depends upon well-considered single guide RNA; for this purpose, many bioinformatics methods and tools are created to support the design of greatly active and precise single guide RNA. Insilico single guide RNA architecture is a crucial point for effective gene editing by means of the CRISPR technique. Persistent attempts have been made to improve in-silico single guide RNA formulation having great on-target effectiveness and decreased off-target effects. This review offers a summary of the CRISPR computational tools to help different researchers pick a specific tool for their work according to pros and cons, along with new thoughts to make new computational tools to overcome all existing limitations.

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.

The CRISPR-associated Cas4 protein from Leptospira interrogans demonstrate versatile nuclease activity

The Cas4 protein is one of the core CRISPR-associated (Cas) proteins implicated in the adaptation module in many variants of the CRISPR-Cas system in prokaryotes against the invading genetic elements. Cas4 is recognized as a DNA exonuclease that contains a RecB nuclease domain and a Fe-S cluster-binding module. In Leptospira interrogans serovar Copenhageni strain Fiocruz L1-130, the cas4 gene is functionally transcribed as an active component of the CRISPR-Cas I-B system. Investigation of nuclease activity of Cas4 (LinCas4) of the L. interrogans illustrated divalent-metal cofactor (Mn2+ or Mg2+) dependent endonuclease activity on the DNA substrate. In agreement, mutation of the selective metal interacting residues (Asp74 and Glu87) curtails the DNA cleavage activity in LinCas4. Computational modeling shows metal-ion interacting residues (Asp74 and Glu87) in the LinCas4 to be a part of the RecB motifs II and III, the same as other Cas4 orthologs. The mutation of a potential DNA interacting residue in the LinCas4 (LinCas4Y132A) or one of the four cysteine residues (LinCas4C18A) involved in coordinating the 4Fe-4S cluster did not perturb its DNase activity. Iron chelation assay of the purified LinCas4 demonstrated it in the apostate conformation. Reconstitution of the Fe-S cluster in the LinCas4 under in vitro condition displayed its coordination with four iron atoms per LinCas4 monomer and was confirmed by the UV and CD spectroscopy studies.

From genome scissors to molecular scalpel: evolution of CRISPR systems

From bizarre palindromic repeats to a bacterial defense mechanism, to genome editing tool, and more, Clustered Regularly Interspaced Short Palindromic Repeats or CRISPR has significantly impacted the way we study genome modification in less than a decade. In this review, we would like to highlight some key players over 30 years of research and explain this biotechnological tool's basic mechanisms. We also refer to the evolution of the CRISPR variants and some of the applications derived from them. The understanding and upgrading of this system will be a valuable tool in the years to come to solve some of the challenges in diverse fields from pharmaceuticals to therapeutics, from basic plant genetics to crop improvement, from metabolic engineering to waste management and industrial processing.

Evolutionary Ecology and Interplay of Prokaryotic Innate and Adaptive Immune Systems

Like many organisms, bacteria and archaea have both innate and adaptive immune systems to defend against infection by viruses and other parasites. Innate immunity most commonly relies on the endonuclease-mediated cleavage of any incoming DNA that lacks a specific epigenetic modification, through a system known as restriction-modification. CRISPR-Cas-mediated adaptive immunity relies on the insertion of short DNA sequences from parasite genomes into CRISPR arrays on the host genome to provide sequence-specific protection. The discovery of each of these systems has revolutionised our ability to carry out genetic manipulations, and, as a consequence, the enzymes involved have been characterised in exquisite detail. In comparison, much less is known about the importance of these two arms of the defence for the ecology and evolution of prokaryotes and their parasites. Here, we review our current ecological and evolutionary understanding of these systems in isolation, and discuss the need to study how innate and adaptive immune responses are integrated when they coexist in the same cell.

CRISPR-mediated host genomic DNA damage is efficiently repaired through microhomology-mediated end joining in Zymomonas mobilis

CRISPR-Cas systems provide bacteria and archaea with adaptive immunity against mobile genetic elements (MGEs) through uptake of invader-derived spacers. De novo adaptation samples spacers from both invaders and hosts, whereas primed adaptation shows higher specificity to sample spacers from invaders in many model systems as well as in the subtype I-F system of Zymomonas mobilis. Self-derived spacers will lead to CRISPR self-interference. However, our in vivo study demonstrated that this species used the microhomology-mediated end joining (MMEJ) pathway to efficiently repair subtype I-F CRISPR-Cas system-mediated DNA breaks guided by the self-targeting spacers. MMEJ repair of DNA breaks requires direct microhomologous sequences flanking the protospacers and leads to DNA deletions covering the protospacers. Importantly, CRISPR-mediated genomic DNA breaks failed to be repaired via MMEJ pathway in presence of higher copies of short homologous DNA. Moreover, CRISPR-cleaved exogenous plasmid DNA was failed to be repaired through MMEJ pathway, probably due to the inhibition of MMEJ by the presence of higher copies of the plasmid DNA in Z. mobilis. Our results infer that MMEJ pathway discriminates DNA damages between in the host chromosome versus mobile genetic element (MGE) DNA, and maintains genome stability post CRISPR immunity in Z. mobilis.

