Figure 3

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The Cas proteins involved in target cleavage of six types. In Type I systems, multiple subunits assemble along the crRNA and initiate the R-loop formation, which then recruits Cas3 for dsDNA cleavage. The complex in Type III exhibits a similar architecture as that in Type I, and the difference is that this complex recognizes and degrades complementary target RNA. Complex in Type IV is less understood; here shows the ribonucleoprotein (RNP) complex of Type IV-B, whose structure has been determined [83]. Cas9 nuclease in Type II mediates dsDNA cleavage guided by crRNA and tracrRNA. The non-targeting DNA cleavage of Cas9 is reported to be RNA-independent in the presence of Mn2+ ions [84]. However, in Type V, Cas12 has both targeting and non-targeting DNA cleavage activity guided by crRNA alone or crRNA and tracrRNA (scoutRNA in Type V-C and Type V-D). Cas13 from Type VI is an RNA-guided RNA nuclease. And target-activated Cas13 complex can cleave surrounding RNA molecules non-specifically [85].

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