The chromatin scaffold protein SAFB1 localizes SUMO-1 to the promoters of ribosomal protein genes to facilitate transcription initiation and splicing

SUMOylation is enriched in the nuclear matrix and required for chromosome segregation

As an important posttranslational modification, SUMOylation plays critical roles in almost all biological processes. Although it has been well-documented that SUMOylated proteins are mainly localized in the nucleus and have roles in chromatin-related processes, we showed recently that the SUMOylation machinery is actually enriched in the nuclear matrix rather than chromatin. Here, we provide compelling biochemical, cellular imaging and proteomic evidence that SUMOylated proteins are highly enriched in the nuclear matrix. We demonstrated that inactivation of SUMOylation by inhibiting SUMO-activating E1 enzyme or KO of SUMO-conjugating E2 enzyme UBC9 have only mild effect on nuclear matrix composition, indicating that SUMOylation is neither required for nuclear matrix formation nor for targeting proteins to nuclear matrix. Further characterization of UBC9 KO cells revealed that loss of SUMOylation did not result in significant DNA damage, but led to mitotic arrest and chromosome missegregation. Altogether, our study demonstrates that SUMOylated proteins are selectively enriched in the nuclear matrix and suggests a role of nuclear matrix in mediating SUMOylation and its regulated biological processes.

SUMOylation regulates pre-mRNA splicing to overcome DNA damage in fungi

Pathogenic fungi are subject to DNA damage stress derived from host immune responses during infection. Small ubiquitin-like modifier (SUMO) modification and precursor (pre)-mRNA splicing are both involved in DNA damage response (DDR). However, the mechanisms of how SUMOylation and splicing coordinated in DDR remain largely unknown. Combining with biochemical analysis, RNA-Seq method, and biological analysis, we report that SUMO pathway participates in DDR and virulence in Fusarium graminearum, a causal agent of Fusarium head blight of cereal crops world-wide. Interestingly, a key transcription factor FgSR is SUMOylated upon DNA damage stress. SUMOylation regulates FgSR nuclear-cytoplasmic partitioning and its phosphorylation by FgMec1, and promotes its interaction with chromatin remodeling complex SWI/SNF for activating the expression of DDR-related genes. Moreover, the SWI/SNF complex was found to further recruit splicing-related NineTeen Complex, subsequently modulates pre-mRNA splicing during DDR. Our findings reveal a novel function of SUMOylation in DDR by regulating a transcription factor to orchestrate gene expression and pre-mRNA splicing to overcome DNA damage during the infection of F. graminearum, which advances the understanding of the delicate regulation of DDR by SUMOylation in pathogenic fungi, and extends the knowledge of cooperation of SUMOylation and pre-mRNA splicing in DDR in eukaryotes.

UHMK1 is a novel splicing regulatory kinase

The U2AF Homology Motif Kinase 1 (UHMK1) is the only kinase that contains the U2AF homology motif, a common protein interaction domain among splicing factors. Through this motif, UHMK1 interacts with the splicing factors SF1 and SF3B1, known to participate in the 3' splice site recognition during the early steps of spliceosome assembly. Although UHMK1 phosphorylates these splicing factors in vitro, the involvement of UHMK1 in RNA processing has not previously been demonstrated. Here, we identify novel putative substrates of this kinase and evaluate UHMK1 contribution to overall gene expression and splicing, by integrating global phosphoproteomics, RNA-seq, and bioinformatics approaches. Upon UHMK1 modulation, 163 unique phosphosites were differentially phosphorylated in 117 proteins, of which 106 are novel potential substrates of this kinase. Gene Ontology analysis showed enrichment of terms previously associated with UHMK1 function, such as mRNA splicing, cell cycle, cell division, and microtubule organization. The majority of the annotated RNA-related proteins are components of the spliceosome but are also involved in several steps of gene expression. Comprehensive analysis of splicing showed that UHMK1 affected over 270 alternative splicing events. Moreover, splicing reporter assay further supported UHMK1 function on splicing. Overall, RNA-seq data demonstrated that UHMK1 knockdown had a minor impact on transcript expression and pointed to UHMK1 function in epithelial-mesenchymal transition. Functional assays demonstrated that UHMK1 modulation affects proliferation, colony formation, and migration. Taken together, our data implicate UHMK1 as a splicing regulatory kinase, connecting protein regulation through phosphorylation and gene expression in key cellular processes.

