Exogenously supplied thymine is only poorly utilized by wild-type cells of Escherichia coli for the synthesis of their DNA. It appears that the lack of incorporation of exogenous thymine is due to a lack of endogenous deoxyribosyl groups, which are required for the synthesis of thymidine.

Data to be presented in this paper indicate that the ability of different thymine auxotrophs to grow on progressively lower thymine concentrations is a function of their capacity to increase the internal pool of deoxyribose 1-phosphate and/or the level of thymidine phosphorylase.

Thymine incorporation in wild-type cells could be observed if the cells possessed an increased deoxyribose 1-phosphate pool.

The kinetics of thymine uptake in wild-type cells, in a mutant lacking thymidine phosphorylase and in thymine auxotrophs show that the initial rates of thymine uptake are almost identical and proportional to the external thymine concentration.

The experiments in vivo led us to conclude that the incorporation of exogenous thymine occurs via thymidine, which is synthesized from thymine and deoxyribose 1-phosphate, catalyzed by thymidine phosphorylase. In accordance with this studies in vitro with purified thymidine phosphorylase showed that thymidine is synthesized from deoxyribose 1-phosphate and thymine rather than through a deoxyribose transfer from a deoxynucleoside.