Superoxide Dismutases in Bacteria and Pathogen Protists

Abstract

Superoxide dismutases (SODs), which catalyze the dismutation of superoxide (O₂^•−) into hydrogen peroxide and oxygen, play a central role in protection against oxidative stress. All aerobic organisms produce superoxide, which is not very reactive, but can in the presence of hydrogen peroxide in vitro generate extremely reactive species such as hydroxyl radicals, via the metal-catalyzed Haber-Weiss reaction (Beauchamp and Fridovich 1970). The ubiquity of SODs suggests that the presence of superoxide in vivo can lead to such active species and, consequently, deleterious lesions. A classic approach to investigating the biological role of an enzyme is to study the phenotype of defective mutants. The first clear evidence of the deleterious effects of superoxide in vivo was provided by cloning the E. coli SOD genes and constructing mutants lacking both Mn- and Fe-containing SODs (Carlioz and Touati 1986), which are subject to oxygen-dependent DNA damage.

The progress of biotechnologies during the past few years allows molecular genetic techniques to be applied to an increasing number of species. One consequence is that SOD genes of numerous microorganisms, including strict anaerobes, archebacteria, rickettsia, cyanobacteria, and bacterial and protist pathogens, have been cloned and sequenced. The development of polymerase chain reaction (PCR), electroporation, new shuttle vectors, and suicide plasmids led to the construction of SOD null mutants in a wide variety of microorganisms by disruption of cloned sod genes and exchange of alleles. The role of environmental factors in increasing oxidative stress and modulating expression of antioxidant defenses has been studied. In this chapter, I...