MUTATION AS A CHEMICAL PROCESS 



227 



the protein in the mature phage. It is known to consist of many en- 

 zymes, some of which are directly involved in the polymerization of 

 DNA. 



The requirement of protein for DNA synthesis is not restricted to cases 

 such as phage development where the DNA is of a new sort not previ- 

 ously synthesized by the bacterium. The synthesis of any DNA seems to 

 require the synthesis of a protein peculiar to it, either concomitantly, 

 or preceding it. When thymineless-histidineless (thy~ his~) bacteria are 

 starved for thymine but supplied with histidine, essentially no DNA but 

 a good amount of protein is synthesized. Upon transfer of such thymine- 

 starved bacteria into a medium containing thymine, but now no histidine, 

 protein synthesis ceases but abundant DNA synthesis begins (Figure 

 8.17). Was the previous synthesis of protein required for the formation 

 of DNA? Prevention of the initial protein synthesis during thymine 

 starvation by chloramphenicol shows that it was. For, when these bac- 

 teria are transferred to medium with thymine but with no histidine or 

 chloramphenicol, protein synthesis still does not take place. Further- 

 more, the vigorous synthesis of DNA observed in the first instance does 

 not take place; only a minimal synthesis characteristic of cells that have 

 not been starved for thymine is observed. The new synthesis of protein 

 during thymine starvation was required for the excess DNA that formed 

 when thymine was subsequently provided. 



Some studies with ultraviolet light similarly implicate protein in the 

 process of DNA replication and in the process of mutation as well. 



10 20 10 20 30 40 



Minutes After Infection 



FIGURE 8.16. A representation of the effect of cfiloramphenicol on the synthesis of 

 protein and DNA by phage-infected bacteria (after Tomizawa and Sunakawa, 1956, 

 J. Gen. Physiol., 39:553). 



