Malaysian Society of RNA Science

The discovery of messenger RNA (mRNA) unfolded over decades through collaborative efforts and key experiments. In 1944, Avery, McLeod, and McCarty identified DNA as the hereditary material, but the mechanism of gene function remained unclear. Watson and Crick’s 1953 proposal that DNA’s base sequence contained genetic information raised questions about how this information translated into biological function. Early theories, such as George Gamow’s idea of direct protein synthesis on DNA, were dismissed. Instead, based on work by Brachet and Caspersson, Crick believed RNA played a role in cytoplasmic protein synthesis. André Boivin (1947) and Alexander Dounce (1952) further suggested RNA as an intermediary between DNA and proteins, though its exact role was undefined.

In the 1950s, ribosomes were identified as RNA-rich particles, with ribosomal RNA initially thought to be the intermediary. However, experiments by Jeener, Monod, Hershey, and Volkin and Astrachan (1956) hinted at a short-lived RNA intermediary. Volkin and Astrachan’s discovery of “DNA-like RNA” in phage-infected E. coli was particularly significant. The concept of a messenger molecule emerged from the PaJaMo experiments (1957–1960), where Jacob, Monod, and Pardee (the Paris group) proposed a “cytoplasmic messenger” (X) carrying genetic information to ribosomes.

A breakthrough came in 1960 when Brenner and Crick linked Volkin and Astrachan’s findings to the Paris group’s messenger hypothesis, proposing that ribosomes were inert “reading heads” and mRNA carried genetic information. Jacob and Monod formally named this molecule “messenger RNA” (mRNA) later that year. The Brenner-Jacob-Meselson experiment (1961) confirmed mRNA’s existence by showing transient RNA copied from phage DNA associated with ribosomes. Simultaneously, Watson’s group and Spiegelman and Hall provided further evidence, with the latter demonstrating sequence complementarity between phage DNA and RNA.

In 1961, Jacob and Monod published a comprehensive review outlining mRNA’s theoretical framework, while Nirenberg and Matthaei’s experiments using synthetic RNA (poly(U)) proved mRNA’s function by directing polyphenylalanine synthesis, cracking the first “word” of the genetic code. The discovery of mRNA thus integrated insights and experiments from multiple researchers, culminating in a unified understanding of its role in gene expression.

Ref: Cobb, M. (2015). Who discovered messenger RNA?. Current Biology, 25(13), R526-R532.

The discovery of tRNA originated from studies on protein synthesis in the 1950s. In 1956, Paul Zamecnik and Mahlon Hoagland were investigating how amino acids are assembled into proteins. They identified an enzyme, later called the pH 5 enzyme, that activated amino acids using ATP, producing aminoacyl-adenylates, an essential step in protein synthesis.

In 1957, Hoagland, Ogata, and Nohara discovered that this enzyme also transferred amino acids onto an unknown RNA molecule. Further experiments confirmed that this RNA was an essential carrier of amino acids in protein formation. Zamecnik and his colleagues initially named it soluble RNA (sRNA), but it was later renamed transfer RNA (tRNA) as its function became clearer.

The connection between tRNA and protein synthesis was further established when, in 1958, Berg, Ofengand, and Lipmann discovered aminoacyl-tRNA synthetases, enzymes that specifically attach amino acids to their corresponding tRNA molecules. This process ensures the accurate delivery of amino acids during protein formation.

In the same year (1958), Hans Zachau in Lipmann’s lab found that amino acids attach to the 3′-end (CCA sequence) of tRNA. Meanwhile, Francis Crick had proposed the Adaptor Hypothesis (1955–1958), suggesting that a molecule must exist to link amino acids to genetic instructions. The discovery of tRNA confirmed Crick’s idea, proving that tRNA acts as an adaptor that translates genetic code into proteins.

By the 1960s, scientists like Robert Holley began sequencing tRNA, leading to the complete sequence of tRNAAla (alanine tRNA) in 1965. This milestone provided deeper insight into how tRNA functions in protein synthesis.

Thus, research on protein synthesis (1950s–1960s) led to the discovery of tRNA, revealing its essential role in delivering amino acids to ribosomes and ensuring the accurate translation of genetic information into proteins. These discoveries laid the foundation for decoding the genetic code, marking a major milestone in molecular biology.

Ref: RajBhandary, Uttam L., and Caroline Köhrer. “Early days of tRNA research: discovery, function, purification and sequence analysis.” Journal of biosciences 31.4 (2006): 439-451.

The discovery of nucleic acids spanned decades, beginning in 1847 when Justus Liebig observed an acidic substance, later identified as inosinic acid, though its significance remained unclear. In 1869, Friedrich Miescher identified a phosphorus-rich material in cell nuclei, naming it nuclein, marking the first connection between this substance and cellular function. By 1889, Richard Altmann isolated the acidic component of nuclein and termed it nucleic acid. His work set the stage for Albrecht Kossel, who identified the chemical building blocks of nucleic acids—phosphoric acid, purine and pyrimidine bases, and a pentose sugar. Kossel’s contributions were fundamental to nucleic acid research and earned him the Nobel Prize in 1910 (https://www.nobelprize.org/prizes/medicine/1910/summary/).

By the early 1900s, researchers classified nucleic acids into two types: yeast nucleic acid, found in plants and containing uracil, and thymus nucleic acid, found in animals and containing thymine. However, early studies mistakenly believed that thymus nucleic acid contained a hexose (six-carbon) sugar instead of a pentose.

A major breakthrough in understanding nucleic acid composition came through Phoebus Levene’s work between 1908 and 1935. In 1909, he and Walter Jacobs investigated yeast nucleic acid, discovering nucleosides (sugar-base subunits) and correctly identifying d-ribose as the sugar in yeast nucleic acid, later known as RNA—which had previously been mistaken for l-xylose. However, they incorrectly assumed that ribose was present in all nucleic acids, failing to recognize that DNA contained deoxyribose, a sugar lacking one oxygen atom.

In 1929, Levene and London corrected this misconception by isolating 2-deoxy-d-ribose from thymus nucleic acid, definitively distinguishing DNA from RNA. This finding clarified that RNA contains ribose, while DNA contains deoxyribose, resolving a longstanding confusion in nucleic acid chemistry.

Levene also played a key role in establishing nucleic acids as polymeric macromolecules rather than simple, repeating tetranucleotide units. In 1908, he and Mandel proposed that nucleotides were linked in a series, leading to the tetranucleotide hypothesis. While Levene initially supported this model, he later recognized that nucleic acids were larger and more complex. Molecular weight measurements and enzymatic digestion experiments by Levene and Gerhard Schmidt confirmed that both DNA and RNA were polymers. Additionally, they pioneered ultracentrifugation to analyze nucleic acid chain lengths, reinforcing the understanding of long, linear macromolecules.

Levene’s work provided experimental proof that RNA and DNA are composed of linked nucleotides, laying the groundwork for recognizing DNA as a long, information-carrying molecule. His insights were instrumental in the eventual discovery of DNA’s double-helix structure by Watson and Crick in 1953.

Ref: Frixione E, Ruiz-Zamarripa L. The “scientific catastrophe” in nucleic acids research that boosted molecular biology. J Biol Chem. 2019;294(7):2249-2255. doi:10.1074/jbc.CL119.007397