The product of the Escherichia coli aidB gene is homologous to human isovaleryl-coenzyme A dehydrogenase (IVD), an enzyme involved in the breakdown of the amino acid leucine. The aidB gene is not expressed constitutively, but its transcription is induced via distinct mechanisms in response to: (i) exposure to alkylating agents; (ii) acetate at a slightly acidic pH; and (iii) anoxia. Induction by alkylating agents is mediated by the transcriptional activator Ada, in its methylated form (meAda); the other forms of induction are Ada independent and require σs, the alternative σs factor mainly expressed during the stationary phase of bacterial growth. In this report we show that, in the absence of any transcriptional factor, aidB is efficiently transcribed in vitro by the σs, but not by the σ70, form of RNA polymerase holoenzyme. In the presence of meAda, levels of transcription by both forms of RNA polymerase are significantly increased. However, σs -dependent transcription of aidB is inhibited both in vitro and in vivo by binding of the transcriptional regulator Lrp (leucine responsive protein) to the aidB promoter region (PaidB)- Lrp acts as a specific repressor for σs -dependent transcription of aidB. Leucine counteracts Lrp binding to PaidB as does binding to PaidB of meAda, which causes Lrp to dissociate from the promoter. The physiological significance of aidB transcription regulation is discussed.