The reason bacteria may end up rid of their plasmid is due to the metabolic stress the plasmid imposes on the cell, this may result in a slowed growth of the cell, thus plasmid free segregants outgrow plasmid bearing segregants.
The cell overcomes this problem through use of 'post segregational killing systems':

ColE1 containing cells secrete colicin, cells that do not contain the ColE1 plasmid do not have the imm gene and thus do not produce the immunity protein and die.
F encodes 2 host killing 'toxin/antitoxin' systems: hok/sok and ccdA/ccdB (ccdA is a stable protein that inhibits DNA gyrase, whereas ccdB is unstable inhibitor of ccdA)

hok/sok killing mechanism operates much the same as ccdA/B but operates on the RNA level. Hok is a host killing peptide, translated from a stable (20min half life) mRNA. Sok is an unstable (5 min half life) antisense that binds to Hok mRNA and prevents it being translated.

	Created by Austin Davies over 8 years ago

Accessory genome lecture block