ROBERTS, BRITTEN, AND BOLTON 93 



procedures outlined above are only a step toward the needed resolution, but they 

 have already given indications that: 



1. The 80 S ribosomes are composed of nucleoprotein of approximately two 

 amino acids per nucleotide. 



2. This composition is unaltered in the smaller disintegration products re- 

 sulting from magnesium deficiency. 



3. Adsorption followed by elution from the DEAE column causes disintegra- 

 tion into smaller particles of different composition containing approximately 

 one amino acid per nucleotide. 



4. The protein of the ribosomes is a special protein or at least a special class 

 of proteins lacking cystine and cysteine. It is therefore doubtful that any of the 

 enzymes that have been reported in the microsome pellet are actually in the 

 ribosome fraction. 



5. The protein of the ribosomes is most certainly not precursor to the non- 

 particulate proteins. Such a relationship is ruled out by the data on the com- 

 position and on the kinetics of formation. 



6. Incorporation of amino acids, sulfur, and phosphorus into nucleoprotein of 

 the ribosomes shows a kinetic delay which indicates that the ribosomes have a 

 macromolecular precursor. 



7. This precursor has properties suggestive of nucleic acid or nucleoprotein 

 of a low protein content. 



These findings when checked and verified will be useful in providing further 

 conditions that must be met by any theory of protein synthesis. The low initial 

 specific radioactivity found in the ribosome fraction differs markedly from the 

 high initial specific radioactivity found in the deoxycholate-insoluble part of the 

 microsome fraction of rat liver [15]. A partial explanation for the difference 

 may be that the nucleoprotein has been stripped clean of adhering newly formed 

 protein by the column; it is not a complete explanation, however, because the 

 nonparticulate protein of the microsome fraction did not have a high initial 

 specific radioactivity. More likely, the difference arises from the difference in 

 the growth rates. If the ribosomes furnish the templates for protein synthesis, 

 and if chains of 150 amino acid residues are produced by the ribosomes, then 

 each of the 10,000 ribosomes of a coli cell must produce one polypeptide chain 

 per 10 seconds to give the observed rate of protein synthesis. If one polypeptide 

 chain adheres to each ribosome, after 4 minutes' exposure to the tracer only 1/24 

 of the newly formed polypeptide chains would be found still adhering to the 

 particles. To show kinetic effects in protein synthesis with these rapidly grow- 

 ing organisms it will be necessary to use much shorter exposures to the tracer. 



The synthesis of the particles themselves appears to be a distinctly different 

 process, as it proceeds at a more leisurely rate. Even after 24 minutes there are 

 still marked departures from the steady-state distribution. These findings are 

 compatible with, but certainly do not prove, the idea that the smaller particles 

 observed in the cell juice are not simply bits broken off during disruption of 



