Functions as a (p)ppGpp synthase. 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. Plays a role in adaptation of Brucella to its intracellular host environment (By similarity). ATP + GTP = AMP + guanosine 3'-diphosphate 5'-triphosphate Belongs to the RelA/SpoT family.