# Structure, Biology, and Therapeutic Application of Toxin-Antitoxin Systems in Pathogenic Bacteria
## 5 Exploitation of TA Systems for the Development of Novel Antibiotic Drugs  
Infectious diseases have been considered one of the leading causes of human mortality worldwide. TA systems have recently received much attention as one of the most promising antibacterial  
Figure 8: Structures of atypical TA proteins from _E. coli_ O157, _S. typhimurium_, _B. abortus_, _E. faecalis_, and _B. subtilis_. (**A**) Structure of the PaaA–ParE complex from _E. coli_ O157; (**B**) structure of the TacT toxin from _S. typhimurium_. The tRNA binding and active sites are mapped onto the electrostatic surface model of TacT. (**C**) Structure of the BrnT toxin from _B. abortus_. The residues related to a ribonuclease activity are indicated. (**D**) Structure of the Fst toxin from _E. faecalis_. Transmembrane segment and acidic residues on the electrostatic surface model of Fst are indicated. (**E**) Structure of the SpoIISA–SpoIISB complex. targets [90; 182; 183; 184; 185] because they are widely distributed in almost all of the most important bacterial pathogens but not in eukaryotic cells. One of the powerful antibacterial strategies is an artificial activation of the toxin, which could be accomplished by using small compounds or peptides as inhibitors of the protein (toxin)-protein (antitoxin) interactions (Figure 9A). However, this field is in the early stages; there are still no drugs designed to target TA interfaces. The identification of drug candidates can be assisted by computational drug design such as molecular docking and virtual screening [186]. Another antibiotic strategy that can be envisioned is to accelerate degradation of the bound antitoxin by Lon or Clp proteases, resulting in a release of the toxin and subsequent cell death (Figure 9B). This approach utilizes the mechanism in which the overproduction of the protease indirectly vitalizes the toxin. In fact, overexpression of Lon protease specifically activates the YoeB toxin from its non-toxic complex with the YefM antitoxin, leading to global translation inhibitions and hence cell death [187]. Antibiotic molecules that bind and activate ClpP protease were identified in other systems [188; 189]. It is conceivable that drug candidates could bind to the promoter DNA of a TA operon and repress its transcription, hindering the continuous supply of the short-lived antitoxin within the cell, which eventually causes cell lethality from the free toxin (Figure 9C). To achieve this, biomolecules can be designed to enhance promoter binding and avoid the formation of a complex with the toxin on the basis of the structure of DNA-binding motif of the antitoxin. The three strategies mentioned above utilize type II TA systems, which are found in a large number of bacteria but not in eukaryotes. To inhibit the synthesis of the antitoxin, we could use antisense RNA that binds to the antitoxin coding mRNA without affecting the toxin translation (Figure 9D). This approach could be applied to all types of TA systems. The toxin molecule itself could be an antibiotic drug candidate, but the therapeutic obstacles are poor oral availability and high costs of bioprocessing as shown in many biomolecular drugs. In addition, the toxins have to be effectively delivered to habitats of pathogenic bacteria in the patient's body without deleterious effects on the human cells or normal commensal organisms. The type I toxin PepA1 from _S. aureus_ is a 30-residue hydrophobic peptide that can cause membrane lysis of both bacterial cells and human erythrocytes [190; 191]. This toxin was sequentially and chemically optimized to enhance the antibacterial activity and reduce the hemolytic side effect [191]. The mechanism of action of PepA1 derivatives can be explained by their structural property such as linear, helical, and cyclic conformations [191]. A more sophisticated approach would make use of recombinant bacteriophages [192], which deliver the desired toxin gene into pathogenic bacteria. The highly expressed toxin will kill the pathogens (Figure 9E).