Interaction of deoxyribonucleic acid with histone f2b and its half-molecules. Circular dichroism studies

Micelle Density Regulated by a Reversible Switch of Protein Secondary Structure

Protein secondary structures may exhibit reversible transitions that occur in an abrupt and controllable manner. In this report, we demonstrate that such transitions may be utilized in the design of a "smart" protein micellar system, in which a stimulus-induced change in protein structure triggers a rapid change in micelle compacticity and size. Specifically, recombinant DNA methods were used to prepare a protein triblock copolymer containing a central hydrophilic block and two hydrophobic end blocks derived from elastin-mimetic peptide sequences. Below the copolymer inverse transition temperature (T(t)), dilute solutions of this amphiphilic protein formed monodispersed micelles in a narrow range of R(H) of approximately 100 nm. When the the temperature was raised above T(t), an abrupt increase in micelle internal density was observed with a concomitant reduction in micelle size. This reversible change in micelle compacticity was triggered by helix-to-sheet protein folding transition. Significantly, these protein polymer-based micelles, which are rapidly responsive to environmental stimuli, establish a new mechanism for the design of controlled drug delivery vehicles.

Structure Distribution in an Elastin-Mimetic Peptide (VPGVG)3 Investigated by Solid-State NMR

Elastin is an extracellular-matrix protein that imparts elasticity to tissues. We have used solid-state NMR to determine a number of distances and torsion angles in an elastin-mimetic peptide, (VPGVG)3, to understand the structural basis of elasticity. C-H and C-N distances between the V6 carbonyl and the V9 amide segment were measured using 13C-15N and 13C-1H rotational-echo double-resonance experiments. The results indicate the coexistence of two types of intramolecular distances: a third of the molecules have short C-H and C-N distances of 3.3 +/- 0.2 and 4.3 +/- 0.2 A, respectively, while the rest have longer distances of about 7 A. Complementing the distance constraints, we measured the (phi, psi ) torsion angles of the central pentameric unit using dipolar correlation NMR. The -angles of P7 and G8 are predominantly ~150, thus restricting the majority of the peptide to be extended. Combining all torsion angles measured for the five residues, the G8 C chemical shift, and the V6-V9 distances, we obtained a bimodal structure distribution for the PG residues in VPGVG. The minor form is a compact structure with a V6-V9 C=O-HN hydrogen bond and can be either a type II -turn or a previously unidentified turn with Pro (phi = -70, psi= 20 +/- 20) and Gly ( phi= -100 +/- 20, psi = -20 +/- 20). The major form is an extended and distorted beta-strand without a V6-V9 hydrogen bond and differs from the ideal parallel and antiparallel beta-strands. The other three residues in the VPGVG unit mainly adopt antiparallel beta-sheet torsion angles. Since (VPGVG)3 has the same 13C and 15N isotropic and anisotropic chemical shifts as the elastin-mimetic protein (VPGXG)n (X = V and K, n = 195), the observed conformational distribution around Pro and Gly sheds light on the molecular mechanism of elastin elasticity.

Design and Construction of Glutamine Binding Proteins with a Self-Adhering Capability to Unmodified Hydrophobic Surfaces as Reagentless Fluorescence Sensing Devices

