Comparison of CRISPR-Cas Immune Systems in Healthcare-Related Pathogens

The ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) and Clostridium difficile have been identified as the leading global cause of multidrug-resistant bacterial infections in hospitals. CRISPR-Cas systems are bacterial immune systems, empowering the bacteria with defense against invasive mobile genetic elements that may carry the antimicrobial resistance (AMR) genes, among others. On the other hand, the CRISPR-Cas systems are themselves mobile. In this study, we annotated and compared the CRISPR-Cas systems in these pathogens, utilizing their publicly available large numbers of sequenced genomes (e.g., there are more than 12 thousands of S. aureus genomes). The presence of CRISPR-Cas systems showed a very broad spectrum in these pathogens: S. aureus has the least tendency of obtaining the CRISPR-Cas systems with only 0.55% of its isolates containing CRISPR-Cas systems, whereas isolates of C. difficile we analyzed have CRISPR-Cas systems each having multiple CRISPRs. Statistical tests show that CRISPR-Cas containing isolates tend to have more AMRs for four of the pathogens (A. baumannii, E. faecium, P. aeruginosa, and S. aureus). We made available all the annotated CRISPR-Cas systems in these pathogens with visualization at a website (https://omics.informatics.indiana.edu/CRISPRone/pathogen), which we believe will be an important resource for studying the pathogens and their arms-race with invaders mediated through the CRISPR-Cas systems, and for developing potential clinical applications of the CRISPR-Cas systems for battles against the antibiotic resistant pathogens.

Characterization of 67 Confirmed Clustered Regularly Interspaced Short Palindromic Repeats Loci in 52 Strains of Staphylococci

Staphylococcus aureus (S. aureus), which is one of the most important species of Staphylococci, poses a great threat to public health. Clustered regularly interspaced short palindromic repeats (CRISPR) and their CRISPR-associated proteins (Cas) are an adaptive immune platform to combat foreign mobile genetic elements (MGEs) such as plasmids and phages. The aim of this study is to describe the distribution and structure of CRISPR-Cas system in S. aureus, and to explore the relationship between CRISPR and horizontal gene transfer (HGT). Here, we analyzed 67 confirmed CRISPR loci and 15 companion Cas proteins in 52 strains of Staphylococci with bioinformatics methods. Comparing with the orphan CRISPR loci in Staphylococci, the strains harboring complete CRISPR-Cas systems contained multiple CRISPR loci, direct repeat sequences (DR) forming stable RNA secondary structures with lower minimum free energy (MFE), and variable spacers with detectable protospacers. In S. aureus, unlike the orphan CRISPRs away from Staphylococcal cassette chromosome mec (SCCmec), the complete CRISPR-Cas systems were in J1 region of SCCmec. In addition, we found a conserved motif 5'-TTCTCGT-3' that may protect their downstream sequences from DNA interference. In general, orphan CRISPR locus in S. aureus differed greatly from the structural characteristics of the CRISPR-Cas system. Collectively, our results provided new insight into the diversity and characterization of the CRISPR-Cas system in S. aureus.

Tenacibaculum maritimum CRISPR loci analysis and evaluation of isolate spoligotyping

AIMS

We performed in silico analysis of CRISPRcas loci from Tenacibaculum maritimum, evaluated spoligotyping as a subtyping method and genotyped uncharacterized Turkish isolates from European sea bass by multilocus sequence typing (MLST).

METHODS AND RESULTS

Spoligotyping was performed with primers designed to allow amplification and sequencing of whole CRISPR-arrays from 23 T. maritimum isolates. Twenty-three completed/draft genomes were also downloaded from the NCBI database and analysed. MLST of Turkish isolates was achieved with a well-established 7-gene scheme. Tenacibaculum maritimum genomes carry a structurally complete but partially defective class II CRISPRcas locus due to known amino acid substitutions in encoded Cas9 proteins. Our spacer identification suggests that the host range of bacteriophage P2559Y and Vibrio phage nt-1 include T. maritimum and that the most recurrent infection recorded by isolates has been with Tenacibaculum phage PTm5. Thirty-eight isolates with this CRISPRcas locus belonged to 25 spoligotypes and to 24 sequence types by MLST, respectively. According to MLST, T. maritimum isolates from Turkey are most related to previously defined sequence types ST3, ST40 and ST41 isolates from Spain, Malta and France.

CONCLUSIONS

The evaluated spoligotyping offers discriminatory power comparable to MLST.

SIGNIFICANCE AND IMPACT OF THE STUDY

Spoligotyping has potential as a quick, easy and cheap tool for subtyping of T. maritimum isolates.

