Genome complexity: origins of DNA folding (Introduction)

by David Turell , Friday, August 11, 2017, 00:23 (2663 days ago) @ David Turell

Apparently all the way back to Archaea:

https://phys.org/news/2017-08-dna-archaea.html

"The archaeal DNA folding, described today in the journal Science, hints at the evolutionary origins of genome folding, a process that involves bending DNA and one that is remarkably conserved across all eukaryotes (organisms that have a defined nucleus surrounded by a membrane).

"Like Eukarya and Bacteria, Arachaea represent one of the three domains of life. But Archaea are thought to include the closest living relatives to an ancient ancestor that first hit on the idea of folding DNA.

"Scientists have long known that cells in all eukaryotes, from fish to trees to people, pack DNA in exactly the same way. DNA strands are wound around a 'hockey puck' composed of eight histone proteins, forming what's called a nucleosome. Nucleosomes are strung together on a strand of DNA, forming a "beads on a string" structure. The universal conservation of this genetic necklace raises the question of its origin.

"If all eukaryotes have the same DNA bending style, "then it must have evolved in a common ancestor," said study co-author John Reeve, a microbiologist at Ohio State University. "But what that ancestor was, is a question no one asked."

***

"The researchers revealed that despite using a single type of histone (and not four as do eukaryotes), the archaea were folding DNA in a very familiar way, creating the same sort of bends as those found in eukaryotic nucleosomes.

"But there were differences, too. Instead of individual beads on a string, the archaeal DNA formed a long superhelix, a single, large curve of already twisty DNA strands.
"In Archaea, you have one single building block," Luger said. "There is nothing to stop it. It's almost like it's a continuous nucleosome, really."

"This superhelix formation, it turns out, is important. When CU Boulder postdoctoral researcher Francesca Mattiroli, together with Thomas Santangelo's lab at Colorado State University, created mutations that interfered with this structure, the cells had trouble growing under stressful conditions. What's more, the cells seemed to not be using a set of their genes properly.

"'It's clear with these mutations that they can't form these stretches," Mattiroli said.
The results suggest that the archaeal DNA folding is an early prototype of the eukaryotic nucleosome.

"'I don't think there's any doubt that it's ancestral," Reeve said."

Comment: this is the best sort of evidence that evolution is a process of common descent. Archaea are the oldest of the three domains of life, and closest to original life forms.