The chemically and genetically remodeling of proteins with ligand binding specificities can be utilized to synthesize various protein-based microsensors for detecting single biomolecules. Here, we describe the construction and characterization of fluorophore-labeled glutamine binding proteins (QBP) and derivatives coupled to the independently designed hydrophobic polypeptide (E12) that can adhere onto solid surfaces via hydrophobic interactions. The single cysteine mutant (N160C QBP) modified with the three environmentally sensitive fluorescent dyes (IAANS, acrylodan, and IANBD ester) showed increased changes in fluorescence intensity induced by glutamine binding. The use of these conjugates as reagentless fluorescence sensors enables us to determine the glutamine concentrations (0.1-50 microM) in homogeneous solution. The fusion of N160C QBP with E12, (Gly4-Ser)n spacers (GSn), and IANBD resulted in the novel fluorescence sensing elements having an adhering capability to hydrophobic surfaces of unmodified microplates. In ELISA and fluorescence experiments for the microplates treated with a series of the conjugates, IANBD-labeled N160C QBP-GS1-E12 displayed the best reproducibility in adhesion onto the hydrophobic surfaces and the precise correlation between fluorescence changes and glutamine concentrations. The performance of the biosensor-attached microplate for glutamine titrations demonstrated that the hydrophobic interaction of E12 with solid surfaces is useful for effective immobilization of proteins that need specific conformational movements in recognizing particular biomolecules. Therefore, the technique using E12 as a surface-linking domain for protein adhesion onto unmodified substrates could be applied effectively to prepare microplates/arrays for a wide variety of high-throughput assays on chemical and biological samples.

Binding of small acid-soluble spore proteins from Bacillus subtilis changes the conformation of DNA from B to A

Small acid-soluble spore proteins (SASPs) appear 3-4 hr after the onset of sporulation in Gram-positive bacteria and constitute up to 20% of the protein of mature spores. Previous studies using Bacillus subtilis deletion mutants lacking SASP-alpha and -beta have shown that such mutations abolish the elevated resistance of spores to UV radiation. Analyses using circular dichroism and Fourier-transform infrared spectroscopy now demonstrate that binding alpha/beta-type SASPs to DNA in vitro causes a structural change in DNA, from the B to the A conformation. This may provide the basis whereby alpha/beta-type SASPs confer increased spore UV resistance in vivo--by changing spore DNA conformation, they alter DNA photochemistry such that UV irradiation produces spore photoproduct instead of the more lethal cyclobutane-type thymine dimers.

Reactivation of affinity-purified estrogen receptors by peptides derived from histone H2B

Purification of estrogen receptors by affinity chromatography over diethylstilbestrol-agarose is associated with a major loss of estradiol binding activity. Histone H2B can restore a significant fraction of the binding activity. Cleavage of the H2B molecule into two halves by cyanogen bromide reveals that the carboxyl terminus is responsible for the major reactivating effects.

Circular dichroism studies on chromatin models. Interactions between DNA and sequential polypeptides containing arginine

The sequential polypeptides (L-Arg-Xaa-Gly)n where Xaa represents amino acid residues Ala, Val and Leu, were employed as models of arginine-rich histones, in studying their interactions with nucleic acids. These polypeptide-DNA complexes were prepared using gradient dialysis and their conformational properties were investigated by circular dichroism spectroscopy. It was found that poly(L-Arg-L-Val-Gly) caused pronounced structural changes in the DNA molecule (conformational transition from B to the more compact and asymmetric C form) as a function of ionic strength and polypeptide: DNA ratio. In contrast the DNA interaction with poly (L-Arg-L-Ala-Gly) and poly (L-Arg-L-Leu-Gly) increased in the order of Ala----Leu, but with slight structural changes in the DNA secondary conformation. Thus, the importance of the composition, amino acid sequence and conformation of the polypeptides which bind to DNA was demonstrated. The significance of the hydrophobic forces besides the arginine-phosphate charge interaction, which modulate the nature of the polypeptide-DNA complexes and their condensation into higher-ordered tertiary structures, as found in chromatin, was also confirmed.

