Abstract

In Escherichia coli, the counterbalances between superoxide dismutase (SOD) and hydroperoxidase are more important for overall sensitivity to oxidative stress such as paraquat (PQ, 0.1 mM), H_2O_2 (1mM), CuSO_4(0.1 mM), CuSO_4 and heat shock (37℃→42℃) than the level of either SOD or HP1 alone. To evaluate the relative coordinate defense, E. coli SOD and/or hydroperoxidase double mutants were transformed with plasmids expressing SOD^+HP1^-, SOD^-HP1^+ and SOD^-HP1^-, and their physiological response to oxidative stess was measured. SOD was found to be more important than hydroperoxide in preventing oxygen-mediated stress except for the case of H_2O_2 treatment in the presence of SOD. Nevertheless, a balance between both enzymes are necessary for an effective defense against oxygen mediated radicals. The average ratio of SOD:HP1 on activity of various E. coli strains under oxidative stress except copper sulphate treatment show about 1.4 in the wild type. When the ratio, however, was reversed in mutant cells encoded with Photobactrium leiognathi CuZnSOD gene, their sensitivities were more increased. This suggests that although HP1 could reduce the H_2O_2 or hydroxyl radicals, it also inhibited the SOD function against ixygen radicals.

• Keywords: Gene expression; HP1; oxgen radical; SOD