Toxic component of a type I toxin-antitoxin (TA) system. Part of the plasmid-stabilizing activity of plasmid R1; when R1 is lost cells die rapidly. When overexpressed kills cells within 5 minutes; causes collapse of the transmembrane potential and arrest of respiration (PubMed:3019679). Its toxic effect is partially neutralized by antisense RNA Sok. Belongs to the Hok/Gef family.