Prophages are associated with extensive CRISPR-Cas auto-immunity

CRISPR-Cas systems require discriminating self from non-self DNA during adaptation and interference. Yet, multiple cases have been reported of bacteria containing self-targeting spacers (STS), i.e. CRISPR spacers targeting protospacers on the same genome. STS has been suggested to reflect potential auto-immunity as an unwanted side effect of CRISPR-Cas defense, or a regulatory mechanism for gene expression. Here we investigated the incidence, distribution, and evasion of STS in over 100 000 bacterial genomes. We found STS in all CRISPR-Cas types and in one fifth of all CRISPR-carrying bacteria. Notably, up to 40% of I-B and I-F CRISPR-Cas systems contained STS. We observed that STS-containing genomes almost always carry a prophage and that STS map to prophage regions in more than half of the cases. Despite carrying STS, genetic deterioration of CRISPR-Cas systems appears to be rare, suggesting a level of escape from the potentially deleterious effects of STS by other mechanisms such as anti-CRISPR proteins and CRISPR target mutations. We propose a scenario where it is common to acquire an STS against a prophage, and this may trigger more extensive STS buildup by primed spacer acquisition in type I systems, without detrimental autoimmunity effects as mechanisms of auto-immunity evasion create tolerance to STS-targeted prophages.

Improving Cancer Immunotherapy with CRISPR-Based Technology

The rapidly evolving field of immunotherapy has attracted great attention in the field of cancer research and already revolutionized the clinical practice standard for treating cancer. Genetically engineered T cells expressing either T cell receptors or chimeric antigen receptors represent novel treatment modalities and are considered powerful weapons to fight cancer. The immune checkpoint blockade, which harnesses the negative control signaling behind the anti-tumor immune response with therapeutic antibodies by blocking cytotoxic T lymphocyte-associated protein 4 or the programmed cell death 1 pathways are another mainstream direction for cancer immunotherapy. In addition to cytotoxic T cells, other immune cell types such as nature killer cells and macrophages also possess the ability to eradicate cancer cells, which may serve as the basis to develop novel cancer immunotherapies. The advent of cutting-edge genome editing technology, especially clustered regularly interspaced palindromic repeats (CRISPR)-based tools, has greatly expedited many biomedical research areas, including cancer immunology and immunotherapy. In this review, the contribution of current CRISPR techniques to basic and translational cancer immunology research is discussed, and the future for cancer immunotherapy in the age of CRISPR is predicted.

Structural basis for inhibition of an archaeal CRISPR-Cas type I-D large subunit by an anti-CRISPR protein

A hallmark of type I CRISPR-Cas systems is the presence of Cas3, which contains both the nuclease and helicase activities required for DNA cleavage during interference. In subtype I-D systems, however, the histidine-aspartate (HD) nuclease domain is encoded as part of a Cas10-like large effector complex subunit and the helicase activity in a separate Cas3' subunit, but the functional and mechanistic consequences of this organisation are not currently understood. Here we show that the Sulfolobus islandicus type I-D Cas10d large subunit exhibits an unusual domain architecture consisting of a Cas3-like HD nuclease domain fused to a degenerate polymerase fold and a C-terminal domain structurally similar to Cas11. Crystal structures of Cas10d both in isolation and bound to S. islandicus rod-shaped virus 3 AcrID1 reveal that the anti-CRISPR protein sequesters the large subunit in a non-functional state unable to form a cleavage-competent effector complex. The architecture of Cas10d suggests that the type I-D effector complex is similar to those found in type III CRISPR-Cas systems and that this feature is specifically exploited by phages for anti-CRISPR defence.

Immunity in Light of Spinoza and Canguilhem

All living organisms are under stress imposed by their surrounding environments. They must adapt to their stressors to live and survive. At the forefront of this adaptation is a defense system called immunity. Immunity, as the most ancient cognitive apparatus with memory function, is present in all living organisms. In previous reports, minimal cognitive function was defined as a “biologized” concept—namely, perception of elements in a milieu, integration of perceived information, reaction according to integrated information, and memory of that experience. In this study, I aim to explore the essential feature of immunity by synthesizing scientific facts and “metaphysicalizing” them with logical reasoning. As a result of my analysis, I have realized the essential element in immunity: the capacity to preserve the existence of organisms by regulating their physiology and pathology. Having further analyzed immunity with special reference to the philosophy of Baruch Spinoza and George Canguilhem, conatus (“appetite”, to be precise) with normative activities is deeply embedded in immunity and may constitute its essential feature. Given that conatus and normativity imply mental elements, including the judgment of good and bad or health and disease, it is possible to conclude that the essential function of immunity includes cognition with normative connotations. This inclusive view encourages us to rethink the fundamental nature and philosophical implications of immunity from the cognitive perspective.

CRISPR-Associated Factor Csa3b Regulates CRISPR Adaptation and Cmr-Mediated RNA Interference in Sulfolobus islandicus

Acquisition of spacers confers the CRISPR–Cas system with the memory to defend against invading mobile genetic elements. We previously reported that the CRISPR-associated factor Csa3a triggers CRISPR adaptation in Sulfolobus islandicus. However, a feedback regulation of CRISPR adaptation remains unclear. Here we show that another CRISPR-associated factor, Csa3b, binds a cyclic oligoadenylate (cOA) analog (5′-CAAAA-3′) and mutation at its CARF domain, which reduces the binding affinity. Csa3b also binds the promoter of adaptation cas genes, and the cOA analog enhances their binding probably by allosteric regulation. Deletion of the csa3b gene triggers spacer acquisition from both plasmid and viral DNAs, indicating that Csa3b acted as a repressor for CRISPR adaptation. Moreover, we also find that Csa3b activates the expression of subtype cmr-α and cmr-β genes according to transcriptome data and demonstrate that Csa3b binds the promoters of cmr genes. The deletion of the csa3b gene reduces Cmr-mediated RNA interference activity, indicating that Csa3b acts as a transcriptional activator for Cmr-mediated RNA interference. In summary, our findings reveal a novel pathway for the regulation of CRISPR adaptation and CRISPR–Cmr RNA interference in S. islandicus. Our results also suggest a feedback repression of CRIPSR adaptation by the Csa3b factor and the cOA signal produced by the Cmr complex at the CRISPR interference stage.

