648 H. O. HALVORSON 
are briefly exposed to labeled glycine and then incubated in N-free medium, there is 
a rapid transfer of label from the free amino acid pool to the protein fraction, a 
synthesis of the inducible enzyme a-glucosidase, and a dilution in the specific activity 
of the glycine free amino acid pool. In E. coli both protein degradation and reincor- 
poration occur at the rate of 5°%/h (ref. 3). Chloramphenicol inhibits reincorporation 
but does not abolish protein degradation. 
Is protein synthesis preferential in non-dividing cells? Several lines of experimen- 
tation provide a negative answer to this question. First, the differential rate of 
synthesis (1 £/A newly formed protein) of f-galactosidase in E. coli and of a-gluco- 
sidase in yeast (HALVORSON, unpublished results) is the same for growing and non- 




Specific activity 
glycine pool 
: 
£ o 
= yAlpha- § 
é 8000 glucosidase = 
8 = 
5 z 
e 3 5 
BSS 
{e) 
{e) 
{e) 
O 
O 50 100 SSO) 
minutes 
Fig. 2. Utilization of [2-!4C]glycine-labeled pools during nitrogen starvation. 
Data from Hatvorson®. 
growing cells. Secondly, JANECEK® has shown in FE. coli that the electrophoretic 
pattern of [C}|phenylalanine incorporation into proteins under limited nitrogen 
supply is identical to that observed in exponentially growing cells. 
Effect of growth rate on protein turnover 
It was initially observed by Poporsky® that, when arginine-labeled EF. coli was trans- 
ferred to unlabeled medium, approximately 8% of the isotope was lost from the cells 
over a long semi-stationary phase. From these data MANDELSTAM! calculated a turn- 
over rate of 5% /h. Protein turnover rates in non-dividing cells of between 2-5 °%/h 
have been reported for E. coli#, 1.11, 7% /h for B. cereus and slime molds!? and 
0.7%/h for yeast®. 
In rapidly growing cultures of bacteria and yeast, the proteins are essentially 
stable and accurate measurements of turnover are difficult. The assumptions and 
limitations of the methods employed have been discussed elsewhere!) 18, 4. Employing 
analysis of the kinetics of dilution of prelabeled pools and also an internal trapping 
method, the rate of protein turnover in rapidly growing cells has been estimated 
to be less than 0.1%/h (refs. 13-16). Fox AND Brown?’, employing a less sensitive 
References p. 653/654 
