Supercomplex Research Articles

Abstract P3204: Right Ventricle Dysfunction Correlates With Reduced Mitochondrial Respiratory Supercomplex Assembly In Human Hearts With Group II Pulmonary Hypertension.

Introduction: RV dysfunction and failure is a common consequence of pressure overload associated with all types of pulmonary hypertension. Metabolic and mitochondrial structure and function alterations are further associated with RV dysfunction and failure. Mitochondrial respiratory supercomplexes (mSC) are physically associated assemblies of electron transport chain (ETC) complexes I, III, and IV which are thought to promote improved bioenergetic capacity and efficiency. We previously found reductions in mSC in animal models of PH. It is currently unknown if mSC are reduced in human right ventricle with PH and RV dysfunction. Methods: Failing hearts were harvested from patients at the time of transplant (n=20). Sample from the RV were frozen and later mitochondrial isolated and solubilized for Clear-native PAGE (CN-PAGE) to determine complex IV activity. Gels were normalized to Coomassie stained protein via far-red imaging. mSC activity was determined as amount of active CIV in the SC region (>1mD). Only patients with a recent right heart catheterization prior to transplant were used for correlation analysis. Pulmonary artery pulsatility index (PA systolic pressure-PA diastolic pressure/ right atrial pressure (PAPi)) and right atrial pressure to pulmonary capillary wedge pressure (RAP:PCWP) were used as indicators of pulmonary right ventricular dysfunction. Results: The majority of patients displayed high mPAP, however PH with RV dysfunction is associated with high mPAP and RAP:PSWP and low PAPi while high mPAP, low RAP:PSWP, and high PAPi indicate PH with normal or preserved RV dysfunction. SC activity as measured as CIV activity in SC region positively correlated with PAPI, (r=.6, P=.004) and negatively with RAP/PCWP (r= - .57, P=.009). There was no correlation with non-SC CIV activity as measured by CN-PAGE. Conclusion: The severity of RV dysfunction in humans with Group II PH is correlated with reduction in activity of mSC in RV mitochondria

Expanding the phenotypic and biochemical spectrum of NDUFAF3-related mitochondrial disease

Recessive variants in NDUFAF3 are a known cause of complex I (CI)-related mitochondrial disorders (MDs). The seven patients reported to date exhibited severe neurologic symptoms and lactic acidosis, followed by a fatal course and death during infancy in most cases.We present a 10-year-old patient with a neurodevelopmental disorder, progressive exercise intolerance, dystonia, basal ganglia abnormalities, and elevated lactate concentration in blood. Trio-exome sequencing revealed compound-heterozygosity for a pathogenic splice-site and a likely pathogenic missense variant in NDUFAF3. Spectrophotometric analysis of fibroblast-derived mitochondria demonstrated a relatively mild reduction of CI activity. Complexome analyses revealed severely reduced NDUFAF3 as well as CI in patient fibroblasts. Accumulation of early sub-assemblies of the membrane arm of CI associated with mitochondrial complex I intermediate assembly (MCIA) complex was observed. The most striking additional findings were both the unusual occurrence of free monomeric CI holding MCIA and other assembly factors.Here we discuss our patient in context of genotype, phenotype and metabolite data from previously reported NDUFAF3 cases. With the atypical presentation of our patient, we provide further insight into the phenotypic spectrum of NDUFAF3-related MDs. Complexome analysis in our patient confirms the previously defined role of NDUFAF3 within CI biogenesis, yet adds new aspects regarding the correct timing of both the association of soluble and membrane arm modules and CI-maturation as well as respiratory supercomplex formation.

Distinct roles of two SEC scaffold proteins, AFF1 and AFF4, in regulating RNA polymerase II transcription elongation.

The super elongation complex (SEC) containing positive transcription elongation factor b plays a critical role in regulating transcription elongation. AFF1 and AFF4, two members of the AF4/FMR2 family, act as central scaffold proteins of SEC and are associated with various human diseases. However, their precise roles in transcriptional control remain unclear. Here, we investigate differences in the genomic distribution patterns of AFF1 and AFF4 around transcription start sites (TSSs). AFF1 mainly binds upstream of the TSS, while AFF4 is enriched downstream of the TSS. Notably, disruption of AFF4 results in slow elongation and early termination in a subset of AFF4-bound active genes, whereas AFF1 deletion leads to fast elongation and transcriptional readthrough in the same subset of genes. Additionally, AFF1 knockdown increases AFF4 levels at chromatin, and vice versa. In summary, these findings demonstrate that AFF1 and AFF4 function antagonistically to regulate RNA polymerase II transcription.

