Hok/Sok
While present on a plasmid, the Hok-Sok system maintains plasmids through mRNA silencing. On the forward strand of the Hok-Sok cassette is the coding region for the Hok mRNA. Hok ("host killing") is a 52 amino acid toxin that is capable of permeating the cell membrane, causing a loss of electrochemical potential and cell death. The translation of Hok mRNA is constantly inhibited by Sok, an inhibitory RNA molecule that is transcribed on the reverse strand of the Hok-Sok cassette. Sok ("suppression of killing") is capable of forming a double-stranded RNA helix with the 5’-end of Hok mRNA, blocking the ribosome binding site and preventing translation. This RNA duplex is then recognized by RNAse III, leading to enzymatic degradation of the Hok transcript.
Below is a simplified diagram of how the Hok-Sok system maintains a plasmid:
How does this lead to plasmid maintenance? Hok mRNA, due to a high degree of secondary structure, has a long half-life, measured at 20 minutes. Sok, on the other hand, has a half-life of only 30 seconds. The cell must thus retain the coding region for Sok in order to produce enough Sok to continue blocking Hok translation. If the plasmid is lost, then both the Hok and Sok coding regions will be lost; however, previously transcribed Hok mRNA will still be present. With previously transcribed Sok rapidly degrading, Hok mRNA will be translated, killing the daughter cell that did not maintain the plasmid.
Our wet project for 2015 was to demonstrate that the Hok-Sok system could maintain a plasmid over many generations as well as typical antibiotic maintenance systems. We hypothesized that the Hok-Sok system could thus serve as a cheap, ethical alternative to antibiotic pressure.
