Interaction of alleles of therelA, relC andspoT genes inEscherichia coli: Analysis of the interconversion of GTP, ppGpp and pppGpp

Mutants in thespoT gene have been isolated as stringent second site revertants of therelC mutation. These show varying degrees of the characteristics associated with thespoT1 gene,viz relative amount and absolute levels of both pppGpp and ppGpp and the decay rate of the latter. The entry of3H-guanosine into GTP and ppGpp pools inspoT + andspoT1 cells either growing exponentially or during amino acid starvation was determined, and the rate of ppGpp synthesis and its decay constant calculated. During exponential growth the ppGpp pool is 2-fold higher, its decay constant 10-fold lower, and its synthesis rate 5-fold lower inspoT - than inspoT + cells; during amino acid starvation the ppGpp pool is 2-fold higher, its decay constant 20-fold lower, and its synthesis rate 10-fold lower inspoT than inspoT + cells. In one of the “intermediate”spoT mutants the rate of entry of3H-guanosine into GTP, ppGpp and pppGpp was measured during amino acid starvation. The data form the basis of a model for the interconversion of the guanosine nucleotides in which the flow is:GDP→GTP→pppGpp→ppGpp→Y. Calculations of the rates of synthesis and conversion of pppGpp and ppGpp under various conditions in variousspoT + andspoT - strains indicate that the ppGpp concentration indirectly controls the rate of pppGpp synthesis.