AF9 targets acetyl-modified STAT6 to diminish purine metabolism and accelerate cell apoptosis during metastasis.

Cell migration and invasion are two important steps for tumour metastasis, and involved the behaviors including metabolism remodeling and anti-apoptosis. However, it's still elusive that cancer cells how to antagonize apoptosis during tumour metastasis. In this study, we observed that super elongation complex (SEC) subunit AF9 depletion exacerbated cell migration and invasion but reduced the apoptosis during invasive migration. Mechanically, AF9 targeted acetyl (Ac)-STAT6 at lysine (K) 284 and blocked STAT6 transactivation on the promoter of such genes involved in regulating purine metabolism and metastasis, in turn induced apoptosis of suspended cells. Of note, AcSTAT6-K284 was not induced by IL4 signaling but decreased by limited nutrition which triggered SIRT6 to remove acetyl group at STAT6-K284. The functional experiments proved that AcSTAT6-K284 attenuated cell migration and invasion depending on AF9 expression level. Animal metastatic study further confirmed the AF9/AcSTAT6-K284 axis existed and blocked kidney renal clear cell carcinoma (KIRC) metastasis. In clinical, both AF9 expression and AcSTAT6-K284 were decreased accompanied by the advanced tumour grade and positively correlated with KIRC patients' survival. Conclusively, we explored an inhibitory axis which not only suppressed tumour metastasis but also could be utilized for drug development to hamper KIRC metastasis.

The Scap-SREBP1-S1P/S2P lipogenesis signal orchestrates the homeostasis and spatiotemporal activation of NF-κB

The nuclear factor κB (NF-κB) pathway plays essential roles in innate and adaptive immunity, but little is known how NF-κB signaling is compartmentalized and spatiotemporally activated in the cytoplasm. Here, we show that the lipogenesis signal cascade Scap-SREBP1-S1P/S2P orchestrates the homeostasis and spatiotemporal activation of NF-κB. SREBP cleavage-activating protein (Scap) and sterol regulatory element-binding protein 1 (SREBP1) form a super complex with inhibitors of NF-κB α (IκBα) to associate NF-κB close to the endoplasmic reticulum (ER). Upon lipopolysaccharide (LPS) stimulation, Scap transports the complex to the Golgi apparatus, where SREBP1 is cleaved by site-1 protease (S1P)/S2P, liberating IκBα for IκB kinase (Ikk)-mediated phosphorylation and subsequent activation of NF-κB. Loss of Scap or inhibition of S1P or S2P diminishes, while SREBP1 deficiency augments, LPS-induced NF-κB activation and subsequent inflammatory responses. Our results reveal the Scap-SREBP1 complex as an additional cytoplasmic checkpoint for NF-κB homeostasis and unveil the Golgi apparatus as the optimal cellular platform for NF-κB activation, providing insights into the crosstalk between lipogenesis signaling and immunity.

SMYD1a protects the heart from ischemic injury by regulating OPA1-mediated cristae remodeling and supercomplex formation

SMYD1, a striated muscle-specific lysine methyltransferase, was originally shown to play a key role in embryonic cardiac development but more recently we demonstrated that loss of Smyd1 in the murine adult heart leads to cardiac hypertrophy and failure. However, the effects of SMYD1 overexpression in the heart and its molecular function in the cardiomyocyte in response to ischemic stress are unknown. In this study, we show that inducible, cardiomyocyte-specific overexpression of SMYD1a in mice protects the heart from ischemic injury as seen by a > 50% reduction in infarct size and decreased myocyte cell death. We also demonstrate that attenuated pathological remodeling is a result of enhanced mitochondrial respiration efficiency, which is driven by increased mitochondrial cristae formation and stabilization of respiratory chain supercomplexes within the cristae. These morphological changes occur concomitant with increased OPA1 expression, a known driver of cristae morphology and supercomplex formation. Together, these analyses identify OPA1 as a novel downstream target of SMYD1a whereby cardiomyocytes upregulate energy efficiency to dynamically adapt to the energy demands of the cell. In addition, these findings highlight a new epigenetic mechanism by which SMYD1a regulates mitochondrial energetics and functions to protect the heart from ischemic injury.

