Colicin A

Colicin A is a type of Colicin, a bacteriocin made by E. Coli which acts against other nearby E. Coli to kill them by forming a pore in the membrane, leading to depolarisation of the membrane which kills the cell.

SynthesisSynthesis

Colicin A is synthesised through a single operon, containing the colicin A lysis protein encoding gene, Cal. The Cal gene is also involved in the expression of the colicin A operon - without the cal gene the amount of colicin A produced decreases. The cal gene product is likely to be an activator of colicin A expression, including its own expression. The N terminal region of the cal gene product is particularly involved in the regulation of col A synthesis. A separate, unidentified, product (which may be a heat-shock protein) could also be involved, and complement the cal gene product. The expression of the col A operon provokes a shut-off of chromosomal protein expression, which is due to the expression of the lysis gene. This shut-off gives the col A production a priority over other cellular proteins in their synthesis.

Synthesis of col A starts immediately after induction. At lower temperatures, like 30'c, there is a lag phase to this synthesis, which does not end in null cal mutants. At 37-42'c the lag is very short, and a large amount of col A is produced very rapidly. The signal to speed up synthesis from the lag phase involves the cal gene product, alongside the other unidentified gene product at normal and high temperatures.

Cal has a self-regulatory role in the expression of both itself and the col A gene, which may be common to all colicin lysis proteins.

Mechanism of uptakeMechanism of uptake

Colicin A binds to the BtuB Vitamin B12 outer membrane receptor of the target cell, and uses the Tol system to translocate across the membrane, specifically TolQRAB, alongside the OmpF protein. Its use of the Tol system means that Colicin A is in the A group of colicins.

Killing ActivitiesKilling Activities

Colicin A is a pore-forming colicin, which means that its cytotoxic domain inserts into the membrane of the target cell, resulting in the depolarisation of the cell membrane. E. coli uses the polarisation of its cell membrane to generate energy, so with this not functioning a number of energy-requiring cellular functions are inhibited, and the cell ultimately dies, after an arrest of motility within 3 minutes.

Col A when present in an E. coli cell is able to affect macromolecular synthesis throughout the cell, affecting many of the processes in the cell. One such affected system is nucleic acid synthesis; it is halted very soon after Col A is added to the system. Colicin A also has a negative impact on the availability of ATP-dependent active transports, such as some permease activities, such as the uptake of labelled isoleucine.