Putrescine catabolism is a metabolic response to several stresses in Escherichia coli.

Genes whose products degrade arginine and ornithine, precursors of putrescine synthesis, are activated by either regulators of the nitrogen-regulated (Ntr) response or σ(S) -RNA polymerase. To determine if dual control regulates a complete putrescine catabolic pathway, we examined expression of patA and patD, which specify the first two enzymes of one putrescine catabolic pathway. Assays of PatA (putrescine transaminase) activity and β-galactosidase from cells with patA-lacZ transcriptional and translational fusions indicate dual control of patA transcription and putrescine-stimulated patA translation. Similar assays for PatD indicate that patD transcription required σ(S) -RNA polymerase, and Nac, an Ntr regulator, enhanced the σ(S) -dependent transcription. Since Nac activation via σ(S) -RNA polymerase is without precedent, transcription with purified components was examined and the results confirmed this conclusion. This result indicates that the Ntr regulon can intrude into the σ(S) regulon. Strains lacking both polyamine catabolic pathways have defective responses to oxidative stress, high temperature and a sublethal concentration of an antibiotic. These defects and the σ(S) -dependent expression indicate that polyamine catabolism is a core metabolic response to stress.