Single-base resolution nucleosome mapping on DNA sequences

Predicting the configuration and energy of DNA in a nucleosome by coarse-grain modelling

We present a novel algorithm which uses a coarse-grained model and an energy minimisation procedure to predict the sequence-dependent DNA configuration in a nucleosome together with its energetic cost.

A deformation energy model reveals sequence-dependent property of nucleosome positioning

We present a deformation energy model for predicting nucleosome positioning, in which a position-dependent structural parameter set derived from crystal structures of nucleosomes was used to calculate the DNA deformation energy. The model is successful in predicting nucleosome occupancy genome-wide in budding yeast, nucleosome free energy, and rotational positioning of nucleosomes. Our model also indicates that the genomic regions underlying the MNase-sensitive nucleosomes in budding yeast have high deformation energy and, consequently, low nucleosome-forming ability, while the MNase-sensitive non-histone particles are characterized by much lower DNA deformation energy and high nucleosome preference. In addition, we also revealed that remodelers, SNF2 and RSC8, are likely to act in chromatin remodeling by binding to broad nucleosome-depleted regions that are intrinsically favorable for nucleosome positioning. Our data support the important role of position-dependent physical properties of DNA in nucleosome positioning.

iNuc-ext-PseTNC: an efficient ensemble model for identification of nucleosome positioning by extending the concept of Chou's PseAAC to pseudo-tri-nucleotide composition

Nucleosome is a central element of eukaryotic chromatin, which composes of histone proteins and DNA molecules. It performs vital roles in many eukaryotic intra-nuclear processes, for instance, chromatin structure and transcriptional regulation formation. Identification of nucleosome positioning via wet lab is difficult; so, the attention is diverted towards the accurate intelligent automated prediction. In this regard, a novel intelligent automated model "iNuc-ext-PseTNC" is developed to identify the nucleosome positioning in genomes accurately. In this predictor, the sequences of DNA are mathematically represented by two different discrete feature extraction techniques, namely pseudo-tri-nucleotide composition (PseTNC) and pseudo-di-nucleotide composition. Several contemporary machine learning algorithms were examined. Further, the predictions of individual classifiers were integrated through an evolutionary genetic algorithm. The success rates of the ensemble model are higher than individual classifiers. After analyzing the prediction results, it is noticed that iNuc-ext-PseTNC model has achieved better performance in combination with PseTNC feature space, which are 94.3%, 93.14%, and 88.60% of accuracies using six-fold cross-validation test for the three benchmark datasets S₁, S₂, and S₃, respectively. The achieved outcomes exposed that the results of iNuc-ext-PseTNC model are prominent compared to the existing methods so far notifiable in the literature. It is ascertained that the proposed model might be more fruitful and a practical tool for rudimentary academia and research.

DNA physical properties outperform sequence compositional information in classifying nucleosome-enriched and -depleted regions

The nucleosome is the fundamental structural unit of eukaryotic chromatin and plays an essential role in the epigenetic regulation of cellular processes, such as DNA replication, recombination, and transcription. Hence, it is important to identify nucleosome positions in the genome. Our previous model based on DNA deformation energy, in which a set of DNA physical descriptors was used, performed well in predicting nucleosome dyad positions and occupancy. In this study, we established a machine-learning model for predicting nucleosome occupancy in order to further verify the physical descriptors. Results showed that (1) our model outperformed several other sequence compositional information-based models, indicating a stronger dependence of nucleosome positioning on DNA physical properties; (2) nucleosome-enriched and -depleted regions have distinct features in terms of DNA physical descriptors like sequence-dependent flexibility and equilibrium structure parameters; (3) gene transcription start sites and termination sites can be well characterized with the distribution patterns of the physical descriptors, indicating the regulatory role of DNA physical properties in gene transcription. In addition, we developed a web server for the model, which is freely accessible at http://lin-group.cn/server/iNuc-force/.

