274 



NUCLEIC ACIDS AND GROWTH 



by experimental means : in liver, for instance, fasting is followed by a decrease in 

 basophilia and in the RNA content (Davidson, 1947), while DNA remains 

 unaffected (Mandel et aL, 1949; Campbell and Kosterlitz, 1950). 



The close correlation between RNA content and protein synthesis is especially 



impressive in growing cul- 



:w> 





Jk: 



■*•■ 







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4a 



tures of microorganisms: early 

 estimations of the RNA con- 

 tent of growing bacteria by 

 Vendrely (1946) have shown 

 that these cells, which undergo 

 extremely rapid protein syn- 

 thesis during growth, may con- 

 lain as much as 11.5% RNA 

 (^dry weight). The cytochemi- 

 cal observations of Malmgren 

 and Heden (1948), using 

 Gaspersson's ultraviolet ab- 

 sorption technique, demon- 

 strated that the RNA content 

 is low in resting cells ; there is 

 a moderate increase in RNA 

 during the lag phase and a con- 

 siderable synthesis of U.V. ab- 

 sorbing substances during the 

 logarithmic phase of growth. 

 The RNA content reverts to 

 the original value just at the 

 time when all cell divisions 

 cease: in this case, cell division 

 seems to be very closely linked 

 to the RNA content. 



The correlation between the 

 synthesis of RNA and the 

 synthesis of protein is especially 

 striking when microorganisms 

 are analyzed during the loga- 

 rithmic phase of growth: in 

 experiments by Caldwell et al. 

 (1950), the RNA content of 

 bacteria remained proportion- 

 al to the growth rate under a variety of experimental conditions (addition of 

 inhibitors, utilization of slow or fast growing mutants and modifications of the 

 nutrient medium). These results have been confirmed by Northrop (1953) and 

 by Wade (1952), on other bacteria. The importance of the culture conditions in 

 such experiments is indicated by Jeener's (1953) experiments on the flagellate 

 Polytomella coeca: a close relationship between the RNA content and protein 



4b 



Fig. 4. Autoradiographs after incorporation oflabelled 

 phenylalanine into mouse tissues, a: pancreas: much 

 stronger activity in the exocrine part (right) than 

 in a Langerhans islet (left), b: intestine: activity is 

 strongest in the Lieberkiihn crypts. Very little activity 

 in the musculosa (below). 



