ROLE OF NUCLEIC ACIDS 85 



deficiency suppresses not only protein synthesis but RNA synthesis as well 

 (Sands and Roberts, 1952; Gale and Folkes, 1953; Borek et al, 1955). In 

 yeast, sulphur starvation prevents the synthesis of both protein and nucleic 

 acid, although the formation of soluble nucleotides is not impaired 

 (Schmidt et al., 1956). Such observations at first suggested that RNA 

 synthesis may depend on protein synthesis. However, chloramphenicol 

 inhibits protein synthesis without preventing RNA formation (Gale, 1953 ; 

 Wisseman et al., 1954) and amino acids stimulate RNA synthesis even 

 when protein synthesis is suppressed by the antibiotic (Gale and Folkes, 

 1953). 



RNA synthesis in Escherichia coli requires the simultaneous presence of 

 all the amino acids even when protein synthesis is 98 per cent inhibited by 

 chloramphenicol (Gros and Gros, 1956; Pardee and Prestidge, 1956; Yeas 

 and Brawerman, 1957). With amino acids requiring strains, under such 

 conditions, the addition of a very small amount of the limiting amino acid 

 will cause the synthesis of a large amount of RNA. One amino acid mole- 

 cule makes possible the polycondensation of at least eight nucleotide 

 residues (Gros and Gros, 1957). These observations can be accounted 

 for by assuming, e.g. that the RNA precursors are amino acid- 

 nucleotide compounds. Further developments will show whether this 

 assumption is correct. Finally, an enzymic synthesis of RNA from 

 ribonucleoside triphosphates has been reported (Chung et al., 1960; 

 Weiss, 1960). 



The possible involvement of DNA in RNA synthesis has not been clearly 

 observed so far in metabolic studies (see, however, Chung et al., 1960). But 

 our present knowledge on the pathways of RNA synthesis must be regarded 

 as quite rudimentary. 



Histochemical research indicates that the nucleus is an active centre 

 of RNA synthesis, and that some RNA probably forms in the vicinity of 

 DNA. The lampbrush chromosomes of amphibian oocytes (Fig. 26) and 

 the giant chromosomes of insect salivary glands display protusions into 

 the nuclear sap, which are called loops in the first case, puffs in the second. 

 These contain RNA. Autoradiography studies, completed by ribonuclease 

 treatment of the preparations, clearly established that the loops of lamp- 

 brush chromosomes and the puffs of giant chromosomes are the sites of a 

 very active incorporation of radioactive precursors into this RNA (Taylor 

 et al., 1955; Ficq and Pavan, 1957; Ficq et al., 1959). This probably 

 reflects a net synthesis of RNA, since corresponding increases in basophilia 

 were observed at similar locations (Pavan and Breuer, 1955). 



In roots of Vicia faba, the RNA which is present in small amounts in 

 chromatin, the DNA containing structure in resting nuclei, incorporates 

 cytidine very rapidly (Woods, 1959; Woods and Taylor, 1959). Human 

 amnion cells in tissue culture incorporate cytidine-^H in the extranucleolar 



