Bacterial persister cells constitute a subpopulation of genetically identical, metabolically slow-growing cells that are highly tolerant of antibiotics and other environmental stresses. Recent studies have demonstrated that gene loci known as toxin-antitoxin (TA) modules play a central role in the persister state. Under normal growth conditions, antitoxins potently inhibit the activities of the toxins. In contrast, under conditions of stress, the antitoxins are selectively degraded, freeing the toxins to inhibit essential cellular processes, such as DNA replication and protein translation. This inhibition results in rapid growth arrest. In this Review, we highlight recent discoveries of these multifaceted TA systems with a focus on the newly uncovered mechanisms, especially conditional cooperativity, that are used to regulate cell growth and persistence. We also discuss the potential for targeting TA systems for antimicrobial drug discovery.

Toxin-antitoxin systems in bacterial growth arrest and persistence

Toxins, Targets, and Triggers: An Overview of Toxin-Antitoxin Biology

Toxin-antitoxin (TA) systems are small genetic elements composed of a toxin gene and its cognate antitoxin. The toxins of all known TA systems are proteins while the antitoxins are either proteins or non-coding RNAs. Based on the molecular nature of the antitoxin and its mode of interaction with the toxin the TA modules are currently grouped into five classes. In general, the toxin is more stable than the antitoxin but the latter is expressed to a higher level. If supply of the antitoxin stops, for instance under special growth conditions or by plasmid loss in case of plasmid encoded TA systems, the antitoxin is rapidly degraded and can no longer counteract the toxin. Consequently, the toxin becomes activated and can act on its cellular targets. Typically, TA toxins act on crucial cellular processes including translation, replication, cytoskeleton formation, membrane integrity, and cell wall biosynthesis. TA systems and their components are also versatile tools for a multitude of purposes in basic research and biotechnology. Currently, TA systems are frequently used for selection in cloning and for single protein expression in living bacterial cells. Since several TA toxins exhibit activity in yeast and mammalian cells they may be useful for applications in eukaryotic systems. TA modules are also considered as promising targets for the development of antibacterial drugs and their potential to combat viral infection may aid in controlling infectious diseases.

https://www.tandfonline.com/doi/pdf/10.4161/mge.26219

Toxin–antitoxin systems: Biology, identification, and application

Toxin–antitoxin (TA) systems are small genetic elements found on plasmids or chromosomes of countless bacteria, archaea, and possibly also unicellular fungi. Under normal growth conditions, the activity of the toxin protein or its translation is counteracted by an antitoxin protein or noncoding RNA. Five types of TA systems have been proposed that differ markedly in their genetic architectures and modes of activity control. Subtle regulatory properties, frequently responsive to environmental cues, impact the behavior of TA systems. Typically, stress conditions result in the degradation or depletion of the antitoxin. Unleashed toxin proteins impede or alter cellular processes including translation, DNA replication, or ATP or cell wall synthesis. TA toxin activity can then result in cell death or in the formation of drug-tolerant persister cells. The versatile properties of TA systems have also been exploited in biotechnology and may aid in combating infectious diseases.

Toxin-antitoxin systems are ubiquitous and versatile modulators of prokaryotic cell fate

Throughout their life, bacteria need to sense and respond to environmental stress. Thus, such stress responses can require dramatic cellular reprogramming, both at the transcriptional as well as the translational level. This review focuses on the protein factors that interact with the bacterial translational apparatus to respond to and cope with different types of environmental stress. For example, the stringent factor RelA interacts with the ribosome to generate ppGpp under nutrient deprivation, whereas a variety of factors have been identified that bind to the ribosome under unfavorable growth conditions to shut-down (RelE, pY, RMF, HPF and EttA) or re-program (MazF, EF4 and BipA) translation. Additional factors have been identified that rescue ribosomes stalled due to stress-induced mRNA truncation (tmRNA, ArfA, ArfB), translation of unfavorable protein sequences (EF-P), heat shock-induced subunit dissociation (Hsp15), or antibiotic inhibition (TetM, FusB). Understanding the mechanism of how the bacterial cell responds to stress will not only provide fundamental insight into translation regulation, but will also be an important step to identifying new targets for the development of novel antimicrobial agents.

The bacterial translation stress response

http://www2.mrc-lmb.cam.ac.uk/groups/passmore/Publications_files/Boggild%20et%20al%202012.pdf

The crystal structure of the intact E. coli RelBE toxin-antitoxin complex provides the structural basis for conditional cooperativity.

Deadenylation is the first and rate-limiting step during turnover of mRNAs in eukaryotes. In the yeast, Saccharomyces cerevisiae, two distinct 3′–5′ exonucleases, Pop2p and Ccr4p, have been identified within the Ccr4-NOT deadenylase complex, belonging to the DEDD and Exonuclease–Endonuclease–Phosphatase (EEP) families, respectively. Ngl3p has been identified as a new member of the EEP family of exonucleases based on sequence homology, but its activity and biological roles are presently unknown. Here, we show using in vitro deadenylation assays on defined RNA species mimicking poly-A containing mRNAs that yeast Ngl3p is a functional 3′–5′ exonuclease most active at slightly acidic conditions. We further show that the enzyme depends on divalent metal ions for activity and possesses specificity towards poly-A RNA similar to what has been observed for cellular deadenylases. The results suggest that Ngl3p is naturally involved in processing of poly-adenylated RNA and provide insights into the mechanistic variations observed among the redundant set of EEP enzymes found in yeast and higher eukaryotes.

Ane Feddersen

Papers

The crystal structure of the intact E. coli RelBE toxin-antitoxin complex provides the structural basis for conditional cooperativity.

Saccharomyces cerevisiae Ngl3p is an active 3′–5′ exonuclease with a specificity towards poly-A RNA reminiscent of cellular deadenylases