Pathogenic Cousins of Mitochondria

Scientists may have identified the closest living relative of the organism that eventually turned into mitochondria, the organelles that power all eukaryotic cells. The sequence of Rickettsia prowazekii, the pathogen that causes typhus, closely resembles that of mitochrondrial DNA, researchers report in tomorrow's issue of Nature. Tracking down the genes that make the bug so deadly may lead to better vaccines against R. prowazekii, which causes high fever and delirium.

It took 6 years for molecular microbiologist Charles Kurland of the University of Uppsala in Sweden and his colleagues to sequence the 1.1-million-base-pair genome of R. prowazekii. Already, preliminary genetic analyses had suggested that its genome might closely resemble that of the predecessor of a prisoner of cells, the mitochondria. Indeed, a comparison with the DNA still present in modern mitochondria has revealed "very strong similarities," says team member Siv Andersson, particularly in genes involved in energy production.

Many of the pathogen's 834 genes closely resemble those that code for proteins used by yeast mitochondria--genes that are located in the nucleus of yeast cells. This suggests "an early evolutionary event where there was an off-loading of these genes" from the early mitochondrion to the nucleus, says Kurland. As the ancestral host nucleus took on these genes, the mitochondria would have become more dependent on the host cell, until eventually they could no longer survive except within the cell.

Assuming that scenario is correct, the work "is a wonderful study in the way genomes evolve to become degenerate," says evolutionary biologist Carl Woese of the University of Illinois, Urbana. It could also lead to practical payoffs, because a half-dozen genes in the Rickettsia genome code for proteins similar to those that make other bacteria virulent. Three resemble genes for toxic polysaccharides in Staphylococcus aureus, which causes boils. The information should help researchers interested in developing new vaccines for typhus find the right proteins to include in their inoculations, Kurland says.