Toxic component of a type II toxin-antitoxin (TA) system. Its mode of function is controversial; it has been proposed to be an mRNA interferase but also an inhibitor of translation initiation. When overproduced in wild-type cells, inhibits bacterial growth and translation by cleavage of mRNA molecules while it has a weak effect on colony forming ability. Overproduction of Lon protease specifically activates YoeB-dependent mRNA cleavage, leading to lethality. YefM binds to the promoter region of the yefM-yeoB operon to repress transcription, YeoB acts as a corepressor. Shown in vitro to be an mRNA interferase that requires translation for substrate cleavage; if the mRNA is mutated so as to not be translatable it is no longer cleaved. Cleavage only occurs within translated regions. Has RNase activity and preferentially cleaves at the 3'-end of purine ribonucleotides. Also shown in vitro to be a translation initiation blocker. Binds to the 70S ribosome and 50S ribosomal subunit; binding is inhibited by hygromycin A and tetracycline, both of which bind to the 30S subunit in the A site. Thus YoeB is located at the interface between 50S and 30S ribosomes and interacts with the A site where it cleaves mRNA, blocking translation initiation. Forms a complex with antitoxin YefM, in which the toxin is inactive. It has been described as being a YefM-YeoB(2) heterotrimer (PubMed:15980067). Also described as a YefM(2)-YoeB heterotrimer (PubMed:16109374 and PubMed:17170003). Binds the 50S ribosomal subunit. Repressed by YefM, more strongly repressed by the YefM(2)YoeB heterotrimer. Induced in persister cells. Ectopic expression of Salmonella or Shigella toxin VapC induces the yefM-yoeB operon and also induces Yoeb toxin activity in a Lon protease-dependent manner. No visible phenotype under standard growth conditions. Delays Lon protease-dependent lethality upon overexpression of Lon, but does not fully suppress it. No loss of ability to form persister cells. Belongs to the YoeB family.