Formation of a nucleosome-free region in SV40 minichromosomes is dependent upon a restricted segment of DNA

Abstract

We have analyzed minichromosomes of SV40 mutants with the electron microscope. In two insertion mutants, containing the SV40 DNA fragment from 0.66 to 0.715 map unit duplicated diametrically opposite to its original site in two alternative orientations, one nucleosome-free region (“gap”) was visualized in 25% of the molecules of each mutant with equal frequency either at the original position or at the insertion point. In a host-substituted mutant (F161F), where the viral DNA sequences spanning 0.67–0.73 map unit are interspersed as four copies between cellular sequences, one gap was visualized in almost 100% of the molecules in the vicinity of the viral DNA sequences. These observations indicate that gap formation is dependent upon SV40 DNA sequences located between 0.67 and 0.715 map unit.