40 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



already found. At the same time, the ribonuclease content of the 

 amoebae increases 2-3 times. Autoradiography observations with 

 radio-iodine-labeled ribonuclease confirm the penetration of the 

 enzyme in the amoebae, including their nucleus. 



According to the recent analysis of Schumaker (1958), ribonu- 

 clease first combines with acidic sites on the membrane ; the latter, 

 afterwards, become ingested through pinocytosis. 



Once it has penetrated into the amoebae, the enzyme produces 

 many reactions. Besides the attack on RNA, complex formation, as 

 in onion roots, with RNA is a possibility which cannot be ruled 

 out. Substances which combine with ribonuclease without being 

 broken down (DNA, synthetic purine polynucleotides of Grun- 

 berg-Manago et al., 1955, 1956) markedly protect the amoebae 

 against the effects of ribonuclease when both are mixed together. 

 The addition to the ribonuclease (in the external medium) of sub- 

 stances which are good substrates for the enzyme, e.g. synthetic 

 pyrimidine polynucleotides of Grunberg-Manago et al. (1955, 1956), 

 and of mononucleotides has little protective value. 



Ribonuclease also exerts a marked inhibitory effect on the diges- 

 tion of the preys ingested by the amoebae. Direct estimations have 

 shown that protease and dipeptidase activities are strongly reduced 

 in the ribonuclease-treated amoebae. Further work is required to 

 establish whether all enzymes or only those dealing with protein 

 metabolism are inhibited in the ribonuclease-treated organisms 

 (Briers et al., 1957). 



Despite the complexity and difficulty of interpretation of the 

 experimental results, the essential fact is that in all the cells in which 

 ribonuclease penetrates inhibition of protein anabolism quickly 

 follows. For instance, in frog and starfish oocytes (Ficq and Errera, 

 1955) treatment with ribonuclease produces a considerable drop in 

 basophilia (after freeze-substitution fixation). There is a parallel 

 inhibition of the incorporation of phenylalanine into the proteins 

 of the RNA-rich nucleoli and cytoplasm. In this case, one is almost 

 certainly dealing with the formation of a RNA-ribonuclease 

 complex, as in the onion root-tips. Basophilia remains normal 

 in Zenker fixed oocytes and chemical methods show no actual 



