72 THE BIOSYNTHESIS OF PROTEINS 



RNA to protein ratio in the 'soluble' fraction was consistently observed. 

 This indicates that the inhibition of protein synthesis in living onion roots 

 caused by ribonuclease is probably due to partial degradation of a soluble 

 RNA fraction by the enzyme (Brachet and Six, 1959). 



The possibility, however, remains that RNase might also interfere with 

 protein synthesis in other ways, for instance, by slowly damaging ribosome 

 RNA or by forming complexes with it. Ribosome RNA is less sensitive to 

 the lytic action of the enzyme than soluble RNA (Shigeura and Chargaff, 

 1960); other basic proteins like histone, salmine and to a lesser extent 

 cytochrome-c indeed partly inhibit protein synthesis in roots (Brachet, 

 1956) and in ascites cells (Becker and Green, 1960) by combining with 

 RNA. 



Finally, in certain types of animal cells, ribonuclease causes deep changes 

 in RNA metabolism. For instance, in ascites cells in a medium supple- 

 mented with free nucleotides, the enzyme first stimulates RNA synthesis 

 (Ledoux, 1956; Pileri et al, 1957), causing a rapid uptake of pyrimidine 

 nucleotides, thus leading to the formation of abnormal RNA, which is later 

 degraded (Ledoux and Vanderhaeghe, 1957). The effect on protein synthe- 

 sis is not striking in this case, however, and respiration is depressed (Ledoux 

 and Baltus, 1954). 



Ribonuclease was also found to inhibit growth of Bacillus megaterium 

 (Groth, 1956) and of £". coli (Jerne and Maaloe, 1957). In many cases, lysis 

 of the bacteria occurs, but it is possible to isolate strains which are not 

 lysed by the enzyme, and which will continue to grow in its presence 

 (Jeener, 1959a). Using a lysogenic strain of Bacillus megaterium resistant 

 to ribonuclease lysis, Jeener showed that the synthesis of phage protein is 

 partly inhibited whereas bacterial growth is not. The phage proteins made 

 in the presence of the enzyme have abnormal immunological and chromato- 

 graphic properties (Jeener et al., 1960). They can be integrated into phage- 

 like structures, but these are poorly adsorbed on receptive bacteria, and they 

 are also very fragile. Incomplete phage particles have indeed been isolated 

 by serological precipitation (Jeener, 1959b). In these experiments, also, a 

 small fraction only of the total RNA could have been degraded, for the 

 RNA/DNA ratio was practically unaffected. But the composition of RNA 

 was changed: a higher content in uridylic acid was consistently found in 

 the bacteria which had been treated with ribonuclease. This suggests that 

 the abnormalities observed in the proteins produced might result from a 

 structural modification of some RNA fraction (Jeener, 1959b). 



It should be mentioned again here that ribonuclease does not affect pro- 

 tein synthesis in intact mitochondria, but it suppresses completely amino 

 acid incorporation into proteins in extracts obtained from mitochondria 

 (Kalf and Simpson, 1959; Kalf et al, 1959). It is probable therefore that 

 protein synthesis in mitochrondria can go on in a medium containing 



