CHEMISTRY AND VIRAL GROWTH 



Losses of this magnitude might well be expected on general 

 grounds. An attempt was made to test this idea by feeding P^^ 

 to infected cells during the first 4 minutes after infection (22). 

 It was found that the labeled DNA content of the infected cells 

 rose to a maximum during the first 25 minutes, and that 70 to 

 90 per cent of this was eventually incorporated into virus. This 

 high efficiency fails to exclude the hypothesis under consideration 

 because of the slow entry of P^^ into viral precursor DNA. It 

 does show, however, that maturation is not the step that limits 

 the efficiency of transfer from parents to offspring. 



4. Finally, perhaps only a small part or none of the 

 parental DNA goes into vegetative phage directly. Half of it, 

 after playing some unspecified role, could then be broken down 

 and fed into the general pool of DNA precursors. This idea was 

 at first supported by Kozloff's finding that parental DNA 

 phosphorus is transferred more efficiently than parental DNA 

 nitrogen (34,35). However, subsequent and technically su- 

 perior experiments showed equal transfer of phosphorus and 

 purine carbon (65), and equal transfer of the four bases (29). 

 Moreover, the intermediates must be nucleotides or larger frag- 

 ments, because free bases or nucleosides, which compete effec- 

 tively with CO2 (33) or glucose (20) as a source of viral DNA 

 carbon, fail to compete during the transfer from C^Mabeled 

 virus (29). In view of the seeming contradictions, Kozloff"'s 

 result ought to be reinvestigated by the isolation of doubly 

 labeled nucleotides. In the meantime, the bulk of evidence is 

 opposed to, but does not disprove, the idea of indirect use of 

 parental materials. 



At the moment, enumeration of these alternatives (the list 

 is not exhaustive) is of value chiefly to show that the relatively 

 low efficiency of transfer does not permit any conclusions about 

 the mechanism of transfer. In addition, it may be pointed out 

 that the transfer of purine and pyrimidine carbon (except cyto- 

 sine) from bacterial DNA to viral DNA is nearly or quite per- 

 fectly efficient (29). Since this may surely be taken as an ex- 

 ample of "indirect" transfer, the assumption that the transfer 



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