Marshall Nirenberg, not a member of the RNA tie club, nevertheless had interest to try to crack the DNA/RNA code. To do this he hit upon a brilliant idea. He produced RNA consisting only of one base – uracil (a nucleotide specific only to RNA). Thy added this to a mixture containing the cellular machinery for protein synthesis, extracted from E.coli (i.e. they destroyed the cell membranes of E.coli and removed the RNA and DNA). They then added the 20 amino acids, but being sure to radiolabel only one of them. They consequently found that in the extract containing the radioactively labeled phenylalanine, the resulting protein was also radioactive leading them to conclude that that they had found the genetic code for phenylalanine: UUU (three uracil bases in a row) on RNA.
Within a few years, his research team had performed similar experiments and found that three-base repeats of adenosine (AAA) produced the amino acid lysine, and cytosine repeats (CCC) produced proline.
This finding led to the race to discover the whole genetic code for all 50 codes and their respective 20 amino acids. He was awarded the Nobel prize in 1968 for this feat.