The salt dependence of chicken and yeast chromatin structure. Effects on internucleosomal organization and relation to active chromatin

The ionic strength dependences of yeast and chicken erythrocyte chromatin structure have been examined by analysis of nuclear DNase I and Staphylococcal nuclease digestions done under various salt and divalent cation concentrations. The basic features of yeast DNase I profiles (intracore/intercore patterns and their 5-base pair offset) remain present under all conditions tested. However, there are changes in specific parts of the patterns. In very low salt, the intercore DNase I pattern is enhanced; even very small intercore bands can be detected. Staphylococcal nuclease intracore cleavage becomes prominent. Increasing salt enhances the large DNase I intracore bands and the frequency of spacer cleavage for both nucleases. Thus, yeast has a salt-dependent higher order structure: a chromatin fiber with a prominent spacer/core distinction in (physiological) salt; a fiber with a decreased distinction between spacer and core, i.e. a more uniform fiber, in very low salt. The salt-dependent bulk changes resemble single gene chromatin changes during gene expression and may provide a model for that process. Above bands 16.5-17.5, chicken and yeast intercore patterns are coincident. Thus, at least a fraction of chicken chromatin has discrete length spacers like yeast does. This fraction may be significant, for the prominence of the intercore pattern, and hence the apparent abundance of discrete spacers, can be significantly enhanced by digestion in very low salt. The major differences between the two chromatins are in the intracore/intercore transition region: the region is larger and more complex in chicken; ionic strength changes affect the chicken transition region more strongly. Since this region of the profile corresponds to digestion near the ends of the core, that part of the nucleosome must differ in structure and in conformational flexibility in the two chromatins.     

Nucleosome core particle self-assembly kinetics and stability at physiological ionic strength

Micrococcal nuclease, DNase I, and trypsin have been employed to study the kinetics of core particle self-assembly by salt jump from 2.0 to 0.2 M NaCl. A few seconds after the initiation of the reassociation reaction, the bulk of core particle DNA becomes protected from digestion by micrococcal nuclease, whereas free DNA, under the same conditions, is completely hydrolyzed. The central and C-terminal regions of core histones are also protected from trypsin digestion immediately after the 2.0-0.2 M NaCl salt jump. Moreover, the extent of degradation produced by trypsin is the same for samples digested a few seconds after the salt jump and for samples digested 20 min after the salt jump. With DNase I, minor structural differences have been detected between samples obtained at different times during the reaction. However, even in this case our results indicate that many of the characteristic histone-DNA contacts within the core particle are made a few seconds after the initiation of the self-assembly reaction. Furthermore, core particles have been labeled with the fluorescent reagent N-(1-pyrenyl)maleimide (NPM), which was previously used as a sensitive probe for nucleosome conformation. Extensive DNase I or trypsin digestion of NPM-labeled core particles in 0.2 M NaCl does not produce significant changes in excimer fluorescence. This allows us to conclude that the covalent continuity of DNA is not required for the maintenance of the folded conformation of the core particle and that the trypsin-resistant domains of core histones play a fundamental role in the stabilization of this structure.     

Conformation of the HMG17-nucleosome complex

The nucleosome core binds more than two molecules of HMG17 at low ionic strength (8.9 mM Tris-HCl/8.9 mM boric acid/0.25 mM Na2EDTA, pH 8.3). Circular dichroism of the complexes showed only minor conformational changes of the nucleosome core DNA on binding of HMG17, with no detectable change in the histone secondary structure. The fluorescence of N-(3-pyrene) maleimide bound to -SH groups at Cys-110 of H3 histones in the core particle suggested that the structure of the histone octamer assembly changed little upon binding of HMG17 to the nucleosome. These observations support the idea that even a high level of HMG17 binding, e.g., four HMGs per nucleosome, alone, does not open up the core particle.     

Self-assembly of single and closely spaced nucleosome core particles.

