## 2 Multi-Level Interactions between TA Systems
### TA Systems Belonging to Different Replicons  
Interactions between TA systems located on different replicons might be viewed as competition between the different systems and, as a consequence, between replicon. This can be viewed as an \"arms-race\" between TA systems from different locations that drives the evolution of TA systems. 2.1 Arms-Race between Chromosomally- and Plasmid-Encoded TA Systems: The Anti-addiction Model and Beyond  
As explained in Figure 1, addiction relies on the capacity of a plasmid-encoded TA systems to inhibit cell growth and/or kill daughter cells devoid of a plasmid copy. \"Protection\" against this can be achieved if a chromosomally-encoded antitoxin is able to neutralize the plasmid-encoded toxin (Figure 2) [59, 60]. On the one hand, this \"anti-addiction\" phenomenon provides selective advantage to the chromosomally-encoded system by allowing the survival of plasmid-free daughter cells. On the other hand, the selective pressure then lies on the plasmidic toxin. Toxins that are not recognized by the chromosomally-encoded antiotoxins would be selected, as they increase the plasmid prevalence in the population. Few modifications appear to be required to modify antitoxin specificity. The group of [PERSON] showed that one specific mutation in the plasmid-encoded Txe toxin allows for recognition by both its cognate antitoxin and a chromosomally-encoded antitoxin [61]. Anti-addiction might thus serve as a driving force for plasmid-encoded toxins evolution. Plasmid-encoded TA systems in which toxins are able to evade chromosomally-encoded antiotoxins have been identified, such as the F plasmid _ccd\({}_{F}\)_ system and the chromosomally-encoded _ccd\({}_{0157}\)_ system in _E. coli_ O157:H7 [62]; and the R100 plasmid _par_ locus and the chromosomally-encoded _mazEF_ and _chpB_ systems [63, 64]. In addition, molecular evolution analyses revealed that CcdB-like toxins encoded by plasmids are under strong negative selection, indicating that these sequences are very constrained, while chromosomally-encoded toxins are under neutral selection, indicating that these sequences are devoid of functions [65]. 2.2 Arms-Race between Chromosomally-, Plasmid- and Phage-Encoded TA Systems: The Phage Defense Model, RM, Abi and Beyond  
TA systems were proposed to be bacterial defenses against phages. Defense systems either use self-/non-self-discrimination systems, like restriction-modification (RM) systems and DNA phosphorothioation (DND) systems, or trigger cell death of infected cells [66]. As recently underlined by the [PERSON] group, RM systems share many features with TA systems [67]. On the other hand, Abi (abortive infection) systems hinder the propagation of phages in bacterial populations by inducing the death of infected cells. It appears that some TA systems, like type I _hok/sok_, type II _rnlAB_ and type III _toxIN_ systems, function as Abi systems [68, 17, 69]. As both ToxI and Sok are RNAs, the alteration of host translation following phage infections probably affects the antitoxin:toxin ratio and leads to toxin activation. In both cases, phage maturation is slowed down and burst size reduced [68, 17]. Note that both _hok/sok_ and _toxIN_ are encoded on plasmids [68, 17]. In the case of the _E. coli rnlAB_ system, the labile RnIB antitoxin is also degraded during phage infection as host protein expression is arrested. Abi occurs upon infection by a _dmd_ T4 mutant (see below). In this case, the RnIA toxin, an endoribonuclease [70], is activated and degrades phage late mRNAs, thus aborting infection [69]. However, as happens in the perpetual arms race between bacteria and phages, phages have found remedies to inhibit toxin activities. Phages have evolved different mechanisms to avoid this, e.g., antitoxin mimicry [71, 72], protease inhibition [73] or the hijacking of TA systems [72]. The T4 phage encodes Dmd, a protein functionally equivalent to RnIB and able to inhibit RnIA endoribonuclease activity (Figure 2). As RnIA can no longer degrade T4 mRNAs, T4 infection is effective.