Superoxide, Hydrogen Peroxide, and Oxygen Tolerance of Oxygen-Sensitive Mutants of Escherichia coli

Oxygen-intolerant mutants of Escherichia coli K12 were selected by a replica plating technique after treatment with the mutagen, N-methyl-N'-nitro-N-nitrosoguanidine, to a lethality of 99.5%. One group of mutants had lost the ability to induce both peroxidase and catalase when exposed to oxygen but retained the ability to induce the manganese-superoxide dismutase. The second group of mutants had lost the ability to induce the activity of all these enzymes. Failure to induce peroxidase and catalase was associated with enhanced susceptibility of the bacteria to the lethal effect of oxygen. When a member of the first group of mutants was prevented from producing the manganese-superoxide dismutase by the presence of puromycin, its susceptibility to the lethal effects of oxygen was greatly increased. Two types of revertants were seen. In one group the ability to induce enzyme activity was recovered and was accompanied by the return of oxygen tolerance. Members of the other group lost the ability to respire and, therefore, no longer produced 02- and H202. These results indicated that enzymic scavenging of both H202 and 02- provides an important defense against oxygen toxicity. The parallel loss of peroxidase and catalase, which was seen in all mutants, suggests that these enzymes constitute a precursor-product pair in E. coli. The parallel loss in two of these mutants of peroxidase, catalase, and the manganese-superoxide dismutase suggests a control linkage for these enzymes, the basis of which remains to be explored. Pasteur was, in 1861, apparently the first to appreciate both the possibility of life in the absence of oxygen and the potential toxicity of oxygen. It is now recognized that the early phases of biological evolution occurred in an anaerobic world and that the extensive oxygenation of the biosphere was a consequence of photosynthesis. Oxygen is exceedingly useful in metabolism that yields energy and in biosyntheses, but organisms that utilize oxygen must maintain defenses against its deleterious effects. Oxygen is toxic because of the reactivities of the intermediates of its reduction (02- and H202) and the entities that may be generated by these active products [1-3]. A defense against the toxicity of oxygen