Cloning of chicken embryo tRNA genes using single stranded nucleosomal DNA highly enriched for tRNA complementary sequences

DNA from chicken embryo nucleosome tetramers (about 760 base pairs in size) was enriched for tRNA genes by RPC-5 chromatography. The enriched DNA was hybridized with chicken embryo total tRNA and the hybridized DNA isolated utilizing a) avidinbiotin interaction, b) diazobenzyloxymethyl paper, and c) high temperature RPC-5 chromatography. The obtained single stranded DNA highly enriched for tRNA complementary sequences was hybridized with total DNA from nucleosome monomers (140--190 base pairs in size) and the excess of non hybridized monomer nucleosome DNA removed by Sepharose 4B chromatography. The hybrid molecules obtained were made fully double stranded by incubation with E. coli DNA polymerase I, DNA ligase, and exonuclease III. DNA was inserted into plasmid pBR322 by G-C joining procedure and the recombinant DNA used to transform the E. coli strain chi 1776. More than 70% of the transformants obtained hybridize to chicken embryo total tRNA.     

Structure-based Analysis of DNA Sequence Patterns Guiding Nucleosome Positioning in vitro

Abstract

Recent studies of genome-wide nucleosomal organization suggest that the DNA sequence is one of the major determinants of nucleosome positioning. Although the search for underlying patterns encoded in nucleosomal DNA has been going on for about 30 years, our knowledge of these patterns still remains limited. Based on our evaluations of DNA deformation energy, we developed new scoring functions to predict nucleosome positioning. There are three principal differences between our approach and earlier studies: (i) we assume that the length of nucleosomal DNA varies from 146 to 147 bp; (ii) we consider the anisotropic flexibility of pyrimidine-purine (YR) dimeric steps in the context of their neighbors (e.g., YYRR versus RYRY); (iii) we postulate that alternating AT-rich and GC-rich motifs reflect sequence-dependent interactions between histone arginines and DNA in the minor groove. Using these functions, we analyzed 20 nucleosome positions mapped in vitro at single nucleotide resolution (including clones 601, 603, 605, the pGUB plasmid, chicken β-globin and three 5S rDNA genes). We predicted 15 of the 20 positions with 1-bp precision, and two positions with 2-bp precision. The predicted position of the ‘601’ nucleosome (i.e., the optimum of the computed score) deviates from the experimentally determined unique position by no more than 1 bp—an accuracy exceeding that of earlier predictions.

Our analysis reveals a clear heterogeneity of the nucleosomal sequences which can be divided into two groups based on the positioning ‘rules’ they follow. The sequences of one group are enriched by highly deformable YR/YYRR motifs at the minor-groove bending sites SHL ±3.5 and ±5.5, which is similar to the α-satellite sequence used in most crystallized nucleosomes. Apparently, the positioning of these nucleosomes is determined by the interactions between histones H2A/H2B and the terminal parts of nucleosomal DNA. In the other group (that includes the ‘601’ clone) the same YR/YYRR motifs occur predominantly at the sites SHL ±1.5. The interaction between the H3/H4 tetramer and the central part of the nucleosomal DNA is likely to be responsible for the positioning of nucleosomes of this group, and the DNA trajectory in these nucleosomes may differ in detail from the published structures.

Thus, from the stereochemical perspective, the in vitro nucleosomes studied here follow either an X-ray-like pattern (with strong deformations in the terminal parts of nucleosomal DNA), or an alternative pattern (with the deformations occurring predominantly in the central part of the nucleosomal DNA). The results presented here may be useful for genome-wide classification of nucleosomes, linking together structural and thermodynamic characteristics of nucleosomes with the underlying DNA sequence patterns guiding their positions.     

Signals in chicken beta-globin DNA influence chromatin assembly in vitro.

