Hok/sok system

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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.[1] It is a type I system because the toxin is neutralised by a complementary RNA, rather than a partnered protein (type II toxin-antitoxin).[2]

secondary structure of sok non-coding RNA
transcript which binds with hok mRNA.

Genes involved

The hok/sok system involves three genes:[3]

  • hok, host killing - a long lived (half-life 20 minutes) toxin
  • sok, suppression of killing - a short lived (half-life 30 seconds) RNA antitoxin
  • mok, modulation of killing - required for hok translation[4]
HOK
Identifiers
SymbolHOK_GEF
PfamPF01848
InterProIPR000021
PROSITEPDOC00481
Available protein structures:
Pfam  structures / ECOD  
PDBRCSB PDB; PDBe; PDBj
PDBsumstructure summary

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.[3]

If a daughter cell has inherited the R1 plasmid, it has inherited the sok gene and a strong

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.[6]

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.[7]

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),[8] since cell death occurs after segregation of the plasmid.[9][10]

Hok toxin

The hok gene codes for a 52

cell lysis.[2][13]

Homologous systems

Further information: Toxin-antitoxin system

Other plasmids

hok/sok

F plasmid which operate in the same way to maintain the stability of the plasmid.[15] The F plasmid contains another homologous toxin-antitoxin system called srnB.[11]

The first type I toxin-antitoxin system to be found in

pAD1 plasmid in Enterococcus faecalis. Here, RNAI encodes a toxic protein Fst while RNAII is the regulatory sRNA.[16]

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.[17] A mok homologue which overlaps each ldr loci has also been found.[3]

IstR RNA works in a similar system in conjunction with the toxic TisB protein.[18]

See also

References

  1. PMID 2981804
    .
  2. ^
    S2CID 10028255
    .
  3. ^
    PMID 17376733
    .
  4. PMID 17065468
    .
  5. S2CID 45453320
    .
  6. PMID 1370544
    .
  7. PMID 1380562
    .
  8. PMID 3517851
    .
  9. PMID 7536849
    .
  10. PMID 3019679
    .
  11. ^
    PMID 2101633
    .
  12. PMID 12413781
    .
  13. PMID 11018145
    .
  14. PMID 3049248
    .
  15. PMID 10361310
    .
  16. PMID 10931358.[dead link
    ]
  17. PMID 12123448.[dead link
    ]
  18. PMID 17499044
    .

Further reading

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