Studies of replicative DNA synthesis using DNA precursors have shown that the DNA that was replicated most recently is that associated with the nuclear matrix. Consequently, precursors arising via the salvage and the de novo metabolic pathways are first incorporated into a small percentage of the total nuclear DNA that is termed nuclear matrix-associated DNA. These results have been substantiated in cell culture, as well as in intact mammalian systems. Furthermore, when DNA precursors were injected intravenously into regenerating rat liver, a significant lag in the incorporation of orotic acid-derived nucleotides (de novo pathway precursors) into nuclear DNA was observed, when compared with deoxythymidine-derived nucleotides (salvage pathway precursors). This lag in incorporation kinetics was also evident at the nuclear matrix level, although, once incorporated into nuclear matrix-associated DNA, the distribution patterns of both precursors into extra-matrix nuclear DNA fractions were identical. To determine the basis for this kinetic lag, we compared the incorporation kinetics of orotic acid and of deoxythymidine into dTTP and into nuclear matrix-associated DNA, respectively. Orotic acid-derived nucleotides entered the cytosolic dTTP pool before being incorporated into nuclear matrix-associated DNA, that is, traversing the classical metabolic route of DNA precursors. Conversely, deoxythymidine-derived nucleotides by-passed the soluble dTTP cellular pool and engaged directly in DNA synthesis at the nuclear matrix. Not only is this the first evidence for nucleotide channelling in an intact mammalian system, but it also forms direct evidence that salvage pathway DNA precursors are channelled to nuclear matrix-associated sites of DNA replication.

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