Ill PFS OF DIFFERENTIATED AND EMBRYONIC CELLS 5O9 



(5) In interpreting these findings, it was frequently pointed out that RNA may 

 act as a "template" or "mold" in the assembling process of amino acids into whole 

 protein molecules (Borsook, 1956a, b). Whether RNA merely orders the sequence 

 of amino acids in the peptide chain or whether it is also involved in the folding of 

 the peptides into the sterically specific configuration of the protein molecule is still 

 a matter of conjecture. 



(a) Nucleic acids as components of the PFS 



The role of RNA as a key component of the PFS has become more evident from 

 studies of protein formation in microbes in general and of adaptive enzymes in 

 particular. Gale and Folkes (1953a, 1953b, 1955a and 1955b) observed that in 

 Staphylococci the amount of cellular protein increases more rapidly if nucleic 

 acid precursors such as purines and pyrimidines are added to the amino acid 

 mixture which maintains bacterial growth. At the same time, the RNA content 

 of the bacteria increases. It is noteworthy that not only does omission of purines 

 and pyrimidines from the medium lead to a marked drop in the accumulation of 

 proteins, but the omission of a single essential amino acid also prevents the accu- 

 mulation of RNA. It is apparent that in this instance formation of proteins and 

 RNA are closely interlinked processes and impairment of either one results in 

 impairment of the other. 



In disrupted staphylococcus cells, formation of enzymes can be greatly stimu- 

 lated by the addition of nucleic acids or purines and pyrimidines. The degree of 

 stimulation by DNA, RNA, or purine-pyrimidine mixtures differs for each of the 

 investigated enzymatic activities. The decisive point in this investigation is the 

 finding that no incorporation of RNA precursors into RNA in preparations of 

 disrupted cells could be observed unless protein synthesis e.g. formation of 

 adaptive enzymes was occurring. The latter finding suggests again a close inter- 

 dependence of the synthesis of proteins and RNA (see also review by Gale, 

 1956). It must be pointed out, however, that it is not clear from these experiments 

 whether RNA acts in this system directly in some catalytic role or indirectly as 

 a reservoir of co-factors needed in the enzymatic production of metabolic energy. 



The importance of RNA in protein formation is fvirther emphasized by the 

 results of investigations carried out with yeast mutants requiring purines for 

 growth (Spiegelman, Halvorson and Ben-Ishai, 1955^). It was found that the pool 

 of purines and pyrimidines could be exhausted by rapid protein synthesis indi- 

 cating again a close connection between purine and protein metabolism. In addi- 

 tion, it was demonstrated that in these "purine free" yeast cells protein synthesis, 

 as measured by the rate of formation of induced enzymes, could be reactivated by 

 replenishing the purine pool in the cell by adding the required compound to 

 the medium. 



Similarly, in mutants of E. coli which can not synthesize certain precursors of 

 RNA, depletion of the pool of purines and pyrimidines in the bacteria causes a 

 marked delay in the onset of enzyme formation while in "normal" strains the 

 formation of adaptive enzymes starts immediately after addition of the inducing 



^ Full data unpublished. 



Literature p. 559 



