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dc.contributor.authorHopkin, Karen A.
dc.identifier.citationSource: Dissertation Abstracts International, Volume: 53-04, Section: B, page: 1828.;Advisors: Howard M. Steinman.
dc.description.abstractSuperoxide dismutase (SOD) is an enzyme widely found in aerobic organisms that protects cells against toxic species formed during the reduction of molecular oxygen. Escherichia coli contains two types of SOD, one containing iron (FeSOD) and the other manganese (MnSOD). These two enzymes are highly homologous in structure and catalyze the dismutation of superoxide radical with similar catalytic efficiency in vitro. In general, MnSOD is inducible by oxidative stress, while FeSOD is produced constitutively.;The major question asked in this thesis was whether subtle structural differences between E. coli Mn and FeSOD might be the basis for differences in enzyme action in vivo. That is, do MnSOD and FeSOD act equivalently as physiological antioxidants in the E. coli cell? Specifically, do they differ in their ability to prevent oxidative damage to different intracellular target molecules? Finally, how much SOD, Fe or Mn, is necessary to offer an E. coli cell the protection it needs to survive.;To answer these questions, we placed each SOD gene on a plasmid under the control of a tac promoter, and introduced the plasmids into a strain of E. coli deleted for both SOD genes. Thus, sod transcription could be regulated by lac inducers, such as isopropyl-{dollar}\beta{dollar}-thiogalactoside (IPTG), and would no longer be responsive to oxidative stress. Using this system, we adjusted the concentration of IPTG in the medium to produce equal amounts of either MnSOD or FeSOD activity in two otherwise isogenic strains. We then imposed various oxidative stresses and asked whether equal levels of the two different metalloenzyme activities corresponded to differences in cellular response.;We have assayed the effects of oxidative stress on gross cell growth and survival, and on specific targets of oxygen toxicity. (1) Cells containing MnSOD were more resistant than the FeSOD strain to the effects of paraquat, a compound that produces superoxide radicals intracellularly. (2) Cells containing MnSOD exhibited mutation rates approximately one-half that of cells containing an equal activity of FeSOD, indicating increased protection of DNA. (3) However, when we examined protection of a superoxide-sensitive enzyme, the order of their efficiency was reversed. The FeSOD enzyme preserved more 6-phosphogluconate dehydratase activity than the MnSOD.;These results indicate that E. coli Mn and FeSOD are not physiologically equivalent, and that they differ in their relative abilities to protect different intracellular targets against oxyradical damage.
dc.publisherProQuest Dissertations & Theses
dc.titleThe physiological roles of manganese and iron superoxide dismutases in Escherichia coli K-12

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