The secret role of histones in the evolution of complex cells

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This is why Tobias WarneckeThose who study archaeal histones at Imperial College London think, “At the beginning of the emergence of eukaryotes, we must change from simple histones… to nucleosomes with octamers. And they seem to Doing something fundamentally different.”

However, that is still a mystery. Among archaeal species, “there are many species with histones, and there are other species without histones. Even those with histones are very different.” Warneck said.In December last year, he published a paper stating Multiple variants of histones Have different functions. The stability and affinity for DNA of histone-DNA complexes vary. But they are not organized stably or regularly like eukaryotic cell nuclei.

Although the diversity of archaeal histones is confusing, it provides an opportunity to understand the different possible ways of constructing gene expression systems. Warnecke said that this is something we can’t gather from the relative “boringness” of eukaryotes: by understanding the combination of archaea systems, “we can also find out the special features of eukaryotic systems.” There are various differences in archaea. The type and configuration of histones may also help us infer what they may be doing before the consolidation of gene regulation.

Protective effects of histones

Because archaea are relatively simple prokaryotes with a small genome, “I don’t think the initial role of histones is to control gene expression, or at least not in the way we are used to extracting them from eukaryotes,” Warnecke said . Instead, he hypothesized that histones might protect the genome from damage.

Archaea usually live in extreme environments, such as hot springs and volcanic craters on the sea floor, characterized by high temperature, high pressure, high salinity, high acidity or other threats. Stabilizing its DNA with histones may make the DNA strands more difficult to melt under those extreme conditions. Histones can also protect archaea from invaders such as bacteriophages or transposable elements. When the invaders wrap them around the protein, it will be difficult for them to integrate into the genome.

Kurdistani agreed. He said: “If you were studying archaea 2 billion years ago, when you consider histones, you would not think of genome tightening and gene regulation in the first place.” In fact, he has tentatively speculated that histones may be archaea. Provides another chemical protection.

Last July, Kurdistani’s team reported that in yeast nucleosomes, there is a catalytic site at the interface of two histone H3 proteins that can bind to and electrochemically reduce copper. In order to reveal the significance of this evolution, the Kurdistans reviewed the massive increase in oxygen on Earth, the Great Oxidation Event, which occurred approximately 2 billion years ago when eukaryotes first evolved. Higher oxygen levels are bound to cause overall oxidation of metals such as copper and iron, which is essential for biochemistry (albeit toxic). Once oxidized, the metal will become less and less easily used by cells, so any cell that keeps the metal in a reduced form will have an advantage.

Kurdistanni said that in a major oxidation event, the ability to reduce copper will be “a very valuable commodity.” It may be particularly attractive to mitochondrial pioneer bacteria, because cytochrome c oxidase is the last enzyme in the chain of reactions used by mitochondria to produce energy, and it requires copper to function.

Because archaea live in extreme environments, they may have found a way to produce and process reduced copper without being killed by them long before the major oxidation event. Kudistani believes that if this is the case, mitochondria may have invaded the archaeal host to steal reduced copper.

Siavash Kurdistani, a biochemist at the University of California, Los Angeles, speculated how the catalytic ability of certain histones supports the generation of eukaryotic endosymbiosis.Photo: Reed Hutchinson/University of California, Los Angeles (UCLA) Generalized Stem Cell Research Center

This hypothesis is very attractive because it can explain why eukaryotes appear when the oxygen content in the atmosphere rises. Cudistani said: “Before that there were 1.5 billion years of life, and there were no signs of eukaryotes.” “So, for me, the idea that oxygen promotes the formation of the first eukaryotic cells should be all attempts The core of the hypothesis that puts forward the reasons for these characteristics.”

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