38. BIOSYNTHESIS OF PROTEINS IN BACTERIAL CELLS 421 



bolic stability in bacterial proteins, must, however, be examined critically. 

 In relation to the incorporation of radioactive amino acids or of O 18 into 

 the protein fraction of bacteria in the absence of net protein synthesis, it 

 must be kept in mind that such incorporation may involve only a certain 

 class of proteins (for example the proteins of the wall, the alcohol-soluble 

 fraction of Cowie et a/. 97 etc.). In any case the rate of this incorporation is 

 slow. 



In connection with the phenomenon of preferential synthesis, Naono's 

 recent experiments, 98 carried out to test Rickenberg's hypothesis of protein 

 removal during diauxic growth, are important to consider. E. coli was first 

 labeled in its protein fraction with S 35 following the method of Hogness et 

 aL 89 Cells were washed and incubated in a medium containing glucose plus 

 lactose, under the conditions described by Rickenberg. 95 /3-Galactosidase 

 was then isolated in the lag phase and in the phase of growth on lactose. The 

 enzyme was highly purified by ammonium sulfate fractionation, by zone 

 electrophoresis on starch columns, and finally by precipitation with anti- 

 serum. Determinations of radioactivity show that the rate of breakdown 

 of pre-existing protein during preferential formation of enzyme is less than 

 1 % of the rate of net synthesis. 



Accordingly, even if the proteins are capable of some renewal, this 

 phenomenon has been detected only under conditions where bacterial 

 growth is inhibited, 92, 94 and it may be inherent in this particular condition. 

 The observation by Halvorson" that some turnover can be observed in 

 nonproliferating yeasts whereas the protein of the actively growing cells 

 shows no turnover at all, strengthens this point of view. It is possible that 

 malnutrition leads to a certain degree of protein degradation. Indeed this 

 has been shown to be the case with the alcohol-soluble protein of E. coli 

 which breaks down completely under sulfur starvation while it is stable 

 under normal conditions. 97 



In tissue cultures, on the contrary, protein turnover can be shown to 

 occur, both in the resting and in the growing state. 100 Therefore it can be 

 concluded that, if any turnover of protein occurs, its rate is negligible com- 

 pared with the over-all rate of protein synthesis during normal growth of 

 bacteria. 



Proteins are not the only macromolecular components of the cell which 

 are metabolically stable under normal conditions, this is also true of the 



97 R. B. Roberts, P. H. Abelson, 1). B. Cowie, E. T. Bolton, and R. J. Britten; in "Stud- 

 ies of Biosynthesis in Escherichia cult" Carnegie Inst. Wash. Publ. 607, p. 318 (1957) . 



98 S. Naono, personal communication (1959). 



99 H. Halvorson, Biochim. et Biophys. Acta 27, 255 (195S). 



100 H. Eagle, K. A. Piez, R. Fleischman, and V. Oyama, J. Biol. Chem. 234(3), 592 

 (1959). 



