Conserved motifs in the CRISPR leader sequence control spacer acquisition levels in Type I-D CRISPR-Cas systems

Structure reveals why genome folding is necessary for site-specific integration of foreign DNA into CRISPR arrays

Bacteria and archaea acquire resistance to viruses and plasmids by integrating fragments of foreign DNA into the first repeat of a CRISPR array. However, the mechanism of site-specific integration remains poorly understood. Here, we determine a 560-kDa integration complex structure that explains how Pseudomonas aeruginosa Cas (Cas1-Cas2/3) and non-Cas proteins (for example, integration host factor) fold 150 base pairs of host DNA into a U-shaped bend and a loop that protrude from Cas1-2/3 at right angles. The U-shaped bend traps foreign DNA on one face of the Cas1-2/3 integrase, while the loop places the first CRISPR repeat in the Cas1 active site. Both Cas3 proteins rotate 100 degrees to expose DNA-binding sites on either side of the Cas2 homodimer, which each bind an inverted repeat motif in the leader. Leader sequence motifs direct Cas1-2/3-mediated integration to diverse repeat sequences that have a 5'-GT. Collectively, this work reveals new DNA-binding surfaces on Cas2 that are critical for DNA folding and site-specific delivery of foreign DNA.

The biology and type I/III hybrid nature of type I-D CRISPR-Cas systems

Prokaryotes have adaptive defence mechanisms that protect them from mobile genetic elements and viral infection. One defence mechanism is called CRISPR-Cas (clustered regularly interspaced short palindromic repeats and CRISPR-associated proteins). There are six different types of CRISPR-Cas systems and multiple subtypes that vary in composition and mode of action. Type I and III CRISPR-Cas systems utilise multi-protein complexes, which differ in structure, nucleic acid binding and cleaving preference. The type I-D system is a chimera of type I and III systems. Recently, there has been a burst of research on the type I-D CRISPR-Cas system. Here, we review the mechanism, evolution and biotechnological applications of the type I-D CRISPR-Cas system.

Insights into the Mechanism of CRISPR/Cas9-Based Genome Editing from Molecular Dynamics Simulations

The CRISPR/Cas9 system is a popular genome-editing tool with immense therapeutic potential. It is a simple two-component system (Cas9 protein and RNA) that recognizes the DNA sequence on the basis of RNA:DNA complementarity, and the Cas9 protein catalyzes the double-stranded break in the DNA. In the past decade, near-atomic resolution structures at various stages of the CRISPR/Cas9 DNA editing pathway have been reported along with numerous experimental and computational studies. Such studies have boosted knowledge of the genome-editing mechanism. Despite such advancements, the application of CRISPR/Cas9 in therapeutics is still limited, primarily due to off-target effects. Several studies aim at engineering high-fidelity Cas9 to minimize the off-target effects. Molecular Dynamics (MD) simulations have been an excellent complement to the experimental studies for investigating the mechanism of CRISPR/Cas9 editing in terms of structure, thermodynamics, and kinetics. MD-based studies have uncovered several important molecular aspects of Cas9, such as nucleotide binding, catalytic mechanism, and off-target effects. In this Review, the contribution of MD simulation to understand the CRISPR/Cas9 mechanism has been discussed, preceded by an overview of the history, mechanism, and structural aspects of the CRISPR/Cas9 system. These studies are important for the rational design of highly specific Cas9 and will also be extremely promising for achieving more accurate genome editing in the future.

A pyrF-Based Efficient Genetic Manipulation Platform in Acinetobacter baumannii To Explore the Vital DNA Components of Adaptive Immunity for I-F CRISPR-Cas

