Nucleohistone/histones

In this connection, it must also be borne in mind that the deoxyribonucleic acids subjected to analysis have probably not been homogeneous. Deoxyribonucleic acids have been fractionated by making use of their different solubilities in normal saline,186 by extracting thymus nucleo-his-tone with sodium chloride solutions of increasing concentration,187 by ion-exchange,187 and also by adsorption of the polynucleotide onto histone immobilized on a kieselguhr support.123 It is possible, however, that these are artefacts, since it has been shown that deoxyribonucleic acid fractions extracted from calf-thymus nucleohistone may or may not vary in composition according to the previous treatment of the material.188... 

Salt bridges between positively charged basic amino acid side chains of histones and the negatively charged DNA phosphates play a major role in stabilizing the DNA-histone complex. Indeed, treatment of chromatin with concentrated NaCl (1-2 m), which is known to disrupt electrostatic bonds, causes a complete dissociation of DNA and histone in the nucleohistone complex. 

CD spectra (200—350 nm) of calf thymus chromatin were compared with those of pure DNA in solution. Some of the differences observed (above approximately 250 nm) were attributed to conformational changes in the bound DNA, in which a large tilting of the bases may occur [(373) and refs, cited therein]. Previously, similar conclusions had been reached on the basis of ORD measurements on native nucleohistone and on nucleohistone dissociated either fully or partially into DNA and histone [(374) see also (375—379)]. CD studies on the state of DNA in native and partially deproteinized gander erythrocyte chromatin and of complexes of ethidium bromide with these systems were reported (380). 

Each prokaryotic chromosome consists of a supercoiled circular DNA molecule com-plexed to a protein core. Each eukaryotic chromosome consists of a single linear DNA molecule that is complexed with histones to form nucleohistone. 

Each eukaryotic chromosome is composed of nucleohistone, a complex formed by winding a single DNA molecule around a histone octomer to form a nucleosome. The DNA of mitochondria and chloroplasts is similar to the chromosomes found in prokaryotes. 

Bradbury et al. (1967) have studied partial nucleoproteins produced by removal of histone fractions from nucleohistone. Figure 12.6 shows a polarized infrared spectrum of one of the nucleoproteins oriented by shearing. The spectrum is that of a film deuterated by D2O vapor. The bands of the in-plane vibrations of cytosine and guanine moieties are highly polarized when the electric vector is placed perpendicular to the fiber axis. The 1452 cm absorption is that of the amide IF band (Miyazawa et al., 1956 Miyazawa, 1962) for the readily deuterated fraction of the partial nucleoprotein. As seen in Fig. 12.6, the 1452 cm band is polarized considerably in the direction parallel to the fiber axis, giving evidence that the extended polypeptide chain is situated so that its axis lies between the angle of the groove in the DNA double helix and the axis of the DNA helices (Bradbury et al., 1967). 

J. N. Davidson, Biochemistry of Nucleic Acids, seventh ed.. Chapman and Hall, London, 1972. D. M. P. Phillips (ed.). Histones and Nucleohistones, Plenum, New York, 1971. 

Histones occur in the chromosomes of organisms with genuine cell nuclei, for example, in the chromosomes of the thymus gland. The biological function of histones is largely unknown. They probably regulate replication or transcription. Histones are complexed with nucleic acids (nucleohistones) in organisms. 

Isolated nuclei are highly permeable to histones, protamines and other biological macromolecules, whereas ATP and Na become tightly bound. The chief components of isolated and disrupted nuclei are DNA-histone complexes (nucleohistones), ribonucleic acids and poorly soluble acidic proteins (residual proteins). Nuclei also contain high concentrations of an arginase and an adenosine S -phosphata e of unknown function. 

At the terminal stage of spermatogenesis of salmon fish, nucleohistones are replaced by nucleoprotamine. Not much is known about the mechanisms by which one kind of basic polypeptide is replaced by another. What seems certain is that protamines are made in the cytoplasm and transferred to the nucleus before the displacement can take place. Prior to the exchange, both types of basic proteins, preformed histones and newly synthesized protamines, are phosphorylated. 