Self-assembly of DNA with the four core histones but in the absence of H1 generates nucleosome core particles which are spaced randomly over large distances. Closely spaced core particles, however, exhibit a preferred short linkage which is not a multiple of 10 base pairs. They bind about 140 base pairs whereas apparently shorter DNA lengths per nucleosome observed after digestion with micrococcal nuclease are the result of degradation from the ends. The DNA length of one superhelical turn in the core particle is 83 +/- 4 base pairs. Single core particles may bind more DNA than closely spaced core particles but probably less than two full turns of 168 base pairs. The internal structures of single and of native core particles are very similar as judged by their amount of DNA, sedimentation coefficient, appearance in the electron microscope, and digestion with DNase I. In addition to core particles, a particle is described which sediments at 9 S and consists of 108 base pairs of DNA bound to the histone octamer. It appears to be the smallest stable "core particle" but it is not a degradation product of the 146-base-pair core particle. Digestion of end-labeled 9 S and nucleosome core particles with DNase I shows distinct differences.     

Redefinition of the cleavage sites of DNase I on the nucleosome core particle

DNase I has been widely used for the footprinting of DNA-protein interactions including analyses of nucleosome core particle (NCP) structure. Our understanding of the relationship between the footprint and the structure of the nucleosome complex comes mainly from digestion studies of NCPs, since they have a well-defined quasi-symmetrical structure and have been widely investigated. However, several recent results suggest that the established consensus of opinion regarding the mode of digestion of NCPs by DNase I may be based on erroneous interpretation of results concerning the relationship between the NCP ends and the dyad axis. Here, we have used reconstituted NCPs with defined ends, bulk NCPs prepared with micrococcal nuclease and molecular modelling to reassess the mode of DNase I digestion. Our results indicate that DNase I cuts the two strands of the nucleosomal DNA independently with an average stagger of 4 nt with the 3'-ends protruding. The previously accepted value of 2 nt stagger is explained by the finding that micrococcal nuclease produces NCPs not with flush ends, but with approximately 1 nt 5'-recessed ends. Furthermore we explain why the DNA stagger is an even and not an odd number of nucleotides. These results are important for studies using DNase I to probe nucleosome structure in complex with other proteins or any DNA-protein complex containing B-form DNA. We also determine the origin of the 10n +/- 5 nt periodicity found in the internucleosomal ladder of DNase I digests of chromatin from various species. The explanation of the 10n +/- 5 nt ladder may have implications for the structure of the 30 nm fibre.     

A nonuniform distribution of excision repair synthesis in nucleosome core DNA

We have investigated the distribution in nucleosome core DNA of nucleotides incorporated by excision repair synthesis occurring immediately after UV irradiation in human cells. We show that the differences previously observed for whole nuclei between the DNase I digestion profiles of repaired DNA (following its refolding into a nucleosome structure) and bulk DNA are obtained for isolated nucleosome core particles. Analysis of the differences obtained indicates that they could reflect a significant difference in the level of repair-incorporated nucleotides at different sites within the core DNA region. To test this possibility directly, we have used exonuclease III digestion of very homogeneous sized core particle DNA to "map" the distribution of repair synthesis in these regions. Our results indicate that in a significant fraction of the nucleosomes the 5' and 3' ends of the core DNA are markedly enhanced in repair-incorporated nucleotides relative to the central region of the core particle. A best fit analysis indicates that a good approximation of the data is obtained for a distribution where the core DNA is uniformly labeled from the 5' end to position 62 and from position 114 to the 3' end, with the 52-base central region being devoid of repair-incorporated nucleotides. This distribution accounts for all of the quantitative differences observed previously between repaired DNA and bulk DNA following the rapid phase of nucleosome rearrangement when it is assumed that linker DNA and the core DNA ends are repaired with equal efficiency and the nucleosome structure of newly repaired DNA is identical with that of bulk chromatin. Furthermore, the 52-base central region that is devoid of repair synthesis contains the lowest frequency cutting sites for DNase I in vitro, as well as the only "internal" locations where two (rather than one) histones interact with a 10-base segment of each DNA strand.     

A partial characterization of DNA fragments protected from nuclease degradation in histone depleted metaphase chromosomes of the Chinese hamster.

