652 H. O. HALVORSON 
Mechanism of protein and RNA degradation 
From the previous discussion it is clear that the degradative enzymes pre-exist to 
the initiation of RNA and protein turnover. This was confirmed by the observations 
that the same content of proteolytic activity exists in extracts of growing and non- 
dividing cells of E. coli®*°. The in vitro rate of degradation was about equal to that 
observed 77 vivo with non-dividing cells. Proteolytic activity in extracts was in- 
creased by conditions leading to a breakdown in cellular structures. An explanation 
for this phenomenon has been recently provided by the demonstration of a latent 
aminopeptidase associated with EF. coli ribosomes®®, which are themselves labile. In 
yeast ribosomal disruption activates the proteolytic actively (HALVORSON, un- 
published results). 
A similar situation has been observed for ribonuclease. This enzyme is almost ex- 
clusively present in bacteria®*~%° and yeast? as a latent, ribosomal-bound enzyme. 
These same particles contain approx. 90% of the RNA and 30% of the protein of 
microbial cells*!. Ribosomal stability is dependant upon the concentration of Mg?+ 
(refs. 27, 41). When the Mg?+ is removed the larger particles dissociate into smaller 
particles, the latent enzymes are released and the fragments eventually undergo 
autodegradation*?. When intact cells are starved for carbon and nitrogen or Mg?* 
(refs. 27, 43), an analogous ribosomal breakdown is observed. When starved cells 
are transferred to complete medium, the larger ribosomal particles are stabilized. 
These findings provide a model for turnover based on the stability of the ribosomal 
particles. In actively growing cells ribosomes are intact and the hydrolytic enzymes 
inactive. When cells are starved, a fall in the level of phosphorylated intermediates 
follows which is linked either to Mg?+ removal or some other process which renders 
the ribosomal particles labile. During the breakdown of these particles, RNA, protein 
and hydrolytic enzymes are liberated. 
Support for this scheme is provided by the observation 7m vivo that in non-growing 
E. coli there is a balanced degradation of both RNA and protein? 74. Degradation 
of the ribosomal components proceeded at the same rate as the soluble components. 
The observation that under some conditions differential breakdown occurs (Table IT) 
suggests that the hydrolases can either be selectively inhibited or that alternate 
degradation mechanisms exist. 
Significance of intracellular turnover 
Intracellular turnover provides a mechanism whereby the phenotype of a cell can 
be changed in non-dividing cells. Under such conditions growth is dependent upon 
the turnover rate; the amino acids and nucleotides required for synthesis and growth 
are supplied by protein and RNA breakdown. In several cases, such as endotropic 
sporulation” and differentiation in slime molds’, cellular morphogenesis is observed 
in non-dividing cells which involves changes in cell morphology, chemical composi- 
tion, enzyme patterns, metabolism and in the types and distribution of macro- 
molecules. It is of interest that these occur in cells with high turnover rates. To a 
lesser extent, some of the precursors required for the synthesis of phage in infected 
cells are also derived from endogenous macromolecules*®. 
Turnover can also lead to selective enrichment of an enzyme in non-dividing cells 
References p. 653/654 
