24 



RIBONUCLEIC ACIDS AND PROTEIN SYNTHESIS 



can still be obtained when RNA has been partially degraded during 

 its isolation (Fig. 10). In a more recent report, Webster (1956) 

 claims to have obtained a net synthesis of proteins and RNA in the 

 particles isolated from pea roots. This synthesis requires the pre- 

 sence of the four nucleotides of RNA in the triphosphate form {i.e. 

 adenosine, guanosine, cytidine and uridine triphosphates), of seven- 

 teen amino acids and of Mg and K ions. The inhibitors of protein 

 synthesis are said to inhibit the concomitant RNA synthesis and 

 the converse is also true. 



Another case where ribonuclease has been found to exert marked 

 inhibitory effects on protein synthesis is that of the in vitro syn- 

 thesis of amylase, which was studied by Straub et al. (1955, 1957). 



Q2. 0.4 0.6 0.6 



Yeast RNA concentration (mg) 



1.0 



Fig. 10. Effect of RNA on glutamate incorporation in a particulate fraction of 

 pea roots (Webster and Johnson, 1955). 



This synthesis apparently occurs preferentially in the mitochon- 

 dria. Nevertheless, it is strongly inhibited by the addition of ribo- 

 nuclease. 



While there is obviously a good deal of evidence in favor of the 

 opinion that cytoplasmic particles, and microsomes in particular, 

 are the major site of protein synthesis in the cell, it would, however, 

 be an exaggeration to believe that they are the only site of protein 

 synthesis. In experiments by Szafarz (1952), it appears that, when 

 the flagellate Polytomella is grown in continuous culture so that all 



