Intracellular Localization of the Proteins Encoded by Some Type II Toxin-Antitoxin Systems in Escherichia coli

Bacterial toxin-antitoxin (TA) systems encode a toxin and an antitoxin that counteracts the toxin. Such TA systems are found abundantly on bacterial chromosomes and on extrachromosomal genetic elements. The toxin is always a protein. Based on the nature of the antitoxin (protein or RNA) and on their mode of regulation, they are classified into six groups (I to VI). In the group II TA systems, both the toxin and the antitoxin are proteins, and the gene specifying the antitoxin precedes the gene specifying for the toxin. Here, we studied the intracellular localization in Escherichia coli cells of the proteins specified by the following type II TA modules: mazEF, chpBIK, mqsRA, and rnlAB. We visualized the localization of these proteins by fusing them with the fluorescent protein mCherry using recombinant DNA technology. We used fluorescence microscopy and image analysis software to obtain and quantify protein distribution data. With the exception of the chpBIK TA module, we found that the localization of each toxin-antitoxin complex was different from the localization of the toxin itself. Our results demonstrate clearly that the presence of the antitoxin shifts the localization of its respective toxin toward the middle of the cell, which could contribute to the reduction of cellular toxicity. IMPORTANCE Bacterial toxin-antitoxin (TA) systems, which were discovered in 1985, have since been studied extensively. These studies have focused particularly on the distribution of these bacterial TA systems on either plasmids or on bacterial chromosomes, their functionality, their targets, their relation to virulence, and their mechanisms of action. Our study, reported here, is the first to clarify the intracellular localization of the proteins specified for some type II TA systems. We have shown that, with the exception of the chpBIK module, each toxin-antitoxin complex was localized in a different part of the cell than the toxin itself. Our results revealed clearly that the presence of the antitoxin changes the localization of the toxin by moving the toxin toward the middle of the cell. Until now, the general view has been that the antagonistic effect of the antitoxins over their cognate toxins is based only on their direct structural interactions. Here, we show that this antagonistic effect is also a function of a specific change in the intracellular localization of the toxin.