In eubacteria ppGpp (guanosine 3'-diphosphate 5'-diphosphate) is a mediator of the stringent response that coordinates a variety of cellular activities in response to changes in nutritional abundance. This enzyme catalyzes the formation of pppGpp which is then hydrolyzed to form ppGpp (By similarity). ATP + GTP = AMP + guanosine 3'-diphosphate 5'-triphosphate Purine metabolism; ppGpp biosynthesis; ppGpp from GTP: step 1/2. Belongs to the RelA/SpoT family.