SUMMARY The ATP-dependent chromatin remodeling complex SWI/SNF regulates transcription and has been implicated in promoter nucleosome eviction. Efficient nucleosome disassembly by SWI/SNF alone in biochemical assays has however not been directly observed. Employing a model system of dinucleosomes rather than mononucleosomes, we demonstrate that remodeling leads to ordered and efficient disassembly of one of the two nucleosomes. An H2A/H2B dimer is first rapidly displaced and then in a slower reaction an entire histone octamer is lost. Nucleosome disassembly by SWI/SNF did not require additional factors such as chaperones or acceptors of histones. Observations in single molecules as well as bulk measurement suggest that a key intermediate in this process is one in which a nucleosome is moved towards the adjacent nucleosome. SWI/SNF recruited by the transcriptional activator Gal4-VP16 preferentially mobilizes the proximal nucleosome and destabilizes the adjacent nucleosome.

Although epigenetic abnormalities have been described in Huntington’s disease (HD), the causal epigenetic mechanisms driving neurodegeneration in HD cortex and striatum remain undefined. Using an epigenetic pathway-targeted drug screen, we report that inhibitors of DNA methyltransferases (DNMTs), decitabine and FdCyd, block mutant huntingtin (Htt)-induced toxicity in primary cortical and striatal neurons. In addition, knockdown of DNMT3A or DNMT1 protected neurons against mutant Htt-induced toxicity, together demonstrating a requirement for DNMTs in mutant Htt-triggered neuronal death and suggesting a neurodegenerative mechanism based on DNA methylation-mediated transcriptional repression. Inhibition of DNMTs in HD model primary cortical or striatal neurons restored the expression of several key genes, including Bdnf, an important neurotrophic factor implicated in HD. Accordingly, the Bdnf promoter exhibited aberrant cytosine methylation in mutant Htt-expressing cortical neurons. In vivo, pharmacological inhibition of DNMTs in HD mouse brains restored the mRNA levels of key striatal genes known to be downregulated in HD. Thus, disturbances in DNA methylation play a critical role in mutant Htt-induced neuronal dysfunction and death, raising the possibility that epigenetic strategies targeting abnormal DNA methylation may have therapeutic utility in HD.

Background: DNA photoproduct deamination contributes to mutations. Results: Cyclobutane pyrimidine dimers (CPDs) deaminate fastest at TCG sites, and the rate depends on the position of the CPD in a nucleosome determined from hydroxyl radical footprinting. Conclusion: Deamination could explain the high C to T mutation rate at TCG sites, which can be further modulated by nucleosomes. Significance: Sequence context and chromatin structure can modulate UV mutagenesis.

Cells devote considerable resources to nutrient homeostasis, involving nutrient surveillance, acquisition, and storage at physiologically relevant concentrations. Many Saccharomyces cerevisiae transcripts coding for proteins with nutrient uptake functions exhibit peak periodic accumulation during M phase, indicating that an important aspect of nutrient homeostasis involves transcriptional regulation. Inorganic phosphate is a central macronutrient that we have previously shown oscillates inversely with mitotic activation of PHO5. The mechanism of this periodic cell cycle expression remains unknown. To date, only two sequence-specific activators, Pho4 and Pho2, were known to induce PHO5 transcription. We provide here evidence that Mcm1, a MADS-box protein, is essential for PHO5 mitotic activation. In addition, we found that cells simultaneously lacking the forkhead proteins, Fkh1 and Fkh2, exhibited a 2.5-fold decrease in PHO5 expression. The Mcm1-Fkh2 complex, first shown to transactivate genes within the CLB2 cluster that drive G 2 /M progression, also associated directly at the PHO5 promoter in a cell cycle-dependent manner in chromatin immunoprecipitation assays. Sds3, a component specific to the Rpd3L histone deacetylase complex, was also recruited to PHO5 in G 1 . These findings provide (i) further mechanistic insight into PHO5 mitotic activation, (ii) demonstrate that Mcm1-Fkh2 can function combinatorially with other activators to yield late M/G 1 induction, and (iii) couple the mitotic cell cycle progression machinery to cellular phosphate homeostasis.Cellular growth and division are controlled by the temporal execution of programmed events that drive cell cycle progression. Several mechanisms that regulate the cell division cycle of S. cerevisiae are orchestrated by the cyclin-dependent kinase (CDK) Cdc28. While in large part this regulation occurs at the posttranscriptional level through targeted protein degradation (65), an important aspect of cell cycle regulation is also mediated at the level of transcription. More than 13% of S. cerevisiae genes are expressed in a cell cycle stage-specific fashion predominantly, yet not exclusively, via the action of one of three distinct classes of sequence-specific DNA-binding factors (16,68,81). These include SBF/MBF, Ace2/Swi5, and Mcm1, which respectively regulate G 1 /S-, M-, and M/G 1 -dependent transcription (81). These stage-specific roles are an approximation since some overlap occurs, most notably for Mcm1, which exhibits important functions throughout the cell cycle (23, 53).Budding yeast Mcm1, along with Agamous and Deficiens in plants and mammalian serum response factor, is a founding member of a family of proteins containing the highly conserved 56-amino-acid MADS box (58,63,80,95). Mcm1 is an essential gene product with diverse cellular roles in minichromosome maintenance, from which its name is derived, as well as cell cycle control, cell type determination, mating, arginine metabolism, and stress tolerance (14,54,78). Eighty amino acids near the N t...

