Hok/sok system
The hok/sok system is a postsegregational killing mechanism employed by the R1 plasmid in "Escherichia coli". It was the first type I toxin-antitoxin pair to be identified through characterisation of a plasmid-stabilising locus. It is a type I system because the toxin is neutralised by a complementary RNA, rather than a partnered protein (type II toxin-antitoxin). Genes involved. The hok/sok system involves three genes: Killing mechanism. When "E. coli" undergoes cell division, the two daughter cells inherit the long-lived hok toxin from the parent cell. Due to the short half-life  of the sok antitoxin, daughter cells inherit only small amounts and it quickly degrades. If a daughter cell has inherited the R1 plasmid, it has inherited the "sok" gene and a strong promoter which brings about high levels of transcription. So much so that in an R1-positive cell, Sok transcript exists in considerable molar excess over Hok mRNA. Sok RNA then indirectly inhibits the translation of "hok" by inhibiting "mok" translation. There is a complementary region where "sok" transcript binds "hok" mRNA directly ("pictured"), but it does not occlude the Shine-Dalgarno sequence. Instead, "sok" RNA regulates the translation  of the "mok" open reading frame, which nearly entirely overlaps that of "hok". It is this translation-coupling which effectively allows "sok" RNA to repress the translation of "hok" mRNA. The "sok" transcript forms a duplex with the leader region of "hok" mRNA and this is recognized by RNase III and degraded. The cleavage products are very unstable and soon decay. Daughter cells without a copy of the R1 plasmid die because they do not have the means to produce more "sok" antitoxin transcript to inhibit translation of the inherited "hok" mRNA. The killing system is said to be postsegregational  (PSK), since cell death occurs after segregation of the plasmid. Hok toxin. The "hok" gene codes for a 52 amino acid toxic protein which causes cell death by depolarization of the cell membrane. It works in a similar way to holin proteins which are produced by bacteriophages before cell lysis. Homologous systems. Other plasmids. "hok/sok" homologues denoted "flmA/B" (FlmA is the protein toxin and FlmB RNA the antisense regulator) are carried on the F plasmid which operate in the same way to maintain the stability of the plasmid. The F plasmid contains another homologous toxin-antitoxin system called  srnB. The first type I toxin-antitoxin system to be found in gram-positive bacteria is the RNAI-RNAII system of the pAD1 plasmid in "Enterococcus faecalis". Here, RNAI encodes a toxic protein Fst while RNAII is the regulatory sRNA. Chromosomal toxin-antitoxin systems. In "E. coli" strain K-12 there are four long direct repeats (ldr) which encode short open reading frames of 35 codons organised in a homologous manner to the "hok/sok" system. One of the repeats encodes LdrD, a toxic protein which causes cell death. An unstable antisense RNA regulator (Rd1D) blocks the translation of the LdrD  transcript. A "mok" homologue which overlaps each "ldr" loci has also been found. IstR RNA works in a similar system in conjunction with the toxic TisB protein.