Colicins are large domain-structured proteins synthesized by Escherichia coli . These toxins do not contain their own export signals and thus their export requires the help of a lysis protein. Colicin-producing strains also synthesize immunity proteins, which specifically prevent exogenous colicins from killing them. To date, more than twenty colicin types have been identified by their corresponding specific immunity. Under conditions of stress, a small fraction of colicinogenic bacteria are induced to produce colicin and lysis proteins, an action mediated by the SOS response.
Lysis protein causes the lysis of the host cell and the release of colicins.
Colicins attach to specific receptors in the outer membrane of nearby E. coli cells that are not immune or resistant to that colicin. Colicins are translocated across the outer membrane, a movement mediated by the Tol or Ton transport systems. In the past 50 years, the molecular biology and biochemistry of colicins have been studied extensively. Only recently have colicins inspired the interest of molecular evolutionists by providing a model system for addressing molecular mechanisms underlying bacteriocin diversification. Extensive studies on the molecular biology of colicin gene clusters have generated a large body of molecular data, which has allowed comparative studies on DNA and amino acid sequences.