VmUS-INDUCED ACQUISITION OF METABOLIC FUNCTION 



27 



virus multiplication (N. Delihas, personal communication). As- 

 suming that the target for ultraviolet inactivation is phage 

 DNA, all the major portions of which are equally sensitive to 

 radiation, it may be suggested that the portion of DNA control- 

 ling hydroxymethylase synthesis is a small fraction (perhaps 

 about 1/20) of the total DNA; but nevertheless this site is quite 

 large ( about 10,000 nucleotide pairs, or a molecular weight of 

 aboutexlO^). 



It is essential, therefore, to insert a large viral polynucleotide into 

 the bacterium. Unfortunately, this cannot yet be done with isolated 

 DNA. Once the DNA has entered the cell, enzyme appears at an al- 

 most constant rate for ten to 15 minutes. It has been shown that protein 

 synthesis is essential for enzyme production (Flaks et al., 1959). Thus, 

 if protein synthesis is blocked by 5-methyl tryptophan, as in Figure 5, 

 or by deprivation of an amino-acid-requiring mutant, enzyme produc- 

 tion does not occur. Enzyme synthesis can be started by later supplying 

 the appropriate amino acid to the inhibited or deprived cell. Thus the 

 prerequisite of protein synthesis prior to synthesis of viral DNA is in- 

 deed explained in part by the requirement of protein synthesis for the 

 appearance of the dCMP hydroxymethylase. Nevertheless, it can still 

 be supposed that the protein synthesized is not the enzyme itself but 

 rather a protein necessary to free the pre-existing inhibited enzyme or 

 otherwise activate an inactive or incomplete enzyme. Although we do 



TRY- 15 MIN 



5MT 



t 



8 12 16 20 24 28 



MINUTES AFTER INFECTION 



Figure 5. The eflFect of 5-methyl tryptophan on formation of hydroxymethyl- 

 ase. 5 MT = 5 methyl tryptophan. Try = tryptophan. Cells were infected 

 in the presence of 5 MT, and tryptophan was added in two systems at dif- 

 ferent times and omitted in a third system. The development of the hydroxy- 

 methylase was determined. 



