- 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 theplasmid 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 toxicprotein which causes cell death bydepolarization of thecell membrane . [cite journal |author=Pecota DC, Osapay G, Selsted ME, Wood TK |title=Antimicrobial properties of the Escherichia coli R1 plasmid host killing peptide |journal=J. Biotechnol. |volume=100 |issue=1 |pages=1–12 |year=2003 |pmid=12413781 |doi=] The translation of "hok"mRNA is, however, inhibited by the transcript of the "sok" gene, which is anantisense regulator. This then binds to the "hok" mRNA forming a duplex which is recognized by theRNase III and degraded. The killing mechanism is obtained through differentialdecay rate s 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.ee also
*
Addiction module References
*cite journal |author=Thisted T, Sørensen NS, Gerdes K |title=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 |journal=J. Mol. Biol. |volume=247 |issue=5 |pages=859–73 |year=1995 |pmid=7536849 |doi=10.1006/jmbi.1995.0186
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