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

Natronobacterium gregoryi Argonaute inhibits class 1 integron integrase-mediated excision and integration

Argonaute (Ago) proteins, ubiquitous in all domains of life, serve as key components in defense against foreign nucleic acids. While eukaryotic Agos (eAgos) are well characterized for guide RNA-mediated RNA targeting, prokaryotic Agos (pAgos) exhibit diverse functions, particularly in protecting bacteria from invasive DNA. The previous study identified Class 1 integron integrase (IntI-1), a tyrosine site-specific recombinase involved in horizontal transfer of antibiotic resistance genes, as a potential interaction partner of Natronobacterium gregoryi Argonaute (NgAgo), a member of pAgos. Here, we demonstrated that this interaction was direct, depended on the PIWI domain, and was independent of the catalytic activity of NgAgo. Notably, no interaction occurred between NgAgo and Cre (another tyrosine site-specific recombinase), highlighting the specificity of NgAgo-IntI-1 interaction. Furthermore, NgAgo could inhibit binding of IntI-1 to its target DNA, and then impede IntI-1-mediated integration and excision. Consistent with the above finding, few pAgos could be found in prokaryotic genomes containing IntI, whereas IntI showed significant co-occurrence with another bacterial defense system, CRISPR-Cas. In summary, our study elucidated a novel defense mechanism of pAgos through interaction with IntI-1 for inhibiting IntI-1-mediated gene excision/integration process.

A mesophilic Argonaute from Clostridium formicaceticum with efficient DNA cleavage activity guided by small DNA

We characterized a new Argonaute protein (pAgo), CfAgo, from the mesophilic bacterium Clostridium formicaceticum. CfAgo possesses DNA-guided DNA endonuclease activity and cleaves DNA targets at the canonical site. It is active from 28°C to 75°C and prefers DNA guides with a 5'-phosphate group and thymidine as the first nucleotide. Cleavage activity is reduced by single-nucleotide mismatches in the seed, central, and 3'-supplementary regions of guides, with stronger mismatch discrimination observed for 5'hydroxylated (5'OH) guides compared to 5'phosphorylated (5'P) guides. Moreover, structural analysis suggests that the MID domain of CfAgo is crucial for recognizing the 5' guide and it influences the binding specificity. CfAgo catalyzes programmable cleavage of double-stranded DNA in AT-rich regions in the presence of Mn2+ and Mg2+ ions at appropriate salt concentrations. These properties could make CfAgo a promising tool for DNA manipulation such as nucleic acid detection and cleavage.

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.