Thymidine Uptake in Bacteria: The Effect of Purine Nucleosides

Abstract

The kinetics of thymidine uptake by Escherichia coli and Bacillus subtilis cells in the presence of adenine and guanine nucleosides was investigated. The initial concentration of thymidine in the growth medium was 0.35 ug/ml while the initial concentration of purine nucleosides ranged from 25 to 250 ug/ml. Adenine nucleosides when present at a concentration more than 50 ug/ml. strongly inhibit thymidine uptake by the bacteria. The duration of the inhibition depends on the initial concentration of adenine nucleoside in the growth medium. At an initial con-centration of deoxyadenosine (or adenosine) of 250 ug/ml the time of inhibition of thymidine uptake was about 60 min. During this period thymidine is almost completely preserved from the action of bacterial thymidine phosphorylase. Guanine nucleosides (guanosine or deoxyguanosine) do not markedly inhibit thymidine uptake by bacteria even at a concentration of 250 ug/ml. It is shown that they do protect thymidine from the phosphorolytic action of the thymidine phosphorylase although much less effectively than adenine nucleosides. It is suggested that some areas in the bacterial membrane where thymidine phosphorylase is located are not available to guanine nucleosides.