A type II toxin–antitoxin system, <i>ECs3274-ECs3275</i>, in enterohemorrhagic <i>Escherichia coli</i> O157

The toxin-antitoxin (TA) genetic module controls various bacterial events. Novel toxins with different functions are still being discovered. This study aimed to determine whether the ECs3274-ECs3275 gene pair encoded by enterohemorrhagic Escherichia coli O157 functions as a TA system. To characterize this putative TA system, we analyzed the growth of E. coli expressing ECs3274, ECs3275, or both; the interaction between ECs3274 and ECs3275 using bacterial adenylate cyclase two-hybrid assays; and the DNA-binding ability of ECs3274 using gel-mobility shift assays. We observed that the ECs3274 antitoxin interacted with the ECs3275 toxin, was destabilized by Lon protease, and repressed its promoter activity via its helix-turn-helix (HTH) motif. These properties are consistent with those of typical type II TA antitoxins. Interestingly, ECs3275 has an HTH motif not observed in other TA toxins and is necessary for ECs3275 toxicity, suggesting that ECs3275 may exert its toxicity by regulating the expression of specific genes.