Wrapping DNA into chromatin provides a wealth of regulatory mechanisms that ensure normal growth and development in eukaryotes. Our understanding of chromatin structure, including nucleosomes and non-histone protein-DNA interactions, has benefited immensely from nuclease and chemical digestion techniques. DNA-bound proteins, such as histones or site-specific factors, protect DNA against nuclease cleavage and generate large nucleosomal or small regulatory factor footprints. Chromatin subject to distinct modes of regulation often coincides with sites of nuclease hypersensitivity or nucleosome positioning. An inherent limitation of cleavage-based analyses has been the inability to reliably analyze regions of interest when levels of digestion depart from single-hit kinetics. Moreover, cleavage-based techniques provide views that are averaged over all the molecules in a sample population. Therefore, in cases of occupancy of multiple regulatory elements by factors, one cannot define whether the factors are bound to the same or different molecules in the population. The recent development of DNA methyltransferase-based, singlemolecule MAP-IT technology overcomes limitations of ensemble approaches and has opened numerous new avenues in chromatin research. Here, we review the strengths, limitations, applications and future prospects of MAP-IT ranging from structural issues to mechanistic questions in eukaryotic chromatin regulation.

COVID-19 has severely impacted socioeconomically disadvantaged populations. To support pandemic control strategies, geographically weighted negative binomial regression (GWNBR) mapped COVID-19 risk related to epidemiological and socioeconomic risk factors using South Korean incidence data (20 January 2020 to 1 July 2020). We constructed COVID-19-specific socioeconomic and epidemiological themes using established social theoretical frameworks and created composite indexes through principal component analysis. The risk of COVID-19 increased with higher area morbidity, risky health behaviours, crowding, and population mobility, and with lower social distancing, healthcare access, and education. Falling COVID-19 risks and spatial shifts over three consecutive time periods reflected effective public health interventions. This study provides a globally replicable methodological framework and precision mapping for COVID-19 and future pandemics.

Non-invasive methods for mapping chromatin structure are necessary for creating an accurate view of genome function and dynamics in vivo. Ectopic induction of cytosine-5 DNA methyltransferases (C5 MTases) in Saccharomyces cerevisiae is a powerful technique for probing chromatin structure with minimal disruption to yeast physiology. Accessibility of MTases to their cognate sites is impaired based on the strength and span of the protein-DNA interaction to be probed. Methylated cytosines that resist chemical deamination are detected positively by the PCR-based technique of bisulfite genomic sequencing. PCR amplicons can be sequenced directly yielding an average m(5)C frequency or accessibility of each target site within the population, a technique termed methyltransferase accessibility protocol (MAP). More recently, the sequencing of cloned molecules in MAP for individual templates (MAPit) enables assignment of the methylation status of each target site along a continuous DNA strand from a single cell. The unique capability to score methylation at multiple sites in single molecules permits detection of inherent structural variability in chromatin. Here, MAPit analysis of the repressed and induced PHO5 promoter of budding yeast, using a C5 MTase with dinucleotide recognition specificity, reveals considerable cell-to-cell heterogeneity in chromatin structure. Substantial variation is observed in the extent to which the MTase gains entry to each of the nucleosomes positioned at PHO5, suggesting differences in their intrinsic thermodynamic stability in vivo. MAPit should be readily adaptable to the analysis of chromatin structure and non-histone protein-DNA interactions in a variety of model systems.

If instructors are to integrate active learning effectively in courses in Science, Technology, Engineering, and Mathematics (STEM), they need an accurate account of when and how they are integrating active learning-and of how students are responding. Without such an account, instructors may perceive that they are incorporating more active learning than observers document, or they may miss opportunities to target aspects of the implementation that may be adjusted to improve effectiveness. This article describes a visual approach to integrating observational data into self-evaluation and peer review of teaching, practices that can lead to adoption of evidence-based active-learning strategies in STEM. While our approach has specific relevance during this period of reform in STEM education, it was designed to be implemented for undergraduate courses across the disciplines. The presentation of observational data in a timeline provides a "big-picture" view of observed class sessions that captures the sequencing of instructional strategies and the "ebbs and flows" of student participation-in a chronological format that coheres with how instructors often visualize a class session. Such a view can help instructors see where these strategies meet their instructional goals, and where these strategies might be refined and improved.