The implication of DNA bending energy for nucleosome positioning and sliding

Nucleosome not only directly affects cellular processes, such as DNA replication, recombination, and transcription, but also severs as a fundamentally important target of epigenetic modifications. Our previous study indicated that the bending property of DNA is important in nucleosome formation, particularly in predicting the dyad positions of nucleosomes on a DNA segment. Here, we investigated the role of bending energy in nucleosome positioning and sliding in depth to decipher sequence-directed mechanism. The results show that bending energy is a good physical index to predict the free energy in the process of nucleosome reconstitution in vitro. Our data also imply that there are at least 20% of the nucleosomes in budding yeast do not adopt canonical positioning, in which underlying sequences wrapped around histones are structurally symmetric. We also revealed distinct patterns of bending energy profile for distinctly organized chromatin structures, such as well-positioned nucleosomes, fuzzy nucleosomes, and linker regions and discussed nucleosome sliding in terms of bending energy. We proposed that the stability of a nucleosome is positively correlated with the strength of the bending anisotropy of DNA segment, and both accessibility and directionality of nucleosome sliding is likely to be modulated by diverse patterns of DNA bending energy profile.

Review fifteen years of search for strong nucleosomes

Don Crothers, Mikael Kubista, Jon Widom, and their teams have been first to look for strong nucleosomes, in a bid to reveal the nucleosome positioning pattern(s) carried by the nucleosome DNA sequences. They were first to demonstrate that the nucleosome stability correlates with 10-11 base sequence periodicity, and that the strong nucleosomes localize preferentially in centromeres. This review describes these findings and their connection to recent discovery of the strong nucleosomes (SNs) with visibly periodic nucleosome DNA sequences.

A deformation energy-based model for predicting nucleosome dyads and occupancy

Nucleosome plays an essential role in various cellular processes, such as DNA replication, recombination, and transcription. Hence, it is important to decode the mechanism of nucleosome positioning and identify nucleosome positions in the genome. In this paper, we present a model for predicting nucleosome positioning based on DNA deformation, in which both bending and shearing of the nucleosomal DNA are considered. The model successfully predicted the dyad positions of nucleosomes assembled in vitro and the in vitro map of nucleosomes in Saccharomyces cerevisiae. Applying the model to Caenorhabditis elegans and Drosophila melanogaster, we achieved satisfactory results. Our data also show that shearing energy of nucleosomal DNA outperforms bending energy in nucleosome occupancy prediction and the ability to predict nucleosome dyad positions is attributed to bending energy that is associated with rotational positioning of nucleosomes.

Reading sequence-directed computational nucleosome maps

Recently developed latest version of the sequence-directed single-base resolution nucleosome mapping reveals existence of strong nucleosomes and chromatin columnar structures (columns). Broad application of this simple technique for further studies of chromatin and chromosome structure requires some basic understanding as to how it works and what information it affords. The paper provides such an introduction to the method. The oscillating maps of singular nucleosomes, of short and long oligonucleosome columns, are explained, as well as maps of chromatin on satellite DNA and occurrences of counter-phase (antiparallel) nucleosome neighbors.

Apoptotic lymphocytes of H. sapiens lose nucleosomes in GC-rich promoters

We analyzed two sets of human CD4⁺ nucleosomal DNA directly sequenced by Illumina (Solexa) high throughput sequencing method. The first set has ∼40 M sequences and was produced from the normal CD4+ T lymphocytes by micrococcal nuclease. The second set has ∼44 M sequences and was obtained from peripheral blood lymphocytes by apoptotic nucleases. The different nucleosome sets showed similar dinucleotide positioning AA/TT, GG/CC, and RR/YY (R is purine, Y - pyrimidine) patterns with periods of 10–10.4 bp. Peaks of GG/CC and AA/TT patterns were shifted by 5 bp from each other. Two types of promoters in H. sapiens: AT and GC-rich were identified. AT-rich promoters in apoptotic cell had +1 nucleosome shifts 50–60 bp downstream from those in normal lymphocytes. GC-rich promoters in apoptotic cells lost 80% of nucleosomes around transcription start sites as well as in total DNA. Nucleosome positioning was predicted by combination of {AA, TT}, {GG, CC}, {WW, SS} and {RR, YY} patterns. In our study we found that the combinations of {AA, TT} and {GG, CC} provide the best results and successfully mapped 33% of nucleosomes 147 bp long with precision ±15 bp (only 31/147 or 21% is expected).

We analyzed nucleosomal DNA of human CD4+ T normal and apoptotic lymphocytes. Dinucleotide positions (pattern) of AA/TT, GG/CC, WW/SS (W is adenine or thymine, S is guanine or cytosine) and RR/YY (R is purine, Y - pyrimidine) of nucleosome sequences in both cell conditions are similar and have period 10–10.4 bp. We successfully mapped 33% of nucleosomes with precision ±15 bp by combination of {AA, TT}, {GG, CC}, {WW, SS} and {RR, YY} patterns. We identified two types of promoters in H. sapience: AT and GC-rich. AT-rich promoters keep nucleosomes around transcription start site when GC-rich promoters lost 80% of nucleosomes during apoptosis at the same region.

