An enzyme that catalyzes reversibly the transfer of a pyrophosphate group from ATP to the 3'-OH group of GDP or GTP with the formation of guanosine 3'-diphosphate 5'-diphosphate or guanosine 3'-diphosphate 5'-triphosphate and AMP. The enzyme, also called stringent factor, is located in the relA gene in stringent strains of bacteria. The above synthesis is induced by mRNA and uncharged tRNA which is bound to the aminoacyl-t-RNA binding site of the ribosome by a codon-specific association. EC 2.7.6.5.
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