Abstract
An estimated 80% of genomic DNA in eukaryotes is packaged as nucleosomes, which, together with the remaining interstitial linker regions, generate higher order chromatin structures [1]. Nucleosome sequences isolated from diverse organisms exhibit ∼10 bp periodic variations in AA, TT and GC dinucleotide frequencies. These sequence elements generate intrinsically curved DNA and help establish the histone-DNA interface. We investigated an important unanswered question concerning the interplay between chromatin organization and genome evolution: do the DNA sequence preferences inherent to the highly conserved histone core exert detectable natural selection on genomic divergence and polymorphism? To address this hypothesis, we isolated nucleosomal DNA sequences from Drosophila melanogaster embryos and examined the underlying genomic variation within and between species. We found that divergence along the D. melanogaster lineage is periodic across nucleosome regions with base changes following preferred nucleotides, providing new evidence for systematic evolutionary forces in the generation and maintenance of nucleosome-associated dinucleotide periodicities. Further, Single Nucleotide Polymorphism (SNP) frequency spectra show striking periodicities across nucleosomal regions, paralleling divergence patterns. Preferred alleles occur at higher frequencies in natural populations, consistent with a central role for natural selection. These patterns are stronger for nucleosomes in introns than in intergenic regions, suggesting selection is stronger in transcribed regions where nucleosomes undergo more displacement, remodeling and functional modification. In addition, we observe a large-scale (∼180 bp) periodic enrichment of AA/TT dinucleotides associated with nucleosome occupancy, while GC dinucleotide frequency peaks in linker regions. Divergence and polymorphism data also support a role for natural selection in the generation and maintenance of these super-nucleosomal patterns. Our results demonstrate that nucleosome-associated sequence periodicities are under selective pressure, implying that structural interactions between nucleosomes and DNA sequence shape sequence evolution, particularly in introns.
Highlights
Sequence-dependent differences in the physical properties of DNA influence its associations with the histone core, as well as the kinetics of nucleosome assembly and stability [2,3,4,5,6,7,8,9]
We asked whether natural selection mediated through the periodic sequence preferences of nucleosomes shapes the evolution of non-protein-coding regions of D. melanogaster by examining the inter- and intra-species genomic variation relative to these fundamental chromatin building blocks
The sequence changes across nucleosome-bound regions on the melanogaster lineage mirror the observed nucleosome dinucleotide periodicities
Summary
Sequence-dependent differences in the physical properties of DNA influence its associations with the histone core, as well as the kinetics of nucleosome assembly and stability [2,3,4,5,6,7,8,9]. One of the most generalizable sequence affinities of the histone octamer is the periodic variation of dinucleotide frequencies across nucleosomal DNA. Peaks of AA/ TT frequency are found over positions where the minor groove bends interiorly, whereas GC dinucleotides peak where the major groove is facing the histone core. Structural data suggest that DNA shape, in particular the narrowing of the minor groove and the associated lowering of its electrostatic potential at AT-rich sequences facilitate contacts with key histone arginines [9,18,19]. GC dinucleotides contract the major groove, which facilitates the tight winding of DNA around the core [9,20]
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