Suppression of oxidative envelope damage by pseudoreversion of a superoxide dismutase-deficient mutant of Escherichia coli

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Abstract
Mutants of Escherichia coli that are devoid of superoxide dismutase (SOD) fail to grow in aerobic minimal medium. This is largely because of the O2- sensitivities of several amino acid biosynthetic pathways, since amino acid supplements can restore growth, albeit at a slow rate. We now report that growth in amino acid-supplemented medium can be further stimulated by the presence of extracellular osmolytes. Osmolytes also partially suppress the amino acid requirements of the SOD mutant. These data suggest that the combination of oxidative injury and turgor pressure permeabilizes the cell envelope and that critical metabolites, including the limiting products of damaged biosynthetic pathways, escape from the cell. External osmolytes may offer protection by countervailing the usual turgor pressure and thus stabilizing the damaged envelope. This model is consistent with the previous observation that deficiency of cell wall components is lethal to SOD mutants. A pseudorevertant that can grow at a moderate rate in normosmotic medium without amino acid supplementation has been obtained (J. A. Imlay and I. Fridovich, Mol. Gen. Genet. 228:410-416, 1991). Analysis suggests that the suppressor mutation allows the envelope either to resist or to tolerate oxidative lesions. Study of the pseudorevertant may illuminate the molecular basis of this oxidative envelope injury.