High sequence specificity of micrococcal nuclease.

The substrate specificity of micrococcal nuclease (EC 3.1.4.7.) has been studied. The enzyme recognises features of nucleotide composition, nucleotide sequence and tertiary structure of DNA. Kinetic analysis indicates that the rate of cleavage is 30 times greater at the 5' side of A or T than at G or C. Digestion of end-labelled linear DNA molecules of known sequence revealed that only a limited number of sites are cut, generating a highly specific pattern of fragments. The frequency of cleavage at each site has been determined and it may reflect the poor base overlap in the 5' T-A 3' stack as well as the length of contiguous A and T residues. The same sequence preferences are found when DNA is assembled into nucleosomes. Deoxyribonuclease 1 (EC 3.1.4.5.) recognises many of the same sequence features. Micrococcal nuclease also mimics nuclease S1 selectively cleaving an inverted repeat in supercoiled pBR322. The value of micrococcal nuclease as a "non-specific" enzymatic probe for studying nucleosome phasing is questioned.     

Sequence specific cleavage of DNA by micrococcal nuclease.

Micrococcal nuclease is shown to cleave DNA under conditions of partial digestion in a specific manner. Sequences of the type 5'CATA and 5'CTA are attacked preferentially, followed by exonucleolytic degradation at the newly generated DNA termini. GC-rich flanking sequences further increase the probability of initial attack. Unexpectedly, long stretches containing only A and T are spared by the nuclease. These results, which were obtained with spared by the nuclease. These results, which were obtained with mouse satellite DNA and two fragments from the plasmid pBR22, do not support the previous contention that it is the regions of high At-content which are initially cleaved by micrococcal nuclease. This specificity of micrococcal nuclease complicates its use in experiments intended to monitor the nucleoprotein structure of a DNA sequence in chromatin.     

Assembly of nucleosome-like structures mediated by cauliflower DNA topoisomerase

Nucleosome-like structures have been efficiently assembled in vitro by interaction of cauliflower histones, pBR322 DNA and cauliflower DNA topoisomerase, as assayed by supercoiling of relaxed circular DNA and by digestion with micrococcal nuclease. The optimum ionic strength for supercoiling was 150 mM KCl and the optimum weight ratio of histone to DNA was approximately 1.0. Four histones, H2A, H2B, H3 and H4, were necessary for the optimum assembling conditions, and the nucleosomes assembled protected DNA fragments of approximately 150 bp in length. It was found that cauliflower DNA topoisomerase acts not only as a DNA-relaxing enzyme but also as a chaperon factor for nucleosome assembly.     

Structure of eukaryotic chromatin. Evaluation of periodicity using endogenous and exogenous nucleases.

DNA isolated from (a) liver chromatin digested in situ with endogenous Ca2+, Mg2+-dependent endonuclease, (b) prostate chromatin digested in situ with micrococcal nuclease or pancreatic DNAase I, and (c) isolated liver chromatin digested with micrococcal nuclease or pancreatic DNAase I has been analyzed electrophoretically on polyacrylamide gels. The electrophoretic patterns of DNA prepared from chromatin digested in situ with either endogenous endonuclease (liver nuclei) or micrococcal nuclease (prostate nuclei) are virtually identical. Each pattern consists of a series of discrete bands representing multiples of the smallest fragment of DNA 200 +/- 20 base pairs in length. The smallest DNA fragment (monomer) accumulates during prolonged digestion of chromatin in situ until it accounts for nearly all of the DNA on the gel; approx. 20% of the DNA of chromatin is rendered acid soluble during this period. Digestion of liver chromatin in situ in the presence of micrococcal nuclease results initially in the reduction of the size of the monomer from 200 to 170 base pairs of DNA and subsequently results in its conversion to as many as eight smaller fragments. The electrophoretic pattern obtained with DNA prepared from micrococcal nuclease digests of isolated liver chromatin is similar, but not identical, to that obtained with liver chromatin in situ. These preparations are more heterogeneous and contain DNA fragments smaller than 200 base pairs in length. These results suggest that not all of the chromatin isolated from liver nuclei retains its native structure. In contrast to endogenous endonuclease and micrococcal nuclease digests of chromatin, pancreatic DNAase I digests of isolated chromatin and of chromatin in situ consist of an extremely heterogeneous population of DNA fragments which migrates as a continuum on gels. A similar electrophoretic pattern is obtained with purified DNA digested by micrococcal nuclease. The presence of spermine (0.15 mM) and spermidine (0.5 mM) in preparative and incubation buffers decreases the rate of digestion of chromatin by endogenous endonuclease in situ approx. 10-fold, without affecting the size of the resulting DNA fragments. The rates of production of the smallest DNA fragments, monomer, dimer, and trimer, are nearly identical when high molecular weight DNA is present in excess, indicating that all of the chromatin multimers are equally susceptible to endogenous endonuclease. These observations points out the effects of various experimental conditions on the digestion of chromatin by nucleases.     

