Conformational effects of histones H1 on DNA structure: Comparative study between H1-1, H1 o, H5 and sperm holothuira φ o

Abstract

Interactions of mammalian histones, H1-1 and H1°, Φ o from holothuria sperm and H5 with poly(dA-dT), poly(dG-dC) and poly(dG-me 5dC) were measured by a nitrocellulose filter binding assay and circular dichroism. All of the proteins bound to every one of the polymers, but differed in the extent of binding, which depended on the polynucleotide/protein ratios and ionic strength. The order of retention of all polymers was Φ o > H1-1 > H1°. The binding of H1° to poly(dG-me 5dC) was remarkably sensitive to ionic strength. The proteins caused changes in the spectral features of the polynucleotides, but differed in the type and extent of the change. Complexes prepared with H1-1 and H1° with all polymers showed a strongly negative ψ, spectrum. Complexes of poly(dA-dT) and Φ o, at a protein/polynucleotide ratio of 0.4, displayed a distinctive spectrum, giving the appearance of a Z-Iike DNA spectrum, at low ionic strength. At higher ionic strength the complexes showed a ψ spectrum. Complexes of poly(dG-me 5dC) in the Z or B conformation with φ o showed spectral features characteristic of a mixture of a Z-Iike and a ψ spectrum. In contrast, H5 reduced the Z-DNA spectral features in the presence of Mg, and produced an inversion of the B spectrum up to a polynucleotide/ protein ratio of 0.24. These findings demonstrate the ability of different proteins to produce changes in the conformation of DNA. This may reflect the ability of chromatin to undergo differential condensation, depending on both the base composition of DNA and the type of H1 histone bound to it.