Interaction of f1 histone with superhelical DNA

Probing hyper-negatively supercoiled mini-circles with nucleases and DNA binding proteins

It is well accepted that the introduction of negative supercoils locally unwinds the DNA double helix, influencing thus the activity of proteins. Despite the use of recent methods of molecular dynamics simulations to model the DNA supercoiling-induced DNA deformation, the precise extent and location of unpaired bases induced by the negative supercoiling have never been investigated at the nucleotide level. Our goals in this study were to use radiolabeled double-stranded DNA mini-circles (dsMCs) to locate the unpaired bases on dsMCs whose topology ranged from relaxed to hyper-negatively supercoiled states, and to characterize the binding of proteins involved in the DNA metabolism. Our results show that the Nuclease SI is nearly ten times more active on hyper-negatively supercoiled than relaxed DNA. The structural changes responsible for this stimulation of activity were mapped for the first time with a base pair resolution and shown to be subtle and distributed along the entire sequence. As divalent cations modify the DNA topology, our binding studies were conducted with or without magnesium. Without magnesium, the dsMCs topoisomers mostly differ by their twist. Under these conditions, the Escherichia coli topoisomerase I weakly binds relaxed dsMCs and exhibits a stronger binding on negatively and hyper-negatively supercoiled dsMCs than relaxed dsMCs, with no significant difference in the binding activity among the supercoiled topoisomers. For the human replication protein A (hRPA), the more negatively supercoiled is the DNA, the better the binding, illustrating the twist-dependent binding activity for this protein. The presence of magnesium permits the dsMCs to writhe upon introduction of negative supercoiling and greatly modifies the binding properties of the hRPA and Escherichia coli SSB on dsMCs, indicating a magnesium-dependent DNA binding behavior. Finally, our experiments that probe the topology of the DNA in the hRPA-dsMC complexes show that naked and hRPA-bound dsMCs have the same topology.

Two DNA-binding sites on the globular domain of histone H5 are required for binding to both bulk and 5 S reconstituted nucleosomes

We have previously shown the existence of two DNA-binding sites on the globular domain of H5 (termed GH5), both of which are required for nucleosome organisation, as judged by the protection of a 166 bp chromatosome intermediate during micrococcal nuclease digestion of chromatin. This supports a model in which GH5 contacts two duplexes on the nucleosome. However, studies of a nucleosome assembled on the 5 S rRNA gene have argued against the requirement for two DNA-binding sites for chromatosome protection, which has implications for the role of linker histones. We have used this proposed difference in the requirement for a second site on the globular domain in the two models as a means of investigating whether bulk and reconstituted 5 S nucleosomes are indeed fundamentally different. GH5 protects a 166 bp chromatosome in both "bulk" and 5 S systems, and in both cases protection is abolished when all four basic residues in site II are replaced by alanine. Binding to four-way DNA junctions, which present a pair of juxtaposed duplexes, is also abolished. Single mutations of the basic residues did not abolish chromatosome protection in either system, or binding to four-way junctions, suggesting that the residues function as a cluster. Both bulk and 5 S nucleosomes thus require a functional second DNA-binding site on GH5 in order to bind properly to the nucleosome. This is likely to reflect a similar mode of binding in each case, in which two DNA duplexes are contacted in the nucleosome. There is no indication from these experiments that linker histones bind fundamentally differently to 5 S and bulk nucleosomes.

