In the stringent response, the synthesis of ribosomal and transfer RNAs in bacteria is inhibited when protein synthesis is blocked by amino acid starvation. This inhibition evidently prevents the unnecessary production of translational machinery when protein synthesis is blocked. During amino acid starvation a regulatory nucleotide, guanosine 3',5'-tetraphosphate (ppGpp), accumulates and 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.