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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



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