Fragmentation of the CRISPR-Cas Type I-B signature protein Cas8b

Pangenomic Study of Fusobacterium nucleatum Reveals the Distribution of Pathogenic Genes and Functional Clusters at the Subspecies and Strain Levels

Fusobacterium nucleatum is a prevalent periodontal pathogen and is associated with many systemic diseases. Our knowledge of the genomic characteristics and pathogenic effectors of different F. nucleatum strains is limited. In this study, we completed the whole genome assembly of the 4 F. nucleatum strains and carried out a comprehensive pangenomic study of 30 strains with their complete genome sequences. Phylogenetic analysis revealed that the F. nucleatum strains are mainly divided into 4 subspecies, while 1 of the sequenced strains was classified into a new subspecies. Gene composition analysis revealed that a total of 517 "core/soft-core genes" with housekeeping functions widely distributed in almost all the strains. Each subspecies had a unique gene cluster shared by strains within the subspecies. Analysis of the virulence factors revealed that many virulence factors were widely distributed across all the strains, with some present in multiple copies. Some virulence genes showed no consistent occurrence rule at the subspecies level and were specifically distributed in certain strains. The genomic islands mainly revealed strain-specific characteristics instead of subspecies level consistency, while CRISPR types and secondary metabolite biosynthetic gene clusters were identically distributed in F. nucleatum strains from the same subspecies. The variation in amino acid sites in the adhesion protein FadA did not affect the monomer and dimer 3D structures, but it may affect the binding surface and the stability of binding to host receptors. This study provides a basis for the pathogenic study of F. nucleatum at the subspecies and strain levels. IMPORTANCE We used F. nucleatum as an example to analyze the genomic characteristics of oral pathogens at the species, subspecies, and strain levels and elucidate the similarities and differences in functional genes and virulence factors among different subspecies/strains of the same oral pathogen. We believe that the unique biological characteristics of each subspecies/strain can be attributed to the differences in functional gene clusters or the presence/absence of certain virulence genes. This study showed that F. nucleatum strains from the same subspecies had similar functional gene compositions, CRISPR types, and secondary metabolite biosynthetic gene clusters, while pathogenic genes, such as virulence genes, antibiotic resistance genes, and GIs, had more strain level specificity. The findings of this study suggest that, for microbial pathogenicity studies, we should carefully consider the subspecies/strains being used, as different strains may vary greatly.

Functional PAM sequence for DNA interference by CRISPR-Cas I-B system of Leptospira interrogans and the role of LinCas11b encoded within lincas8b

Pathogenic species of Leptospira are recalcitrant for genetic manipulation using conventional tools, and therefore there is a need to explore techniques of higher efficiency. Application of endogenous CRISPR-Cas tool is emerging and efficient; nevertheless, it is limited by a poor understanding of interference machinery in the bacterial genome and its associated protospacer adjacent motif (PAM). In this study, interference machinery of CRISPR-Cas subtype I-B (Lin_I-B) from L. interrogans was experimentally validated in E. coli using the various identified PAM (TGA, ATG, ATA). The overexpression of the Lin_I-B interference machinery in E. coli demonstrated that LinCas5, LinCas6, LinCas7, and LinCas8b can self-assemble on cognate CRISPR RNA to form an interference complex (LinCascade). Moreover, a robust interference of target plasmids containing a protospacer with a PAM suggested a functional LinCascade. We also recognized a small open reading frame within lincas8b that independently co-translates LinCas11b. A mutant variant of LinCascade^-Cas11b that lacks LinCas11b co-expression erred to mount target plasmid interference. At the same time, LinCas11b complementation in LinCascade^-Cas11b rescued target plasmid interference. Thus, the present study establishes Leptospira subtype I-B interference machinery to be functional and, soon, may pave the way for scientists to harness it as a programmable endogenous genetic manipulation tool.

CRISPR/Cas Technologies and Their Applications in Escherichia coli

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas) systems have revolutionized genome editing and greatly promoted the development of biotechnology. However, these systems unfortunately have not been developed and applied in bacteria as extensively as in eukaryotic organism. Here, the research progress on the most widely used CRISPR/Cas tools and their applications in Escherichia coli is summarized. Genome editing based on homologous recombination, non-homologous DNA end-joining, transposons, and base editors are discussed. Finally, the state of the art of transcriptional regulation using CRISPRi is briefly reviewed. This review provides a useful reference for the application of CRISPR/Cas systems in other bacterial species.

Diverse CRISPR-Cas Complexes Require Independent Translation of Small and Large Subunits from a Single Gene

CRISPR-Cas adaptive immune systems provide prokaryotes with defense against viruses by degradation of specific invading nucleic acids. Despite advances in the biotechnological exploitation of select systems, multiple CRISPR-Cas types remain uncharacterized. Here, we investigated the previously uncharacterized type I-D interference complex and revealed that it is a genetic and structural hybrid with similarity to both type I and type III systems. Surprisingly, formation of the functional complex required internal in-frame translation of small subunits from within the large subunit gene. We further show that internal translation to generate small subunits is widespread across diverse type I-D, I-B, and I-C systems, which account for roughly one quarter of CRISPR-Cas systems. Our work reveals the unexpected expansion of protein coding potential from within single cas genes, which has important implications for understanding CRISPR-Cas function and evolution.

