Incorporation of thymine-containing DNA precursors in wild-type and mutant T4-infected plasmolysed cells.

T4-infected cells, plasmolysed 15 min after infection, incorporate low concentrations (less than 20 microM) of deoxythymidine (TdR) into DNA at a significantly greater rate than dTMP, dTTP or thymine. At higher concentrations (greater than 40 microM), dTMP incorporation rate is high, approaching that of TdR at 200 microM. TdR is selectively incorporated at all concentrations tested, and is not inhibited by the other thymine containing DNA precursors. Incorporation of low concentrations of TdR requires the T4-induced thymidine kinase (tk) and is not significantly affected by the presence or absence of T4-induced thymidylate synthetase (td). We show that, in T4-infected plasmolysed cells, exogenously added TdR is preferentially incorporated into short DNA fragments during short pulse times. To explain these and other data a model is proposed in which thymidine plays a modulatory role between leading and lagging strand precursor feeds.