Regulation of SUMOylation on RNA metabolism in cancers

Post-translational modifications of proteins play very important roles in regulating RNA metabolism and affect many biological pathways. Here we mainly summarize the crucial functions of small ubiquitin-like modifier (SUMO) modification in RNA metabolism including transcription, splicing, tailing, stability and modification, as well as its impact on the biogenesis and function of microRNA (miRNA) in particular. This review also highlights the current knowledge about SUMOylation regulation in RNA metabolism involved in many cellular processes such as cell proliferation and apoptosis, which is closely related to tumorigenesis and cancer progression.

hucMSC-Derived Exosomes Alleviate the Deterioration of Colitis via the miR-146a/SUMO1 Axis

Human umbilical cord mesenchymal stem cell-derived exosome (hucMSC-Ex) plays an important role in tissue repair and immunomodulation, leading to the mitigation of inflammatory bowel disease. However, the preventive function of hucMSC-Ex in the onset and progression of colitis-associated colon cancer (CAC) is poorly understood. In the current study, dextran sodium sulfate/azoxymethane-induced colitis mouse model was established, and the mice disease activity index, body weight, colon length, tumor counts, survival curve, tissue H&E/immunohistochemistry, and cytokines expression were analyzed to evaluate the effects of hucMSC-Ex on CAC. In addition, miR-146a mimics were transfected into colonic epithelial cells (fetal human cells) to evaluate their role in the hucMSC-Ex-mediated regulation of SUMO1. The results showed that hucMSC-Ex inhibits the expression of SUMO1 to reduce the process of CAC progression. Further analysis indicated that miR-146a targets and inhibits SUMO1 expression and its binding to β-catenin. In conclusion, our findings showed that hucMSC-Ex is effective in alleviating the deterioration of colitis via the miR-146a-mediated inhibition of SUMO1, which is crucial in this disease process.

SUMO Modification of OsFKBP20-1b Is Integral to Proper Pre-mRNA Splicing upon Heat Stress in Rice

OsFKBP20-1b, a plant-specific cyclophilin protein, has been implicated to regulate pre-mRNA splicing under stress conditions in rice. Here, we demonstrated that OsFKBP20-1b is SUMOylated in a reconstituted SUMOylation system in E.coli and in planta, and that the SUMOylation-coupled regulation was associated with enhanced protein stability using a less SUMOylated OsFKBP20-1b mutant (5KR_OsFKBP20-1b). Furthermore, OsFKBP20-1b directly interacted with OsSUMO1 and OsSUMO2 in the nucleus and cytoplasm, whereas the less SUMOylated 5KR_OsFKBP20-1b mutant had an impaired interaction with OsSUMO1 and 2 in the cytoplasm but not in the nucleus. Under heat stress, the abundance of an OsFKBP20-1b-GFP fusion protein was substantially increased in the nuclear speckles and cytoplasmic foci, whereas the heat-responsiveness was remarkably diminished in the presence of the less SUMOylated 5KR_OsFKBP20-1b-GFP mutant. The accumulation of endogenous SUMOylated OsFKBP20-1b was enhanced by heat stress in planta. Moreover, 5KR_OsFKBP20-1b was not sufficiently associated with the U snRNAs in the nucleus as a spliceosome component. A protoplast transfection assay indicated that the low SUMOylation level of 5KR_OsFKBP20-1b led to inaccurate alternative splicing and transcription under heat stress. Thus, our results suggest that OsFKBP20-1b is post-translationally regulated by SUMOylation, and the modification is crucial for proper RNA processing in response to heat stress in rice.

Regulation of cold-induced thermogenesis by the RNA binding protein FAM195A

Homeothermic vertebrates produce heat in cold environments through thermogenesis, in which brown adipose tissue (BAT) increases mitochondrial oxidation along with uncoupling of the electron transport chain and activation of uncoupling protein 1 (UCP1). Although the transcription factors regulating the expression of UCP1 and nutrient oxidation genes have been extensively studied, only a few other proteins essential for BAT function have been identified. We describe the discovery of FAM195A, a BAT-enriched RNA binding protein, which is required for cold-dependent thermogenesis in mice. FAM195A knockout (KO) mice display whitening of BAT and an inability to thermoregulate. In BAT of FAM195A KO mice, enzymes involved in branched-chain amino acid (BCAA) metabolism are down-regulated, impairing their response to cold. Knockdown of FAM195A in brown adipocytes in vitro also impairs expression of leucine oxidation enzymes, revealing FAM195A to be a regulator of BCAA metabolism and a potential target for metabolic disorders.

