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Search indexed bioRxiv preprints in genomics, neuroscience, cell biology and bioinformatics. Read source abstracts and check manuscript versions; preprints are not peer reviewed.

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bioRxiv2017

High-frequency off-target mutagenesis induced by CRISPR/Cas9 in Arabidopsis and its prevention by improving specificity of the tools

Specificity of CRISPR/Cas9 tools has been a major concern along with the reports of their successful applications. We report unexpected observations of high frequency off-target mutagenesis induced by CRISPR/Cas9 in T1 Arabidopsis mutants although the sgRNA was predicted to have a high specificity score. We also present evidence that the off-target effects were further exacerbated in the T2 progeny. To prevent the off-target effects, we tested and optimized two strategies in Arabidopsis, including introduction of a mCherry cassette for a simple and reliable isolation of Cas9-free mutants and the use of highly specific mutant SpCas9 variants. Optimization of the mCherry vectors and subsequent validation found that fusion of tRNA with the mutant rather than the original sgRNA scaffold significantly improves editing efficiency. We then examined the editing efficiency of eight high-specificity SpCas9 variants in combination with the improved tRNA-sgRNA fusion strategy. Our results suggest that highly specific SpCas9 variants require a higher level of expression than their wild-type counterpart to maintain high editing efficiency. Additionally, we demonstrate that T-DNA can be inserted into the cleavage sites of CRISPR/Cas9 targets with high frequency. Altogether, our results suggest that in plants, continuous attention should be paid to off-target effects induced by CRISPR/Cas9 in current and subsequent generations, and that the tools optimized in this report will be useful in improving genome editing efficiency and specificity in plants and other organisms.

molecular biology↗

bioRxiv2017

Csde1 binds transcripts involved in protein homeostasis and controls their expression in erythropoiesis

Expression of the RNA-binding protein Csde1 (Cold shock domain protein e1) is strongly upregulated during erythropoiesis compared to other hematopoietic lineages. In the severe congenital anemia Diamond Blackfan Anemia (DBA), however, Csde1 expression is impaired. Reduced expression of Csde1 in healthy erythroblasts impaired their proliferation and differentiation, which suggests an important role for Csde1 in erythropoiesis. To investigate the cellular pathways controlled by Csde1 in erythropoiesis, we identified the transcripts that physically associate with Csde1 in erythroid cells. These mainly encoded proteins involved in ribogenesis, mRNA translation and protein degradation, but also proteins associated with the mitochondrial respiratory chain and mitosis. Crispr/Cas9-mediated deletion of the first cold shock domain of Csde1 affected RNA expression and/or protein expression of Csde1-bound transcripts. For instance, protein expression of Pabpc1 was enhanced while Pabpc1 mRNA expression was reduced indicating more efficient translation of Pabpc1 followed by negative feedback on mRNA stability. Overall, the effect of reduced Csde1 function on mRNA stability and translation of Csde1-bound transcripts was modest. Clones with complete loss of Csde1, however, could not be generated. We suggest that Csde1 is involved in feed-back control in protein homeostasis and that it dampens stochastic changes in mRNA expression.

molecular biology↗

bioRxiv2017

Csde1 cooperates with Strap to control translation of erythroid transcripts

Erythropoiesis is regulated at many levels, including control of mRNA translation. Changing environmental conditions, such as hypoxia, or the availability of nutrients and growth factors, require a rapid response enacted by the enhanced or repressed translation of existing transcripts. Csde1 is an RNA-binding protein required for erythropoiesis and strongly upregulated in erythroblasts relative to other hematopoietic progenitors. The aim of this study is to identify the Csde1-containing protein complexes, and investigate their role in regulating the translation of Csde1-bound transcripts. We show that Strap, also called Unrip, was the protein most strongly associated with Csde1 in erythroblasts. Strap is a WD40 protein involved in signaling and RNA splicing, but its role is unknown when associated with Csde1. Reduced expression of Strap did not alter the pool of transcripts bound by Csde1. Instead, it reduced the mRNA and/or protein expression of several Csde1-bound transcript, that encode for proteins essential for translational regulation during hypoxia, such as Hmbs, eIF4g3 and Pabpc4. Also affected by Strap knockdown were Vim, a Gata-1 target crucial for erythrocyte enucleation, and Elavl1, which stabilizes Gata-1 mRNA. Thus, we found that the Csde1/Strap complex is at the crossroad of multiple pathways governing translation in erythroblasts.

