Hydrogenosomes in the rumen fungus Neocallimastix patriciarum

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Abstract
Sedimentable hydrogenase activity was demonstrated in cell-free extracts from both zoospores and vegetative growth of the anaerobic rumen fungus Neocallimastix patriciarum. Electron micrographs of the fraction enriched in hydrogenase activity contained finely granular microbody-like organelles, about 0.5 .mu.m in diameter and having an equilibrium density of about 1.2 g .cntdot. ml-1 in sucrose, 1.12 g .cntdot. ml-1 in Percoll and 1.27-1.28 g .cntdot. ml-1 in Metrizamide. These organelles, which are sedimentable at 105 g-min, bear no similarity to mitochondria, but are morphologically similar to hydrogen-evolving organelles possessed by certain anaerobic protozoa and termed ''hydrogenosomes''. Other typical hydrogenosomal enzymes, namely ''malic'' enzyme, pyruvate:ferredoxin oxidoreductase and NADPH:ferrodoxin oxidoreductase, were enriched in the same particle fraction as hydrogenase. The synthesis of pyruvate:ferredoxin oxidoreductase was found to be suppressed when the organism was cultured under an atmosphere of CO2, and an alternative pathway is proposed for growth under these conditions.