Analysis of DNA double- and single-strand breaks by two dimensional electrophoresis: action of micrococcal nuclease on chromatin and DNA, and degradation in vivo of lens fiber chromatin.

We describe a novel system for two dimensional electrophoresis at neutral and alkaline pH for determining the double-stranded and single-stranded lengths of DNA. With this system we analysed the mode of micrococcal nuclease digestion of DNA in cellular and SV40 viral chromatin and of supercoiled SV40 DNA. The enzyme reaction occurred in two steps : the enzyme first introduced single-strand breaks, then converted these to double-strand breaks by an adjacent cleavage on the opposite strand. Digestion of cellular chromatin DNA occurred by a similar mechanism. Chromatin fragments produced by limited micrococcal nuclease action contained many single-strand breaks, which may be important when this method is used to prepare chromatin fragments for biochemical and biophysical studies. Nucleosome monomer to tetramer produced at later stages of digestion contained few if any single-strand breaks.     

Heterogeneity of chromatin fragments produced by micrococcal nuclease action.

Digestion of calf thymus chromatin with micrococcal nuclease produces a mixture of apparently well defined nucleoprotein fragments which have been partially resolved by sedimentation on linear (5-20%) sucrose gradients. Sedimentation patterns reveal a predominant peak at the 11S position, three slower components, which have not previously been reported, at the 3.4S, 5.3S and 8.6S positions, and three faster components at the 17S, 22S and 26S positions. DNA isolated from the 3S to 12S region of gradients has been resolved on polyacrylamide gels into nine to ten discrete components ranging from 47 to 156 base pairs in length. A nearly identical pattern of small DNA products was obtained from chromatin digested in intact nuclei. These data suggest that chromatin contains either several types of subunits or predominently a single type of subunit which can be asymmetrically cleaved at any one of four or more sites.     

Subunit organization of Euglena chromatin

Digestion of $\text{Euglena}$ nuclei or extracted chromatin with micrococcal nuclease results in the identification of a repeating structure. The DNA repeat size, analyzed on agarose and polyacrylamide gels, is found to be 225±13 base pairs. DNase I digestion produces a serie of fragments multiples of roughly 10 bases. Eventhough pressure shearing is necessary to disrupt the though pellicule of the phytoflagellate, we confirm that, in $\text{Euglena}$, chromatin organization is similar to that of other eukaryotes.     

