To confirm that NgAgo uses ssDNA guides, we analyzed the nucleic acids purified from NgAgo expressed in Escherichia coli, as those bacteria produce both 5′-phosphorylated ssRNAs and ssDNAs, and the nucleic acids of proper length (13–25 nt) naturally bind to prokaryotic Argonautes11,12. Nuclease digestion confirmed that DNA but not RNA binds NgAgo (Fig. 1a). To test whether the NgAgo expressed by E. coli can cleave target DNA at 37 °C in vitro, we performed an in vitro plasmid cleavage assay. We designed three 5′ phosphorylated 24-nucleotide (nt) ssDNA guides, among which two guides were complementary to each other (‘FW’ and ‘RV’ guides) and corresponded to a target site in plasmid pACYCDuet-eGFP, which had no homologous sequence to the NgAgo-encoding plasmid pGEX6P-1 (Fig. 1b). The other guide had a random sequence without overlap with pACYCDuet-eGFP (noncomplementary (‘NC’) guide). We also designed a pair of 5′-phosphorylated RNA guides corresponding
to the same target site. Before the cleavage assay, we replaced the native nucleic acids bound to the purified) NgAgo with our designed guides by incubating NgAgo with the guides at 55 °C for 1 h.
NgAgo could not catalyze cleavage without guide or with the NC guide (Fig. 1b). When supplied with either of the complementary FW or RV guides, NgAgo could nick the negatively supercoiled plasmid at 37 °C; when supplied with both FW and RV guides, NgAgo linearized the plasmid. However, neither ssDNA guides without 5′ phosphorylation nor 5′-phosphorylated ssRNA guides led to plasmid cleavage (Fig. 1c). Thus, NgAgo can cleave DNA double helix targets at
37 °C with 5′ phosphorylated ssDNA guide.