MICROSOMES AND RNA PARTICLES 19 



and protein synthesis were inhibited while DNA synthesis remained 

 unimpaired. 



Similar conclusions have been drawn by Spiegelman and his co- 

 workers (1955) from a series of extensive experiments on induced 

 enzyme synthesis. They found that strong interference with DNA 

 synthesis has no striking effect on enzyme formation, whereas a 

 50 % inhibition of RNA synthesis completely suppresses induced 

 enzyme synthesis. 



Recent experiments of Chantrenne (1956a, b) confirm in a very 

 convincing way that the synthesis of a specific enzyme protein is 

 associated with the synthesis of a new, possibly specific RNA. He 

 found that, when non-respiring yeast cells synthesize catalase under 

 the inducing action of oxygen, new RNA molecules are built up. 

 Under Chantrenne's (1956a, b) experimental conditions, adenine 

 is incorporated into RNA of adapting cells at a faster rate than in 

 non-adapted cells; furthermore, this incorporation occurs pref- 

 erentially in one particular cell fraction. 



The evidence, as it now stands, is thus strongly in favor of 

 idea that, in microorganisms, fresh RNA synthesis occurs wk^^l-^-^'^Z ^ 

 new protein synthesis has been induced. ' ''^ -« .: 



4. THE ROLE OF THE MICROSOMES AND\ 

 RIBONUCLEOPROTEIN PARTICLES IN PROTEIN SY 



We have already seen that RNA is mainly localized in the 

 and in the cytoplasm. A little more will be said now about cyto- 

 plasmic RNA, while the subject of nuclear RNA will be left for 

 Chapter 3. 



Thanks to the pioneer work of Claude (1943), it is known that 

 the bulk of cytoplasmic RNA, in a homogenate, is associated with 

 small particles, the microsomes. Electron microscopy has helped 

 considerably in our understanding of the real nature of these micro- 

 somes (see Brachet, 1957, and Haguenau, 1958, for detailed re- 

 views of the question). In short, the microsomes present in a homog- 

 enate are breakdown products of elaborate cytoplasmic structures, 

 known to electron microscopists as the endoplasmic reticulum or 



References p. 50/54 



