ELAV links paused Pol II to alternative polyadenylation in the Drosophila nervous system

Choice of Alternative Polyadenylation Sites, Mediated by the RNA-Binding Protein Elavl3, Plays a Role in Differentiation of Inhibitory Neuronal Progenitors

Alternative polyadenylation (APA) is a widespread mechanism involving about half of the expressed genes, resulting in varying lengths of the 3′ untranslated region (3′UTR). Variations in length and sequence of the 3′UTR may underlie changes of post-transcriptional processing, localization, miRNA targeting and stability of mRNAs. During embryonic development a large array of mRNAs exhibit APA, with a prevalence of the longer 3′UTR versions in differentiating cells. Little is known about polyA+ site usage during differentiation of mammalian neural progenitors. Here we exploit a model of adherent neural stem (ANS) cells, which homogeneously and efficiently differentiate into GABAergic neurons. RNAseq data shows a global trend towards lengthening of the 3′UTRs during differentiation. Enriched expression of the longer 3′UTR variants of Pes1 and Gng2 was detected in the mouse brain in areas of cortical and subcortical neuronal differentiation, respectively, by two-probes fluorescent in situ hybridization (FISH). Among the coding genes upregulated during differentiation of ANS cells we found Elavl3, a neural-specific RNA-binding protein homologous to Drosophila Elav. In the insect, Elav regulates polyA+ site choice while interacting with paused Pol-II promoters. We tested the role of Elavl3 in ANS cells, by silencing Elavl3 and observed consistent changes in 3′UTR length and delayed neuronal differentiation. These results indicate that choice of the polyA+ site and lengthening of 3′UTRs is a possible additional mechanism of posttranscriptional RNA modification involved in neuronal differentiation.

Analysis of alternative cleavage and polyadenylation in mature and differentiating neurons using RNA-seq data

Background

Most eukaryotic protein-coding genes exhibit alternative cleavage and polyadenylation (APA), resulting in mRNA isoforms with different 3' untranslated regions (3' UTRs). Studies have shown that brain cells tend to express long 3' UTR isoforms using distal cleavage and polyadenylation sites (PASs).

Methods

Using our recently developed, comprehensive PAS database PolyA_DB, we developed an efficient method to examine APA, named Significance Analysis of Alternative Polyadenylation using RNA-seq (SAAP-RS). We applied this method to study APA in brain cells and neurogenesis.

Results

We found that neurons globally express longer 3' UTRs than other cell types in brain, and microglia and endothelial cells express substantially shorter 3' UTRs. We show that the 3' UTR diversity across brain cells can be corroborated with single cell sequencing data. Further analysis of APA regulation of 3' UTRs during differentiation of embryonic stem cells into neurons indicates that a large fraction of the APA events regulated in neurogenesis are similarly modulated in myogenesis, but to a much greater extent.

Conclusion

Together, our data delineate APA profiles in different brain cells and indicate that APA regulation in neurogenesis is largely an augmented process taking place in other types of cell differentiation.

Post-transcriptional Processing of mRNA in Neurons: The Vestiges of the RNA World Drive Transcriptome Diversity

Neurons are morphologically complex cells that rely on the compartmentalization of protein expression to develop and maintain their extraordinary cytoarchitecture. This formidable task is achieved, at least in part, by targeting mRNA to subcellular compartments where they are rapidly translated. mRNA transcripts are the conveyor of genetic information from DNA to the translational machinery, however, they are also endowed with additional functions linked to both the coding sequence (open reading frame, or ORF) and the flanking 5′ and 3′ untranslated regions (UTRs), that may harbor coding-independent functions. In this review, we will highlight recent evidences supporting new coding-dependent and -independent functions of mRNA and discuss how nuclear and cytoplasmic post-transcriptional modifications of mRNA contribute to localization and translation in mammalian cells with specific emphasis on neurons. We also describe recently developed techniques that can be employed to study RNA dynamics at subcellular level in eukaryotic cells in developing and regenerating neurons.

QAPA: a new method for the systematic analysis of alternative polyadenylation from RNA-seq data

Alternative polyadenylation (APA) affects most mammalian genes. The genome-wide investigation of APA has been hampered by an inability to reliably profile it using conventional RNA-seq. We describe 'Quantification of APA' (QAPA), a method that infers APA from conventional RNA-seq data. QAPA is faster and more sensitive than other methods. Application of QAPA reveals discrete, temporally coordinated APA programs during neurogenesis and that there is little overlap between genes regulated by alternative splicing and those by APA. Modeling of these data uncovers an APA sequence code. QAPA thus enables the discovery and characterization of programs of regulated APA using conventional RNA-seq.

