Uncurved-DNA signals are important for translational positioning of nucleosomes

Structural constraints revealed in consistent nucleosome positions in the genome of S. cerevisiae

Background

Recent advances in the field of high-throughput genomics have rendered possible the performance of genome-scale studies to define the nucleosomal landscapes of eukaryote genomes. Such analyses are aimed towards providing a better understanding of the process of nucleosome positioning, for which several models have been suggested. Nevertheless, questions regarding the sequence constraints of nucleosomal DNA and how they may have been shaped through evolution remain open. In this paper, we analyze in detail different experimental nucleosome datasets with the aim of providing a hypothesis for the emergence of nucleosome-forming sequences.

Results

We compared the complete sets of nucleosome positions for the budding yeast (Saccharomyces cerevisiae) as defined in the output of two independent experiments with the use of two different experimental techniques. We found that < 10% of the experimentally defined nucleosome positions were consistently positioned in both datasets. This subset of well-positioned nucleosomes, when compared with the bulk, was shown to have particular properties at both sequence and structural levels. Consistently positioned nucleosomes were also shown to occur preferentially in pairs of dinucleosomes, and to be surprisingly less conserved compared with their adjacent nucleosome-free linkers.

Conclusion

Our findings may be combined into a hypothesis for the emergence of a weak nucleosome-positioning code. According to this hypothesis, consistent nucleosomes may be partly guided by nearby nucleosome-free regions through statistical positioning. Once established, a set of well-positioned consistent nucleosomes may impose secondary constraints that further shape the structure of the underlying DNA. We were able to capture these constraints through the application of a recently introduced structural property that is related to the symmetry of DNA curvature. Furthermore, we found that both consistently positioned nucleosomes and their adjacent nucleosome-free regions show an increased tendency towards conservation of this structural feature.

Localization of histone H1 binding sites within the nucleosome by UV-induced H1-DNA crosslinking in vivo

In our previous paper (Belikov et al., (1993), Nucl. Acids Res., 21, 4796-4802) we had studied DNA-protein architecture of so-called Alu-repeats in D. melanogaster ribosomal nontranscribed spacer using DNase I genomic footprinting and UV-induced DNA-protein crosslinking. Our data indicated precise positioning of two non-histone proteins (rABP50 and rABP 70), histone octamer, and histone H1 within the sequences of Alu-repeats. The data on the histone H1 binding sites within Alu-repeat region was presented, but not discussed as the authors could not reach a consensus in its evaluation. Here, the authors use these data to present a model of placement of the linker histone(s) within nucleosome. Our model places one contact of the globular domain of linker histone in the major groove on the inside of DNA superhelix just within the end of the core particle (site +6.5) and the second, close to the dyad (site -1). C-terminal tail binds to the internucleosomal spacer and N-terminal tail interacts simultaneously with the adjacent gyres thus stabilizing DNA superhelix.