Credit: B. DODSON

By the 1950s, scientists generally assumed that converting genetic information into the substance of life was a matter of translation. A DNA sequence, made from a combination of four kinds of nucleotides, was translated into a protein sequence, which was made from a combination of twenty kinds of amino acids. But how did this translation occur, and what machinery was involved? These were the big questions at that time.

How the answers were found includes an unexpected subplot that involves the joining of two groups of scientists with very different backgrounds and interests — biochemists and molecular biologists. Biochemists were traditional laboratory types, concerned with dissecting the cellular machinery. Molecular biologists were a new breed who were immersed in the detailed structure of large molecules and in information processing.

By 1952, Paul Zamecnik and his fellow biochemists at Harvard's Department of Medicine had successfully broken open animal cells and separated the key components by ultracentrifugation. When in combination, two of these components were capable of carrying out protein synthesis in the test tube. These were ribosomes — cytoplasmic particles on which the final linking of amino acids to make proteins occurred — and a soluble fraction that was rich in a variety of molecules whose functions were not yet known. In addition, the nucleotide ATP was found to supply the energy essential for the process.

I joined Zamecnik's group in 1952 and began looking for evidence for the presumed initial step in protein synthesis — the energizing, or activation, of amino acids. Using techniques learnt the previous year working in the laboratories of Fritz Lipmann, a pioneer in biochemical energetics, I found that the soluble fraction was rich in a set of enzymes that attached the adenosine monophosphate part of ATP to amino acids, creating aminoacyl-AMPs. This modification provided the amino acids with the energy they would need to react with each other to form a chain. I reported these findings at a meeting organized by molecular biologists in 1955. Interest was lukewarm — the audience was more interested in how amino acids were arranged in specific sequences (how they were ordered) than in how they were energized.

In 1956, Zamecnik and his colleague Mary Stephenson made a surprising discovery: in the presence of ATP, amino acids were also attached to a small quantity of RNA found in the soluble fraction. Although most of the total cellular RNA was in the ribosomes, around 10% was found in the soluble fraction and was presumed to be ‘junk’ — fragments of the larger RNA from the ribosomes, perhaps produced in the process of rupturing the cells and extracting their contents. We called it soluble RNA (sRNA).

In the meantime, researchers at the Cavendish Laboratory in Cambridge, England — the vanguard of those who came to be known as molecular biologists — were engaged in the study of the structure of proteins and DNA. 1953 was the bright year of revelation — James Watson and Frances Crick unveiled the structure of DNA. By this time, scientists generally believed that RNA copies of single strands of DNA, acting as templates prescribing the sequences of amino acids in proteins, existed on ribosomes. Frances Crick turned his attention to how amino acids might be ordered on such presumed templates. As there is no chemical similarity or complementarity between amino acids and nucleotides, and thus no means by which they could directly interact, Crick suggested that amino acids might be first attached to short single strands of RNA nucleotides, thereby making the amino acids ‘recognizable’ to complementary sequences of nucleotides on the templates. In its simplest form, 20 specific enzymes would catalyse the attachment of 20 different kinds of amino acids to 20 different RNA ‘adaptor’ molecules. These would then be ordered by complementary nucleotide pairing on single-stranded RNA templates on ribosomes. Francis circulated this ‘adaptor hypothesis’ among 20 fellow molecular biologists of the RNA Tie Club in 1955, but it was not formally published until 1958 (Symp. Soc. Exp. Biol. 12, 138–163; 1958).

Back in Boston, unaware of Francis's brilliant imaginative leap, I was pursuing the peculiar fact that amino acids seemed to bind to an RNA component of the cellular fraction that catalysed the ATP-dependent activation of amino acids. (It was intriguing to assume that the natural function of the activating enzymes was to transfer their aminoacyl-AMPs to these sRNA molecules. This later proved to be true.) Were these sRNA-bound amino acids essential intermediates in protein synthesis? With thumping heart, I did the key experiment. I first briefly incubated the soluble fraction with amino acids and ATP, to attach amino acids to sRNA, then removed unattached amino acids and ATP before incubating the fraction with ribosomes. To my delight, the amino acids on sRNA were rapidly transferred to their final linkage in protein on ribosomes! From that moment on, we had little doubt that sRNA (later to be renamed transfer RNA or tRNA) was the physical link between activated amino acids and their ordered arrangement in proteins (Biochim. Biophys. Acta 24, 106–107; 1957).

In late 1956 Jim Watson, recently appointed to the Department of Biology at Harvard, learned of our findings through the efficient Boston–Cambridge (Massachusetts) grapevine and paid us a visit. After hearing our account of the discovery of sRNA, he asked if we knew of Francis Crick's adaptor hypothesis. Acknowledging our ignorance and somewhat miffed that a molecular biologist had foretold the existence of the intermediate we had discovered, we couldn't help but admire Francis's prescience. An image arose before me: we explorers, slashing and sweating our way through a dense jungle, rewarded at last by a vision of a beautiful temple — looking up to see Francis, on gossamer wings of theory, gleefully pointing it out to us!

And so it was that tRNAs and their companion activating enzymes (which came to be known as aminoacyl-tRNA synthetases), framed by the adaptor hypothesis, brought the classical biochemists and the molecular biologists together, snug in the same discipline, all speaking the same language.