SAIL: a new conserved anti-fibrotic lncRNA in the heart

Long non-coding RNAs (lncRNAs) account for a large proportion of genomic transcripts and are critical regulators in various cardiac diseases. Though lncRNAs have been reported to participate in the process of diverse cardiac diseases, the contribution of lncRNAs in cardiac fibrosis remains to be fully elucidated. Here, we identified a novel anti-fibrotic lncRNA, SAIL (scaffold attachment factor B interacting lncRNA). SAIL was reduced in cardiac fibrotic tissue and activated cardiac fibroblasts. Gain- and loss-of-function studies showed that knockdown of SAIL promoted proliferation and collagen production of cardiac fibroblasts with or without TGF-β1 (transforming growth factor beta1) treatment, while overexpression of SAIL did the opposite. In mouse cardiac fibrosis induced by myocardial infarction, knockdown of SAIL exacerbated, whereas overexpression of SAIL alleviated cardiac fibrosis. Mechanically, SAIL inhibited the fibrotic process by directly binding with SAFB via 23 conserved nucleotide sequences, which in turn blocked the access of SAFB to RNA pol II (RNA polymerase II) and reduced the transcription of fibrosis-related genes. Intriguingly, the human conserved fragment of SAIL (hSAIL) significantly suppressed the proliferation and collagen production of human cardiac fibroblasts. Our findings demonstrate that SAIL regulates cardiac fibrosis by regulating SAFB-mediated transcription of fibrotic related genes. Both SAIL and SAFB hold the potential to become novel therapeutic targets for cardiac fibrosis.

SUMO and Transcriptional Regulation: The Lessons of Large-Scale Proteomic, Modifomic and Genomic Studies

One major role of the eukaryotic peptidic post-translational modifier SUMO in the cell is transcriptional control. This occurs via modification of virtually all classes of transcriptional actors, which include transcription factors, transcriptional coregulators, diverse chromatin components, as well as Pol I-, Pol II- and Pol III transcriptional machineries and their regulators. For many years, the role of SUMOylation has essentially been studied on individual proteins, or small groups of proteins, principally dealing with Pol II-mediated transcription. This provided only a fragmentary view of how SUMOylation controls transcription. The recent advent of large-scale proteomic, modifomic and genomic studies has however considerably refined our perception of the part played by SUMO in gene expression control. We review here these developments and the new concepts they are at the origin of, together with the limitations of our knowledge. How they illuminate the SUMO-dependent transcriptional mechanisms that have been characterized thus far and how they impact our view of SUMO-dependent chromatin organization are also considered.

Chemical Decorations of "MARs" Residents in Orchestrating Eukaryotic Gene Regulation

Genome organization plays a crucial role in gene regulation, orchestrating multiple cellular functions. A meshwork of proteins constituting a three-dimensional (3D) matrix helps in maintaining the genomic architecture. Sequences of DNA that are involved in tethering the chromatin to the matrix are called scaffold/matrix attachment regions (S/MARs), and the proteins that bind to these sequences and mediate tethering are termed S/MAR-binding proteins (S/MARBPs). The regulation of S/MARBPs is important for cellular functions and is altered under different conditions. Limited information is available presently to understand the structure–function relationship conclusively. Although all S/MARBPs bind to DNA, their context- and tissue-specific regulatory roles cannot be justified solely based on the available information on their structures. Conformational changes in a protein lead to changes in protein–protein interactions (PPIs) that essentially would regulate functional outcomes. A well-studied form of protein regulation is post-translational modification (PTM). It involves disulfide bond formation, cleavage of precursor proteins, and addition or removal of low-molecular-weight groups, leading to modifications like phosphorylation, methylation, SUMOylation, acetylation, PARylation, and ubiquitination. These chemical modifications lead to varied functional outcomes by mechanisms like modifying DNA–protein interactions and PPIs, altering protein function, stability, and crosstalk with other PTMs regulating subcellular localizations. S/MARBPs are reported to be regulated by PTMs, thereby contributing to gene regulation. In this review, we discuss the current understanding, scope, disease implications, and future perspectives of the diverse PTMs regulating functions of S/MARBPs.

