IRON AND OXYGEN REGULATION OF ESCHERICHIA-COLI MNSOD EXPRESSION - COMPETITION BETWEEN THE GLOBAL REGULATORS FUR AND ARCA FOR BINDING TO DNA

Manganese superoxide dismutase (the sodA gene product) in Escherichia coli, is negatively regulated by two global regulators, ArcA (aerobic respiration control) and Fur (ferric uptake regulation), coupling its expression to aerobic metabolism and the intracellular iron pool. Footprinting analyses were carried out on the sodA promoter region with purified Fur protein and with ArcA protein overproduced in crude extracts. ArcA is able to bind in vitro in the absence of the in vivo triggering signal. The binding occurs in one step and study of contacts within the operator sequence reveals binding on one side of the double helix. The DNA protection extends to a much larger region (about 65 bp) than would be expected for a 27 kDa protein, suggesting polymerization. Both Fur and ArcA footprints encompass the -35 and -10 promoter region and there is considerable overlap of their binding sequences, in agreement with in vivo results suggesting that either regulator alone can block sodA transcription. Furthermore, competition experiments show that Fur and ArcA binding to the sodA promoter are mutually exclusive and that ArcA can easily displace Fur, but not vice versa. The biological significance of this in vitro behaviour is discussed.