molecular biology↗

bioRxiv2017

From Cohorts to Molecules: Adverse Impacts of Endocrine Disrupting Mixtures

Convergent evidence associates endocrine disrupting chemicals (EDCs) with major, increasingly-prevalent human disorders. Regulation requires elucidation of EDC-triggered molecular events causally linked to adverse health outcomes, but two factors limit their identification. First, experiments frequently use individual chemicals, whereas real life entails simultaneous exposure to multiple EDCs. Second, population-based and experimental studies are seldom integrated. This drawback was exacerbated until recently by lack of physiopathologically meaningful human experimental systems that link epidemiological data with results from model organisms.\n\nWe developed a novel approach, integrating epidemiological with experimental evidence. Starting from 1,874 mother-child pairs we identified mixtures of chemicals, measured during early pregnancy, associated with language delay or low-birth weight in offspring. These mixtures were then tested on multiple complementary in vitro and in vivo models. We demonstrate that each EDC mixture, at levels found in pregnant women, disrupts hormone-regulated and disease-relevant gene regulatory networks at both the cellular and organismal scale.

molecular biology↗

bioRxiv2017

Clinically-guided mutation screening of two families with hereditary retinal disease

Hereditary retinal disease (HRD) is a series of Mendelian diseases affecting the retina in the eye. The genetic basis of HRD is very complicated, with more than 100 disease-causing genes being identified. Though NGS has allowed rapid and large-scale mutation screening of Mendelian disease, the cost of NGS still prevents its universal application all over the world, for an accurate molecular diagnosis. Here, by clinical guidance from patient phenotypes, we performed targeted molecular diagnosis by direct Sanger sequencing of the most likely candidate gene in two families diagnosed with HRD. Then we identified two novel protein-truncating variants in the gene CRB1. Our results demonstrated the notion that molecular diagnosis and clinical diagnosis can be mutually supplemented and clinically guided direct sequencing is a cost-effective approach for molecular diagnosis and subsequent genetic counseling.

molecular biology↗

bioRxiv2017

Darwin Assembly: fast, efficient, multi-site bespoke mutagenesis.

Engineering proteins for designer functions and biotechnological applications almost invariably requires (or at least benefits from) multiple mutations to non-contiguous residues. Several methods for multiple site-directed mutagenesis exist, but there remains a need for fast and simple methods to efficiently introduce such mutations - particularly for generating large, high quality libraries for directed evolution. Here, we present Darwin Assembly, which can deliver high quality libraries of over 108 transformants, targeting multiple (> 10) distal sites with minimal wild-type contamination (lower than 0.25% of total population) and which takes a single working day from purified plasmid to library transformation. Darwin Assembly uses commercially available enzymes, can be readily automated, and offers a cost-effective route to highly complex and customizable library generation.

molecular biology↗

bioRxiv2017

Systematic Dissection of Sequence Elements Controlling σ70 Promoters Using a Genomically-Encoded Multiplexed Reporter Assay in E. coli

Promoters are the key drivers of gene expression and are largely responsible for the regulation of cellular responses to time and environment. In E. coli, decades of studies have revealed most, if not all, of the sequence elements necessary to encode promoter function. Despite our knowledge of these motifs, it is still not possible to predict the strength and regulation of a promoter from primary sequence alone. Here we develop a novel multiplexed assay to study promoter function in E. coli by building a site-specific genomic recombination-mediated cassette exchange (RMCE) system that allows for the facile construction and testing of large libraries of genetic designs integrated into precise genomic locations. We build and test a library of 10,898 {sigma}70 promoter variants consisting of all combinations of a set of eight -35 elements, eight -10 elements, three UP elements, eight spacers, and eight backgrounds. We find that the -35 and -10 sequence elements can explain approximately 74% of the variance in promoter strength within our dataset using a simple log-linear statistical model. Simple neural network models explain greater than 95% of the variance in our dataset by capturing nonlinear interactions with the spacer, background, and UP elements.

molecular biology↗

bioRxiv2017

Molecular Identification of some Indian Muscid flies (Diptera: Muscidae) based on mitochondrial gene COII

