One cannot rule them all: Are bacterial toxins-antitoxins druggable?

Chan, Wai Ting; Balsa, Dolors; Espinosa, Manuel

Employment of the toxins from TAs to use in a number of biotechnological applications have been proposed and, in several cases, successfully developed for commercial purposes such as positive selection plasmid vectors (Stieber, Gabant and Szpirer 2008) . In this instance, Escherichia coli strains harbour plasmid vectors expressing the lethal toxin gene product, which is counteracted by expression of the cognate antitoxin in trans. Cloning of genes is achieved by transformation of a strain devoid of the antitoxin, thus allowing the selection of recombinants in which only the inactivated toxin genes are recovered (Bernard et al., 1994) . Other applications of the TAs include the controlled release of genetically modified organisms into the biosphere. Balance of the benefits versus possible risks has been tackled by creating genetic traps, termed active containment systems that avoid bacterial escape. They are based on the controlled expression of lethal genes that would be turned 'on' in the case of escape into the environment. Toxins with a broad spectrum, such as MazF or Kid, have been chosen as good candidates as genetic traps (Ramos et al., 1994; Guan et al., 2013) and, in some cases, combination of two toxins acting on different targets and controlled by different regulatory signals has been considered as the best alternative because the risk of escape in these containment conditions was estimated to be lower than 10 −8 (Torres et al., 2003) . Further applications in biosensors and bioremediations, the GeneGuard system, have been developed within a synthetic biology framework, to be used in a variety of bacteria (Wright et al., 2014) . In the food industry, employment of TAs to construct plasmid vectors in which the number of plasmidfree cells is decreased as well as vectors to overproduce plasmidencoded proteins in the absence of antibiotic selection have proved to be a useful approach (Pecota et al., 1997) . Excellent reviews on the biotechnological applications of TAs have been published (Suzuki et al., 2005; Inouye 2006; Stieber, Gabant and Szpirer 2008; Unterholzner, Poppenberger and Rozhon 2013) .