A small proportion (0.1-0.5%) of the total DNA content of native Chinese hamster metaphase chromosomes is protected from nucleolytic degradation following the removal of histones by extraction with either 0.2 N HCl or 2 M NaCl, and remains attached to the nonhistone protein core. Acid extraction followed by DNase I digestion leads to small fragments of 10-30 bases. Salt extraction followed by micrococcal nuclease digestion gives approx. 140 b.p. fragments which are undistinguishable in size from nucleosome core DNA fragments. Furthermore, DNase I treatment of salt extracted chromosomes gives DNA fragments containing single strands which are multiples of 10 bases in length, again characteristic of the nucleosome structure. Reassociation kinetics using the 32P-labelled 140 b.p. fragments as probes suggests they are enriched for rapidly reassociating sequences.     

Wrapping of genomic polydA.polydT tracts around nucleosome core particles.

Five human clones containing genomic regions of polydA have been isolated by their ability to form intermolecular triple helices with agarose cross-linked polyU. All of these clones contain Alu repetitive DNA sequences. End-labelled DNA fragments containing these sequences have been successfully reconstituted onto nucleosome core particles by salt exchange. The structure of these has been examined by digesting with DNase I, hydroxyl radicals or diethylpyrocarbonate. DNase I cleavage of the polydA tracts is poor in the free DNA but is markedly enhanced at certain positions when complexed with nucleosome cores. Phased digestion patterns are observed which continue through the (A)n blocks and reveal an average helical periodicity of about 10 base pairs. The distance between adjacent maxima varies between 8-12 base pairs, suggesting that the exact helical repeat is not necessarily constant. One fragment containing the sequence (TA)11T34 reveals a 12 base pair repeat within the (AT)n region. A pUC19 polylinker fragment containing a block of A69.T69 cloned into the Smal site could also be reconstituted onto nucleosome cores and reveals the same phased DNaseI digestion pattern. The DNase I cleavage pattern is not identical at each of the maxima, suggesting that the structural distortions imposed by the core particles are not constant along the DNA.     

Use of selectively trypsinized nucleosome core particles to analyze the role of the histone "tails" in the stabilization of the nucleosome

Using immobilized trypsin and an appropriate fractionation procedure, we have been able to prepare, for the first time, nucleosome core particles containing selectively trypsinized histone domains. The particles thus obtained: [(H3T-H4T)2-2(H2AT-H2BT)].DNA; [(H3-H4)2-2(H2AT-H2BT)].DNA; [H3T-H4T)2-2(H2A-H2B)].DNA (where T means trypsinized), together with the non-trypsinized controls have been characterized using the following techniques: analytical ultracentrifugation, circular dichroism, thermal denaturation and DNAse I digestion. The major aim of this study was to analyze the role of the amino-terminal regions (the histone "tails") on the stability of the nucleosome in solution. The data obtained from this analysis clearly show that stability of the nucleosome core particle to dissociation (below a salt concentration of 0.7 M-NaCl) is not affected by the presence or the absence of any of the N-terminal regions of the histones. Furthermore, these histone regions make very little contribution, if any, to the conformational transition that nucleosomes undergo in this range of salt concentrations. They play, however, a very important role in determining the thermal stability of the particle, as reflected in the dramatic alterations exhibited by the melting profiles upon selective removal of these tails by trypsinization. The melting data can be explained by a simple hypothesis that ascribes interaction of H2A/H2B and H3/H4 tails to particular regions of the nucleosomal DNA.     

Periodicity of exonuclease III digestion of chromatin and the pitch of deoxyribonucleic acid on the nucleosome

