DNA of a circular minichromosome linearized by restriction enzymes or other reagents is resistant to further cleavage: an influence of chromatin topology on the accessibility of DNA.
ABSTRACT The accessibility of DNA in chromatin is an essential factor in regulating its activities. We studied the accessibility of the DNA in a ∼170 kb circular minichromosome to DNA-cleaving reagents using pulsed-field gel electrophoresis and fibre-fluorescence in situ hybridization on combed DNA molecules. Only one of several potential sites in the minichromosome DNA was accessible to restriction enzymes in permeabilized cells, and in growing cells only a single site at an essentially random position was cut by poisoned topoisomerase II, neocarzinostatin and γ-radiation, which have multiple potential cleavage sites; further sites were then inaccessible in the linearized minichromosomes. Sequential exposure to combinations of these reagents also resulted in cleavage at only a single site. Minichromosome DNA containing single-strand breaks created by a nicking endonuclease to relax any unconstrained superhelicity was also cut at only a single position by a restriction enzyme. Further sites became accessible after ≥95% of histones H2A, H2B and H1, and most non-histone proteins were extracted. These observations suggest that a global rearrangement of the three-dimensional packing and interactions of nucleosomes occurs when a circular minichromosome is linearized and results in its DNA becoming inaccessible to probes.
Article: Precise localization of the alpha-globin gene cluster within one of the 20- to 300-kilobase DNA fragments released by cleavage of chicken chromosomal DNA at topoisomerase II sites in vivo: evidence that the fragments are DNA loops or domains.[show abstract] [hide abstract]
ABSTRACT: We have mapped the position of the alpha-globin gene cluster in the 20- to 300-kilobase fragments of chromosomal DNA isolated from growing chicken HD3 erythroblastoid cells exposed to 4'-demethylepipodophyllotoxinthenylidene beta-D-glucoside. This epipodophyllotoxin traps functioning topoisomerase II molecules, the denaturation of which cleaves DNA and reveals their reaction sites. The DNA fragments, prepared by centrifugation in sucrose gradients, bind selectively to glass-fiber filters and are protected from lambda 5'-exonuclease, properties compatible with the presence of a topoisomerase II subunit bound to their 5' ends. Restriction enzyme cleavage of the fragments and hybridization with cloned alpha-globin-region probes reveal additional distinctive bands not seen in control DNA, allowing the localization of fragment ends near this gene cluster. The terminal regions of fragments from sucrose gradients or from field-inversion electrophoresis gels were also used to probe cloned regions of the gene cluster. Both approaches show that this cluster of three genes, which is not expressed in these cells, is located at a specific position in a approximately 20-kilobase DNA fragment. The upstream end of this fragment lies in a region that contains a site of DNA attachment to the nuclear matrix mapped by both in vivo and in vitro methods, and its downstream end is flanked by approximately 80% A + T sequences characteristic of matrix-attachment regions. These observations suggest that the DNA fragments are formed because topoisomerase II molecules can specifically and readily integrate into DNA at matrix-attachment regions and that the fragments represent entire DNA loops or domains.Proceedings of the National Academy of Sciences 11/1991; 88(19):8515-9. · 9.68 Impact Factor
Article: Isolation, characterization, and structure of the folded interphase genome of Drosophila melanogaster.[show abstract] [hide abstract]
ABSTRACT: The intact interphase genome of Drosophila melanogaster has been isolated by sucrose gradient centrifugation after gentle lysis of tissue culture cells in 0.9 M NaCl-0.4% nonidet P40. The non-viscous folded DNA sediments as a single broad 5000S peak in a complex with RNA (a fraction of the nuclear nascent RNA) and protein (all of the four intranuclesome histones: H2A, H2B, H3, and H4). The folded DNA is supercoiled and can be relaxed to slower sedimenting forms either by intercalating ethidium or by nicking with DNAase I. Incomplete DNAase treatment gives partially relaxed complexes, indicating that each nick relaxes only a stretch of DNA (defined as a supercoiled DNA loop) without affecting the superhelical content of the rest of the genome. The concentration of superhelices in the Drosophila folded DNA is the same as in the E. coli and SV40 closed circular DNAs-that is, about one negative turn every 200 base pairs (bp) in 0.15 M NaCl at 26 degrees C. The estimated average size of the supercoiled DNA loops, about 85,000 bp, equals the size of the larger Drosophila chromomeres. Ethidium intercalation in 0.9 M NaCl both removes the negative superhelical turns and dissociates the four histones from the DNA. The four histones are dissociated in equimolar concentrations, and the relative proportion of histones displaced from the DNA is a function of ethidium concentration. The histones are completely dissociated from the folded DNA at the ethidium concentration. The histones are completely dissociated from the folded DNA at the ethidium concentration which removes all of the negative superhelices. Thus the data strongly suggest that the rotation of the Watson Crick helix which accompanies ethidium intercalation causes the loss of nucleosomes from the DNA. The results are interpreted in terms of a model for the folded Drosophila genome which has the DNA constrained (by both protein-DNA and RNA- DNA interactions) into independent supercoiled loops containing on the average 400 nucleosomes per loop. Each nucleosome is composed of a histone core with the DNA wound around it in a 360 degrees left-handed toroidal supercoil; each nucleosome toroidal supercoil plus its relaxed internucleosome DNA contains, on the average, 200 bp.Cell 12/1976; 9(3):393-407. · 32.40 Impact Factor
Article: Chromatin structure, not DNA sequence specificity, is the primary determinant of topoisomerase II sites of action in vivo.[show abstract] [hide abstract]
ABSTRACT: In the studies reported here we have used topoisomerase II as a model system for analyzing the factors that determine the sites of action for DNA-binding proteins in vivo. To localize topoisomerase II sites in vivo we used an inhibitor of the purified enzyme, the antitumor drug VM-26. This drug stabilizes an intermediate in the catalytic cycle, the cleavable complex, and substantially stimulates DNA cleavage by topoisomerase II. We show that lysis of VM-26 treated tissue culture cells with sodium dodecyl sulfate induces highly specific double-strand breaks in genomic DNA, and we present evidence indicating that these double-strand breaks are generated by topoisomerase II. Using indirect end labeling to map the cleavage products, we have examined the in vivo sites of action of topoisomerase II in the 87A7 heat shock locus, the histone repeat, and a tRNA gene cluster at 90BC. Our analysis reveals that chromatin structure, not sequence specificity, is the primary determinant in topoisomerase II site selection in vivo. We suggest that chromatin organization may provide a general mechanism for generating specificity in a wide range of DNA-protein interactions in vivo.Molecular and Cellular Biology 11/1991; 11(10):4973-84. · 5.53 Impact Factor