New research supports RNA-protein as the origin of the living world

The researchers were inspired by ribosomes, which show strands of ribosomal translational RNA. Image credit: Omikron/Science Photo Library

On May 11, a new study published in Nature said chemists had solved a key problem in the theory of the origin of life, demonstrating that RNA molecules could bind short-chain amino acids together.

Bill Martin, who studies molecular evolution at the University of Düsseldorf in Germany, said the discovery opens up vast and fundamental new avenues for exploring early chemical evolution.

The study supports another claim based on the “RNA world” hypothesis, which holds that the first organisms were based on RNA strands before DNA and the proteins it encoded evolved. Standard theory suggests that in the “RNA world,” life may exist in the form of complex strands of primitive RNA that can both replicate themselves and compete with other strands. Later, these “RNA enzymes” may have evolved the ability to make proteins and eventually convert their genetic information into more stable DNA.

But how this process happens remains questionable, in part because the efficiency of catalysts composed of RNA alone is far lower than that of the protein enzymes found in all living cells today. Thomas Carell, a corresponding author of the study and an organic chemist at the University of Munich in Germany, believes that despite the discovery of RNA catalysts, their catalytic power is poor.

In studying this conundrum, the researchers were inspired by the role that RNA plays in all modern organisms in making proteins: strands of RNA encoding genes (usually copied from SEQUENCEs of DNA bases) pass through ribosomes, which produce amino acids at a time to form the corresponding proteins.

Unlike most enzymes, the ribosome itself is made up not only of proteins, but also of RNA fragments that play an important role in the synthesis of proteins. In addition, ribosomes contain modified versions of standard RNA nucleosides A, C, G, U.

By linking two segments of RNA commonly found in living cells, the researchers constructed a synthetic RNA molecule that included two modified nucleosides. At the first specific nucleoside site, the synthetic molecule can bind to one amino acid, which then shifts sideways to bind to a neighboring second specific nucleoside. Subsequently, the researchers isolated the original RNA strands and introduced a new strand of RNA that carries its own amino acids and forms strong covalent bonds with amino acids previously attached to the second strand.

Step by step, this process produces a short chain of amino acids, a mini protein, a peptide, that attaches to the RNA. The formation of chemical bonds between amino acids requires energy, which the researchers provide by excitation of amino acids in solution with various reactants.

Martin said it was a very exciting discovery. “Not only does it point to a new path for the formation of RNA-based peptides, the discovery also reveals new evolutionary implications for naturally occurring RNA-modified bases.” Martin added that the results suggest that RNA played an important role in the origin of life.

Loren Williams, a biophysical chemist at the Georgia Institute of Technology in the United States, agrees, arguing that if the origin of RNA and the origin of proteins are linked, and their emergence is not independent, then this will fundamentally turn to support the “RNA-protein world” and move away from the “RNA world”.

To prove that this is a reasonable origin of life, scientists must go through several steps further. The peptides formed on the team’s RNA are made up of random sequences of amino acids, rather than being determined by information stored in the RNA. Carell said larger RNA structures can fold into shapes that “recognize” specific amino acids at specific locations, producing defined structures. These complex RNA-peptide mixtures may have catalytic properties and become more efficient under evolutionary pressures. Carell believes that if molecules can be replicated, similar micro-organisms will be produced. (Source: China Science Daily Xin Yu)

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