Inorganic polyphosphate kinase is required to stimulate protein degradation and for adaptation to amino acid starvation in Escherichia coli

Inorganic polyphosphate (polyP) kinase was studied for its roles in physiol ogical responses to nutritional deprivation in Escherichia coli. A mutant l acking polyp kinase exhibited an extended lag phase of growth, when shifted from a rich to a minimal medium (nutritional downshift). Supplementation o f amino acids to the minimal medium abolished the extended growth lag of th e mutant. Levels of the stringent response factor, guanosine 5'-diphosphate 3'-diphosphate, increased in response to the nutritional downshift, but, u nlike in the wild type, the levels were sustained in the mutant. These resu lts suggested that the mutant was impaired in the induction of amino acid b iosynthetic enzymes. The expression of an amino acid biosynthetic gene, his G, was examined by using a transcriptional lacZ fusion. Although the mutant did not express the fusion in response to the nutritional downshift; North ern blot analysis revealed a significant increase of hisG-lacZ mRNA. Amino acids generated by intracellular protein degradation are very important for the synthesis of enzymes at the onset of starvation. In the wild type, the rate of protein degradation increased in response to the nutritional downs hift whereas it did not in the mutant. Supplementation of amino acids at lo w concentrations to the minimal medium enabled the mutant to express the hi sG-lacZ fusion. Thus, the impaired regulation of protein degradation result s in the adaptation defect, suggesting that polyp kinase is required to sti mulate protein degradation.