DNA synthesis in L-cells in the presence of 5-fluorodeoxyuridine.

After 16 h of incubation with 10-minus6 M FdUrd, the rate of (32P) orthophosphate uptake into DNA isolated from L-cells amounted to 15% of that of an untreated culture, although cell division had stopped several hours earlier. All 4 deoxynucleotides were present in this DNA but its nucleotide composition, as measured by enzymatic digestion and chromatography, reflected a decreased thymidine precursor pool in FdUrd-treated cells. Sedimentation analysis in alkaline sucrose gradients revealed that the DNA formed in the presence of FdUrd had a sedimentation coefficient of 10 S which corresponded to a single-stranded molecular weight of 5.5.105. This DNA could be "chased" into a high molecular weight DNA if the FdUrd block was bypassed with added dThd or BrdUrd. Other analyses failed to detect RNA covalently linked to the DNA fragments at a level of more than 5% RNA or about 90 ribonucleotides. The accumulation of these DNA fragments could be explained by assuming that in the presence of limiting precursor pool the rate of DNA chain initiation is greater than the rate of chain elongation.