Toxic component of a type II toxin-antitoxin (TA) system. A translation-dependent mRNA interferase. Overexpression causes cessation of cell growth and inhibits cell proliferation via inhibition of translation; this blockage is overcome by subsequent expression of antitoxin YafN. Overexpression causes cleavage of a number of mRNAs in a ribosome-dependent fashion. YafO binding to the 50S ribosomal subunit in the translation complex induces mRNA cleavage 3' to the region protected by the ribosome; YafO alone is not able to digest mRNA. Probably forms a complex with the antitoxin YafN which inhibits the mRNA interferase activity. Induced by amino acid starvation, glucose starvation, the DNA cross-linker mitomycin C (SOS response) and when translation is blocked. Induction is decreased in the absence of the Lon protease suggesting, by homology to other toxin-antitoxin systems, that Lon may degrade the YafN antitoxin. Transcription is negatively regulated by the cognate locus, probably by YafN. A member of the dinB-yafNOP operon; it has 2 promoters, 1 upstream of dinB and 1 specific for yafN-yfaO.