Fig. 1

The evolutionary history of eukaryotic RNAi: assembly from diverse archaeal and bacterial ancestors. The “bacterial” and “archaeal” components of the RNAi protein machinery are assumed to have evolved from the proto-mitochondrial endosymbiont and its archaeal host, respectively. This scenario rests on the fact that RNase III is a protein that is nearly ubiquitous in bacteria but rare in archaea, and the (DNA-dependent) RNA polymerase that is thought to be the ancestor of the RNAi RdRp so far has been identified only in bacteriophages (not in archaeal viruses). However, it cannot be ruled out that these genes have been acquired by the mesophilic archaeal ancestor of eukaryotes (presumably, a member of the Lokiarchaeota) prior to endosymbiosis. RIII, RNAse III