The ribonucleotide reductase activities in vitamin B12-sufficient and -deficient cells of Euglena gracilis were measured. We found that the cells progress into vitamin B12 deficiency the enzyme activity increases, reaching a maximum value of 20-fold in advanced deficiency. No signigicant differences in the activities were found to result as a consequence of different growth conditions. We propose that the increased activity in vitamin B12-deficient cells is due to an increase in enzyme protein.

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