RNA, then, was the molecule that directed protein production. At some point Nirenberg hypothesized that if he could introduce a specific, known RNA triplet into a cell-free system, and if the system responded by synthesizing a distinct amino acid, then he would have a key to unlocking the genetic code. Others at the NIH were making strings of synthetic nucleotides, long-chain molecules that repeated the same base: AAAAA ... (also known as poly-A); TTTTT ... (poly-T); and so on.
Nirenberg got hold of a quantity of poly-U (in RNA, uracil replaces DNA’s thymine), and he wrote up an experimental protocol for Matthaei to carry out. And so it happened that late one night in May 1961, Matthaei added a quantity of poly-U into a cell-free system.
It was a historic moment: the cell sap reacted by churning out the amino acid phenylalanine. One codon had been deciphered, and the triplet UUU became the first word in the chemical dictionary of life.
“That was really staggering,” Nirenberg recalls today.
He announced the result in August 1961 at a biochemical congress in Moscow. Soon afterward Nirenberg had competition: Nobelist Severo Ochoa of the New York University School of Medicine set up his own lab and started deciphering the code, too. Ochoa continued until 1964, when at a meeting of the American Chemical Society both he and Nirenberg spoke. At that point, each scientist had discovered the base compositions, but not the sequences, of many of the codons. Ochoa spoke first and reported the compositions of some of them. “I was the next speaker,” Nirenberg remembers. “I described a simple assay that could be used to determine the nucleotide sequences of RNA codons. Ochoa then stopped working on the genetic code.”
By 1966, with the aid of key contributions from Holley and Khorana, Nirenberg had identified both the compositions and base sequences of all the genetic code’s 64 trinucleotides. For this achievement, he shared the Nobel Prize in 1968; however, he somehow became the Forgotten Father of the Genetic Code.
Why? “Personality, I guess,” Nirenberg says. “I’m shy, retiring. I like to work, and I’ve never gone out of my way to try to publicize myself. Crick told me I was stupid because I never was after the limelight.” In addition, Watson and Crick’s discovery yielded a simple, visually stunning image: a gleaming molecular spiral staircase. The genetic code, in contrast, was a mazeworks of forbidding chemical names, codons and complex molecular functions—a publicist’s nightmare.
In Nirenberg’s own mind, anyway, he had better things to do than burnish his reputation. He turned his talents to the brain. In particular, he wanted to discover how axons and dendrites found one another during embryonic development and how they wired up correctly.
To find out, he and his colleagues established thousands of nerve cell lines, including cells that were hybrids of muscle and nerve. He found that by electrically stimulating a nerve cell, he could record a response across a synapse with striated muscle cells—the cell-level equivalent of Luigi Galvani’s getting frog muscles to move in the 18th century. Experiments on fruit flies revealed the existence of four new genes, NK-1 through NK-4, that regulate the differentiation of embryonic nerve cells called neuroblasts.
Nirenberg has racked up 71 publications in neurobiology over the past 20 years. But for all that productivity, those studies will likely never eclipse his cracking of the language of A, T, G and C. That he is not well known for it does not seem to faze him. “Deciphering the genetic code was fantastic fun,” he says. “I mean, it was really thrilling.” Fame may be fleeting, but the genetic code will endure for as long as there is life.
This article was originally published with the title The Forgotten Code Cracker.