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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.

Sequence and Features