Antitoxin component of a type II toxin-antitoxin (TA) system. A labile antitoxin (half-life of 5.9 minutes) that inhibits the endonuclease activity of cognate toxin LsoA but not that of non-cognate toxin RnlA. Can form a complex with cognate toxin LsoA. This plasmid is only found in Sakai-derived strains of E.coli O157.