IV. CELLULAR CONTROL OF DNA BIOSYNTHESIS 175 



and RNA synthesis ceases. From these observations it was concluded 

 that synthesis of protein and/or RNA was required for sustained DNA 

 synthesis (Goldstein et al., 1959; Maal0e and Hanawalt, 1961). More 

 careful investigation of the synthesis of DNA in cells starved for an 

 essential amino acid revealed that it could proceed to a limited extent 

 in individual cells before ceasing. (In these studies the cells were also 

 starved of uracil; however, the similarity of the behavior of the culture 

 as a whole to experiments of others in which only amino acids were 

 limited makes it most probable that the effects observed were the direct 

 result of amino acid starvation.) The amounts of DNA made (ca. 50% 

 of the initial cell content), as well as the number of cells which at any 

 time during starvation could be shown (by autoradiography) to retain 

 the capacity for DNA synthesis, were consistent with the hypothesis that 

 each cell continued to finish the DNA replication cycle upon which it 

 had embarked (Hanawalt et al.. 1961). Experiments in which DNA 

 formed under these conditions is labeled by means of a density marker 

 are also consistent with this hypothesis (Hanawalt et al., 1961 ; Nakada 

 and Ryan, 1961). It was suggested that such cells could not commence 

 a second cycle of DNA replication without a short respite from the 

 starvation to which they were subject. It was concluded that protein 

 and/or RNA synthesis were required to initiate a new cycle of DNA 

 replication. The evidence presented appears conclusive in showing that 

 DNA synthesis can proceed at least to a limited extent in the absence 

 of RNA and protein synthesis. Moreover, the hypothesis that a cell 

 engaged in DNA synthesis will finish a cycle of replication despite the 

 absence of RNA or protein synthesis is extremely attractive. 



Subsequent experiments have shown, however, that the amount of 

 DNA made will depend upon the growth conditions prior to starvation 

 and that, under certain conditions, cells can more than double their 

 content of DNA or carry on synthesis beyond the end of one cycle. 

 Moreover, quantitative autoradiography indicates that those bacteria 

 which are able to retain their ability to synthesize DNA for a long time 

 may have a relatively slow rate of DNA synthesis (Schaechter, 19611. 

 Thus, an alternative possibility is that all of the cells retain the capacity 

 to synthesize a certain amount of DNA upon deprivation of an essential 

 amino acid but some cells exhaust this capacity quickly, whereas others 

 take a longer time. 



Another possibility, which has not been properly resolved, is that 

 under certain starvation conditions bacteria may complete one replica- 

 tion cycle in addition to that in which they were engaged when starva- 

 tion began (possibly by using an endogenous reservoir of the essential 

 amino acid). 