Competition between HMG-I(Y), HMG-1 and histone H1 on four-way junction DNA

High mobility group proteins HMG-I(Y) and HMG-1, as well as histone H1, all share the common property of binding to four-way junction DNA (4H), a synthetic substrate commonly used to study proteins involved in recognizing and resolving Holliday-type junctions formed during in vivo genetic recombination events. The structure of 4H has also been hypothesized to mimic the DNA crossovers occurring at, or near, the entrance and exit sites on the nucleosome. Furthermore, upon binding to either duplex DNA or chromatin, all three of these nuclear proteins share the ability to significantly alter the structure of bound substrates. In order to further elucidate their substrate binding abilities, electrophoretic mobility shift assays were employed to investigate the relative binding capabilities of HMG-I(Y), HMG-1 and H1 to 4H in vitro. Data indicate a definite hierarchy of binding preference by these proteins for 4H, with HMG-I(Y) having the highest affinity (Kd approximately 6.5 nM) when compared with either H1 (Kd approximately 16 nM) or HMG-1 (Kd approximately 80 nM). Competition/titration assays demonstrated that all three proteins bind most tightly to the same site on 4H. Hydroxyl radical footprinting identified the strongest site for binding of HMG-I(Y), and presumably for the other proteins as well, to be at the center of 4H. Together these in vitro results demonstrate that HMG-I(Y) and H1 are co-dominant over HMG-1 for binding to the central crossover region of 4H and suggest that in vivo both of these proteins may exert a dominant effect over HMG-1 in recognizing and binding to altered DNA structures, such as Holliday junctions, that have conformations similar to 4H.

Histone H1 preferentially binds to superhelical DNA molecules of higher compaction

In chromatin, the physiological amount of H1 is one molecule per nucleosome or, roughly, one molecule per 200 bp of DNA. We observed that at such a stoichiometry, H1 selectively binds to supercoiled DNA with magnitude of sigma > or = 0.012 (both negative and positive), leaving relaxed, linear, or nicked DNA molecules unbound. When negative and positive DNA topoisomers of varying superhelicity are simultaneously present in the binding mixture, H1 selectively binds to the molecules with highest superhelicity; less supercoiled forms are gradually involved in binding upon increasing the amount of input protein. We explain this topological preference of H1 as the consequence of an increased probability for more than one H1-DNA contact provided by the supercoiling. The existence of simultaneous contacts of H1 with both intertwined DNA strands in the supercoiled DNA molecules is also inferred by topoisomerase relaxation of H1-DNA complexes that had been prefixed with glutaraldehyde.

Deoxyribonuclease I-facilitated electrotransfer of protein-DNA complexes from electrophoretic gels to nitrocellulose membranes

A simple and reproducible technique for efficient transfer of protein-DNA complexes from electrophoretic gels to nitrocellulose membranes is described. Transfer of DNA-protein complexes may be difficult, especially when the DNA is of high molecular mass. A considerable improvement in the efficiency of transfer can be achieved by directly digesting the DNA in the gel by DNase I. The method is illustrated in the example of the preferential binding of histone H1 to superhelical plasmid DNA.

Histone structure and the organization of the nucleosome

Chromatin structure is now believed to be dynamic and intimately related with cellular processes such as transcription. Over the past few years, high-resolution structures for the histones have become available. These structures and their implications for nucleosome organization are reviewed here.

H1 binding unwinds DNA. Evidence from topological assays

The preference of the linker histones to bind to superhelical DNA in comparison with linear or relaxed molecules suggests that these proteins might, in turn, change the twist and/or writhe of DNA molecules upon binding. In order to explore such a possibility, we looked for changes in the linking number of plasmid pBR322 caused by H1 binding, using assays that involve nicking and resealing of DNA strands. Two types of enzymes were used, eukaryotic topoisomerase I and prokaryotic DNA ligase. The results revealed that H1 binding causes unwinding of the DNA, with the unwinding angle being approximately 10 degrees . The globular domain of histone H1 is also capable of unwinding DNA, but to a lesser degree.

Linker histones affect patterns of digestion of supercoiled plasmids by single-strand-specific nucleases

The effect of histone H1 binding on the cleavage of superhelical plasmids by single-strand-specific nucleases was investigated. Mapping of P1 cleavage sites in pBR322, achieved by EcoRI digestion after the original P1 attack, showed an intriguing phenomenon: preexisting susceptible sites became "protected," whereas some new sites appeared at high levels of H1. Similar results were obtained with another single-strand-specific nuclease, S1. Disappearance of cutting at preexisting sites and appearance of new sites was also observed in a derivative plasmid that contains a 36-bp stretch of alternating d(AT) sequence that is known to adopt an altered P1-sensitive conformation. On the other hand, H1 titration of a dimerized version of the d(AT)18-containing plasmid led to protection of all preexisting sites except the d(AT)18 inserts, which were still cut even at high H1 levels; in this plasmid no new sites appeared. The protection of preexisting sites is best explained by long-range effects of histone H1 binding on the superhelical torsion of the plasmid. The appearance of new sites, on the other hand, probably also involves a local effect of stabilization of specific sequences in Pl-sensitive conformation, due to direct H1 binding to such sequences. That such binding involves linker histone N- and/or C-terminal tails is indicated by the fact that titration with the globular domain of H5, while causing disappearance of preexisting sites, does not lead to the appearance of any new sites.

