On the Catabolism of Deoxyribonucleosides in Cells and Cell Extracts of Escherichia coli

Extracts of Escherichia coli cells differ considerably from whole cells in their deoxyribosidecatabolizing activities. Whole cells catalyze an efficient transfer of the deoxyribosyl group from thymine to adenine, whereas phosphorolysis of the deoxyribonucleosides proceeds more slowly. The nucleoside purine pyrimidine deoxyribosyltransferase activity is shown to depend on the presence in the cells of thymidine phosphorylase as well as purine nucleoside phosphorylase. Cell extracts show a considerable loss of the nucleoside purine pyrimidine deoxyribosyltransferase activity, but catalyze the phosphorolysis of the deoxyribonucleosides much more efficiently than whole cells. Whole cells readily catabolize the deoxyribose moiety of thymidine while cell extracts have partially lost this capacity. The loss seems to be due to a destruction of deoxyribomutase activity during sonic treatment of the cells, while thymidine phosphorylase and deoxyriboaldolase activities are resistant to this treatment. All the above mentioned enzymes, involved in catabolism of deoxyribonucleosides, are released in the medium when the cells are subjected to osmotic shock‐treatment. They are all induced by growth of the culture in the presence of deoxyribonucleosides. These observations have led to the assumption that the enzymes concerned with breakdown of deoxyribonucleosides in E. coli structurally and functionally constitute a complex, located at or near the cell membrane, and depend on the intact structure of the cells for proper function. Uptake of exogenous thymine into cellular DNA is probably controlled by this complex of enzymes, since the cellular pool of deoxyribosyl groups, necessary for the formation of thymidine from thymine, may be regulated through an interplay between these enzymes.