Antitoxin component of a type II toxin-antitoxin (TA) system. Functions as an mRNA interferase antitoxin; overexpression prevents YafO-mediated cessation of cell growth and inhibition of cell proliferation. Probably forms a complex with the mRNA interferase YafO 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 this protein. A member of the dinB-yafNOP operon; it has 2 promoters, 1 upstream of dinB and 1 specific for yafN-yfaO. Belongs to the phD/YefM antitoxin family.