420 F. GROS 



containing an inducer for /3-galactosidase, the newly formed enzyme, after 

 extensive purification, shows no radioactivity. This experiment and some 

 variants of it, led to the conclusion that in normal^ growing E. coli, the 

 proteins are metabolically stable. A similar conclusion was reached by Rot- 

 man and Spiegelman 90 in studying /3-galactosidase formation in E. coli 

 previously labeled by incorporation of lactate-C 14 . 



The lack of turnover in microbial proteins has also been illustrated by 

 Koch and Levy's experiments, 91 based on quite a different principle. Glycine 

 is the precursor both of the protein glycine and of the purines of nucleic 

 icids. Bacteria were therefore grown in the presence of glycine-C 14 plus a 

 mixture of nonradioactive purines in order to prevent labeling of the RNA 

 fraction. Such cells with labeled proteins were transferred to a simple min- 

 eral medium without any radioactive tracer or purines and allowed to 

 grow for a few generations. If protein turnover takes place, the glycine 

 liberated by the breakdown of the protein would lead to labeling in the 

 purines of the nucleic acid. The specific radioactivities of the isolated purines 

 showed however, that the protein turnover was quite negligible. 



During the last few years 9294 doubt has been thrown on these conclusions 

 regarding the stability of bacterial protein. It has, for example, been ob- 

 served that amino acid-requiring mutants of E. coli, when starved of their 

 essential metabolites, still incorporate into their proteins radioactivity from 

 other amino acids 92 or from H 2 18 . 94 Moreover, the pool of free amino acids 

 in E. coli rises appreciably above the normal value when bacteria are al- 

 lowed to metabolize glucose in a medium where net protein synthesis is 

 not possible. Finally Rickenberg 96 interprets his finding of preferential syn- 

 thesis of enzymes as suggesting a metabolic instability of the protein in 

 E. coli; when grown in the presence of a mixture of glucose and lactose, E. 

 coli utilizes glucose first and lactose only after glucose exhaustion. 96 During 

 the lag period which separates growth on glucose from growth on lactose, 

 the bacteria show no increase in total protein but they begin to synthesize 

 /3-galactosidase very rapidly. Rickenberg's interpretation of these results 

 is that preexisting protein could undergo turnover and liberate endogenous 

 products which would be reutilized for the formation of /3-galactosidase. 



These arguments which might appear to disprove the concept of meta- 



90 B. Rotman and S. Spiegelman, J. Bacteriol. 68, 419 (1954). 



91 A. L. Koch and H. R. Levy, J. Biol. Chem. 217, 947 (1955). 



92 J. Mandelstam, Nature 179, 179 (1957). 



93 J. Mandelstam, Biochem. J. 69, 110 (1958). 



94 E. Borek, L. Ponticorvo, and D. Rittenberg, Proc. Natl. Acad. Sci. U. S. 44, 369 

 (1958). 



95 H. V. Rickenberg and C. T. Lester, J. Gen. Microbiol. 13, 279 (1955). 



96 J. Monod, "Recherches sur la croissance des cultures bacte>iennes." Herman et 

 Cie. Paris, 1942. 



