RNA silencing plays crucial roles in both bacteria and eukaryotes, yet its machinery appears to differ in these two kingdoms. A couple of Argonaute protein homologs have been reported in some archaeal species in recent years. As Argonaute protein is the key component of eukaryotic RNA silencing pathways, such findings suggested the possibility of existence of eukaryotic RNA silencing like pathways in Archaea, which present the life forms between prokaryotes and eukaryotes. To further explore such hypothesis, we systematically screened 71 fully sequenced archaaeal genomes, and identified some proteins containing homologous regions to the functional domains of eukaryotic RNA silencing pathway key proteins. The phylogenetic relationships of these proteins were analyzed. The conserved func-tional amino acids between archaeal and eukaryotic Piwi domains suggested their functional similarity. Our results provide new clues to the evolution of RNA silencing pathways.