Nuclear reassemblyin vitro is independent of nucleosome/chromatin assembly

It was show11 that nuclear reassembly was induced by small pieces of DNA fragments in cell-free extracts ofXenopus. In an attempt to learn the relationship between the nuclear reassembly and nucleosome/chromatin assembly, limited amounts of CM-Cellulose are used to eliminate the capacity of the egg extract S-150 to assemble chromatin. while the forming of nucleosomes is checked with DNA supercoiling by plasmid DNA pBR322 incubated in the extract, and further analysed by micrococcal nuclease digestion. This depleted extract is then used to induce nuclear reassembly around demembraned sperms with membrane vesicles. It is found that CM-Cellulose depletes histones H2A and H2B efficiently and blocks the assembly of nucleosomes, the demembraned sperms are yet reconstituted into nuclei in the treated S-150, although the chromatin in reassembled nuclei does not produce protected DNA fragments when digested with micrococcal nuclease. It suggests that in the cell-free system ofXenopus, DNA can be formed into nuclei without assembly of nucleosomes or chromatin.     

Nucleosome structure in Aspergillus nidulans

The structure of chromatin from Aspergillus nidulans was studied using micrococcal nuclease and DNAase I. Limited digestion with micrococcal nuclease revealed a nucleosomal repeat of 154 base pairs for Aspergillus and 198 base pairs for rat liver. With more extensive digestion, both types of chromatin gave a similar quasi-limit product with a prominent fragment at 140 base pairs. The similarity of the two limit digests suggests that the structure of the 140 base pair nucleosome core is conserved. This implies that the difference in nucleosome repeat lengths between Aspergillus and rat liver is caused by a difference in the length of the DNA between two nucleosome cores. Digestion of Aspergillus chromatin with DNAase I produced a pattern of single-stranded fragments at intervals of 10 bases which was similar to that produced from rat liver chromatin.     

The relaxation of specificity of BanI restriction endonuclease from Bacillus aneurinolyticus IAM 1077

The restriction endonuclease BanI from Bacillus aneurinolyticus IAM 1077, which recognizes 5′-GGPyPuCC-3′ and cleaves between G and G within this sequence, has decreased substrate specificity at high nuclease concentrations. The relaxation of its specificity was enhanced during modified reactions: digestion of pBR322 DNA or lambda DNA in the presence of high glycerol and dimethyl-sulfoxide (DMSO) produced additional fragments in addition to the inherent fragments. Therefore, it is required to check the reaction conditions carefully for generation of inherent fragments.     

Removal of histone H1 exposes linker DNA in chromatin to DNAse I

After removal of histone H1 about 40% of DNA in chromatin acquires the sensitivity of naked DNA to DNAse I. Digestion of H1-depleted chromatin with DNAse I leads to a qualitative change in the digestion pattern, generating DNA fragments of approx. 200 b.p. and multiples, similar to those obtained with micrococcal nuclease. Both effects are reversed upon reconstitution of purified H1 to H1-depleted chromatin.     

DNA fragments associated with chromosome scaffolds

Following extensive digestion of HeLa metaphase chromosomes with either Hae III endonuclease or micrococcal nuclease, nonhistone protein scaffolds may be isolated. Scaffolds isolated after Hae III digestion have about 1.5% of the chromosomal DNA attached to them. This DNA is heterogeneous in size, ranging from about 0.2 to 20 kbp. It can be cleaved with either Eco RI or Hae III - Eco RI, producing a series of repeated fragments, of which the most abundant is 1.7 kbp in length. The 1.7-kdp fragment is tandemly repeated and is enriched (about 50-fold) in the scaffold-associated DNA. It is located primarily on human chromosome 1 and is probably a component of human satellites II and III. Scaffolds isolated after micrococcal nuclease digestion have about 0.1% of chromosomal DNA attached. This DNA is present in two size classes - fragments larger than 10 kbp and fragments approximately 0.2 kbp long. Restriction enzyme digestion of this DNA gives no prominent repeated fragments. Its reassociation kinetics are similar to those of total DNA, indicating that it is not enriched in either highly repetitive or middle repetitive sequences.     

The photoaddition of trimethylpsoralen to Drosophila melanogaster nuclei: a probe for chromatin substructure.