Molecular Regulation of Alternative Polyadenylation (APA) within the Drosophila Nervous System

Alternative polyadenylation (APA) is a widespread gene regulatory mechanism that generates mRNAs with different 3'-ends, allowing them to interact with different sets of RNA regulators such as microRNAs and RNA-binding proteins. Recent studies have shown that during development, neural tissues produce mRNAs with particularly long 3'UTRs, suggesting that such extensions might be important for neural development and function. Despite this, the mechanisms underlying neural APA are not well understood. Here, we investigate this problem within the Drosophila nervous system, focusing on the roles played by general cleavage and polyadenylation factors (CPA factors). In particular, we examine the model that modulations in CPA factor concentration may affect APA during development. For this, we first analyse the expression of the Drosophila orthologues of all mammalian CPA factors and note that their expression decreases during embryogenesis. In contrast to this global developmental decrease in CPA factor expression, we see that cleavage factor I (CFI) expression is actually elevated in the late embryonic central nervous system, suggesting that CFI might play a special role in neural tissues. To test this, we use the UAS/Gal4 system to deplete CFI proteins from neural tissue and observe that in this condition, multiple genes switch their APA patterns, demonstrating a role of CFI in APA control during Drosophila neural development. Furthermore, analysis of genes with 3'UTR extensions of different length leads us to suggest a novel relation between 3'UTR length and sensitivity to CPA factor expression. Our work thus contributes to the understanding of the mechanisms of APA control within the developing central nervous system.

Poly(A)-ClickSeq: click-chemistry for next-generation 3΄-end sequencing without RNA enrichment or fragmentation

The recent emergence of alternative polyadenylation (APA) as an engine driving transcriptomic diversity has stimulated the development of sequencing methodologies designed to assess genome-wide polyadenylation events. The goal of these approaches is to enrich, partition, capture and ultimately sequence poly(A) site junctions. However, these methods often require poly(A) enrichment, 3΄ linker ligation steps, and RNA fragmentation, which can necessitate higher levels of starting RNA, increase experimental error and potentially introduce bias. We recently reported a click-chemistry based method for generating RNAseq libraries called ‘ClickSeq’. Here, we adapt this method to direct the cDNA synthesis specifically toward the 3΄UTR/poly(A) tail junction of cellular RNA. With this novel approach, we demonstrate sensitive and specific enrichment for poly(A) site junctions without the need for complex sample preparation, fragmentation or purification. Poly(A)-ClickSeq (PAC-seq) is therefore a simple procedure that generates high-quality RNA-seq poly(A) libraries. As a proof-of-principle, we utilized PAC-seq to explore the poly(A) landscape of both human and Drosophila cells in culture and observed outstanding overlap with existing poly(A) databases and also identified previously unannotated poly(A) sites. Moreover, we utilize PAC-seq to quantify and analyze APA events regulated by CFIm25 illustrating how this technology can be harnessed to identify alternatively polyadenylated RNA.

Regulation by 3'-Untranslated Regions

3'-untranslated regions (3'-UTRs) are the noncoding parts of mRNAs. Compared to yeast, in humans, median 3'-UTR length has expanded approximately tenfold alongside an increased generation of alternative 3'-UTR isoforms. In contrast, the number of coding genes, as well as coding region length, has remained similar. This suggests an important role for 3'-UTRs in the biology of higher organisms. 3'-UTRs are best known to regulate diverse fates of mRNAs, including degradation, translation, and localization, but they can also function like long noncoding or small RNAs, as has been shown for whole 3'-UTRs as well as for cleaved fragments. Furthermore, 3'-UTRs determine the fate of proteins through the regulation of protein-protein interactions. They facilitate cotranslational protein complex formation, which establishes a role for 3'-UTRs as evolved eukaryotic operons. Whereas bacterial operons promote the interaction of two subunits, 3'-UTRs enable the formation of protein complexes with diverse compositions. All of these 3'-UTR functions are accomplished by effector proteins that are recruited by RNA-binding proteins that bind to 3'-UTR cis-elements. In summary, 3'-UTRs seem to be major players in gene regulation that enable local functions, compartmentalization, and cooperativity, which makes them important tools for the regulation of phenotypic diversity of higher organisms.

