That’s why Tobias Warnecke, who research archaeal histones at Imperial College London, thinks that “there’s something special that must have happened at the dawn of eukaryotes, where we transition from just having simple histones … to having octameric nucleosomes. And they seem to be doing something qualitatively different.”

What that’s, nonetheless, continues to be a thriller. In archaeal species, there are “quite a few that have histones, and there are other species that don’t have histones. And even those that do have histones vary quite a lot,” Warnecke stated. Last December, he printed a paper exhibiting that there are diverse variants of histone proteins with completely different features. The histone-DNA complexes differ in their stability and affinity for DNA. But they aren’t as stably or frequently organized as eukaryotic nucleosomes.

As puzzling as the variety of archaeal histones is, it gives a chance to know the completely different attainable methods of constructing methods of gene expression. That’s one thing we can not glean from the relative “boringness” of eukaryotes, Warnecke says: Through understanding the combinatorics of archaeal methods, “we can also figure out what’s special about eukaryotic systems.” The number of completely different histone sorts and configurations in archaea may additionally assist us deduce what they may have been doing earlier than their position in gene regulation solidified.

A Protective Role for Histones

Because archaea are comparatively easy prokaryotes with small genomes, “I don’t think that the original role of histones was to control gene expression, or at least not in a manner that we are used to from eukaryotes,” Warnecke stated. Instead, he hypothesizes that histones may need protected the genome from injury.

Archaea usually reside in excessive environments, like scorching springs and volcanic vents on the seafloor, characterised by excessive temperatures, excessive pressures, excessive salinity, excessive acidity or different threats. Stabilizing their DNA with histones could make it tougher for the DNA strands to soften in these excessive situations. Histones additionally may shield archaea in opposition to invaders, equivalent to phages or transposable components, which might discover it tougher to combine into the genome when it’s wrapped across the proteins.

Kurdistani agrees. “If you were studying archaea 2 billion years ago, genome compaction and gene regulation are not the first things that would come to mind when you are thinking about histones,” he stated. In truth, he has tentatively speculated a few completely different sort of chemical safety that histones may need supplied the archaea.

Last July, Kurdistani’s workforce reported that in yeast nucleosomes, there’s a catalytic web site on the interface of two histone H3 proteins that may bind and electrochemically scale back copper. To unpack the evolutionary significance of this, Kurdistani goes again to the huge improve in oxygen on Earth, the Great Oxidation Event, that occurred across the time that eukaryotes first developed greater than 2 billion years in the past. Higher oxygen ranges will need to have triggered a world oxidation of metals like copper and iron, that are essential for biochemistry (though poisonous in extra). Once oxidized, the metals would have turn out to be much less accessible to cells, so any cells that saved the metals in lowered type would have had a bonus.

During the Great Oxidation Event, the power to scale back copper would have been “an extremely valuable commodity,” Kurdistani stated. It may need been significantly engaging to the micro organism that had been forerunners of mitochondria, since cytochrome c oxidase, the final enzyme in the chain of reactions that mitochondria use to supply vitality, requires copper to operate.

Because archaea reside in excessive environments, they may have discovered methods to generate and deal with lowered copper with out being killed by it lengthy earlier than the Great Oxidation Event. If so, proto-mitochondria may need invaded archaeal hosts to steal their lowered copper, Kurdistani suggests.

The speculation is intriguing as a result of it may clarify why the eukaryotes appeared when oxygen ranges went up in the environment. “There was 1.5 billion years of life before that, and no sign of eukaryotes,” Kurdistani stated. “So the idea that oxygen drove the formation of the first eukaryotic cell, to me, should be central to any hypotheses that try to come up with why these features developed.”