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

Table 1 The core proteins and domains comprising the RNA/DNA-guided immune systems^a

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(?)	Dicer: ERCC4-like SF2 helicase RNase III PAZ 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

^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

ISSN: 1745-6150