No abstract

Argonaute proteins are programmable nucleases that have defense and regulatory functions in both eukaryotes and prokaryotes. All known prokaryotic Argonautes (pAgos) characterized so far act on DNA targets. Here, we describe a new class of pAgos that uniquely use DNA guides to process RNA targets. The biochemical and structural analysis of Pseudooceanicola lipolyticus pAgo (PliAgo) reveals an unusual organization of the guide binding pocket that does not rely on divalent cations and the canonical set of contacts for 5’-end interactions. Unconventional interactions of PliAgo with the 5’-phosphate of guide DNA define its new position within pAgo and shift the site of target RNA cleavage in comparison with known Argonautes. The specificity for RNA over DNA is defined by ribonucleotide residues at the cleavage site. The analysed pAgos sense mismatches and modifications in the RNA target. The results broaden our understanding of prokaryotic defense systems and extend the spectrum of programmable nucleases with potential use in RNA technology.

Prokaryotic Argonautes (pAgos) are programmable nucleases involved in cell defense against invading DNA. In vitro, pAgos can bind small single-stranded guide DNAs to recognize and cleave complementary DNA. In vivo, pAgos preferentially target plasmids, phages and multicopy genetic elements. Here, we show that CbAgo nuclease from Clostridium butyricum can be used for genomic DNA engineering in bacteria. We demonstrate that CbAgo loaded with plasmid-derived guide DNAs can recognize and cleave homologous chromosomal loci, and define the minimal length of homology required for this targeting. Cleavage of plasmid DNA at an engineered site of the I-SceI meganuclease increases guide DNA loading into CbAgo and enhances processing of homologous chromosomal loci. Analysis of guide DNA loading into CbAgo also reveals off-target sites of I-SceI in the Escherichia coli genome, demonstrating that pAgos can be used for highly sensitive detection of double-stranded breaks in genomic DNA. Finally, we show that CbAgo-dependent targeting of genomic loci with plasmid-derived guide DNAs promotes homologous recombination between plasmid and chromosomal DNA, depending on the catalytic activity of CbAgo. Specific targeting of plasmids with Argonautes can be used to integrate plasmid-encoded sequences into the chromosome thus enabling genome editing.