Nucleosomal TATA-switch: Competing orientations of TATA on the nucleosome

RBPJ binds to consensus and methylated cis elements within phased nucleosomes and controls gene expression in human aortic smooth muscle cells in cooperation with SRF

Given our previous demonstration that RBPJ binds a methylated repressor element and regulates smooth muscle cell (SMC)-specific gene expression, we used genome-wide approaches to identify RBPJ binding regions in human aortic SMC and to assess RBPJ's effects on chromatin structure and gene expression. RBPJ bound to consensus cis elements, but also to TCmGGGA sequences within Alu repeats that were less transcriptionally active as assessed by DNAse hypersensitivity, H3K9 acetylation, and Notch3 and RNA Pol II binding. Interestingly, RBPJ binding was frequently detected at the borders of open chromatin, and a large fraction of genes induced or repressed by RBPJ depletion were associated with this cluster of RBPJ binding sites. RBPJ binding dramatically co-localized with serum response factor (SRF) and RNA seq experiments in RBPJ- and SRF-depleted SMC demonstrated that these factors interact functionally to regulate the contraction and inflammatory gene programs that help define SMC phenotype. Finally, we showed that RBPJ bound preferentially to phased nucleosomes independent of active chromatin marks and to cis elements positioned at the beginning and middle of the nucleosome dyad. These novel findings add important insight into RBPJ's role in chromatin structure and gene expression in SMC.

Transcription factors operate TATA switches via rotational remodeling of local columnar chromatin structure

Our earlier study on the nucleosomes containing TBP binding sites (TATA boxes) indicated that generally the same sequence, which harbors the TATA box, encodes simultaneously an alternative rotational setting of the box, so that the TATA element is either exposed (position "minor groove out") or hidden in position "minor groove in". The sequence elements (dinucleotides) residing on the inner surface of DNA in contact with histone octamers are identified by calculating YR tracks in the promoter regions of the genes - periodically reappearing YR elements, at distances of 10-11 bases from one another. Non-YR elements of the YR tracks are also verified by nucleosome mapping procedure based on alternation of runs of purines with runs of pyrimidines. The tracks observed in yeast promoter regions are found to split into two, passing further downstream either through the TATA element or 4-6 bases toward the tail of the box. The points of splitting which play the role of TATA switches are located in close vicinity or within the transcription factor binding sites. This suggests the regulatory function of transcription factors changing the YR tracks by over- or under-twisting DNA induced by their binding.

Structure of silent transcription intervals and noise characteristics of mammalian genes

Mammalian transcription occurs stochastically in short bursts interspersed by silent intervals showing a refractory period. However, the underlying processes and consequences on fluctuations in gene products are poorly understood. Here, we use single allele time-lapse recordings in mouse cells to identify minimal models of promoter cycles, which inform on the number and durations of rate-limiting steps responsible for refractory periods. The structure of promoter cycles is gene specific and independent of genomic location. Typically, five rate-limiting steps underlie the silent periods of endogenous promoters, while minimal synthetic promoters exhibit only one. Strikingly, endogenous or synthetic promoters with TATA boxes show simplified two-state promoter cycles. Since transcriptional bursting constrains intrinsic noise depending on the number of promoter steps, this explains why TATA box genes display increased intrinsic noise genome-wide in mammals, as revealed by single-cell RNA-seq. These findings have implications for basic transcription biology and shed light on interpreting single-cell RNA-counting experiments.

Sequence structure of Lowary/Widom clones forming strong nucleosomes

Lowary and Widom selected from random sequences those which form exceptionally stable nucleosomes, including clone 601, the current champion of strong nucleosome (SN) sequences. This unique sequence database (LW sequences) carries sequence elements which confer stability on the nucleosomes formed on the sequences, and, thus, may serve as source of information on the structure of "ideal" or close to ideal nucleosome DNA sequence. An important clue is also provided by crystallographic study of Vasudevan and coauthors on clone 601 nucleosomes. It demonstrated that YR·YR dinucleotide stacks (primarily TA·TA) follow one another at distances 10 or 11 bases or multiples thereof, such that they all are located on the interface between DNA and histone octamer. Combining this important information with alignment of the YR-containing 10-mers and 11-mers from LW sequences, the bendability matrices of the stable nucleosome DNA are derived. The matrices suggest that the periodically repeated TA (YR), RR, and YY dinucleotides are the main sequence features of the SNs. This consensus coincides with the one for recently discovered SNs with visibly periodic DNA sequences. Thus, the experimentally observed stable LW nucleosomes and SNs derived computationally appear to represent the same entity - exceptionally stable SNs.

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.

The conformational state of the nucleosome entry-exit site modulates TATA box-specific TBP binding

The TATA binding protein (TBP) is a critical transcription factor used for nucleating assembly of the RNA polymerase II machinery. TBP binds TATA box elements with high affinity and kinetic stability and in vivo is correlated with high levels of transcription activation. However, since most promoters use less stable TATA-less or TATA-like elements, while also competing with nucleosome occupancy, further mechanistic insight into TBP's DNA binding properties and ability to access chromatin is needed. Using bulk and single-molecule FRET, we find that TBP binds a minimal consensus TATA box as a two-state equilibrium process, showing no evidence for intermediate states. However, upon addition of flanking DNA sequence, we observe non-specific cooperative binding to multiple DNA sites that compete for TATA-box specificity. Thus, we conclude that TBP binding is defined by a branched pathway, wherein TBP initially binds with little sequence specificity and is thermodynamically positioned by its kinetic stability to the TATA box. Furthermore, we observed the real-time access of TBP binding to TATA box DNA located within the DNA entry–exit site of the nucleosome. From these data, we determined salt-dependent changes in the nucleosome conformation regulate TBP's access to the TATA box, where access is highly constrained under physiological conditions, but is alleviated by histone acetylation and TFIIA.

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.