Exonuclease III has previously been shown to pause about every 10 nucleotides along the 3' strands while it invades the nucleosome core. Here, the exact periodicity of this digestion, i.e., the spacing of the pauses, was determined. Results showed that the exonuclease digests the first 20 nucleotides at the edge of the nucleosome core with a periodicity of approximately 11 nucleotides; in contrast, DNA closer to the center of the particle is digested with a smaller periodicity of about 10 nucleotides. These figures differ from the known periodicity of DNase I digestion, approximately 10 and 10.5 nucleotides at the edge and in the center of the nucleosome, respectively. Moreover, as shown by sedimentations in sucrose gradients, the structure of the nucleosome does not appear to be significantly altered by the gradual destruction of its DNA moiety by the exonuclease. Such stability of the nucleosome, along with other complementary observations, indicates that the transition in the digestion periodicity of the exonuclease may not be the consequence of a structural rearrangement of the particle upon trimming. This transition may rather be ascribed to the properties of the native nucleosome and to the intrinsic mechanism of action of the enzyme. Finally, evidence is presented which suggests that the exonuclease 10-nucleotide periodicity of digestion of the inner region of the nucleosome reflects a 10 base pair/turn pitch of the DNA in that region.     

Location of the primary sites of micrococcal nuclease cleavage on the nucleosome core

The positions and relative frequencies of the primary cleavages made by micrococcal nuclease on the DNA of nucleosome core particles have been found by fractionating the double-stranded products of digestion and examining their single-stranded compositions. This approach overcomes the problems caused by secondary events such as the exonucleolytic and pseudo-double-stranded actions of the nuclease and, combined with the use of high resolution gel electrophoresis, enables the cutting site positions to be determined with a higher precision than has been achieved hitherto. The micrococcal nuclease primary cleavage sites lie close (on average, within 0.5 nucleotide) to those previously determined by Lutter (1981) for the nucleases DNase I and DNase II. These similarities show that the accessible regions are the same for all three nucleases, the cleavage sites being dictated by the structure of the nucleosome core. The differences in the final products of the digestion are explained in terms of secondary cleavage events of micrococcal nuclease. While the strongly protected regions of the nucleosome core DNA are common to all three nucleases, there are differences in the relative degrees of cutting at the more exposed sites characteristic of the particular enzyme. In particular, micrococcal nuclease shows a marked polarity in the 3'-5' direction in the cutting rates as plotted along a single strand of the nucleosomal DNA. This is explained in terms of the three-dimensional structure of the nucleosome where, in any accessible region of the double helix, the innermost strand is shielded by the outermost strand on the one side and the histone core on the other. The final part of the paper is concerned with the preference of micrococcal nuclease to cleave at (A,T) sequences in chromatin.     

DNase II digestion of the nucleosome core: precise locations and relative exposures of sites.

The precise locations and relative exposures of the DNase II-accessible sites in the nucleosome core DNA are determined using techniques previously employed for the enzyme DNase I. It is found that there are a number of similarities between the site exposure patterns for the two enzymes but that in general the DNase II seems to discriminate less among adjacent sites' accessibilities than does DNase I. The two enzymes attack essentially the same positions in the DNA, the average difference between the precise location of the site being less than one-half base for the two enzymes. Such close similarities in the digestion patterns of two enzymes with such different mechanisms of scission show that the patterns reflect the structure of the nucleosome core and not merely the properties of the particular enzyme used.     

DNase I sensitivity of ribosomal genes in isolated nucleosome core particles

The level of chromatin structure at which DNase I recognizes conformational differences between inert and activated genes has been investigated. Bulk and ribosomal DNA's of Tetrahymena pyriformis were differentially labeled in vivo with [14C]- and [3H]-thymidine, respectively, utilizing a defined starvation-refeeding protocol. The 3H-labeled ribosomal genes were shown to be preferentially digested by DNase I in isolated nuclei. Staphylococcal nuclease digested the ribosomal genes more slowly than bulk DNA, probably owing to the higher GC content of rDNA. DNase I and staphylococcal nuclease digestions of purified nucleosomes and of nucleosome core particles isolated from dual-labeled, starved-refed nuclei were indistinguishable from those of intact nuclei. We conclude from these studies that DNase I recognizes an alteration in the internal nucleosome core structure of activated ribosomal genes.     

A 145-base pair DNA sequence that positions itself precisely and asymmetrically on the nucleosome core.

