11:19pm Saturday 25 May 2019



Taking their hints from relics of this evolution left behind in modern cells, researchers at the Salk Institute for Biological Studies concluded that after only two waves of “matching” and some last minute fiddling, all 20 commonly used amino acids were firmly linked with their respective codons, setting the stage for the emergence of proteins with unique, defined sequences and properties.

ribosome  structures

Analysis of ribosome structures (shown on the left) from four different species revealed a non-random affinity between anticodon-containing RNA triplets and their respective amino acids (shown on the right) .

Courtesy of David Johnson, Salk Institute for Biological Studies.

Their findings, which will be published in next week’s online edition of the Proceedings of the National Academy of Sciences, provide the first in vivo data shedding light on the origin and evolution of the genetic code.



Cells provide a dazzling variety of functions that cover all of our body’s needs, yet they make do with a very limited number of molecular building blocks. With few exceptions, all known forms of life use the same common 20 amino acids—and only those 20—to keep alive organisms as diverse as humans, earthworms, tiny daisies, and giant sequoias.

ribosome structures

Ribosomes bring together all the mechanical machinery necessary for translation. They couple tRNAs to their proper codons on the mRNA and facilitate the formation of peptide bonds between amino acids.

Image: Courtesy of Jamie Simon, Salk Institute for Biological Studies

Each of the 20 amino acids is matched to its own carrier molecule known as transfer RNA (tRNA). During protein synthesis, which is coordinated by so-called ribosomes, amino acids are brought out one by one by their respective tRNAs and inserted in the growing protein chain according to the instructions spelled out in the universal language of life—the genetic code. The code is “read” with the help of anticodons embedded in each tRNA, which pair up with their codon-counterparts.

Several hypotheses have been put forward to explain why codons are selectively assigned to specific amino acids. “One of the theories, the stereochemical hypothesis, gained some traction when researchers could show that short codon- or anticodon-containing polynucleotide molecules like to interact with their respective amino acids,” says graduate student and first author David B. F. Johnson.


“Also, the ribosome emerged from an early evolutionary stage of life to help with the translation of the genetic code before the last universal common ancestor,” explains Wang, “and therefore is more likely to serve as a molecular fossil that preserved biological evidence.”

When Wang and Johnson probed bacterial ribosomes for imprints of the genetic code, they found evidence that direct interactions between amino acids and nucleotide triplet anticodons helped establish matching pairs. “We now believe that the genetic code was established in two different stages,” says Johnson.



The work was supported in part by the Searle Scholar Program, the Beckman Young Investigator Program and the National Institutes of Health Director’s New Innovator Award.



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