PEPTIDES AND PROTEINS ' IO7 



amylase by pigeon pancrease is enhanced by the addition of 

 amino-acids, and the greater the number of amino-acids, the 

 greater their effect. Gale has recently studied the effect of 

 other amino-acids, and of purines and pyrimidines on the 

 absorption, accumulation and further metabolism of glu- 

 tamic acid by Staph, aureus, and obtained evidence that an 

 increase in cellular combined glutamic acid is indicative of 

 the synthesis of new protein [14]. Protein synthesis only 

 occurred when the cells were suspended in a medium which 

 contained, in addition to glutamic acid and glucose, all the 

 amino-acids to which Staph, aureus is exacting. 



Studies of actively dividing embryonic cells and cells 

 engaged in rapid protein synthesis led Caspersson and inde- 

 pendently Brachet to propose that protein synthesis is pre- 

 ceded by the synthesis of pentose nucleic acids and that 

 these substances then participate in and control the synthesis 

 of proteins. By using the ultraviolet light microscope tech- 

 nique (p. 130), Malmgren and Heden measured the nucleic 

 acid content of cells at various stages during the growth of 

 cultures of Esch. coli and Bacillus cereus. Their results indi- 

 cated that the lag phase was a period of intense nucleic acid 

 synthesis and in consequence the cellular concentration of 

 nucleic acid reached a maximum during the early part of the 

 lag phase: thereafter it gradually declined and became mini- 

 mal during the stationary phase. Malmgren and Heden con- 

 cluded that Caspersson's and Brachet's hypothesis also 

 applied to bacteria and that a culture only passed out of 

 the lag phase when a critical intracellular concentration of 

 nucleic acid has been attained [28]. Other workers using 

 Staph, aureus have provided further evidence in support of 

 this conclusion [14, 29]. The rate of protein synthesis by 

 washed cells of Staph, aureus can be directly correlated with 

 the nucleic acid content of the cells at the time of harvesting 

 [14]. During the growth of bacterial cultures it is the pentose 

 nucleic acid content of the cell which alters: the desoxy- 

 pentose nucleic acid content remains approximately con- 

 stant [29]. It is interesting to note that after irradiation with 

 ultraviolet light, bacteria are unable to develop the usual 

 adaptive increase in activity when they are incubated in the 



