PROTEIN TURNOVER IN MICROORGANISMS 647 
incorporation in Escherichia coli? and Bacillus cereus* led to the demonstration in 
staphylococci that in the presence of chloramphenicol, glutamic acid and glycine 
were involved in de novo cell wall synthesis rather than protein synthesis?: °. 
Examples of turnover due either to cell turnover or to intracellular degradation 
and resynthesis exist in the microbial literature. In order to demonstrate the latter, it 
is necessary to show that cell turnover does not occur or is insignificant. An example 
of this has been provided in yeast® as shown in Fig. 1. When phenylalanine- or leucine- 
labeled yeast are incubated under conditions of carbon and nitrogen starvation, 
protein breakdown leads to a linear replenishment of the cold trichloroacetic acid- 
a—=x« Phenylalanine 
o—o Leucine 
e—e Alpha-. 
Glucosidase 




Intracellular pool 
‘lo Release 

Medium 

200 300 
minutes 
Fig. 1. Kinetics of replenishment of the free amino acid pool and the release of components into 
the medium. Data from HALvorson®. 
soluble pool at the rate of 0.76°,/h. Similar results were observed with a number 
of amino acids examined suggesting that overall protein degradation is taking place. 
The initial replenishment may involve the internal pool since exchange with the 
medium is minimal and addition of glucose leads to a rapid reincorporation. During 
the extended protein degradation, the level of the intracellular amino acids rises 
and eventually amino acids appear in the medium. It is not clear whether the amino 
acids flow through the expandable pool during their excretion. 
Cell lysis could be ignored in the experiment in Fig. 1 since: (a) only traces of 
amino acids are released to the medium; (b) an intracellular inducible enzyme, 
a-glucosidase, appeared in the medium only at the rate of 0.03°%/h; and (c) single 
cell isolates from 5-h starved cells showed only 2 out of 124 cells to be non-viable. 
MANDELSTAM® arrived at similar conclusions from the failure to detect /-galacto- 
sidase release during protein turnover in EF. coli. From these experiments it seems 
clear that the replenishment of the metabolic pool in non-dividing cells arises prima- 
rily from intracellular protein degradation. 
Synthetic capacities under conditions of limited growth 
The rate of protein turnover can be measured either by measurements of protein 
degradation (k,) in the presence of a sufficient trap for amino acids or by direct 
measurement of reincorporation (,). This is illustrated in Fig. 2. When yeast cells 
References p. 653/654 
