Functions as a (p)ppGpp synthase; GDP can be used instead of GTP, resulting in an increase of (p)ppGpp synthesis (PubMed:18067544). The enzyme binds ATP, then GDP or GTP and catalysis is highly cooperative (PubMed:26460002). 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. Probably has a minor role in the stringent response (PubMed:18067544). ATP + GTP = AMP + guanosine 3'-diphosphate 5'-triphosphate ATP + GDP = AMP + guanosine 3',5'-bis(diphosphate) Allosterically regulated by its own products; pppGpp simulates synthesis 10-fold more than ppGpp. 2 pppGpp molecules bind in a regulatory cleft in the middle of the tetramer in an asymmetric manner. There is a specific contact of Lys-25 to the gamma-phosphate of pppGpp, explaining why pppGpp stimulates activity but ppGpp does not (PubMed:26460002). Purine metabolism; ppGpp biosynthesis; ppGpp from GTP: step 1/2. Homotetramer (PubMed:26460002). Expressed during exponential growth through the transition to the stationary phase, but not during entry of cells into stationary phase (PubMed:18067544). Protein continuously expressed with a peak at 2 hours after inoculation, decreasing until 3.5 hours as cells enter stationary phase (at protein level) (PubMed:22950019). No visible phenotype, double yjbM-ywaC and triple relA-yjbM-ywaC mutants are also viable (PubMed:18067544). The synthase activity of YjbM is greater than that of YwaC (PubMed:18067544). Belongs to the RelA/SpoT family.