We have confirmed the result that chicken beta-globin gene chromatin, which possesses the characteristics of active chromatin in erythroid cells, has shortened internucleosome spacings compared with bulk chromatin or that of the ovalbumin gene, which is inactive. To understand how the short (approximately 180-bp) nucleosome repeat arises specifically on beta-globin DNA, we have studied chromatin assembly of cloned chicken beta-globin DNA in a defined in vitro system. With chicken erythrocyte core histones and linker histone H5 as the only cellular components, a cloned 6.2-kb chicken beta-globin DNA fragment assembled into chromatin possessing a regular 180 +/- 5-bp repeat, very similar to what is observed in erythroid cells. A 2-kb DNA subfragment containing the beta A gene and promoter region, but lacking the downstream intergenic region between the beta A and epsilon genes, failed to generate a regular nucleosome array in vitro, suggesting that the intergenic region facilitates linker histone-induced nucleosome alignment. When the beta A gene was placed on a plasmid that contained a known chromatin-organizing signal, nucleosome alignment with a 180-bp periodicity was restored, whereas nucleosomes on flanking plasmid sequences possessed a 210-bp spacing periodicity. Our results suggest that the shortened 180-bp nucleosome spacing periodicity observed in erythroid cells is encoded in the beta-globin DNA sequence and that nucleosome alignment by linker histones is facilitated by sequences in the beta A-epsilon intergenic region.     

Characterization of Dnmt1 Binding and DNA Methylation on Nucleosomes and Nucleosomal Arrays

The packaging of DNA into nucleosomes and the organisation into higher order structures of chromatin limits the access of sequence specific DNA binding factors to DNA. In cells, DNA methylation is preferentially occuring in the linker region of nucleosomes, suggesting a structural impact of chromatin on DNA methylation. These observations raise the question whether DNA methyltransferases are capable to recognize the nucleosomal substrates and to modify the packaged DNA. Here, we performed a detailed analysis of nucleosome binding and nucleosomal DNA methylation by the maintenance DNA methyltransferase Dnmt1. Our binding studies show that Dnmt1 has a DNA length sensing activity, binding cooperatively to DNA, and requiring a minimal DNA length of 20 bp. Dnmt1 needs linker DNA to bind to nucleosomes and most efficiently recognizes nucleosomes with symmetric DNA linkers. Footprinting experiments reveal that Dnmt1 binds to both DNA linkers exiting the nucleosome core. The binding pattern correlates with the efficient methylation of DNA linkers. However, the enzyme lacks the ability to methylate nucleosomal CpG sites on mononucleosomes and nucleosomal arrays, unless chromatin remodeling enzymes create a dynamic chromatin state. In addition, our results show that Dnmt1 functionally interacts with specific chromatin remodeling enzymes to enable complete methylation of hemi-methylated DNA in chromatin.     

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.     

Purification and nucleosome mapping analysis of native yeast plasmid chromatin

There is much evidence indicating the importance in gene regulation of the positions of nucleosomes with respect to DNA sequence. Low resolution chromatin structures have been described for many genes, but there is a dearth of detailed high resolution chromatin structures. In the cases where they are available, high resolution maps have revealed much more complex chromatin structures, with multiple alternative nucleosome positions. The discovery that ATP-dependent chromatin remodelling machines are recruited to genes, with their ability to mobilise nucleosomes on DNA and to alter nucleosomal conformation, emphasises the necessity for obtaining high resolution nucleosome maps, so that the details of these remodelling reactions can be defined in vivo. Here, we describe protocols for purifying plasmid chromatin from cells of the yeast Saccharomyces cerevisiae and for mapping nucleosome positions on the plasmid using the monomer extension mapping method. This method requires purified chromatin, but is capable of mapping relatively long stretches of chromatin in great detail. Typically, it reveals very complex chromatin structures.     

A phenylalanine tRNA gene from Neurospora crassa: conservation of secondary structure involving an intervening sequence.

We have isolated and sequenced a tRNAPhe gene from Neurospora crassa. Hybridization analyses suggest that trnaPhe is the only tRNA encoded on the cloned 5 kb DNA fragment. The tRNAPhe gene contains an intervening sequence 16 nucleotides in length located one nucleotide 3' to the anticodon position. The tRNAPhe coding region of Neurospora and yeast are 91% conserved, whereas their intervening sequences are only 50% identical. The pattern of sequence conservation is consistent with a proposed secondary structure for the tRNA precursor in which the anticodon is base paired with the middle of the intervening sequence and the splice points are located in adjacent single-stranded loops. The DNA sequence following the tRNAPhe coding region is similar to sequences following other genes transcribed by RNA polymerase III in that it is AT-rich and includes a tract of A residues in the coding strand. In contrast, the sequence preceding the Neurospora tRNAPhe coding region does not resemble sequences preceding other sequenced tRNA genes.     

