Characterisation of the stbD/E toxin-antitoxin system of pEP36, a plasmid of the plant pathogen Erwinia pyrifoliae

Here we report that pEP36 possesses a functional hybrid toxin-antitoxin module, stbD/EpEP36, with the toxin showing homology to the RelE/ParE proteins and the antidote belonging to the Phd/YefM antitoxin family. Bacteria expressing the StbEpEP36 toxin arrest cell growth and enter a viable but non-culturable stage. However, they maintain their typical cell length and do not show filamentation. Pulse-chase experiments revealed that StbEpEP36 acts as a global inhibitor of protein synthesis while it does not interfere with DNA and RNA synthesis. The StbDpEP36 antitoxin is capable of neutralising StbEpEP36 toxicity. Additional experiments show that the stbD/EpEP36 module can stabilise plasmids at least 20-fold. Thus the toxin-antitoxin system may contribute to the remarkable stability of pEP36.