A 145-bp DNA sequence, cloned from Escherichia coli, was reconstituted into nucleosome core particles by a number of methods. The behaviour of the resulting complex upon sucrose gradient sedimentation and nucleoprotein gel electrophoresis closely resembled that of control bulk nucleosome core particles. DNase I digestion of the 32P-end-labelled complex revealed the 10-bp periodicity of cleavages expected for DNA bound on a histone surface. The narrow cleavage sites observed (1 bp wide) imply that the sequence occupies a single preferred position on the nucleosome core, accurate to the level of single base pairs. By relating the digestion pattern observed to the pattern of site protection found for random sequence nucleosomes, the DNA position was found to be offset by 17 bp from that in the normal core particle. A number of experiments argue against the involvement of length or end effects and suggest that it is some feature of the DNA sequence itself that determines this precise positioning of DNA on the nucleosome.     

Nucleosome core binding region of chromosomal protein HMG-17 acts as an independent functional domain.

Chromosomal proteins HMG-14 and HMG-17 have a modular structure. Here we examine whether the putative nucleosome-binding domain in these proteins can function as an independent module. Mobility shift assays with recombinant HMG-17 indicate that synthetic molecules can be used to analyze the interaction of this protein with the nucleosome core. Peptides corresponding to various regions of the protein have been synthesized and their interaction with nucleosome cores analyzed by mobility shift, thermal denaturation and DNase I digestion. A 30 amino acid long peptide, corresponding to the putative nucleosome-binding domain of HMG-17, specifically shifts the mobility of cores as compared to free DNA, elevates the tm of both the premelt and main melt of the cores and protects from DNase I digestion the same nucleosomal DNA sites as the intact protein. The binding of both the peptide and the intact protein is lost upon digestion of the histone tails by trypsin. The nucleosomal binding sites of the peptide appear identical to those of the intact protein. Thus, a region of the protein can acts as an independent functional domain. This supports the notion that HMG-14 and HMG-17 are modular proteins. This finding is relevant to the understanding of the function and evolution of HMG-14/-17, the only nucleosome core particle binding proteins known to date.     

The mechanism of nucleosome assembly onto oligomers of the sea urchin 5 S DNA positioning sequence

We have used a model system composed of tandem repeats of Lytechinus variegatus 5 S rDNA (Simpson, R. T., Thoma, F., and Brubaker, J. M. (1985) Cell 42, 799-808) reconstituted into chromatin with chicken erythrocyte core histones to investigate the mechanism of chromatin assembly. Nucleosomes are assembled onto the DNA template by mixing histone octamers and DNA in 2 M NaCl followed by stepwise dialysis into very low ionic strength buffer over a 24-h period. By 1.0 M NaCl, a defined intermediate composed of arrays of H3.H4 tetramers has formed, as shown by analytical and preparative ultracentrifugation. Digestion with methidium propyl EDTA.Fe(II) indicates that these tetramers are spaced at 207 base pair intervals, i.e. one/repeat length of the DNA positioning sequence. In 0.8 M NaCl, some H2A.H2B has become associated with the H3.H4 tetramers and DNA. Surprisingly, under these conditions DNA is protected from methidium propyl EDTA.Fe(II) digestion almost as well as in the complete nucleosome, even though these structures are quite deficient in H2A.H2B. By 0.6 M NaCl, nucleosome assembly is complete, and the MPE digestion pattern is indistinguishable from that observed for oligonucleosomes at very low ionic strength. Below 0.6 M NaCl, the oligonucleosomes are involved in various salt-dependent conformational equilibria: at approximately 0.6 M, a 15% reduction in S20,w that mimics a conformational change observed previously with nucleosome core particles; at and above 0.1 M, folding into a more compact structure(s); at and above 0.1 M NaCl, a reaction involving varying amounts of dissociation of histone octamers from a small fraction of the DNA templates. In low ionic strength buffer (less than 1 mM NaCl), oligonucleosomes are present as fully loaded templates in the extended beads-on-a-string structure.     