AF4/FMR2 and IL-10 gene single nucleotide polymorphisms are correlated with disease susceptibility and immune infiltration in ankylosing spondylitis

To explore the correlation of polymorphisms of AF4/FMR2 family genes and IL-10 gene with genetic susceptibility to ankylosing spondylitis (AS) and identify the high-risk factors of AS. This case-control study was conducted among 207 AS patients and 321 healthy individuals. The tag single nucleotide polymorphisms (SNPs) rs340630, rs241084, rs10865035, rs1698105, and rs1800896 of the AF4/FMR2 family gene and IL-10 gene of the AS patients were genotyped, and the distribution frequencies of the genotypes and alleles were analyzed to explore the relationship between different genetic models and AS and the gene-gene and gene-environment interactions. Gender ratio, smoking history, drinking history, hypertension, erythrocyte sedimentation rate and C-reactive protein differed significantly between the case group and the control group (P < 0.05). The dominant model and recessive model of AFF1 rs340630, the recessive model of AFF3 rs10865035, and the recessive model of IL-10 rs1800896 were significantly different between the two groups (P=0.031, 0.010, 0.031, and 0.019, respectively). Gene-environment interaction analysis suggested that the interaction model incorporating AFF1 rs340630, AFF2 rs241084, AFF3 rs10865035, AFF4 rs1698105, IL-10 rs1800896, smoking history and drinking history was the best model. The genes related with AF4/FMR2 and IL-10 were enriched in the biological processes of AF4 super extension complex, interleukin family signal transduction, cytokine stimulation and apoptosis. The expression levels of AF4/FMR2 and IL-10 were positively correlated with immune infiltration (r > 0). The SNPs of AF4/FMR2 and IL-10 genes are associated with the susceptibility to AS, and the interactions of AF4/FMR2 and IL-10 genes with the environmental factors contributes causes AS through immune infiltration.

Imidazopyridine Amides: Synthesis, Mycobacterium smegmatis CIII2CIV2 Supercomplex Binding, and In Vitro Antimycobacterial Activity.

Q203 (telacebec) is an imidazopyridine amide (IPA) targeting the respiratory CIII2CIV2 supercomplex of the mycobacterial electron transport chain (ETC). Aiming for a better understanding of the molecular mechanism of action of IPA, 27 analogues were prepared through a seven-step synthetic scheme. Oxygen consumption assay was designed to test the inhibition of purified Mycobacterium smegmatis CIII2CIV2 by these compounds. The assay results generally supported structure-activity relationship information obtained from the structure of M. smegmatis CIII2CIV2 bound to Q203. The IC50 of Q203 and compound 27 was 99 ± 32 and 441 ± 138 nM, respectively. All IPAs including Q203 showed no inhibition of mitochondrial ETC, proving their selectivity against mycobacteria. In vitro Mycobacterium tuberculosis growth inhibition and M. smegmatis CIII2CIV2 binding did not correlate perfectly. These observations suggest that further investigation into the mechanisms of resistance in different mycobacterial species is needed to understand the lack of the correlation pattern between CIII2CIV2 inhibition and cellular activity.

Structural insights into cardiolipin replacement by phosphatidylglycerol in a cardiolipin-lacking yeast respiratory supercomplex

Cardiolipin is a hallmark phospholipid of mitochondrial membranes. Despite established significance of cardiolipin in supporting respiratory supercomplex organization, a mechanistic understanding of this lipid-protein interaction is still lacking. To address the essential role of cardiolipin in supercomplex organization, we report cryo-EM structures of a wild type supercomplex (IV1III2IV1) and a supercomplex (III2IV1) isolated from a cardiolipin-lacking Saccharomyces cerevisiae mutant at 3.2-Å and 3.3-Å resolution, respectively, and demonstrate that phosphatidylglycerol in III2IV1 occupies similar positions as cardiolipin in IV1III2IV1. Lipid-protein interactions within these complexes differ, which conceivably underlies the reduced level of IV1III2IV1 and high levels of III2IV1 and free III2 and IV in mutant mitochondria. Here we show that anionic phospholipids interact with positive amino acids and appear to nucleate a phospholipid domain at the interface between the individual complexes, which dampen charge repulsion and further stabilize interaction, respectively, between individual complexes.

