Abstract
Abstract The increased protein turnover that accompanies amino acid starvation was examined in isogenic auxotrophs of Escherichia coli K-12 which differ only in the genetic locus responsible for control of RNA synthesis under conditions of amino acid starvation. The multiple auxotrophs investigated were CP78 (stringent RNA control) and CP79 (relaxed RNA control). The protein degradation which occurs following amino acid starvation in the stringent CP78 consists of two processes. (a) A chloramphenicol-insensitive breakdown which plays a large role early in the starvation period due to the presence of a pool of labile proteins, and (b) a chloramphenicol-sensitive proteolysis which causes the bulk of the observed protein degradation. Under normal conditions of amino acid starvation, there is considerably less protein degradation in the relaxed CP79 relative to the stringent CP78. This decreased level of degradation in the relaxed CP79 appears to result from a deficiency in the chloramphenicol-sensitive proteolytic process. However, under conditions of simultaneous starvation for both glucose and amino acids, proteolysis occurs to the same extent in CP78 and CP79. Whereas the withdrawal of glucose from the starvation medium has no effect on proteolysis in the stringent CP78, it increases degradation in CP79 to the level occurring in CP78. These results are discussed in terms of the pleiotropic nature of the control exercised by the RNA control locus.
Highlights
This delayed effect on proteolysis indicates that either (a) the relaxed mutation in CP79 reduces protein breakdown via an indirect time-dependent process or (b) the effect is immediate but is obscured by the fact that the degradation early in the starvation differs in some basic way from that occurring at later periods
Control experiments indicated that chloramphenicol and rifampicin were effective under these conditions; that is, the latter inhibited RNA synthesis in starved CP79 and the former relaxed RNA synthesis in starved CP78
When chloramphenicol is tested in the stable protein system, the apparent delay in the inhibition of proteolysis disappears just as was observed for the effect of the relaxed mutation. It appears that the protein breakdown following amino acid starvation in stringent bacteria consists of two processes. (a) A chloramphenicol-resistant degradation which plays a large role early in the starvation due to the presence of a pool of labile proteins, and (b) a chloramphenicolsensitive proteolysis which represents the major part of the observed degradation
Summary
The increased protein turnover that accompanies amino acid starvation was examined in isogenic auxotrophs of Escherichia coli K-12 which differ only in the genetic locus responsiblefor control of RNA synthesis under conditions of amino acid starvation. This delayed effect on proteolysis indicates that either (a) the relaxed mutation in CP79 reduces protein breakdown via an indirect time-dependent process or (b) the effect is immediate but is obscured by the fact that the degradation early in the starvation differs in some basic way from that occurring at later periods.
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