SITES INCLUDING THOSE OF ORIGIN AND TERMINATION OF REPLICATION ARE NOT FREELY AVAILABLE TO SINGLE-CUT RESTRICTION ENDONUCLEASES IN THE SUPERCOMPACT FORM OF SIMIAN VIRUS-40 MINICHROMOSOME

Simian virus 40 minichromosomes, isolated from both virions (MV) and infected cells (MI), have highly compact structures in buffer containing 0.15 M NaCl and sediment with S values of about 90-100 and 115-130, respectively. Under the electron microscope, also, MI appear the more compact of the two. Only 30-35% of the sites of origin and termination of replication in MV are freely available to the restriction endonucleases Bgl 1 and Bam H1. MV are similarly resistant to Eco R1 and Hpa II. In contrast, almost no sites in MI are available to any of the above single-cut endonucleases. In 0.6 M NaCl, MV and MI change to relaxed structures of 45-55 S and 50-60 S, respectively, containing 20-24 nucleosomes per genome, and become more sensitive to Bgl 1, Bam H1, Eco R1, and Hpa II. © 1979.