Fig. 1

Functional overview of CRISPR/Cas9 and CRISPR/Cas13 systems. a Schematic of Cas9 mechanism in genome editing. This system requires the recruitment of CRISPR-associated protein Cas9 (blue) to the target site recognized by the guide RNA (gRNA: orange). Target site cleavage by Cas9 is ensured by the presence of the protospacer adjacent motif (PAM) (green), a sequence that immediately follows the target site. The PAM will determine the Cas9 cleavage site, which lies about three nucleotides upstream of the PAM. b Schematic of the Cas13 RNA cleavage mechanism. This system requires the pre-assembly of Cas13 (green) with the CRISPR-RNA (crRNA: red) to recognize target RNAs. Upon RNA-binding, Cas13 will undergo a conformational change and induce the catalytic activity of its nuclease domains, resulting in the cleavage of target transcripts. c Comparisons of Cas9 size with different Cas13 subtypes (a–d). Polypeptide sizes are indicated as the number of amino acids. d Relative structural representation of different Cas13 subtype-compatible crRNAs. All four subtype crRNAs carry a direct repeat to facilitate the binding with their corresponding Cas13 enzyme, as well as a spacer sequence specific for the target transcript. Cas13b-compatible crRNAs carry a direct repeat at the 3′-end while compatible crRNAs for Cas13a, c, and d carry the direct repeat at the 5′-end