Cleavage and polyadenylation: Ending the message expands gene regulation

Cleavage and polyadenylation (pA) is a fundamental step that is required for the maturation of primary protein encoding transcripts into functional mRNAs that can be exported from the nucleus and translated in the cytoplasm. 3'end processing is dependent on the assembly of a multiprotein processing complex on the pA signals that reside in the pre-mRNAs. Most eukaryotic genes have multiple pA signals, resulting in alternative cleavage and polyadenylation (APA), a widespread phenomenon that is important to establish cell state and cell type specific transcriptomes. Here, we review how pA sites are recognized and comprehensively summarize how APA is regulated and creates mRNA isoform profiles that are characteristic for cell types, tissues, cellular states and disease.

Transcription Impacts the Efficiency of mRNA Translation via Co-transcriptional N6-adenosine Methylation

Transcription and translation are two main pillars of gene expression. Due to the different timings, spots of action, and mechanisms of regulation, these processes are mainly regarded as distinct and generally uncoupled, despite serving a common purpose. Here, we sought for a possible connection between transcription and translation. Employing an unbiased screen of multiple human promoters, we identified a positive effect of TATA box on translation and a general coupling between mRNA expression and translational efficiency. Using a CRISPR-Cas9-mediated approach, genome-wide analyses, and in vitro experiments, we show that the rate of transcription regulates the efficiency of translation. Furthermore, we demonstrate that m⁶A modification of mRNAs is co-transcriptional and depends upon the dynamics of the transcribing RNAPII. Suboptimal transcription rates lead to elevated m⁶A content, which may result in reduced translation. This study uncovers a general and widespread link between transcription and translation that is governed by epigenetic modification of mRNAs.

•

Transcription rates of mRNAs positively correlate with rates of their translation

•

Dynamics of RNA polymerase II impact the deposition of m⁶A on mRNAs

•

Suboptimal transcription enhances m⁶A modification of mRNAs

•

Excessive m⁶A modification is detrimental for the translation process

The Nrd1-like protein Seb1 coordinates cotranscriptional 3' end processing and polyadenylation site selection

Termination of RNA polymerase II (RNAPII) transcription is associated with RNA 3' end formation. For coding genes, termination is initiated by the cleavage/polyadenylation machinery. In contrast, a majority of noncoding transcription events in Saccharomyces cerevisiae does not rely on RNA cleavage for termination but instead terminates via a pathway that requires the Nrd1-Nab3-Sen1 (NNS) complex. Here we show that the Schizosaccharomyces pombe ortholog of Nrd1, Seb1, does not function in NNS-like termination but promotes polyadenylation site selection of coding and noncoding genes. We found that Seb1 associates with 3' end processing factors, is enriched at the 3' end of genes, and binds RNA motifs downstream from cleavage sites. Importantly, a deficiency in Seb1 resulted in widespread changes in 3' untranslated region (UTR) length as a consequence of increased alternative polyadenylation. Given that Seb1 levels affected the recruitment of conserved 3' end processing factors, our findings indicate that the conserved RNA-binding protein Seb1 cotranscriptionally controls alternative polyadenylation.

Tissue-specific regulation of alternative polyadenylation represses expression of a neuronal ankyrin isoform in C. elegans epidermal development

Differential mRNA polyadenylation plays an important role in shaping the neuronal transcriptome. In C. elegans, several ankyrin isoforms are produced from the unc-44 locus through alternative polyadenylation. Here, we identify a key role for an intronic polyadenylation site (PAS) in temporal- and tissue-specific regulation of UNC-44/ankyrin isoforms. Removing an intronic PAS results in ectopic expression of the neuronal ankyrin isoform in non-neural tissues. This mis-expression underlies epidermal developmental defects in mutants of the conserved tumor suppressor death-associated protein kinase dapk-1 We have previously reported that the use of this intronic PAS depends on the nuclear polyadenylation factor SYDN-1, which inhibits the RNA polymerase II CTD phosphatase SSUP-72. Consistent with this, loss of sydn-1 blocks ectopic expression of neuronal ankyrin and suppresses epidermal morphology defects of dapk-1 These effects of sydn-1 are mediated by ssup-72 autonomously in the epidermis. We also show that a peptidyl-prolyl isomerase PINN-1 antagonizes SYDN-1 in the spatiotemporal control of neuronal ankyrin isoform. Moreover, the nuclear localization of PINN-1 is altered in dapk-1 mutants. Our data reveal that tissue and stage-specific expression of ankyrin isoforms relies on differential activity of positive and negative regulators of alternative polyadenylation.