Taxonomic identification of flies having medical and veterinary importance is often complicated due to the lookalike morphological characters. Molecular identification of five Indian muscid flies (Musca domestica, Musca sorbens, Musca crassirostris, Stomoxys calcitrans and Haematobia irritans) has been attempted on the basis of mitochondrial COII gene. Sequences of 500-520 bp were analysed and found to be A+T rich. Rate of transitions was higher than transversions. The average haplotype diversity was 0.833 and nucleotide diversity was 0.02547 within the different species, which were calculated with the DnaSP Version 5.0. The genetic distances calculated by K2P model, showed the interspecific distances range more than 8.2%, while the intraspecific distance range had not exceed 0.8%. The 1000 bootstrapped Neighbour-joining tree and Maximum likelihood tree were constructed to establish the phylogenetic relationship between the different muscid species. The results show the robustness of COII gene as a diagnostic marker. The data obtained from this study would be worthy for medical and veterinary entomologists for precise identification of imperative muscid species.

molecular biology↗

bioRxiv2017

Rapid high-resolution measurement of DNA replication timing by droplet digital PCR

Genomes are replicated in a reproducible temporal pattern. Current methods for assaying allele replication timing are time consuming and/or expensive. These include high-throughput sequencing which can be used to measure DNA copy number as a proxy for allele replication timing. Here, we use droplet digital PCR to study DNA replication timing at multiple loci in budding yeast and human cells. We establish that the method has temporal and spatial resolutions comparable to the high-throughput sequencing approaches, while being faster than alternative locus-specific methods. Furthermore, the approach is capable of allele discrimination. We apply this method to determine relative replication timing across timing transition zones in cultured human cells. Finally, multiple samples can be analysed in parallel, allowing us to rapidly screen kinetochore mutants for perturbation to centromere replication timing. Therefore, this approach is well suited to the study of locus-specific replication and the screening of cis- and trans-acting mutants to identify mechanisms that regulate local genome replication timing.

molecular biology↗

bioRxiv2017

DNA lesions proximity modulates damage tolerance pathways

The genome of all organisms is constantly threatened by numerous agents that cause DNA damages. When the replication fork encounters an unrepaired DNA lesion, two DNA damage tolerance pathways are possible: error-prone translesion synthesis (TLS) that requires specialized DNA polymerases, and error-free Damage Avoidance (DA) that relies on homologous recombination. The balance between these two mechanisms is essential since it defines the level of mutagenesis during lesion bypass, allowing genetic variability and adaptation to the environment, but also introducing the risk of generating genome instability. Here we report that the mere proximity of replication-blocking lesions that arise in Escherichia colis genome during a genotoxic stress, leads to a strong increase in the use of the error-prone TLS. We show that this increase is caused by the local inhibition of homologous recombination due to the overlapping of single-stranded DNA regions generated downstream the lesions. This increase in TLS is independent of SOS activation, but its mutagenic effect is additive with the one of SOS. Hence, the combination of SOS induction and lesions proximity leads to a strong increase in TLS that becomes the main lesion tolerance pathway used by the cell during a genotoxic stress.

molecular biology↗

bioRxiv2017

A 20S proteasome receptor for degradation of intrinsically disordered proteins

Degradation of intrinsically disordered proteins (IDPs) by the 20S proteasome, unlike ubiquitin-dependent 26S proteasomal degradation, does not require proteasomal targeting by polyubiquitin. However, how these proteins are recognized by the proteasome was unknown. We report here on a mechanism of 20S proteasome targeting. Analysis of protein interactome datasets revealed that the proteasome subunit PSMA3 interacts with many IDPs. By employing in vivo and cell-free experiments we demonstrated that the PSMA3 C-terminus binds p21, c-Fos and p53, all IDPs and 20S proteasome substrates. A 69 amino-acids long fragment is autonomously functional in interacting with IDP substrates. Remarkably, this fragment in isolation blocks the degradation of a large number of IDPs in vitro and increases the half-life of proteins in vivo. We propose a model whereby the PSMA3 C-terminal region plays a role of substrate receptor in the process of proteasomal degradation of many IDPs.

molecular biology↗

bioRxiv2017

Molecular identification of stone loaches of Choman River system, Tigris Basin, based on the Cytochrome b gene with an overview of the Cobitoidea Superfamily