Noncanonical role of singleminded-2s in mitochondrial respiratory chain formation in breast cancer

Dysregulation of cellular metabolism is a hallmark of breast cancer progression and is associated with metastasis and therapeutic resistance. Here, we show that the breast tumor suppressor gene SIM2 promotes mitochondrial oxidative phosphorylation (OXPHOS) using breast cancer cell line models. Mechanistically, we found that SIM2s functions not as a transcription factor but localizes to mitochondria and directly interacts with the mitochondrial respiratory chain (MRC) to facilitate functional supercomplex (SC) formation. Loss of SIM2s expression disrupts SC formation through destabilization of MRC Complex III, leading to inhibition of electron transport, although Complex I (CI) activity is retained. A metabolomic analysis showed that knockout of SIM2s leads to a compensatory increase in ATP production through glycolysis and accelerated glutamine-driven TCA cycle production of NADH, creating a favorable environment for high cell proliferation. Our findings indicate that SIM2s is a novel stabilizing factor required for SC assembly, providing insight into the impact of the MRC on metabolic adaptation and breast cancer progression.

HIGD2A silencing impairs hepatocellular carcinoma growth via inhibiting mitochondrial function and the MAPK/ERK pathway

BackgroundThe Hypoxia inducible gene domain family member 2A (HIGD2A) protein is indispensable for the assembly of the mitochondrial respiratory supercomplex, which has been implicated in cell proliferation and cell survival under hypoxic conditions. Because the liver has a naturally low oxygen microenvironment, the role of HIGD2A in the development of hepatocellular carcinoma (HCC) remains largely unknown.MethodsGene expression data and clinical information were obtained from multiple public databases. A lentivirus-mediated gene knockdown approach was conducted to explore the function and mechanism of HIGD2A activity in HCC cells. In vivo and in vitro assays were performed to investigate the biological roles of HIGD2A.ResultsHIGD2A was overexpressed in HCC tissues and cell lines and was associated with a worse prognosis. Silencing HIGD2A expression significantly attenuated cell proliferation and migration, caused S-phase cell cycle arrest, and decreased tumor formation in nude mice. Mechanistically, HIGD2A depletion greatly decreased cellular ATP levels by disrupting mitochondrial ATP production. Moreover, HIGD2A knockdown cells displayed impaired mitochondrial function, such as mitochondrial fusion, increased expression of the mitochondrial stress response protein, and decreased oxygen consumption. Furthermore, knockdown of HIGD2A markedly attenuated the activation of the MAPK/ERK pathway.ConclusionsHIGD2A promoted liver cancer cell growth by fueling mitochondrial ATP synthesis and activating the MAPK/ERK pathway, suggested that targeting HIGD2A may represent a new strategy for HCC therapy.

Zebrafish ELL-associated factors Eaf1/2 modulate erythropoiesis via regulating gata1a expression and WNT signaling to facilitate hypoxia tolerance

EAF1 and EAF2, the eleven-nineteen lysine-rich leukemia (ELL)-associated factors which can assemble to the super elongation complex (AFF1/4, AF9/ENL, ELL, and P-TEFb), are reported to participate in RNA polymerase II to actively regulate a variety of biological processes, including leukemia and embryogenesis, but whether and how EAF1/2 function in hematopoietic system related hypoxia tolerance during embryogenesis remains unclear. Here, we unveiled that deletion of EAF1/2 (eaf1−/− and eaf2−/−) caused reduction in hypoxia tolerance in zebrafish, leading to reduced erythropoiesis during hematopoietic processes. Meanwhile, eaf1−/− and eaf2−/− mutants showed significant reduction in the expression of key transcriptional regulators scl, lmo2, and gata1a in erythropoiesis at both 24 h post fertilization (hpf) and 72 hpf, with gata1a downregulated while scl and lmo2 upregulated at 14 hpf. Mechanistically, eaf1−/− and eaf2−/− mutants exhibited significant changes in the expression of epigenetic modified histones, with a significant increase in the binding enrichment of modified histone H3K27me3 in gata1a promoter rather than scl and lmo2 promoters. Additionally, eaf1−/− and eaf2−/− mutants exhibited a dynamic expression of canonical WNT/β-catenin signaling during erythropoiesis, with significant reduction in p-β-Catenin level and in the binding enrichment of both scl and lmo2 promoters with the WNT transcriptional factor TCF4 at 24 hpf. These findings demonstrate an important role of Eaf1/2 in erythropoiesis in zebrafish and may have shed some light on regeneration medicine for anemia and related diseases and on molecular basis for fish economic or productive traits, such as growth, disease resistance, hypoxia tolerance, and so on.

