Most Colicins Kill by One of Two Different Mechanisms

The Col plasmids allow the strains of E. coli that possess them to kill other related bacteria. There are two basic approaches to this. The first is to damage the victim's cell membrane. A gene on the ColE1 plasmid encodes the colicin E1 protein that inserts itself through the membrane of the target cell and creates a channel allowing vital cell contents, including essential ions to leak out and protons to flood into the cell (Fig. 16.18).The influx of protons collapses the proton motive force.The energy derived from the proton motive force drives the production of ATP and the uptake of many nutrients, without which the bacteria quickly die. A single molecule of colicin El that penetrates the membrane is enough to kill the target cell. Colicin I and colicin V operate by a similar mechanism.

Colicin M and Pesticin A1122 destroy the peptidoglycan of the cell wall rather than puncturing the cytoplasmic membrane. These colicins need to penetrate only as far as the outer surface of the cytoplasmic membrane, i.e. the site of peptidoglycan assembly. Without the peptidoglycan, the bacterial cell loses shape and eventually bursts. Pesticin A1122 is made by Yersinia pestis and kills Y. pseudotuberculosis, Y. enterocolitica, plasmid free Y. pestis and many strains of E. coli (although curiously not E. coli K12).

ColE plasmid Small multicopy plasmid that carries genes for colicins of the E group. Used as the basis of many widely used cloning vectors

Bacteria are Immune to their own Colicins 445

FIGURE 16.18 Some Colicins Damage the Cell Membrane

When colicin El protein attacks a bacterial cell, it punctures a hole through the outer membrane, cell wall, and inner membrane. The hole allows protons to leak into the bacteria and vital ions to leak out. A single channel abolishes energy generation.

FIGURE 16.19 Colicin Immunity System

In order to protect itself, a colicin-making cell also produces an immunity protein (right). This protein is also encoded by the colicin plasmid. It blocks the active site of the colicin thus preventing the cell from killing itself. The immunity protein is specific and only inhibits one type of colicin. If a cell lacks immunity protein, the colicin is able to kill the cell (left).

FIGURE 16.18 Some Colicins Damage the Cell Membrane

When colicin El protein attacks a bacterial cell, it punctures a hole through the outer membrane, cell wall, and inner membrane. The hole allows protons to leak into the bacteria and vital ions to leak out. A single channel abolishes energy generation.

Ions

Vital ions leak out and protons leak in membrane o o ° o o o o o 0

Ions

Inner membrane

Cell wall

Inner membrane

Cell wall

Vital ions leak out and protons leak in membrane

FIGURE 16.19 Colicin Immunity System

In order to protect itself, a colicin-making cell also produces an immunity protein (right). This protein is also encoded by the colicin plasmid. It blocks the active site of the colicin thus preventing the cell from killing itself. The immunity protein is specific and only inhibits one type of colicin. If a cell lacks immunity protein, the colicin is able to kill the cell (left).

Sensitive cell

Immune cell

Sensitive cell

Immune cell

The second approach is to degrade the nucleic acids of the victim. The ColE2 and ColE3 plasmids both encode nucleases, enzymes that degrade nucleic acids. The colicin E2 and E3 proteins are very similar over their N-terminal 75% and as a result they share the same receptor on the surface of sensitive bacteria. They differ in the C-terminus and have different nucleic acid targets. Colicin E2 is a deoxyribonuclease that cuts up the chromosome of the target cell. Colicin E3 is a ribonuclease that snips the 16S rRNA of the small ribosomal subunit at a specific sequence, releasing a fragment of 49 nucleotides from the 3' end. This abolishes protein synthesis and though much more specific than colicin E2, is just as lethal. Again, a single colicin molecule that enters the victim is enough to kill the target cell.

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