Cloning of the sodA gene from Corynebacterium melassecola and role of superoxide dismutase in cellular viability

The sodA gene encoding the Corynebacterium melassecola manganese-cofactored superoxide dismutase (SOD) has been cloned in Escherichia coli and sequenced. The gene is transcribed monocistronically; the predicted polypeptide is 200 amino acids long and associates in a homotetrameric, manganese dependent form, able to complement an SOD-deficient E. coli mutant. A second open reading frame, coding for a putative 217 amino acid protein with high homology to peptide methionine sulfoxide reductases from various origins, has been identified immediately upstream of sodA in the opposite transcription orientation, The sodA gene was inactivated by insertion of an integrative vector carrying a kanamycin resistance gene. The growth rate of the SOD deficient integrant was only slightly affected in BHI rich medium as well as in BMCG chemically defined medium, but was strongly affected by the presence of the redox-cycling agent paraquat. The SOD deficiency had, on the other hand, a deleterious effect on viability as soon as the culture entered the stationary phase of growth in BHI medium. Surprisingly, SOD deficiency was able to rescue the dramatic loss of viability observed for the wild-type strain in BMCG synthetic medium when glucose was not the limiting growth factor.