Turnover of cellular
protein has been estimated in Escherichia coli during continuous exponential growth and in the absence of extensive experimental manipulation. Estimation is based upon the cumulative release into carrier pools of free leucine-1-(14)C over a number of time intervals after its pulsed incorporation into
protein. Breakdown rates obtained with other labeled
amino acids are similar to those obtained with
leucine. Two kinetically separate processes have been shown. First, a very rapid turnover of 5% of the
amino acid label occurs within 45 sec after its incorporation, most likely indicating maturative cleavages within the
proteins after their assembly. A slower heterogeneous rate of true
protein turnover follows, falling by 39% in the remaining
proteins for each doubling of turnover time. At 36 C, the total breakdown rate of cellular
protein is 2.5 and 3.0% per hr over a threefold range of growth rate in
glucose and
acetate medium, respectively. This relatively constant breakdown rate is maintained during slower growth by more extensive
protein replacement, one fifth of the
protein synthesized at any time in the
acetate medium being replaced after 4.6 doubling times. Intracellular proteolysis thus appears to be a normal and integral reaction of the growing cell. The total rate equals minimal estimates obtained by others for arrested or decelerated growth but is kinetically more heterogeneous. Quantitatively proteolysis is not directly affected by growth arrestment per se as caused by
alpha-methylhistidine,
chloramphenicol, or uncouplers of oxidative phosphorylation, but qualitatively it can gradually become more homogeneous kinetically as a secondary event of
starvation. Under more extreme conditions as with extensive washing, prolonged phosphorylative uncoupling, or acidification of the growth medium, the proteolytic rate can increase severalfold.