Type II Toxin-Antitoxin Systems in Escherichia coli

Abstract

The toxin-antitoxin (TA) system is widespread in prokaryotes and archaea, comprising toxins and antitoxins that counterbalance each other. Based on the nature and mode of action of antitoxins, they are classified into eight groups (type I to VIII). Both the toxins and the antitoxins are proteins in type II TA systems, and the antitoxin gene is usually upstream of the toxin gene. Both genes are organized in an operon and expression of which is regulated at the transcriptional level by the antitoxin-toxin complex, which binds the operon DNA through the DNA-binding domain of the antitoxin. The TA system plays a crucial role in various cellular processes, such as programmed cell death, cell growth, persistence, and virulence. Currently, Type II TA systems have been used as a target for developing new antibacterial agents for treatment. Therefore, the focus of this review is to understand the unique response of Type II TA in Escherichia coli to stress and its contribution to the maintenance of resistant strains. Here, we review the Type II TA system in E. coli and describe their regulatory mechanisms and biological functions. Understanding how TA promotes phenotypic heterogeneity and pathogenesis mechanisms may help to develop new treatments for infections caused by pathogens rationally.

Keywords: E. coli; bacterial persistence; biofilm formation; phage infection; type II toxin-antitoxin.

Figure 1

Type II toxin-antitoxin (TA) locus of E. coli K-12. Diagram of the genes and control loops of a typical type II TA locus. The red arrow to the right indicates the TA operon promoter. When the free toxin concentration is low, the promoter is repressed by the antitoxin during rapid growth, especially by the TA complex that binds tightly to the promoter region. In contrast, promoter activity is inhibited by free toxins, a regulatory phenomenon known as conditional synergy.

Figure 2

The activity of toxins in the TA system. (a) Topoisomerase is inactivated by the amylase FicT. the DNA cleavage enzyme is poisoned by direct binding of CcdB. (b) Translation is the target of numerous toxins that act at every level of protein synthesis. VapC toxin cleaves the tRNA anticodon stem-loop or the stem-loop toxin loop of 23s ribosomal RNA. MazF toxin degrades free mRNA and ribosomal RNA, and RelE toxin cleaves translational mRNAs at the ribosomal A site. HipA toxin phosphorylates aminoacyl- tRNA synthetase phosphorylates and prevents tRNA charging. doc phosphorylates elongation factors and prevents tRNA delivery to the ribosome. (c) MbcT toxin degrades NAD+. (d) ζ toxin phosphorylates precursors of peptidoglycan synthesis. Adapted with permission from Springer Nature from Jurėnas D, Fraikin N, Goormaghtigh F, Van Melderen L. Biology and evolution of bacterial toxin-antitoxin systems. Nat Rev Microbiol. 2022;20(6):335–350.

Figure 3

Shows the rationale for the role of TA modules in their biological functions. (a) Post-isolation killing mechanism, type II system-mediated plasmid addiction relies on differential stabilization between toxin (red) and antitoxin (green) proteins. When the unstable antitoxin is no longer replenished, the toxin will be released from the antitoxin-toxin complex. It will be able to kill these cells, thus contributing to plasmid maintenance. (b) In abortive infection, in phage-infected cells, transcription of host genes is repressed, the unstable antitoxin is not replenished, and the toxin will be released from the antitoxin-toxin complex and be able to prevent phage multiplication. (c) Persister formation, where persisters constitute a subpopulation of cells in the bacterial population that exhibit tolerance to antibiotics and other environmental stress conditions due to a phenotypic shift to a dormant state.