Molecular data and phylogenetic relationships of four Choman loaches species (Oxynoemacheilus chomanicus, O. zagrosensis, O. kurdistanicus and Turcinoemacheilus kosswigi) recently morphologically described from western Iran were evaluated with 64 species from the Cobitoidea superfamily based on their cytochrome b sequences to exhibit the placement of the Choman loaches species within the Cobitoidea superfamily. A comparative analysis of Kimura-2-parameter (K2P) distances was accomplished using sequence divergences of Cobitoidea to calculate intra and interspecific in superfamily, family and genus taxa. The average intraspecific K2P genetic distances of Choman loaches species was 0.005 whereas this value was 0.016 for the Cobitoidea superfamily. Molecular phylogenetic relationships were assessed using Maximum likelihood and Bayesian methods. Dendrograms obtained by these methods revealed all four Choman loaches species as distinct species among other reported Nemacheilidae Spp. These species were clustered with Oxynoemacheilus and Turcinoemacheilus genera within other species in the Nemacheilidae family. The phylogenetic analysis revealed that Cobitoidea superfamily consists of nine families ((Gyrinocheilidae + Botiidae) + ((Catostomidae + Vaillentellidae) + ((Nemacheilidae + Cobitidae) + ((Ellopostomidae + Gastromyzontidae) + Balitoridae)))) and indicated Nemacheilidae is a valid and distinct family from Balitoridae.

molecular biology↗

bioRxiv2017

Development of real-time polymerase chain reaction assays allowing molecular detection of Echinococcus felidis, Echinococcus granulosus sensu stricto, and Echinococcus canadensis in carnivore feces samples, animal and human hydatid cyst material from Uganda and Kenya

First evaluations on field samples, including carnivore feces, animal and human hydatid cyst material from Uganda and Kenya, showed specific amplification of two target regions of the mitochondrial genome of Echinococcus species according to melt and high-resolution melt curve analyses of the developed real-time PCR assays. Consecutive sequencing of PCR products revealed that, apart from Echinococcus felidis, sequences of two other tapeworm species, Echinococcus granulosus sensu stricto and Echinococcus canadensis, which are also endemic in East Africa, were detected by the developed real-time PCR assays.

molecular biology↗

bioRxiv2017

Impact of sequence variant detection and bacterial DNA extraction methods on the measurement of microbial community composition in human stool

BackgroundThe human gut microbiome has been widely studied in the context of human health and metabolism, however the question of how to analyze this community remains contentious. This study compares new and previously well established methods aimed at reducing bias in bioinformatics analysis (QIIME 1 and DADA2) and bacterial DNA extraction of human fecal samples in 16S rRNA marker gene surveys.\n\nResultsAnalysis of a mock DNA community using DADA2 identified more chimeras (QIIME 1: 0.70% of total reads vs DADA2: 1.96%), fewer sequence variants, (QIIME 1: 1297.4 + 98.88 vs. DADA2: 136.27 + 11.35, mean + SD) and correct taxa at a higher resolution of classification (i.e. genus-level) than open reference OTU picking in QIIME 1. Additionally, the extraction of whole cell mock community bacterial DNA using four commercially available kits resulted in varying DNA yield, quality and bacterial community composition. Of the four kits compared, ZymoBIOMICS DNA Miniprep Kit provided the greatest yield, with a slight enrichment of Enterococcus. However, QIAamp Fast DNA Stool Mini Kit resulted in the highest DNA quality. Mo Bio PowerFecal DNA Kit had the most dramatic effect on the mock community composition, resulting in an increased proportion of members of the family Enterobacteriaceae and genus Eshcerichia as well as members of genera Lactobacillus and Pseudomonas. The presence of a sterile fecal matrix had a slight, but inconsistent effect on the yield, quality and taxa identified after extraction with all four DNA extraction kits. Extraction of bacterial DNA from native stool samples revealed a distinct effect of the DNA stabilization reagent DNA/RNA Shield on community composition, causing an increase in the detected abundance of members of orders Bifidobacteriales, Bacteroidales, Turicibacterales, Clostridiales and Enterobacteriales.\n\nConclusionThese results confirm that the DADA2 algorithm is superior to sequence clustering by similarity to determine microbial community structure. Additionally, commercially available kits used for bacterial DNA extraction from fecal samples have some effect on the proportion of high abundance members detected in a microbial community, but it is less significant than the effect of using DNA stabilization reagent, DNA/RNA Shield.

molecular biology↗

bioRxiv2017

Near-Atomic Cryo-EM Imaging of a Small Protein Displayed on a Designed Scaffolding System

