Table 2.
Characteristics of Cas13 effectors in type VI CRISPR systems.
| Cas Effectors | Cas13a (VI-A) | Cas13b (VI-B) | Cas13d (VI-D) | Cas13x (VI-X) |
|---|---|---|---|---|
| Cas protein size | ~1250 aa | ~1150 aa | ~930 aa | ~775–805 aa |
| Protospacer-flanking site (PFS) |
A, U and C | A, U and G | no PFS constraints | no PFS constraints |
| Architecture | REC and NUC lobes | pyramidal (binary complex) | REC and NUC lobes | REC and NUC lobes |
| pre-crRNA processing site | Helical-1 and HEPN-2 domains | RRI-2 domain | HEPN-2 domain | not process |
| Direct repeat lengths | 35–39 nt | 36 nt | 36 nt | 36 nt |
| Orientation (repeat to spacer) | 5′–3′ | 3′–5′ | 5′–3′ | 3′–5′ |