Heavily Armed Ancestors: CRISPR Immunity and Applications in Archaea with a Comparative Analysis of CRISPR Types in Sulfolobales

Prokaryotes are constantly coping with attacks by viruses in their natural environments and therefore have evolved an impressive array of defense systems. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) is an adaptive immune system found in the majority of archaea and about half of bacteria which stores pieces of infecting viral DNA as spacers in genomic CRISPR arrays to reuse them for specific virus destruction upon a second wave of infection. In detail, small CRISPR RNAs (crRNAs) are transcribed from CRISPR arrays and incorporated into type-specific CRISPR effector complexes which further degrade foreign nucleic acids complementary to the crRNA. This review gives an overview of CRISPR immunity to newcomers in the field and an update on CRISPR literature in archaea by comparing the functional mechanisms and abundances of the diverse CRISPR types. A bigger fraction is dedicated to the versatile and prevalent CRISPR type III systems, as tremendous progress has been made recently using archaeal models in discerning the controlled molecular mechanisms of their unique tripartite mode of action including RNA interference, DNA interference and the unique cyclic-oligoadenylate signaling that induces promiscuous RNA shredding by CARF-domain ribonucleases. The second half of the review spotlights CRISPR in archaea outlining seminal in vivo and in vitro studies in model organisms of the euryarchaeal and crenarchaeal phyla, including the application of CRISPR-Cas for genome editing and gene silencing. In the last section, a special focus is laid on members of the crenarchaeal hyperthermophilic order Sulfolobales by presenting a thorough comparative analysis about the distribution and abundance of CRISPR-Cas systems, including arrays and spacers as well as CRISPR-accessory proteins in all 53 genomes available to date. Interestingly, we find that CRISPR type III and the DNA-degrading CRISPR type I complexes co-exist in more than two thirds of these genomes. Furthermore, we identified ring nuclease candidates in all but two genomes and found that they generally co-exist with the above-mentioned CARF domain ribonucleases Csx1/Csm6. These observations, together with published literature allowed us to draft a working model of how CRISPR-Cas systems and accessory proteins cross talk to establish native CRISPR anti-virus immunity in a Sulfolobales cell.

Application of different types of CRISPR/Cas-based systems in bacteria

As important genome editing tools, CRISPR/Cas systems, especially those based on type II Cas9 and type V Cas12a, are widely used in genetic and metabolic engineering of bacteria. However, the intrinsic toxicity of Cas9 and Cas12a-mediated CRISPR/Cas tools can lead to cell death in some strains, which led to the development of endogenous type I and III CRISPR/Cas systems. However, these systems are hindered by complicated development and limited applications. Thus, further development and optimization of CRISPR/Cas systems is needed. Here, we briefly summarize the mechanisms of different types of CRISPR/Cas systems as genetic manipulation tools and compare their features to provide a reference for selecting different CRISPR/Cas tools. Then, we show the use of CRISPR/Cas technology for bacterial strain evolution and metabolic engineering, including genome editing, gene expression regulation and the base editor tool. Finally, we offer a view of future directions for bacterial CRISPR/Cas technology.

Development of both type I-B and type II CRISPR/Cas genome editing systems in the cellulolytic bacterium Clostridium thermocellum

The robust lignocellulose-solubilizing activity of C. thermocellum makes it a top candidate for consolidated bioprocessing for biofuel production. Genetic techniques for C. thermocellum have lagged behind model organisms thus limiting attempts to improve biofuel production. To improve our ability to engineer C. thermocellum, we characterized a native Type I-B and heterologous Type II Clustered Regularly-Interspaced Short Palindromic Repeat (CRISPR)/cas (CRISPR associated) systems. We repurposed the native Type I-B system for genome editing. We tested three thermophilic Cas9 variants (Type II) and found that GeoCas9, isolated from Geobacillus stearothermophilus, is active in C. thermocellum. We employed CRISPR-mediated homology directed repair to introduce a nonsense mutation into pyrF. For both editing systems, homologous recombination between the repair template and the genome appeared to be the limiting step. To overcome this limitation, we tested three novel thermophilic recombinases and demonstrated that exo/beta homologs, isolated from Acidithiobacillus caldus, are functional in C. thermocellum. For the Type I-B system an engineered strain, termed LL1586, yielded 40% genome editing efficiency at the pyrF locus and when recombineering machinery was expressed this increased to 71%. For the Type II GeoCas9 system, 12.5% genome editing efficiency was observed and when recombineering machinery was expressed, this increased to 94%. By combining the thermophilic CRISPR system (either Type I-B or Type II) with the recombinases, we developed a new tool that allows for efficient CRISPR editing. We are now poised to enable CRISPR technologies to better engineer C. thermocellum for both increased lignocellulose degradation and biofuel production.