Superoxide dismutase activities in Rhizobium phaseoli bacteria and bacteroids

Superoxide dismutase activity in free-living Rhizobium phaseoli is due to the presence of two different enzymes containing manganese or iron. Under usual culture conditions, the manganese-enzyme appears largely predominant but the induction of the iron-superoxide dismutase can be obtained by addition of methyl viologen to the culture media. The corresponding bacteroid, extracted from French-bean nodules, contains only a manganese-superoxide dismutase whose characteristics are similar to those of the bacterial enzyme. However, the activity of the microsymbiont is slightly lower than that of free-living cells. The presence of an active superoxide dismutase in the bacteroids suggests a significant formation of superoxide anion by their metabolism; this can be correlated with the existence of a large oxygen demand by the microsymbionts within the nodule, as suggested by their important oxygen uptake in vitro.