G-quadruplexes Sequester Free Heme in Living Cells.

Heme is an essential cofactor for many enzymes, but free heme is toxic and its levels are tightly regulated. G-quadruplexes bind heme avidly in vitro, raising the possibility that they may sequester heme in vivo. If so, then treatment that displaces heme from quadruplexes is predicted to induce expression of genes involved in iron and heme homeostasis.

Layer-specific chromatin accessibility landscapes reveal regulatory networks in adult mouse visual cortex.

Mammalian cortex is a laminar structure, with each layer composed of a characteristic set of cell types with different morphological, electrophysiological, and connectional properties. Here, we define chromatin accessibility landscapes of major, layer-specific excitatory classes of neurons, and compare them to each other and to inhibitory cortical neurons using the Assay for Transposase-Accessible Chromatin with high-throughput sequencing (ATAC-seq). We identify a large number of layer-specific accessible sites, and significant association with genes that are expressed in specific cortical layers.

The Werner syndrome RECQ helicase targets G4 DNA in human cells to modulate transcription.

The Werner syndrome (WS) is a prototypic adult Mendelian progeroid syndrome in which signs of premature aging are associated with genomic instability and an elevated risk of cancer. The WRN RECQ helicase protein binds and unwinds G-quadruplex (G4) DNA substrates in vitro, and we identified significant enrichment in G4 sequence motifs at the transcription start site and 5' ends of first introns (false discovery rate < 0.001) of genes down-regulated in WS patient fibroblasts.

Adult mouse cortical cell taxonomy revealed by single cell transcriptomics.

Nervous systems are composed of various cell types, but the extent of cell type diversity is poorly understood. We constructed a cellular taxonomy of one cortical region, primary visual cortex, in adult mice on the basis of single-cell RNA sequencing. We identified 49 transcriptomic cell types, including 23 GABAergic, 19 glutamatergic and 7 non-neuronal types.

Regulation of gene expression by the BLM helicase correlates with the presence of G-quadruplex DNA motifs.

Bloom syndrome is a rare autosomal recessive disorder characterized by genetic instability and cancer predisposition, and caused by mutations in the gene encoding the Bloom syndrome, RecQ helicase-like (BLM) protein. To determine whether altered gene expression might be responsible for pathological features of Bloom syndrome, we analyzed mRNA and microRNA (miRNA) expression in fibroblasts from individuals with Bloom syndrome and in BLM-depleted control fibroblasts. We identified mRNA and miRNA expression differences in Bloom syndrome patient and BLM-depleted cells.

G quadruplexes are genomewide targets of transcriptional helicases XPB and XPD.

G4 motifs are greatly enriched near promoters, suggesting that quadruplex structures may be targets of transcriptional regulation. Here we show, by ChIP-Seq analysis of human cells, that 40% of the binding sites of the transcription-associated helicases, XPB and XPD, overlap with G4 motifs. The highly significant overlap of XPB and XPD binding sites with G4 motifs cannot be explained by GC richness or parameters of the genomewide analysis, but instead suggests that these proteins are recruited to quadruplex structures that form in genomic DNA (G4 DNA).

PGBD5: a neural-specific intron-containing piggyBac transposase domesticated over 500 million years ago and conserved from cephalochordates to humans.

Background: piggyBac domain (PGBD) transposons are found in organisms ranging from fungi to humans. Three domesticated piggyBac elements have been described. In the ciliates Paramecium tetraurelia and Tetrahymena thermophila, homologs known as piggyMacs excise internal eliminated sequences from germline micronuclear DNA during regeneration of the new somatic macronucleus.

The G4 genome.

Recent experiments provide fascinating examples of how G4 DNA and G4 RNA structures--aka quadruplexes--may contribute to normal biology and to genomic pathologies. Quadruplexes are transient and therefore difficult to identify directly in living cells, which initially caused skepticism regarding not only their biological relevance but even their existence. There is now compelling evidence for functions of some G4 motifs and the corresponding quadruplexes in essential processes, including initiation of DNA replication, telomere maintenance, regulated recombination in immune evasion and the immune response, control of gene expression, and genetic and epigenetic instability.

What role (if any) does the highly conserved CSB-PGBD3 fusion protein play in Cockayne syndrome?

The PGBD3 piggyBac transposon inserted into CSB intron 5 early in the primate lineage. As a result of alternative splicing, the human CSB gene now encodes three proteins: CSB, a CSB-PGBD3 fusion protein that joins the N-terminal CSB domain to the C-terminal PGBD3 transposase domain, and PGBD3 transposase. The fusion protein is as highly conserved as CSB, suggesting that it is advantageous in health; however, expression of the fusion protein in CSB-null cells induces a constitutive interferon (IFN) response.

Tethering of the conserved piggyBac transposase fusion protein CSB-PGBD3 to chromosomal AP-1 proteins regulates expression of nearby genes in humans.

The CSB-PGBD3 fusion protein arose more than 43 million years ago when a 2.5-kb piggyBac 3 (PGBD3) transposon inserted into intron 5 of the Cockayne syndrome Group B (CSB) gene in the common ancestor of all higher primates. As a result, full-length CSB is now coexpressed with an abundant CSB-PGBD3 fusion protein by alternative splicing of CSB exons 1-5 to the PGBD3 transposase.

The conserved Cockayne syndrome B-piggyBac fusion protein (CSB-PGBD3) affects DNA repair and induces both interferon-like and innate antiviral responses in CSB-null cells.

Cockayne syndrome is a segmental progeria most often caused by mutations in the CSB gene encoding a SWI/SNF-like ATPase required for transcription-coupled DNA repair (TCR). Over 43Mya before marmosets diverged from humans, a piggyBac3 (PGBD3) transposable element integrated into intron 5 of the CSB gene. As a result, primate CSB genes now generate both CSB protein and a conserved CSB-PGBD3 fusion protein in which the first 5 exons of CSB are alternatively spliced to the PGBD3 transposase.

Ubiquitin recognition by the Cockayne syndrome group B protein: binding will set you free.

Transcription-coupled nucleotide excision repair (TC-NER) requires the coordinated efforts of many proteins. In this issue of Molecular Cell, Anindya et al. (2010) show that the proteins assemble at the site of DNA damage but cannot begin repair until the Cockayne syndrome group B protein (CSB) binds ubiquitin.