Hok/sok system

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".

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. [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 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.

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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