Acinetobacter baumannii is an important pathogenic bacterium with multidrug resistance which causes infections with high mortality rates. In-depth genetic analysis of A. baumannii virulence and drug-resistant genes is highly desirable. In this study, we utilized the conserved pyrF-flanking fragment to rapidly generate uracil auxotrophy hosts with pyrF deleted in model and clinical A. baumannii strains and then introduced the pyrF gene as the selectable and counterselectable marker to establish a series of gene manipulation vectors. For gene deletion with the suicide pyrF-based plasmid, the second-crossover colonies screened with the pyrF/5-fluoroorotic acid (5-FOA) system were obtained more quickly and efficiently than those screened with the sacB/sucrose system. By using the replicative plasmid, the recognized protospacer-adjacent motif (PAM) bias for type I-F CRISPR was experimentally revealed in A. baumannii AYE. Interestingly, interference recognized only the PAM-CC sequence, whereas adaptation priming tolerates 4 PAM sequences. Furthermore, we also performed a rapid and extensive modification of the I-F CRISPR-Cas elements and revealed that the role of double-nucleotide sequence mutants at the end of the repeat could be critical during both CRISPR interference and priming; we also found strong biases for A and demonstrated that adaptation could tolerate certain sequence and size variations of the leader in A. baumannii. In conclusion, this pyrF-based genetic manipulation system was readily applicable and efficient for exploring the genetic characteristics of A. baumannii. IMPORTANCE In this study, we developed the widely applicable and efficient pyrF-based selection and counterselection system in A. baumannii for gene manipulation. In most cases, this pyrF/5-FOA genetic manipulation system was very effective and enabled us to obtain marker-free mutants in a very short period of time. Utilizing this system and the separate mechanism of interference and/or primed adaptation, our experiments revealed some recognition mechanism differences for the key DNA elements of PAM, leader, and repeat in the priming adaptation process of the I-F CRISPR-Cas systems of A. baumannii, which provided some new and original insights for the study of the molecular mechanisms of these processes and laid a foundation for further studies.

DNA Motifs and an Accessory CRISPR Factor Determine Cas1 Binding and Integration Activity in Sulfolobus islandicus

CRISPR-Cas systems empower prokaryotes with adaptive immunity against invasive mobile genetic elements. At the first step of CRISPR immunity adaptation, short DNA fragments from the invaders are integrated into CRISPR arrays at the leader-proximal end. To date, the mechanism of recognition of the leader-proximal end remains largely unknown. Here, in the Sulfolobus islandicus subtype I-A system, we show that mutations destroying the proximal region reduce CRISPR adaptation in vivo. We identify that a stem-loop structure is present on the leader-proximal end, and we demonstrate that Cas1 preferentially binds the stem-loop structure in vitro. Moreover, we demonstrate that the integrase activity of Cas1 is modulated by interacting with a CRISPR-associated factor Csa3a. When translocated to the CRISPR array, the Csa3a-Cas1 complex is separated by Csa3a binding to the leader-distal motif and Cas1 binding to the leader-proximal end. Mutation at the leader-distal motif reduces CRISPR adaptation efficiency, further confirming the in vivo function of leader-distal motif. Together, our results suggest a general model for binding of Cas1 protein to a leader motif and modulation of integrase activity by an accessory factor.

Distribution and phasing of sequence motifs that facilitate CRISPR adaptation

CRISPR-associated proteins (Cas1 and Cas2) integrate foreign DNA at the "leader" end of CRISPR loci. Several CRISPR leader sequences are reported to contain a binding site for a DNA-bending protein called integration host factor (IHF). IHF-induced DNA bending kinks the leader of type I-E CRISPRs, recruiting an upstream sequence motif that helps dock Cas1-2 onto the first repeat of the CRISPR locus. To determine the prevalence of IHF-directed CRISPR adaptation, we analyzed 15,274 bacterial and archaeal CRISPR leaders. These experiments reveal multiple IHF binding sites and diverse upstream sequence motifs in a subset of the I-C, I-E, I-F, and II-C CRISPR leaders. We identify subtype-specific motifs and show that the phase of these motifs is critical for CRISPR adaptation. Collectively, this work clarifies the prevalence and mechanism(s) of IHF-dependent CRISPR adaptation and suggests that leader sequences and adaptation proteins may coevolve under the selective pressures of foreign genetic elements like plasmids or phages.

Cooperation between Different CRISPR-Cas Types Enables Adaptation in an RNA-Targeting System

CRISPR-Cas systems are immune systems that protect bacteria and archaea against their viruses, bacteriophages. Immunity is achieved through the acquisition of short DNA fragments from the viral invader’s genome.

