Mao na Tobias Warnecke, who studies archaeal histones at Imperial College London, thinks that “something special must have happened at the dawn of eukaryotes, where we moved from just having simple histones… to having octameric nucleosomes. And they seem to be doing something different. ”
What that is, however, is still a mystery. Of the archaeal species, there are “several with histones, and there are other species without histones. And even those with histone vary, ”Warnecke said. Last December, he published a paper showing that there was different variations in histone proteins with different functions. Histone-DNA complexes vary in their strength and relevance to DNA. But they are not as robust or regularly organized as eukaryotic nucleosomes.
Not surprisingly the diversity of archaeal histones, it provides an opportunity to identify different possible ways of constructing gene expression systems. That’s something we can’t get out of the relative “boredom” of eukaryotes, Warnecke says: By understanding combinatorial archaeological systems, “we can also learn what’s special about eukaryotic systems.” Different histone species and archaea configurations may also help us determine what they may have been doing prior to their role in gene regulation.
A Protective Role for Histones
Because archaea are relatively simple prokaryotes with a small genome, “I don’t think the original role of histones was to inhibit gene expression, or at least not in the way we’re used to from eukaryotes,” he said. by Warnecke. However, he thinks histones may be protecting the genome from damage.
Archaea often live in harsh environments, such as hot springs and volcanic vents on the sea floor, characterized by high temperatures, high pressure, high salt, high acidity or other threats. Strengthening their DNA with histones can make it more difficult to digest DNA fibers in severe conditions. Histones can also protect the archaea against invaders, such as phages or immovable elements, which are found to be more difficult to integrate into the genome when they are packed with proteins.
Kurdistani agrees. “If you studied archaea 2 billion years ago, genome integration and gene regulation aren’t the first things you think about when you think about histones,” he said. In fact, he has a tentative hypothesis about a very different chemical protection that might be offered by archaea histones.
Last July, Kurdistani’s group reported that in the yeast nucleosome, there is a catalytic interface area of the two histone proteins of 0.3 that binds and electrochemically extracts copper. To decipher the significance of its evolution, Kurdistani goes back to the massive rise in oxygen on Earth, the Great Occasion of Oxidation, which occurred at a time when eukaryotes first evolved more than 2 billion years ago. Higher oxygen levels cause a global oxidation of metals such as copper and iron, which are essential for biochemistry (even if excessively toxic). Once oxidized, metals can become less usable to cells, so any cells that prevent metals from depleting may have an advantage.
During the Oxidation Event, the ability to reduce copper could have been “an even more valuable product,” Kurdistani said. This may be especially attractive to mitochondria-predominant bacteria, because cytochrome c oxidase, the last enzyme in the chain of reactions used by mitochondria to produce energy, requires copper to function.
Since archaea live in harsh environments, they may have found ways to produce and control depleted copper that had not been killed before the Great Oxidation Event. If so, the proto-mitochondria may have invaded archaeal hosts to steal their reduced copper, Kurdistani suggests.
The hypothesis is interesting because it could explain why eukaryotes show an increase in oxygen levels in the air. “There were 1.5 billion years of life before that, and there was no sign of eukaryotes,” Kurdistani said. “That’s why the idea that oxygen triggers the formation of the first eukaryotic cell, to me, should be crucial to any hypothesis trying to figure out why these forms developed.