Guanosine 3',5'-tetraphosphate is an important regulatory nucleotide in the E. coli stringent response to amino acid starvation. When ppGpp accumulates, it somehow inhibits rRNA and tRNA synthesis. A class of mutants, called relaxed, does not accumulate ppGpp under these conditions and does not show the stringent response; that is, amino acid starvation does not cause inhibition of rRNA or tRNA synthesis. The action of ppGpp in controlling rRNA gene transcription is not yet known.