On mechanisms determining the interrelationships between DNA and histone components of chromatin

The relative affinity of histones for DNA was studied by the analysis of competitive histone binding to DNA in whole histone/DNA mixtures at physiological and low ionic strengths as well as in water. Use of polyphosphate in similar experiments, as a model of DNA deprived of hydrophobic functional groups allowed us to reject the hypothesis that hydrophobic DNA-histone interaction plays a decisive role in the determination of the relative affinity of histones for DNA, because the orders of histone preference for DNA and for polyphosphate were the same. The relative histone affinity for DNA does not depend on the secondary structure of DNA or on the ionic strength of salt solutions, though the differences in the histone affinities for DNA decrease on lowering the salt concentration. The binding orders of the first and the last molecules of histone type to DNA, studied at various DNA/histone ratios in the medium of physiological ionic strength, are the following: H3+H4, H2A+H2B, H1 and H3+H4, H2A, H2B, H1. In water the binding orders of the first and the last histone molecules to DNA are identical: H3+H4, H2A, H2B+H1. It is concluded that the relative histone affinity for DNA in water/salt solutions is determined by non-ionic interactions between histones bound to DNA. The folding of DNA induced by histone-histone interaction seems to lead to the increase in the correlation between amino acid residues in the histone regions bound to DNA and the ionic DNA-histone interaction becoming stronger.

Infrared linear dichroism investigations of deoxyribonucleic complexes with histones H2B and H3

Complexes between DNA and histones H2B and H3 were studies by means of infrared linear dichroism in a wide range of histone to DNA ratios and of different relative humidities. The measurement of the dichroic ratios allows one to determine the secondary structure of DNA in the complexes. It is shown that the progressive addition of histone H2B or H3 to DNA inhibits the structural B leads to A transition and DNA remains in a B-type form at low relative humidity. A new simple method is proposed to evaluate the amount of A or B forms of DNA when both structures are present. It is found that the B leads to A transition is fully inhibited when only one molecule of H2B or H3 histone is bound per about three or four turns of DNA helix, respectively. It is proposed that about four to three turns of DNA helix represent the "critical length of DNA" (minimum "cooperative unit") for the B leads to A transition.

Interactions of histones and histone peptides with DNA Thermal denaturation and solubility studies

The interactions of DNA with the five histone components (H1, H2B, H2A, H3 and H4) and with a number of histone fragments (N-H1 (1--72), C-H1 (73--216), N-H2B (l--59), C-H2B, (63--125), N-H2A (1-39), C-H2A (58--129), N-H4 (1--84) and C-H4 (85--102) have been studied by using the techniques of thermal denaturation and solubility behaviour. Complexes in 10(-3) M phosphate buffer, 2 - 10(-5) M Na(2)-EDTA, pH 7.0 were prepared by the direct mixing method. For lysine-rich histones (H1 and H2B) it has been found that the main characteristics which governs the interaction with DNA are located in the very lysine-rich part of the molecules, i.e. in the C-H1 and N-H2B segments. These regions are also responsible for a cooperative distribution of the histone along the DNA molecules in the artificial complexes. It appears from our studies that the tertiary structure of the moderately, arginine-rich histone (H2A) is an essential feature for its interaction with DNA. The two arginine-rich histones (H3 and H4) complexed with DNA behave in a similar way, both in thermal denaturation and in DNA precipitation. In the case of C-H4, a marked shift of the melting profile has been observed which is correlated with the presence in the peptide of the hydrophilic cluster Lys-Arg-Gln-Gly-Arg-Thr. Our results suggest that large segments rich in lysine and basic clustering within histones give rise to different modes of electrostatic interaction with DNA.

Interaction of DNA with poly(L-Lys-L-Ala-Gly) and poly(L-Lys-L-Ala-L-Pro). Circular dichroism and thermal denaturation studies

