Hok/sok system

The host killing/suppressor of killing system, also known as hok/sok system, in molecular biology, is a postsegregational killing system of the plasmid R1 of Escherichia coli.



In simple words, the system is controlled by two genes, hok and sok, coding respectively what can be thought of as a long-lived poison and a short-lived antidote. After cell division, daughter cells without a copy of the plasmid dies, as the poison is still active from the parent cell, while the short-lived antidote is not stopping the poison anymore. Only cells with a plasmid can produce more antidote and survive. For this reason, the killing system is "postsegregational", since cell death occurs after segregation of the plasmid.

The hok gene codes for a 52 amino acid toxic protein which causes cell death by depolarization of the cell membrane. The translation of hok mRNA is, however, inhibited by the transcript of the sok gene, which is an antisense regulator and binds to the hok mRNA, forming a duplex which is recognized by the RNase III and degraded. The killing mechanism is obtained through differential decay rates of the hok and sok transcripts: while hok mRNA is quite stable, sok-RNA is rapidly degraded, which would allow hok to be expressed; however the higher rate of transcription of sok compensate, leaving hok mRNA untranslated in plasmid-containing cells. The loss of plasmid causes the hok mRNA not to be inhibited anymore by sok antisense, leading to protein expression and cell death.