PROTEIN TURNOVER IN MICROORGANISMS 651 
Metabolic regulation of turnover 
The synthesis of protein and nucleic acids are coupled to energy-yielding reactions. 
Thus restricting the energy supply reduces the rate of reincorporation of pool com- 
ponents into macromolecules. Of particular interest was the finding that the break- 
down of protein and nucleic acid in non-dividing cells is suppressed by agents which 
inhibit energy-yielding reactions?: ® 38, 34, An example of this is shown in Table III. 
When prelabeled yeast is incubated with 2,4-dinitrophenol (DNP), azide or arsenate, 
protein and nucleic acid degradation is almost completely abolished. MANDELSTAM?® 
has observed a similar effect with DNP in EF. coli. Azide inhibited protein break- 
down only after a 45-min delay suggesting that it causes a gradual accumulation 
within the cell of some inhibitory metabolite. 
TABLE III 
EFFECT OF ENERGY INHIBITORS ON PROTEIN AND NUCLEIC ACID BREAKDOWN IN YEAST 
Data from HALvorRson® 

; a Starvation period 
Incubation conditions (h) p —— eS 


Protein Nucleic acid 
4° 10-4 WM dinitrophenol I 89 
2 gI 
4 92 98 
2:10-? M azide 5 100 
2: 10-2 M arsenate 5 100 


The above inhibition of the degradative system(s) can be understood either if its 
activation or function required energy or if its activity were regulated by metabolites 
whose levels are controlled by inhibitors. An energy dependence of the degradative 
system(s) would seem unlikely on several grounds. First, as will be discussed later, 
RNA and protein degradation appear to be mediated by strictly hydrolytic reactions. 
Secondly, at least for RNA degradation, conditions can be established in which 
these inhibitors have no effect. Horrucui?’ observed a 5°%/h breakdown of RNA in 
E. coli suspended in phosphate-free medium which was unaffected by DNP and 
azide and stimulated by arsenate. 
If the degradative system(s) is regulated by the intracellular concentration of low 
molecular-weight metabolites then conditions which alter the steady-state level of 
metabolic pools should influence degradation. Examples of this occurs during glucose 
metabolism under reduced growth rate or during selective starvation when anabolic 
reactions are unlinked from catabolic reactions. Phosphorylated metabolites may 
play an essential role in this regulation since inhibitors of oxidative phosphorylation 
inhibit breakdown and phosphate deficiency both stimulates this reaction and 
renders the system insensitive to such inhibitors. These observations suggest that 
the activation of the degradative reactions may be linked to reduced levels of some 
phosphorylated metabolites. 
References p. 653/654 