Alternative polyadenylation of mRNA precursors

Alternative polyadenylation (APA) is an RNA-processing mechanism that generates distinct 3' termini on mRNAs and other RNA polymerase II transcripts. It is widespread across all eukaryotic species and is recognized as a major mechanism of gene regulation. APA exhibits tissue specificity and is important for cell proliferation and differentiation. In this Review, we discuss the roles of APA in diverse cellular processes, including mRNA metabolism, protein diversification and protein localization, and more generally in gene regulation. We also discuss the molecular mechanisms underlying APA, such as variation in the concentration of core processing factors and RNA-binding proteins, as well as transcription-based regulation.

What Is the Impact of mRNA 5' TL Heterogeneity on Translational Start Site Selection and the Mammalian Cellular Phenotype?

A major determinant in the efficiency of ribosome loading onto mRNAs is the 5′ TL (transcript leader or 5′ UTR). In addition, elements within this region also impact on start site selection demonstrating that it can modulate the protein readout at both quantitative and qualitative levels. With the increasing wealth of data generated by the mining of the mammalian transcriptome, it has become evident that a genes 5′ TL is not homogeneous but actually exhibits significant heterogeneity. This arises due to the utilization of alternative promoters, and is further compounded by significant variability with regards to the precise transcriptional start sites of each (not to mention alternative splicing). Consequently, the transcript for a protein coding gene is not a unique mRNA, but in-fact a complexed quasi-species of variants whose composition may respond to the changing physiological environment of the cell. Here we examine the potential impact of these events with regards to the protein readout.

Alternative polyadenylation coupled to transcription initiation: Insights from ELAV-mediated 3' UTR extension

Transcription initiation and mRNA maturation were long considered co-occurring but separately regulated events of gene control. In the past decade, gene promoters, the platforms of transcription initiation, have been assigned additional functions such as the regulation of splicing and 3' end processing. In a recent study, Oktaba and Zhang and al. reveal that neural 3' UTR extension is dependent on promoter sequences. In Drosophila neurons, promoter regions of a subset of genes recruit the RNA-binding protein ELAV, which is required for subsequent ELAV-mediated alternative polyadenylation. Intriguingly, RNA Polymerase II pausing at promoters seems to facilitate ELAV recruitment. How transcription initiation and alternative polyadenylation, processes separated by an entire gene length, are functionally linked, remains unsolved. In this article, I summarize recent findings and discuss possible mechanisms.

Alternative Polyadenylation: Another Foe in Cancer

Advancements in sequencing and transcriptome analysis methods have led to seminal discoveries that have begun to unravel the complexity of cancer. These studies are paving the way toward the development of improved diagnostics, prognostic predictions, and targeted treatment options. However, it is clear that pieces of the cancer puzzle are still missing. In an effort to have a more comprehensive understanding of the development and progression of cancer, we have come to appreciate the value of the noncoding regions of our genomes, partly due to the discovery of miRNAs and their significance in gene regulation. Interestingly, the miRNA-mRNA interactions are not solely dependent on variations in miRNA levels. Instead, the majority of genes harbor multiple polyadenylation signals on their 3' UTRs (untranslated regions) that can be differentially selected on the basis of the physiologic state of cells, resulting in alternative 3' UTR isoforms. Deregulation of alternative polyadenylation (APA) has increasing interest in cancer research, because APA generates mRNA 3' UTR isoforms with potentially different stabilities, subcellular localizations, translation efficiencies, and functions. This review focuses on the link between APA and cancer and discusses the mechanisms as well as the tools available for investigating APA events in cancer. Overall, detection of deregulated APA-generated isoforms in cancer may implicate some proto-oncogene activation cases of unknown causes and may help the discovery of novel cases; thus, contributing to a better understanding of molecular mechanisms of cancer. Mol Cancer Res; 14(6); 507-17. ©2016 AACR.

Mediator and TREX-2: Emerging links between transcription initiation and mRNA export

Nuclear pore proteins interact dynamically with chromatin to regulate gene activities. A key question is how nucleoporin interactions mechanistically alter a gene's intranuclear position and transcriptional output. We reported recently on a direct interaction between the nuclear pore-associated TREX-2 complex and promoter-bound Mediator. This highlights how nuclear-pore associated adaptors gain regulatory access to the core transcription machinery. In this Extra View, we discuss an additional implication that arises from our work and the recent literature: how promoter elements may regulate mRNA metabolism beyond transcription initiation.

