436 F. GROS 



tyrosine pool by one-half, whereas this level is not affected in the case of 

 the other amino acids studied. 



Thus, 5-fluorouracil by being incorporated very rapidly into an "acceptor 

 RNA fraction" (probably the sRNA) modifies certain of its properties, such 

 as the capacity to bind proline and tyrosine, as to "adapt" these amino- 

 acids to the proper template. As a possible consequence of this fact, the 

 protein, newly synthesized in the presence of the analog, contains about 20 

 to 30 % less of these two amino acids than does normal protein. These two 

 amino acids are probably replaced by others (probably arginine) and this 

 explains why certain enzymes are not synthesized (or certain enzyme activ- 

 ities are not increased) after the addition of 5-fluorouracil to the culture. 



3. Nucleic Acid Synthesis in the Absence of Protein Synthesis 



It has just been shown that the cell machinery involved in the synthesis 

 of protein can function, at least for a limited period of time, in the absence 

 of nucleic acid synthesis: for instance, enzymes can be manufactured in a 

 bacterium no longer able to make either RNA or DNA as a result of base 

 starvation. The reverse is also true: a cell in which protein synthesis is 

 actively prevented can go on making RNA or DNA in quite large amounts. 

 This situation was first described by Gale in his classic observations on 

 bacteria treated by chloramphenicol. This type of uncoupling is of interest 

 mainly for two reasons: (1) it sheds some light on the mechanism of particle 

 formation; (2) it has led to the observation that free amino acids are in- 

 volved in RNA synthesis. 27 • 2S 



Bacteria treated with chloramphenicol can double (or triple) their con- 

 tent of nucleic acid; while synthesis of cytoplasmic protein is almost com- 

 pletely inhibited. This observation was first made in S. aureus lbb and has 

 been extended to other sensitive strains. 156 Chloramphenicol is not the 

 only agent able to produce this type of uncoupling. Aureomycin 155 and 

 erythromycin 157 have similar effects within a certain range of concentration. 

 This is also true for some amino acid analogs such as 5-methyltryptophan 

 which reduces protein synthesis by about 80%, and leaves the rate of 

 nucleic acid synthesis unchanged. Finally, in some very exceptional cases 

 a net increase in RNA content occurs without protein formation, e.g., in 

 amino acid auxotrophs lacking their essential amino acid. This is found 

 only in some strains of E. coli K12 which specifically require methionine. 158 

 While cytoplasmic proteins and enzymes are not synthesized in the pres- 



155 E. F. Gale, 2nd Intern. Congr. Biochem., Paris p. 1 (1952). 



156 C. L. Wisseman, J. E. Smadel, F. E. Hahn, and H. E. Hopps, J. Bacteriol. 67, 662 

 (1954). 



167 T. D. Brock and M. L. Brock, Biochim. el Biophys. Acta 33, 274 (1955). 

 158 E. Borek, A. Ryan, and J. Rockenbach, J. Bacteriol. 69, 460 (1955). 



