The Relationship of Cobalt Requirement to Propionate Metabolism in Rhizobium

Cobalt is known to be essential for symbiotic nitrogen fixation by legumes (l-5), but a requirement of the element for growth of leguminous plants supplied with adequate fixed nitrogen has not been shown. Ahmed and Evans (3) provided evidence that cobalt was required for the bacteria within the nodules of inoculated legumes but was not required for the leguminous plants per se. Lowe, Evans, and Ahmed (6) and Lowe and Evans (7) subsequently showed that cobalt is necessary for normal growth of five species of Rhixobium. More recently, Kliewer and Evans (8-10) analyzed nodules of a variety of symbionts and cells from several species of Rhizobium and observed the presence of cobamide coenzymes in each of the organisms. In addition, they (9) isolated and identified the dimethylbenaimidazolylcobamide coenzyme in extracts of cells isolated from soybean nodules and in cells of a pure culture of Rhizobium meliloti and presented data (10) showing that cobalt deficiency not only limited the growth of Rhizobium japonicum and R. meliloti but also limited the synthesis of cobamide coenzymes in these organisms. The Bn coenzyme has been shown to be involved in propionate metabolism in animals (11-13) and in Propionibacterium sherma&i (14, 15). In addition, Arnstein and White (16) have shown that Bn deficiency in Ochromonas malhamensis resulted in a reduced capacity of the organism to oxidize propionate. Ayers (17) has described similar effects of Blz deficiency in a Flavobacterium species. On the basis of these reports, experiments were conducted to determine whether Rhizobium would metabolize propionate and the possible effect of cobalt deficiency on propionate utilization in these bacteria. In a preliminary communication (18), evidence was presented showing that the rate of oxidation of propionate by cell suspensions of R. meliloti was strikingly affected by the concentration of cobalt in the culture medium. In contrast, the oxidation of glutamate was not affected by cobalt deficiency. In this paper, evidence is presented showing that enzymes necessary for the catalysis of the conversion of propionate to succinyl coenzyme A by a pathway involving methylmalonyl coenzyme A are widely distributed in Rhizobium and that cobalt deficiency in R. meliloti results in insufficient Blz coenzyme for the function of methylmalonyl coenzyme A mutase in cell-free extracts.