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.
Put simply, the system is controlled by two genes, hok and sok, which code what can be thought of as a long-lived poison, and a short-lived antidote, respectively. After cell division, daughter cells without a copy of the plasmid die because the poison from the parent cell is still active while the short-lived antidote is not. 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.[1] The translation of hok mRNA is, however, inhibited by the transcript of the sok gene, which is an antisense regulator. This then 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 compensates for this, leaving hok mRNA untranslated in plasmid-containing cells. The loss of plasmid causes the hok mRNA not to be inhibited by sok antisense, which leads to protein expression and cell death.
See also
References
- Thisted T, Sørensen NS, Gerdes K (1995). "Mechanism of post-segregational killing: secondary structure analysis of the entire Hok mRNA from plasmid R1 suggests a fold-back structure that prevents translation and antisense RNA binding". J. Mol. Biol. 247 (5): 859–73. doi:. PMID 7536849.
- ^ Pecota DC, Osapay G, Selsted ME, Wood TK (2003). "Antimicrobial properties of the Escherichia coli R1 plasmid host killing peptide". J. Biotechnol. 100 (1): 1–12. doi:. PMID 12413781.
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