Adaptation of thymidine utilization to changing rates of DNA synthesis in the cell cycle

In synchronous cultures of P-815 murine mastocytoma and of Chinese hamster ovary (CHO) cells, the relative contribution of exogenous thymidine to DNA synthesis was studied by comparing rates of (3H)thymidine incorporation with the rate of DNA synthesis as derived from incorporation of (3H)thymidine (10(-5) M) in the presence of amethopterin. In synchronous P-815 cultures, time-dependent variations of DNA synthesis rates were in close agreement with those of (3H)thymidine incorporation rates at concentrations of the precursor ranging from 5 x 10(-8) to 10(-5) M. Similarly, in synchronous CHO cell cultures prepared by two different methods, time-dependent changes in DNA synthesis rate were almost identical with those of the rate of incorporation of (3H)thymidine supplied at 5 x 10(-8) M. Thus, at a given thymidine concentration in the medium, the proportion of thymine residues in DNA that were derived from exogenous thymidine remained nearly constant, even though rates of cellular DNA synthesis underwent pronounced changes. This indicates that in the synchronous culture systems used, utilization of exogenous thymidine is efficiently adapted to changing rates of DNA synthesis.