Corrigendum: Composition and Diversity of CRISPR-Cas13a Systems in the Genus Leptotrichia

[This corrects the article DOI: 10.3389/fmicb.2019.02838.].

[Design of Guide RNA for CRISPR/Cas Plant Genome Editing]

CRISPR/Cas technology of genome editing is a powerful tool for making targeted changes in the DNA of various organisms, including plants. The choice of the precise nucleotide sequence (protospacer) in the gene to be edited is important in the design of guide RNA, which can be carried out by specialized software. We review and compare all the known on-line and off-line resources for guide RNA design, with special attention paid to tools capable of searching for off-target edits sites in plant genomes. The use of Cas12a may be preferable to Cas9. Techniques allowing C→T and G→A base editing without DNA cleavage are discussed along with the basic requirements for the design of effective and highly specific guide RNAs. Ways for improving guide RNA design software are presented. We also discuss the lesser risks of off-target editing in plant genomes as opposed to animal genomes. Examples of edited plant genomes including those that do not lead to the creation of transgenic plants are reviewed.

Composition and Diversity of CRISPR-Cas13a Systems in the Genus Leptotrichia

Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas13a, previously known as CRISPR-C2c2, is the most recently identified RNA-guided RNA-targeting CRISPR-Cas system that has the unique characteristics of both targeted and collateral single-stranded RNA (ssRNA) cleavage activities. This system was first identified in Leptotrichia shahii. Here, the complete whole genome sequences of 11 Leptotrichia strains were determined and compared with 18 publicly available Leptotrichia genomes in regard to the composition, occurrence and diversity of the CRISPR-Cas13a, and other CRISPR-Cas systems. Various types of CRISPR-Cas systems were found to be unevenly distributed among the Leptotrichia genomes, including types I-B (10/29, 34.4%), II-C (1/29, 2.6%), III-A (6/29, 15.4%), III-D (6/29, 15.4%), III-like (3/29, 7.7%), and VI-A (11/29, 37.9%), while 8 strains (20.5%) had no CRISPR-Cas system at all. The Cas13a effectors were found to be highly divergent with amino acid sequence similarities ranging from 61% to 90% to that of L. shahii, but their collateral ssRNA cleavage activities leading to impediment of bacterial growth were conserved. CRISPR-Cas spacers represent a sequential achievement of former intruder encounters, and the retained spacers reflect the evolutionary phylogeny or relatedness of strains. Analysis of spacer contents and numbers among Leptotrichia species showed considerable diversity with only 4.4% of spacers (40/889) were shared by two strains. The organization and distribution of CRISPR-Cas systems (type I-VI) encoded by all registered Leptotrichia species revealed that effector or spacer sequences of the CRISPR-Cas systems were very divergent, and the prevalence of types I, III, and VI was almost equal. There was only one strain carrying type II, while none carried type IV or V. These results provide new insights into the characteristics and divergences of CRISPR-Cas systems among Leptotrichia species.

Dynamics of Cas10 Govern Discrimination between Self and Non-self in Type III CRISPR-Cas Immunity

Adaptive immune systems must accurately distinguish between self and non-self in order to defend against invading pathogens while avoiding autoimmunity. Type III CRISPR-Cas systems employ guide RNA to recognize complementary RNA targets, which triggers the degradation of both the invader's transcripts and their template DNA. These systems can broadly eliminate foreign targets with multiple mutations but circumvent damage to the host genome. To explore the molecular basis for these features, we use single-molecule fluorescence microscopy to study the interaction between a type III-A ribonucleoprotein complex and various RNA substrates. We find that Cas10-the DNase effector of the complex-displays rapid conformational fluctuations on foreign RNA targets, but is locked in a static configuration on self RNA. Target mutations differentially modulate Cas10 dynamics and tune the CRISPR interference activity in vivo. These findings highlight the central role of the internal dynamics of CRISPR-Cas complexes in self versus non-self discrimination and target specificity.