Derivatives of the furocoumarin, psoralen, can penetrate intact cells or nuclei and cross-link opposite strands of the chromosomal DNA under the influence of long wave-length ultraviolet light. The potential of trioxsalen (4,5',8-trimethylpsoralen) as a probe for chromatin structure has been investigated. The DNA in both embryo nuclei and tissue culture cells from Drosophila melanogaster was found to be about 90% protected from trioxsalen binding relative to purified DNA. Digestion of trioxsalen-treated nuclei by micrococcal nuclease and gel electrophoresis of the resulting DNA gave the same type of band pattern that is characteristic of native, untreated nuclei are digestion. Nuclease digestion was therefore used to examine the distribution of bound trioxsalen in the DNA. The resulting DNA fragments were analyzed both by radioactivity measurements and quantitative electron microscopy. The nuclease cleaved intact photoreacted nuclei in such a way that preferential excision of trioxsalen containing regions of the DNA occurred, but, when acting upon purified DNA that contained bount trioxsalen, it attacked the trioxsalen-free regions preferentially. It was thus concluded that trixosalen binds at the sites corresponding to the regular nuclease-sensitive regions of the chromatin in nuclei.     

A site of discontinuity in the interaction between DNA and histones in nucleosomes of sea urchin embryo chromatin.

Digestion of chromatin DNA in nuclei of sea urchin embryos with pancreatic nuclease and with micrococcal nuclease give additional details concerning the interaction between DNA and histones. A specific site of hydrolysis appears to be located on the nucleosome in such a position as to split the DNA unit length in two equivalent fragments of about 60–70 base pairs in length. The complete digestion of chromatin DNA appears to depend on the low stability of the nucleosome containing the split DNA fragments.     

Nuclear reassembly in vitro is independent of nucleosome/chromatin assembly

It was show11 that nuclear reassembly was induced by small pieces of DNA fragments in cell-free extracts ofXenopus. In an attempt to learn the relationship between the nuclear reassembly and nucleosome/chromatin assembly, limited amounts of CM-Cellulose are used to eliminate the capacity of the egg extract S-150 to assemble chromatin. while the forming of nucleosomes is checked with DNA supercoiling by plasmid DNA pBR322 incubated in the extract, and further analysed by micrococcal nuclease digestion. This depleted extract is then used to induce nuclear reassembly around demembraned sperms with membrane vesicles. It is found that CM-Cellulose depletes histones H2A and H2B efficiently and blocks the assembly of nucleosomes, the demembraned sperms are yet reconstituted into nuclei in the treated S-150, although the chromatin in reassembled nuclei does not produce protected DNA fragments when digested with micrococcal nuclease. It suggests that in the cell-free system ofXenopus, DNA can be formed into nuclei without assembly of nucleosomes or chromatin. Projrrt supported by the National Natural Science Foundation of China (Grant No. 39730240)     

Nascent DNA in nucleosome like structures from chromatin

We have used chromatin sensitivity to cleavage by micrococcal nuclease as a probe for differences between chromatin containing nascent DNA and that containing bulk DNA. Micrococcal nuclease digested the nascent DNA in chromatin of swimming blastulae of sea urchins more rapidly to acid-soluble nucleotides than the DNA of bulk chromatin. A part of the nascent DNA occurred in micrococcal nuclease-resistant structures which were either different from, or temporary modifications of, the bulk nucleosomes. This was inferred from the size differences between bulk and nascent DNA fragments in 10% polyacrylamide gels after micrococcal nuclease digestion of nuclei from a mixture of ¹⁴C-thymidine long- and ³H-thymidine pulse-labeled embryos. Bulk monomer and dimer DNA fragments contained about 170 and 410 base pairs (bp), respectively, when 18% of the bulk DNA had been rendered acid-soluble. At this level of digestion, “nascent monomer DNA” fragments of about 150 bp as well as 305 bp “large nascent DNA fragments” were observed. Increasing levels of digestion indicated that the large nascent DNA fragment was derived from a chromatin structure which was more resistant to micrococcal nuclease cleavage than bulk dimer chromatin subunits. Peaks of ³H-thymidine-labeled DNA fragments from embryos which had been pulse-labeled and then chased or labeled for several minutes overlapped those of ¹⁴C-thymidine long-labeled monomer, dimer and trimer fragments. This indicated that the chromatin organization at or near the replication fork which had temporarily changed during replication had returned to the organization of its nonreplicating state.     