Current single particle electron cryo-microscopy (cryo-EM) techniques can produce images of large protein assemblies and macromolecular complexes at atomic level detail without the need for crystal growth. However, proteins of smaller size, typical of those found throughout the cell, are not presently amenable to detailed structural elucidation by cryo-EM. Here we use protein design to create a modular, symmetrical scaffolding system to make protein molecules of typical size amenable to cryo-EM. Using a rigid continuous alpha-helical linker, we connect a small 17 kDa protein (DARPin) to a protein subunit that was designed to self-assemble into a cage with cubic symmetry. We show that the resulting construct is amenable to structural analysis by single particle cryo-EM, allowing us to identify and solve the structure of the attached small protein at near-atomic detail, ranging from 3.5 to 5 [A] resolution. The result demonstrates that proteins considerably smaller than the theoretical limit of 50 kDa for cryo-EM can be visualized clearly when arrayed in a rigid fashion on a symmetric designed protein scaffold. Furthermore, because the amino acid sequence of a DARPin can be chosen to confer tight binding to various other protein or nucleic acid molecules, the system provides a future route for imaging diverse macromolecules, potentially broadening the application of cryoEM to proteins of typical size in the cell.\n\nSignificance statementNew electron microscopy methods are making it possible to view the structures of large proteins and nucleic acid complexes at atomic detail, but the methods are difficult to apply to molecules smaller than about 50 kDa, which is larger than the size of the average protein in the cell. The present work demonstrates that a protein much smaller than that limit can be successfully visualized when it is attached to a large protein scaffold designed to hold 12 copies of the attached protein in symmetric and rigidly defined orientations. The small protein chosen for attachment and visualization can be modified to bind to other diverse proteins, opening up a new avenue for imaging cellular proteins by cryo-EM.

molecular biology↗

bioRxiv2017

Post-catalytic spliceosome structure reveals mechanism of 3'-splice site selection

Introns are removed from eukaryotic mRNA precursors by the spliceosome in two transesterification reactions - branching and exon ligation. Following branching, the 5'-exon remains paired to U5 snRNA loop 1, but the mechanism of 3'-splice site recognition during exon ligation has remained unclear. Here we present the 3.7[A] cryo-EM structure of the yeast P complex spliceosome immediately after exon ligation. The 3'-splice site AG dinucleotide is recognised through non-Watson-Crick pairing with the 5'-splice site and the branch point adenosine. A conserved loop of Prp18 together with the -finger and the RNaseH domain of Prp8 clamp the docked 3'-splice site and 3'-exon. The step 2 factors Prp18 and Slu7 and the C-terminal domain of Yju2 stabilise a conformation competent for 3'-splice site docking and exon ligation. The structure accounts for the strict conservation of the GU and AG dinucleotides of the introns and provides insight into the catalytic mechanism of exon ligation.

molecular biology↗

bioRxiv2017

RNA virus interference via CRISPR/Cas13a system in plants

CRISPR/Cas systems confer immunity against invading nucleic acids and phages in bacteria and archaea. CRISPR/Cas13a (known previously as C2c2) is a class 2 type VI-A ribonuclease capable of targeting and cleaving single stranded RNA (ssRNA) molecules of the phage genome. Here, we employ CRISPR/Cas13a to engineer interference with an RNA virus, Turnip Mosaic Virus (TuMV), in plants. CRISPR/Cas13a produced interference against green fluorescent protein (GFP) expressing TuMV in transient assays and stable overexpression lines of Nicotiana benthamiana. crRNAs targeting the HC-Pro and GFP sequences exhibited better interference than those targeting other regions such as coat protein (CP) sequence. Cas13a can also process pre-crRNAs into functional crRNAs. Our data indicate that CRISPR/Cas13a can be used for engineering interference against RNA viruses, providing a potential novel mechanism for RNA-guided immunity against RNA viruses, and for other RNA manipulations in plants.

molecular biology↗

bioRxiv2017

Interplay of target site architecture and miRNA abundance determine miRNA activity and specificity

The recognition that the miRNA seed sequence is a major determinant of miRNA activity has greatly advanced the ability to predict miRNA targets. However, it has remained unclear to what extent miRNAs act redundantly when they are members of the same family and thus share a common seed. Using in vivo studies in C. elegans, we uncover features that drive specific target repression by individual miRNA family members. We find that seed-distal complementarity to a specific family member promotes specificity. However, the extent and robustness of specificity are greatly increased by seed match imperfections, such as bulges and G:U wobble base pairs. Depending on the seed match architecture, specificity may be overcome by increasing the levels of a miRNA lacking seed-distal complementarity. Hence, in contrast to a binary distinction between functional and non-functional target sites, our data support a model where functionality depends on a combination of target site quality and miRNA abundance. This emphasizes the importance of studying miRNAs under physiological conditions in their endogenous contexts.

molecular biology↗