510 MOLECULAR MECHANISMS OF DIFFERENTIATION 5 



substrate. In the mutant the RNA necessary for the initiation of protein synthesis 

 has to be manufactured before protein synthesis can begin. Similar results were 

 obtained by Pardee (1954). 



After irradiation with ultraviolet light, RNA synthesis in E. coli as measured by 

 uptake of ^^P is impaired (Spiegelman, Halvorson and Ben-Ishai, 1955). Under 

 these same conditions the production of adaptive enzymes, such as galactosidase 

 was almost completely abolished while overall growth persisted at the rate obser- 

 ved in the non-irradiated cells. When the synthesis of RNA was inhibited by a 

 purine analog, 5-hydroxy-uridine, again no effect on the overall growth rate was 

 observed but the formation of adaptive enzymes was again abolished. Even after 

 enzyme synthesis had started the analog caused an abrupt cessation of the synthetic 

 activity. This again may indicate that it is the process of RNA synthesis and not 

 merely the presence of RNA which is responsible for the formation of the adaptive 

 enzyme, while the formation of proteins necessary for overall growth is unaffected 

 by the analog and does not seem to depend on the synthesis of RNA. This interpre- 

 tation is not fully conclusive since it has not been established that 5-hydroxy- 

 uridine inhibits RNA synthesis only. 



These experiments provide evidence for the close relationship between the 

 formation of nucleic acid and the synthesis of proteins (adaptive enzymes). In this 

 instance it is not the mere presence of nucleic acids in high concentration which is 

 the essential factor for synthesis of new types of proteins, but a concomitant rapid 

 synthesis of the RNA molecule is required. 



The importance of nucleic acids for protein formation in cells of more highly 

 organized animals has become apparent in recent years. As is well known, a connec- 

 tion between RNA and protein formation in animal cells had long been postulated 

 by Brachet (1950) and Caspersson (1950). These authors based their assumption 

 on rather tenuous and circumstantial evidence such as the occurrence of large 

 quantities of RNA in cells with a rapid rate of protein formation. 



These initial interpretations have now been substantiated in various ways. 

 Without exception it has been found that enzymatic removal of nucleic acids 

 from the protein forming centers, discussed above, abolished their capacity to 

 continue protein synthesis. 



From the microsome moiety, a fraction can be separated which is very rich in 

 RNA and which shows an incorporation of labelled amino acids into its protein 

 fraction which is several times higher than the incorporation into the ;zon-RNA 

 containing microsomal proteins. Again the incorporation can be abolished by 

 pretreatment of the microsome fraction with RNase (Hultin, 1950 and Littlefield, 

 Keller, Gross and Zamecnik, 1955). Hultin (1955) points out that in the case of 

 liver cells from newly hatched chicks the incorporation patterns of the soluble and 

 of the microsomal RNA proteins are contrary to what might be expected. The 

 RNA component obtained from microsomes shows the lower and the soluble RNA 

 the higher rate of incorporation while the reverse holds true for the corresponding 

 protein fractions. The implications of this interesting observation are obscure. 



Important data concerning the relationship of the RNA to protein synthesis 

 were obtained by a comparison of the protein forming capacities of pancreas, 

 liver and kidney (Allfrey, Daly and Mirsky, 1953). In comparing the composition 