The configurational relationship between histones and DNA remains ambiguous, mainly because no clear-cut results can be obtained with standard physicochemical analysis. Sharp patterns are obtained with X-ray diffraction of DNA, but X-ray diffraction studies of histones provide only diffuse images in which the ring shape and the orientation of the arc are difficult to define. However, it seems well established that the DNA maintains its structure inside the nucleoprotein. Studies of nucleohistones by infrared spectroscopy or rotatory dispersion have also yielded results that are not easily interpreted. 

Bartley, J.A., Chalkley, R. The binding of deoxyribonucleic acid and histone in native nucleohistone. J. biol. Chem. 247, 3647-3655 (1972)... 

Despite the vast number of chemical and physical experiments, the available information is still insufficient to explain the detailed structiu e of the nucleohistones, with the exception of several DNA-repressor protein complexes. Some general tendencies, however, have been found the a-helix conformation of the polypeptides is predominant in histones, as has been shown by ORD experiments lysine-rich histones mainly occupy the major groove of B-DNA,< > while protamines are bound to DNA in the minor groove (for further details see Otto et alP ). 

After dissociation of nucleohistone into DNA and histone in concentrated NaCl solutions, the nucleohistone complex can be restored by interaction between DNA and histone in dilute salt... 

Fig. 86. Structure of nucleohistone shown as a section cut perpendicularly to the long axis of the particles. Darkly shaded circles denote DNA, lightly shaded circles regions containing histone, a) Isotropic solutions, structure of one multistranded particle 4 DNA molecules are located in the center of the micelle b, c) other types of possible structures (Luzzati and Nicolaieff, 1963).

The hypotheses concerning possible function of histones as genetic regulators (repressors) are based on several assumptions. First, it is evident that before there can be any serious discussion that histones fulfill such functions, in addition to our knowledge of the structure of nucleohistones and the nature of the links between DNA and histones, the following must also be proved. 

Littau and co-workers (1965) studied the chromatin structure of thymus lymphocytes by biochemical and electron-microscopic methods. They found that nucleohistone chromatin masses consist of two types of complexes compact and diffuse. The formation of compact complexes, as they showed, is associated with the lysine-rich histone, which forms cross-linkages between the DNA strands. The diffuse histone, in which synthesis of messenger RNA mainly takes place, has no such cross-linkages. It contains arginine-rich histone which is linked to DNA along its strands. 

The compact nucleohistone corresponds to the heterochromatin of chromosomes and the diffuse to the euchromatin. We know that heterochromatin zones of the chromosomes are inert. In the poly-tene chromosomes of insects, histones from different poles of the... 

However, in their attempt to find such an RNA—histone complex in a number of animal tissues, Commerford and Delihas (1966) were unable to confirm Bonner s findings exactly. In pea nucleohistone about 100 RNA—histone nucleotides were present for every thousand DNA nucleotides. In nucleohistone of the liver and intestinal mucous membrane only two RNA—histone nucleotides were present per thousand DNA nucleotides. Commerford and Delihas consider that this ratio alone is insufficient ground for assigning to this RNA fraction the role of a specific carrier of repressors. 

Bonner, J., Ts o, P. O. P. (Eds.) The nucleohistones. San Francisco Holden-Day Inc. 1964. Busch, H. Histones and other nuclear proteins. New York Academic Press Inc. 1965. Murray, K. The basic proteins of cell nuclei. Ann. Rev. Biochem. 34, 209— 246 (1965). Vendrely, R., Vendrely, C. Biochemistry of histones and protamines. In Protoplasma-logia, Vol. V/3 c. Berlin-Heidelberg-New York Springer 1966. 

Phillips, D, M. P, Histones and nucleohistones. London and New York Plenum Press 1971. 

---