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Cobamides and Ribonucleotide Reduction

III. FACTORS INFLUENCING THE LEVEL OF COBAMIDE-DEPENDENT RIBONUCLEOSIDE TRIPHOSPHATE REDUCTASE IN LACTOBACILLUS LEICHMANNII

From the ¹ From the Department of Biochemistry, John Curtin School of Medical Research, Australian National University Canberra, Australian Capital Territory, Australia

The specific and total activities of ribonucleoside triphosphate reductase in extracts of Lactobacillus leichmannii are increased as the period of linear growth of the culture before harvesting is increased. Extracts from cells harvested toward the end of the linear phase have the highest total and specific activity, and the activity of extracts is lower the longer the time of harvesting is delayed beyond this point. Stationary phase cells give extracts without significant reductase activity. The increase in specific and total activities as the period of linear phase growth is extended before harvesting appears to indicate a rapid rate of synthesis of reductase since chloramphenicol and actinomycin D arrest this increase. The decrease in reductase activity of the extracts from cells harvested in the stationary phase does not appear to result from the presence of an increased amount of proteolytic enzymes in the extracts nor from incomplete liberation of enzyme from such cells. The specific and total activities of the reductase in extracts of L. leichmannii harvested at the end of the linear phase of growth are lower when higher concentrations of deoxyribosyl compounds or of cyanocobalamin are present in the growth medium. This is interpreted as indicating that deoxyribosyl compounds act as corepressors in the synthesis of the reductase and that cyanocobalamin indirectly represses such synthesis through its ability to increase the intracellular pool of acid-soluble deoxyribosyl compounds. Neither thymine nor deoxyribose causes repression of the synthesis of this enzyme.

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