Difference between revisions of "Team:UMaryland/Description"

Line 105: Line 105:
  
 
<p>
 
<p>
−
Antibiotic resistance is a necessary selection factor for transgenic bacteria using plasmids as vectors. This staple of genetic engineering has been met with opposition with valid claims that the addition of antibiotics to the environment harms native species and poses a risk to unwanted antibiotic resistance through lateral gene transfer.
+
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.</p>
−
<p>
+
 
−
The Hok/Sok system has naturally evolved in bacteria as a means of plasmid retention, and is capable of addressing the issue by providing a selection factor for plasmid retention without the dangers of antibiotics and risk of lateral gene transfer. The Hok (host killing) gene codes for a mRNA which lies dormant in its initial secondary structure. As it is degraded by exonuclease, it assumes a translatable secondary structure which produces an apoptosis triggering protein. The Sok (suppression of killing) gene codes for a mRNA transcript that binds to the Hok mRNA, preventing it from being translated. The complex is eventually degraded by nuclease. Hok has a half life of 20 minutes, while Sok has a half life of 30 seconds. As long as both genes are present, the cell remains alive. After cell division, should the cell not retain the plasmid of interest which contains Hok/Sok, Hok mRNA remains the cytoplasm for 20 minutes, while remaining Sok is degraded. Since the cell does not contain a Sok gene, no Sok is being produced to save the cell from being killed by Hok. This system is very similar to current antibiotic resistance systems, only without the necessity for antibiotics themselves, resolving the issue of environmentally safe plasmid retention.  
+
<p>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.</p>
 +
 
 
<br>
 
<br>
  

Revision as of 21:31, 10 September 2015