High-affinity binding sites for histone H1 in plasmid DNA

The interaction of histone H1 isolated from chicken erythrocytes with restriction fragments from plasmids pBR322 and pUC19 was studied by gel electrophoresis. Certain restriction fragments exhibited unusually high affinity for the histone, forming high molecular mass complexes at protein to DNA ratios at which the other fragments did not show evidence for binding. The highly preferred fragments are intrinsically curved, as judged by their electrophoretic mobility in polyacrylamide gels, by computer modeling, and by imaging with scanning force microscopy. However, control experiments with either curved portions of the same fragments or highly curved kinetoplast DNA fragments showed that the presence of curvature alone was not sufficient for preferential binding. By using various restriction fragments centered around the highly preferred sequence, it was found that the high-affinity binding required in addition the presence of specific sequences on both sides of the region of curvature. Thus, both curvature and the presence of specific sites seem to be required to generate high affinity.

Binding of histones H1 and H5 and their globular domains to four-way junction DNA

We have compared chicken erythrocyte linker histones H1 and H5 binding to a synthetic four-way DNA junction. Each histone binds to form a single complex, with an affinity which permits competition against a large excess of linear duplex DNA. The affinity of H5 is higher than that of H1. The globular domain from either protein will also bind strongly, but in this case multiple binding occurs. Binding of intact H1 is inhibited by cations: Mg2+ and spermidine are very effective, Na+ much less so. This inhibition is not likely to be a general ion-competition effect, for Mg2+ is much less effective in inhibiting the binding of H1 to linear DNA. Instead, the inhibition of binding may be due to ion-dependent changes in the conformation of the four-way junction, which are known to occur under similar conditions. These results strongly suggest that the angle formed between the arms of the DNA junction could be a major determinant in the interaction of H1 with DNA crossovers.

Histones H1 and H5 interact preferentially with crossovers of double-helical DNA

The interaction of the linker histones H1 and H5 from chicken erythrocyte chromatin with pBR322 was studied as a function of the number of superhelical turns in circular plasmid molecules. Supercoiled plasmid DNA was relaxed with topoisomerase I so that a population with a narrow distribution of topoisomers, containing from zero to five superhelical turns, was obtained. None of the topoisomers contained alternative non-B-DNA structures. Histone-DNA complexes formed at either 25 or 100 mM NaCl final concentration and at histone-DNA molar ratios ranging from 10 to 150 were analyzed by agarose gel electrophoresis. The patterns of disappearance of individual topoisomer bands from the gel were interpreted as an indication of preference of the linker histones for crossovers of double-helical DNA. This preference was observed at both salt concentrations, being more pronounced under conditions of low ionic strength. Isolated H5 globular domain also caused selective disappearance of topoisomers from the gel, but it did so only at very high peptide-DNA molar ratios. The observed preference of the linker histones for crossovers of double-helical DNA is viewed as a part of the mechanism involved in the sealing of the two turns of DNA around the histone octamer.

Theoretical analysis of gel electrophoretic data for interaction of lysine rich histone with supercoiled DNA

We demonstrate the possibility of using gel electrophoresis as a technique for the quantitative analysis of interaction of lysine rich histone with DNA. On the basis of theoretical framework for extended ligand binding to one-dimensional lattices such as DNA we have set up systems of equations which relate the ligand-to-DNA ratio to the observed gel migration distance of the complex. From the analysis of experimental data for gel electrophoresis of supercoiled DNA in the presence of lysine rich histones we have found that the observed variation of electrophoretic mobilities of the histone-DNA complexes at low histone-to-DNA ratios can be described by a non-cooperative binding behaviour. At this limit we have estimated the intrinsic binding constant to be of the order of 10(3) M-1.

