646 DYNAMIC ASPECTS — AMINO ACID POOL TURNOVER 
THE FUNCTION AND CONTROL OF INTRACELLULAR PROTEIN 
TURNOVER IN MICROORGANISMS 
HARLYN O. HALVORSON 
Department of Bacteriology, University of Wisconsin, Madison, Wisc. (U.S.A.) 
Microorganisms contain metabolic pools of nucleotides and amino acids which are 
on the main line of synthesis of macromolecules. As had just been described in this 
Symposium by Dr. Cowie, these endogenous metabolic pools are derived either 
from exogenously provided components or synthesized 7 vivo from NH, and glucose. 
The recent demonstrations in bacteria, yeast and mammalian cells that under cer- 
tain conditions nucleic acids and proteins are labile and are degraded to their con- 
stituent precursors (see review by MANDELSTAM?!) provides yet another mechanism 
for internally replenishing these metabolic pools. Under conditions permitting pro- 
tein and nucleic acid synthesis, a dynamic turnover exists due to the simultaneous 
breakdown and reutilization of the degradation products for synthesis. 
Two of the central questions raised by the demonstration of protein and nucleic 
acid turnover are: 
1. What are the control mechanisms governing its function? 
2. Does this turnover have any significance to the physiology of the cell? 
Demonstration of intracellular protein turnover 
For the present discussion we will define protein turnover as the coupled degradation 
of protein to amino acids and their re-incorporation into newly synthesized protein. 
Given a population of microbial cells, at least three mechanisms can be envisaged 
for such turnover: 
cell lysis or secretion remaining 
1. Cell turnover: Protein > AA > protein 
proteolysis intact cells 

2. Exchange incorporation: Protein = AA 
Oo 
Protein 
As 
3. Intracellular degradation and vesynthesis: Ks k, 
AA x 
Exchange incorporation? was proposed to account for a chloramphenicol-resistant 
incorporation of glutamic acid and glycine into the hot trichloroacetic acid-soluble 
fraction of staphylococci. Presumably amino acids in the protein were replaced by 
identical free amino acids. Failure to demonstrate such chloramphenicol-resistant 
References p. 653/654 
