Dimerization of simian virus 40 T-antigen hexamers activates T-antigen DNA helicase activity

Chromosomal DNA replication in higher eukaryotes takes place in DNA synthesis factories containing numerous replication forks. We explored the role of replication fork aggregation in vitro, using as a model the simian virus 40 (SV40) large tumor antigen (T antigen), essential for its DNA helicase and origin-binding activities. Previous studies have shown that T antigen binds model DNA replication forks primarily as a hexamer (TAgH) and to a lesser extent as a double hexamer (TAgDH). We find that DNA unwinding in the presence of ATP or other nucleotides strongly correlates with the formation of TAgDH-DNA fork complexes. TAgH- and TAgDH-fork complexes were isolated, and the TAgDH-bound fork was denatured at a 15-fold-higher rate during the initial times of unwinding. TAgDH bound preferentially to a DNA substrate containing a 50-nucleotide bubble, indicating the bridging of each single-stranded DNA/duplex DNA junction, and this DNA molecule was also unwound at a high rate. Both the TAgH- and TAgDH-fork complexes were relatively stable, with the half-life of the TAgDH-fork complex greater than 40 min. Our data therefore indicate that the linking of two viral replication forks serves to activate DNA replication.