178 KARL C. I.AHK 



It- has also Ui'cii louml that in syiichroiious ciiHuics of hactciia in 

 which the DNA synthetic cych' consists of a step of >ynth('>is foHowcd 

 by a ])('riotl of no synthesis, chloi'aniphcnicol blocks synthesis of cyto- 

 plasmic protein but will allow DNA synthesis to continue until the 

 intfact'llular content of the latter is more than iloubleil. Moreover, DNA 

 continues to l)e synthesized (to a reduced extent) in steps separated by 

 periods ecjual to those found in non-treated cultures (Mai'tiyama anrl 

 Lark. 1961). Periodic DNA synthesis was followed undei- these condi- 

 tions for three cycles (diu-ing which the cellular DNA content doubled. 

 This would argue that protein synthesis did not control the initiation of 

 (or the timing of) a new step of DNA synthesis. 



In these experiments, the rate of DNA .synthesis (that is, the amount 

 made in a definite time interval) was found to dejiend upon the time at 

 whii'h chloramphenicol was added to the culture. Thus, it appeare(| that 

 the rate of DNA synthesis was increased if protein synthesis was allowed 

 to proceed during the initial stages of the cell cycle. 



A similar study was nuide on bacteria with synchi'onous division 

 cycles in which no cyclic synthesis of DNA occurred (Doudney, 1960). 

 In this system DNA synthesis was inhibited only if chloramphenicol was 

 added at a certain stage of the cell cycle. The author concluded that 

 DNA synthesis depended upon protein synthesis occurring during that 

 particular phase of the cell cycle. This conclusion, however, appears 

 imwarranted since cells subjected to chloram])henicol 10 minutes earlier 

 were perfectly capable of synthesizing DNA for the next 40 mimites, 

 that is, during and after the period in which it was postulated that 

 protein synthesis was required. This occurred despite the fact that 

 l)rotein synthesis was blocked. 



An extension of experiments on the starvation of amino acirl auxo- 

 trophs has been carried out by Billen (1960b), who has shown that the 

 recovery of the ability to synthesize DNA upon subsecjuent refeeding 

 can be inhibiterl by irradiation or by chloi'amphenicol. As observed by 

 others, chloramphenicol was not found to be as effective as amino acid 

 starvation in blocking DNA synthesis. However, it blocked resumption 

 of DNA synthesis previously inhil)ited by amino acid starvation. These 

 experiments suggest that during the process of amino acid starvation 

 proteins necessary for DNA synthesis are lost oi' destroyed and that 

 chloramphenicol or irradiation may destioy or inhibit the mechanism 

 for replacing these.'* Experiments have shown that cells thus treated 

 retain all of the active enzymes necessary for DNA .synthesis, i.e., all 

 of the kinases and the polymera.se (Billen, 1960b, 1962a). Therefore, if a 



^ Thi.s might occur in a inaiinor similar to tliat in which sulfur-starved cells lose 

 alcofiol-sohiblo ]irotcins (Kolx-rts cl (iL. 19.5.5). 