Novel nucleosomal particles containing core histones and linker DNA but no histone H1

Eukaryotic chromosomal DNA is assembled into regularly spaced nucleosomes, which play a central role in gene regulation by determining accessibility of control regions. The nucleosome contains ∼147 bp of DNA wrapped ∼1.7 times around a central core histone octamer. The linker histone, H1, binds both to the nucleosome, sealing the DNA coils, and to the linker DNA between nucleosomes, directing chromatin folding. Micrococcal nuclease (MNase) digests the linker to yield the chromatosome, containing H1 and ∼160 bp, and then converts it to a core particle, containing ∼147 bp and no H1. Sequencing of nucleosomal DNA obtained after MNase digestion (MNase-seq) generates genome-wide nucleosome maps that are important for understanding gene regulation. We present an improved MNase-seq method involving simultaneous digestion with exonuclease III, which removes linker DNA. Remarkably, we discovered two novel intermediate particles containing 154 or 161 bp, corresponding to 7 bp protruding from one or both sides of the nucleosome core. These particles are detected in yeast lacking H1 and in H1-depleted mouse chromatin. They can be reconstituted in vitro using purified core histones and DNA. We propose that these ‘proto-chromatosomes’ are fundamental chromatin subunits, which include the H1 binding site and influence nucleosome spacing independently of H1.     

Nucleosomal arrays can be salt-reconstituted on a single-copy MMTV promoter DNA template: their properties differ in several ways from those of comparable 5S concatameric arrays

Subsaturated nucleosomal arrays were reconstituted on a single-copy MMTV promoter DNA fragment by salt dialysis procedures and studied by atomic force microscopy. Up to an occupation level of approximately eight nucleosomes on this 1900 bp template, salt reconstitution produces nucleosomal arrays which look very similar to comparably loaded 5S rDNA nucleosomal arrays; i.e., nucleosomes are dispersed on the DNA template. Thus, at these occupation levels, the single-copy MMTV template forms arrays suitable for biophysical analyses. A quantitative comparison of the population features of subsaturated MMTV and 5S arrays detects differences between the two: a requirement for higher histone levels to achieve a given level of nucleosome occupation on MMTV templates, indicating that nucleosome loading is thermodynamically less favorable on this template; a preference for pairwise nucleosome occupation of the MMTV (but not the 5S) template at midrange occupation levels; and an enhanced salt stability for nucleosomes on MMTV versus 5S arrays, particularly in the midrange of array occupation. When average occupation levels exceed approximately eight nucleosomes per template, MMTV arrays show a significant level of mainly intramolecular compaction; 5S arrays do not. Taken together, these results show clearly that the nature of the underlying DNA template can affect the physical properties of nucleosomal arrays. DNA sequence-directed differences in the physical properties of chromatin may have important consequences for functional processes such as gene regulation.     

On the occurrence of nucleosome phasing in chromatin.

We have found that DNAase I digestion of yeast, HeLa and chicken erythrocyte nuclei produces a pattern of DNA fragments spaced 10 bases apart and extending to at least 300 bases. This "extended ladder" of DNA fragments is most clearly seen with yeast, and least clearly with chicken erythrocytes. The appearance of regular and discrete bands at sizes much larger than the repeat size shows that the core particles (140 bp of DNA + H2A, H2B, H3 H4) in at least some fraction of chromatin are spaced in a particular fashion, by discrete lengths of spacer DNA, and not randomly. Based on the abundance of small repeats in yeast and from experiments with nucleosome oligomers, we conclude that the extended ladder and nucleosomal phasing probably arise mainly from regions in the chromatin in which nucleosome cores are closely packed or closely spaced (140-160 bp X n). Contributions from less closely packed but still accurately phased nucleosomes, however, cannot be entirely excluded.     

Dynamic properties of nucleosomes during thermal and ATP-driven mobilization

The fundamental subunit of chromatin, the nucleosome, is not a static entity but can move along DNA via either thermal or enzyme-driven movements. Here we have monitored the movements of nucleosomes following deposition at well-defined locations on mouse mammary tumor virus promoter DNA. We found that the sites to which nucleosomes are deposited during chromatin assembly differ from those favored during thermal equilibration. Taking advantage of this, we were able to track the movement of nucleosomes over 156 bp and found that this proceeds via intermediate positions spaced between 46 and 62 bp. The remodeling enzyme ISWI was found to direct the movement of nucleosomes to sites related to those observed during thermal mobilization. In contrast, nucleosome mobilization driven by the SWI/SNF and RSC complexes were found to drive nucleosomes towards sites up to 51 bp beyond DNA ends, with little respect for the sites favored during thermal repositioning. The dynamic properties of nucleosomes we describe are likely to influence their role in gene regulation.