Universal full-length nucleosome mapping sequence probe

For the computational sequence-directed mapping of the nucleosomes, the knowledge of the nucleosome positioning motifs - 10-11 base long sequences - and respective matrices of bendability, is not sufficient, since there is no justified way to fuse these motifs in one continuous nucleosome DNA sequence. Discovery of the strong nucleosome (SN) DNA sequences, with visible sequence periodicity allows derivation of the full-length nucleosome DNA bendability pattern as matrix or consensus sequence. The SN sequences of three species (A. thaliana, C. elegans, and H. sapiens) are aligned (512 sequences for each species), and long (115 dinucleotides) matrices of bendability derived for the species. The matrices have strong common property - alternation of runs of purine-purine (RR) and pyrimidine-pyrimidine (YY) dinucleotides, with average period 10.4 bases. On this basis the universal [R,Y] consensus of the nucleosome DNA sequence is derived, with exactly defined positions of respective penta- and hexamers RRRRR, RRRRRR, YYYYY, and YYYYYY.

Strong nucleosomes reside in meiotic centromeres of C. elegans

Recently discovered strong nucleosomes (SNs) are characterized by strongly periodical DNA sequence, with visible rather than hidden sequence periodicity. In a quest for possible functions of the SNs, it has been found that the SNs concentrate within centromere regions of A. thaliana chromosomes . They, however, have been detected in Caenorhabditis elegans as well, although the holocentric chromosomes of this species do not have centromeres. Scrutinizing the SNs of C. elegans and their distributions along the DNA sequences of the chromosomes, we have discovered that the SNs are located mainly at the ends of the chromosomes of C. elegans. This suggests that, perhaps, the ends of the chromosomes fulfill some function(s) of centromeres in this species, as also indicated by the cytogenetic studies on meiotic chromosomes in spermatocytes of C. elegans, where the end-to-end association is observed. The centromeric involvement of the SNs, also found in A. thaliana, opens new horizons for the chromosome and centromere structure studies.

Visible periodicity of strong nucleosome DNA sequences

Fifteen years ago, Lowary and Widom assembled nucleosomes on synthetic random sequence DNA molecules, selected the strongest nucleosomes and discovered that the TA dinucleotides in these strong nucleosome sequences often appear at 10-11 bases from one another or at distances which are multiples of this period. We repeated this experiment computationally, on large ensembles of natural genomic sequences, by selecting the strongest nucleosomes--i.e. those with such distances between like-named dinucleotides, multiples of 10.4 bases, the structural and sequence period of nucleosome DNA. The analysis confirmed the periodicity of TA dinucleotides in the strong nucleosomes, and revealed as well other periodic sequence elements, notably classical AA and TT dinucleotides. The matrices of DNA bendability and their simple linear forms--nucleosome positioning motifs--are calculated from the strong nucleosome DNA sequences. The motifs are in full accord with nucleosome positioning sequences derived earlier, thus confirming that the new technique, indeed, detects strong nucleosomes. Species- and isochore-specific variations of the matrices and of the positioning motifs are demonstrated. The strong nucleosome DNA sequences manifest the highest hitherto nucleosome positioning sequence signals, showing the dinucleotide periodicities in directly observable rather than in hidden form.

DNA conformation and energy in nucleosome core: a theoretical approach

DNA conformation in complex with proteins is far from its canonical B-form. The affinity of complex formation and structure of DNA depend on its attachment configuration and sequence. In this article, we develop a mechanical model to address the problem of DNA structure and energy under deformation. DNA in nucleosome core particle is described as an example. The structure and energy of nucleosomal DNA is calculated based on its sequence and positioning state. The inferred structure has remarkable similarity with X-ray data. Although there is no sequence-specific interaction of bases and the histone core, we found considerable sequence dependency for the nucleosomal DNA positioning. The affinity of nucleosome formation for several sequences is examined and the differences are compatible with observations. We argue that structural energy determines the natural state of nucleosomal DNA and is the main reason for affinity differences in vitro. This theory can be utilized for the DNA structure and energy determination in protein-DNA complexes in general. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:JBSD:17.

