PRECURSOR-PRODUCT RELATIONSHIPS BETWEEN THYMIDINE NUCLEOTIDES AND DNA IN MAMMALIAN-CELLS .1. STUDIES ON INTACT-CELLS

To identify the thymidine nucleotide serving as proximate precursor for DNA synthesis, kinetics of incorporation of [3H]thymidine into individual thymidine nucleotides and into DNA of Chinese hamster ovary cell cultures were analyzed after prelabeling of nucleotides or of DNA with [14C]thymidine. Labeling kinetics of nucleotides were determined in synchronous S-phase cell populations. The rate constant for incorporation of thymidine into dTMP was higher by a factor of approximately 10 than that for incorporation into dTDP and dTTP. On the other hand, kinetics of labeling of dTDP and dTTP were not significantly different from each other, indicating a high rate of interconversion of these 2 nucleotides. Rate constants for labeling of dTDP and dTTP were in good agreement with that of the proximate DNA precursor as derived from the time course of DNA labeling. Labeled, and possibly also total, intracellular dTDP and dTTP may represent single-pool compartments. Due to the rapid interconversion of dTDP and dTTP, these data obtained with intact cells are compatible with the assumption that either dTDP or dTTP, or possibly both of these intermediates, are used as the proximate precursor(s) for DNA synthesis.