In Silico Processing of the Complete CRISPR-Cas Spacer Space for Identification of PAM Sequences

Despite extensive exploration of the diversity of CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats, CRISPR associated) systems, biological applications have been mostly confined to Class 2 systems, specifically the Cas9 and Cas12 (formerly Cpf1) single effector proteins. A key limitation of exploring and utilizing other CRISPR-Cas systems with unique functionalities, particularly Class I types and their multi-protein effector complex, is the knowledge of the system's protospacer adjacent motif (PAM) sequence identity. In this work, the authors developed a systematic pipeline, named CASPERpam, that enables a comprehensive assessment of the PAM sequences of all the available CRISPR-Cas systems in the NCBI database of bacterial genomes. The CASPERpam analysis reveals that within the 30 389 assemblies previously screened for CRISPR arrays, there exists 26 364 spacers that match somewhere in the viral, bacterial, and plasmid databases of NCBI, using the constraints of 95% sequence identity and 95% sequence coverage for blast hits. When grouping these results by species, the authors identified putative PAM sequences for 1049 among 1493 unique species. The remaining species either have insufficient data or an undetermined result from the analysis. Finally, the authors assigned a confidence score to each species' PAM prediction and generate categories that largely cover the revealed diversity of PAM motifs, providing a baseline for further experimental studies including PAM assays. The authors envision CASPERpam is a useful bioinformatic tool for understanding and harnessing the diversity of CRISPR-Cas systems.

DnaQ exonuclease-like domain of Cas2 promotes spacer integration in a type I-E CRISPR-Cas system

CRISPR-Cas systems constitute an adaptive immune system that provides acquired resistance against phages and plasmids in prokaryotes. Upon invasion of foreign nucleic acids, some cells integrate short fragments of foreign DNA as spacers into the CRISPR locus to memorize the invaders and acquire resistance in the subsequent round of infection. This immunization step called adaptation is the least understood part of the CRISPR-Cas immunity. We have focused here on the adaptation stage of Streptococcus thermophilus DGCC7710 type I-E CRISPR4-Cas (St4) system. Cas1 and Cas2 proteins conserved in nearly all CRISPR-Cas systems are required for spacer acquisition. The St4 CRISPR-Cas system is unique because the Cas2 protein is fused to an additional DnaQ exonuclease domain. Here, we demonstrate that St4 Cas1 and Cas2-DnaQ form a multimeric complex, which is capable of integrating DNA duplexes with 3'-overhangs (protospacers) in vitro We further show that the DnaQ domain of Cas2 functions as a 3'-5'-exonuclease that processes 3'-overhangs of the protospacer to promote integration.

Cas4 Nucleases Define the PAM, Length, and Orientation of DNA Fragments Integrated at CRISPR Loci

To achieve adaptive and heritable immunity against viruses and other mobile genetic elements, CRISPR-Cas systems must capture and store short DNA fragments (spacers) from these foreign elements into host genomic CRISPR arrays. This process is catalyzed by conserved Cas1/Cas2 integration complexes, but the specific roles of another highly conserved protein linked to spacer acquisition, the Cas4 nuclease, are just now emerging. Here, we show that two Cas4 nucleases (Cas4-1 and Cas4-2) play critical roles in CRISPR spacer acquisition in Pyrococcus furiosus. The nuclease activities of both Cas4 proteins are required to process protospacers to the correct size. Cas4-1 specifies the upstream PAM (protospacer adjacent motif), while Cas4-2 specifies the conserved downstream motif. Both Cas4 proteins ensure CRISPR spacer integration in a defined orientation leading to CRISPR immunity. Collectively, these findings provide in vivo evidence for critical roles of Cas4 nucleases in protospacer generation and functional spacer integration at CRISPR arrays.

Determining the Specificity of Cascade Binding, Interference, and Primed Adaptation In Vivo in the Escherichia coli Type I-E CRISPR-Cas System

In clustered regularly interspaced short palindromic repeat (CRISPR)-Cas (CRISPR-associated) immunity systems, short CRISPR RNAs (crRNAs) are bound by Cas proteins, and these complexes target invading nucleic acid molecules for degradation in a process known as interference. In type I CRISPR-Cas systems, the Cas protein complex that binds DNA is known as Cascade. Association of Cascade with target DNA can also lead to acquisition of new immunity elements in a process known as primed adaptation. Here, we assess the specificity determinants for Cascade-DNA interaction, interference, and primed adaptation in vivo, for the type I-E system of Escherichia coli Remarkably, as few as 5 bp of crRNA-DNA are sufficient for association of Cascade with a DNA target. Consequently, a single crRNA promotes Cascade association with numerous off-target sites, and the endogenous E. coli crRNAs direct Cascade binding to >100 chromosomal sites. In contrast to the low specificity of Cascade-DNA interactions, >18 bp are required for both interference and primed adaptation. Hence, Cascade binding to suboptimal, off-target sites is inert. Our data support a model in which the initial Cascade association with DNA targets requires only limited sequence complementarity at the crRNA 5' end whereas recruitment and/or activation of the Cas3 nuclease, a prerequisite for interference and primed adaptation, requires extensive base pairing.IMPORTANCE Many bacterial and archaeal species encode CRISPR-Cas immunity systems that protect against invasion by foreign DNA. In the Escherichia coli CRISPR-Cas system, a protein complex, Cascade, binds 61-nucleotide (nt) CRISPR RNAs (crRNAs). The Cascade complex is directed to invading DNA molecules through base pairing between the crRNA and target DNA. This leads to recruitment of the Cas3 nuclease, which destroys the invading DNA molecule and promotes acquisition of new immunity elements. We made the first in vivo measurements of Cascade binding to DNA targets. Thus, we show that Cascade binding to DNA is highly promiscuous; endogenous E. coli crRNAs can direct Cascade binding to >100 chromosomal locations. In contrast, we show that targeted degradation and acquisition of new immunity elements require highly specific association of Cascade with DNA, limiting CRISPR-Cas function to the appropriate targets.

