DNase-seq to Study Chromatin Accessibility in Early Xenopus tropicalis Embryos

Transcriptional regulatory elements are typically found in relatively nucleosome-free genomic regions, often referred to as "open chromatin." Deoxyribonuclease I (DNase I) can digest nucleosome-depleted DNA (presumably bound by transcription factors), but DNA in nucleosomes or higher-order chromatin fibers is less accessible to the nuclease. The DNase-seq method uses high-throughput sequencing to permit the interrogation of DNase hypersensitive sites (DHSs) across the entire genome and does not require prior knowledge of histone modifications, transcription factor binding sites, or high quality antibodies to identify potentially active regions of chromatin. Here, discontinuous iodixanol gradients are used as a gentle preparation of the nuclei from Xenopus embryos. Short DNase I digestion times are followed by size selection of digested genomic DNA, yielding DHS fragments. These DNA fragments are subjected to real-time quantitative polymerase chain reaction (qPCR) and sequencing library construction. A library generation method and pipeline for analyzing DNase-seq data are also described.