Complete Genomic Analysis of Enterococcus faecium Heat-Resistant Strain Developed by Two-Step Adaptation Laboratory Evolution Method

Stress resistance is an important trait expected of lactic acid bacteria used in food manufacturing. Among the various sources of stress, high temperature is a key factor that interrupts bacterial growth. In this regards, constant efforts are made for the development of heat-resistant strains, but few studies were done accompanying genomic analysis to identify the causal factors of the resistance mechanisms. Furthermore, it is also thought that tolerance to multiple stresses are equally important. Herein, we isolated one Enterococcus faecium strain named BIOPOP-3 and completed a full-length genome sequence. Using this strain, a two-step adaptive laboratory evolution (ALE) method was applied to obtain a heat-resistant strain, BIOPOP-3 ALE. After sequencing the whole genome, we compared the two full-length sequences and identified one non-synonymous variant and four indel variants that could potentially confer heat resistance, which were technically validated by resequencing. We experimentally verified that the evolved strain was significantly enhanced in not only heat resistance but also acid and bile resistance. We demonstrated that the developed heat-resistant strain can be applied in animal feed manufacturing processes. The multi-stress-resistant BIOPOP-3 ALE strain developed in this study and the two-step ALE method are expected to be widely applied in industrial and academic fields. In addition, we expect that the identified variants which occurred specifically in heat-resistant strain will enhance molecular biological understanding and be broadly applied to the biological engineering field.

Structural insights into assembly, operation and inhibition of a type I restriction-modification system

Type I restriction-modification (R-M) systems are widespread in prokaryotic genomes and provide robust protection against foreign DNA. They are multisubunit enzymes with methyltransferase, endonuclease and translocase activities. Despite extensive studies over the past five decades, little is known about the molecular mechanisms of these sophisticated machines. Here, we report the cryo-electron microscopy structures of the representative EcoR124I R-M system in different assemblies (R₂M₂S₁, R₁M₂S₁ and M₂S₁) bound to target DNA and the phage and mobile genetic element-encoded anti-restriction proteins Ocr and ArdA. EcoR124I can precisely regulate different enzymatic activities by adopting distinct conformations. The marked conformational transitions of EcoR124I are dependent on the intrinsic flexibility at both the individual-subunit and assembled-complex levels. Moreover, Ocr and ArdA use a DNA-mimicry strategy to inhibit multiple activities, but do not block the conformational transitions of the complexes. These structural findings, complemented by mutational studies of key intermolecular contacts, provide insights into assembly, operation and inhibition mechanisms of type I R-M systems.

The evolution of host resistance and parasite infectivity is highest in seasonal resource environments that oscillate at intermediate amplitudes

Seasonal environments vary in their amplitude of oscillation but the effects of this temporal heterogeneity for host-parasite coevolution are poorly understood. Here, we combined mathematical modelling and experimental evolution of a coevolving bacteria-phage interaction to show that the intensity of host-parasite coevolution peaked in environments that oscillate in their resource supply with intermediate amplitude. Our experimentally parameterized mathematical model explains that this pattern is primarily driven by the ecological effects of resource oscillations on host growth rates. Our findings suggest that in host-parasite systems where the host's but not the parasite's population growth dynamics are subject to seasonal forcing, the intensity of coevolution will peak at intermediate amplitudes but be constrained at extreme amplitudes of environmental oscillation.

Comparative genomic provides insight into the virulence and genetic diversity of Vibrio parahaemolyticus associated with shrimp acute hepatopancreatic necrosis disease

Acute hepatopancreatic necrosis disease (AHPND) is an important shrimp disease of economic importance which causes mass mortality of cultivated penaeid shrimps in Southeast Asian countries, Mexico and South America. This disease was originally caused by Vibrio parahaemolyticus (VP_AHPND) which is reported to harbour a transferable plasmid carrying the virulent PirAB-like toxin genes (pirAB^vp). However, little is known about the pathogenicity of VP_AHPND. To extend our understanding, comparative genomic analyses was performed in this study to identify the genetic differences and to understand the phylogenetic relationship of VP_AHPND strains. Seven Vibrio parahaemolyticus strains (five VP_AHPND strains and two non-VP_AHPND strains) were sequenced and 31 draft genomes of V. parahaemolyticus were retrieved from NCBI database and incorporated into the genomic comparison to elucidate their genomic diversity. The study showed that the genome sizes of the VP_AHPND strains were approximately 5 Mbp. Ten sequence types (STs) were identified among the VP_AHPND strains using in silico-Multilocus Sequence Typing analysis (MLST) and ST 970 was the predominant ST. Phylogenetic analysis based on MLST and single nucleotide polymorphisms (SNP) showed that the VP_AHPND strains were genetically diverse. Based on the comparative genomic analysis, several functional proteins were identified from diiferent categories associated with virulence-related proteins, secretory proteins, conserved domain proteins, transporter proteins, and phage proteins. The CRISPR analysis showed that VP_AHPND strains contained less number of CRISPRs elements than non-VP_AHPND strains while six prophages regions were identified in the genomes, suggested the lack of CRISPR might promote prophage insertion. The genomic information in this study provide improved understanding of the virulence of these VP_AHPND strains.

