PROTEIN TURNOVER IN MICROORGANISMS 653 
in several ways. If, under conditions of turnover, an inducer is added’ or depression 
established, such as phosphate starvation for alkaline phosphatase synthesis‘, 
reincorporation is accompanied by an increased differential rate of synthesis of a 
particular enzyme. A second type of enrichment can occur following extensive turn- 
over if the newly synthesized enzyme is selectively stable to degradation!’ 19, An 
example of this may occur in the case of /-galactosidase in EF. colt. 
The interesting question remains as to whether turnover exists in rapidly growing 
cultures and if so, whether it has any physiological significance. This problem is 
technically difficult and any turnover if it occurs is at a low rate. In recent years 
several types of RNA have been shown to participate in protein synthesis and in 
turnover, at least in part, 7m vivo. Of particular interest are the observations of 
SYLVEN et al.4® who observed a cyclic variation in the activities of peptidase and 
proteinase during the synchronous growth of yeast. The highest cellular levels of 
enzyme occurred immediately prior to cell division, declining to minimum levels 
during subsequent budding. These enzymes could provide alternating conditions 
for intracellular protein degradation associated with a particular stage in cellular 
division. Similar variations have been observed in the phosphatase activity of 
yeast*? and in the metaphosphate content of Corynebacterium diphtheriae*® during 
synchronous cell division. 
ACKNOWLEDGEMENTS 
This work was supported in part by research grants from the National Institute of 
Health, National Science Foundation and the U.S. Air Force Office of Scientific 
Research of the Air Research and Development Command. 
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