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

pEP36 is a plasmid ubiquitously present in Erwinia pyrifoliae, a pathogen which causes black stem blight of Asian pear. pEP36 is highly stable in its host, even in the absence of selective pressure. The plasmid is closely related to pEA29, which is widespread in E. amylovora, the causative agent of fire blight of apple and pear trees. Here we report that pEP36 possesses a functional hybrid toxin-antitoxin module, stbD/E(pEP36), with the toxin showing homology to the RelE/ParE proteins and the antidote belonging to the Phd/YefM antitoxin family. Bacteria expressing the StbE(pEP36) 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 StbE(pEP36) acts as a global inhibitor of protein synthesis while it does not interfere with DNA and RNA synthesis. The StbD(pEP36) antitoxin is capable of neutralising StbE(pEP36) toxicity. Additional experiments show that the stbD/E(pEP36) module can stabilise plasmids at least 20-fold. Thus the toxin-antitoxin system may contribute to the remarkable stability of pEP36.