Evolution and Biological Roles of Alternative 3'UTRs

More than half of human genes use alternative cleavage and polyadenylation to generate alternative 3' untranslated region (3'UTR) isoforms. Most efforts have focused on transcriptome-wide mapping of alternative 3'UTRs and on the question of how 3'UTR isoform ratios may be regulated. However, it remains less clear why alternative 3'UTRs have evolved and what biological roles they play. This review summarizes our current knowledge of the functional roles of alternative 3'UTRs, including mRNA localization, mRNA stability, and translational efficiency. Recent work suggests that alternative 3'UTRs may also enable the formation of protein-protein interactions to regulate protein localization or to diversify protein functions. These recent findings open an exciting research direction for the investigation of new biological roles of alternative 3'UTRs.

Subcellular RNA profiling links splicing and nuclear DICER1 to alternative cleavage and polyadenylation

Alternative cleavage and polyadenylation (APA) plays a crucial role in the regulation of gene expression across eukaryotes. Although APA is extensively studied, its regulation within cellular compartments and its physiological impact remains largely enigmatic. Here, we used a rigorous subcellular fractionation approach to compare APA profiles of cytoplasmic and nuclear RNA fractions from human cell lines. This approach allowed us to extract APA isoforms that are subjected to differential regulation and provided us with a platform to interrogate the molecular regulatory pathways that shape APA profiles in different subcellular locations. Here, we show that APA isoforms with shorter 3' UTRs tend to be overrepresented in the cytoplasm and appear to be cell-type-specific events. Nuclear retention of longer APA isoforms occurs and is partly a result of incomplete splicing contributing to the observed cytoplasmic bias of transcripts with shorter 3' UTRs. We demonstrate that the endoribonuclease III, DICER1, contributes to the establishment of subcellular APA profiles not only by expected cytoplasmic miRNA-mediated destabilization of APA mRNA isoforms, but also by affecting polyadenylation site choice.

Global insights into alternative polyadenylation regulation

Alternative pre-mRNA processing greatly increases the coding capacity of the human genome and regulatory factors involved in RNA processing play critical roles in tissue development and maintenance. Indeed, abnormal functions of RNA processing factors have been associated with a wide range of human diseases from cancer to neurodegenerative disorders. While many studies have emphasized the importance of alternative splicing (AS), recent high-throughput sequencing efforts have also allowed global surveys of alternative polyadenylation (APA). For the majority of pre-mRNAs, as well as some non-coding transcripts such as lncRNAs, APA selects different 3'-ends and thus modulates the availability of regulatory sites recognized by trans-acting regulatory effectors, including miRs and RNA binding proteins (RBPs). Here, we compare the available technologies for assessing global polyadenylation patterns, summarize the roles of auxiliary factors on APA, and discuss the impact of differential polyA site (pA) selection in the determination of cell fate, transformation and disease.

Epitope-tagged yeast strains reveal promoter driven changes to 3'-end formation and convergent antisense-transcription from common 3' UTRs

Epitope-tagging by homologous recombination is ubiquitously used to study gene expression, protein localization and function in yeast. This is generally thought to insulate the regulation of gene expression to that mediated by the promoter and coding regions because native 3′ UTR are replaced. Here we show that the 3′ UTRs, CYC1 and ADH1, contain cryptic promoters that generate abundant convergent antisense-transcription in Saccharomyces cerevisiae. Moreover we show that aberrant, truncating 3′ –end formation is often associated with regulated transcription in TAP-tagged strains. Importantly, the steady-state level of both 3′ –truncated and antisense transcription products is locus dependent. Using TAP and GFP-tagged strains we show that the transcriptional state of the gene-of-interest induces changes to 3′ –end formation by alternative polyadenylation and antisense transcription from a universal 3′ UTR. This means that these 3′ UTRs contains plastic features that can be molded to reflect the regulatory architecture of the locus rather than bringing their own regulatory paradigm to the gene-fusions as would be expected. Our work holds a cautionary note for studies utilizing tagged strains for quantitative biology, but also provides a new model for the study of promoter driven rewiring of 3′ –end formation and regulatory non-coding transcription.

