The discovery of a new mechanism by which a known bacterium infects
numerous plants could one day help uncover a new class of anti-cancer drugs
for humans, according to a study released Wednesday.
Researchers looking at the single-cell animal that causes brown spot
disease in common green beans isolated a molecule, called syringolin A, that
boosts the bacterium's virulence and accounts for its ability to infect.

A genetically-modified form of the bacterium - Pseudomonas syringae
pv. syringae (Pss) - in which the molecule was removed lost the capacity to
attack the plants, confirming the pathogen's powers.

But what surprised lead scientist Robert Dudler of the Munich
Technical University and his colleagues was how the offending molecule
worked.

Syringolin A, they found, enters plant host cells and prevents a
protein structure called proteasome - which plays a critical role in
breaking down other, undesirably proteins - from doing its job.

"To activate the plant defence response mechanism you need the
proteasome," Dudler told AFP. "Syringolin A seems to be a natural form of
proteasome inhibitor."

What has been a blight for vegetable farmers, however, could be a boon
in medicine.

"The discovery of a novel family of inhibitory natural products, which
we refer to as sybractins, may also have implications for the development of
anti-cancer drugs," he said.

Previous research has shown that Syringolin A has the capacity to
shrink certain cancers, opening the possibility that this and similar
molecules might be developed as new cancer therapies.

The equivalent of Syringolin A is found in other pathogenic bacteria,
notably one which causes melioidosis, or Whitmore's disease, in humans.

This holds the promise, says Dudler, that these organisms are capable
of manufacturing proteasome inhibitors in the same sybractin class.

The pathogen responsible for Whitmore's disease, Burkholderia
pseudomallei, has been considered as a potential agent for biological
warfare and biological terrorism, according to the US Centers for Disease
Control and Prevention in Atlanta, Georgia.

Pss is an important model in molecular plant pathology. Its complete
genome was sequenced in 2005 in the United States.



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