Wild-type Escherichia coli deprived of a required amino acid shows a stringent response involving various alterations in the cell's normal physiology, including the arrest of net synthesis of stable (ribosomal and transfer) RNA, lipids, phospholipids and peptidoglycans1. The stringent response is found in cells of the Rel+ (stringent) but not Rel− (relaxed) phenotype. The intracellular concentration of guanosine 5′-diphosphate-3′-diphosphate (ppGpp) and guanosine 5′-triphosphate-3′-diphosphate (pppGpp) increases dramatically in amino acid-deprived Rel+ cells due to the enzymatic activity of the relA gene product2, which also stimulates ppGpp synthesis in vitro3. The correlation between stringent response and increase in the intracellular concentration of ppGpp has been interpreted as evidence that this nucleotide has a causal role in stringency1. However, as recent findings show that the correlation does not always hold, ppGpp cannot be considered to act as a unique effector molecule and the idea of a causal role for ppGpp in stringent control has become questionable4,5. Here we describe experiments that cast further doubt on the proposed biological role of this nucleotide, as they show that stringent control can occur in the absence of ppGpp accumulation.