Histone H1-DNA interactions and their relation to chromatin structure and function

The belief that histone H1 interacts primarily with DNA in chromatin and much less with the protein component has led to numerous studies of artificial H1-DNA complexes. This review summarizes and discusses the data on different aspects of the interaction between the linker histone and naked DNA, including cooperativity of binding, preference for supercoiled DNA, selectivity with respect to base composition and nucleotide sequence, and effect of H1 binding on the conformation of the underlying DNA. The nature of the interaction, the structure of the complexes, and the role histone H1 exerts in chromatin are also discussed.

Altered catalytic activity of and DNA cleavage by DNA topoisomerase II from human leukemic cells selected for resistance to VM-26

The simultaneous development of resistance to the cytotoxic effects of several classes of natural product anticancer drugs, after exposure to only one of these agents, is referred to as multiple drug resistance (MDR). At least two distinct mechanisms for MDR have been postulated: that associated with P-glycoprotein and that thought to be due to an alteration in DNA topoisomerase II activity (at-MDR). We describe studies with two sublines of human leukemic CCRF-CEM cells approximately 50-fold resistant (CEM/VM-1) and approximately 140-fold resistant (CEM/VM-1-5) to VM-26, a drug known to interfere with DNA topoisomerase II activity. Each of these lines is cross-resistant to other drugs known to affect topoisomerase II but not cross-resistant to vinblastine, an inhibitor of mitotic spindle formation. We found little difference in the amount of immunoreactive DNA topoisomerase II in 1.0 M NaCl nuclear extracts of the two resistant and parental cell lines. However, topoisomerase II in nuclear extracts of the resistant sublines is altered in both catalytic activity (unknotting) of and DNA cleavage by this enzyme. Also, the rate at which catenation occurs is 20-30-fold slower with the CEM/VM-1-5 preparations. The effect of VM-26 on both strand passing and DNA cleavage is inversely related to the degree of primary resistance of each cell line. Our data support the hypothesis that at-MDR is due to an alteration in topoisomerase II or in a factor modulating its activity.

A DNA-binding protein from Ustilago maydis prefers duplex DNA without chain interruptions

Using a nitrocellulose filter binding assay, we have partially purified a protein from mitotic cells of Ustilago maydis that binds preferentially to covalently closed circular duplex DNA. DNA containing single- or double-strand breaks is bound poorly by the protein. Once formed, the DNA-protein complex is stable, resisting dissociation in high salt. However, when a DNA strand is broken, the complex appears to dissociate. The protein binds equally well to form I DNA of phi X174 or the plasmid pBR322, but has a higher affinity for a hybrid plasmid containing a cloned region of Drosophila melanogaster satellite DNA.

Isolation of Drosophila proteins that bind selectively to left-handed Z-DNA

An affinity column for isolating Z-DNA binding proteins was made by attaching brominated poly(dG-dC) to Sephadex. Proteins from Drosophila nuclei were prepared and those that could bind to Escherichia coli B-DNA were removed from the solution. The remaining proteins were passed over the Z-DNA affinity column and then eluted with NaCl. Using both direct and competitive filter binding assays, we found that the eluted proteins bind to brominated poly(dG-dC) (Z-DNA) and poly(dG-m5dC) but not to poly(dG-dC) (B-DNA), native or denatured E. coli or calf thymus DNA, or brominated oligonucleotides. The proteins also bind to negatively supercoiled plasmids carrying Z-DNA sequences but not to relaxed or linearized plasmids in which the Z-DNA conformation is no longer present. Gel analysis reveals a mixture of several large proteins up to approximately 150,000 daltons.

Cytophotometric determinations of DNA, histone, arginine, lysine, and their concentrations in eu- and heterochromatin of the cell nucleus of dysplasias, carcinoma in situ, and carcinoma of the human cervix uteri

In the cell nuclei of dysplasias, carcinoma in situ, and carcinoma the amount of DNA, histone, arginine, lysine, and their condensed areas were determined by cytophotometry. Moreover their concentrations in the euchromatic and heterochromatic areas of the nuclei were measured. Statistical analysis showed that the quantitative relations between parameters did not change during carcinogenesis. The significantly increased amount of lysine underlines the role of the lysine-rich histones in producing denser coiling of the chromatin fibril in heterochromatin.

