Reassessing the Role of the Type II MqsRA Toxin-Antitoxin System in Stress Response and Biofilm Formation: mqsA Is Transcriptionally Uncoupled from mqsR

Abstract

There is growing controversy regarding the role of chromosomal toxin-antitoxin systems in bacterial physiology. mqsRA is a peculiar toxin-antitoxin system, as the gene encoding the toxin precedes that of the antitoxin. This system was previously shown to play a role in stress response and biofilm formation. In this work, we identified two promoters specifically driving the constitutive expression of the antitoxin, thereby decoupling the expression of antitoxin from the toxin. We also showed that mqsRA contributes neither to the regulation of biofilm formation nor to the sensitivity to oxidative stress and bile salts. Finally, we were unable to confirm that the MqsA antitoxin is a global regulator. Altogether, our data are ruling out the involvement of the mqsRA system in Escherichia coli regulatory networks.

Type II Toxin-Antitoxin Systems: Evolution and Revolutions

“…However, the authors found that yefM-yoeB-induced cleavage at 42°C was not correlated with growth inhibition and that yefM-yoeB did not give any fitness advantage or disadvantage when cells were grown at this temperature (114). Finally, another study showed that the MqsA antitoxin is rapidly degraded by Lon under oxidative stress (115), which was not reproduced by a subsequent study (53). To conclude, antitoxin proteolysis under stress conditions, leading to TA system activation, is far from being a general rule, and further research is needed to better understand the conditions leading to TA systems induction and phenotypes, probably on a case-by-case basis.…”

“…Later on, the mqsRA system in E. coli was found to regulate a variety of biological processes such as biofilm formation and response to oxidative stress, notably by the capacity of the antitoxin MqsA to regulate expression of master regulators such as csgD or rpoS (51,52). However, none of these phenotypes or regulations could be reproduced in an independent study (53). All of these observations call into question the implication of TA modules in various stress responses, whether through PCD, translation inhibition, or pleiotropic regulation.…”

“…3B). For instance, the rnlA and mqsA antitoxin genes are transcribed from additional constitutive promoters located in the toxin coding sequence (53,90). Another layer of complexity is found in the hicAB system: the hicAB autoregulated promoter produces a transcript truncated at the position of the hicA ribosome-binding site, thus only supporting synthesis of the HicB antitoxin (91).…”

“…With time and in a common effort from independent groups, these models were questioned or disproved by different studies, shedding light on conceptual issuesnotably, the absolute necessity that TA systems have to carry out important functions in cellular physiology since they are so widespread in bacterial chromosomes-as well as major experimental flaws like strain genotype issues, fluorescent reporter issues, and questionable data analysis (46,47,53,60,61). We thus propose to go back to the basics, ask crucial questions that still remain unanswered, and provide suggestions which could, hopefully, improve our understanding of TA systems.…”

Pseudomonas putidachromosomal toxin-antitoxin systems carry neither clear fitness benefits nor big costs

“…Non-growing bacteria are known to be protected from environmental insults 51-53 even if they do not actively combat with stress, as appears to be the case, for example, with most antibiotic-tolerant persister cells 54 . However, despite the potential of TA-encoded toxins to protect bacteria, the deletion of whole TA loci does not usually affect the bacterial fitness 20,27-30,55 , leaving the question of TA systems’ importance in stress tolerance still open.…”