On the Regulation of Guanosine Tetraphosphate Levels in Stringent and Relaxed Strains of Escherichia coli

We have examined the intracellular levels of the guanosine tetraphosphate, guanosine 5'-diphosphate 2'- or 3'-diphosphate (ppGpp), in an isogenic pair of stringent and relaxed strains of Escherichia coli. As previously observed, relaxed strains do not accumulate ppGpp during amino acid deprivation, while stringent strains do. However, under all other culture conditions examined, relaxed and stringent cells regulate their ppGpp levels similarly. During carbon deprivation or step-down transitions both strains accumulate ppGpp. Neither strain accumulates appreciable amounts during phosphate deprivation or after treatment with colicin E1. The basal levels of ppGpp observed in both stringent and relaxed strains during balanced growth are, in most cases, the same, and vary inversely with the growth rate and RNA content of the cells.