Genome complexity: histone evolution and role (Introduction)

by David Turell @, Monday, May 10, 2021, 23:06 (1052 days ago) @ David Turell

They came from Archaea in a simpler form and play a major role in decoding DNA in eukaryotes:

https://www.quantamagazine.org/dnas-histone-spools-hint-at-how-complex-cells-evolved-20...

"In the biology of complex cells, or eukaryotes, the ballet of molecules that transcribe and translate genomic DNA into proteins holds centerstage, but that dance would be impossible without the underappreciated work of histone proteins gathering up the DNA into neat bundles and unpacking just enough of it when needed.

"Histones, as linchpins of the apparatus for gene regulation, play a role in almost every function of eukaryotic cells. “In order to get complex, you have to have genome complexity, and evolve new gene families, and you have to have a cell cycle,” explained William Martin, an evolutionary biologist and biochemist at Heinrich Heine University in Germany. “And what’s in the middle of all this? Managing your DNA.”

"New work on the structure and function of histones in ancient, simple cells has now made the longstanding, central importance of these proteins to gene regulation even clearer. Billions of years ago, the cells called archaea were already using histones much like our own to manage their DNA — but they did so with looser rules and much more variety. From those similarities and differences, researchers are gleaning new insights, not only into how the histones helped to shape the origins of complex life, but also into how variants of histones affect our own health today.

"Eukaryotes arose about 2 billion years ago, when a bacterium that could metabolize oxygen for energy took up residence inside an archaeal cell. That symbiotic partnership was revolutionary because energy production from that proto-mitochondrion suddenly made expressing genes much more metabolically affordable, Martin argues. The new eukaryotes suddenly had free rein to expand the size and diversity of their genomes and to conduct myriad evolutionary experiments, laying the foundation for the countless eukaryotic innovations seen in life today. “Eukaryotes are an archaeal genetic apparatus that survives with the help of bacterial energy metabolism,” Martin said.

***

"The four primary histones of eukaryotes — H2A, H2B, H3 and H4 — assemble into octamers with two copies of each. These octamers, called nucleosomes, are the basic units of eukaryotic DNA packaging.

"By curving the DNA around the nucleosome, the histones prevent it from clumping together and keep it functional.

***

"Their observations confirmed that the structures of archaeal nucleosomes are less fixed. Eukaryotic nucleosomes are always stably wrapped by about 147 base pairs of DNA, and always consist of just eight histones. (For eukaryotic nucleosomes, “the buck stops at eight,” Luger said.) Their equivalents in archaea wind up between 60 and 600 base pairs. These “archaeasomes” sometimes hold as few as three histone dimers, but the largest ones consist of as many as 15 dimers.

***

"Kurdistani’s conjecture also suggests an alternative hypothesis for why eukaryotic genomes got so big. The histones’ copper-reducing activity only occurs at the interface of the two H3 histones inside an assembled nucleosome wrapped with DNA. “I think there’s a distinct possibility that the cell wanted more histones. And the only way to do that was to expand this DNA repertoire,” Kurdistani said. With more DNA, cells could wrap more nucleosomes and enable the histones to reduce more copper, which would support more mitochondrial activity. “It wasn’t just that histones allowed for more DNA, but more DNA allowed for more histones,” he said.

***

"Although Warnecke is not convinced that the viral histones tell us much about the origin of eukaryotic histones, he is fascinated by their possible functions. One possibility is that they help to compact the viral DNA; another idea is that they could be disguising the viral DNA from the host’s defenses.

"Histones have had myriad roles since the dawn of time. But it was really in the eukaryotes that they became the linchpins for complex life and countless evolutionary innovations. That’s why Martin calls the histone “a basic building block that never could realize its full potential without the help of mitochondria.'”

Comment: A long paper describing how Archaeal histones might have evolved into eukaryotic forms. This paper exposes how important the histones are to genomic functioning.


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