2 Bending of DNA in Nucleoprotein Complexes

Abstract

I. INTRODUCTION
When DNA is packaged, whether in a phage head or a eukaryotic chromosome, it is often tightly bent. In chromatin, the elementary unit of the structure is the nucleosome in which the DNA wraps approximately twice around an octamer of the histone proteins H3, H4, H2A, and H2B, as a left-handed superhelix with a diameter of 86 Å (Finch et al. 1977; Richmond et al. 1984). The path of the DNA between histone octamers is not known, but it appears that variable lengths of “linker” DNA separate individual nucleosomes (Prunell and Kornberg 1982; Widom and Klug 1985). A single copy of the fifth histone, H1, is associated with the linker in a way that is not fully understood. When the linker DNA is digested away by nucleases, the H1 drops off leaving the so-called nucleosome core particle, consisting of about 145 bp wrapped in approximately 1.8 superhelical turns around the histone octamer. The two copies of each histone in the octamer are related by a unique axis of twofold symmetry, or dyad, that at one end passes through the midpoint of DNA. Although in the chromosome the histone octamers are associated with a great variety of DNA sequences, studies of nucleosome positioning in both reconstituted and naturally occurring systems have shown that these proteins can adopt well-defined, even precise locations with respect to the primary DNA sequence (Simpson and Stafford 1983; Edwards and Firtel 1984; Palen and Cech 1984; Ramsay et al. 1984; Drew and Travers 1985a; Rhodes...