In the Driver's Seat: The Case for Transcriptional Regulation and Coupling as Relevant Determinants of the Circadian Transcriptome and Proteome in Eukaryotes

Circadian clocks drive daily oscillations in a variety of biological processes through the coordinate orchestration of precise gene expression programs. Global expression profiling experiments have suggested that a significant fraction of the transcriptome and proteome is under circadian control, and such output rhythms have historically been assumed to rely on the rhythmic transcription of these genes. Recent genome-wide studies, however, have challenged this long-held view and pointed to a major contribution of posttranscriptional regulation in driving oscillations at the messenger RNA (mRNA) level, while others have highlighted extensive clock translational regulation, regardless of mRNA rhythms. There are various examples of genes that are uniformly transcribed throughout the day but that exhibit rhythmic mRNA levels, and of flat mRNAs, with oscillating protein levels, and such observations have largely been considered to result from independent regulation at each step. These studies have thereby obviated any connections, or coupling, that might exist between the different steps of gene expression and the impact that any of them could have on subsequent ones. Here, we argue that due to both biological and technical reasons, the jury is still out on the determination of the relative contributions of each of the different stages of gene expression in regulating output molecular rhythms. In addition, we propose that through a variety of coupling mechanisms, gene transcription (even when apparently arrhythmic) might play a much relevant role in determining oscillations in gene expression than currently estimated, regulating rhythms at downstream steps. Furthermore, we posit that eukaryotic genomes regulate daily RNA polymerase II (RNAPII) recruitment and histone modifications genome-wide, setting the stage for global nascent transcription, but that tissue-specific mechanisms locally specify the different processes under clock control.

The end of the message: multiple protein-RNA interactions define the mRNA polyadenylation site

Recent studies have significantly reshaped current models for the protein–RNA interactions involved in poly(A) site recognition. Here, Shi and Manley review the recent advances in this area and provide a perspective for future studies.

The key RNA sequence elements and protein factors necessary for 3′ processing of polyadenylated mRNA precursors are well known. Recent studies, however, have significantly reshaped current models for the protein–RNA interactions involved in poly(A) site recognition, painting a picture more complex than previously envisioned and also providing new insights into regulation of this important step in gene expression. Here we review the recent advances in this area and provide a perspective for future studies.

Concentration and Localization of Coexpressed ELAV/Hu Proteins Control Specificity of mRNA Processing

Neuronally coexpressed ELAV/Hu proteins comprise a family of highly related RNA binding proteins which bind to very similar cognate sequences. How this redundancy is linked to in vivo function and how gene-specific regulation is achieved have not been clear. Analysis of mutants in Drosophila ELAV/Hu family proteins ELAV, FNE, and RBP9 and of genetic interactions among them indicates that they have mostly independent roles in neuronal development and function but have converging roles in the regulation of synaptic plasticity. Conversely, ELAV, FNE, RBP9, and human HuR bind ELAV target RNA in vitro with similar affinities. Likewise, all can regulate alternative splicing of ELAV target genes in nonneuronal wing disc cells and substitute for ELAV in eye development upon artificially increased expression; they can also substantially restore ELAV's biological functions when expressed under the control of the elav gene. Furthermore, ELAV-related Sex-lethal can regulate ELAV targets, and ELAV/Hu proteins can interfere with sexual differentiation. An ancient relationship to Sex-lethal is revealed by gonadal expression of RBP9, providing a maternal fail-safe for dosage compensation. Our results indicate that highly related ELAV/Hu RNA binding proteins select targets for mRNA processing through alteration of their expression levels and subcellular localization but only minimally by altered RNA binding specificity.

The core promoter: At the heart of gene expression

The identities of different cells and tissues in multicellular organisms are determined by tightly controlled transcriptional programs that enable accurate gene expression. The mechanisms that regulate gene expression comprise diverse multiplayer molecular circuits of multiple dedicated components. The RNA polymerase II (Pol II) core promoter establishes the center of this spatiotemporally orchestrated molecular machine. Here, we discuss transcription initiation, diversity in core promoter composition, interactions of the basal transcription machinery with the core promoter, enhancer-promoter specificity, core promoter-preferential activation, enhancer RNAs, Pol II pausing, transcription termination, Pol II recycling and translation. We further discuss recent findings indicating that promoters and enhancers share similar features and may not substantially differ from each other, as previously assumed. Taken together, we review a broad spectrum of studies that highlight the importance of the core promoter and its pivotal role in the regulation of metazoan gene expression and suggest future research directions and challenges.

RNA metabolism. Promoters drive alternative polyadenylation

The RNA-binding protein Elav is recruited in the fruit fly to promoter regions, possibly by paused RNA polymerase II, to promote alternative polyadenylation.