Structure Basis for Directional R-loop Formation and Substrate Handover Mechanisms in Type I CRISPR-Cas System

Type I CRISPR systems feature a sequential dsDNA target searching and degradation process, by crRNA-displaying Cascade and nuclease-helicase fusion enzyme Cas3, respectively. Here we present two cryo-EM snapshots of the Thermobifida fusca type I-E Cascade: (1) unwinding 11 bp of dsDNA at the seed-sequence region to scout for sequence complementarity, and (2) further unwinding of the entire protospacer to form a full R-loop. These structures provide the much-needed temporal and spatial resolution to resolve key mechanistic steps leading to Cas3 recruitment. In the early steps, PAM recognition causes severe DNA bending, leading to spontaneous DNA unwinding to form a seed-bubble. The full R-loop formation triggers conformational changes in Cascade, licensing Cas3 to bind. The same process also generates a bulge in the non-target DNA strand, enabling its handover to Cas3 for cleavage. The combination of both negative and positive checkpoints ensures stringent yet efficient target degradation in type I CRISPR-Cas systems.

Major and minor crRNA annealing sites facilitate low stringency DNA protospacer binding prior to Type I-A CRISPR-Cas interference in Sulfolobus

The stringency of crRNA-protospacer DNA base pair matching required for effective CRISPR-Cas interference is relatively low in crenarchaeal Sulfolobus species in contrast to that required in some bacteria. To understand its biological significance we studied crRNA-protospacer interactions in Sulfolobus islandicus REY15A which carries multiple, and functionally diverse, interference complexes. A range of mismatches were introduced into a vector-borne protospacer that was identical to spacer 1 of CRISPR locus 2, with a cognate CCN PAM sequence. Two important crRNA annealing regions were identified on the 39 bp protospacer, a strong primary site centered on nucleotides 3 - 7 and a weaker secondary site at nucleotides 21 - 25. Multiple mismatches introduced into remaining protospacer regions did not seriously impair interference. Extending the study to different protospacers demonstrated that the efficacy of the secondary site was greatest for protospacers with higher G+C contents. In addition, the interference effects were assigned specifically to the type I-A dsDNA-targeting module by repeating the experiments with mutated protospacer constructs that were transformed into an S. islandicus mutant lacking type III-Bα and III-Bβ interference gene cassettes, which showed similar interference levels to those of the wild-type strain. Parallels are drawn to the involvement of 2 annealing sites for microRNAs on some eukaryal mRNAs which provide enhanced binding capacity and specificity. A biological rationale for the relatively low crRNA-protospacer base pairing stringency among the Sulfolobales is considered.

Protospacer recognition motifs: mixed identities and functional diversity

Protospacer adjacent motifs (PAMs) were originally characterized for CRISPR-Cas systems that were classified on the basis of their CRISPR repeat sequences. A few short 2-5 bp sequences were identified adjacent to one end of the protospacers. Experimental and bioinformatical results linked the motif to the excision of protospacers and their insertion into CRISPR loci. Subsequently, evidence accumulated from different virus- and plasmid-targeting assays, suggesting that these motifs were also recognized during DNA interference, at least for the recently classified type I and type II CRISPR-based systems. The two processes, spacer acquisition and protospacer interference, employ different molecular mechanisms, and there is increasing evidence to suggest that the sequence motifs that are recognized, while overlapping, are unlikely to be identical. In this article, we consider the properties of PAM sequences and summarize the evidence for their dual functional roles. It is proposed to use the terms protospacer associated motif (PAM) for the conserved DNA sequence and to employ spacer acqusition motif (SAM) and target interference motif (TIM), respectively, for acquisition and interference recognition sites.