| Abstract: | To investigate the cellular proteins involved in simian virus 40 (SV40) replication, extracts derived from human 293 cells have been fractionated into multiple components. When such fractions are combined with the virus-encoded T antigen (TAg) and SV40 origin containing plasmid DNA, efficient and complete replication is achieved, while each fraction alone is inactive. At present, a minimum of eight such cellular components have been identified. Previous experiments have demonstrated one of these to be the cell-cycle-regulated proliferating-cell nuclear antigen (PCNA). As PCNA has been identified as a processivity factor for DNA polymerase δ, we suggest that both polymerases α and β are involved in this system. Three further fractions have been identified. One is a partially purified fraction which, under certain conditons, is required with TAg for the formation of a pre-synthesis complex of proteins at the replication origin. The second of these factors, RF-A, is a complex of three polypeptides which may function as a eucaryotic SSB. The third, RF-C, is a factor which is required, with PCNA, for coordinated leading- and lagging-strand synthesis at the replication fork. Complete synthesis and segregation of the daughter molecules also requires the presence of topoisomerases I and II. These results suggest a model for DNA synthesis which involves multiple stages prior to and during replicative DNA synthesis. |
