Prokaryotic Argonautes defend genomes against invasive DNA.

Argonaute proteins are central players in small RNA-mediated silencing mechanisms such as RNA interference (RNAi), microRNA repression and piRNA-mediated transposon silencing. In eukaryotes, Argonautes bind small RNAs that guide them to RNA targets in order to regulate gene expression and repress invasive genomic elements. Although Argonaute proteins are conserved in all life forms from bacteria to eukaryotes, until now studies have focused on the biological functions of eukaryotic Argonautes. Here we highlight two recent studies that discover the functions of prokaryotic Argonautes in defence against exogenous DNA.