Circular dichroism studies of lysine-rich histone f-1-deoxyribonucleic acid complexes. Effect of salts and dioxane upon conformation

Abstract

Nucleoprotein complexes reconstituted from calf thymus DNA and lysine-rich histone (f-1 fraction) in a variety of neutral salts and in dioxane were examined by circular dichroism. The alteration of the DNA ellipticity bands is interpreted as a reflection of conformational change of the DNA in the complexes. Gradient dialysis of DNA and f-1 in various ratios into 0.14 or 0.3 M uni-univalent salt solutions results in circular dichroic spectra which can be approximately ordered according to the lyotropic series. This sequence was found to be, in order of the effect on the DNA circular dichroic spectrum (that is, beginning the sequence with salts that alter the circular dichroism spectra the most): guanidine'HC1 > NHdC1, NH40Ac >> CsCl > KC1, KF > NaOAc, NaBr, NaF, NaCl > LiCl >> NaC104, KCNS, NaI. Most of the effect is exerted through cations.Blkali metal ions enhance the ellipticity changes of complexes to various extents. Ammonium and guanidinium ions cause the largest changes in the circular dichroism; these specific ion effects are remarkable because of the relatively low salt concentration at which they operate. Most anions (OAc-, F-, C1-, Br-) have little effect, except for C104-, CNS-, and I-, which dissociate the complexes. Very low MgClz concentration ( 5 0.006 M ) augments conformational changes; at higher concentration MgClz causes dissociation. Addition of dioxane (up to 40% by volume) enhances distortion of the DNA ellipticity. Different types of complexes result (1) from direct mixing of f-1 histone and DNA in 0.14 iM KaF, (2) from dialysis into 0.01 M NaF, and (3) from dialysis into 0.14 M NaF. Filtration experiments indicate that histone binds to DNA in a partially cooperative manner. f-1-DNA interaction is modified by salt concentration, specific salt effects, dioxane, and manner of complex formation. This complicated behavior indicates that charge attraction between histone and DNA, even when supplemented by hydrophobic bonding, is insufficient to account for the observed circular dichroic changes. Specific association of complexes may be involved, mediated by the presence of various salts.

Water-soluble lysine-containing polypeptides. III. Sequential lysine-glycine polypeptides. A circular dichroism and electron microscopy study of annealed complexes with DNA, sonicated DNA, and denatured DNA

“…This characteristic cd spectrum is also produced when D N A is dissolved in water soluble organic polymers, such as polyethylene oxide (PEO) (8)(9)(10)(11)(12)(13)(14). It is also found in dehydrated films of DNA (15), in con~plexes of DNA with several random and sequential lysine-containing polypeptides (i6-221, and in complexes of histone H-1 (F-1) with D N A (23)(24)(25)(26), This characteristic cd spectrum has beer! said to be produced by $-state DNA (8).…”

“…Several explanations of the way it is produced have been offered. The>-are: 41) polypeptide prornoted solvent excl~lsion from the DNA; (2) polypeptide promoted induction of longrange order in the DNA: (3) polypeptide promoted increases in the helix-helix interactions of the DNA brought on by aggregation of the DNA; (4) polypepticte promoted increases in the production of a liquid crystalline state in the complexes (14,23).…”