SUMMARY: Mutants of which were prototrophic in a defined medium below 32° required either thymidine (mutant ) or low concentration of thymine (mutant ) for growth at 37°. No thymidylate synthetase was produced by these mutants at 37°. Normal enzyme activity was found in bacteria grown at 25°. Absence of thymidylate synthetase activity in bacteria grown at 37° could not be accounted for either by a heat-sensitive enzyme protein or by an inhibitor of this enzyme activity.

The incorporation of thymine by prototrophic bacteria of was negligible at either temperature. Mutant and its prototrophic revertant both readily took up thymine at 37° but not at 25°. Thymine uptake by both prototrophic and bacteria was markedly enhanced by the presence of any of the deoxyribonucleosides except thymidine. Moreover, thymidine antagonized thymine uptake. No thymidine phosphorylase activity was detected in , but phosphorolytic decomposition of deoxyuridine was found and that of deoxyadenosine seemed likely. The phosphorolytic reaction of deoxyuridine resulted in an accumulation of deoxyribose by bacteria, whilst both wild-type and bacteria catabolized this sugar. The secondary mutation in bacteria presumably resulted in a defect in synthesis of 1,5-phosphodeoxyribo mutase or deoxyribose 5-phosphate aldolase (E.C. Although no thymidine phosphorylase was detected in , resting suspensions of this bacterium transferred the deoxyribosyl moiety from thymidine to thymine. This indicates the presence of the enzyme nucleoside purine (pyrimidine) deoxyribosyl transferase in This particular enzyme may also be involved in the increased thymine uptake in the presence of different deoxyribonucleosides.


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