Toxin YafQ Reduces Escherichia coli Growth at Low Temperatures

Identification and characterization of VapBC toxin-antitoxin system in Bosea sp. PAMC 26642 isolated from Arctic lichens

Toxin-antitoxin (TA) systems are genetic modules composed of a toxin interfering with cellular processes and its cognate antitoxin, which counteracts the activity of the toxin. TA modules are widespread in bacterial and archaeal genomes. It has been suggested that TA modules participate in the adaptation of prokaryotes to unfavorable conditions. The Bosea sp. PAMC 26642 used in this study was isolated from the Arctic lichen Stereocaulon sp. There are 12 putative type II TA loci in the genome of Bosea sp. PAMC 26642. Of these, nine functional TA systems have been shown to be toxic in Escherichia coli The toxin inhibits growth, but this inhibition is reversed when the cognate antitoxin genes are coexpressed, indicating that these putative TA loci were bona fide TA modules. Only the BoVapC1 (AXW83_01405) toxin, a homolog of VapC, showed growth inhibition specific to low temperatures, which was recovered by the coexpression of BoVapB1 (AXW83_01400). Microscopic observation and growth monitoring revealed that the BoVapC1 toxin had bacteriostatic effects on the growth of E. coli and induced morphological changes. Quantitative real time polymerase chain reaction and northern blotting analyses showed that the BoVapC1 toxin had a ribonuclease activity on the initiator tRNA^fMet, implying that degradation of tRNA^fMet might trigger growth arrest in E. coli Furthermore, the BoVapBC1 system was found to contribute to survival against prolonged exposure at 4°C. This is the first study to identify the function of TA systems in cold adaptation.

A Primary Physiological Role of Toxin/Antitoxin Systems Is Phage Inhibition

Toxin/antitoxin (TA) systems are present in most prokaryote genomes. Toxins are almost exclusively proteins that reduce metabolism (but do not cause cell death), and antitoxins are either RNA or proteins that counteract the toxin or the RNA that encodes it. Although TA systems clearly stabilize mobile genetic elements, after four decades of research, the physiological roles of chromosomal TA systems are less clear. For example, recent reports have challenged the notion of TA systems as stress-response elements, including a role in creating the dormant state known as persistence. Here, we present evidence that a primary physiological role of chromosomally encoded TA systems is phage inhibition, a role that is also played by some plasmid-based TA systems. This includes results that show some CRISPR-Cas system elements are derived from TA systems and that some CRISPR-Cas systems mimic the host growth inhibition invoked by TA systems to inhibit phage propagation.

Evaluating the Potential for Cross-Interactions of Antitoxins in Type II TA Systems

The diversity of Type-II toxin–antitoxin (TA) systems in bacterial genomes requires tightly controlled interaction specificity to ensure protection of the cell, and potentially to limit cross-talk between toxin–antitoxin pairs of the same family of TA systems. Further, there is a redundant use of toxin folds for different cellular targets and complexation with different classes of antitoxins, increasing the apparent requirement for the insulation of interactions. The presence of Type II TA systems has remained enigmatic with respect to potential benefits imparted to the host cells. In some cases, they play clear roles in survival associated with unfavorable growth conditions. More generally, they can also serve as a “cure” against acquisition of highly similar TA systems such as those found on plasmids or invading genetic elements that frequently carry virulence and resistance genes. The latter model is predicated on the ability of these highly specific cognate antitoxin–toxin interactions to form cross-reactions between chromosomal antitoxins and invading toxins. This review summarizes advances in the Type II TA system models with an emphasis on antitoxin cross-reactivity, including with invading genetic elements and cases where toxin proteins share a common fold yet interact with different families of antitoxins.

Pseudomonas putida Responds to the Toxin GraT by Inducing Ribosome Biogenesis Factors and Repressing TCA Cycle Enzymes

The potentially self-poisonous toxin-antitoxin modules are widespread in bacterial chromosomes, but despite extensive studies, their biological importance remains poorly understood. Here, we used whole-cell proteomics to study the cellular effects of the Pseudomonas putida toxin GraT that is known to inhibit growth and ribosome maturation in a cold-dependent manner when the graA antitoxin gene is deleted from the genome. Proteomic analysis of P. putida wild-type and ΔgraA strains at 30 °C and 25 °C, where the growth is differently affected by GraT, revealed two major responses to GraT at both temperatures. First, ribosome biogenesis factors, including the RNA helicase DeaD and RNase III, are upregulated in ΔgraA. This likely serves to alleviate the ribosome biogenesis defect of the ΔgraA strain. Secondly, proteome data indicated that GraT induces downregulation of central carbon metabolism, as suggested by the decreased levels of TCA cycle enzymes isocitrate dehydrogenase Idh, α-ketoglutarate dehydrogenase subunit SucA, and succinate-CoA ligase subunit SucD. Metabolomic analysis revealed remarkable GraT-dependent accumulation of oxaloacetate at 25 °C and a reduced amount of malate, another TCA intermediate. The accumulation of oxaloacetate is likely due to decreased flux through the TCA cycle but also indicates inhibition of anabolic pathways in GraT-affected bacteria. Thus, proteomic and metabolomic analysis of the ΔgraA strain revealed that GraT-mediated stress triggers several responses that reprogram the cell physiology to alleviate the GraT-caused damage.