Scientists have found an organelle - an enclosed free-floating specialised structure - inside a primitive cell for the first time.
By Dr David Whitehouse
BBC News Online science editor
Prokaryotic cells, such as bacteria, are relatively simple and have no nuclei.
The organelles are the dark spots
It is believed they evolved first then absorbed other prokaryotes and became eukaryotes - complex cells that have nuclei and structures like the energy-producing mitochondria.
Finding a self-contained organelle inside a prokaryote is a puzzle as it suggests that the evolution of cells - the basic building blocks of higher organisms - may have to be reconsidered.
The organelle in question may also have a role in human diseases, such as malaria and African sleeping sickness.
Two types of cell
Biologists recognise two types of cell in nature that are fundamentally different because of their size and internal construction.
Prokaryotes are relatively small cells that contain regions inside them where genes congregate but no membrane separates them from the rest of the cell. They lack so-called organelles such as chloroplasts and mitochondria.
More complicated are the eukaryotes, the cells that comprise all other living things. They have their genetic material enclosed in a membrane and have other enclosed structures (organelles) within them as well.
Professor Roberto Docampo, of the University of Illinois at Urbana-Champaign, US, has been studying the unicellular organism Agrobacterium tumefaciens. It is responsible for gall disease in many plants.
It is also a geneticist's favourite as its method of DNA transfer can be used to make GM crops.
The organelle he found inside the bacteria is practically identical to an organelle he found inside unicellular eukaryotes. This particular organelle helps the bacteria regulate its acidic content.
According to Professor Docampo, the work is important for several reasons. He told BBC News Online that an organelle had never been found in a prokaryote before.
He says it is significant that the same organelle is found in the more complicated eukaryotes implying that it may have a common evolutionary origin for both types of cell.
"It appears that this organelle has been conserved in evolution from prokaryotes to eukaryotes, since it is present in both," he says. "This argues against the belief that all eukaryotic organelles were formed when early eukaryotes swallowed prokaryotes."
Finally, the organelle is known to be present in a number of pathogenic organisms, including those that cause malaria, toxoplasmosis, African sleeping sickness and Chagas disease among others.
This may provide scientists with a technique to tackle these diseases. Because the organelle is not present in animal cells, it may be a useful target for chemotherapy for those diseases.