Mechanism of the in vitro breakdown of guanosine 5'-diphosphate 3'-diphosphate in Escherichia coli.

Degradation of guanosine tetraphosphate (ppGpp) involves an enzyme associated with the ribosomal fraction from spoT+ strains of Escherichia coli. Double-label experiments with pp[3h]gpp, pp[3H]Gpp, or pp[3H]Gpp as substrate strongly suggest that ppG is the degradation product and that the enzyme releases two phosphates coordinately from the 3' position of ppGpp. In the absence of pppA this reaction proceeds in an uncoupled fashion, yielding ppG and PPi, but in the presence of pppA the decay is considerably enhanced and a pppA-ppi exchange reaction occurs in which the 3'-pyrophosphoryl group of ppGpp displaces the gamma and beta phosphates of pppA. Sodium PPi at 4 m7 inhibits decay of ppGpp regardless of whether or not pppA is present.