70 THE BIOSYNTHESIS OF PROTEINS 



amino acids on to soluble RNA (Heidelberger et ah, 1956; Canellakis, 

 1957; Hecht et al, 1958; Edmonds and Abrams, 1957; Paterson and Le 

 Page, 1957; Okazaki and Okazaki, 1959; Harbers and Heidelberger, 1959; 

 Vincent and Baltus, 1959). The requirement for purines and pyrimidines 

 for protein synthesis might very well find its explanation in this end chain 

 metabolism of soluble RNA, or in other reactions that are still to be dis- 

 covered. 



3. Importance of the Structural Integrity of Ribonucleic Acids 



(a) Effects of rihonuclease. Highly purified ribonuclease from beef pan- 

 creas was found to inhibit completely the incorporation of labelled amino 

 acids into proteins of homogenized pancreas or liver (Siekevitz, 1952; 

 AWir &y et al., 1953; Zamecnik and Keller, 1954). In similar preparations, 

 ribonuclease was without any effect on other complex biosynthetic pro- 

 cesses such as lipid formation. The selective action of ribonuclease upon 

 amino acid incorporation thus provided a very direct evidence to the 

 participation of some RNase sensitive substance, most probably RNA, in 

 protein formation. Observations of the same kind were made on homo- 

 genates of animal or plant cells. Disrupted bacteria are able to incorporate 

 amino acids into their proteins under appropriate conditions; here again 

 the incorporating system is inactivated by ribonuclease, which destroys a 

 large part of the RNA in the preparation (Gale, 1956, 1957; Gale and 

 Folkes, 1955). Protoplasts (i.e. bacteria which have been stripped of their 

 cell wall and protected from osmotic effects by an adequate concentration of 

 sugar or salts) keep the capacity of making proteins, including inducible 

 enzymes (Wiame et ah, 1955 ; Landman and Spiegelman, 1955 ; Spiegelman, 

 1957). In the presence of ribonuclease, incorporation of amino acids into 

 protoplasts protein is completely inhibited (Lester, 1953; Beljanski, 1954). 

 Protoplasts are very fragile structures, and ribonuclease easily causes their 

 disruption, followed by dispersal of their content (Brenner, 1955). It can 

 be shown, however, that the inhibition of protein synthesis occurs before 

 the disruption of the protoplasts (Bridoux and Hanotier, 1956). Spiegel- 

 man and Landman (1955) established conditions which ensure stability and 

 preserve the metabolic activity of the protoplasts in the presence of ribonu- 

 clease for a longer time. The enzyme invariably suppresses protein forma- 

 tion under these conditions, in a rather selective manner. 



More striking still are the effects of ribonuclease on otherwise intact 

 living cells. Crude ribonuclease preparations had been shown to inhibit 

 cell division when injected into an amphibian egg (Thomas et ah, 1946). 

 This experiment was later repeated with highly purified enzyme on divid- 

 ing eggs at the two and four cells stages; it was observed that micro- 

 injection of ribonuclease into one blastomere inhibits the division not only 

 of the injected cell but of the other blastomeres as well, as if ribonuclease 



