INTERVENTION OF RNA IN LIVING CELLS 43 



so-called "dark phase", i.e., when no complete infectious virus 

 particles can be recovered from the infected host cells. A very inter- 

 esting hypothesis, proposed by Casterman and Jeener (1955) and 

 by Le Clerc (1956), gives a satisfactory explanation of the present 

 facts. Both tobacco mosaic virus and influenza virus would, im- 

 mediately after infection, break down into RNA and protein. The 

 RNA would, as already discussed earlier, play the same genetic 

 role as DNA in phage infection. During that initial period, where 

 RNA would be separated from the protein constituent of the virus, 

 RNA would of course be extremely sensitive to ribonuclease. Its 

 destruction would result in the loss of infectivity. Such an explana- 

 tion is obviously in keeping with all that has been said earlier in 

 this chapter on the genetic role of RNA in plant viruses. 



But there is an alternative, perhaps more probable, explanation. 

 On penetration of the virus, the infected host cell would build a new 

 specific RNA. This RNA synthesis would be necessary for virus 

 multiplication and it would be inhibited by ribonuclease. There is, 

 in fact, good evidence for the view that in the E. coli-T phages 

 system, synthesis of new RNA molecules in the bacterium imme- 

 diately follows the injection of phage DNA (Volkin et al., 1958). 

 Jeener's (1959) recent experiments clearly show that, in a compa- 

 rable system, ribonuclease inhibits phage multiplication, although 

 the latter contains no RNA. It has also been reported by Tamm 

 (1948) that ribonuclease inhibits the multiplication of another DNA- 

 containing virus, that of vaccinia. Further experiments are obviously 

 needed before all the facts already known about the effects of 

 ribonuclease on virus multiplication can be adequately explained. 



To summarize, ribonuclease can penetrate into a number of cells 

 and organisms. The enzyme does not interfere with the energy- 

 producing mechanisms and it has complex effects on RNA metab- 

 olism. Enzyme-substrate complex formation is usually followed, 

 sooner or later, by an enzymatic breakdown of the RNA. In all 

 cases studied so far, incorporation of amino acids into proteins, 

 protein synthesis, mitotic activity and growth have been drastically 

 inhibited. Addition of foreign RNA, in many instances, exerts favor- 

 able effects on the ribonuclease-treated cells. The experiments 



References p. 50/54 



