Graduate Thesis Or Dissertation
 

Some properties of ribonucleotide reductase in Rhizobium species

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  • Ribonucleotide reductase from Rhizobium meliloti has been partially purified and characterized. The enzymatic reduction of ribonucleotides to deoxyribonucleotides is dependent upon B₁₂ coenzyme and dihydrolipoate. B₁₂ coenzyme was more effective than B₁₂ coenzyme analogues in the reductase reaction. The affinity of the R. meliloti reductase system for B₁₂ coenzyme was approximately ten-fold less than that of Lactobacillus, the only reductase system reported to be B₁₂ coenzyme dependent. Certain guanosine, adenosine and cytidine phosphates were effective substrates in the reductase reaction. The uridine phosphates were reduced very slowly. The optimum concentrations of the different ribonucleotides as substrates were considerably different. Ribonucleoside diphosphates, in general, were effective at lower substrate concentrations and had lower K values than the respective ribonucleoside mon- or triphosphates. The rates of reduction of ribonucleotides at optimum substrate concentrations were not significantly stimulated by magnesium and ATP. Dihydrolipoate supplied the electrons for ribonucleotide reductions and the reductase system appears to be specific for this reductant. In preliminary experiments, extracts of R. meliloti contained a thioredoxin system which may function as the natural electron donor for ribonucleotide reduction. B₁₂ coenzyme-dependent ribonucleotide reductases have been identified in other species of Rhizobium. Reductase activity in the extracts of legume nodules also was dependent on B₁₂ coenzyme. The amount of reductase activity in the Rhizobium species and in nodules was related to the apparent growth rate of the bacteria. Cultures of R. meliloti grown on a mineral medium deficient in cobalt had a slower rate of growth than those grown on a medium containing cobalt. Cells from the deficient cultures contained a substantially greater reductase apoenzyme activity than those grown with adequate cobalt. Supplementing the cobalt deficient medium with deoxyribose compounds did not result in an increase in growth rate to that of normal cells nor cause the apoenzyme activity to be repressed.
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