Complexes of DNA with polypeptides composed of Lys, Ala, and Gly in both a sequential order, poly(L-lysine-L-alanine-glycine), and a statistical distribution, poly(L-lysine36-L-alanine28-glycine), were prepared using gradient dialysis. These polypeptide-DNA complexes were studied using ultraviolet absorption (UV) and circular dichroism (CD) to probe the conformation, binding, and melting behavior of DNA in the complex. Complexes with the sequential polypeptide showed no structural change in the DNA; however, the complexes with the random polypeptide yield CD spectra similar to phi DNA [Maniatis, T., Venable, Jr., J.S., and Lerman, L.S. (1974), J. Mol. Biol. 84, 37]. A second sequential polypeptide, poly(L-Lys-L-Ala-L-Pro)n, -DNA complex was also studied. It was found to exhibit pronounced structural changes as a function of ionic strength and poly-peptide-DNA ratio, more similar to the random sequence that the ordered sequence of the Lys, Ala, Gly polymer. Thus the importance of the composition and amino acid sequence in polypeptides which bind to DNA, even in such simple systems, is demonstrated. Evidence from thermal denaturation, employing simultaneous monitoring of CD and UV changes, supports a model in which specific polypeptides cause condensation of the DNA in the complex into an asymmetric tertiary structure. The relevance of these model systems to chromatin is discussed.

Prediction of the conformation of the histones

The secondary structures of the histones, H1, H2A, H2B, H3, and H4 have been predicted utilizing the predictive scheme of Chou and Fasman (Biochemistry 13:211, 222[1974]) and a new set of conformational parameters based on the X-ray data of 29 protein structures. The alpha-helical, beta-sheet, reverse beta-turns, and random coil regions of these proteins are carefully delineated. Structures are specified which are most probably under various environmental conditions, i.e., for changes in ionic strength, association between histones and in association with DNA. Potential conformational changes within these histones are also predicted.

A study of histone-histone interactions by affinity chromatography

Homologous whole histone from calf thymus was adsorbed on Sepharose 4B columns with covalently coupled histone fractions H2a, H2b, H3 or H4 in 0.01 M phosphate buffer, pH 6.7 - 1 M NaCl. The adsorbed histones were eluted from the columns with 5 M urea in the same buffer. Electrophoretic analysis has shown that the different columns exhibit selective affinity to the histone fractions: the H2b column to histone H2b and H2a (with only weak affinity to histones H3 and H4), the H2a column to histones H2b and H3 (moderate affinity to histone H2b), and the H4 column to histone H3, H4 and H2b (weak affinity to histone H2a). Histone H1 displayed no fixation by either of the columns tested.

Chromatin and nucleosome structure

Chromatin nucleosomes (mononucleosomes through pentanucleosomes) have been isolated by staphylococcal nuclease digestion of calf thymus nuclei. The peak value ellipticity is the same for all oligomers, 1900 deg cm2, mol-1 at 280-nm, 23 degrees C. The dh280/dT vs T show a progressive increase in Tm of the main thermal band (73.5 degrees C, monomer; 79 degrees C, pentamer). Very small amounts of free DNA can be observed in the melting profiles, and shoulders at 60 degrees C and 93 degrees C appear and increase in magnitude as the particle size increases. The magnitude of the change, delta[theta]280, increases with oligomer size. This pattern could result from an initial unfolding of an asymmetric assembly of nucleosomes (polynucleosome superhelix) in addition to the denaturation of the internal nucleosome structure, and a subsequent or simultaneous denaturation of the double strand DNA. The extent of this unfolding appears to depend upon the size of the oligomer and therefore implies interactions between asymmetrically assembled neighboring nucleosomes.

Interactions between arginine-rich histones and deoxyribonucleic acids. II. Circular dichroism

