58 THE BIOSYNTHESIS OF PROTEINS 



DNA is required for protein synthesis in isolated nuclei, and that DNA is 

 involved personally in the process. However, further data from the same 

 laboratory (AUfrey and Mirsky, 1957) showed that after inhibition by 

 deoxyribonuclease, amino acid incorporation into nuclear proteins can be 

 restored by partially degraded DNA just as well as by native DNA. Even 

 RNA would restore the amino acid uptake. More strangely still, polyadeny- 

 lic acid, heparin, chondroitinsulphate and as unnatural a polyanionic 

 compound as polyethylene sulphonate could restore much of the incorpor- 

 ating capacity of deoxyribonuclease inactivated nuclei. If polyethylene 

 sulphonate is added immediately after the depolymerization of DNA has 

 occurred, it is possible to remove 75 per cent of the DNA without change 

 in nuclear ability to incorporate amino acids into protein (Allfrey and 

 Mirsky, 1959). 



Moreover, protein synthesis is not the only function of isolated nuclei to 

 be impaired by deoxyribonuclease; the enzyme deeply damages other 

 metabolic processes in nuclei and especially oxidative phosphorylation 

 (Allfrey and Mirsky, 1959). Again, exogeneous DNA, RNA and poly- 

 anionic substances restore ATP production. The effects on amino acid 

 incorporation might thus be a mere consequence of the inhibition of 

 phosphorylation. 



The experiments with isolated nuclei cannot be taken any more as 

 evidence of DNA being directly involved in the synthesis of nuclear 

 proteins. To the contrary they underline that the presence of intact DNA 

 is unimportant for the incorporation process, since DNA can be sub- 

 stituted for by any one of several polyanionic substances including synthe- 

 tic resins. 



It is not known how deoxyribonuclease inhibits phosphorylation. A 

 possibility is that deoxyribonuclease would bind reversibly some negatively 

 charged compounds like phospholipids or perhaps some enzyme involved 

 in the process, and could be displaced by an excess of exogeneous poly- 

 anionic compounds (cf. Sekiguchi and Sibatani, 1958). 



Whatever the explanation of deoxyribonuclease inhibition proves to be, 

 one must admit that there is at present no evidence that DNA is directly 

 involved in the synthesis of even nuclear proteins. 



2. Bacteria 



The most recent studies on DNA formation indicate that at the level 

 of individual bacteria in exponentially growing cultures the synthesis of 

 DNA is essentially continuous (Schaechter et al., 1959; Young and Fitz- 

 James, 1959; McFall and Stent, 1959). Under the usual laboratory con- 

 ditions, protein synthesis and DNA synthesis thus occur simultaneously 

 in bacteria. It is possible, however, to dissociate protein synthesis from 

 DNA formation in a number of different ways. Thymidine starvation of 



