# Emerging Roles of Toxin-Antitoxin Modules in Bacterial Pathogenesis
## 3 The Targets of Toxins are Diverse  
The intracellular targets of the toxin factors of TA complexes are diverse. However, numerous type II toxins possess sequence-specific RNA cleavage (endoribonuclease) activity that diminishes protein synthesis and hence cellular metabolism in response to disparate stress and nutritional stimuli. These signals include, for example, amino acid starvation, oxidative stress, and exposure to antibiotics that inhibit transcription or translation. Certain endoribonuclease toxins cleave free mRNA whereas others hydrolyze only ribosome-associated transcripts [4; 36].  
MazEF is one of the most widespread and well-characterized type II TA complexes (Figure 2). The prototypical MazF toxin of _E. coli_ specifically cleaves 5'-ACA-3' motifs in mRNA transcripts. This cleavage is independent both of the ribosome and of translation and is inhibited by the MazE antitoxin [37; 38]. MazF homologues encoded by other species possess sequence specificities that differ both from that of the _E. coli_ archetype and from each other. For example, _M. tuberculosis_ specifies ten MazF homologues that recognize diverse cleavage motifs in mRNA [39; 40]. Moreover, certain MazF toxins also have been shown to cleave both 16S or 23S rRNA and, more recently, tRNA [40]. The cleavage selectivity of MazF homologues and other endoribonuclease toxins may allow targeting of specific pools of RNA whose degradation is advantageous in response to discrete metabolic or environmental stimuli [41].  
Like MazF, the type II RelE toxin of _E. coli_ (Figure 2) is a sequence-specific endoribonuclease that is counteracted by the cognate RelB antitoxin [42]. MazF and RelE homologues form evolutionarily distinct subgroups of toxins. Unlike MazF, mRNA cleavage by the canonical RelE encoded by _E. coli_ is ribosome-dependent, although RelE homologues exhibit differences in this dependency [4; 42]. Similar to the MazF family of endoribonucleases, different RelE homologues exhibit varied cleavage specificities [43]. Other toxin families, including the VapC group, also are sequence-specific endoribonucleases [44]. However, not all type II toxins cleave RNA. Instead, certain toxins dampen protein synthesis by inhibiting the translation machinery without mRNA degradation. For example, the HipA toxin inhibits protein synthesis by phosphorylating glutamyl-transfer RNA synthase, whereas the Doc toxin inactivates the essential translation elongation factor EF-Tu by phosphorylation [45; 46; 47]. Moreover, other type II toxins interfere with the action of topoisomerases that are crucial in maintaining chromosome topology, disrupt cell wall biosynthesis, or interfere with DNA replication [4; 19; 48; 49].  
The targets of type I toxins have been less well-investigated than those of type II toxins. The cell membrane generally is the primary target of the former, but type I toxins that mediate DNA or RNA cleavage also have been described [50]. The archetypal type I Hok toxin induces cellular damage by depolarizing the cytoplasmic membrane [51]. The unrelated Fst toxin similarly perturbs membrane integrity, and also impairs chromosome segregation and cell division [52]. In contrast, the type I BsrG toxin does not permeabilized the cell membrane but instead disrupts cell envelope biosynthesis [53]. Although the properties of the ToxIN and homologous complexes are the most well-defined among type III TA complexes, the intracellular target of the ToxN endoribonuclease remains uncertain [54]. The prototypical type IV toxin CbtA binds and interferes with polymerization of the MreB and FtsZ cytoskeletal proteins that are involved in maintenance of cell shape and in cell division, respectively. The CbeA antitoxin does not interact directly with CbtA but instead suppresses toxicity by stabilizing MreB and FtsZ [55]. In the most well-defined type V complex, the GhoT protein toxin disrupts the cell membrane to produce lysed or 'ghost' cells. The GhoS antitoxin is a ribonuclease that cleaves the toxin mRNA and disables synthesis of GhoT [18].