CRISPR-Cas immune systems adapt to new threats by acquiring new spacers from invading nucleic acids such as phage genomes. However, some CRISPR-Cas loci lack genes necessary for spacer acquisition despite variation in spacer content between microbial strains. It has been suggested that such loci may use acquisition machinery from cooccurring CRISPR-Cas systems within the same strain. Here, following infection by a virulent phage with a double-stranded DNA (dsDNA) genome, we observed spacer acquisition in the native host Flavobacterium columnare that carries an acquisition-deficient CRISPR-Cas subtype VI-B system and a complete subtype II-C system. We show that the VI-B locus acquires spacers from both the bacterial and phage genomes, while the newly acquired II-C spacers mainly target the viral genome. Both loci preferably target the terminal end of the phage genome, with priming-like patterns around a preexisting II-C protospacer. Through gene deletion, we show that the RNA-cleaving VI-B system acquires spacers in trans using acquisition machinery from the DNA-cleaving II-C system. Our observations support the concept of cross talk between CRISPR-Cas systems and raise further questions regarding the plasticity of adaptation modules.

Novel Strategy to Combat Antibiotic Resistance: A Sight into the Combination of CRISPR/Cas9 and Nanoparticles

Antibiotic resistance is a significant crisis that threatens human health and safety worldwide. There is an urgent need for new strategies to control multidrug-resistant (MDR) bacterial infections. The latest breakthrough in gene-editing tools based on CRISPR/Cas9 has potential application in combating MDR bacterial infections because of their high targeting ability to specifically disrupt the drug resistance genes that microbes use for infection or to kill the pathogen directly. Despite the potential that CRISPR/Cas9 showed, its further utilization has been hampered by undesirable delivery efficiency in vivo. Nanotechnology offers an alternative way to overcome the shortcomings of traditional delivery methods of therapeutic agents. Advances in nanotechnology can improve the efficacy and safety of CRISPR/Cas9 components by using customized nanoparticle delivery systems. The combination of CRISPR/Cas9 and nanotechnology has the potential to open new avenues in the therapy of MDR bacterial infections. This review describes the recent advances related to CRISPR/Cas9 and nanoparticles for antimicrobial therapy and gene delivery, including the improvement in the packaging and localizing efficiency of the CRISPR/Cas9 components in the NP (nanoparticle)/CRISPR system. We pay particular attention to the strengths and limitations of the nanotechnology-based CRISPR/Cas9 delivery system to fight nosocomial pathogens.We highlight the need for more scientific research to explore the combinatorial efficacy of various nanoparticles and CRISPR technology to control and prevent antimicrobial resistance.

Identification of a Type IV-A CRISPR-Cas System Located Exclusively on IncHI1B/IncFIB Plasmids in Enterobacteriaceae

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) are diverse immune systems found in many prokaryotic genomes that target invading foreign DNA such as bacteriophages and plasmids. There are multiple types of CRISPR with arguably the most enigmatic being Type IV. During an investigation of CRISPR carriage in clinical, multi-drug resistant, Klebsiella pneumoniae, a Type IV-A3 CRISPR-Cas system was detected on plasmids from two K. pneumoniae isolates from Egypt (isolated in 2002-2003) and a single K. pneumoniae isolate from the United Kingdom (isolated in 2017). Sequence analysis of all other genomes available in GenBank revealed that this CRISPR-Cas system was present on 28 other plasmids from various Enterobacteriaceae hosts and was never found on a bacterial chromosome. This system is exclusively located on IncHI1B/IncFIB plasmids and is associated with multiple putative transposable elements. Expression of the cas loci was confirmed in the available clinical isolates by RT-PCR. In all cases, the CRISPR-Cas system has a single CRISPR array (CRISPR1) upstream of the cas loci which has several, conserved, spacers which, amongst things, match regions within conjugal transfer genes of IncFIIK/IncFIB(K) plasmids. Our results reveal a Type IV-A3 CRISPR-Cas system exclusively located on IncHI1B/IncFIB plasmids in Enterobacteriaceae that is likely to be able to target IncFIIK/IncFIB(K) plasmids presumably facilitating intracellular, inter-plasmid competition.

