Transcription forms and remodels supercoiling domains unfolding large-scale chromatin structures

DNA supercoiling is an inherent consequence of twisting DNA and is critical for regulating gene expression and DNA replication. However, DNA supercoiling at a genomic scale in human cells is uncharacterized. To map supercoiling we used biotinylated-trimethylpsoralen as a DNA structure probe to show the genome is organized into supercoiling domains. Domains are formed and remodeled by RNA polymerase and topoisomerase activities and are flanked by GC-AT boundaries and CTCF binding sites. Under-wound domains are transcriptionally active, enriched in topoisomerase I, “open” chromatin fibers and DNaseI sites, but are depleted of topoisomerase II. Furthermore DNA supercoiling impacts on additional levels of chromatin compaction as under-wound domains are cytologically decondensed, topologically constrained, and decompacted by transcription of short RNAs. We suggest that supercoiling domains create a topological environment that facilitates gene activation providing an evolutionary purpose for clustering genes along chromosomes

Effects of DNA supercoiling on chromatin architecture

Disruptions in chromatin structure are necessary for the regulation of eukaryotic genomes, from remodelling of nucleosomes at the base pair level through to large-scale chromatin domains that are hundreds of kilobases in size. RNA polymerase is a powerful motor which, prevented from turning with the tight helical pitch of the DNA, generates over-wound DNA ahead of itself and under-wound DNA behind. Mounting evidence supports a central role for transcription-dependent DNA supercoiling in disrupting chromatin structure at all scales. This supercoiling changes the properties of the DNA helix in a manner that substantially alters the binding specificity of DNA binding proteins and complexes, including nucleosomes, polymerases, topoisomerases and transcription factors. For example, transient over-wound DNA destabilises nucleosome core particles ahead of a transcribing polymerase, whereas under-wound DNA facilitates pre-initiation complex formation, transcription factor binding and nucleosome core particle association behind the transcribing polymerase. Importantly, DNA supercoiling can also dissipate through DNA, even in a chromatinised context, to influence both local elements and large chromatin domains. We propose a model in which changes in unconstrained DNA supercoiling influences higher levels of chromatin organisation through the additive effects of DNA supercoiling on both DNA-protein and DNA-nucleosome interactions. This model links small-scale changes in DNA and chromatin to the higher-order fibre and large-scale chromatin structures, providing a mechanism relating gene regulation to chromatin architecture in vivo

Infant Feeding Decision-Making and the Influences of Social Support Persons Among First-Time African American Mothers

BACKGROUND: While breast milk is considered the gold standard of infant feeding, a majority of African American mothers are not exclusively breastfeeding their newborn infants. PURPOSE: The overall goal of this critical ethnographic research study was to describe infant feeding perceptions and experiences of African American mothers and their support persons. METHODS: Twenty-two participants (14 pregnant women and eight support persons) were recruited from public health programs and community based organizations in northern California. Data were collected through field observations, demographic questionnaires, and multiple in-person interviews. Thematic analysis was used to identify key themes. RESULTS: Half of the mothers noted an intention to exclusively breastfeed during the antepartum period. However, few mothers exclusively breastfed during the postpartum period. Many participants expressed guilt and shame for not being able to accomplish their antepartum goals. Life experiences and stressors, lack of breastfeeding role models, limited experiences with breastfeeding and lactation, and changes to the family dynamic played a major role in the infant feeding decision making process and breastfeeding duration. CONCLUSION: Our observations suggest that while exclusivity goals were not being met, a considerable proportion of African American women were breastfeeding. Future interventions geared towards this population should include social media interventions, messaging around combination feeding, and increased education for identified social support persons. Public health measures aimed at reducing the current infant feeding inequities would benefit by also incorporating more culturally inclusive messaging around breastfeeding and lactation