Nucleic Acids and Amino Acid Incorporation 177 



presence of the complete mixture of amino acids, decreases 

 with time and incorporation eventually ceases ; the amount of 

 amino acid incorporated when the process ceases corresponds 

 to only a small proportion of the corresponding residues 

 initially present in the protein fraction. In condition 1, no 

 change in the protein content of the preparation can be 

 demonstrated and it has been shown elsewhere (Gale and 

 Folkes, 1955a; Gale, 1956a and b) that such incorporation 

 cannot be taken as a measure of protein synthesis or as an 

 indication that such synthesis is occurring unless supporting 

 evidence is forthcoming. As a working hypothesis, it has 

 been suggested that incorporation under condition 1 takes 

 place as a result of an exchange reaction between amino acid 

 added to the medium and corresponding residues in certain 

 of the proteins present in the preparation. It may be that 

 such exchange incorporation is an activity of a part or parts 

 of the protein-synthesizing mechanism and that this activity 

 can occur when total protein synthesis is not possible. 



Effect of Nucleic Acid Removal 



Nucleic acid can be removed from the disrupted cells by 

 extraction with M-NaCl or incubation with ribo- or deoxyribo- 

 riuclease. After such treatment the preparation is no longer 

 able to synthesize protein unless the incubation mixture is 

 supplemented by appropriate mixtures of nucleic acids or 

 their precursors (Gale and Folkes, 19556). The treatment 

 also results in a decrease in incorporation of amino acids under 

 condition 1 ; the degree of decrease varies with the amount of 

 nucleic acid removed and also with the particular amino acid 

 whose incorporation is being studied. The incorporation of 

 glycine is particularly sensitive; removal of nucleic acid to 

 the point where the content is less than 10 per cent of that in 

 the initial cell results in reduction of glycine incorporation to 

 10-15 per cent of that which takes place in the intact cell. 



Incorporation in nucleic acid-depleted preparations can be 

 restored by addition of staphylococcal nucleic acid to the 