Weight loss increases skeletal muscle mitochondrial energy efficiency in obese mice.

Weight loss from an overweight state is associated with a disproportionate decrease in whole-body energy expenditure that may contribute to the heightened risk for weight regain. Evidence suggests that this energetic mismatch originates from lean tissue. Although this phenomenon is well documented, the mechanisms have remained elusive. We hypothesized that increased mitochondrial energy efficiency in skeletal muscle is associated with reduced expenditure under weight loss. Wildtype (WT) male C57BL6/N mice were fed with high fat diet for 10 weeks, followed by a subset of mice that were maintained on the obesogenic diet (OB) or switched to standard chow to promote weight loss (WL) for additional 6 weeks. Mitochondrial energy efficiency was evaluated using high-resolution respirometry and fluorometry. Mass spectrometric analyses were employed to describe the mitochondrial proteome and lipidome. Weight loss promoted ~50% increase in the efficiency of oxidative phosphorylation (ATP produced per O2 consumed, or P/O) in skeletal muscle. However, weight loss did not appear to induce significant changes in mitochondrial proteome, nor any changes in respiratory supercomplex formation. Instead, it accelerated the remodeling of mitochondrial cardiolipin (CL) acyl-chains to increase tetralinoleoyl CL (TLCL) content, a species of lipids thought to be functionally critical for the respiratory enzymes. We further show that lowering TLCL by deleting the CL transacylase tafazzin was sufficient to reduce skeletal muscle P/O and protect mice from diet-induced weight gain. These findings implicate skeletal muscle mitochondrial efficiency as a novel mechanism by which weight loss reduces energy expenditure in obesity.

The super elongation complex drives transcriptional addiction in MYCN-amplified neuroblastoma.

MYCN amplification in neuroblastoma leads to aberrant expression of MYCN oncoprotein, which binds active genes promoting transcriptional amplification. Yet, how MYCN coordinates transcription elongation to meet productive transcriptional amplification and which elongation machinery represents MYCN-driven vulnerability remain to be identified. We conducted a targeted screen of transcription elongation factors and identified the super elongation complex (SEC) as a unique vulnerability in MYCN-amplified neuroblastomas. MYCN directly binds EAF1 and recruits SEC to enhance processive transcription elongation. Depletion of EAF1 or AFF1/AFF4, another core subunit of SEC, leads to a global reduction in transcription elongation and elicits selective apoptosis of MYCN-amplified neuroblastoma cells. A combination screen reveals SEC inhibition synergistically potentiates the therapeutic efficacies of FDA-approved BCL-2 antagonist ABT-199, in part due to suppression of MCL1 expression, both in MYCN-amplified neuroblastoma cells and in patient-derived xenografts. These findings identify disruption of the MYCN-SEC regulatory axis as a promising therapeutic strategy in neuroblastoma.

Assembly and functional role of mitochondrial respiratory supercomplex

Assembly and functional role of mitochondrial respiratory supercomplex

Loss of respiratory complex I subunit NDUFB10 affects complex I assembly and supercomplex formation.

The orchestrated activity of the mitochondrial respiratory or electron transport chain (ETC) and ATP synthase convert reduction power (NADH, FADH2) into ATP, the cell's energy currency in a process named oxidative phosphorylation (OXPHOS). Three out of the four ETC complexes are found in supramolecular assemblies: complex I, III, and IV form the respiratory supercomplexes (SC). The plasticity model suggests that SC formation is a form of adaptation to changing conditions such as energy supply, redox state, and stress. Complex I, the NADH-dehydrogenase, is part of the largest supercomplex (CI+CIII2+CIVn). Here, we demonstrate the role of NDUFB10, a subunit of the membrane arm of complex I, in complex I and supercomplex assembly on the one hand and bioenergetics function on the other. NDUFB10 knockout was correlated with a decrease of SCAF1, a supercomplex assembly factor, and a reduction of respiration and mitochondrial membrane potential. This likely is due to loss of proton pumping since the CI P P -module is downregulated and the P D -module is completely abolished in NDUFB10 knock outs.

