Here we present the 15 pseudochromosomes of sweet potato, Ipomoea batatas, the seventh most important crop in the world and the fourth most significant in China. By using a novel haplotyping method based on genome assembly, we have produced a half haplotype-resolved genome from ~296 Gb of paired-end sequence reads amounting to roughly 67-fold coverage. By phylogenetic tree analysis of homologous chromosomes, it was possible to estimate the time of two recent whole-genome duplication events as occurring about 0.8 and 0.5 million years ago. This half haplotype-resolved hexaploid genome represents the first successful attempt to investigate the complexity of chromosome sequence composition directly in a polyploid genome, using sequencing of the polyploid organism itself rather than any of its simplified proxy relatives. Adaptation and application of our approach should provide higher resolution in future genomic structure investigations, especially for similarly complex genomes.
Long-range gene regulation involves physical proximity between enhancers and promoters to generate precise patterns of gene expression in space and time. However, in some cases, proximity coincides with gene activation, whereas, in others, preformed topologies already exist before activation. In this study, we investigate the preformed configuration underlying the regulation of the Shh gene by its unique limb enhancer, the ZRS, in vivo during mouse development. Abrogating the constitutive transcription covering the ZRS region led to a shift within the Shh-ZRS contacts and a moderate reduction in Shh transcription. Deletion of the CTCF binding sites around the ZRS resulted in the loss of the Shh-ZRS preformed interaction and a 50% decrease in Shh expression but no phenotype, suggesting an additional, CTCF-independent mechanism of promoter-enhancer communication. This residual activity, however, was diminished by combining the loss of CTCF binding with a hypomorphic ZRS allele, resulting in severe Shh loss of function and digit agenesis. Our results indicate that the preformed chromatin structure of the Shh locus is sustained by multiple components and acts to reinforce enhancer-promoter communication for robust transcription. genetics | 3D genome | gene regulation | development | limb
The combinatorial action of co-localizing chromatin modifications and regulators determines chromatin structure and function. However, identifying co-localizing chromatin features in a high-throughput manner remains a technical challenge. Here we describe a novel reChIP-seq approach and tailored bioinformatic analysis tool, normR that allows for the sequential enrichment and detection of co-localizing DNA-associated proteins in an unbiased and genome-wide manner. We illustrate the utility of the reChIP-seq method and normR by identifying H3K4me3 or H3K27me3 bivalently modified nucleosomes in primary human CD4+ memory T cells. We unravel widespread bivalency at hypomethylated CpG-islands coinciding with inactive promoters of developmental regulators. reChIP-seq additionally uncovered heterogeneous bivalency in the population, which was undetectable by intersecting H3K4me3 and H3K27me3 ChIP-seq tracks. Finally, we provide evidence that bivalency is established and stabilized by an interplay between the genome and epigenome. Our reChIP-seq approach augments conventional ChIP-seq and is broadly applicable to unravel combinatorial modes of action.
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