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Protein Degradation in Escherichia coli

II. STRAIN DIFFERENCES IN THE DEGRADATION OF PROTEIN AND NUCLEIC ACID RESULTING FROM STARVATION
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      Based on leucine exchange measurements of cells labeled with [14C]leucine, we had previously reported (J. Biol. Chem., 245, 2889 (1970)) that only a limited class of protein of Escherichia coli B is subject to a first order process of rapid degradation. The rate of degradation (half-life of 60 min or faster) and the total amount of protein undergoing degradation (2 to 7%) was the same during growth and during various kinds of starvation. We extend this finding and report here that the release of this “rapidly degrading protein” is unaltered not only during growth and starvation but also during growth in an enriched medium (step-up), in a poor medium (step-down), and during the diauxic phase of induced β-galactosidase synthesis.
      The extent of this observed degradation (less than 10%) is lower than reported by other workers (20 to 35%). This discrepancy is not a consequence of experimental manipulation or cellular damage but reflects differences in bacterial strains. Although strains ML and K-12 under conditions of growth release radioactivity similar in amount and half-life to the rapid protein degradation process observed in strain B, during several conditions of starvation, degradation of an additional class of cellular protein can be measured by leucine exchange in these two strains but not in B. The starvation-induced protein degradation occurs at a rate of 2.5 to 6% per hour, and the amount of the cellular protein that degrades as a result of starvation amounts to 20 to 40% of the total bacterial protein. Simultaneous with this starvation-induced protein degradation is the excretion of nucleic acid degradation products into the medium amounting to 40 to 60% of the ordinarily stable nucleic acid of growing cells. Starvation-induced nucleic acid degradation occurs in all strains equally. With the findings of these strain differences, much of the conflict in the literature can be explained satisfactorily.
      We conclude that a continuous process of protein degradation occurs in bacterial cells at all times, and an additional process of protein degradation concomitant with nucleic acid degradation is initiated following starvation of nutrients. Some of the properties of the degradation processes under starvation conditions are as follows. While almost all of the protein degradation products released in the presence of carrier leucine are acid soluble, a large portion of nucleic acid degradation products released from the cells are precipitable by cold trichloroacetic acid. Both protein and nucleic acid degradation occur under starvation of either glucose, nitrogen, or phosphate. Inhibition of protein synthesis by chloramphenicol at 100 µg per ml inhibits the starvation-induced protein degradation, but does not affect the degradation of the rapidly degrading protein.

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