Nucleosome positioning and sensitivity suggest novel functional organization of the purple sea urchin (Strongylocentrotus purpuratus) genome
The eukaryotic nucleosome is the fundamental subunit of chromatin and plays functional roles in DNA templated processes including replication and transcription. In eukaryotic promoters, nucleosome organization is generally thought to be highly structured, with nucleosomes occupying canonical positions flanking the transcription start site (TSS), thereby regulating access of the transcriptional machinery to the underlying DNA. We sought to determine whether this canonical distribution is present in the purple sea urchin, Strongylocentrotus purpuratus, an important developmental model organism.  We used titrations of micrococcal nuclease to produce high throughput maps of nucleosome distribution, sensitivity to digestion, and subnucleosomal footprints from female urchin gonad tissue. Unlike yeast, flies, zebrafish, maize, or humans, urchins lack a nucleosome depleted region over TSSs. Urchin promoters are dominated by strongly positioned and highly occupied +1 and +2 nucleosomes which are most prominent in highly expressed genes. Additionally, urchin promoters exhibit distinct patterns of sensitivity to nuclease digestion, with heightened sensitivity upstream of the TSS and limited resistance to nuclease digestion. Discretely positioned sensitive nucleosomes were enriched in promoters of highly expressed genes, suggesting a relationship between nucleosome sensitivity and transcriptional regulation. Moreover, urchin promoters were enriched for small fragment-protected footprints associated with known regulatory motifs. Collectively, we present a comprehensive overview of the unique interplay between nucleosome positioning, chromatin sensitivity, cis-regulatory interactions, and gene expression in sea urchins. Our study not only provides a reproducible methodology applicable to other organisms, but also advances our understanding of functional chromatin organization and gene regulation in urchins. Overall design: MNase-seq of wild caught Strongylocentrotus purpuratus female gonad tissue.