CATABOLITE REGULATION OF 2 ESCHERICHIA-COLI OPERONS ENCODING NITRITE REDUCTASES - ROLE OF THE CRA PROTEIN

The Escherichia coli nil and nrf operons, which encode alternative nit rite reductases expressed during anaerobic growth, are subject to cata bolite regulation. Transcription from the nir promoter is maximal when bacteria are grown in rich media such as Lennox broth supplemented wi th glucose. Conversely, expression of the nrf operon is suppressed by rich media, but stimulated during growth in minimal medium with glycer ol and fumarate. The role of the catabolite repressor-activator (Cra) protein in catabolite regulation of the nir and nrf promoters was inve stigated. Transcription from the nir promoter was re pressed by Cra wh en cells were grown in minimal medium with glycerol and fumarate. Crud e protein extracts from a strain overproducing Cra encoded on a multic opy plasmid retarded a nir promoter fragment in a mobility shift assay , confirming that the observed Cra-dependent repression was due to the direct interaction of Cra with the regulatory region of the nil-opero n. Furthermore, the inclusion of fructose l-phosphate, an effector of Cra DNA-binding activity, in the assay decreased the ability of Cra to retard the nir promoter fragment. In contrast, transcription from the nrf promoter was not regulated by Cra under any of the growth conditi ons tested.