Nucleosome Structure and Dynamics: The DNA Minicircle Approach

DNA supercoiling in eukaryotic viruses was first described by Vinograd and co-workers more than 30 years ago (see ref. 1 for a review), but its origin was recognized to result from DNA complexation with host histones to form minichromosomes only 10 years later (2 ). Because they are in contact with endogenous topoisomerases, minichromosomes are relaxed, that is, the linker DNAs that connect nucleosomes to one-another along the minichromosomes are free from torsional constraint. DNA supercoiling therefore results from nucleosome removal during protein extraction. Negative supercoiling of SV40 DNA was found to correspond to a reduction in the DNA linking number, Lk, of approx 26 (3 ), and the number of nucleosomes in the minichromosomes was approximately the same (4 ,5 ). Thus, each nucleosome reduces Lk by one (see Subheading 3.1. , for Lk definition).