Toxic component of a type I toxin-antitoxin (TA) system (PubMed:17462020). Involved in the degradation and recycling of damaged RNA (PubMed:17462020). It is itself a target for degradation by the ATP-dependent protease Lon (PubMed:17462020). Purifies with ribosomes. Repressed by LexA, induced by DNA damage (PubMed:10760155, PubMed:17462020). During the SOS response (at protein level when expressed as a fusion protein) (PubMed:17462020). The mRNA stability and translation is controlled by the small regulatory RNA symR, which probably blocks ribosome binding (PubMed:17462020). Expression of the proteinaceous toxin (putative endonuclease) is controlled by antisense sRNA SymR (PubMed:17462020). Overexpression of this protein is toxic for the cell, affecting colony-forming ability and protein synthesis. Belongs to the SymE family. Extended N-terminus. Extended N-terminus.