On the geometry of forms on supermanifolds

This paper provides a rigorous account on the geometry of forms on supermanifolds, with a focus on its algebraic-geometric aspects. First, we introduce the de Rham complex of differential forms and we compute its cohomology. We then discuss three intrinsic definitions of the Berezinian sheaf of a supermanifold - as a quotient sheaf, via cohomology of the super Koszul complex or via cohomology of the total de Rham complex. Further, we study the properties of the Berezinian sheaf, showing in particular that it defines a right D-module. Then we introduce integral forms and their complex and we compute their cohomology, by providing a suitable Poincaré lemma. We show that the complexes of differential forms and integral forms are quasi-isomorphic and their cohomology computes the de Rham cohomology of the reduced space of the supermanifold. The notion of Berezin integral is then introduced and put to good use to prove the superanalog of Stokes' theorem and Poincaré duality, which relates differential and integral forms on supermanifolds. Finally, a different point of view is discussed by introducing the total tangent supermanifold and (integrable) pseudoforms in a new way. In this context, it is shown that a particular class of integrable pseudoforms having a distributional dependence supported at a point on the fibers are isomorphic to integral forms. Within the general overview, several new proofs of results are scattered.

BioEssays 4/2023

BioEssaysVolume 45, Issue 4 2370031 COVER PICTUREFree Access BioEssays 4/2023 First published: 16 March 2023 https://doi.org/10.1002/bies.202370031AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Mediator is a multi-protein complex with essential roles in transcriptional regulation. In article 2200178 Hidehisa Takahashi and colleagues take a closer look at one of the subunits of this complex, namely MED26. MED26 is involved in recruiting the super elongation complex (SEC) to polyadenylated genes and little elongation complex (LEC) to non-polyadenylated genes. LEC recruitment is accompanied by the association between two nuclear bodies: Cajal bodies and histone locus bodies. Based on these observations, the authors hypothesize that MED26-containing Mediator, via its disordered region, orchestrates multiple transcription processes through the organization of nuclear bodies. Volume45, Issue4April 20232370031 RelatedInformation

Structure, stability, and properties of cyclo[18]carbon-Zinc super sandwich complexes (C18-Zn-C18)

The unparalleled geometric and electronic structures along with the fascinating properties of cyclo[18]carbon, a new member in the carbon allotrope family, is found to be a riveting molecule. Density functional theory (DFT) calculations were accomplished to probe the structure, stability and properties of sandwich structures of 3d-transition series metal atoms with C18 rings. Results reveal that among all the 3d-transition metal atoms only the Zn atom forms a stable super sandwich-like structure with C18 (i.e., C18-Zn-C18). Ab initio molecular dynamic simulation was carried out to attain the comprehensive understanding of the stability of the C18-Zn-C18 complex. Result indicates that the non-covalent interaction plays a vital role in stabilizing the C18-Zn-C18 super sandwich complex which is further supported by atoms in molecules and extended transition state-natural orbital for chemical valence analysis. The Kohn–Sham band structure and density of state calculations were performed to investigate the periodic tube-like structures obtained using C18 and Zn. The calculated band gap using HSE06 in conjunction with TZP of Zn@C18 is found to be 0.49 eV.

MED26-containing Mediator may orchestrate multiple transcription processes through organization of nuclear bodies.

Mediator is a coregulatory complex that plays essential roles in multiple processes of transcription regulation. One of the human Mediator subunits, MED26, has a role in recruitment of the super elongation complex (SEC) to polyadenylated genes and little elongation complex (LEC) to non-polyadenylated genes, including small nuclear RNAs (snRNAs) and replication-dependent histone (RDH) genes. MED26-containing Mediator plays a role in 3' Pol II pausing at the proximal region of transcript end sites in RDH genes through recruitment of Cajal bodies (CBs) to histone locus bodies (HLBs). This finding suggests that Mediator is involved in the association of CBs with HLBs to facilitate 3' Pol II pausing and subsequent 3'-end processing by supplying 3'-end processing factors from CBs. Thus, we argue the possibility that Mediator is involved in the organization of nuclear bodies to orchestrate multiple processes of gene transcription.