The Partial Dissociation of Nucleohistone by Salts

Abstract

The circular dichroism of calf thymus deoxyribonucleohistone was compared with that of samples partially depleted of histone in solutions of sodium chloride. Differences in the near ultraviolet circular dichroism of DNA and deoxyribonucleohistone were associated with the presence of the f2b, f2a2 and f 3 histone fractions in the complex. The near ultraviolet circular dichroic spectra of both DNA and deoxyribonucleohistone were very sensitive to the solvent environment. Thus differences in the circular dichroic spectra of DNA and deoxyribonucleohistone cannot necessarily be associated with long-range effects, such as supercoiling, but may be due to short-range effects, such as might be caused by histone-DNA interactions.The absorbance and the circular dichroic melting profiles of deoxyribonucleohistone were biphasic. A new dichroic species was formed during the melting of the first of these steps and evidence is presented that this is a new histone-DNA complex produced by the migration of some or all of the fractions f2b, f2a2 and f 3 from denatured t o native regions of the DNA during the melting process. Thus the biphasic melting profile of deoxyribonucleohistone does not necessarily imply that these fractions are arranged heterogeneously on the DNA.Studies on the hydrodynamic properties of nucleohistone have shown that certain histone fractions maintain nucleohistone in a more compact conformation than DNA [l]. It has been suggested that this conformation is a supercoil [2]. Marked differences are also found in the optical rotatory dispersion and circular dichroic spectra of DNA and nucleohistone [3-51. I n order to see whether the histone fractions reponsible for the characteristic difference in hydrodynamic properties were also responsible for the difference in optical properties, we have investigated the circular dichroic spectra of nucleohistone partially depleted of specific histone fractions by salt.The histone fractions which are implicated in the supercoiling of nucleohistone also produce biphasic melting profiles in the complex [1,6]. Therefore we have studied the circular dichroism of native and partially depleted nucleohistone as a function of temperature in order to gain further information about the contribution of specific histone fractions to the conformation of the native nucleoprotein. Apreliminary account of this work has been published [7]. EXPERIMENTAL PROCEDUREThe preparation, characterisation and experimental methods, other than circular dichroism, have been described previously [11. The solvent used in all cases was 0.7 mM sodium phosphate, p H 6.8, except where otherwise statcd. Deoxyribonucleohistone samples that have been partially depleted of histone will be identified by the molarity of the sodium chloride solution used for the depletion. Thus a sample depleted in 0.7 M NaCl is designated 0.7 deoxyribonucleohistone. The extinction coefficients for the heat-treated samples were obtained by phosphorus analysis [S]. Circular dichroic spectra were measured in a Jouan Dicrographe Model C...

Differential scanning calorimetry of nuclei reveals the loss of major structural features in chromatin by brief nuclease treatment.

“…This analysis used a combination of circular dichroism and calorimetry, and the samples were analyzed at low salt concentrations to maintain optical clarity. It is unfortunate that these and many other studies (13)(14)(15)(16)(17)(18)(19)(20)(21) of nucleosome and chromatin melting cannot be strictly compared with our results because of the substantial differences in salt concentrations. Weischet et al (12) found the melting of core particles to be quite sensitive to salts in the range 0.1-10 mM.…”

Water-Soluble Lysine-containing Polypeptides. II. The Interaction of Several Sequential Lysine–Glycine Polypeptides with DNA. A Circular Dichroism Study of DNA Conformation in Annealed Complexes

“…At lysine/DNA-phosphate ratios of approximately 0.5 it was noted that in each case the D N A portion of the spectrum was reminiscent of spectra seen in chromatin isolated from various sources (19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29). That is, the single band a t 275 nm in the spectrum of the native DNA shifted over to a smaller positive band a t approximately 282 nm upon complex formation with polymers I11 and IV (Figs.…”

Experimental Differential Light-Scattering Correction to the Circular Dichroism of Bacteriophage T2

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