Interaction of histone H1 with superhelical DNA. Sedimentation and electron microscopical studies at low salt concentration

Complexes of histones H1 with superhelical SV40 DNA obtained by direct mixing were studied in 0.1 SSC buffer corresponding to 0.02 M Na+. Depending on the molar input ratio H1/DNA three classes of sedimenting species were observed: (1) a component sedimenting similar to superhelical DNA with a sedimentation coefficient s2o,w of 25 S observable up to 335 Mol H1/Mol DNA (w/w = 2); (2) a component with s2o,w = 120 S appearing at 135 Mol H1/Mol DNA and (3) growing amounts of heterogeneous aggregates greater than 1000 S. Electron micrographs revealed the 25 S component to consist of double-fibers formed from one DNA molecule and the 120 S component to consist of bundles of several such double-fibers. The aggregates represent cable-like structures. The addition of ethidium bromide to 25 S complexes induces the formation of bundles, if H1 is present in a quantity which alone is not sufficient to bring about this effect. This result indicates that ethidium bromide effects a redistribution of H1 molecules and that H1 is responsible for the bundle formation.

Interaction between lysine-rich histones and DNA

Using a membrane filter retention technique we have studied the interaction between DNA and lysine rich histone H5 in vitro. It is found that, depending on the ionic conditions, H5 can bind DNA in a random or cooperative manner and exhibits a preference to DNA with high molecular weight and/or high A + T content, as also observed with H1. The presence of 6 M urea in the assay mixture does not impair the selectivity of H5 to A + T rich DNA but partly affects its selectivity to DNA size. In contrast to H1, H5 does not discriminate between the superhelical and relaxed forms of circular SV40 DNA.

Translocation of the pre-synaptic complex formed upon DNA uptake by Streptococcus sanguis and its inhibition by ethidium bromide

Donor DNA in its initially bound, single-stranded form exists in a chromosomally-unassociated complex where it is resistant to exogenous DNase I but sensitive to micrococcal nuclease. Most of the complexes are readily recuperable from the supernatant of recipients converted into spheroplasts. Subsequent to formation of this superficially located complex, donor DNA progressively associates with the recipient chromosome into which it is eventually integrated. Treatment of recipients with ethidium bromide at various times after initial DNA binding almost immediately halts translocation of whatever donor material is not yet synapsed with the chromosome. On the other hand, donor DNA that has already synapsed experiences no difficulty in becoming genetically integrated. Some degradation occurs to DNA that fails to undergo translocation as a result of ethidium bromide treatment, the acid-soluble products appearing in the culture medium. DNA in untranslocated complexes surviving treatment is not appreciably different in single-strand length from that in untreated complexes. When these surviving complexes are isolated from a cell lysate, the contained DNA can be shown by spectrofluorometry to have bound the drug.

Histones: metabolism in simian virus 40-infected cells and incorporation into virions

The infection of confluent monkey cells with simian virus 40 induced the synthesis of both cellular DNA and histones. However, during the course of infection, cellular histone synthesis was uncoupled from cellular DNA replication and became coupled to viral DNA replication. The synthesis of all five host histone species was induced after virus infection and they appeared to be more highly phosphorylated than their couterparts in uninfected cells. At late times after infection, the cells contained twice as much histones as did uninfected cells. All the histone species except H1 were incorporated into virions. Compared to cellular histones, virion histones were enriched in the arginine-rich species H3 and H4. Although both old and newly synthesized cellular histones were incorporated into virions, there were about 5 times more newly synthesized than old histone polypeptides in virions.

