Structure of transcribing RNA polymerase II-nucleosome complex, bioRxiv, 2018-10-07
Transcription of eukaryotic protein-coding genes requires passage of RNA polymerase II (Pol II) through chromatin. Pol II passage is impaired by nucleosomes and requires elongation factors that help Pol II to efficiently overcome the nucleosomal barrier1-4. How the Pol II machinery transcribes through a nucleosome remains unclear because structural studies have been limited to Pol II elongation complexes formed on DNA templates lacking nucleosomes5. Here we report the cryo-electron microscopy (cryo-EM) structure of transcribing Pol II from the yeast Saccharomyces cerevisiae engaged with a downstream nucleosome core particle (NCP) at an overall resolution of 4.4 Å with resolutions ranging from 4-6 Å in Pol II and 6-8 Å in the NCP. Pol II and the NCP adopt a defined orientation that could not be predicted from modelling. Pol II contacts DNA of the incoming NCP on both sides of the nucleosomal dyad with its domains ‘clamp head’ and ‘lobe’. Comparison of the Pol II-NCP structure to known structures of Pol II complexes reveals that the elongation factors TFIIS, DSIF, NELF, PAF1 complex, and SPT6 can be accommodated on the Pol II surface in the presence of the oriented nucleosome. Further structural comparisons show that the chromatin remodelling enzyme Chd1, which is also required for efficient Pol II passage6,7, could bind the oriented nucleosome with its motor domain. The DNA-binding region of Chd1 must however be released from DNA when Pol II approaches the nucleosome, and based on published data8,9 this is predicted to stimulate Chd1 activity and to facilitate Pol II passage. Our results provide a starting point for a mechanistic analysis of chromatin transcription.
biorxiv biochemistry 100-200-users 2018Bright and photostable chemigenetic indicators for extended in vivo voltage imaging, bioRxiv, 2018-10-06
Imaging changes in membrane potential using genetically encoded fluorescent voltage indicators (GEVIs) has great potential for monitoring neuronal activity with high spatial and temporal resolution. Brightness and photostability of fluorescent proteins and rhodopsins have limited the utility of existing GEVIs. We engineered a novel GEVI, Voltron, that utilizes bright and photostable synthetic dyes instead of protein-based fluorophores, extending the combined duration of imaging and number of neurons imaged simultaneously by more than tenfold relative to existing GEVIs. We used Voltron for in vivo voltage imaging in mice, zebrafish, and fruit flies. In mouse cortex, Voltron allowed single-trial recording of spikes and subthreshold voltage signals from dozens of neurons simultaneously, over 15 minutes of continuous imaging. In larval zebrafish, Voltron enabled the precise correlation of spike timing with behavior.
biorxiv neuroscience 200-500-users 2018Bazam A rapid method for read extraction and realignment of high throughput sequencing data, bioRxiv, 2018-10-04
AbstractBackgroundAs costs of high throughput sequencing have fallen, we are seeing vast quantities of short read genomic data being generated. Often, the data is exchanged and stored as aligned reads, which provides high compression and convenient access for many analyses. However, aligned data becomes outdated as new reference genomes and alignment methods become available. Moreover, some applications cannot utilise pre-aligned reads at all, necessitating conversion back to raw format (FASTQ) before they can be used. In both cases, the process of extraction and realignment is expensive and time consuming.FindingsWe describe Bazam, a tool that efficiently extracts the original paired FASTQ from reads stored in aligned form (BAM or CRAM format). Bazam extracts reads in a format that directly allows realignment with popular aligners with high concurrency. Through eliminating steps and increasing the accessible concurrency, Bazam facilitates up to a 90% reduction in the time required for realignment compared to standard methods. Bazam can support selective extraction of read pairs from focused genomic regions, further increasing efficiency for targeted analyses. Bazam is additionally suitable as a base for other applications that require efficient paired read information, such as quality control, structural variant calling and alignment comparison.ConclusionsBazam offers significant improvements for users needing to realign genomic data.
biorxiv bioinformatics 200-500-users 2018High-performance GFP-based calcium indicators for imaging activity in neuronal populations and microcompartments, bioRxiv, 2018-10-04
AbstractCalcium imaging with genetically encoded calcium indicators (GECIs) is routinely used to measure neural activity in intact nervous systems. GECIs are frequently used in one of two different modes to track activity in large populations of neuronal cell bodies, or to follow dynamics in subcellular compartments such as axons, dendrites and individual synaptic compartments. Despite major advances, calcium imaging is still limited by the biophysical properties of existing GECIs, including affinity, signal-to-noise ratio, rise and decay kinetics, and dynamic range. Using structure-guided mutagenesis and neuron-based screening, we optimized the green fluorescent protein-based GECI GCaMP6 for different modes of in vivo imaging. The jGCaMP7 sensors provide improved detection of individual spikes (jGCaMP7s,f), imaging in neurites and neuropil (jGCaMP7b), and tracking large populations of neurons using 2-photon (jGCaMP7s,f) or wide-field (jGCaMP7c) imaging.
biorxiv neuroscience 200-500-users 2018Microscopy-based chromosome conformation capture enables simultaneous visualization of genome organization and transcription in intact organisms, bioRxiv, 2018-10-04
Eukaryotic chromosomes are organized in multiple scales, from nucleosomes to chromosome territories. Recently, genome-wide methods identified an intermediate level of chromosome organization, topologically associating domains (TADs), that play key roles in transcriptional regulation. However, these methods cannot directly examine the interplay between transcriptional activation and chromosome architecture while maintaining spatial information. Here, we present a multiplexed, sequential imaging approach (Hi-M) that permits the simultaneous detection of chromosome organization and transcription in single nuclei. This allowed us to unveil the changes in 3D chromatin organization occurring upon transcriptional activation and homologous chromosome un-pairing during the awakening of the zygotic genome in intact Drosophila embryos. Excitingly, the ability of Hi-M to explore the multi-scale chromosome architecture with spatial resolution at different stages of development or during the cell cycle will be key to understand the mechanisms and consequences of the 4D organization of the genome.
biorxiv cell-biology 100-200-users 2018scRNA-seq mixology towards better benchmarking of single cell RNA-seq analysis methods, bioRxiv, 2018-10-04
AbstractSingle cell RNA sequencing (scRNA-seq) technology has undergone rapid development in recent years, bringing with new challenges in data processing and analysis. This has led to an explosion of tailored analysis methods for scRNA-seq data to address various biological questions. However, the current lack of gold-standard benchmark datasets makes it difficult for researchers to systematically evaluate the performance of the many methods available. Here, we designed and carried out a realistic benchmark experiment that included mixtures of single cells or ‘pseudo cells’ created by sampling admixtures of cells or RNA from up to 5 distinct cancer cell lines. Altogether we generated 14 datasets using droplet and plate-based scRNA-seq protocols, compared multiple data analysis methods in combination for tasks ranging from normalization and imputation, to clustering, trajectory analysis and data integration. Evaluation across 3,913 analyses (methods × benchmark dataset combinations) revealed pipelines suited to different types of data for different tasks. Our dataset and analysis present a comprehensive comparison framework for benchmarking most common scRNA-seq analysis tasks.
biorxiv bioinformatics 100-200-users 2018