50 years ago protein synthesis met molecular biology: the discoveries of amino acid activation and transfer RNA

Two key biochemical insights emerged nearly 50 years ago with the discoveries, by Mahlon Hoagland and Paul Zamecnik, of the two initial steps in the synthesis of protein: the activation of amino acids and their subsequent linkage to a hitherto unidentified cellular RNA fraction—tRNA (1–3). These laboratory events coincided with Francis Crick’s brilliant prediction that such “adaptor” RNAs must exist (i.e., before amino acids could be ordered as prescribed by DNA they must be given an identity that could be recognized by a nucleotide sequence) (4). Hoagland had intended to become a surgeon but had contracted tuberculosis while a medical student at Harvard. Two years later, recovered, he completed his MD training but, finding himself still not up to the physical demands of surgery, he turned to biochemistry. He spent three years as a post-doc at the Massachusetts General Hospital (MGH) in cancer research, a year with Kaj Linderstrom-Lang at the Carlsberg Laboratory in Copenhagen and a year in Fritz Lipmann’s lab at the MGH—all seminal experiences (5). He then joined the group headed by Paul Zamecnik at the MGH. Using a technique involving amino acid-dependent exchange of radioactive pyrophosphate with ATP, catalyzed by enzymes in cell-free fractions of rat liver, Hoagland found that amino acid activation involved the formation of a mixed anhydride bond between the -phosphorous of ATP and the carboxyl group of the amino acid (2). Importantly, the activated amino acid adenylate remained bound to the enzyme that catalyzed its formation. Hoagland then turned his attention to a curious earlier finding of Zamecnik, viz. that C-amino acids became covalently linked to a nonsedimentable RNA species. Hoagland followed up this lead and found that the attachment of amino acids to this “soluble RNA” (soon renamed tRNA) seemed to be catalyzed by the same enzymes that had activated them and that, once linked to the RNA, the amino acids were rapidly transferred to peptide linkage in proteinclearly indicating that this special RNA was behaving as an intermediate in protein synthesis (3). Hoagland has engagingly written (6) about how, in 1956, Jim Watson had visited his lab at the MGH and told him of Francis Crick’s earlier astonishingly prescient prediction of the existence of “adaptor” RNA (which Crick had not yet published, see ref. 4). Thus, the two seemingly disparate schools, biochemistry and molecular biology, which had managed to slide past one another as two ships in the night, now henceforth sailed on in tandem. Following a faculty position at Harvard Medical School, Hoagland went on to become Chairman of the Department of Biochemistry at Dartmouth Medical School and in 1970 became President and Scientific Director of the Worcester Foundation for Biomedical Research, retiring in 1985. Now 84, he remains intensely active, with a passionate interest and extraordinary skill in educating the lay public (e.g., 7–11) and also deeply absorbed in the joy of wood sculpture (perhaps a “surgeon” after all). Paul Zamecnik continued a productive and highly creative research career at MGH through the 1960s and 70s. In 1979, he moved to the Worcester Foundation, bringing Hoagland and Zamecnik together once again, but now in the positions of institute director and active investigator, respectively (Fig. 1). In 1997, Zamecnik returned to MGH as Physician Emeritus, thus closing a career circle. Now 92, he continues an active research program on antisense DNA (e.g., 12), a field that he single-handedly opened (13). The careers of Mahlon Hoagland and Paul Zamecnik (5, 14) are extraordinary in both scientific significance and longevity. As the 50th anniversary of their breakthroughs in protein synthesis approaches, The FASEB Journal warmly salutes them.