Abstract
Escherichia coli cells maintain a strong defense against oxidative stress caused by reactive oxygen species such as superoxide radical, hydrogen peroxide and hydroxyl radical. Responding to the elevated flux of superoxide radicals, the expression of more than 30 proteins becomes elevated over the basal levels in E. coli. We have isolated eight strains of Escherichia coli K12 with superoxide-inducible gene soi::lacZ fusion by using Mud(Ap,lac) phage, which is randomely integrated into the E. coli chromosome. The expression of these soi::lacZ genes depended on the increased level of superoxide and the function of soxRS gene. The soi-9 gene was identified as the ydbK gene, which product was predicted as a putative pyruvate oxidoreductase. The ydbK gene was conserved among bacteria such as Salmonella typhimurium. Among YdbK proteins of these species, the ferredoxin:oxidoreductase motif, ferric ion cluster motif, TPP binding motif were conserved. The deficiency in the function of the ydbK gene enhanced the sensitivity of E. coli cells to the superoxide radical-generating drugs, menadione and methyl viologen. Induction of the ydbK gene is necessary for the protection of cells from superoxide radical, even if E. coli cells have two normal superoxide dismutase, SodA and SodB. The ydbK promoter was cloned, and we constructed ydbK::lacZ reporter plasmid. Cells carrying this plasmid showed enhanced LacZ activity in the response to the high level of superoxide radical, depending on soxS gene function.
