Protection of Methanosarcina barkeri against oxidative stress: identification and characterization of an iron superoxide dismutase

Methanosarcina barkeri is a methanogenic archaeon that can only grow under strictly anoxic conditions but which can survive oxidative stress. We have recently reported that the organism contains a monofunctional catalase, We describe here that it also possesses an active iron superoxide dismutase. T he enzyme was purified in three steps over 130-fold in a 14% yield to a spe cific activity of 1500 U/mg. SDS-PAGE revealed the presence of only one ban d, at an apparent molecular mass of 25 kDa. The primary structure determine d from the cloned and sequenced gene revealed similarity to iron- and manga nese superoxide dismutases. The highest similarity was to the iron superoxi de dismutase from Methanobacterium thermoautotrophicum. The enzyme from M. barkeri was found to contain, per mol, 1 mol iron, but no manganese in agre ement with the general observation that anaerobically growing organisms onl y contain iron superoxide dismutase. The enzyme was not inhibited by cyanid e (10 mM), which is a property shared by all iron- and manganese superoxide dismutases. The presence of superoxide dismutase in M, barkeri is notewort hy since a gene encoding superoxide dismutase (sod) has not been found in A rchaeoglobus fulgidus, a sulfate-reducing archaeon most closely related to the Methanosarcinaceae.