Toxic component of a type II toxin-antitoxin (TA) system. It has been proposed to be an mRNA interferase but also an inhibitor of translation initiation. Has an in vitro RNase activity and preferentially cleaves at the 3'-end of purine ribonucleotides. YefM binds to the promoter region of the yefM-yeoB operon to repress transcription, YeoB acts as a corepressor (By similarity). Forms a complex with antitoxin YefM, in which the toxin is inactive. Deletion of the tomB-hha operon causes reduced infection of the mouse bladder but not of the kidney. Belongs to the YoeB family.