Part:BBa_K1783001
Hok/Sok Type I Toxin/Antitoxin Cassette
Codes for the Hok/Sok Type I toxin-antitoxin system. This system naturally evolved in bacteria as a method to maintain plasmids. Hok-Sok is noted for its presence on the R1 plasmid, which contains genes that confer antibiotic resistance. Hok-Sok is believed to be used to maintain these plasmids in the absence of antibiotics.
The Host Killing gene creates a transcript with a strong secondary structure that conceals its ribosome binding site. As RNAse degrades the transcript, it assumes a new secondary structure, revealing the ribosome binding site. The protein that is translated drastically imbalances the plasma membrane electrochemical gradients, leading to cell death.
The Suppression Of Killing gene encondes for the Sok mRNA which complements and binds to the Hok mRNA. The resulting complex is unable to be translated and is rapidly degraded by RNase. Hok has a half life of 20 minutes, while Sok has a half life of 30 seconds.
Should a cell resulting from a division not receive the Hok/Sok plasmid from its parent, the mRNA transcripts produced in its parent remain in the cytoplasm. Sok rapidly degrades, but Hok still remains with its relatively longer half life. Without the plasmid to encode for the Sok antitoxin, the Hok is able to be translated, thus killing the cell. The Hok/Sok system is thus an effective internal pressure on the cell to maintain its plasmid.
K1783001 is a twin of K513000, which was designed by VIT iGEM team but never implemented.
Usage and Biology
Hok/Sok is naturally found in the R1 plasmid of E. coli, as well as in plasmids of other bacterial species. It is used naturally to maintain antibiotic resistance plasmids in the absence of antibiotics.
Sequence and Features
- 10COMPATIBLE WITH RFC[10]
- 12COMPATIBLE WITH RFC[12]
- 21COMPATIBLE WITH RFC[21]
- 23COMPATIBLE WITH RFC[23]
- 25COMPATIBLE WITH RFC[25]
- 1000COMPATIBLE WITH RFC[1000]
The hok gene is found on the forward strand of the cassette. Mok, the target of sok, is found on the same strand, in a different coding region and is co-transcribed with hok. The hok/mok transcript is capable of folding into a very stable secondary structure (t1/2 = 20 minutes). Due to the high inherent secondary structure of hok, which is responsible for its long half-life, the hok/sok cassette is difficult to PCR amplify, requiring buffers capable of destabilizing the base-pair stacking and H-bond interactions that make up the complex. In addition, the cassette can also slightly interfere with sequencing reactions, leading to a loss in signal intensity immediately after the end of the cassette.
Sok is found on the reverse strand of the cassette and is complementary to the 5' end of the hok/mok transcript.
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