Summary Trimethylation of Histone H3 at Lysine 4 (H3K4me3) is a chromatin modification known to mark the transcription start sites of active genes. Here we show that H3K4me3 domains that spread more broadly over genes in a given cell type preferentially mark genes essential for the identity and function of that cell type. Using the broadest H3K4me3 domains as a discovery tool in neural progenitor cells, we identify novel regulators of these cells. Machine learning models reveal that the broadest H3K4me3 domains represent a distinct entity, characterized by increased marks of elongation. Broadest H3K4me3 domains also have more paused polymerase at their promoters, suggesting a unique transcriptional output. Indeed, genes marked by broadest H3K4me3 domains exhibit enhanced transcriptional consistency rather than increased transcriptional levels, and perturbation of H3K4me3 breadth leads to changes in transcriptional consistency. Thus, H3K4me3 breadth contains information that could ensure transcriptional precision at key cell identity/function genes.

Chromatin accessibility, a crucial component of genome regulation, has primarily been studied in homogeneous and simple systems, such as isolated cell populations or early-development models. Whether chromatin accessibility can be assessed in complex, dynamic systems in vivo with high sensitivity remains largely unexplored. In this study, we use ATAC-seq to identify chromatin accessibility changes in a whole animal, the model organism Caenorhabditis elegans, from embryogenesis to adulthood. Chromatin accessibility changes between developmental stages are highly reproducible, recapitulate histone modification changes, and reveal key regulatory aspects of the epigenomic landscape throughout organismal development. We find that over 5000 distal noncoding regions exhibit dynamic changes in chromatin accessibility between developmental stages and could thereby represent putative enhancers. When tested in vivo, several of these putative enhancers indeed drive novel cell-type-and temporal-specific patterns of expression. Finally, by integrating transcription factor binding motifs in a machine learning framework, we identify EOR-1 as a unique transcription factor that may regulate chromatin dynamics during development. Our study provides a unique resource for C. elegans, a system in which the prevalence and importance of enhancers remains poorly characterized, and demonstrates the power of using whole organism chromatin accessibility to identify novel regulatory regions in complex systems.

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Chromatin accessibility, a crucial component of genome regulation, has primarily been studied in homogeneous and simple systems, such as isolated cell populations or early-development models. Whether chromatin accessibility can be assessed in complex, dynamic systems in vivo with high sensitivity remains largely unexplored. In this study, we use ATAC-seq to identify chromatin accessibility changes in a whole animal, the model organism Caenorhabditis elegans, from embryogenesis to adulthood. Chromatin accessibility changes between developmental stages are highly reproducible, recapitulate histone modification changes, and reveal key regulatory aspects of the epigenomic landscape throughout organismal development. We find that over 5000 distal noncoding regions exhibit dynamic changes in chromatin accessibility between developmental stages and could thereby represent putative enhancers. When tested in vivo, several of these putative enhancers indeed drive novel cell-type-and temporal-specific patterns of expression. Finally, by integrating transcription factor binding motifs in a machine learning framework, we identify EOR-1 as a unique transcription factor that may regulate chromatin dynamics during development. Our study provides a unique resource for C. elegans, a system in which the prevalence and importance of enhancers remains poorly characterized, and demonstrates the power of using whole organism chromatin accessibility to identify novel regulatory regions in complex systems.

Shigella flexneri, a causative agent of bacterial dysentery, possesses two predicted iron-sulfur cluster biosynthesis systems called Suf and Isc. S. flexneri strains containing deletion mutations in the entire suf operon (UR011) or the iscSUA genes (UR022) were constructed. Both mutants were defective in surviving exposure to oxidative stress. The suf mutant showed growth that was comparable to that of the parental strain in both iron-replete and iron-limiting media; however, the isc mutant showed reduced growth, relative to the parental strain, in both media. Although the suf mutant formed wild-type plaques on Henle cell monolayers, the isc mutant was unable to form plaques on Henle cell monolayers because the strain was noninvasive. Expression from both the suf and isc promoters increased in iron-limiting media and in the presence of hydrogen peroxide. Iron repression of the suf promoter was mediated by Fur, and increased suf expression in iron-limiting media was enhanced by the presence of IscR. Iron repression of the isc promoter was mediated by IscR. Hydrogen peroxide-dependent induction of suf expression, but not isc expression, was mediated by OxyR. Furthermore, IscR was a positive regulator of suf expression in the presence of hydrogen peroxide and a negative regulator of isc expression in the absence of hydrogen peroxide. Expression from the S. flexneri suf and isc promoters increased when Shigella was within Henle cells, and our data suggest that the intracellular signal mediating this increased expression is reduced iron levels.As a facultative intracellular pathogen, Shigella flexneri spends a significant portion of its life cycle within the epithelial cells lining the human colon. Invasion of and intracellular survival/replication of the bacteria within these epithelial cells requires the ability to sense the environment and initiate an appropriate metabolic strategy during infection. Global analysis of Shigella transcription during epithelial cell infection indicated that a variety of metabolic genes, including the suf genes, are precisely regulated when Shigella is intracellular (16,27).The Shigella flexneri suf and isc loci encode predicted ironsulfur (Fe-S) cluster biosynthesis systems. Iron-sulfur clusters are essential for a wide variety of biological processes, including redox reactions, substrate binding and activation, iron storage, protein structure, and regulation of gene expression (11). The S. flexneri isc locus contains iscR, iscS, iscU, iscA, hscB, hscA, and fdx and is highly conserved with the Escherichia coli isc locus. E. coli IscS catalyzes the desulfurization of L-cysteine for the recruitment of S for Fe-S cluster formation (4). IscU and IscA are predicted to form scaffolds for Fe-S cluster assembly based on similarities with Azobacter vinelandii NifU and IscA Nif (1, 11, 13). The chaperones HscB and HscA aid in Isc-mediated Fe-S protein maturation, although the specifics are not entirely clear (for a review, see reference 11). Deletion of the isc locus in E. coli reduced the gro...