42 RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



according to Groth (1956), ribonuclease strongly inhibits growth 

 and amino acid incorporation into the bacterial proteins; simul- 

 taneously, the RNA content of the treated bacteria decreases 

 significantly. A detailed reinvestigation of the same problem by 

 Jeener (1959) shows that the situation is very complex in the case 

 of both lysogenic and normal bacteria. Among the latter, different 

 mutant strains, distinguishable by their sensitivity to different con- 

 centrations of ribonuclease, have been detected and isolated. As in 

 onion root-tips, ribonuclease does not necessarily break down the 

 RNA. Whenever the enzyme is active on a given strain, its primary 

 effect is an antimitotic one. 



There is excellent evidence also that ribonuclease strongly inter- 

 feres with virus multiplication. As shown first by Kleczkowski 

 and Kleczkowski (1954). The enzyme inhibits the multiplication of 

 Rhizobium bacteriophage. The addition of the enzyme after the 

 phage has already combined with the bacteria does not prevent the 

 phage from multiplying, but it decreases the rate of multiplication 

 of both the phage and the bacteria. Experiments on tobacco mo- 

 saic virus by Casterman and Jeener (1955) and by Bawden and 

 Harrison (1955) have confirmed that ribonuclease inhibits virus 

 synthesis. The enzyme does not act directly on the virus RNA, 

 which is protected against the enzyme by its protein shell. If ribonu- 

 clease is injected into tobacco leaves before the virus is added, it 

 completely prevents the infection. If, on the other hand, the enzyme 

 is injected into already infected leaves, virus multiplication is only 

 prevented during the first few hours after the injection (Hamers- 

 Casterman and Jeener, 1957). 



Essentially similar observations have been made on influenza 

 (Le Clerc, 1956; Le Clerc and Brachet, 1957) and avian pest (Zillig 

 et al., 1955) viruses. In the case of influenza virus, Le Clerc's care- 

 ful analysis clearly shows that ribonuclease does not act directly on 

 the virus ; nor does the enzyme inhibit amino acid incorporation in 

 the host cells (chorio-allantoic membrane). Ribonuclease only stops 

 the growth of the virus when it is added shortly after the infection. 

 The general conclusion of these experiments is the same : ribonu- 

 clease inhibits multiplication of the virus when the latter is in the 



