This was a stunning discovery, says Allen, because it was thought that the urea cycle originated with the metazoan (animal) branch of life.
There it has played an important role in facilitating a wide range of physiological innovations in vertebrates.
For example, urea synthesis enables rapid control of minerals and salts in the blood in animals such as sharks, skates, rays and bony fish, and ammonia detoxification associated with water retention in amphibians and mammals.
The latter was likely a prerequisite for life on land, and subsequently enabled the biochemical pathways necessary for processing a high-protein diet.
Allen and others have now shown that the urea cycle originated hundreds of millions of years before the appearance of metazoans.
The team used RNA interference techniques to partially silence a key urea cycle enzyme in diatoms.
Paper co-author Alisdair Fernie of the Max-Planck Institute of Molecular Plant Physiology evaluated the metabolite profile of diatoms with and without an impaired urea cycle.
Then Allen analyzed the data and found that urea cycle metabolites are critical for cellular recycling of carbon and nitrogen.
The metabolites are also important for facilitating the rapid onset of exponential growth characteristic of diatom recovery from nutrient starvation.
"It appears that the animal urea cycle, critical for cellular export of carbon and nitrogen wastes, was co-opted from an ancestral pathway that originally evolved as a nitrogen and carbon recycling and recovery mechanism," says Allen.
"This is a very interesting finding we didn't expect to see, and essentially changes the way we view diatoms relative to animals and plants."
The work also sug
|Contact: Cheryl Dybas|
National Science Foundation