The effect of H1 histone on the action of DNA-relaxing enzyme

The action of DNA-relaxing enzyme on H1-DNA complexes was investigated. Complexes of superhelical and relaxed closed circular duplex DNA with H1 were treated with mammalian relaxing enzyme, deproteinized, and electrophoresed on agarose gels. At relatively low ratios of H1 to superhelical DNA, molecules of superhelical density intermediate between those of the starting material and relaxed DNA, the normal product, were generated. At relatively high H1 histone concentrations (H1:DNA greater than 0.4 w/w), the superhelical DNA was not relaxed. Further, no superhelical turns were introduced into relaxed closed duplex DNA at any concentration of H1 tested. Thus, the binding of H1 histone to DNA prevents the action of the relaxing enzyme. Moreover, H1 histone does not appear to unwind the DNA duplex upon binding. The implications of these observations and the previously demonstrated specificity of H1 histone for superhelical DNA are discussed in relation to the structure of chromatin.

Selective interaction of 5'-bromodeoxyuridine substituted DNA with different chromosomal proteins

Chromosomal proteins selectively interact with 5'-bromodeoxyuridine (BrdUrd) substituted DNA relative to unsubstituted DNA. The relative affinities of chromosomal proteins for BrdUrd-DNA and unsubstituted DNA were measured by both thermal chromatography on hydroxylapatite and selective retention on nitrocellulose filters. Certain chromosomal proteins have a high affinity for hydroxylapatite; thus, during thermal chromatography of chromatin, the single-stranded DNA component percolates across a bed of adsorbed proteins as it elutes. We have measured the relative affinities of Brd-Urd-DNA and normal DNA for chromosomal proteins by chromatographing appropriate mixtures on hydroxylapatite. The results show that, under these conditions, the histone components, rather than the nonhistone chromatin proteins, retard the BrdUrd-substituted DNA. In addition, the individual histones vary in the degree of their affinity for BrdUrd-DNA in the order H3 greater than H4 greater than H2A greater than H2B greater than H1. We have used the property that protein-DNA complexes have a preferential affinity for nitrocellulose filters over naked DNA to measure the selective binding of BrdUrd-DNA and unsubstituted DNA's to both histone and nonhistone chromosomal proteins at low temperatures. The histones selectively retained BrdUrd-DNA on filters in the order H4 greater than H2A greater than H3 greater than H2B greater than H1. Using this assay, the nonhistones displayed greater selectivity toward BrdUrd-DNA than the histone fraction. We interpret these results to mean BrdUrd-containing DNA has a specific affinity for certain chromosomal proteins with BrdUrd-DNA may be the basis for selective inhibition of cytodifferentiation by the thymidine analogue, BrdUrd.

Studies on the interaction of H1 histone with superhelical DNA: characterization of the recognition and binding regions of H1 histones

The very lysine rich histone, H1, isolated from a variety of sources interacts preferentially with superhelical DNA compared to relaxed DNA duplexes. The nature of this specific interaction has been investigated by studying the ability of various purified fragments of H1 histone from calf thymus to recognize and bind superhelical DNA. The data suggest that the globular region of the H1 histone molecule (amino acid residues 72-106) is involved in the recognition of superhelical DNA. Thus, the H1 histone carboxy-terminal fragment, 72-212, resembles native H1 histone both quantitatively and qualitatively in its ability to discriminate between and bind to superhelical and relaxed DNA while the H1 histone carboxy-terminal fragment, residues 106-212, has lost this specificity, binding superhelical and relaxed DNA equally well. Furthermore, under conditions in which the globular region of the intact H1 histone has been unfolded, the molecule loses its ability to discriminate between superhelical and relaxed DNA, and binds both forms of DNA equally.

Histones bind more tightly to bromodeoxyuridine-substituted DNA than to normal DNA

Using a membrane filter assay, we have obtained results from both kinetic and competition experiments indicating that histones bind more strongly to bromodeoxyuridine-substituted DNA than to normal DNA. At 37 degrees C in our standard buffer of 0.2 M ionic strength, the rate of dissociation of histones H1, H2, and h4 from BrdU-substituted DNA is respectively 7, 4, and 2 times slower than it is from normal DNA. Competition experiments show an even greater difference between BrdU-substituted and normal DNA with respect to histone binding. The tighter binding of histones to BrdU-substituted DNA is of interest because of the known effects of BrdU on eukaryotic chromosome condensation and staining, virus induction, and the inhibition of differentiation.

