38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 449 



However, though no strong argument permits at present to reject such 

 an hypothesis, there is a set of observations showing that the primary 

 site of protein synthesis is RNA rather than DXA. 



(1) Kinetic studies show that in animal cells amino acids are incorporated 

 first into "RNA rich" particles (Chapter 37). 



(2) The rate of protein synthesis is under certain conditions directly 

 proportional to the RXA content of the cell. 139 ' 189 



3. Transfer of Information from the DXA 



Until now the scheme of protein synthesis which is tentatively proposed 

 can be summarized as follows: activated amino acids are transferred to the 

 sRNA, 30 which adapts them sterically to a template composed of ribonu- 

 cleoprotein particles. At that stage, the amino acids are incorporated into 

 a peptide chain which is stripped off the template. The free template can 

 then act catalytically. 



If it be assumed that RXA is a template for the assembly of the peptide 

 chains, DXA must act at some appropriate stage in controlling the sequence 

 of the amino acids in this chain, since it has been observed that some muta- 

 tions lead to the formation of altered proteins e.g., hemoglobin, 200 al- 

 kaline phosphatase 201 or /3-galactosidase. 202 The most logical assumption 

 is that DXA controls protein specificity through the RXA by forming 

 RXA replicates of its own structure. While nothing is known concern- 

 ing the biochemical relationships between DXA and RXA synthesis, there 

 are many indications, however, suggesting that the nucleus is the RXA 

 forming site 203, 204 or that the presence of DXA is necessary for RXA syn- 

 thesis. 



4. Regulation of the Rate of Protein Synthesis 



The rate of protein synthesis depends on at least two types of factors: 

 the concentration of precursors immediately available (amino acids), and 

 the RXA content. 



If, as is suggested by certain experiments, amino acids are necessary as 

 such for the synthesis of RXA, 27 ■ 28 it is possible to visualize a mechanism 

 for controlling the rate of protein synthesis. When this rate tends to in- 



200 V. M. Ingram, Nature 180, 326 (1957). 



201 C. Levinthal, Revs. Modern Phys. 31, 249 (1959); Garen, A. in "Symposium on 

 Microbial Genetics," London (April I960)." Cambridge Univ. Press, London and 

 New York, 1960. 



202 J. Monod, Angew. ('hem. 71, 685, (1959). 



203 1). Mazia, "Enzymes, Units of Biological Structure and Function" (A. E. Gaebler, 

 ed.), p. 261. Academic Press, New York, 1956. 



204 L. Goldstein and W. Plaut, Proc. Natl. Acad. Sci. U. S. 41, 874 (1955). 