Formation and Stoichiometry of CRISPR-Cascade Complexes with Varying Spacer Lengths Revealed by Native Mass Spectrometry

The adaptive immune system of bacteria and archaea against viral DNA is based on clustered, regularly interspaced, short palindromic repeats (CRISPRs) which are encoded in the host genome and translated into CRISPR RNAs (crRNAs) containing single spacer sequences complementary to foreign DNA. crRNAs assemble with CRISPR-associated (Cas) proteins forming surveillance complexes that base-pair with viral DNA and mediate its degradation. As specificity of degradation is provided by the crRNA spacer sequence, genetic engineering of the CRISPR system has emerged as a popular molecular tool, for instance, in gene silencing and programmed DNA degradation. Elongating or shortening the crRNA spacer sequence are therefore promising ventures to modify specificity toward the target DNA. However, even though the stoichiometry of wild-type complexes is well established, it is unknown how variations in crRNA spacer length affect their stoichiometry. The CRISPR-associated antiviral defense surveillance complexes of Streptococcus thermophilus (StCascade complexes) contain crRNA and five protein subunits. Using native mass spectrometry, we studied the formation and stoichiometry of StCascade complexes assembled on a set of crRNAs with different spacer lengths. We assigned all relevant complexes and gained insights into the stoichiometry of the complexes as well as their preferred assembly. We found that stable complexes, which incorporate or lose a (Cas7)₂(Cse2)₁-module, assemble on crRNA varied in length by 12-nucleotide units, while varying crRNA length in six-nucleotide units results in heterogeneous mixtures of complexes. Combining our results from the various variants, we generated an assembly pathway revealing general features of I-E type Cascade complex formation.

Cas3 Protein-A Review of a Multi-Tasking Machine

Cas3 has essential functions in CRISPR immunity but its other activities and roles, in vitro and in cells, are less widely known. We offer a concise review of the latest understanding and questions arising from studies of Cas3 mechanism during CRISPR immunity, and highlight recent attempts at using Cas3 for genetic editing. We then spotlight involvement of Cas3 in other aspects of cell biology, for which understanding is lacking—these focus on CRISPR systems as regulators of cellular processes in addition to defense against mobile genetic elements.

Anti-CRISPR proteins targeting the CRISPR-Cas system enrich the toolkit for genetic engineering

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas adaptive immune defense systems, which are widely distributed in bacteria and Archaea, can provide sequence-specific protection against foreign DNA or RNA in some cases. However, the evolution of defense systems in bacterial hosts did not lead to the elimination of phages, and some phages carry anti-CRISPR genes that encode products that bind to the components mediating the defense mechanism and thus antagonize CRISPR-Cas immune systems of bacteria. Given the extensive application of CRISPR-Cas9 technologies in gene editing, in this review, we focus on the anti-CRISPR proteins (Acrs) that inhibit CRISPR-Cas systems for gene editing. We describe the discovery of Acrs in immune systems involving type I, II, and V CRISPR-Cas immunity, discuss the potential function of Acrs in inactivating type II and V CRISPR-Cas systems for gene editing and gene modulation, and provide an outlook on the development of important biotechnology tools for genetic engineering using Acrs.

A CRISPR RNA Is Closely Related With the Size of the Cascade Nucleoprotein Complex

The currently known prokaryotic adaptive immune system against mobile genetic elements is based on clustered regularly interspaced short palindromic repeats (CRISPR). CRISPR-associated (Cas) proteins and the transcribed short CRISPR RNA (crRNA) molecule form a heterologous ribonucleoprotein complex that neutralizes invading foreign nucleic acids, wherein the crRNA molecule base-pairs with the exogenous genetic elements. In the ribonucleoprotein complexes of the type I CRISPR system, a helical backbone of six identical subunits is commonly found. However, it is not clear how this ribonucleoprotein complex is assembled and what is the determinant factor for its size. We elucidated the crystal structure of the Csy3 subunit of the type I-F ribonucleoprotein complex from Zymomonas mobilis (ZmCsy3), in which seven ZmCsy3 protomers in the asymmetric unit form a molecular helix that is part of a filamentous structure in the entire crystal system. This ZmCsy3 helical structure is remarkably similar to the crRNA-bound hexameric Csy3 backbone from Pseudomonas aeruginosa, with conserved interactions between neighboring subunits. The monomeric ZmCsy3 in solution is transformed into different oligomeric states depending on the added crRNAs. These results suggest that a crRNA and Csy3 subunit play a determinant role in the stepwise formation of the functional Cascade ribonucleoprotein complex and the recruitment of other subunits, and crRNA functions as a molecular ruler for determining the size of the Cascade silencing complex.

A Stable Genetic Transformation System and Implications of the Type IV Restriction System in the Nitrogen-Fixing Plant Endosymbiont Frankia alni ACN14a

Genus Frankia is comprised primarily of nitrogen-fixing actinobacteria that form root nodule symbioses with a group of hosts known as the actinorhizal plants. These plants are evolutionarily closely related to the legumes that are nodulated by the rhizobia. Both host groups utilize homologs of nodulation genes for root-nodule symbiosis, derived from common plant ancestors. The corresponding endosymbionts, Frankia and the rhizobia, however, are distantly related groups of bacteria, leading to questions about their symbiotic mechanisms and evolutionary history. To date, a stable system of electrotransformation has been lacking in Frankia despite numerous attempts by research groups worldwide. We have identified type IV methyl-directed restriction systems, highly-expressed in a range of actinobacteria, as a likely barrier to Frankia transformation. Here we report the successful electrotransformation of the model strain F. alni ACN14a with an unmethylated, broad host-range replicating plasmid, expressing chloramphenicol-resistance for selection and GFP as a marker of gene expression. This system circumvented the type IV restriction barrier and allowed the stable maintenance of the plasmid. During nitrogen limitation, Frankia differentiates into two cell types: the vegetative hyphae and nitrogen-fixing vesicles. When the expression of egfp under the control of the nif gene cluster promoter was localized using fluorescence imaging, the expression of nitrogen fixation in nitrogen-limited culture was localized in Frankia vesicles but not in hyphae. The ability to separate gene expression patterns between Frankia hyphae and vesicles will enable deeper comparisons of molecular signaling and metabolic exchange between Frankia-actinorhizal and rhizobia-legume symbioses to be made, and may broaden potential applications in agriculture. Further downstream applications are possible, including gene knock-outs and complementation, to open up a range of experiments in Frankia and its symbioses. Additionally, in the transcriptome of F. alni ACN14a, type IV restriction enzymes were highly expressed in nitrogen-replete culture but their expression strongly decreased during symbiosis. The down-regulation of type IV restriction enzymes in symbiosis suggests that horizontal gene transfer may occur more frequently inside the nodule, with possible new implications for the evolution of Frankia.

