Addiction module

Found first in Escherichia coli on low copy number plasmids, addiction modules are responsible for a process called the postsegregational killing effect.

When bacteria lose these plasmid(s) (or other extrachromosomal elements), the cured cells are selectively killed because the unstable antitoxin is degraded faster than the more stable toxin.

Proteic addiction modules use proteins as toxins and antitoxins, as opposed to RNA or other methods.

In protein-based addiction modules, the genes encoding the toxin and antitoxin lie adjacent to each other and are continuously expressed under one operon.

The antitoxin is generally less stable than the toxin due to its degradation by proteases already present in the cell.

The antitoxin in proteic addiction modules functions by binding directly to the toxin and preventing its mode of action.

Shokeen, Sonia; Greenfield, Tony J; Ehli, Erik A; Rasmussen, Jessica; Perrault, Brian E; Weaver, Keith E. (March 2009).

A small dimer representing the structures of two toxin molecules is associated with the c-terminal domains of the associated antitoxin molecules in the ccdAB addiction module.
An example of a proteic addiction module. The CcdB toxin molecules (in red) are bound to CcdA antitoxin molecules (c-terminal regions shown in green) to form a complete CcdAB complex.