High-Frequency Promoter Firing Links THO Complex Function to Heavy Chromatin Formation

A R-loop sensing pathway mediates the relocation of transcribed genes to nuclear pore complexes

Nuclear pore complexes (NPCs) have increasingly recognized interactions with the genome, as exemplified in yeast, where they bind transcribed or damaged chromatin. By combining genome-wide approaches with live imaging of model loci, we uncover a correlation between NPC association and the accumulation of R-loops, which are genotoxic structures formed through hybridization of nascent RNAs with their DNA templates. Manipulating hybrid formation demonstrates that R-loop accumulation per se, rather than transcription or R-loop-dependent damages, is the primary trigger for relocation to NPCs. Mechanistically, R-loop-dependent repositioning involves their recognition by the ssDNA-binding protein RPA, and SUMO-dependent interactions with NPC-associated factors. Preventing R-loop-dependent relocation leads to lethality in hybrid-accumulating conditions, while NPC tethering of a model hybrid-prone locus attenuates R-loop-dependent genetic instability. Remarkably, this relocation pathway involves molecular factors similar to those required for the association of stalled replication forks with NPCs, supporting the existence of convergent mechanisms for sensing transcriptional and genotoxic stresses.

The THO Complex as a Paradigm for the Prevention of Cotranscriptional R-Loops

Different proteins associate with the nascent RNA and the RNA polymerase (RNAP) to catalyze the transcription cycle and RNA export. If these processes are not properly controlled, the nascent RNA can thread back and hybridize to the DNA template forming R-loops capable of stalling replication, leading to DNA breaks. Given the transcriptional promiscuity of the genome, which leads to large amounts of RNAs from mRNAs to different types of ncRNAs, these can become a major threat to genome integrity if they form R-loops. Consequently, cells have evolved nuclear factors to prevent this phenomenon that includes THO, a conserved eukaryotic complex acting in transcription elongation and RNA processing and export that upon inactivation causes genome instability linked to R-loop accumulation. We revise and discuss here the biological relevance of THO and a number of RNA helicases, including the THO partner UAP56/DDX39B, as a paradigm of the cellular mechanisms of cotranscriptional R-loop prevention.

Colla corii asini might upregulate ZNF471 and THOC5 by KRAB domain-containing zinc-finger protein pathway and THO complex subunit 5 pathway to improve anemia of pregnant women with β-thalassemia

Pregnant patients with β-thalassemia are more likely to have progressive anemia which expose them to risk of adverse pregnancy outcomes, blood transfusion, and iron overload. Results from our previous study indicated that Colla corii asini (CCA, E'jiao), a natural ingredient of traditional Chinese medicine, could significantly increase hemoglobin level of pregnant women with β- thalassemia, but the underlying molecular mechanism was unclear. Thus, we applied high-throughput transcriptome sequencing to study the transcriptomic change before and after the CCA treatment. Twenty eligible pregnant women were recruited and randomized to either the CCA treatment group or the blank control group in a 3:1 ratio. Patients in the treatment group orally received daily 15 g CCA powder for 4 weeks. We analyzed the therapeutic effect indexes and the transcriptomic change in subjects' peripheral blood before and after treatment. We found that β CD 41-42(-TTCT)/β^A was the main genotype of the subjects. The regulatory impact of CCA treatment became more evident among the subjects of genotype β CD 41-42(-TTCT)/β^A. Gene ontogenesis analysis revealed that the top five molecular functions of differentially expressed genes were involved in membrane functionality and cellular structure. We further identified two consistent upregulated genes ZNF471 and THOC5 in the effective treatment group, which were engaged in Kruppel-associated box (KRAB) domain-containing zinc-finger protein pathway and THOC5 pathway, respectively. Based on our current findings, we hypothesize that the anti-anemia effect of CCA on pregnant women with β-thalassemia might be related to translation regulation of spectrin synthesis, membrane stability, and eventually prolonged the life span of erythrocytes.