Sexual Selection in Bacteria?

A main mechanism of lateral gene transfer in bacteria is transformation, where cells take up free DNA from the environment which subsequently can be recombined into the genome. Bacteria are also known to actively release DNA into the environment through secretion or lysis, which could aid uptake via transformation. Various evolutionary benefits of DNA uptake and DNA release have been proposed but these have all been framed in the context of natural selection. Here, we interpret bacterial DNA uptake and release in the context of sexual selection theory, which has been central to our understanding of the bewildering diversity of traits associated with sexual reproduction in the eukaryote world but has never been applied to prokaryotes. Specifically, we explore potential scenarios where bacteria releasing DNA into the environment could compete for successful uptake by other cells, or where bacteria could selectively take up DNA to enhance their fitness. We conclude that there is potential for sexual selection to act in bacteria, and that this might in part explain the considerable diversity in transformation-related behaviours.

Spermidine strongly increases the fidelity of Escherichia coli CRISPR Cas1-Cas2 integrase

Site-selective CRISPR array expansion at the origin of bacterial adaptive immunity relies on recognition of sequence-dependent DNA structures by the conserved Cas1-Cas2 integrase. Off-target integration of a new spacer sequence outside canonical CRISPR arrays has been described in vitro However, this nonspecific integration activity is rare in vivo Here, we designed gel assays to monitor fluorescently labeled protospacer insertion in a supercoiled 3-kb plasmid harboring a minimal CRISPR locus derived from the Escherichia coli type I-E system. This assay enabled us to distinguish and quantify target and off-target insertion events catalyzed by E. coli Cas1-Cas2 integrase. We show that addition of the ubiquitous polyamine spermidine or of another polyamine, spermine, significantly alters the ratio between target and off-target insertions. Notably, addition of 2 mm spermidine quenched the off-target spacer insertion rate by a factor of 20-fold, and, in the presence of integration host factor, spermidine also increased insertion at the CRISPR locus 1.5-fold. The observation made in our in vitro system that spermidine strongly decreases nonspecific activity of Cas1-Cas2 integrase outside the leader-proximal region of a CRISPR array suggests that this polyamine plays a potential role in the fidelity of the spacer integration also in vivo.

Simulations reveal challenges to artificial community selection and possible strategies for success

Multispecies microbial communities often display "community functions" arising from interactions of member species. Interactions are often difficult to decipher, making it challenging to design communities with desired functions. Alternatively, similar to artificial selection for individuals in agriculture and industry, one could repeatedly choose communities with the highest community functions to reproduce by randomly partitioning each into multiple "Newborn" communities for the next cycle. However, previous efforts in selecting complex communities have generated mixed outcomes that are difficult to interpret. To understand how to effectively enact community selection, we simulated community selection to improve a community function that requires 2 species and imposes a fitness cost on one or both species. Our simulations predict that improvement could be easily stalled unless various aspects of selection are carefully considered. These aspects include promoting species coexistence, suppressing noncontributors, choosing additional communities besides the highest functioning ones to reproduce, and reducing stochastic fluctuations in the biomass of each member species in Newborn communities. These considerations can be addressed experimentally. When executed effectively, community selection is predicted to improve costly community function, and may even force species to evolve slow growth to achieve species coexistence. Our conclusions hold under various alternative model assumptions and are therefore applicable to a variety of communities.

Noncoding RNA

Regulatory RNAs, present in many bacterial genomes and particularly in pathogenic bacteria such as Staphylococcus aureus, control the expression of genes encoding virulence factors or metabolic proteins. They are extremely diverse and include noncoding RNAs (sRNA), antisense RNAs, and some 5' or 3' untranslated regions of messenger RNAs that act as sensors for metabolites, tRNAs, or environmental conditions (e.g., temperature, pH). In this review we focus on specific examples of sRNAs of S. aureus that illustrate how numerous sRNAs and associated proteins are embedded in complex networks of regulation. In addition, we discuss the CRISPR-Cas systems defined as an RNA-interference-like mechanism, which also exist in staphylococcal strains.