Circular dichroism (CD) was used to investigate the conformations of arginine-rich histones, H3 (III or f3) and H4 (IV or f2a1), and DNA in the complexes prepared by four different methods: (A) NaCl gradient dialysis with urea; (B) NaCl gradient dialysis without urea; (C) direct mixing in 2.5 x 10(-4) M EDTA, pH 8.0; and (D) direct mixing in 0.01 M sodium phosphate, pH 7.0. Using the CD spectrum of native chromatin as a criterion to judge the closeness of a complex to its native state, it was observed that a complex made by direct mixing at low ionic strength (methods C and D) is better than the ones made by NaCl gradient dialysis with or without urea (methods A and B). It is explained as a result of lack of ordered secondary structures in histones due to the presence of urea in method A or due to nonspecific aggregation in NaCl without urea (method B). Compared with all the earlier reports in literature on the CD of histone-DNA complexes, the CD spectra of arginine-rich histone-DNA complexes prepared by methods C and D are closest to that of native chromatin both in shape and in amplitude. These results imply (a) that arginine-rich histones play an important role in maintaining the conformation of chromatin and (b) that the binding of these two histones to DNA prepared by methods C and D are close to that in native chromatin. Noticeable variation in conformation of free and bound histone and histone-bound DNA has also been observed in histone H3 with one or two cysteine residues, and in reduced or oxidized state even when the complexes were prepared and examined in the same condition. CD spectra of arginine-rich histones in 0.01 M phosphates, pH 7.0, indicate the presence of alpha-helix which could be responsible for a favorable binding of the less basic regions of these histones to DNA under this condition as demonstrated by thermal denaturation (Yu, S. .S, Li H. J., and Shih, T. Y. (1976), Bio-chemistry, the preceding paper in this issue). To preserve or generate alpha-helical structures in histones seems to be a critical step in reconstituting good histone-DNA complexes.

Basic polypeptides as histone models. Effect of conformation, base composition and methylation of nucleic acids on the interaction with H1 and histone models and on the circular dichroism of complexes

Interaction of histone H 1 and models simulating histone chains was followed by monitoring the melting curves of supernatants after the sedimentation of aggregated complexes. In a mixture of two DNAs the histones reacted selectively with (A+T)-rich and non-methylated DNA, respectively. H 1 and (Ala-Lys-Pro)n also interacted preferentially with DNA in a mixture with double stranded RNA whereas (Lys30,Ala70)n did not show any selectivity. (G+C)-rich DNA in complexes showed CD spectra the intensity of which decreased with increasing DNA methylation to values comparable with these of complexes of (A+T)-rich DNA. In complexed with double stranded RNA only the polymer (Lys30,Ala70) displayed CD pattern similar to spectra of complexes with DNA. It was concluded that formation and structure of complexes depend selectively on the DNA conformation and base composition.

A contribution of nonhistone proteins to the conformation of chromatin

1. Changes in circular dichroism (CD) spectra and thermal melting profiles of guinea pigliver DNA reassociated with histones and/or nonhistone proteins from the cerebral of liver chromatin are described. 2. In the DNA-histone complex, positive ellipiticity in the CD spectrum at 260-300 nm is progressively lod by a red-shift of the crossover point at around 260 nm. DNA in this complex is thermally stabilised to a considerable extent, but not to such a full extent as is shown with DNA in native chromatin. 3. DNA-nonhistone complex in 0.14 M NaCl is, in contrast to DNA-histone complex, not precipitable by centrifugation at 20 000 X g. DNA in this complex shows only a slight reduction in ellipticity at 260-300 nm, and a very weak thermal stabilisation. 4. Characteristics in the CD spectrum of the native chromatin are most satisfactorily reproduced in the DNA-histone-nonhistone complex. These include a large decrease in ellipticity at 260-300 nm, a red-shift of the crossover point at around 260 nm, and a slight negative band at around 305 nm. Also, DNA in this complex is thermally stabilised to the extent comparable with DNA in the native chromatin. 5. Addition of nonhistone proteins to the preformed DNA-histone complex in 3 M urea renders a half of the complex, named DNA-histone(-nonhistone), unprecipitable upon centrifugation at 20 000 X g in 0.14 M NaCl. CD spectrum and thermal melting profile of the precipitable DNA-histone(-nonhistone) complex are similar to those of the DNA-histone-nonhistone complex, while in the unprecipitable DNA-histone(-nonhistone) comples, the ellipticity at 260-300 nm is significantly elevated and the highest melting transition (at 80 degrees C) is lacking. 6. The CD spectrum of native cerebral chromatin closely resembles that of unprecipitable DNA-histone(-nonhistone) complex, while in liver chromatin, the spec.trum is an intermediate between those of the unprecipitable and pn of chromatin by nonhistone proteins. Cerebral nonhistone proteins bind to DNA and to the DNA-histone complex more extensively than liver nonhistone proteins. 7. It is concluded that, although the basic conformation of DNA in native chromatin is determined largely by histones, nonhistone proteins also play an individual role. There is also an indication that nonhistone proteins exert an organ-specific modification of chromatin superstructure.

