Turnover of protein in Escherichia coli starving for nitrogen

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1. 1. Rates of degradation and synthesis. Degradation of pre-labeled protein in starving cells, measured by the exchange into the medium of [ 14C]leucine, proceeds at a rate of 2–3 %/h for 6–8 h at 37°, then abruptly shuts off almost totally. Meanwhile the incorporation of amino acids into peptide bonds, and the amounts of β-galactosidase (EC 3.2.1.23) and alkaline phosphatase (EC 3.1.3.1.) formed, decrease continuously (to less than 0.3 % synthesis of protein/h). Furthermore, at any time, only about 0.05 as much ribosomal protein is made, relative to soluble protein, as is formed in growing cells. 2. 2. Lack of exchange of amino acids in auxotrophs. When an auxotroph is starved for an amino acid as well as for nitrogen, its uptake of other amino acids is severely inhibited. Nevertheless, synthesis of enzyme proteins, as a result of turnover, continues about as well as in cultures starved only for nitrogen. 3. 3. Effect of inhibitors on proteolysis in cells. Degradation of protein is shut off when synthesis of protein is inhibited by chloramphenicol, 2,4-dinitrophenol, 5-methyltryptophan or proflavine at any time during starvation. It continues, however, when amino acid analogues (ethionine, norleucine, p-fluorophenylalanine) are added to the medium, suggesting that a small amount of peptide bond synthesis, but not necessarily of any particular protein, is required for breakdown of protein to continue. 4. 4. It is suggested that the level of amino acids or amino-acyl sRNA may determine both the activity of pre-existing proteases and the permeability to more amino acids in starving cells.