[962 J. D.WATSON 



Crick to finish formulating some general principles on viral structure 10 . Our 

 main idea was that the finite nucleic acid content of viruses severely restrict- 

 ed the number of amino acids they could code for. As a consequence, the 

 protein coat could not be constructed from a very large number of different 

 protein molecules. Instead it must be constructed from a number of identical 

 small sub-units arranged in a regular manner. These ideas already held for 

 Tobaa Mosaic Virus, a rod-shaped virus, and we were very pleased when 

 I ). I . 1 ). C Jaspar 1 ', then working with us at the Cavendish, took some elegant 

 diffraction pictures o\' Bushy Stunt Virus crystals and extended experimental 

 support to the spherical viruses. 



Structural Studies on Ribosomes 



At that time almost no structural studies had been done with ribosomes. They 

 were chiefly characterized by their sedimentation constants; those from 

 higher organisms 12 in the 70S-80S range, while those from bacteria 13 appeared 

 smaller and to be of two sizes (30s and 50s). Because the bacterial particles 

 seemed smaller, they seemed preferable for structural studies. Thus when 

 Alfred Tissieres and I came to Harvard's Biological Laboratories in 1956, we 

 initiated research on the ribosomes of the commonly studied bacteria Esche- 

 richia coli. We hoped that their structure would show similarities with the 

 small spherical RNA viruses. Then we might have a good chance to crys- 

 tallize them and to eventually use X-ray diffraction techniques to establish 

 their 3 -dimensional structure. 



Ribosome sub-units 



But from the beginning of our Harvard experiments, it was obvious that 

 ribosome structure would be more complicated than RNA virus structure. 

 1 Vpending upon the concentration of divalent cations (in all our experi- 

 ments Mg •"), 4 classes of E. coli ribosomes were found, characterized by 

 sedimentation constants of 30s, 50s, 70s, and 100s. Our first experiments in 

 io~ 4 M Mg revealed 30s and 50s ribosomes. At the same time Bolton 14 , at 

 the Carnegie Institute of Washington employing higher Mg 1-4 " levels, saw 

 faster sedimenting ribosomes and suggested that they were observing ag- 

 gregates of the smaller particles. Soon after, our experiments 15 revealed that, 

 as the Mg ++ concentration is raised, one 30s particle and one 50s particle 



s-114 



