Colicin

A colicin is a type of bacteriocin.

Colicins are composed of three globular domains. One domain regulates the target and binds to the receptor on the sensitive cell. The second is involved with translocation, co-opting the machinery of the target cell. The third is the 'killing' domain and may produce a pore in the target cell membrane, or act as a nuclease to chop up the DNA or RNA of the target cell. Because they target specific receptors and use specific translocation machinery, cells can make themselves resistant to the colicin by repressing or deleting the genes for these proteins. Such resistant cells may suffer the lack of a key nutrient (such as iron or vitamin B) but benefit by not being killed. Colicins exhibit a '1-hit killing kinetic' which doesn't necessarily mean a single molecule is sufficient to kill, but certainly that it only takes a small number. In his Nobel Laureate speech, Salvador E. Luria, 1969, speculated that colicins could only be this toxic by causing a domino effect that destabilized the cell membrane He was not entirely correct, but pore-forming colicins do de-polarize the membrane and thus eliminate the energy source for the cell. The colicins are highly effective toxins.

Virtually all colicins are carried on plasmids. There are two general classes of colicinogenic plasmids, large, low-copy number plasmids, and small high copy number plasmids. The larger plasmids carry other genes as well as the colicin operon. The colicin operons are generally organized with several major genes. These include an immunity gene, a colicin structural gene, and a BRP (bacteriocin release protein), or lysis, gene. The immunity gene is often produced constitutively, while the BRP is generally produced only as a read-through of the stop codon on the colicin structural gene. The colicin itself is repressed by the SOS response and may be regulated in other ways as well.

Research indicates that retaining the colicin plasmid is very important for cells that live with their relatives, because if a cell loses the immunity gene, it quickly becomes subject to destruction by circulating colicin. At the same time, colicin is only released from a producing cell by the use of the lysis protein, which results in that cell's death. This suicidal production mechanism would appear to be very costly, except for the fact that it is regulated by the SOS response, which responds to significant DNA damage. In short, colicin production may only occur in terminally-ill cells. Still these matters require further research.