Preincubation of T antigen with DNA overcomes repression of SV40 DNA replication by nucleosome assembly.

Circular duplex DNA containing the SV40 replication origin was assembled into chromosomes in vitro with core histones and nucleosome assembly factor from HeLa cells. Their ability to serve as a template for replication was examined by incubating them with SV40 T antigen and HeLa cell extract. Nucleosome assembly of the template prevented DNA replication. Replication of chromosomes was severely inhibited at more than two-thirds of physiological nucleosome density. When the DNA was preincubated with T antigen and then assembled into chromosomes, however, inhibition of DNA replication was greatly reduced. These results suggest that nucleosome assembly of the template inhibits initiation of SV40 DNA replication and that the inhibition can be overcome by formation of an initiation complex before nucleosome assembly.