Cas4 Nucleases Can Effect Specific Integration of CRISPR Spacers

Clustered regularly interspaced short palindromic repeat (CRISPR)-Cas systems incorporate short DNA fragments from invasive genetic elements into host CRISPR arrays in order to generate host immunity. Recently, we demonstrated that the Csa3a regulator protein triggers CCN protospacer-adjacent motif (PAM)-dependent CRISPR spacer acquisition in the subtype I-A CRISPR-Cas system of Sulfolobus islandicus However, the mechanisms underlying specific protospacer selection and spacer insertion remained unclear. Here, we demonstrate that two Cas4 family proteins (Cas4 and Csa1) have essential roles (i) in recognizing the 5' PAM and 3' nucleotide motif of protospacers and (ii) in determining both the spacer length and its orientation. Furthermore, we identify amino acid residues of the Cas4 proteins that facilitate these functions. Overexpression of the Cas4 and Csa1 proteins, and also that of an archaeal virus-encoded Cas4 protein, resulted in strongly reduced adaptation efficiency, and the former proteins yielded a high incidence of PAM-dependent atypical spacer integration or of PAM-independent spacer integration. We further demonstrated that in plasmid challenge experiments, overexpressed Cas4-mediated defective spacer acquisition in turn potentially enabled targeted DNA to escape subtype I-A CRISPR-Cas interference. In summary, these results define the specific involvement of diverse Cas4 proteins in in vivo CRISPR spacer acquisition. Furthermore, we provide support for an anti-CRISPR role for virus-encoded Cas4 proteins that involves compromising CRISPR-Cas interference activity by hindering spacer acquisition.IMPORTANCE The Cas4 family endonuclease is an essential component of the adaptation module in many variants of CRISPR-Cas adaptive immunity systems. The Crenarchaeota Sulfolobus islandicus REY15A carries two cas4 genes (cas4 and csa1) linked to the CRISPR arrays. Here, we demonstrate that Cas4 and Csa1 are essential to CRISPR spacer acquisition in this organism. Both proteins specify the upstream and downstream conserved nucleotide motifs of the protospacers and define the spacer length and orientation in the acquisition process. Conserved amino acid residues, in addition to those recently reported, were identified to be important for these functions. More importantly, overexpression of the Sulfolobus viral Cas4 abolished spacer acquisition, providing support for an anti-CRISPR role for virus-encoded Cas4 proteins that inhibit spacer acquisition.

Characterization of CRISPR-Cas systems in Leptospira reveals potential application of CRISPR in genotyping of Leptospira interrogans

Leptospirosis is a zoonotic disease caused by pathogenic Leptospira. However, understanding of the pathogenic mechanism of Leptospira is still elusive due to the limited number of genetic tools available for this microorganism. Currently, the reason for the genetic inaccessibility of Leptospira is still unknown. It is well known that as an acquired immunity of bacteria, Clustered Regularly Interspaced Short Palindromic Repeat-CRISPR-associated gene (CRISPR-Cas) systems can help bacteria against invading mobile genetic elements. In this study, the occurrence and diversity of CRISPR-Cas systems in 41 genomes of Leptospira strains were investigated. Three subtypes (subtype I-B, subtype I-C and subtype I-E) of CRISPR-Cas systems were identified in both pathogenic and intermediate Leptospira species but not in saprophytic species. Noteworthy, the majority of pathogenic species harbor two different types of CRISPR-Cas systems (subtype I-B and subtype I-E). Furthermore, Cas2 protein of subtype I-C in L. interrogans exhibited a metal-dependent DNase activity in a nonspecific manner. CRISPR spacers in subtype I-B are highly conserved within the same serovars and hypervariable across different serovars of L. interrogans. Based on the subtype I-B CRISPR arrays, the serotypes of different L. interrogans strains were easily identified. Investigation of the origin of CRISPR spacers showed that 192 spacers (23.5%) matched to mobile genetic elements, indicating CRISPR-Cas systems may play an important role in the defense of foreign invading DNA.

Structural insights into a high fidelity variant of SpCas9

The RNA-guided endonucleases of the CRISPR-Cas9 system, including the most widely used Cas9 from Streptococcus pyogenes (SpCas9), are becoming a robust genome editing tool in model organisms and hold immense promise for therapeutic applications. Many strategies have been employed to overcome the limitations caused by SpCas9's off-target effects and its stringent requirement for the protospacer adjacent motif (PAM) sequence. However, the structural mechanisms underlying these strategies remain undefined. Here, we present crystal structure of a SpCas9 variant, xCas9 3.7 that has broad PAM compatibility and high DNA targeting specificity, in complex with a single-guide RNA and its double-stranded DNA targets. Structural comparison revealed that salt bridge-stabilized R1335 is critical for the stringent selection of PAM sequence by SpCas9. Unrestricted rotamerization of this residue by the E1219V mutation in xCas9 3.7 lessens the stringency for PAM recognition and allows SpCas9 to recognize multiple PAM sequences as further supported by biochemical data. Compared to those in wild-type (WT) SpCas9, REC2 and REC3 domains in xCas9 3.7 undergo striking conformational changes, leading to reduced contact with DNA substrate. SpCas9 mutants engineered to display less interaction with DNA and have conformationally more flexible REC2 and REC3 domains display enhanced specificity for DNA substrates in both biochemical and cellular assays. Taken together, our findings reveal the structural mechanisms underlying the broadened PAM compatibility and high DNA fidelity of xCas9 3.7, which can assist rational engineering of more efficient SpCas9 variants and probably other Cas9 orthologs.

The Effect of DNA Topology on Observed Rates of R-Loop Formation and DNA Strand Cleavage by CRISPR Cas12a

Here we explored the mechanism of R-loop formation and DNA cleavage by type V CRISPR Cas12a (formerly known as Cpf1). We first used a single-molecule magnetic tweezers (MT) assay to show that R-loop formation by Lachnospiraceae bacterium ND2006 Cas12a is significantly enhanced by negative DNA supercoiling, as observed previously with Streptococcus thermophilus DGCC7710 CRISPR3 Cas9. Consistent with the MT data, the apparent rate of cleavage of supercoiled plasmid DNA was observed to be >50-fold faster than the apparent rates for linear DNA or nicked circular DNA because of topology-dependent differences in R-loop formation kinetics. Taking the differences into account, the cleavage data for all substrates can be fitted with the same apparent rate constants for the two strand-cleavage steps, with the first event >15-fold faster than the second. By independently following the ensemble cleavage of the non-target strand (NTS) and target strand (TS), we could show that the faster rate is due to NTS cleavage, the slower rate due to TS cleavage, as expected from previous studies.

