Transcriptional activator nuclear factor I stimulates the replication of SV40 minichromosomes in vivo and in vitro.

SV40 DNA replication in vivo is greatly stimulated by cis-acting transcriptional elements. We studied a model viral chromosome containing a single binding site for the cellular transcriptional activator, nuclear factor I (NF-I/CTF), located adjacent to the replication origin. The presence of the NF-I recognition site increased replication efficiency over 20-fold in vivo. Purified NF-I had little effect on the replication efficiency in the standard SV40 cell-free system when the template was introduced as naked DNA. However, NF-I specifically prevented the repression of DNA replication that occurred when the template was preassembled into chromatin. Our data support a model in which the binding of a transcriptional activator perturbs the local distribution of nucleosomes, thereby increasing the accessibility of the origin region.