A proteolytic system in growing and non-growing cells ofEscherichia coli

Abstract

(1) Escherichia coli cells contain a protease system splitting its own proteins as well as casein labelled with131I at a slightly alkaline pH. The rate of proteolysis is not markedly influenced by ethylenediaminotetraacetic acid or by iodoacetic acid, p-chlormercuribenzoate diminishes the rate of hydrolysis of131I casein but its effect cannot be reversed by cysteine. (2) The proteolytic system is not present in the cells in the form of an enzymogen and its activity is not diminished even after 55 passages on a synthetic medium containing ammonium chloride as the sole nitrogen source. (3) In living cells proteins labelled with36S methionine are degraded at a rate not exceeding 0.2%/hr. at 29–30°, both in a growing culture and in the normal stationary phase. Similarly, the protease activity in cell-free extracts does not fundamentally change in the course of development of the culture. (4) Proteins in extracts are split at a rate of 1.8–2.3%/hr. at 32° according to results based on colorimetric estimations with tyrosine, or at a rate of 1.1–1.3%/hr, at 32° as shown by the course of degradation of35S proteins. No resynthesis of proteins from liberated amino acids took place in the extracts under the described experimental conditions. (5) The decomposition of labelled proteins in cell-free systems was stimulated by dinitrophenol and sodium azide.