I7() KAHL (i. LARK 



The bacteria investigated have been shown to synthesize DNA 

 throughout the majority of their (li^•ision cycle. When a siniihir experi- 

 ment was cari'ied out on animal cells grown in tissue culture, which have 

 a defined period, N, of DNA synthesis, they were found, after 48 hours 

 of amino acid deprivation, to be arrested in a state in which the DNA 

 content of the nucleus was that characteristic of cells which have just 

 divided (Paul, 1962) (see Fig. 8). 



Similarly, when an amino acid analog, ethionine, was fed to rats, it 

 was found that the DNA content of individual liver cells was one-half 

 that (4C -^ 2C) found in control animals or in animals in which the 

 inhibition was ovei'come by feeding methionine (Holzner et al., 1959). 

 The results of this latter expei-iment are similar to the finding of 

 Schneider et al. (1960) that injections of certain amino acid analogs 

 will result in a loss of ability to incorporate thymidine-H'' into regen- 

 erating liver. This may be due to inability to form active DNA- 

 synthesizing enzymes, or possibly to an effect similar to that observed 

 in bacteria starved for amino acids. Unfortunately, no enzyme assays 

 were carried out and thus it is impossible to distinguish beween the two 

 hypotheses (Schneider et al., 1960). Similar observations to these were 

 made by Lieberman and Ove (1962), who studied DNA synthesis in cells 

 from rabbit kidney cortex. 



Further experiments w^ould seem to be required in the animal cell 

 systems in order to determine at what stage in d such cells are halted; 

 whether or not individual cells are all halted in the same stage; and the 

 number of DNA cycles which can take place following the onset of 

 starvation. However, these findings would tend to support the hypothesis 

 that amino acid deprivation allow'ed cells to finish DNA duplication and 

 division but prevented initiation of a new cycle of DNA biosynthesis. 



The assumption that protein synthesis is required for the initiation of 

 a new DNA synthetic cycle needs to be examined in greater detail. 

 Although the effects of chloramphenicol on E. coli (Billen, 1959b; Neid- 

 hardt and Gros, 1957; Doudney, 1961a) tend to support the observations 

 on cells starved for amino acids, frequently the magnitude of the chlor- 

 amphenicol effect does not parallel that of amino acid starvation. This 

 is exemplified by experiments with Lactobacillus acidophilus (Okazaki 

 and Okazaki, 1959) for which it has been shown that deletion of essential 

 amino acids from the medium results in suppression of DNA synthesis 

 following a pattern observed in the starved auxotrophs of E. coli 

 described above (residual synthesis of 80-100% of existing intracellular 

 content). However, if RNA and protein synthesis were inhibited by a 

 combination of uracil starvation and the addition of chloramphenicol, no 

 inhibition of DNA synthesis was observed. A similar inhibition of RNA 



