Synthesis of Guanosine Tetra- and Pentaphosphate Requires the Presence of a Codon-Specific, Uncharged Transfer Ribonucleic Acid in the Acceptor Site of Ribosomes

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Abstract
The nucleotides ppGpp and pppGpp accumulate when wild-type, but not rel - , strains of Escherichia coli are starved for required amino acids. These compounds are synthesized on ribosomes, in the presence of the product of the rel gene, from GDP and GTP; ATP is used as the phosphate donor. The signal for making these compounds is the presence of an uncharged tRNA in the ribosomal acceptor site. These compounds are not accumulated if the ribosomes are actively engaged in protein synthesis.