Quantitative analysis of chromatin structure by circular dichroism

Quantitative analysis of the circular dichroism of nucleohistones and protein-free DNA was carried out in order to determine the structure and the role of the linker region DNA in chromatin, in terms of the conformational change of chromatin as a function of the ionic strength. It is shown clearly that the circular dichroism of Hl-depleted chromatin isolated from calf thymus is determined only by the ratio of the core region to the linker region and demonstrated by the linear combination of the spectrum of protein-free DNA and that of the nucleosome core in 5 mm-Tris · HCl, 1 mm-EDTA (pH 7.8). The calculated spectrum for the linker region in the H1-depleted chromatin was in good agreement with that of protein-free DNA. From the difference spectra between nucleohistones and protein-free DNA, it is suggested that the chromatin has an additional winding of DNA other than 146 base-pairs of DNA around the histone core. By decreasing the ionic strength to values lower than 5 mm-Tris · HCl, 1 mm-EDTA, the ellipticity of H1-depleted chromatin increased greatly between 250 nm and 300 nm while the increase was small in the case of chromatin and the nucleosome core. Nucleosomes with linker region DNA but without histone H1 also show great increase in ellipticity in this range of wavelengths as the ionic strength is decreased. Therefore, the linker region in H1-depleted chromatin plays an important role in the conformational changes brought about by changes in the ionic strength, and the conformational changes caused in the DNA of chromatin by decreasing the ionic strength are suppressed by the presence of histone H1.     

Precise location of DNase I cutting sites in the nucleosome core determined by high resolution gel electrophoresis.

The precise locations of the DNase I cutting sites in the nucleosome core have been determined by analysis of the DNA products of a DNase I digestion of 32P end-labelled mucleosome cores on a high resolution gel electrophoresis system. This system is capable of resolving fragments of mixed sequence DNA differing by one base into the region of 160 bases in length. The DNase I cutting sites in the core are found to be spaced at multiples of about 10.4 (i.e. clearly different from 10.0) bases along the DNA, but show significant variations about this value. In addition to the location of the sites, the stagger between individual sites on opposite strands has been determined and is found to be inconsistent with at least one proposed mechanism for nuclease cleavage of chromatin DNA. Finally, a calculated distribution of fragment lengths in a DNase I digest of nuclei has been determined from the data obtained from the nucleosome core and found to be in reasonable agreement with the observed distribution. The periodicity of 10.4 is discussed with respect to the number of base pairs per turn of chromatin DNA and the number of superhelical turns of DNA per nucleosome.     

A correlation between nucleosome spacer region susceptibility to DNase I and histone acetylation.

Hepatoma tissue culture (HTC) cell nuclei were digested with either DNase I or micrococcal nuclease and the nucleohistone digestion products fractionated by gel electrophoresis or exclusion chromatography. Under appropriate conditions, gel electrophoresis demonstrates that for both nucleases, only cleavages within the nucleosome spacer regions and not within the nucleosome core lead to freely migrating nucleohistone particles. These particles consist of nucleosome cores, nucleosomes and nucleosome oligomers. Following DNase I digestion and fractionation by exclusion chromatography, analysis of the histones indicates a direct relationship between increased spacer region susceptibility to nuclease and increased nucleosomal histone acetylation. Evidently digestion sites outside the regions of DNA protected by core histones can reflect the degree of acetylation of core histones. Such a relationship is not found when micrococcal nuclease is used to digest the samples.     

The roles of H1, the histone core and DNA length in the unfolding of nucleosomes at low ionic strength.

Calf thymus nucleosomes exhibit two different and independent hydrodynamic responses to diminishing salt concentration. One change is gradual over the range 40 to 0.2 mM Na+ and is accompanied by decreases in contact-site cross-linking efficiency. The other change is abrupt, being centered between 1 and 2 mM Na+. We found only one abrupt change in sedimentation rate for particles ranging in DNA content fom 144 to 230 base pairs. This response to decreasing ionic strength is similar for particles of both 169 and 230 base pairs. Core particles (144 base pairs) exhibit a somewhat diminished response. The abrupt change is blocked by formaldehyde or dimethylsuberimidate cross-linking. The blockage by dimethylsuberimidate demonstrates that the abrupt conformational change requires the participation of the core histones. H1 completely blocks the abrupt but not the gradual conformational change. Thus H1 uncouples the different responses to low ionic strength and exerts an important constraint on the conformational states available to the nucleosome core.