Ethidium bromide as a probe of conformational heterogeneity of DNA in chromatin. The role of histone H1

The accessibility and the tertiary structure of the DNA inside chromatin were studied by using ethidium bromide (EB) as a fluorescent probe. The exclusion model of binding was refined by introductina a parameter alpha (0less than alpha less than 1) which measures the accessibility of the DNA and by taking into account when necessary the existence of two sets of binding sites. We were thus able to fit predicted and experimental isotherms and then to describe completely EB binding to native or partially histone depleted chromatin under various conditions. Itn native chromatin 95% of the DNA (alpha = 0.95) appears to be accessible to EB but two sets of sites are present. The first one corresponds to alpha = 0.13 and is characterized by an affinity constant which is higher by two orders of magnitude than that relative to pure DNA. The second set corresponds to alpha = 0.82 and the corresponding binding constant is only three or four times lower than that of pure DNA. The sites with high affinity are still present after treatment with formaldehyde but disappear after removal of histon H1. By comparison with chromatin treated with deoxycholate of with artifical complexes between H1 and DNA, high affinity sites were found only when all of the histons are bound to DNA. An alpha value around 0.8 is still obtained in 1 M NaC1 treated chromatin, pointing to the fact that histones H3 and H4 are preventing 20% of the DNA to intercalate EB.

Transition from noncooperative to cooperative and selective binding of histone H1 to DNA

A transition from noncooperative to cooperative binding of DNA and histone h1 occurs between 20 and 40mMNaCI in 5 mM Tris-HCI, pH 7.5. Below 20mM NaCI in mixtures in H1 and excess DNA, H1 binds to all of the DNA molecules, causing them to sediment faster, and does not distinguish between DNA molecules that differ in size or base composition. However, at NaCI concentrations above the narrow transition range, H1 binds to only some of the DNA molecules and leaves the rest free. If the DNA molecules in a mixture are the same size, H1 selectively binds those that have the highest content of adenosine (A)+thymidine (T). By means of competion experiments at salt concentrations spanning the transition range, it is demonstrated that H1 selectivity requires cooperativity. A high degree of selectivity based on A + T content can be produced by cooperative binding: for average DNA sizes of 2 X10(6) daltons, more than ten molecules of calf lymphocyte DNA (57% a+ t) are chosen per molecule of Escherichia coli DNA (50% A+T).

The androgenic regulation of prostate proteins with a high affinity for deoxyribonucleic acid. Evidence for a prostate deoxyribonucleic acid-unwinding protein

1. When testosterone is injected into castrated rats in vivo, a significant increase in the incorporation of [35S]methionine into prostate proteins may be detected under conditions in vitro. 2. Studies based on DNA-cellulose chromatography show that the synthesis of prostate proteins with a high affinity for DNA is particularly enhanced by androgenic stimulation. 3. These changes in protein synthesis are negated when the anti-androgen, cyproterone acetate, is administered concomitantly with testosterone in vivo. 4. Two assays were developed for measuring the strand separation of prostate DNA; first, the retention of 3H-labelled native DNA on nitrocellulose membranes, and second, the activation of native DNA as a template for 9S prostate DNA polymerase. On the basis of these criteria, DNA-unwinding activity is present in the prostate gland and it is regulated by androgens in a steroid-and tissue-specific manner. 5. The results are discussed in the context of the mechanism of action of androgens, particularly since the changes provoked in DNA-unwinding activity by androgens precede the onset of DNA replication and mitosis.

A sedimentation study of the interaction of superhelical SV40 DNA with H1 histone

By moving boundary sedimentation it is shown that the interaction of H1 histone with superhelical circular SV40 DNA results in the formation of giant heterogeneous aggregates. The size of these aggregates grows with increasing H1 concentration. s20,w values of some 10 000 S were measured. As compared with open relaxed circular DNA a preferential interaction of superhelical DNA with H1 histone is observed, irrespective of the sign of the superhelical turns which was reversed by the addition to DNA of ethidium bromide. The addition to the H1 complexed aggregates of ethidium bromide effects a progressive breakdown of the aggregates. Furthermore, the superhelicity of DNA is not changed by the addition of small amounts of H1 histone.