Writing a wrong: Coupled RNA polymerase II transcription and RNA quality control

Processing and maturation of precursor RNA species is coupled to RNA polymerase II transcription. Co-transcriptional RNA processing helps to ensure efficient and proper capping, splicing, and 3' end processing of different RNA species to help ensure quality control of the transcriptome. Many improperly processed transcripts are not exported from the nucleus, are restricted to the site of transcription, and are in some cases degraded, which helps to limit any possibility of aberrant RNA causing harm to cellular health. These critical quality control pathways are regulated by the highly dynamic protein-protein interaction network at the site of transcription. Recent work has further revealed the extent to which the processes of transcription and RNA processing and quality control are integrated, and how critically their coupling relies upon the dynamic protein interactions that take place co-transcriptionally. This review focuses specifically on the intricate balance between 3' end processing and RNA decay during transcription termination. This article is categorized under: RNA Turnover and Surveillance > Turnover/Surveillance Mechanisms RNA Processing > 3' End Processing RNA Processing > Splicing Mechanisms RNA Processing > Capping and 5' End Modifications.

Pervasive transcription fine-tunes replication origin activity

RNA polymerase (RNAPII) transcription occurs pervasively, raising the important question of its functional impact on other DNA-associated processes, including replication. In budding yeast, replication originates from Autonomously Replicating Sequences (ARSs), generally located in intergenic regions. The influence of transcription on ARSs function has been studied for decades, but these earlier studies have neglected the role of non-annotated transcription. We studied the relationships between pervasive transcription and replication origin activity using high-resolution transcription maps. We show that ARSs alter the pervasive transcription landscape by pausing and terminating neighboring RNAPII transcription, thus limiting the occurrence of pervasive transcription within origins. We propose that quasi-symmetrical binding of the ORC complex to ARS borders and/or pre-RC formation are responsible for pausing and termination. We show that low, physiological levels of pervasive transcription impact the function of replication origins. Overall, our results have important implications for understanding the impact of genomic location on origin function.

Condensin Depletion Causes Genome Decompaction Without Altering the Level of Global Gene Expression in Saccharomyces cerevisiae

Condensins are broadly conserved chromosome organizers that function in chromatin compaction and transcriptional regulation, but to what extent these two functions are linked has remained unclear. Here, we analyzed the effect of condensin inactivation on genome compaction and global gene expression in the yeast Saccharomyces cerevisiae by performing spike-in-controlled genome-wide chromosome conformation capture (3C-seq) and mRNA-sequencing analysis. 3C-seq analysis shows that acute condensin inactivation leads to a global decrease in close-range intrachromosomal interactions as well as more specific losses of interchromosomal tRNA gene clustering. In addition, a condensin-rich interaction domain between the ribosomal DNA and the centromere on chromosome XII is lost upon condensin inactivation. Unexpectedly, these large-scale changes in chromosome architecture are not associated with global changes in mRNA levels. Our data suggest that the global transcriptional program of proliferating S. cerevisiae is resistant to condensin inactivation and the associated profound changes in genome organization.

Intron or no intron: a matter for nuclear pore complexes

Nuclear pore complexes (NPCs) have been shown to regulate distinct steps of the gene expression process, from transcription to mRNA export. In particular, mRNAs expressed from intron-containing genes are surveyed by a specific NPC-dependent quality control pathway ensuring that unspliced mRNAs are retained within the nucleus. In this Extra View, we summarize the different approaches that have been developed to evaluate the contribution of various NPC components to the expression of intron-containing genes. We further present the mechanistic models that could account for pre-mRNA retention at the nuclear side of NPCs. Finally, we discuss the possibility that other stages of intron-containing gene expression could be regulated by nuclear pores, in particular through the regulation of mRNA biogenesis factors by the NPC-associated SUMO protease Ulp1.

Nuclear pore components affect distinct stages of intron-containing gene expression

Several nuclear pore-associated factors, including the SUMO-protease Ulp1, have been proposed to prevent the export of intron-containing messenger ribonucleoparticles (mRNPs) in yeast. However, the molecular mechanisms of this nuclear pore-dependent mRNA quality control, including the sumoylated targets of Ulp1, have remained unidentified. Here, we demonstrate that the apparent 'pre-mRNA leakage' phenotype arising upon ULP1 inactivation is shared by sumoylation mutants of the THO complex, an early mRNP biogenesis factor. Importantly, we establish that alteration of THO complex activity differentially impairs the expression of intronless and intron-containing reporter genes, rather than triggering bona fide 'pre-mRNA leakage'. Indeed, we show that the presence of introns within THO target genes attenuates the effect of THO inactivation on their transcription. Epistasis analyses further clarify that different nuclear pore components influence intron-containing gene expression at distinct stages. Ulp1, whose maintenance at nuclear pores depends on the Nup84 complex, impacts on THO-dependent gene expression, whereas the nuclear basket-associated Mlp1/Pml39 proteins prevent pre-mRNA export at a later stage, contributing to mRNA quality control. Our study thus highlights the multiplicity of mechanisms by which nuclear pores contribute to gene expression, and further provides the first evidence that intronic sequences can alleviate early mRNP biogenesis defects.

