Scientific understanding of the molecular basis of life increased dramatically after Oswald T. Avery's 1944 discovery that deoxyribonucleic acid (DNA) was the "transforming principle" and Francis Crick and James Watson's 1953 description of DNA's "double helix" structure. The process, however, by which DNA replicates itself during cellular reproduction, or how DNA expresses its genetic information, was still a mystery in the late 1950s. Many scientists theorized that the answer lay in the interaction between DNA and ribonucleic acid (RNA), but how this occurred, or how RNA translated information from DNA, was still then unknown. Such questions were very much in vogue at the time among both bacterial geneticists such as Joshua Lederberg" and protein chemists such as Christian Anfinsen.
In 1959, the RNA question also piqued Marshall Nirenberg's intellectual curiosity. Like formally trained geneticists, Nirenberg wanted to know if RNA was the chemical liaison or "messenger" between DNA, the genetic material, and proteins, the building blocks of cells. Nirenberg, however, had no formal training in molecular genetics. He had only attended evening courses about genetics, intended for scientists at the National Institutes of Health who were interested in conducting interdisciplinary research. Many of Nirenberg's colleagues felt that it was naive for a biochemist untrained in molecular genetics to commence a brand-new area of research; at least one felt that Nirenberg was committing "professional suicide."
In 1960, Nirenberg was joined by J. Heinrich Matthaei, a postdoctoral researcher from the University of Bonn in Germany then studying at Cornell University. They studied nucleotides--adenosine, cytosine, guanine, and thymine--the basic molecular units that are strung together to form long linear molecules of DNA and (with uracil substituted for thymine) RNA. Following recent work by the Swiss geneticist Alfred Tissieres, Nirenberg and Matthaei set out to create a "cell-free environment" that would let them examine how the minute mechanisms of RNA actually work, free of the normal biological processes of cells that could obscure molecular activities. After working with several possible organisms to serve as models, they settled on Escherichia coli, a common bacterium found in abundance in the human colon.
Nirenberg and Matthaei created a synthetic RNA molecule outside the bacterium and introduced this RNA to E. coli. They found that their synthetic RNA specified that phenylalanine, an amino acid, be added to the end of a growing strand of linked amino acids, the precursor to proteins. Nirenberg and Matthaei concluded that traces of uracil had directed the synthesis of phenylalanine. On the RNA strand, synthetic RNA made of multiple batches of three units of uracil directed an amino acid chain composed entirely of phenylalanine. One three-unit batch of uracil could be read as UUU (poly-U), which was a three-letter shorthand method or "code word" for identifying phenylalanine. Nirenberg and Matthaei quickly realized that this was the messenger that they had been looking for. Their experiments proved that "messenger RNA," which transcribes genetic information from DNA, directs protein synthesis. That is, messenger RNA transmits the DNA messages that prescribe the assembly of amino acids into the complex proteins that drive living processes.
In August 1961, Nirenberg and Matthaei published their now-classic essay, "The Dependence of Cell-Free Protein Synthesis in E. Coli upon Naturally Occurring or Synthetic Polyribonucleotides," in the Proceedings of the National Academy of Sciences. In that same month, Nirenberg presented a version of his findings with the poly-U experiments to a small group of about thirty scientists at the International Congress of Biochemistry in Moscow. Viewers can see the original draft of Nirenberg's address to the Moscow Congress in the Documents section. Francis Crick, who was in attendance at the Moscow meeting, had heard that Nirenberg and Matthaei had found a clue that might unravel one of the central mysteries of molecular genetics. Crick arranged to have the young scientist deliver his paper again, this time to the assembled body of about a thousand people. By the end of the Moscow conference, the discovery made the obscure and mild-mannered NIH scientist a veritable celebrity.
By January 1962, interviews and photographs featuring Nirenberg and Matthaei appeared in scientific journals, newspapers, and weekly magazines around the world. UUU was described as the first word in the chemical dictionary of life, and the key to deciphering the entire genetic code. The poly-U experiments also made Nirenberg a much sought-after speaker at research institutes. He was invited to participate in the first-ever symposium devoted exclusively to the RNA code, which was held at Indiana University in January 1962. Viewers can see original documents related to this historic symposium in the Documents section. Despite the professional and social demands made upon him, Nirenberg also found time to encourage young scientists. In many cases, he invited them to observe and work with him.