Growth inhibition by bacterial Cas2Em proteins expressed in mammalian cells

BACKGROUND

Clustered regularly interspaced short palindromic repeats (CRISPRs) and the CRISPR-associated (Cas) proteins are bacterial adaptive immune system for survival. In our previous study, we demonstrate that polyploid giant bacterial cells (PGBC) induced by Cas2 protein is a step required by new spacer acquisition reaction catalyzed by Cas1/Cas2 complex. We also demonstrated that a carboxyl terminal domain on Cas2Em (the protein Cas2 cloned from Elizabethkingia meningoseptica) is sufficient and enough for PGBC. Thus, the potential role of Cas2Em in microbial-host interaction was explored in this study.

METHODS

The impacts of Cas2Em on growth of both CHO-K1 and Hela cells were investigated. The subcellular localization and potential molecular target of Ca2Em were studied.

RESULTS

The growth of mammalian cells were inhibited by Cas2Em protein via G1 arresting and apoptosis. In addition, we also demonstrated that Cas2Em was tightly associated with nuclear outer membrane and could be immunoprecipitated with 14-3-3γ through a 30 amino acid domain (homology of CLK2).

CONCLUSION

Cas2Em significantly suppressed the growth of mammalian cells indicating Cas2 proteins play an important role in mammalian cells.

Applications of CRISPR-Cas systems in lactic acid bacteria

As a phenotypically and phylogenetically diverse group, lactic acid bacteria are found in a variety of natural environments and occupy important roles in medicine, biotechnology, food and agriculture. The widespread use of lactic acid bacteria across these industries fuels the need for new and functionally diverse strains that may be utilized as starter cultures or probiotics. Originally characterized in lactic acid bacteria, CRISPR-Cas systems and derived molecular machines can be used natively or exogenously to engineer new strains with enhanced functional attributes. Research on CRISPR-Cas biology and its applications has exploded over the past decade with studies spanning from the initial characterization of CRISPR-Cas immunity in Streptococcus thermophilus to the use of CRISPR-Cas for clinical gene therapies. Here, we discuss CRISPR-Cas classification, overview CRISPR biology and mechanism of action, and discuss current and future applications in lactic acid bacteria, opening new avenues for their industrial exploitation and manipulation of microbiomes.

Fidelity of prespacer capture and processing is governed by the PAM-mediated interactions of Cas1-2 adaptation complex in CRISPR-Cas type I-E system

Prokaryotes deploy CRISPR-Cas-based RNA-guided adaptive immunity to fend off mobile genetic elements such as phages and plasmids. During CRISPR adaptation, which is the first stage of CRISPR immunity, the Cas1-2 integrase complex captures invader-derived prespacer DNA and specifically integrates it at the leader-repeat junction as spacers. For this integration, several variants of CRISPR-Cas systems use Cas4 as an indispensable nuclease for selectively processing the protospacer adjacent motif (PAM) containing prespacers to a defined length. Surprisingly, however, a few CRISPR-Cas systems, such as type I-E, are bereft of Cas4. Despite the absence of Cas4, how the prespacers show impeccable conservation for length and PAM selection in type I-E remains intriguing. Here, using in vivo and in vitro integration assays, deep sequencing, and exonuclease footprinting, we show that Cas1-2/I-E-via the type I-E-specific extended C-terminal tail of Cas1-displays intrinsic affinity for PAM containing prespacers of variable length in Escherichia coli Although Cas1-2/I-E does not prune the prespacers, its binding protects the prespacer boundaries from exonuclease action. This ensures the pruning of exposed ends by exonucleases to aptly sized substrates for integration into the CRISPR locus. In summary, our work reveals that in a few CRISPR-Cas variants, such as type I-E, the specificity of PAM selection resides with Cas1-2, whereas the prespacer processing is co-opted by cellular non-Cas exonucleases, thereby offsetting the need for Cas4.