In eubacteria ppGpp (guanosine 3'-diphosphate 5'-diphosphate) is a mediator of the stringent response which coordinates a variety of cellular activities in response to changes in nutritional abundance. This enzyme catalyzes both the synthesis and degradation of ppGpp. The second messengers ppGpp and c-di-GMP together control biofilm formation in response to translational stress; ppGpp represses biofilm formation while c-di-GMP induces it. ppGpp activates transcription of CsrA-antagonistic small RNAs CsrB and CsrC, which down-regulate CsrA's action on translation during the stringent response (PubMed:21488981). ATP + GTP = AMP + guanosine 3'-diphosphate 5'-triphosphate guanosine 3',5'-bis(diphosphate) + H2O = diphosphate + GDP + H(+) Purine metabolism; ppGpp biosynthesis; ppGpp from GDP: step 1/1. Purine metabolism; ppGpp biosynthesis; ppGpp from GTP: step 1/2. Interacts with ObgE/CgtA (AC P42641); this association is not required for pre-50S ribosome binding. Under both amino acid and carbon starvation conditions about half of the protein dissociates from the ribosome. Is associated with pre-50S ribosomal subunits in a salt-dependent manner. In the presence of wild-type relA, spoT cannot be deleted because ppGpp accumulation is toxic. In the double relA/spoT deletion (a ppGpp0 mutant) there is a very large increase in biofilm formation (in a csrA-disrupted background) (PubMed:19460094). The ppGpp0 mutant makes decreased levels of CsrA and its inhibitory small RNAs (sRNA) CsrB and CsrC (PubMed:21488981). The double relA/spoT deletion obviates persister cell formation in a hipA7 mutant (PubMed:14622409, PubMed:26051177). Belongs to the RelA/SpoT family.