# Emerging Roles of Toxin-Antitoxin Modules in Bacterial Pathogenesis
## 5 Persistence Emerges in Response to the Activation of Diverse TA Modules
### Toxin Activity and Persister Cell Formation in E. coli
In this situation, transcriptional autorepression of TA loci is abolished and thereby a high rate of TA transcription and toxin production is promoted which explains the elevated levels of TA mRNAs observed in persisters [96; 97]. However, when the Lon-mediated degradation rate of antiotoxins returns to a normal level, the rapid production and accumulation of the antiotoxins results in quenching of toxin activity and resuscitation of cell growth. This pathway of toxin activation is supported by mathematical modeling [106]. A similar result was obtained by [PERSON] and co-workers who found that _E. coli_ enters a dormant state once the toxin  
Figure 4: Schematic representation of a complex and multilayered protein network engaged in persistence formation in _E. coli_ via TA complexes. The central part illustrates Lon protease as an example of antitoxin degrading proteases, which also include ClpXP and ClpAP. MazEF is shown as an exemplar type II module in which the toxin is an endoribonuclease. Only the relevant, most well-described interactions are displayed. Colored boxes denote toxins (red), antioxins (blue), Lon protease (yellow), other proteins (dark grey), small molecules (light grey), and cellular processes and structures (purple). Arrows, stimulation; broken lines ending in balls, inhibition. level crosses a threshold, and that the length of the dormancy depends on the toxin's level [107]. Mathematical modeling of the cooperation between multiple TAs in promoting persistence frequency also has been presented [108]. Nevertheless, the above model is suitable only for persistence mediated by type II TA systems. Moreover, persisters are still formed without (p)ppGpp, although at lower levels. Additionally, the MqsR, MazF, GhoT, and YafQ toxins are able to induce persistence in the absence of (p)ppGpp, poly(P), and Lon [97; 102]. Apparently, the persistence model which involves (p)ppGpp, poly(P), and Lon is valid only in a limited number of cases. It has been proposed that the ability of a cell to enter the persister state is inversely related to its growth rate. Thus, cells activate the production of toxins in response to stress as a strategy to reduce growth and to induce persistence in a small subpopulation of cells. Therefore, it appears that persistence does not solely depend on (p)ppGpp but rather on the growth rate of the bacterial culture [109]. Accordingly, very limited overproduction, resulting in minimal inhibition of cell growth, of not only TA toxins but also unrelated, non-TA proteins, such as the DnaJ chaperone of _E. coli_ and PnnrC inner membrane protein of _Salmonella enterica_ subsp. _ enterica_ serovar Typhimurium (_S. typhimurium_), lead to the production of persister cells tolerant of ampicillin and ciprofloxacin [99]. Therefore, expression of many genes in addition to TA modules can drive the production of dormant cells [99; 110; 111]. Transcriptional control of TA gene expression plays a vital role in preventing inadvertent toxin activation [6] and potentially in controlling entry into and exit from the persistent state [10]. Antitoxins encoded by type II cassettes are sequence-specific DNA binding proteins that typically recognize their promoter regions poorly, which results in weak, but significant, transcriptional repression of the downstream TA genes. Under these conditions, the levels of antitoxin are sufficient to sequester the toxin molecules. Similarly, when the TA ratio is balanced, toxin interaction with the antitoxin enhances the DNA binding affinity of the latter, which induces strong repression (Figure 1) and maintains an effective TA stoichiometry. However, an increased concentration of toxin, such as occurs during persistence, destabilizes DNA binding by the antitoxin, which results in strong transcriptional de-repression of the TA genes. Sufficient antitoxin is then produced to capture the excess toxin and to re-establish repression. Thus, cells can switch from persistence into a growing phase.