INTERVENTION OF RNA IN LIVING CELLS 37 



roots (Figs. 13 and 14). The effects are exactly parallel to those 

 on the incorporation of amino acids into proteins, so that the 

 inhibition is almost complete within 3 hours ; it is usually irre- 

 versible. Chemical estimations of the protein content of the treated 

 roots have further shown that, as might be expected, protein syn- 

 thesis is almost completely inhibited. 



Another interesting finding is shown in Figs. 13 and 14. The 

 addition of yeast RNA to the treated roots partially restores growth, 

 at least for a few hours. RNA alone exerts first a stimulation, 

 and then an inhibition of growth in untreated roots. The results 

 which we obtained with the living root-tip cells are thus in perfect 

 agreement with the in vitro experiments of Webster and Johnson 

 (1955) on the microsomal fraction of pea roots. 



Addition of RNA can even entirely restore the incorporation of 

 amino acids into proteins of ribonuclease-treated roots, provided 

 that the ribonuclease treatment was such as to produce a 60-70 % 

 inhibition of the incorporation. In these experiments, RNA isolated 

 from onion roots was about 4 times more active than yeast RNA 

 in reactivating the protein synthesis. (Sels-Brygier, 1958). 



The mode of action of ribonuclease, in the case of living onion 

 roots, is certainly complex and is not yet completely understood. 

 It is certain, however, that the inhibition of protein synthesis is not 

 a result of an indirect effect of the enzyme on energy-producing 

 reactions. The oxygen consumption remains essentially normal even 

 after a 3-h ribonuclease treatment and the ATP level shows a slight 

 increase. It is also unlikely that ribonuclease produces a marked 

 breakdown of RNA in the living cells. No consistent results were 

 obtained when the RNA content and the incorporation of precur- 

 sors (=^2P, adenine) into the RNA molecule were studied in the 

 treated roots. Depending upon the species of onions and the ribo- 

 nuclease preparations used, stimulation or inhibition of RNA 

 synthesis and RNA metabolism were obtained. Breakdown of 

 the RNA could, however, be obtained after longer times of action 

 (4-6 h). 



Very recently, we were able to show that ribonuclease quickly 

 breaks down the RNA which is present in the supernatant fraction 



References p. 50/54 



