Addiction module

addiction modules are toxin-antitoxin systems. Each consists of a pair of genes that specify two components: a stable toxin and an unstable antitoxin that interferes with the lethal action of the toxin. Found first in E. coli on low copy number plasmids, they are responsible for what is 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 is the more stable toxin. The cells are “addicted” to the short-lived product, because its de novo synthesis is essential for cell survival. Thus, “addiction modules” were implicated in maintaining the stability of extrachromosomal elements. Toxin–antitoxin systems, some of which are homologous to these extrachromosomal “addiction modules,” also occur in the chromosomes of many bacteria.