Bacterial Argonaute samples the transcriptome to identify foreign DNA

Eukaryotic Argonautes bind small RNAs and use them as guides to find complementary RNA targets and induce gene silencing. Though homologs of eukaryotic Argonautes are present in many bacteria and archaea their small RNA partners and functions are unknown. We found that the Argonaute of Rhodobacter sphaeroides (RsAgo) associates with 15-19 nt small RNAs that correspond to the majority of transcripts. RsAgo also binds single-stranded 22-24 nt DNA molecules that are complementary to the small RNAs and enriched in sequences derived from exogenous plasmids as well as genome-encoded foreign nucleic acids such as transposons and phage genes. Expression of RsAgo in the heterologous E. coli system leads to formation of plasmid–derived small RNA and DNA and plasmid degradation. In a R. sphaeroides mutant lacking RsAgo, expression of plasmid-encoded genes is elevated, while other genes are unaffected. Our results indicate that RNAi-related processes found in eukaryotes are also conserved in bacteria and target foreign nucleic acids. Overall design: Small RNA and small DNA from purified RsAgo expressed in Rhodobacter sphaeroides ATCC17025 ("strain 25") and E. coli BL21(DE3) Total small RNA (13-30nt) from R. sphaeroides ATCC17025 and ATCC17029 (wild-type; RsAgo mutant; containing either empty or RsAgo-expressing plasmid) Total ribosomal RNA-depleted RNA from wild type or RsAgo mutant R. sphaeroides ATCC17025