Immunity to bacteriocins

Some diffusible substances released by bacteria into the medium inhibit the growth of other sensitive, generally closely related, bacteria. These substances, which are proteinaceous in nature, act at low concentrations. They have been called bacteriocins. A common characteristic of bacteriocin-producing strains is their ability to resist their own bacteriocin through specific-immunity peptides or proteins. The best-documented bacteriocins are the colicins produced by various members of the Enterobacteriaceae.

Colicinogeny appears to be the result of selection pressure among the Enterobacterial Colicins arc-encoded by Col plasmids and their production is induced by agents which damage DNA. In most Col plasmids three genes have been identified: the colicin structural gene, the immunity gene, which protects the producing cells against the action of their own colicin, and the lysis gene, whose expression is required for efficient colicin release. Once secreted into the medium, colicins require at least three steps to kill sensitive cells of Escherichia coli: 1 ) binding to a specific receptor located in the outer membrane; 2) translocation across the membrane; and 3) interaction with their specific target in the cell. Three different domains linearly organized along the polypeptide chain are involved in each of these steps.

Pore-forming colicins kill sensitive bacteria by forming voltage-dependent ion channels in their inner membranes. This membrane depolarization causes a series of metabolic effects such as inhibition of active transport and of protein and nucleic acid synthesis, decrease of internal ATP concentrations, and leakage of potassium. Cell death results from all these events.

Pore-forming colicins form well-defined channels in the planar lipid bilayer. These channels are characterized by their sensitivity to the difference of the electrical potential across the membrane. The channel domain is composed of 10 a helices and has to rearrange in order to insert into the membrane. The voltage-independent insertion of a hydrophobic helical hairpin, buried in the core of the structure, and the voltage-dependent insertion of a large voltage-responsive segment leads to channel formation.

Immunity genes are constitutively expressed and are transcribed in the opposite direction to the colicin genes. Immunity proteins are produced in very low amounts and protect the cell against external colicin only. The amount of colicin reaching its target, the inner membrane, depends on the number of uptake sites (400-1000); a very low amount of immunity protein is thus sufficient to protect the cell. Immunity proteins to channel-forming colicins are membrane proteins of 11-18 kDa that span the cytoplasmic membrane three to four times. They protect the cells against the colicin they produce- by specifically interacting with the C-terminal domain of the colicins. Immunity proteins act in the cytoplasmic membrane by specifically preventing pore formation. The high specificity of the immunity protein suggests that a molecular complex forms between the colicin and its cognate immunity protein. The specific determinant for immunity protein recognition has been found to be the hydrophobic hairpin of the colicin channel domain. The immunity protein diffuses freely in the membrane and then recognizes within the lipid bilayer the colicin channel domain by interacting with its hydrophobic hairpin (Figure 1).

Colicins which do not form channels have enzymatic activity: some hydrolyze the bacterial chromosome while others cleave the ribosomal RNA or inhibit murein synthesis. For instance, colicin E.3 is a

Periplasm

Periplasm

Cytoplasm

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