Cytoplasmic & Nuclear Structure & Metabolism 7 



found that microsomes (the smallest cytoplasmic particulates, 

 which have also the highest RNA content) are most active in 

 the incorporation of radioactive amino acid into proteins. 

 More recently, Gale and Folkes (1954, 1955) have found, in 

 bacteria lysed by ultrasonics, that protein synthesis will 

 only take place if RNA is left intact. Indeed this process is 

 brought to a stop if the nucleic acid fraction is extracted 

 by various means. In our laboratory also (Brachet, 1954, 

 1955a and b), it has been shown that ribonuclease, by specific- 

 ally attacking or binding the RNA of normal, living cells 

 (onion roots, amoebae, star-fish or amphibian eggs, etc.), has 

 a powerful inhibitory action on the incorporation of amino 

 acids into proteins, on the growth of the cell and on its overall 

 protein synthesis. 



It is now a generally accepted fact, as pointed out by 

 Borsook (1955), Gale (1955) and Mirsky (1955), that nucleic 

 acids are directly and fundamentally involved in protein 

 synthesis. This is clear at least in the case of RNA (Gale and 

 Folkes, 1954, 1955; Brachet, 1954, 1955 a and b), but appears 

 less evident for DNA; some experiments of Allfrey (1954) and 

 AUfrey and Mirsky (1955) do indicate that desoxyribonuclease 

 inhibits amino acid incorporation into the proteins of isolated 

 thymus nuclei; the inhibition, however, is not so strong as 

 that by ribonuclease for the whole cell although ribonuclease 

 does not inhibit amino acid incorporation into the proteins of 

 isolated nuclei. 



In view of the high nucleic acid content of cell nuclei and 

 because of the now well-established importance of these 

 compounds in protein synthesis, Caspersson's idea (1941, 

 1950) of a particularly important function of the nucleus in 

 protein synthesis has been brought into focus again and several 

 laboratories have initiated experiments on this problem. A 

 simple method, used chiefly by Mirsky and co-workers (Daly, 

 Allfrey and Mirsky, 1952) and by Davidson and co-workers 

 (Crosbie, Smellie and Davidson, 1953; Smellie, Mclndoe and 

 Davidson, 1953), consists in injecting a radioactive amino 

 acid into a living animal and then determining the specific 