Transcription factors modulate c-Fos transcriptional bursts

Transcription is a stochastic process occurring mostly in episodic bursts. Although the local chromatin environment is known to influence the bursting behavior on long timescales, the impact of transcription factors (TFs)--especially in rapidly inducible systems--is largely unknown. Using fluorescence in situ hybridization and computational models, we quantified the transcriptional activity of the proto-oncogene c-Fos with single mRNA accuracy at individual endogenous alleles. We showed that, during MAPK induction, the TF concentration modulates the burst frequency of c-Fos, whereas other bursting parameters remain mostly unchanged. By using synthetic TFs with TALE DNA-binding domains, we systematically altered different aspects of these bursts. Specifically, we linked the polymerase initiation frequency to the strength of the transactivation domain and the burst duration to the TF lifetime on the promoter. Our results show how TFs and promoter binding domains collectively act to regulate different bursting parameters, offering a vast, evolutionarily tunable regulatory range for individual genes.

Sumoylation of the THO complex regulates the biogenesis of a subset of mRNPs

Assembly of messenger ribonucleoparticles (mRNPs) is a pivotal step in gene expression, but only a few molecular mechanisms contributing to its regulation have been described. Here, through a comprehensive proteomic survey of mRNP assembly, we demonstrate that the SUMO pathway specifically controls the association of the THO complex with mRNPs. We further show that the THO complex, a key player in the interplay between gene expression, mRNA export and genetic stability, is sumoylated on its Hpr1 subunit and that this modification regulates its association with mRNPs. Altered recruitment of the THO complex onto mRNPs in sumoylation-defective mutants does not affect bulk mRNA export or genetic stability, but impairs the expression of acidic stress-induced genes and, consistently, compromises viability in acidic stress conditions. Importantly, inactivation of the nuclear exosome suppresses the phenotypes of the hpr1 non-sumoylatable mutant, showing that SUMO-dependent mRNP assembly is critical to allow a specific subset of mRNPs to escape degradation. This article thus provides the first example of a SUMO-dependent mRNP-assembly event allowing a refined tuning of gene expression, in particular under specific stress conditions.

THOC5, a member of the mRNA export complex: a novel link between mRNA export machinery and signal transduction pathways in cell proliferation and differentiation

Cell growth, differentiation, and commitment to a restricted lineage are guided by a timely expressed set of growth factor/cytokine receptors and their down-stream transcription factor genes. Transcriptional control mechanisms of gene expression during differentiation have been mainly studied by focusing on the cis- and trans-elements in promoters however, the role of mRNA export machinery during differentiation has not been adequately examined. THO (Suppressors of the transcriptional defects of hpr1 delta by overexpression) complex 5 (THOC5) is a member of THO complex which is a subcomplex of the transcription/export complex (TREX). THOC5 is evolutionarily conserved in higher eukaryotes, however the exact roles of THOC5 in transcription and mRNA export are still unclear. In this review, we focus on recently uncovered aspects of the role of THOC5 in signal transduction induced by extracellular stimuli. THOC5 is phosphorylated by several protein kinases at multiple residues upon extracellular stimuli. These include stimulation with growth factors/cytokines/chemokines, or DNA damage reagents. Furthermore, THOC5 is a substrate for several oncogenic tyrosine kinases, suggesting that THOC5 may be involved in cancer development. Recent THOC5 knockout mouse data reveal that THOC5 is an essential element in the maintenance of stem cells and growth factor/cytokine-mediated differentiation/proliferation. Furthermore, depletion of THOC5 influences less than 1% of total mRNA export in the steady state, however it influences more than 90% of growth factor/cytokine induced genes. THOC5, thereby contributes to the 3' processing and/or export of immediate-early genes induced by extracellular stimuli. These studies bring new insight into the link between the mRNA export complex and immediate-early gene response. The data from these studies also suggest that THOC5 may be a useful tool for studying stem cell biology, for modifying the differentiation processes and for cancer therapy.