PROTEIN TURNOVER IN MICROORGANISMS 649 
method, estimated the rate of protein turnover in FE. coli to be approx. 2.7°%/h. 
One clear exception has appeared. UrBA* has shown that in growing B. cereus the 
turnover rate is 1.4%/h. 
Although the turnover rates in growing cells are uncertain, one can conclude 
from the present data that the absolute rate of protein turnover is appreciably 
greater in non-dividing cells than in growing cells. 
Degradation of cellular components 
Do all of the cellular proteins participate in intracellular turnover? That heterogeneity 
towards protein turnover exists is suggested by the findings that under conditions 
in which protein turnover is taking place, certain inducible enzymes are stable in 
the absence of their inducer!: 18, 19 whereas others2® 21, as well as the alcohol-soluble 
proteins of EF. coli??, are labile. 
TABLE I 
TURNOVER OF PROTEIN AND NUCLEIC ACID FRACTIONS IN NITROGEN-STARVED BACTERIA 


% breakdown* % synthesis* 



Fractions —~ — —— —_—__—___— — ———_—_— 
RNA Protein RNA Protein 
Soluble 5 25 14.5 16.5 
Ribosomal 24 21.5 6.0 21.5 

* 4h starved. Data from MANDELSTAM AND HALvVoRSON??. 
Turnover can involve a significant proportion of cellular protein. Approximately 
half of the total protein of B. cereus* and one quarter of the protein of EF. colz? are 
subject to turnover. Such turnover involves both structural (ribosomal particles) 
and soluble proteins”: 24. As shown in Table I, in non-dividing EF. coli the proteins 
of the ribosomes and of the soluble fraction are equally labile and both are degraded 
at approx. 5°%/h. This process is approximately balanced by the rate of resynthesis. 
Ribosomal nucleic acid is degraded at about the same rate as protein, however, the 
rate of synthesis of RNA occurs at a rate of only 1.5°%%/h. Intracellular stability may 
depend upon starvation conditions. In E. coli the alcohol-soluble protein is labile 
during sulfur starvation”? but stable in nitrogen starvation.! 
Imitation of turnover 
At least two mechanisms can be involved to explain the increased turnover rate in 
non-dividing cells: 
1. Synthesis of intracellular degradation system(s) by induction or depression. 
2. Activation of the pre-existing degradative system(s) because of the removal of 
an endogenous inhibitor or unmasking from a latent form. 
Some insight into this problem can be achieved by examining the conditions 
which lead to intracellular breakdown and turnover. In general, specific starvations, 
References p. 653/654 
