Arthur J. Villasanta – Fourth Estate Contributor
Chapel Hill, NC, United States (4E) – Life on Earth didn’t originate from nucleic acids as is widely accepted but from an intimate partnership between the nucleic acids (genetic instructions for all organisms) and small proteins called peptides.
This, according to two new papers (one in Molecular Biology and Evolution, the other in Biosystems) from biochemists and biologists at the University of North Carolina at Chapel Hill and the University of Auckland.
Their “peptide-RNA” hypothesis contradicts the widely-held “RNA-world” hypothesis that states life originated from nucleic acids and only later evolved to include proteins.
The widely accepted RNA-world theory posits that RNA (or the molecule that plays roles in coding, regulating, and expressing genes) elevated itself from the primordial soup of amino acids and cosmic chemicals. RNA eventually gave rise to short proteins called peptides and then to single-celled organisms.
But it’s still a mystery how amino acid building blocks were first assembled according to coded nucleic acid templates into the proteins that formed the machinery of all cells.
The new papers show how recent experimental studies of two enzyme superfamilies surmount the tough theoretical questions about how complex life emerged on Earth more than four billion years ago.
The special attributes of the ancestral versions of these enzyme superfamlies, and the self-reinforcing feedback system they would have formed with the first genes and proteins, would have kick-started early biology and driven the first life forms toward greater diversity and complexity, said researchers.
Considered the machines of life, proteins catalyze and synchronize the chemical reactions inside cells.
“Until now, it has been thought to be impossible to conduct experiments to penetrate the origins of genetics,” said co-author Dr. Charles Carter, PhD, professor of biochemistry and biophysics at the UNC School of Medicine.
“But we have now shown that experimental results mesh beautifully with the ‘peptide-RNA’ theory, and so these experiments provide quite compelling answers to what happened at the beginning of life on Earth.”
Co-author Peter Wills, PhD, professor of physics at the University of Auckland, said that compared to the RNA-world hypothesis, “what we’ve outlined is simply a much more probable scenario for the origin of life. We hope our data and the theory we’ve outlined in these papers will stimulate discussion and further research on questions relevant to the origins of life.”
The two scientists are fully aware that the RNA-world hypothesis still dominates the origin-of-life research field.
“That theory is so alluring and expedient that most people just don’t think there’s any alternative,” said Carter. “But we are very confident there is.”
Before there was life on Earth, there were simple chemicals. Somehow, they produced both amino acids and nucleotides that eventually became the proteins and nucleic acids necessary to create single cells.
And the single cells became plants and animals. Research this century has revealed how the primordial chemical soup created the building blocks of life. There is also widespread scientific consensus on the historical path by which cells evolved into plants and animals.
Carter and Wills argue that RNA could not kick-start this process alone because it lacks a property they call “reflexivity.” It can’t enforce the rules by which it’s made.
RNA needed peptides to form the reflexive feedback loop necessary to lead eventually to life forms.
At the heart of the peptide-RNA theory are enzymes so ancient and important that their remnants are still found in all living cells and even in some sub-cellular structures, including mitochondria and viruses.
There are 20 of these ancient enzymes called aminoacyl-tRNA synthetases (aaRSs).
Each of them recognizes one of the 20 amino acids that serve as the building blocks of proteins. (Proteins, considered the machines of life, catalyze and synchronize the chemical reactions inside cells.)
In modern organisms, an aaRS effectively links its assigned amino acid to an RNA string containing three nucleotides complementary to a similar string in the transcribed gene. The aaRSs thus play a central role in converting genes into proteins, a process called translation that is essential for all life forms.
The 20 aaRS enzymes belong to two structurally distinct families, each with 10 aaRSs.
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