FOR the first time, an 鈥渙rganelle鈥� has been found in a bacterium, overturning the dogma that these specialised compartments are unique to the complex cell types found in plants and animals.
The findings pose intriguing questions about how cells evolved. Complex or eukaryotic cells have a sophisticated structure with numerous membrane-bound compartments, whereas bacteria or prokaryotic cells are usually regarded as little more than a bag of chemicals surrounded by a rigid wall. They do have a few primitive structures, but proper organelles, surrounded by a membrane and partly independent from the rest of the cell, were thought to be the preserve of complex cells.
Now work by Roberto Docampo鈥檚 team at the University of Illinois, Urbana-Champaign, suggests that at least one kind of organelle evolved in prokaryotes before the evolution of eukaryotic cells. 鈥淚t certainly will change the textbook knowledge,鈥� says Mikl贸s M眉ller of the Rockefeller University in New York. The new organelle is very similar to one found in some parasites and fungi, down to the presence of a key enzyme, which suggests a common origin.
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The structures were actually first spotted in 1904 and subsequently seen in many bacteria, fungi and parasites, but scientists thought the speckles appearing under the microscope were merely sites of mineral deposition. It wasn鈥檛 until 1994 that Docampo鈥檚 team showed that the structures in one eukaryote (the parasite Trypanosoma cruzi) were in fact vesicles enclosed by a membrane, and that the membrane contained with enzymes called pyrophosphatases, which pump ions into the compartment. Docampo thinks the organelles, which he named acidocalcisomes, play a role in storing energy, as well as regulating pH and calcium concentrations.
Now Docampo鈥檚 team has shown that the similar looking structures in bacterial cells are indeed the same as those in more complex cells. The membranes of the acidocalcisome in the bacterium Agrobacterium tumefaciens are also studded with pyrophosphatase, they found, and the bacterial version of the enzyme closely resembles the one in the Trypanosoma parasite. 鈥淗aving a homologue enzyme in a similar organelle is a strong argument for the common origin of the two organelles,鈥� Docampo says in The Journal of Biological Chemistry (DOI: 10.1074/ JBC.M304548200).
How the organelle evolved is not clear. Docampo thinks it is unlikely to have arisen by one prokaryote ancestor engulfing another, the kind of event that almost certainly gave rise to organelles such as mitochondria and chloroplasts (see Graphic). Preliminary evidence suggests that the organelle developed within the bacterium, he says.
Docampo鈥檚 work is not the first to suggest the division between prokaryotic and eukaryotic cells is less clear-cut than the textbooks make out. Other recent work suggests some bacteria also have a primitive cytoskeleton composed of protein filaments, another characteristic that supposedly evolved only in complex cells.