Colicin genes clusters are located on plasmids by operons encodind colicin functions and are composed of a toxin-encoding gene; an immunity gene which encodes a protein conferring self-specific immunity to the cell against its own colicin by binding to and inactivating the encoded toxin ; and, frequently, a lysis gene, whose product contributes to the release of toxin into the environment via lysis or pseudo-lysis of the producing cell.
The lysis gene is sometimes absent, especially when more than one colicin gene cluster coexist in the same cell.
A direct and specific interaction within inner membrane proteins between the immunity protein and their corresponding specific pore-forming domains (the C-terminal domain) of its cognate colicin achieves cell protection
Colicins targeting intracellular enzymes such as nuclease are inactivated by direct binding of the nuclease-specific immunity proteins (Ims) to the nuclease colicin C-terminal domain in the cytoplasm with very high affinity leading to protein complex (71-kDa heterodimer) releasing into the extracellular medium.