Requirement of a Cytoplasmic Fraction for Synthesis of SV40 Deoxyribonucleic Acid in Isolated Nuclei*.

About 50% of the SV40 DNA in the process of replication (sv40(ri) dna) completed replication in lysates of infected BSC-1 cells by conversion to covalently closed, superhelical SV40 DNA (SV40(I) DNA). Fractionation of the lysate into nuclear and cytoplasmic components blocked 99% of the synthesis of SV40(I) DNA in the purified nuclei. The reconstituted system, made by adding back the cytoplasmic fraction before incubation at 30 degrees, completely restored the in vitro level of SV40(I) DNA synthesis. Preliminary characterization of the activity found in the cytoplasmic fraction suggested it was a soluble, heat-labile protein (or proteins) with a minimum molecular weight of about 30,000 and an active sulfhydryl group. The activity was present in both infected and uninfected monkey cells, and at a lower level in mouse, hamster, and human cell lines. Neither serum starvation nor cycloheximide treatment of cells diminished the activity in the cytoplasmic fraction. Purified cytoplasmic DNA polymerase from KB cells did not substitute for the cytoplasmic fraction which was required for elongation of newly synthesized DNA strands. In the absence of the cytoplasmic fraction, conversion of 4 S DNA into longer strands was inhibited, and SV40(RI) DNA appeared to be broken specifically at the replication forks.