Abnormal scaffold attachment factor 1 expression and localization in spinocerebellar ataxias and Huntington's chorea

SAFB1 is a DNA and RNA binding protein that is highly expressed in the cerebellum and hippocampus and is involved in the processing of coding and non-coding RNAs, splicing and dendritic function. We analyzed SAFB1 expression in the post-mortem brain tissue of spinocerebellar ataxia (SCA), Huntington's disease (HD), Multiple sclerosis (MS), Parkinson's disease patients and controls. In SCA cases, the expression of SAFB1 in the nucleus was increased and there was abnormal and extensive expression in the cytoplasm where it co-localized with the markers of Purkinje cell injury. Significantly, no SAFB1 expression was found in the cerebellar neurons of the dentate nucleus in control or MS patients; however, in SCA patients, SAFB1 expression was increased significantly in both the nucleus and cytoplasm of dentate neurons. In HD, we found that SAFB1 expression was increased in the nucleus and cytoplasm of striatal neurons; however, there was no SAFB1 staining in the striatal neurons of controls. In PD substantia nigra, we did not see any changes in neuronal SAFB1 expression. iCLIP analysis found that SAFB1 crosslink sites within ATXN1 RNA were adjacent to the start and within the glutamine repeat sequence. Further investigation found increased binding of SAFB1 to pathogenic ATXN1-85Q mRNA. These novel data strongly suggest SAFB1 contributes to the etiology of SCA and Huntington's chorea and that it may be a pathological marker of polyglutamine repeat expansion diseases.

SAFB2 Enables the Processing of Suboptimal Stem-Loop Structures in Clustered Primary miRNA Transcripts

Many microRNAs (miRNAs) are generated from primary transcripts containing multiple clustered stem-loop structures that are thought to be recognized and cleaved by the Microprocessor complex as independent units. Here, we uncover an unexpected mode of processing of the bicistronic miR-15a-16-1 cluster. We find that the primary miR-15a stem-loop is not processed on its own but that the presence of the neighboring primary miR-16-1 stem-loop on the same transcript can compensate for this deficiency in cis. Using a CRISPR/Cas9 screen, we identify SAFB2 (scaffold attachment factor B2) as an essential co-factor in this miR-16-1-assisted pri-miR-15 cleavage and describe SAFB2 as an accessory protein of the Microprocessor. Notably, SAFB2-mediated cleavage expands to other clustered pri-miRNAs, indicating a general mechanism. Together, our study reveals an unrecognized function of SAFB2 in miRNA processing and suggests a scenario in which SAFB2 enables the binding and processing of suboptimal Microprocessor substrates in clustered primary miRNA transcripts.

Scaffold attachment factor B suppresses HIV-1 infection of CD4+ T cells by preventing binding of RNA polymerase II to HIV-1's long terminal repeat

The 5' end of the HIV, type 1 (HIV-1) long terminal repeat (LTR) promoter plays an essential role in driving viral transcription and productive infection. Multiple host and viral factors regulate LTR activity and modulate HIV-1 latency. Manipulation of the HIV-1 LTR provides a potential therapeutic strategy for combating HIV-1 persistence. In this study, we identified an RNA/DNA-binding protein, scaffold attachment factor B (SAFB1), as a host cell factor that represses HIV-1 transcription. We found that SAFB1 bound to the HIV-1 5' LTR and significantly repressed 5' LTR-driven viral transcription and HIV-1 infection of CD4⁺ T cells. Mechanistically, SAFB1-mediated repression of HIV-1 transcription and infection was independent of its RNA- and DNA-binding capacities. Instead, by binding to phosphorylated RNA polymerase II, SAFB1 blocked its recruitment to the HIV-1 LTR. Of note, SAFB1-mediated repression of HIV-1 transcription from proviral DNA maintained HIV-1 latency in CD4⁺ T cells. In summary, our findings reveal that SAFB1 binds to the HIV-1 LTR and physically interacts with phosphorylated RNA polymerase II, repressing HIV-1 transcription initiation and elongation. Our findings improve our understanding of host modulation of HIV-1 transcription and latency and provide a new host cell target for improved anti-HIV-1 therapies.

