Effects of reduced energy production on protein degradation, guanosine tetraphosphate, and RNA synthesis in Escherichia coli.

In Escherichia coli (strain A33, ATCC No. 27873) protein degradation increases and RNA synthesis falls under conditions that reduce the cell’s capacity to generate ATP (e.g. glucose or phosphate starvation). To test whether reduced energy production might signal these responses, cellular ATP content was lowered to different extents with various concentrations of inhibitors of electron transport. Concentrations of cyanide or azide which caused a moderate reduction (30 to 50%) in ATP content led to the cessation of growth, a decrease in RNA synthesis, and a doubling in the rate of breakdown of normal cell proteins. Under these conditions, the degradation of abnormal cell proteins containing canavanine was not altered. In contrast, concentrations of cyanide or azide that lowered ATP by 80% or more reduced the degradation of both normal and abnormal proteins below rates found in growing cells. Thus, two distinct effects of reduced ATP production were demonstrated: 1) a moderate inhibition of energy metabolism accelerated the breakdown of normal cell proteins in similar fashion to starvation; 2) a drastic reduction of ATP levels inhibited the degradation of all cell proteins. Roth rel A’ and rel Acells increase protein degradation when treated with these inhibitors of respiration. Under these conditions both strains show a marked accumulation of guanosine-3’-diphosphate-5’-diphosphate (ppGpp). Thus in addition to the well established changes in ppGpp synthesis determined by levels of charged tRNA, ppGpp content also depends on the intracellular energy levels. The accumulation of ppGpp when ATP production was inhibited resulted primarily from a 6to g-fold decrease in the degradation of this nucleotide, in accord with earlier reports on cells deprived of glucose. Addition of tetracycline, an inhibitor of ppGpp synthesis, to cells treated with cyanide or azide caused a rapid decrease in ppGpp levels and lowered protein catabolism toward the levels found in growing cells. Tetracycline also reduced protein breakdown in cells deprived of a carbon source or