Infrared linear dichroism investigations of deoxyribonucleic acid complexes with poly(L-arginine) and poly(L-lysine)

Complexes between DNAs from various sources and poly(L-lysine) and poly(L-arginine) were studied by means of infrared linear dichroism. The measurements of dichroic ratios allowed us to determine the orientation of the phosphate group of DNA in the complexes with basic polypeptides. At high relative humidities (higher than 90%, B form), the bisector of the less than OPO in the complexes forms an angle with respect to the helical axis which has a value lower by about 4 degrees than in the corresponding DNA sample. This change of orientation of the phosphate group of DNA indicates a modification of the B form upon binding of polylysine or polyarginine. The structural transitions B leads to A and B leads to C measured as a function of relative humidities were not affected by formation of complexes with both basic polypeptides. Similar results were obtained for complexes prepared by direct mixing or by salt gradient dialysis. The presence of A and C forms was observed in complexes of DNA with poly(L-lysine) and poly(L-arginine) at lower relative humidity. Thus, the conformational flexibility of DNA in complexes with polylysine and polyarginine is not changed despite a substantial increase in the Tm (melting temperature). These results are considered as a model for the understanding of interactions between DNA and histones particularly of the binding of the N-terminal fragment, lysine or arginine rich.

Complexes of DNA with histones f2a2 and f3. Circular dichroism studies

Two histones from calf thymus, the slightly lysine-rich histone f2a2 and the arginine-rich f3, were combined separately, with homologous DNA. The complexes were reconstituted by means of guanidine hydrochloride gradient dialysis, and their circular dichroic (CD) spectra were examined in 0.14 M NaCl. The CD spectra of f2a2-DNA complexes are characterized by a positive band at 272 nm which is blue-shifted and greatly enhanced relative to the corresponding band for native DNA. This type of CD change was noted previously with f2a1-DNA and f2b-DNA complexes. In contrast, f3 histone causes only minor distortions in the DNA CD spectrum, and their character depends upon the state of the two sulfhydryl groups in f3. When the cysteines are reduced, f3-DNA complexes have a slightly increased positive band with a small blue shift; when oxidized disulfide is the predominant form, this CD band becomes slightly smaller than native DNA value. This laboratory has now examined complexes reconstituted from DNA and all five histones of calf thymus. The sum of the CD spectra of these complexes, although very similar to the CD curve for reconstituted complexes containing whole histone, does not approximate that of chromatin; the consequence of this observation is discussed.

Conformational changes in chromatin from density inhibited WI-38 fibroblasts stimulated to proliferate

