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Table 1 The core proteins and domains comprising the RNA/DNA-guided immune systemsa

From: Evolution of RNA- and DNA-guided antivirus defense systems in prokaryotes and eukaryotes: common ancestry vs convergence

pAgo: innate immunity in prokaryotes Eukaryotic RNAi: innate immunity (piRNA branch: adaptive immunity) CRISPR-Cas: adaptive immunity in prokaryotes
Adaptation/spacer acquisition
NA NA Cas1: unique α-helical fold
Cas2: RRM
Guide RNA processing/maturation and amplification
pAgo: RNase H, PAZ
additional nucleases(?)
ERCC4-like SF2 helicase
RdRp (double-psi beta barrel, DdRp homolog)
Class1: multi-RRM complexes
Class2: [RNase III]; uncharacterized domains of effector proteins
Target recognition and cleavage
pAgo: RNase H, PAZ
additional nucleases(?)
eAgo: RNase H, PAZ Class 1: SF2 helicase, HD nuclease, PolB-like/RRM nuclease
Class 2: TnpB/RuvC (RNAse H fold), HEPN
  1. aOnly the key, evolutionarily conserved domains are included for each system. The domains that are homologous between different classes of RNA/DNA-guided systems are shown in bold type. For Class 2 CRISPR-Cas, RNase III is shown in brackets, to indicate that this is not a Cas protein