Transcription Start Site Mapping Using Super-low Input Carrier-CAGE

Cap analysis of gene expression (CAGE) is a method used for single-nucleotide resolution detection of RNA polymerase II transcription start sites (TSSs). ...

Imperial College London

MRC London Institute of Medical Sciences

Isoleucyl-tRNA synthetase (IleRS) is unusual among aminoacyl-tRNA synthetases in having a tRNA-dependent pre-transfer editing activity. Alongside the typical bacterial IleRS (such as Escherichia coli IleRS), some bacteria also have the enzymes (eukaryote-like) that cluster with eukaryotic IleRSs and exhibit low sensitivity to the antibiotic mupirocin. Our phylogenetic analysis suggests that the ileS1 and ileS2 genes of contemporary bacteria are the descendants of genes that might have arisen by an ancient duplication event before the separation of bacteria and archaea. We present the analysis of evolutionary constraints of the synthetic and editing reactions in eukaryotic/eukaryote-like IleRSs, which share a common origin but diverged through adaptation to different cell environments. The enzyme from the yeast cytosol exhibits tRNA-dependent pre-transfer editing analogous to E. coli IleRS. This argues for the presence of this proofreading in the common ancestor of both IleRS types and an ancient origin of the synthetic site-based quality control step. Yet surprisingly, the eukaryote-like enzyme from Streptomyces griseus IleRS lacks this capacity; at the same time, its synthetic site displays the 10(3)-fold drop in sensitivity to antibiotic mupirocin relative to the yeast enzyme. The discovery that pre-transfer editing is optional in IleRSs lends support to the notion that the conserved post-transfer editing domain is the main checkpoint in these enzymes. We substantiated this by showing that under error-prone conditions S. griseus IleRS is able to rescue the growth of an E. coli lacking functional IleRS, providing the first evidence that tRNA-dependent pre-transfer editing in IleRS is not essential for cell viability. 

Naturally Occurring Isoleucyl-tRNA Synthetase without tRNA-dependent Pre-transfer Editing.

Core promoters across the genome.

Cap analysis of gene expression (CAGE) is a methodology for genome-wide quantitative mapping of mRNA 5' ends to precisely capture transcription start sites at a single nucleotide resolution. In combination with high-throughput sequencing, CAGE has revolutionized our understanding of the rules of transcription initiation, led to discovery of new core promoter sequence features, and discovered transcription initiation at enhancers genome-wide. The biggest limitation of CAGE is that even the most recently improved version (nAnT-iCAGE) still requires large amounts of total cellular RNA (5 µg), preventing its application to scarce biological samples such as those from early embryonic development or rare cell types. Here, we present SLIC-CAGE, a Super-Low Input Carrier-CAGE approach to capture 5' ends of RNA polymerase II transcripts from as little as 5-10 ng of total RNA. This dramatic increase in sensitivity is achieved by specially designed, selectively degradable carrier RNA. We demonstrate the ability of SLIC-CAGE to generate data for genome-wide promoterome with 1000-fold less material than required by existing CAGE methods, by generating a complex, high-quality library from mouse embryonic day 11.5 primordial germ cells.

SLIC-CAGE: high-resolution transcription start site mapping using nanogram-levels of total RNA.

One of the fundamental processes that determine cellular fate is regulation of gene transcription. Understanding these regulatory processes is therefore essential for understanding cellular responses to changes in environmental conditions. At the core promoter, the regulatory region containing the transcription start site (TSS), all inputs regulating transcription are integrated. Here, we used Cap Analysis of Gene Expression (CAGE) to analyze the pattern of TSSs at four different environmental conditions (limited in ethanol, limited in nitrogen, limited in glucose and limited in glucose under anaerobic conditions) using the Saccharomyces cerevisiae strain CEN.PK113-7D. With this experimental setup, we were able to show that the TSS landscape in yeast is stable at different metabolic states of the cell. We also show that the spatial distribution of transcription initiation events, described by the shape index, has a surprisingly strong negative correlation with measured gene expression levels, meaning that genes with higher expression levels tend to have a broader distribution of TSSs. Our analysis supplies a set of high-quality TSS annotations useful for metabolic engineering and synthetic biology approaches in the industrially relevant laboratory strain CEN.PK113-7D, and provides novel insights into yeast TSS dynamics and gene regulation.

Saccharomyces cerevisiae displays a stable transcription start site landscape in multiple conditions.

Aminoacyl-tRNA synthetases (aaRSs), the enzymes responsible for coupling tRNAs to their cognate amino acids, minimize translational errors by intrinsic hydrolytic editing. Here, we compared norvaline (Nva), a linear amino acid not coded for protein synthesis, to the proteinogenic, branched valine (Val) in their propensity to mistranslate isoleucine (Ile) in proteins. We show that in the synthetic site of isoleucyl-tRNA synthetase (IleRS), Nva and Val are activated and transferred to tRNA at similar rates. The efficiency of the synthetic site in pre-transfer editing of Nva and Val also appears to be similar. Post-transfer editing was, however, more rapid with Nva and consequently IleRS misaminoacylates Nva-tRNA at slower rate than Val-tRNA. Accordingly, an Escherichia coli strain lacking IleRS post-transfer editing misincorporated Nva and Val in the proteome to a similar extent and at the same Ile positions. However, Nva mistranslation inflicted higher toxicity than Val, in agreement with IleRS editing being optimized for hydrolysis of Nva-tRNA. Furthermore, we found that the evolutionary-related IleRS, leucyl- and valyl-tRNA synthetases (I/L/VRSs), all efficiently hydrolyze Nva-tRNAs even when editing of Nva seems redundant. We thus hypothesize that editing of Nva-tRNAs had already existed in the last common ancestor of I/L/VRSs, and that the editing domain of I/L/VRSs had primarily evolved to prevent infiltration of Nva into modern proteins.

On the Mechanism and Origin of Isoleucyl-tRNA Synthetase Editing against Norvaline.

CRISPR/Cas9 is a powerful genome-editing tool, but spurious off-target edits present a barrier to therapeutic applications. To understand how CRISPR/Cas9 discriminates between on-targets and off-targets, we have developed a single-molecule assay combining optical tweezers with fluorescence to monitor binding to λ-DNA. At low forces, the Streptococcus pyogenes Cas9 complex binds and cleaves DNA specifically. At higher forces, numerous off-target binding events appear repeatedly at the same off-target sites in a guide-RNA-sequence-dependent manner, driven by the mechanical distortion of the DNA. Using single-molecule Förster resonance energy transfer (smFRET) and cleavage assays, we show that DNA bubbles induce off-target binding and cleavage at these sites, even with ten mismatches, as well as at previously identified in vivo off-targets. We propose that duplex DNA destabilization during cellular processes (for example, transcription, replication, etc.) can expose these cryptic off-target sites to Cas9 activity, highlighting the need for improved off-target prediction algorithms.

Nevena Cvetesic

Institute of Clinical Sciences,
Faculty of Medicine,
Institute of Clinical Sciences, Faculty of Medicine

Imperial College London

Transcription Start Site Mapping Using Super-low Input Carrier-CAGE

Nevena Cvetesic

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Imperial College London

Transcription Start Site Mapping Using Super-low Input Carrier-CAGE

Institute of Clinical Sciences,
Faculty of Medicine,
Institute of Clinical Sciences, Faculty of Medicine