The DarT/DarG Toxin-Antitoxin ADP-Ribosylation System as a Novel Target for a Rational Design of Innovative Antimicrobial Strategies

Nucleotidyltransferase toxin MenT extends aminoacyl acceptor ends of serine tRNAs to control Mycobacterium tuberculosis growth

Toxins of toxin-antitoxin systems use diverse mechanisms to inhibit bacterial growth. In this study, we characterize the translation inhibitor toxin MenT3 of Mycobacterium tuberculosis, the bacterium responsible for tuberculosis in humans. We show that MenT3 is a robust cytidine specific tRNA nucleotidyltransferase in vitro, capable of modifying the aminoacyl acceptor ends of most tRNA but with a marked preference for tRNASer, to which long stretches of cytidines are added. Furthermore, transcriptomic-wide analysis of MenT3 targets in M. tuberculosis identifies tRNASer as the sole target of MenT3 and reveals significant detoxification attempts by the essential CCA-adding enzyme PcnA in response to MenT3. Finally, under physiological conditions, only in the presence the native menAT3 operon, an active pool of endogenous MenT3 targeting tRNASer in M. tuberculosis is detected, likely reflecting the importance of MenT3 during infection.

Identification and characterization of a novel type II toxin-antitoxin system in Aeromonas veronii

The bacterial type II toxin-antitoxin (TA) system is a rich genetic element that participates in various physiological processes. Aeromonas veronii is the main bacterial pathogen threatening the freshwater aquaculture industry. However, the distribution of type II TA system in A. veronii was seldom documented and its roles in the life activities of A. veronii were still unexplored. In this study, a novel type II TA system AvtA-AvtT was predicted in a fish pathogen Aeromonas veronii biovar sobria with multi-drug resistance using TADB 2.0. Through an Escherichia coli host killing and rescue assay, we demonstrated that AvtA and AvtT worked as a genuine TA system, and the predicted toxin AvtT actually functioned as an antitoxin while the predicted antitoxin AvtA actually functioned as a toxin. The binding ability of AvtA with AvtT proteins were confirmed by dot blotting analysis and co-immunoprecipitation assay. Furthermore, we found that the toxin and antitoxin labelled with fluorescent proteins were co-localized. In addition, it was found that the transcription of AvtAT bicistronic operon was repressed by the AvtAT protein complex. Deletion of avtA gene and avtT gene had no obvious effect on the drug susceptibility. This study provides first characterization of type II TA system AvtA-AvtT in aquatic pathogen A. veronii.

Targeting the Ubiquitin-Proteasome System and Recent Advances in Cancer Therapy

Ubiquitination is a reversible post-translational modification based on the chemical addition of ubiquitin to proteins with regulatory effects on various signaling pathways. Ubiquitination can alter the molecular functions of tagged substrates with respect to protein turnover, biological activity, subcellular localization or protein-protein interaction. As a result, a wide variety of cellular processes are under ubiquitination-mediated control, contributing to the maintenance of cellular homeostasis. It follows that the dysregulation of ubiquitination reactions plays a relevant role in the pathogenic states of human diseases such as neurodegenerative diseases, immune-related pathologies and cancer. In recent decades, the enzymes of the ubiquitin-proteasome system (UPS), including E3 ubiquitin ligases and deubiquitinases (DUBs), have attracted attention as novel druggable targets for the development of new anticancer therapeutic approaches. This perspective article summarizes the peculiarities shared by the enzymes involved in the ubiquitination reaction which, when deregulated, can lead to tumorigenesis. Accordingly, an overview of the main pharmacological interventions based on targeting the UPS that are in clinical use or still in clinical trials is provided, also highlighting the limitations of the therapeutic efficacy of these approaches. Therefore, various attempts to circumvent drug resistance and side effects as well as UPS-related emerging technologies in anticancer therapeutics are discussed.

MenT nucleotidyltransferase toxins extend tRNA acceptor stems and can be inhibited by asymmetrical antitoxin binding

Mycobacterium tuberculosis, the bacterium responsible for human tuberculosis, has a genome encoding a remarkably high number of toxin-antitoxin systems of largely unknown function. We have recently shown that the M. tuberculosis genome encodes four of a widespread, MenAT family of nucleotidyltransferase toxin-antitoxin systems. In this study we characterize MenAT1, using tRNA sequencing to demonstrate MenT1 tRNA modification activity. MenT1 activity is blocked by MenA1, a short protein antitoxin unrelated to the MenA3 kinase. X-ray crystallographic analysis shows blockage of the conserved MenT fold by asymmetric binding of MenA1 across two MenT1 protomers, forming a heterotrimeric toxin-antitoxin complex. Finally, we also demonstrate tRNA modification by toxin MenT4, indicating conserved activity across the MenT family. Our study highlights variation in tRNA target preferences by MenT toxins, selective use of nucleotide substrates, and diverse modes of MenA antitoxin activity.