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Fig. 2 | Genome Biology

Fig. 2

From: Diversity of CRISPR-Cas immune systems and molecular machines

Fig. 2

Diversity of CRISPR-Cas molecular machines. Two main classes of CRISPR-Cas systems exist, which are defined by the nature of their Cas effector nucleases, either constituted by multiprotein complexes (class 1), or by a single signature protein (class 2). For class 1 systems, the main types of CRISPR-Cas systems include type I and type III systems. Illustrated here as an example, the Escherichia coli K12 type I-E system (upper left) targets sequences flanked by a 5′-located PAM. Guide RNAs are generated by Cascade, in a Cas6-defined manner and typically contain an eight-nucleotide 5′ handle derived from the CRISPR repeat, a full spacer sequence, and a 3′ hairpin derived from the CRISPR repeat. Following nicking of the target strand, the 3′ to 5′ Cas3 exonuclease destroys the target DNA in a directional manner. In the Pyrococcus furiosus DSM 3638 type III-B system (lower left), a short crRNA guide directs the Cmr complex towards complementary single-stranded RNA in a PAM-independent manner. For the canonical type II-A Streptococcus thermophilus LMD-9 system (upper right), a dual crRNA–tracrRNA guide generated by Cas9 and RNase III targets a 3′-flanked PAM DNA complementary sequence for the genesis of a precise double-stranded break using two nickase domains (RuvC and HNH). For the Francisella novicida U112 type V system (lower right), a single guide RNA targets complementary dsDNA flanked by a 5′-PAM using Cpf1, which generates a staggered dsDNA break. Cascade CRISPR-associated complex for antiviral defense, CRISPR clustered regularly interspaced short palindromic repeat, crRNA CRISPR RNA, dsDNA double-stranded DNA, L leader, nt nucleotide, PAM protospacer adjacent motif, ssRNA single-stranded RNA, tracrRNA trans-activating CRISPR RNA

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