Imprecise Spacer Acquisition Generates CRISPR-Cas Immune Diversity through Primed Adaptation

Many prokaryotes possess CRISPR-Cas adaptive immune systems to defend against viruses and invading mobile genetic elements. CRISPR-Cas immunity relies on genetic memories, termed spacers, for sequence-specific recognition of infections. The diversity of spacers within host populations is important for immune resilience, but we have limited understanding of how CRISPR diversity is generated. Type I CRISPR-Cas systems use existing spacers to enhance the acquisition of new spacers through primed CRISPR adaptation (priming). Here, we present a pathway to priming that is stimulated by imprecisely acquired (slipped) spacers. Slipped spacers are less effective for immunity but increase priming compared with canonical spacers. The benefits of slipping depend on the relative rates of phage mutation and adaptation during defense. We propose that slipped spacers provide a route to increase population-level spacer diversity that pre-empts phage escape mutant proliferation and that the trade-off between adaptation and immunity is important in diverse CRISPR-Cas systems.

Type IV CRISPR RNA processing and effector complex formation in Aromatoleum aromaticum

Type IV CRISPR-Cas modules belong to class 1 prokaryotic adaptive immune systems, which are defined by the presence of multisubunit effector complexes. They usually lack the known Cas proteins involved in adaptation and target cleavage, and their function has not been experimentally addressed. To investigate RNA and protein components of this CRISPR-Cas type, we located a complete type IV cas gene locus and an adjacent CRISPR array on a megaplasmid of Aromatoleum aromaticum EbN1, which contains an additional type I-C system on its chromosome. RNA sequencing analyses verified CRISPR RNA (crRNA) production and maturation for both systems. Type IV crRNAs were shown to harbour unusually short 7 nucleotide 5'-repeat tags and stable 3' hairpin structures. A unique Cas6 variant (Csf5) was identified that generates crRNAs that are specifically incorporated into type IV CRISPR-ribonucleoprotein (crRNP) complexes. Structures of RNA-bound Csf5 were obtained. Recombinant production and purification of the type IV Cas proteins, together with electron microscopy, revealed that Csf2 acts as a helical backbone for type IV crRNPs that include Csf5, Csf3 and a large subunit (Csf1). Mass spectrometry analyses identified protein-protein and protein-RNA contact sites. These results highlight evolutionary connections between type IV and type I CRISPR-Cas systems and demonstrate that type IV CRISPR-Cas systems employ crRNA-guided effector complexes.

CasPDB: an integrated and annotated database for Cas proteins from bacteria and archaea

Clustered regularly interspaced short palindromic repeats (CRISPR) and associated proteins (Cas) constitute CRISPR–Cas systems, which are antiphage immune systems present in numerous bacterial and most archaeal species. In recent years, CRISPR–Cas systems have been developed into reliable and powerful genome editing tools. Nevertheless, finding similar or better tools from bacteria or archaea remains crucial. This requires the exploration of different CRISPR systems, identification and characterization new Cas proteins. Archives tailored for Cas proteins are urgently needed and necessitate the prediction and grouping of Cas proteins into an information center with all available experimental evidence. Here, we constructed Cas Protein Data Bank (CasPDB), an integrated and annotated online database for Cas proteins from bacteria and archaea. The CasPDB database contains 287 reviewed Cas proteins, 257 745 putative Cas proteins and 3593 Cas operons from 32 023 bacteria species and 1802 archaea species. The database can be freely browsed and searched. The CasPDB web interface also represents all the 3593 putative Cas operons and its components. Among these operons, 328 are members of the type II CRISPR–Cas system.

Evolutionary emergence of infectious diseases in heterogeneous host populations

The emergence and re-emergence of pathogens remains a major public health concern. Unfortunately, when and where pathogens will (re-)emerge is notoriously difficult to predict, as the erratic nature of those events is reinforced by the stochastic nature of pathogen evolution during the early phase of an epidemic. For instance, mutations allowing pathogens to escape host resistance may boost pathogen spread and promote emergence. Yet, the ecological factors that govern such evolutionary emergence remain elusive because of the lack of ecological realism of current theoretical frameworks and the difficulty of experimentally testing their predictions. Here, we develop a theoretical model to explore the effects of the heterogeneity of the host population on the probability of pathogen emergence, with or without pathogen evolution. We show that evolutionary emergence and the spread of escape mutations in the pathogen population is more likely to occur when the host population contains an intermediate proportion of resistant hosts. We also show that the probability of pathogen emergence rapidly declines with the diversity of resistance in the host population. Experimental tests using lytic bacteriophages infecting their bacterial hosts containing Clustered Regularly Interspaced Short Palindromic Repeat and CRISPR-associated (CRISPR-Cas) immune defenses confirm these theoretical predictions. These results suggest effective strategies for cross-species spillover and for the management of emerging infectious diseases.

The probability that an epidemic will break out is highly dependent on the ability of the pathogen to acquire new adaptive mutations and to induce evolutionary emergence. Forecasting pathogen emergence thus requires a good understanding of the interplay between the epidemiology and evolution taking place at the onset of an outbreak. Here, we provide a comprehensive theoretical framework to analyze the impact of host population heterogeneity on the probability of pathogen evolutionary emergence. We use this model to predict the impact of the fraction of susceptible hosts, the inoculum size of the pathogen, and the diversity of host resistance on pathogen emergence. Our experiments using lytic bacteriophages and CRISPR-resistant bacteria support our theoretical predictions and demonstrate that manipulating the diversity of resistance alleles in a host population may be an effective way to limit the emergence of new pathogens.

