# Structure, Biology, and Therapeutic Application of Toxin-Antitoxin Systems in Pathogenic Bacteria
## 3 TA Modules from Pathogenic Bacteria
### Gram-Positive Bacteria  
_Streptococcus pyogenes_ is the causative agent of many human diseases such as superficial skin infections, pharyngitis, erysipelas, and cellulitis. Several type II TA families are extensively distributed throughout _S. pyogenes_ species: five HicBA, five MazEF, two omega-epsilon-zeta (\(\omega\)-\(\varepsilon\)-\(\zeta\)), seven ParDE, and 19 RelBE pairs (see Supplementary Table S1). In particular, \(\omega\)-\(\varepsilon\)-\(\zeta\), TA module was discovered as a PSK system on plasmid pSM19035 from _S. pyogenes_[130]. The \(\omega\)-\(\varepsilon\)-\(\zeta\), system not only stabilizes resistance plasmids but also promotes virulence and is transcriptionally modulated by another repressor protein designated \(\omega\)[131; 132]. The \(\varepsilon\) antitoxin acts solely as an inhibitor of the \(\zeta\) toxin, not a transcription repressor, which distinguishes it from many other antitoxins. When the \(\varepsilon\) antitoxin is continuously degraded by AAA+ proteases and not replenished by protein synthesis, the \(\zeta\) toxin dissociates with it and eventually causes cell death [133]. _Streptococcus pneumoniae_ is a significant human pathogenic bacterium that asymptomatically infects the respiratory tract and becomes pathogenic in immune-compromised individuals [134]. This bacterium has been recognized as the main cause of community acquired pneumonia and meningitis in children and the elderly. _S. pneumoniae_ species harbour type II TA pairs including five HicBA, one \(\omega\)-\(\varepsilon\)-\(\zeta\), one PezAT, four Phd-Doc, and 17 RelBE (see Supplementary Table S1). There are four functionally well-characterized type II TA operons, namely _relBE2_, _pezAT_, _yefM-yoeB_, and _phd-doc_, and several putative TA operons depending on the strains [4; 135]. The pneumococcal RelE2 toxin shows a similar toxicity for both _S. pneumoniae_ and _E. coli_, but its cognate antitoxin RelB2 seems to neutralize the toxicity in a certain growth time [136], which contrasts with those of the _E. coli_ RelB antitoxins. In solution state, RelB2 alone is present as a dimer, but the RelBE complex is formed predominantly in a ReB2:RelE2 stoichiometry of 4:2 [137]. This heterohexamer can bind to one operator DNA molecule. In contrast, the _E. coli_ RelBE complex is assumed to exist as a heterotetramer or a heterotrimer in solution [97; 138]. RelE1, one of putative toxins of _S. pneumoniae_, has no RNase activity [57] and is not harmful to both _S. pneumoniae_ and _E. coli_[135]. The pneumococcal YoeB toxin was reported to inhibit the cellular growth of _E. coli_[139]. The toxicity is much lower in a Lon protease-deficient strain than in the wild-type strain [139], possibly because Lon protease degrades the cognate antitoxin YefM and thus increases the toxicity of YoeB. Similar to other type II antitoxins, YefM is thermally less stable than the YefM-YoeB complex, which was demonstrated by a lower melting temperature of YefM (\(\sim\)-45 \({}^{\circ}\)C) [139]. The _phd-doc_ pair is most recently identified as the fourth _bona fide_ TA operon in _S. pneumoniae_, showing typical features of type II TA systems [135]; (i) overexpression of the Doc toxin strongly inhibits the growth of _S. pneumoniae_, which is resumed by co-expression with the Phd antitoxin; and (ii) the _phd-doc_ promoter is slightly and almost completely repressed by Phd and Phd-Doc complex, respectively. The PezAT TA system located on the chromosome of _S. pneumoniae_ is another version of the pneumococcal plasmid-encoded \(\varepsilon\)-\(\zeta\), system. PezT inhibits the cell growth of _E. coli_ for the first 3 h, which is subsequently resuscitated without the need for co-expression with the PezA antitoxin [140]. This growth profile is similar to the profile yielded by the \(\zeta\) toxin [141]. The toxicity of PezT can be neutralized by the antitoxin PezA, which is a multi-domain protein. PezAT modules located on pneumococcal pathogenicity islands could provide the host with virulence factors and antibiotic resistance [142; 143]. _E.