SV40 viral minichromosome: preferential exposure of the origin of replication as probed by restriction endonucleases.

Isolated SV40 minichromosomes [1-3] were treated with different single-cut restriction endonucleases to probe the arrangement of nucleosomes in relation to the SV70 DNA sequence. While Eco RI and Bam HI each cut 22-27% of the SV40 minichromosomes under limit-digest conditions, Bgl I, which cuts SV40 DNA at or very near the origin of replication [4,5], cleaves 90-95% of the minichromosomes in a preparation. Similar results were obtained with minichromosomes which had been fixed with formaldehyde before endonuclease treatment. One possible interpretation of these findings is that the arrangement of nucleosomes in the compact SV40 minichromosomes is nonrandom at least with regard to sequences near the origin of DNA replication.