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DNA-shredding Bacteria

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Image: MARK PALLEN and NICK LOMAN

The most common culprit of food poisoning in the U.S. is a bacterium called Campylobacter jejuni (right). And researchers from Yale University have now uncovered its secret weapon: a small molecule that leaves a cell's DNA in shreds.

It was already known that the bacterium makes a toxin called CDT, which consists of three different proteins. In this recent study, the scientists introduced these proteins separately into cells and found that only one of them, CdtB, had a toxic effect. As they report in today¿s issue of Science, the protein left the cells' nuclei, which contain the DNA, in pieces. At lower amounts, it made the cells enlarge, similar to the effect of the CDT toxin.

When the researchers compared the gene sequence encoding CdtB with other genes, they found it had striking similarity to a DNA-shredding enzyme called DNase I. So they mutated the CdtB-gene in an area that renders DNase I dysfunctional, and found that the change had the same effect: the mutant CdtB protein no longer led to fragmented cell nuclei. Also, when they mixed the mutant protein together with the two other toxin components, cells remained unharmed, whereas the mix of all three normal proteins made them enlarge. Presumably, CdtB damages the DNA first, which only becomes visible when there is a lot of the protein. Because of this damage, the cells stop dividing and enlarge before they ultimately die.

Food poisoning is bad enough, but when the attacker damages the DNA of the intestinal cells, it can even become dangerous: if these cells survive mutated, intestinal cancer may result.

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