Multiple levels of meaning in DNA sequences, and one more

If we define a genetic code as a widespread DNA sequence pattern that carries a message with an impact on biology, then there are multiple genetic codes. Sequences involved in these codes overlap and, thus, both interact with and constrain each other, such as for the triplet code, the intron-splicing code, the code for amphipathic alpha helices, and the chromatin code. Nucleosomes preferentially are located at the ends of exons, thus protecting splice junctions, with the N9 positions of guanines of the GT and AG junctions oriented toward the histones. Analysis of protein-coding sequences reveals numerous traces of tandem repeats, apparently formed by triplet expansion, which in effect is a genome inflation ``code''. Our data are consistent with the hypothesis that expansion of simple tandem repetition of certain aggressive triplets has been a characteristic of life from its emergence. Such expanding triplets appear to be the major factor underlying observed codon usage biases.

Predicting nucleosome positions in yeast: using the absolute frequency

Nucleosome is the basic structure of chromatin in eukaryotic cells, and they form the chromatin fiber interconnected by sections of linker DNA. Nucleosome positioning is of great significance for gene transcription regulation. In this paper, we consider the difference of absolute frequency of nucleotides between the nucleosome forming and nucleosome inhibiting sequences. Based on the 2-mer absolute frequency of nucleotides in genome, a new model is constructed to distinguish nucleosome DNA and linker DNA. When used to predict DNA potential for forming nucleosomes in S. cerevisiae, the model achieved a high accuracy of 96.05%. Thus, the model is very useful for predicting nucleosome positioning.

Structural features based genome-wide characterization and prediction of nucleosome organization

Background

Nucleosome distribution along chromatin dictates genomic DNA accessibility and thus profoundly influences gene expression. However, the underlying mechanism of nucleosome formation remains elusive. Here, taking a structural perspective, we systematically explored nucleosome formation potential of genomic sequences and the effect on chromatin organization and gene expression in S. cerevisiae.

Results

We analyzed twelve structural features related to flexibility, curvature and energy of DNA sequences. The results showed that some structural features such as DNA denaturation, DNA-bending stiffness, Stacking energy, Z-DNA, Propeller twist and free energy, were highly correlated with in vitro and in vivo nucleosome occupancy. Specifically, they can be classified into two classes, one positively and the other negatively correlated with nucleosome occupancy. These two kinds of structural features facilitated nucleosome binding in centromere regions and repressed nucleosome formation in the promoter regions of protein-coding genes to mediate transcriptional regulation. Based on these analyses, we integrated all twelve structural features in a model to predict more accurately nucleosome occupancy in vivo than the existing methods that mainly depend on sequence compositional features. Furthermore, we developed a novel approach, named DLaNe, that located nucleosomes by detecting peaks of structural profiles, and built a meta predictor to integrate information from different structural features. As a comparison, we also constructed a hidden Markov model (HMM) to locate nucleosomes based on the profiles of these structural features. The result showed that the meta DLaNe and HMM-based method performed better than the existing methods, demonstrating the power of these structural features in predicting nucleosome positions.

Conclusions

Our analysis revealed that DNA structures significantly contribute to nucleosome organization and influence chromatin structure and gene expression regulation. The results indicated that our proposed methods are effective in predicting nucleosome occupancy and positions and that these structural features are highly predictive of nucleosome organization.

The implementation of our DLaNe method based on structural features is available online.

Thirty years of multiple sequence codes

An overview is presented on the status of studies on multiple codes in genetic sequences. Indirectly, the existence of multiple codes is recognized in the form of several rediscoveries of Second Genetic Code that is different each time. A due credit is given to earlier seminal work related to the codes often neglected in literature. The latest developments in the field of chromatin code are discussed, as well as perspectives of single-base resolution studies of nucleosome positioning, including rotational setting of DNA on the surface of the histone octamers.

"Anticipated" nucleosome positioning pattern in prokaryotes

Linguistic (word count) analysis of prokaryotic genome sequences, by Shannon N-gram extension, reveals that the dominant hidden motifs in A+T rich genomes are T(A)(T)A and G(A)(T)C with uncertain number of repeating A and T. Since prokaryotic sequences are largely protein-coding, the motifs would correspond to amphipathic alpha-helices with alternating lysine and phenylalanine as preferential polar and non-polar residues. The motifs are also known in eukaryotes, as nucleosome positioning patterns. Their existence in prokaryotes as well may serve for binding of histone-like proteins to DNA. In this case the above patterns in prokaryotes may be considered as "anticipated" nucleosome positioning patterns which, quite likely, existed in prokaryotic genomes before the evolutionary separation between eukaryotes and prokaryotes.

