Sleeping sickness bug swallowed a plant

JOHN WHITFIELD

Trypanosoma brucei use the plant genes to break down sugars. © SPL

The parasite that causes sleeping sickness once swallowed a plant, say researchers. They have found plant-like genes in the organism, suggesting that drugs based on herbicides might be able to kill it¹.

Sleeping sickness, which affects around half a million people in sub-Saharan Africa each year, is caused by a microbe called Trypanosoma brucei. Closely related bugs also cause Chagas' disease in South America and leishmaniasis. All three can be fatal.

More than a billion years ago the ancestor of trypanosomes probably merged with a type of green algae, says Fred Opperdoes of the Catholic University of Louvain in Belgium. Taking a plant on board would have enabled it to harness the Sun's energy: "It would have had a tremendous advantage," he says.

When trypanosomes became parasites, they no longer needed to photosynthesize. The passenger degenerated, and some of its genes passed to those of the trypanosome.

The genes "are vital for the survival of trypanosomes", says Opperdoes. "For 10-15 years I've been convinced that they are ideal drug targets, but I never realized where they came from."

"It's a very good idea to search for such targets," says malaria researcher Hassan Jomaa of Justus-Liebig University in Giessen, Germany. The genes' exotic origins should reduce the risk of side effects, as animals do not share them, Jomaa explains.

Several microorganisms, including the malaria parasite, seem to have absorbed others in the past. Both the cellular powerhouses called mitochondria and chloroplasts, which plants use to turn sunlight into chemical energy, are thought to have originally been free-living bacteria.

Mix and match

Opperdoes and his colleagues found the leftover plant genes by analysing the genomes of T. brucei and Leishmania mexicana. So far, they have found 16 genes that have their closest relatives in plants, and suspect that more wait to be discovered.

Plants use the equivalent genes to photosynthesize, using carbon dioxide to make sugars. Trypanosomes use them to break sugars down, in a unique cellular system.

The finding shows how easy it is for microbes to mix and match genes from different sources for different uses, says evolutionary biologist Miklos Muller of Rockefeller University in New York.

1. Hannaert, V. et al. Plant-like traits associated with metabolism of Trypanosoma parasites. Proceedings of the National Academy of Sciences, published online, doi:10.1073/pnas.0335769100 (2003). |Article|

© Nature News Service / Macmillan Magazines Ltd 2003