Ethidium binding sites on plasmid DNA determined by photoaffinity labeling

Photoaffinity labeling of pBR322 with ethidium monoazide (8-azido-3-amino-5-ethyl-6-phenylphenanthridinium chloride) was used to provide evidence for the sequence specificity of ethidium binding to native DNA. DNA-drug interactions were examined at concentrations of eight covalently bound ethidium drugs per molecule of pBR322 (4363 base pairs). Restriction enzyme cutting was blocked by the covalent binding of a drug molecule at (or near) the enzyme recognition sequence. This phenomenon was observed with all restriction enzymes tested and was not limited to specific regions of the pBR322 molecule. Double-digestion experiments indicated that a drug molecule may bind 2 to 3 base pairs outside the recognition sequence and still block restriction enzyme digestion. Intact plasmid was treated with [3H]ethidium monoazide and digested with restriction enzymes. The amount of covalently-linked ethidium analog was quantitated for different restriction fragments and the G-C content of each fragment was determined from the DNA sequence. In approximately half of the fragments the drug appeared to preferentially bind at a G-C base pair. However, no preference for specific sequences such as 5'-C-G-3' was detected, as had been suggested by previous modeling studies with ethidium bromide. The other fragments were located in specific map regions of the plasmid and did not bind drug with a strict dependence on GC content suggesting that binding specificity may depend on more than one structural feature of the DNA.     

Reduced repeat length of nascent nucleosomal DNA is generated by replicating chromatin in vivo

Micrococcal nuclease digestion of nuclei from sea urchin embryos revealed transient changes in chromatin structure which resulted in a reduction in the repeat length of nascent chromatin DNA as compared with bulk DNA. This was considered to be entirely the consequence of in vivo events at the replication fork (Cell 14, 259, 1978). However, a micrococcal nuclease-generated sliding of nucleosome cores relative to nascent DNA, which might account for the smaller DNA fragments, was not excluded. In vivo [3H]thymidine pulse-labeled nuclei were fixed with a formaldehyde prior to micrococcal nuclease digestion. This linked chromatin proteins to DNA and thus prevented any in vitro sliding of histone cores. All the nascent DNAs exhibiting shorter repeat lengths after micrococcal nuclease digestion, were resolved at identical mobilities in polyacrylamide gels of DNA from fixed and unfixed nuclei. We conclude that these differences in repeat lengths between nascent and bulk DNA was generated in vivo by changes in chromatin structure during replication, rather than by micrococcal nuclease-induced sliding of histone cores in vitro.     

Organization of 5S genes in chromatin of Xenopus laevis.

The chromatin organization of the genes coding for 5S RNA in Xenopus laevis has been investigated with restriction endonucleases and micrococcal nuclease. Digestion of nuclei from liver, kidney, blood and kidney cells maintained in culture with micrococcal nuclease reveals that these Xenopus cells and tissues have shorter nucleosome repeat lengths than the corresponding cells and tissues from other higher organisms. 5S genes are organized in nucleosomes with repeat lengths similar to those of the bulk chromatin in liver (178 bp) and cultured cells (165 bp); however, 5S gene chromatin in blood cells has a shorter nucleosome repeat (176 bp) than the bulk of the genome in these cells (184 bp). From an analysis of the 5S DNA fragments produced by extensive restriction endonuclease cleavage of chromatin in situ, no special arrangement of the nucleosomes with respect to the sequence of 5S DNA can be detected. The relative abundance of 5S gene multimers follows a Kuhn distribution, with about 57% of all HindIII sites cleaved. This suggests that HindIII sites can be cleaved both in the nucleosome core and linker regions.