purine nucleotide biosynthesis purine metabolism ppGpp biosynthesis MetaCyc:PPGPPMET-PWY guanosine 5'-diphosphate 3'-diphosphate biosynthesis 5'-ppGpp-3' biosynthesis pathway guanosine tetraphosphate biosynthesis GO:0015970 Biosynthesis of ppGpp (guanosine tetraphosphate, guanosine 5'-diphosphate 3'-diphosphate), a derivative of guanine riboside. The stringent response to amino acid starvation, whereby stable RNA synthesis is curtailed in favour of transcription of amino acid biosynthetic genes, is controlled by the alarmone ppGpp. UPa:UPA00908 KEGG:map00230