Quiescent confluent monolayers of WI-38 human diploid fibroblasts can be stimulated to proliferate by replacing the old medium with fresh medium plus 10% serum. Circular dichroism spectra of chromatin from stimulated cells between 2 and 10 hrs after stimulation show an increase in positive ellipticity maxima and a blue shift in the 250-300 nm region. These changes are reversed when the stimulated cells enter DNA synthesis (which, in the present conditions, begins to increase at 12-15 hrs and reaches a peak at 20 hrs). The circular dichroism changes occurring 3 hrs after stimulation have been studied in greater detail. They consist in a 35% (average) increase in positive ellipticity and a blue shift in the 250-300 nm region. Changes in the gamma less than 244 nm region are less consistent. The differences between chromatins of stimulated and unstimulated cells are abolished when both chromatins are washed with 0.25 M NaC1. This procedure removes 10-12% of chromosomal proteins, which chromatograph with non-histone proteins. DNA, RNA and histones could not be detected in the 0.25 M NaC1 extract. In gel electrophoretic profiles of radioactively labelled chromosomal proteins from stimulated and unstimulated WI-38 cells there were no detectable differences between histones. The non-histone proteins of stimulated cells showed one radioactive peak which was increased above the level of non-histone proteins from control cells. These results show that structural changes occur in the chromatin of WI-38 cells stimulated to proliferate several hrs before the onset of DNA synthesis. The fact that differences in the chromatins can be abolished by washing with 0.25 M NaC1 could give a clue as to the mechanisms responsible for these structural changes.

Interaction of histone f2al fragments with deoxyribonucleic acid. Circular dichroism and thermal denaturation studies

The glycine-arginine-rich histone, f2al (IV) (102 amino acids), from calf thymus was cleaved at residue 84 with cyanogen bromide. Complexes containing homologous DNA and each f2al fragment were reconstituted by means of Gdn-HC1 gradient dialysis. The circular dichroic (CD) spectra of these complexes were all examined in 0.14 M NaC1. The CD spectra of the DNA-f2al fragment complexes did not differ appreciably from that of DNA alone in the wavelength region above 240 nm. However, intact f2al-DNA complexes yield CD spectra which differ significantly (enhanced, blue-shifted, 273-nm band) from that of native DNA (Shih and Fasman, 1971). The small C-terminal fragment (85-102) was bound weakly to DNA under the conditions used. However, the large basic N-terminal fragment (1-83) was bound as well to DNA as was whole f2al, but produced no CD distortion. The conformation of the N-terminal fragment, unlike intact f2al, was not changed upon increasing the ionic strength to 0.14 M NaF. These results complement previous studies on f2al and its N-terminal CNBr fragment (Ziccardi and Schumaker, 1973). Thermal denaturation of the complexes in 2.5 X 10(-4) M EDTA was monitored simultaneously by changes in the absorption and CD spectra. All complexes showed a thermal transition at 45 degrees (Tml), attributable to the melting of free, double-stranded DNA. In addition, f2al-DNA and N fragment-DNA complexes displayed melting phenomena at 88 and 78 degrees (Tm2), respectively, caused by the denaturation of the histone-bound DNA. This difference in Tm2 constitutes further evidence that loss of the 18-amino-acid carboxyl end segment of f2al prohibits the unique type of interaction which occurs between DNA and the intact histone.

Investigation of huge negative circular dichroism spectra of some nucleoproteins

Under certain conditions of preparation, DNA, whether free or complexed with polylysine or histone KAP (I, fl), produce huge negative circular dichroism (CD) spectra with maxima at about 270nm. In order to investigate the cause of these spectra, reconstituted polylysine-DNA complex was used as a model system. It was found that the CD change of DNA in the complex is not a linear function of the fraction of base pairs bound. Such a CD spectrum is not changed despite dilution up to 128 folds for as long as 12 hours. Difference CD spectra taken between free DNA and any of the complexes are qualitatively the same, and are similar to those of free DNA and nucleohistone KAP (Fasman et al., Biochemistry 9, 2814-2822, 1970), free DNA and direct mixed polylysine-DNA complexes, or free DNA in high salt (Chang et al., Biochemistry12, 3028-3032, 1973). The suggestion is made that this CD spectrum might be caused by specific conformational changes in DNA, perhaps belonging to the family of B to C transitions followed by a further structural distortion of DNA due to aggregation of the nucleoprotein molecules.

A model for particulate structure in chromatin

A model is proposed for the structure of nuclease-resistant chromatin particles. The model is novel in that it proposes that the DNA in such a particle is wound about a protein core, made up of the hydrophobic regions of histone molecules.