DNA synthesis in isolated HeLa cell nuclei. Optimalization of the system and characterization of the product

DNA replication in isolated nuclei from synchronized HeLa cells has been studied in an effort to optimalize the system and characterize the product. The synthesis was highly dependent on the four deoxyribonucleoside triphosphates, ATP, and Mg2+. Optimum pH was about 7.8. The system was further stimulated by monovalent ions with NH4Cl and Tris-HCl (each 65 mM) being the most effective. The four ribonucleoside triphosphates and glycerol gave a slight but very reproducible and additive stimulation. Low concentrations of spermine and spermidine (0.2-1.5 X 10(-4) M) were also slightly stimulatory (10-15%) whereas higher concentrations were inhibitory. The reaction product was DNase sensitive, and banded at 1.699 g/ml in neutral CsCl together with bulk HeLa nuclear DNA. When studied by neutral CsCl and alkaline Cs2SO4 gradients, the incorporation of [3H]TTP was mainly (more than 85%) due to further elongation of strands initiated in vivo as evidenced by BrdUrd labeling.