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 sigma 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 -galactosidase from cells with patAlacZ transcriptional and translational fusions indicate dual control of patA transcription and putrescine-stimulated patA translation. Similar assays for PatD indicate that patD transcription required sigma S-RNA polymerase, and Nac, an Ntr regulator, enhanced the sigma S-dependent transcription. Since Nac activation via sigma 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 sigma 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 sigma S-dependent expression indicate that polyamine catabolism is a core metabolic response to stress.