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. Foot printing analyses were carried out on the sodA promoter region with pu rified Fur protein and with ArcA protein overproduced in crude extract s. ArcA is able to bind in vitro in the absence of the in vivo trigger ing signal. The binding occurs in one step and study of contacts withi n the operator sequence reveals binding on one side of the double heli x. The DNA protection extends to a much larger region (about 65 bp) th an 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 agre ement with in vivo results suggesting that either regulator alone can block sodA transcription. Furthermore, competition experiments show th at Fur and ArcA binding to the sodA promoter are mutually exclusive an d that ArcA can easily displace Fur, but not vice versa. The biologica l significance of this in vitro behaviour is discussed.