Introduction by Martin Redfern (MR)

Michael Gray (MG) interviewed by Richard Black (RB)

MR: Every multicellular organism known has one feature in common with all others: inside its cells are tiny organelles called mitochondria. These are the chemical powerhouses of cells, using the chemical energy of food such as sugars to produce a molecule known as ATP, the high energy fuel of processes throughout the cell. All so-called 'eukaryotes' including all plants and animals possess mitochondria. And, as long as a century ago, biologists studying cells under the microscope noticed striking similarities between them and primitive bacteria. For a start, mitochondria contain their own genes. When similar genes turned up in free-living bacteria, it strengthened the evidence that these vital components of our cells may once have been infections picked up by our most primitive single-celled ancestors. Perhaps it was not a one-sided relationship. The bacteria brought their chemical power generators; the cells gave protection and supplied food. The partnership flourished, and, over billions of years, became an inseparable relationship. In the process, the mitochondria seem to have tried out almost every genetic variation imaginable. Indeed, they've been described as the molecular playground of nature. The differences between mitochondrial genes in different species have been widely used as a sort of molecular clock to plot evolutionary relationships. At the same time, there's been a search for the child that missed out on the playground: the most primitive mitochondrion most like the ancestral symbiotic bacterium. Now, scientists under Professor Michael Gray of Dalhousie University in Nova Scotia think they've found it, as he explained to Richard Black.

MG: This is in an organism that is called Reclinomonas americana, it's an organism relatively new to science; it was only described in 1993 by workers at the American Type Culture Collection. It's a freshwater protozoon, it lives in ponds, lakes, in sediments, it eats bacteria, and what was interesting about this organism is that the structure of its flagellum (the apparatus that it uses to swim with) looked very much like that in a small class of eukaryotic cells that actually don't have mitochondria, and that are thought to perhaps have diverged away from the main line of eukaryotic evolution before the mitochondrial endosymbiont entered. So we thought that there might be some reason to look at this organism as being the closest one to that group that is thought never to have had mitochondria.

RB: So what does the mitochondria in this organism, in this protozoon, actually look like then?

MG: Well, to us it was rather spectacular, it looks for all intents and purposes like a shrunken bacterial genome. It has none of the bizarre characteristics that are found in other mitochondrial DNAs, in animal mitochondrial DNAs for example, and so if you didn't know any better, if you didn't know where it came from, you might at first glance think that it came from a bacterium, which is of course exactly what the endosymbiont hypothesis predicts.

RB: Has it got more genes than you'd find in a typical mitochondrion in other organisms, or less genes? MG: It has more, in fact. It's a genome that's about four times as large as the mitochondrial genome in human cells; it has all of the genes that have been found in all of the other mitochondrial DNAs that have been studied to date, and it has a bunch of new ones, so it has genes that have never been found in mitochondrial DNA before, and ones that we might have suspected should have been there at the beginning RB: And yet it's performing the same function as other mitochondria, is it?

MG: Yes, it is, and this is one of the strange features of mitochondrial DNA evolution. We see that there has been a lot of gene loss, gene transfer, gene replacement, the mitochondrial DNA has been reorganised fairly radically, and yet the biology of the system has been able to accommodate to this and to produce a structure, an organelle, that carries out basically the same function in all eukaryotes. So it gives us an idea that genomes are very, very flexible entities.

RB: And I suppose one question you would ask would be, "Well, if the mitochondria of other organisms have changed so much, why has this one not changed very much?"

MG: Yes, that's a good question. We don't know the answer to that, it is the case that the protists, the unicellular eukaryotes that we're working with constitute about 95% of the biological diversity within this group. So, when people look at plants and animals and fungi, they're really only looking at a very small proportion of this big group. And so it's been relatively unexplored, and so we're essentially through the Organelle Genome Megasequencing Project trying to explore diversity within this group. We would be very happy to find another one, for example, that is twice as big as the one we've just reported.

RB: Having said that the DNA in this particular mitochondrion resembles bacterial DNA, is it particularly close to any kind of bacteria, because that in turn would be quite fascinating, it would give some clue as to the kind of bacterium that might have formed the original mitochondrion.

MG: Well, that question has actually been fairly well settled in recent years. We know fairly precisely where the endosymbiont came from, and that's from a group of bacteria called the alpha- Proteobacteria, and a subdivision of that specifically, and these contain organisms that are all intracellular parasites, and the one that might be best known to your listeners is the organism Rickettsia that causes Rocky Mountain spotted fever. So that's the organism that our mitochondria are most closely related to.

RB: In this particular mitochondrion that you've been looking at, the one from Reclinomonas americana, you said that it contains many more genes than the typical mitochondrion, so is this turning up any surprises in terms of what those genes actually do?

MG: Yes, perhaps the biggest surprise is that the enzyme that is actually responsible for expressing mitochondrial DNA, that enzyme is called RNA polymerase, and in bacteria the enzyme is made up of four different proteins, and so we would have expected that the mitochondrial RNA polymerase would be like a bacterial RNA polymerase. Instead, it isn't: in most organisms, it's a single protein, the gene is in the nucleus not in the mitochondrial DNA, and it looks more like a bacteriophage, a bacterial viral, RNA polymerase. And so where this came from has been a mystery. And it turns out that Reclinomonas is the first organism whose mitochondrial DNA contains the four bacterial RNA polymerase genes that we would have suspected should be there. So at some very early point in mitochondrial DNA evolution this bacterial enzyme must have been discarded, and a new one from somewhere picked up. And so these kind of studies give us some idea of the dynamics and the pathways by which the organelle has evolved.

RB: Now, finally, Michael, you've obviously found something which is rather like the original mitochondrion, but perhaps it's not the granddaddy of them all, perhaps you're going to go back to other protozoons or other organisms, and see if you can find something which is even more primitive.

MG: Well, this has given us a lot of encouragement to look further. I think you're right, we may be just at the tip of the iceberg at the moment, and if our funding is renewed we'll certainly continue the search.

MR: And maybe come up with that "mother of all mitochondria".