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, it is probably the hydrolysis activity that is required for optimal growth (Probable). ATP + GTP = AMP + guanosine 3'-diphosphate 5'-triphosphate Purine metabolism; ppGpp biosynthesis; ppGpp from GTP: step 1/2. Decreased growth rate; growth is almost completely restored in a triple relA-yjbM-ywaC mutant (PubMed:18067544). Belongs to the RelA/SpoT family.