Human nucleosomes: special role of CG dinucleotides and Alu-nucleosomes

Background

The periodical occurrence of dinucleotides with a period of 10.4 bases now is undeniably a hallmark of nucleosome positioning. Whereas many eukaryotic genomes contain visible and even strong signals for periodic distribution of dinucleotides, the human genome is rather featureless in this respect. The exact sequence features in the human genome that govern the nucleosome positioning remain largely unknown.

Results

When analyzing the human genome sequence with the positional autocorrelation method, we found that only the dinucleotide CG shows the 10.4 base periodicity, which is indicative of the presence of nucleosomes. There is a high occurrence of CG dinucleotides that are either 31 (10.4 × 3) or 62 (10.4 × 6) base pairs apart from one another - a sequence bias known to be characteristic of Alu-sequences. In a similar analysis with repetitive sequences removed, peaks of repeating CG motifs can be seen at positions 10, 21 and 31, the nearest integers of multiples of 10.4.

Conclusions

Although the CG dinucleotides are dominant, other elements of the standard nucleosome positioning pattern are present in the human genome as well.

The positional autocorrelation analysis of the human genome demonstrates that the CG dinucleotide is, indeed, one visible element of the human nucleosome positioning pattern, which appears both in Alu sequences and in sequences without repeats. The dominant role that CG dinucleotides play in organizing human chromatin is to indicate the involvement of human nucleosomes in tuning the regulation of gene expression and chromatin structure, which is very likely due to cytosine-methylation/-demethylation in CG dinucleotides contained in the human nucleosomes. This is further confirmed by the positions of CG-periodical nucleosomes on Alu sequences. Alu repeats appear as monomers, dimers and trimers, harboring two to six nucleosomes in a run. Considering the exceptional role CG dinucleotides play in the nucleosome positioning, we hypothesize that Alu-nucleosomes, especially, those that form tightly positioned runs, could serve as "anchors" in organizing the chromatin in human cells.

Nucleosome DNA sequence structure of isochores

Background

Significant differences in G+C content between different isochore types suggest that the nucleosome positioning patterns in DNA of the isochores should be different as well.

Results

Extraction of the patterns from the isochore DNA sequences by Shannon N-gram extension reveals that while the general motif YRRRRRYYYYYR is characteristic for all isochore types, the dominant positioning patterns of the isochores vary between TAAAAATTTTTA and CGGGGGCCCCCG due to the large differences in G+C composition. This is observed in human, mouse and chicken isochores, demonstrating that the variations of the positioning patterns are largely G+C dependent rather than species-specific. The species-specificity of nucleosome positioning patterns is revealed by dinucleotide periodicity analyses in isochore sequences. While human sequences are showing CG periodicity, chicken isochores display AG (CT) periodicity. Mouse isochores show very weak CG periodicity only.

Conclusions

Nucleosome positioning pattern as revealed by Shannon N-gram extension is strongly dependent on G+C content and different in different isochores. Species-specificity of the pattern is subtle. It is reflected in the choice of preferentially periodical dinucleotides.

Nucleosome positioning pattern derived from oligonucleotide compositions of genomic sequences

Availability of nucleosome positioning pattern(s) is crucial for chromatin studies. The matrix form of the pattern has been recently derived (I. Gabdank, D. Barash, E. N. Trifonov. J Biomol Struct Dyn 26, 403-412 (2009), and E. N. Trifonov. J Biomol Struct Dyn 27, 741-746 (2010)). In its simplified linear form it is described by the motif CGRAAATTTYCG. Oligonucleotide components of the motif (say, triplets GRA, RAA, AAA, etc.) would be expected to appear in eukaryotic sequences more frequently. In this work we attempted the reconstruction of the bendability patterns for 13 genomes by a novel approach-extension of highest frequency trinucleotides. The consensus of the patterns reconstructed on the basis of trinucleotide frequencies in 13 eukaryotic genomes is derived: CRAAAATTTTYG. It conforms to the earlier established sequence motif. The reconstruction, thus, attests to the universality of the nucleosome DNA bendability pattern.