Nucleosomes are preferentially positioned at exons in somatic and sperm cells.

Nucleosome positioning is constrained at eukaryotic transcription start sites and implicated in transcriptional regulation. Moreover, recent observations indicate that chromatin structure, transcription and splicing are functionally intertwined, and that modified nucleosomes with trimethylation of lysine 36 in histone subunit 3 (H3K36me3) are enriched at internal exons and the downstream flanking intronic regions of highly expressed genes. However, the position of nucleosomes in the interior of genes has been thought to be largely random. Here we show, by analysis of data sets from human sperm and T cells and medaka (Japanese killifish, Oryzias latipes) blastulae, that internal exons of genes are characterized by sharply elevated average nucleosome occupancy in comparison to flanking intronic sequences. We also show that the preferential positioning of nucleosomes at internal exons is independent of their modification status, and of the GC content, conservation or the expression level of the exon. These findings show that the location of exons is recorded in the chromatin structure and may be inherited across generations. Such embedded information may underpin transcriptionally coupled exon recognition and splice site selection.