228 



CELL HEREDITY 



250 



200 - 



150 - 



100 



Hours 



FIGURE 8.17. The role of protein in DNA synthesis. The left half of the figure shows 

 the change in the protein content of E. coli fhy his starved for thymine but provided 

 with histidine (H 7 ) in the presence or absence of chloramphenicol. The DNA in 

 these cells does not increase by more than 5 per cent. During the 2 hours involved, the 

 viable cell number remains constant; only afterwards does the thymineless death char- 

 acteristic of fhy bacteria set in. The right half of the figure shows the behavior 

 of protein and DNA when the cells are washed and suspended in medium with thymine 

 but devoid of histidine (H T ). Here again they do not die for at least two hours. 

 In the absence of histidine, no protein synthesis occurs. But DNA increases to an extent 

 dependent upon whether the previous starvation for thymine took place in the absence 

 or presence of chloramphenicol, without or with the synthesis of protein (from Nakada, 

 1960, Biophys. Biochim. Acta., 44:241). 



When bacteria are irradiated with mild doses of ultraviolet light, DNA 

 synthesis is temporarily arrested and the length of the delay is a function 

 of the dose used. Protein synthesis can be blocked at the same time if 

 specific inhibitors such as chloramphenicol are added after irradiation or 

 if amino acid auxotrophs are starved for their requirement. In these 

 cases, DNA formation does not begin until protein synthesis is allowed 

 again. Witkin has shown that some UV-induced mutations require the 

 presence of amino acids for their realization. Even when amino acids 



