PERIPLASMIC COPPER-ZINC SUPEROXIDE-DISMUTASE PROTECTS HAEMOPHILUS-DUCREYI FROM EXOGENOUS SUPEROXIDE

Haemophilus ducreyi causes chancroid, a sexually transmitted genital u lcer disease implicated in increased heterosexual transmission of HIV. As part of an effort to identify H. ducreyi gene products involved in virulence and pathogenesis, we created random TnphoA insertion mutati ons in an H. ducreyi 35 000 library cloned in Escherichia coil. Insert s encoding exported or secreted PhoA fusion proteins were characterize d by DNA sequencing. One such clone encoded a Cu-Zn superoxide dismuta se (SOD) enzyme. The Cu-Zn SOD was periplasmic in H. ducreyi and accou nted for most of the detectable SOD activity in whole-cell lysates of H. ducreyi grown in Vitro. To investigate the function of the Cu-Zn SO D, we created a Cu-Zn SOD-deficient H. ducreyi strain by inserting a c at cassette into the sodC gene. The wild-type and Cu-Zn SOD null mutan t strains were equally resistant to excess cytoplasmic superoxide indu ced by paraquat, demonstrating that the Cu-Zn SOD did not function in the detoxification of cytoplasmic superoxide. However, the Cu-Zn SOD n ull strain was significantly more susceptible to killing by extracellu lar superoxide than the wild type. This result suggests that the H. du creyi Cu-Zn SOD may play a role in bacterial defence against oxidative killing by host immune cells during infection.