An internal segment of the (p)ppGpp synthetase gene, relA, of Streptomyces coelicolor A3(2) was amplified from genomic DNA using the polymerase chain reaction and used as a hybridization probe to isolate the complete gene from a cosmid library. relA lies downstream of a gene (apt) that apparently encodes adenine phosphoribosyltransferase and is transcribed from two promoters, relAp1 and relAp2, and by transcriptional readthrough from apt. While the level of relAp2 transcripts remained relatively constant, relAp1 activity apparently peaked during transition phase, following a decline in readthrough transcription from apt. Disruption of relA using an att- derivative of the temperate phage phi C31 abolished ppGpp synthesis on amino acid depletion. When grown on agar, the disruptants grew more slowly than a control lysogen made with an att+ phage vector and gave smaller colonies that sporulated normally. The relA mutation had no consistent or marked effect on actinorhodin production in either liquid- or agar-grown cultures, indicating that elevated levels of (p)ppGpp are not essential for triggering the onset of antibiotic production.