Toxic component of a type II toxin-antitoxin (TA) system. A stable (half-life over 20 minutes) endoribonuclease that degrades mRNA. Degradation may be translation-stimulated. Overexpression in the absence of cognate antitoxin LsoB causes retarded growth and mRNA degradation, this effect is mitigated upon coexpression with antitoxin LsoB or enterobacteria phage T4 Dmd. Degrades late enterobacteria phage T4 mRNAs, protecting the host against T4 reproduction. Can form a complex with cognate antitoxin LsoB and with enterobacteria phage T4 antitoxin Dmd. This plasmid is only found in Sakai-derived strains of E.coli O157.