Identification and sequence analysis of two novel cryptic plasmids isolated from the vaginal mucosa of South African women

The vaginal mucosa is dominated by Gram positive, rod shaped lactobacilli which serve as a natural barrier against infection. In both healthy- and bacterial vaginosis (BV)-infected women Lactobacillus crispatus and Lactobacillus jensenii have been found to be the predominant Lactobacillus species. Many studies have been conducted to assess factors influencing lactobacilli dominance in the vaginal microbiome. In the present study two plasmids, pLc4 and pLc17, isolated from vaginal Lactobacillus strains of both healthy and BV-infected women were characterized. The smaller plasmid, pLc4 (4224 bp), was detected in both L. crispatus and L. jensenii strains, while pLc17 was only detected in L. crispatus. Based on its nucleotide sequence pLc4 appears highly novel, with its replication protein having 44% identity to the replication initiation protein of pSMQ173b_03. Phylogenetic analysis with other Rolling Circle Replication plasmids confirmed that pLc4 shows a low degree of similarity to these plasmids. Plasmid pLc17 (16,663 bp) appears to carry both a RCR replicon and a theta replicon, which is rare in naturally occurring plasmids. pLc4 was maintained at a high copy number of 29, while pLc17 appears to be a medium copy number plasmid maintained at 11 copies per chromosome. While sequence analysis is a valuable tool to study cryptic plasmids, further function-based analysis will be required in order to fully elucidate the role of these plasmids within the vaginal milieu.

Bioinformatic evidence of widespread priming in type I and II CRISPR-Cas systems

CRISPR-Cas systems provide bacteria and archaea with adaptive immunity against invading genetic elements, such as plasmids, bacteriophages and archaeal viruses. They consist of cas genes and CRISPR loci, which store genetic memories of previously encountered invaders as short sequences termed spacers. Spacers determine the specificity of CRISPR-Cas defence and immunity can be gained or updated by the addition of new spacers into CRISPR loci. There are two main routes to spacer acquisition, which are known as naïve and primed CRISPR adaptation. Naïve CRISPR adaptation involves the de novo formation of immunity, independent of pre-existing spacers. In contrast, primed CRISPR adaptation (priming) uses existing spacers to enhance the acquisition of new spacers. Priming typically results in spacer acquisition from locations near the site of target recognition by the existing (priming) spacer. Primed CRISPR adaptation has been observed in several type I CRISPR-Cas systems and it is potentially widespread. However, experimental evidence is unavailable for some subtypes, and for most systems, priming has only been shown in a small number of hosts. There is also no current evidence of priming by other CRISPR-Cas types. Here, we used a bioinformatic approach to search for evidence of priming in diverse CRISPR-Cas systems. By analysing the clustering of spacers acquired from phages, prophages and archaeal viruses, including strand and directional biases between subsequently acquired spacers, we demonstrate that two patterns of primed CRISPR adaptation dominate in type I systems. In addition, we find evidence of a priming-like pathway in type II CRISPR-Cas systems.

Multifaceted Impacts of Bacteriophages in the Plant Microbiome

Plant-associated bacteria face multiple selection pressures within their environments and have evolved countless adaptations that both depend on and shape bacterial phenotype and their interaction with plant hosts. Explaining bacterial adaptation and evolution therefore requires considering each of these forces independently as well as their interactions. In this review, we examine how bacteriophage viruses (phages) can alter the ecology and evolution of plant-associated bacterial populations and communities. This includes influencing a bacterial population's response to both abiotic and biotic selection pressures and altering ecological interactions within the microbiome and between the bacteria and host plant. We outline specific ways in which phages can alter bacterial phenotype and discuss when and how this might impact plant-microbe interactions, including for plant pathogens. Finally, we highlight key open questions in phage-bacteria-plant research and offer suggestions for future study.

The CRISPR/Cas9 system sheds new lights on the biology of protozoan parasites

The CRISPR/Cas9 system, a natural defence system of bacterial organisms, has recently been used to modify genomes of the most important protozoa parasites. Successful genome manipulations with the CRISPR/Cas9 system are changing the present view of genetics in parasitology. The application of this system offers a major chance to overcome the current restriction in culturing, maintaining and analysing protozoan parasites, and allows dynamic analysis of parasite genes functions, leading to a better understanding of pathogenesis. CRISPR/Cas9 system will have a significant influence on the process of developing novel drugs and treatment strategies against protozoa parasites.

Enzymes Involved in Posttranscriptional RNA Metabolism in Gram-Negative Bacteria

Gene expression in Gram-negative bacteria is regulated at many levels, including transcription initiation, RNA processing, RNA/RNA interactions, mRNA decay, and translational controls involving enzymes that alter translational efficiency. In this review, we discuss the various enzymes that control transcription, translation, and RNA stability through RNA processing and degradation. RNA processing is essential to generate functional RNAs, while degradation helps control the steady-state level of each individual transcript. For example, all the pre-tRNAs are transcribed with extra nucleotides at both their 5' and 3' termini, which are subsequently processed to produce mature tRNAs that can be aminoacylated. Similarly, rRNAs that are transcribed as part of a 30S polycistronic transcript are matured to individual 16S, 23S, and 5S rRNAs. Decay of mRNAs plays a key role in gene regulation through controlling the steady-state level of each transcript, which is essential for maintaining appropriate protein levels. In addition, degradation of both translated and nontranslated RNAs recycles nucleotides to facilitate new RNA synthesis. To carry out all these reactions, Gram-negative bacteria employ a large number of endonucleases, exonucleases, RNA helicases, and poly(A) polymerase, as well as proteins that regulate the catalytic activity of particular RNases. Under certain stress conditions, an additional group of specialized endonucleases facilitate the cell's ability to adapt and survive. Many of the enzymes, such as RNase E, RNase III, polynucleotide phosphorylase, RNase R, and poly(A) polymerase I, participate in multiple RNA processing and decay pathways.