Toxic component of a type II toxin-antitoxin (TA) system (PubMed:17263853). A sequence-specific mRNA endoribonuclease that inhibits translation elongation and induces bacterial stasis (PubMed:19210620). Cleavage occurs between the second and third residue of the Lys codon followed by a G or A (5'AAA(G/A)3'), is reading-frame dependent and occurs within the 5' end of most mRNAs (PubMed:19210620). Ribosome-binding confers the sequence specificity and reading frame-dependence (PubMed:19210620). When overexpressed in liquid media YafQ partially inhibits protein synthesis, with a reduction in growth rate and colony growth rate. This effect is counteracted by coexpression with cognate antitoxin DinJ (PubMed:17263853). YafQ and DinJ together bind their own promoter, and repress its expression (PubMed:24898247). Cell death governed by the MazE-MazF and DinJ-YafQ TA systems seems to play a role in biofilm formation (PubMed:19707553). Monomer in the absence of antitoxin (PubMed:24898247). Forms a heterotetramer with antitoxin DinJ, with 2 YafQ-DinJ dimers associated via the N-terminus of the DinJ antitoxins (YafQ-(DinJ)2-YafQ) (PubMed:17263853, PubMed:24898247). In this complex the toxin activity is inhibited. Binds the 70S ribosome via the 50S ribosomal subunit (PubMed:19210620). May function as a drug tolerance determinant in biofilm, but not stationary phase planktonic cells. By the DNA damaging agent mitomycin C. Cells missing yafQ show increased biofilm sensitivity to the antibiotics cefazolin (a beta-lactam inhibitor) and tobramycin (a protein synthesis inhibitor). There is no difference in antibiotic sensitivity in stationary phase planktonic cells (PubMed:19307375). Belongs to the RelE toxin family. YafQ subfamily.