Automatic Adjustment in Bacterial Cells 325 



and Gros (1957), although chloramphenicol treatment in- 

 creased the RNA content of the cells by about 70 per cent it 

 also increased the DNA content by 50 per cent. Unlike the 

 additional RNA, however, the DNA was stable in the absence 

 of the drug. Earner and Cohen (1957) have managed to double 

 the DNA content of a bacterial mutant unable to synthesize 

 thymine and phenylalanine by preincubation in a minimal 

 medium containing phenylalanine followed by incubation in 

 the presence of thymine alone. They conclude that the pre- 

 incubation with the amino acid may stimulate the formation 

 of a protein possibly essential to the subsequent synthesis of 

 DNA. 



In what has been cited above the controls are in a sense 

 imposed on the cell from without, but analogous mechanisms 

 will be involved in the kinds of control a cell can exercise on 

 its own processes. 



In the work described in section 2 the variations during the 

 cellular growth cycle of certain enzymes have been referred to. 

 Variations of the RNA and DNA during growth cycles also 

 occur, and the detailed investigation of these and their 

 relations to the enzymic changes will undoubtedly prove to 

 be of great interest. 



6. Adaptive Phenomena 



Reversible adjustments in the quantitative relations of the 

 cell functions have been evident throughout the foregoing 

 discussion. Sometimes the establishment of a steady state is 

 rapid and is nearly complete as soon as the newly formed cell 

 material outweighs the old. Sometimes, however, the change 

 is sluggish, and physicochemical explanations of the slow 

 reversibility can, in fact, be offered (Dean and Hinshelwood, 

 1955). 



A slowly reversible change in the enzymic composition of 

 the cell would constitute a natural explanation of resistance 

 induced by drugs, and of the emergence in presence of sub- 

 strates such as lactose or D-arabinose of bacterial strains which 



