Missing Mitochondria: Strange Eukaryotic Cell Lacks 'Powerhouse'

By Botkin-Kowacki, Eva | The Christian Science Monitor, May 12, 2016 | Go to article overview

Missing Mitochondria: Strange Eukaryotic Cell Lacks 'Powerhouse'


Botkin-Kowacki, Eva, The Christian Science Monitor


In high school biology, you may have learned that one key difference between a eukaryotic cell and a prokaryotic cell is that one has mitochondria and the other doesn't.

But that may no longer be true.

A team of scientists have found a eukaryote with no mitochondria.

"Eukaryotic cells are defined by having mitochondria, so it's a big surprise for us that it's possible for a eukaryotic cell to still be viable without mitochondria," says Anna Karnkowska, a researcher at the Biodiversity Research Centre at the University of British Columbia, in an interview with The Christian Science Monitor.

To hunt for mitochondrial material, Dr. Karnkowska and colleagues sequenced the genome of Monocercomonoides sp., single-celled eukaryotes that resides in the guts of vertebrates. They were looking for proteins that are clear markers of the presence of mitochondria, but found none, according to the team's study published Thursday in the journal Current Biology.

Mitochondria are often called "the powerhouse of the cell" because they supply the cell with energy in the form of adenosine triphosphate (ATP). So how does a eukaryote survive without them?

Most energy generation requires oxygen, but not all eukaryotes live in oxygen-rich environments. This particular organism is anaerobic, meaning that it lives in the absence of oxygen - not all that surprising, considering it is a microbe that lives deep inside other animals.

So the mitochondria in anaerobes are already not classic mitochondria in the sense that they don't act exactly the ones in our cells, explains Karnkowska. But mitochondria serve other purposes, too.

Mitochondria are also involved in the vital formation of iron- sulfur proteins. "The eukaryotic cell basically cannot exist without iron-sulfur proteins," she says.

So the team hunted for evidence of a mitochondrial iron-sulfur cluster assembly pathway. This mechanism would be indispensable in a eukaryotic cell.

Or so the scientists thought, until they found no evidence of such a mechanism in this eukaryote.

That doesn't mean the organism is surviving without iron-sulfur proteins. However, that piece of the puzzle is a bit more complex.

Prokaryotes can assemble iron-sulfur clusters without mitochondria. So Karnkowska and her colleagues suspect that this eukaryote picked up a prokaryote's system through a process called horizontal gene transfer, also known as lateral gene transfer. Horizontal gene transfer (HGT) happens when an organism receives genes directly from another organism rather than from parent to offspring.

Once Monocercomonoides acquired that system, a cytosolic sulfur mobilization system (SUF), from a bacteria via horizontal gene transfer, Karnkowska suggests, the mitochondria likely became unnecessary for the eukaryote to function, so the organelle disappeared over generations.

Among scientists studying eukaryotic evolution, this finding may not be shocking, says Gertraud Burger, a researcher at the Robert- Cedergren Center for Bioinformatics and Genomics at the University of Montreal who was not part of the study. …

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