In 1961, Marshall Nirenberg and Heinrich Matthaei discovered the genetic code of living beings, which confirms that the information contained in DNA is expressed in proteins according to a defined pattern. These researchers at the U.S. National Institutes of Health (NIH) made one of the scientific milestones that changed our view of how life develops from a biological point of view. With a very creative experiment, simple and full of beauty. The best minds were attacking the problem from different angles, mostly genetic, but it was the biochemical approach that came up with the solution.
The experiment aimed to synthesize proteins in vitro from its main components: amino acids. Proteins are molecules that exert a specific function in or outside the cell and trigger a large part of the physiological effects that enable life.
To achieve their synthesis, they used the contents of bacterial lysis Escherichia coli to which they were incorporating various RNA molecules and the 20 amino acids that exist. In each experiment, 19 amino acids were “cold” (non-radioactive) and one was “hot” (radioactively labeled with Carbon-14 for later detection). The “hot” amino acid was changed in each round of the experiment, trying to determine which amino acids would be incorporated into a protein after the addition of a particular type of RNA.
The first key experiments were performed with poly-U (RNA composed solely of uridine bases, provided by Leon Heppel and Maxine Singer.
At 3 a.m. on May 27, 1961, Matthaei used phenylalanine as the “hot” amino acid. After one hour, the control tube (without poly-U) showed a background level of 70 counts, while the tube with added poly-U showed 38.000 counts per milligram of protein.
Subsequent experiments showed that the 19 “cold” amino acids were not necessary and that the protein product had the biochemical characteristics of polyphenylalanine, demonstrating that a chain of repeated uracil bases produced a protein chain consisting only of the repetitive amino acid phenylalanine. Although the experiment did not determine the number of bases per codon, it was consistent with the triplet of UUU codons in the RNA encoding only phenylalanine.
In analogous experiments with other RNAs, they found that poly-C directed polyproline synthesis. Nirenberg recalls that the laboratories of Severo Ochoa and James Watson had previously performed similar experiments with poly-A, but failed to detect protein synthesis because poly-L-lysine (unlike most proteins) is soluble in trichloroacetic acid.
Furthermore, using synthetic RNAs that randomly incorporated two bases in different ratios, they produced proteins containing more than one type of amino acid, from which they were able to deduce the nature of triplet of the genetic code and reduce codon possibilities for other amino acids. Nirenberg’s group finally deciphered all amino acid codons in 1966, although this required additional ingenious experimental methods.
Through these experiments it was concluded that the information of 3 nucleotides present in RNA (and therefore, according to the central dogma of biology, also in DNA) was what provided an amino acid to be incorporated into the protein chain and thus synthesize each of the proteins present in living beings.
One more of the brilliant ideas that have marked a before and after in biotechnology and that have allowed projects such as OLIGOFASTX to exist today.
Header image: Creative Commons license
Russell P. (2010). iGenetics: A Molecular Approach, 3rd edition. Pearson/Benjamin Cummings.
^ Leavitt, Sarah A. (2004). “Deciphering the Genetic Code: Marshall Nirenberg. The Coding Craze”. Stetten Museum, Office of NIH History. Archived from the original on 9 February 2020. Retrieved 2009-10-05.
^ Yanofsky C. (2007). “Establishing the Triplet Nature of the Genetic Code” (PDF). Cell. 128 (5): 815–818. doi:10.1016/j.cell.2007.02.029. PMID 17350564. Retrieved 2018-01-24.
^ Crick FH, Barnett L, Brenner S, Watts-Tobin RJ (December 1961). “General nature of the genetic code for proteins” (PDF). Nature. 192 (4809): 1227–32. Bibcode:1961Natur.192.1227C. doi:10.1038/1921227a0. PMID 13882203. S2CID 4276146.
^ Matthaei, H.J., Jones, O.W., Martin, R.G., and Nirenberg, M.W. Vol. 48 No. 4 (1962). “Characteristics and Composition of RNA Coding Units”. Proceedings of the National Academy of Sciences of the United States of America. 48 (4): 666–677. Bibcode:1962PNAS…48..666M. doi:10.1073/pnas.48.4.666. PMC 220831. PMID 14471390.