Prokaryotic Argonaute Protein from Natronobacterium gregoryi Requires RNAs To Activate for DNA Interference In Vivo

Prokaryotic Argonautes for in vivo biotechnology and molecular diagnostics

Prokaryotic Argonautes (pAgos) are an emerging class of programmable endonucleases that are believed to be more flexible than existing CRISPR-Cas systems and have significant potential for biotechnology. Current applications of pAgos include a myriad of molecular diagnostics and in vitro DNA assembly tools. However, efforts have historically been centered on thermophilic pAgo variants. To enable in vivo biotechnological applications such as gene editing, focus has shifted to pAgos from mesophilic organisms. We discuss what is known of pAgos, how they are being developed for various applications, and strategies to overcome current challenges to in vivo applications in prokaryotes and eukaryotes.

A broadly distributed predicted helicase/nuclease confers phage resistance via abortive infection

There is strong selection for the evolution of systems that protect bacterial populations from viral attack. We report a single phage defense protein, Hna, that provides protection against diverse phages in Sinorhizobium meliloti, a nitrogen-fixing alpha-proteobacterium. Homologs of Hna are distributed widely across bacterial lineages, and a homologous protein from Escherichia coli also confers phage defense. Hna contains superfamily II helicase motifs at its N terminus and a nuclease motif at its C terminus, with mutagenesis of these motifs inactivating viral defense. Hna variably impacts phage DNA replication but consistently triggers an abortive infection response in which infected cells carrying the system die but do not release phage progeny. A similar host cell response is triggered in cells containing Hna upon expression of a phage-encoded single-stranded DNA binding protein (SSB), independent of phage infection. Thus, we conclude that Hna limits phage spread by initiating abortive infection in response to a phage protein.

A Programmable, DNA-Exclusively-Guided Argonaute DNase and Its Higher Cleavage Specificity Achieved by 5′-Hydroxylated Guide

Argonaute proteins exist widely in eukaryotes and prokaryotes, and they are of great potential for molecular cloning, nucleic acid detection, DNA assembly, and gene editing. However, their overall properties are not satisfactory and hinder their broad applications. Herein, we investigated a prokaryotic Argonaute nuclease from a mesophilic bacterium Clostridium disporicum (CdAgo) and explored its overall properties, especially with 5′-hydroxylated (5′-OH) guides. We found that CdAgo can exclusively use single-stranded DNA (ssDNA) as guide to cleave ssDNA and plasmid targets. Further, we found the length of the efficient guide is narrower for the 5′-OH guide (17–20 nt) than for the 5′-phosphorylated guide (5′-P, 14–21 nt). Furthermore, we discovered that the 5′-OH guides can generally offer stronger mismatch discrimination than the 5′-P ones. The 5′-OH guides offer the narrower length range, higher mismatch discrimination and more accurate cleavage than the 5′-P guides. Therefore, 5′-OH-guide-directed CdAgo has great potential in biological and biomedical applications.