[962 J.D.WATSON 



30s particles. It is clear that this enzyme it present in .1 free active form, would 

 be rapidly lcth.il to its host cell. Thus its presence in latent form is expected. 

 Hut why it is stuck to ribosomes is still a complete mystery. 



Chemical Intermediates in Protein Synthesis 



Our early experiments with ribosomes were almost unrelated to the efforts 

 of biochemists. At that time our research objects seemed very different. The 

 enzymologically oriented biochemists hoped to find the intermediates and 

 enzymes necessary for peptide bond formation. On the contrary, those of 

 us with a genetic orientation wanted to see the template and discover how 

 it picked out the correct amino acid. Very soon, however, these separate 

 paths came together, partly because of a breakthrough in the nature of the 

 ammo acid intermediates, and partly from an incisive thought by Crick. 



The biochemical advances arose from work in Paul Zamccnik's laboratory 

 at the Massachusetts General Hospital. There was developed a reproducible 

 in vitro system 23 containing ribosomes, supernatant factors, and ATP which 

 incorporated amino acids into protein. Using these systems Hoagland made 

 two important discoveries. Firstly, he 24 showed that amino acids are ini- 

 tially activated by ATP to form high-energy AA-AMP complexes. Sec- 

 ondly, he demonstrated 25 that the activated amino acids arc then transferred 

 to low molecular weight RNA molecules (now known as soluble or transfer 

 RNA), again in an activated form. These amino-acyl-sRNA compounds 

 then function as the direct intermediate for peptide bond formation (Fig. 4). 



It had previously been obvious that amino acid activation would have to 

 occur. However, Hoagland's second discovery (in 1956) of the involvement 

 of a hitherto undiscovered RNA form (sRNA) was unanticipated by almost 

 everybody. Several years previously (in 1954), Leslie Orgel and I spent a 

 quite frustrating fall attempting to construct hypothetical RNA structures 

 which contained cavities complementary in shape to the amino acid side 

 groups. Not only did plausible configurations for the RNA backbone fail to 

 result in good cavities, but even when we disregarded the backbone, we also 

 failed to find convincing holes which might effectively distinguish between 

 such amino acids as valine and isolcucinc. Crick, at the same time (early 

 1955) sensed the same dilemma, and suggested a radical solution to the 

 paradox. He proposed 26 that the amino acids do not combine with the tem- 

 plate. Instead each should first combine with a specific adaptor molecule, 



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