Cervical cancer stem-like cells: systematic review and identification of reference genes for gene expression

Cervical cancer is the fourth most common cancer affecting women worldwide. Among many factors, the presence of cancer stem cells, a subpopulation of cells inside the tumor, has been associated with a worse prognosis. Considering the importance of gene expression studies to understand the biology of cervical cancer stem cells (CCSC), this work identifies stable reference genes for cervical cancer cell lines SiHa, HeLa, and ME180 as well as their respective cancer stem-like cells. A literature review was performed to identify validated reference genes currently used to normalize RT-qPCR data in cervical cancer cell lines. Then, cell lines were cultured in regular monolayer or in a condition that favors tumor sphere formation. RT-qPCR was performed using five reference genes: ACTB, B2M, GAPDH, HPRT1, and TBP. Stability was assessed to validate the selected genes as suitable reference genes. The evaluation validated B2M, GAPDH, HPRT1, and TBP in these experimental conditions. Among them, GAPDH and TBP presented the lowest variability according to the analysis by Normfinder, Bestkeeper, and ΔCq methods, being therefore the most adequate genes to normalize the combination of all samples. These results suggest that B2M, GAPDH, HPRT1, and TBP are suitable reference genes to normalize RT-qPCR data of established cervical cancer cell lines SiHa, HeLa, and ME180 as well as their derived cancer stem-like cells. Indeed, GAPDH and TBP seem to be the most convenient choices for studying gene expression in these cells in monolayers or spheres.

The Regulation of Chromatin by Dynamic SUMO Modifications

Protein modification by the small ubiquitin-related modifier (SUMO) protein regulates numerous cellular pathways and mounting evidence reveals a critical role for SUMO in modulating gene expression. Dynamic sumoylation of transcription factors, chromatin-modifying enzymes, histones, and other chromatin-associated factors significantly affects the transcriptional status of the eukaryotic genome. Recent studies have employed high-throughput ChIP-Seq analyses to gain clues regarding the role of the SUMO pathway in regulating chromatin-based transactions. Indeed, the global distribution of SUMO across chromatin reveals an important function for SUMO in controlling transcription, particularly of genes involved in protein synthesis. These newly appreciated patterns of genome-wide sumoylation will inform more directed studies aimed at analyzing how the dynamics of gene expression are controlled by posttranslational SUMO modification.

Circular RNAs are abundantly expressed and upregulated during human epidermal stem cell differentiation

The expression patterns of endogenous circular RNA (circRNA) molecules during epidermal stem cell (EpSC) differentiation have not previously been explored. Here, we show that circRNAs are abundantly expressed in EpSCs and that their expression change dramatically during differentiation in a coordinated manner. Overall, circRNAs are expressed at higher levels in the differentiated cells, and many upregulated circRNAs are derived from developmental genes, including four different circRNAs from DLG1. The observed changes in circRNA expression were largely independent of host gene expression, and circRNAs independently upregulated upon differentiation are more prone to AGO2 binding and have more predicted miRNA binding sites compared to stably expressed circRNAs. In particular, upregulated circRNAs from the HECTD1 and ZNF91 genes have exceptionally high numbers of AGO2 binding sites and predicted miRNA target sites, and circZNF91 contains 24 target sites for miR-23b-3p, which is known to play important roles in keratinocyte differentiation. We also observed that upregulated circRNAs are less likely to be flanked by homologues inverted Alu repeats compared to stably expressed circRNAs. This coincide with DHX9 being upregulated in the differentiated keratinocytes. Finally, none of the circRNAs upregulated upon differentiation were also upregulated upon DNMT3A or DNMT3B knockdown, making it unlikely that epigenetic mechanisms are governing the observed circRNA expression changes. Together, we provide a map of circRNA expression in EpSCs and their differentiated counterparts and shed light on potential function and regulation of differentially expressed circRNAs.

Regulation of tRNA synthesis by posttranslational modifications of RNA polymerase III subunits

RNA polymerase III (RNAPIII) transcribes tRNA genes, 5S RNA as well as a number of other non-coding RNAs. Because transcription by RNAPIII is an energy-demanding process, its activity is tightly linked to the stress levels and nutrient status of the cell. Multiple signaling pathways control RNAPIII activity in response to environmental cues, but exactly how these pathways regulate RNAPIII is still poorly understood. One major target of these pathways is the transcriptional repressor Maf1, which inhibits RNAPIII activity under conditions that are detrimental to cell growth. However, recent studies have found that the cell can also directly regulate the RNAPIII machinery through phosphorylation and sumoylation of RNAPIII subunits. In this review we summarize post-translational modifications of RNAPIII subunits that mainly have been identified in large-scale proteomics studies, and we highlight several examples to discuss their relevance for regulation of RNAPIII.

Regulation of Cellular Processes by SUMO: Understudied Topics

SUMO plays a multiple role in maintenance of cellular homeostasis, both under normal conditions and under cell stress . Considerable effort has been devoted to unraveling the functions of SUMO in regulation of transcription and preservation of genome stability. However, it is clear from high-throughput SUMO proteome studies that SUMO likely regulates many more cellular processes. The function of SUMO in these processes has hardly been explored. This review will focus on the emerging function of SUMO in regulation of several of these processes.

