Bacterial Argonaute Proteins Aid Cell Division in the Presence of Topoisomerase Inhibitors in Escherichia coli

Abstract

Prokaryotic Argonaute (pAgo) proteins are guide-dependent nucleases that function in host defense against invaders. Recently, it was shown that TtAgo from Thermus thermophilus also participates in the completion of DNA replication by decatenating chromosomal DNA. Here, we show that two pAgos from cyanobacteria Synechococcus elongatus (SeAgo) and Limnothrix rosea (LrAgo) are active in heterologous Escherichia coli and aid cell division in the presence of the gyrase inhibitor ciprofloxacin, depending on the host double-strand break repair machinery. Both pAgos are preferentially loaded with small guide DNAs (smDNAs) derived from the sites of replication termination. Ciprofloxacin increases the amounts of smDNAs from the termination region and from the sites of genomic DNA cleavage by gyrase, suggesting that smDNA biogenesis depends on DNA replication and is stimulated by gyrase inhibition. Ciprofloxacin enhances asymmetry in the distribution of smDNAs around Chi sites, indicating that it induces double-strand breaks that serve as a source of smDNA during their processing by RecBCD. While active in E. coli, SeAgo does not protect its native host S. elongatus from ciprofloxacin. These results suggest that pAgo nucleases may help to complete replication of chromosomal DNA by promoting chromosome decatenation or participating in the processing of gyrase cleavage sites, and may switch their functional activities depending on the host species.

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