Complexes of Histone F1 with DNA in 0.15 M NaCl

Abstract

Aggregated complexes of histone F1 with DNA of different composition and origin were prepared by gradual dialysis from a high salt concentration to 0.15M NaCl. Analysis of the DNA composition both in the supernatant and in the precipitate showed that in equilibrium complexes in this medium, histone Fl selectively prefers sequences of native DNA rich in adenine and thymine. The mechanism of the selective binding is discussed.Study of complexes reconstituted from isolated DNA and from histone fractions represents one of the possible approaches to the investigation of the structure of chromatin and of chromosomes. Progress in this field appears t o be essential for a better understanding of the mechanism of genetic processes in higher organisms. It was pointed out before [1,2] that the hydrodynamic and the optical properties of native and reconstituted complexes in their soluble form tally. It is an advantage of the reconstitution procedure that one can independently alter the character of the components of the complex, to study their interaction and the effect on the final structure of the complex.Recently the view was advanced [3] that native nucleohistone molecularly dispersed at Iow ionic strength partly loses the assumed superhelix structure. A structural transition on decreasing the ionic strength of the medium was also observed with complexes reconstituted from DNA and from the lysine-rich fraction F1 [4,5]. This histone fraction appears to be very interesting since it differs significantly in its properties from other fractions [6] and some authors assume that it substantially affects the structure of chromosomes [7].In the present paper we studied the interaction of histone F1 with DNA of different composition and origin in a medium of approximately physiological ionic strength (0.15M NaC1). It was intended to contribute to the elucidation of the origin of specific complexes of histone F1 * DNA under conditions resembling those in vivo and hence to contribute to determining the biological function of this histone fraction. A similar problem was examined in a greater number of papers [S] and some of their results indicate that a certain specificity of the binding of Abbreviation. CD, circular dichroism. 7.histones to DNA exists [9,10]. It was of interest in this context to note the interactions of DNA with synthetic polypeptides [S, 111. Still, the binding of histones to DNA is generally considered to be rather nonspecific, perhaps also because most of the studies conducted on reconstituted complexes as well as on native nucleohistones were done with soluble nucleohistones, i.e. under conditions rather different from physiological. The results of analysis of DNA composition both in the supernatant and in the aggregated phase presented here indicate that histone F1 in aggregated complexes prepared by the equilibrium method selectively prefers sequences of native DNA rich in adenine plus thymine.

Preferential and cooperative binding of histone I to chromosomal mammalian DNA.

“…The simplest explanation is that we enriched preferential binding sites since they are not present on all DNA fragments of 2 X 105 daltons. The melting temperature of the retained DNA was 50 lower (which corresponds to 12% more A + T) and reannealing of the DNA (Cot plot) indicated a lack of highly repetitive DNA fragments (G + Crich), in agreement with Sponar and Sormova (19).…”

Involvement of histone H1 in the organization of the chromosome fiber.

“…4), however, and the finding of a similar transition of histone H1 complexes made with large DNA fragments (12) leads us to believe that histone H1 is also bound cooperatively on large fiber fragments and most likely also on native fibers at high ionic strength. The binding sites of histone H1 could be DNA sequences rich in A+T (20), because model studies showed a selective histone H1 interaction with DNA species of high A+T content (21).…”

Complexes of histone F1 with DNA in 0.15M NaCl. Circular dichroism and structure of the complexes

“…Preparations of DNA, their isolation, fractionation, and characterization, as well as isolation and characterization of histone F1 are described elsewhere. 8 The complexes were prepared from isolated components (DNA and F1) dissolved at a given weight ratio in 1.5-3M NaCl by gradual dialysis down to 0.4M NaCl (4 hr), 0.3M NaCl (2 hr), and 0.15M NaCl (overnight) at 4°C. All solutions were buffered with 0.013M sodium phosphate (pH 6.8).…”