SUMO and Chromatin Remodeling

Many of the known SUMO substrates are nuclear proteins, which regulate gene expression and chromatin dynamics. Sumoylation, in general, appears to correlate with decreased transcriptional activity, and in many cases modulation of the chromatin template is implicated. Sumoylation of the core histones is associated with transcriptional silencing, and transcription factor sumoylation can decrease gene expression by promoting recruitment of chromatin modifying enzymes. Additionally, sumoylation of transcriptional corepressors and chromatin remodeling enzymes can influence interactions with other transcriptional regulators, and alter their enzymatic activity. In some cases, proteins that are components of transcriptional corepressor complexes have been shown to be SUMO E3 ligases, further emphasizing the integration of sumoylation with the regulation of chromatin remodeling. Despite the evidence suggesting that sumoylation is primarily repressive for access to chromatin, recent analyses suggest that protein sumoylation on the chromatin template may play important roles at highly expressed genes. Elucidating the dynamic interplay of sumoylation with other post-translational modifications of histones and chromatin associated proteins will be key to fully understanding the regulation of access to the chromatin template.

Roles of Sumoylation in mRNA Processing and Metabolism

SUMO has gained prominence as a regulator in a number of cellular processes. The roles of sumoylation in RNA metabolism, however, while considerable, remain less well understood. In this chapter we have assembled data from proteomic analyses, localization studies and key functional studies to extend SUMO's role to the area of mRNA processing and metabolism. Proteomic analyses have identified multiple putative sumoylation targets in complexes functioning in almost all aspects of mRNA metabolism, including capping, splicing and polyadenylation of mRNA precursors. Possible regulatory roles for SUMO have emerged in pre-mRNA 3' processing, where SUMO influences the functions of polyadenylation factors and activity of the entire complex. SUMO is also involved in regulating RNA editing and RNA binding by hnRNP proteins, and recent reports have suggested the involvement of the SUMO pathway in mRNA export. Together, these reports suggest that SUMO is involved in regulation of many aspects of mRNA metabolism and hold the promise for exciting future studies.

The increasing diversity of functions attributed to the SAFB family of RNA-/DNA-binding proteins

RNA-binding proteins play a central role in cellular metabolism by orchestrating the complex interactions of coding, structural and regulatory RNA species. The SAFB (scaffold attachment factor B) proteins (SAFB1, SAFB2 and SAFB-like transcriptional modulator, SLTM), which are highly conserved evolutionarily, were first identified on the basis of their ability to bind scaffold attachment region DNA elements, but attention has subsequently shifted to their RNA-binding and protein–protein interactions. Initial studies identified the involvement of these proteins in the cellular stress response and other aspects of gene regulation. More recently, the multifunctional capabilities of SAFB proteins have shown that they play crucial roles in DNA repair, processing of mRNA and regulatory RNA, as well as in interaction with chromatin-modifying complexes. With the advent of new techniques for identifying RNA-binding sites, enumeration of individual RNA targets has now begun. This review aims to summarise what is currently known about the functions of SAFB proteins.

Roles for SUMO in pre-mRNA processing

When the small ubiquitin-like modifier (SUMO)-1 protein is localized on the genome, it is found on proteins bound to the promoters of the most highly active genes and on proteins bound to the DNA-encoding exons. Inhibition of the SUMO-1 modification leads to reductions in initiation of messenger RNA (mRNA) synthesis and splicing. In this review, we discuss what is known about the SUMOylation of factors involved in transcription initiation, pre-mRNA processing, and polyadenylation. We suggest a mechanism by which SUMO modifications of factors at the promoters of high-activity genes trigger the formation of an RNA polymerase II complex that coordinates and integrates the stimulatory signals for each process to catalyze an extremely high level of gene expression. WIREs RNA 2016, 7:105-112. doi: 10.1002/wrna.1318 For further resources related to this article, please visit the WIREs website.

SUMO-regulated transcription: challenging the dogma

The small ubiquitin-like modifier SUMO regulates many aspects of cellular physiology to maintain cell homeostasis, both under normal conditions and during cell stress. Components of the transcriptional apparatus and chromatin are among the most prominent SUMO substrates. The prevailing view is that SUMO serves to repress transcription. However, as we will discuss in this review, this model needs to be refined, because recent studies have revealed that SUMO can also have profound positive effects on transcription.