We imaged transcription in living cells using a locus-specific reporter system, which allowed precise, single-cell kinetic measurements of promoter binding, initiation and elongation. Photobleaching of fluorescent RNA polymerase II revealed several kinetically distinct populations of the enzyme interacting with a specific gene. Photobleaching and photoactivation of fluorescent MS2 proteins used to label nascent messenger RNAs provided sensitive elongation measurements. A mechanistic kinetic model that fits our data was validated using specific inhibitors. Polymerases elongated at 4.3 kilobases min −1 , much faster than previously documented, and entered a paused state for unexpectedly long times. Transcription onset was inefficient, with only 1% of polymerase-gene interactions leading to completion of an mRNA. Our systems approach, quantifying both polymerase and mRNA kinetics on a defined DNA template in vivo with high temporal resolution, opens new avenues for studying regulation of transcriptional processes in vivo.Transcription by RNA polymerase II (Pol II) is at the core of gene expression and hence is the basis of all cellular activities. Little information exists about the kinetics of this process in live cells 1 , as understanding of gene expression regulation comes from studies using purified proteins. For instance, the subunits of the elongating Pol II are well known 2 and the crystal structure of this enzyme explains much of its behavior in vitro 3,4 . mRNA transcription can be deconstructed into a succession of steps: promoter assembly, clearanceCorrespondence should be addressed to R.H.S. (rhsinger@aecom.yu.edu). AUTHOR CONTRIBUTIONS All data were initially acquired by X.D. and Y.S.-T. Subsequent data were obtained by V.d.T. (Fig. 4a,b and Fig. 5a) and Y.B. (Fig. 9b). S.M.S. was responsible for the microscopy, built the wide-field microscope for live-cell imaging and wrote analysis software. X.D. performed the kinetic modeling. R.D.P. provided consultation on model formulation and testing, and training in the use of the ProcessDB software. R.H.S. supervised the project. COMPETING INTERESTS STATEMENTThe authors declare competing financial interests: details accompany the full-text HTML version of the paper at http:// www.nature.com/nsmb/. HHS Public AccessAuthor manuscript Nat Struct Mol Biol. Author manuscript; available in PMC 2016 July 12. Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript and escape 5 , followed by elongation and termination. The process of transcriptional initiation involves several structural changes in the polymerase as the nascent transcript elongates 6 . Early in initiation, the polymerase can produce abortive transcripts 7,8 . These abortive cycles have been observed with a single prokaryote polymerase (RNAP) releasing several transcripts without escaping the promoter 9,10 . The elongation step can be regulated by pausing for various times, as demonstrated using prokaryotic polymerases in vitro 11,12 .For eukaryotic cells, attempts have been mad...

Live-cell single mRNA imaging is a powerful tool, but has been restricted in higher eukaryotes to artificial cell lines and reporter genes. We describe an approach that enables live-cell imaging of single endogenous labeled mRNA molecules transcribed in primary mammalian cells and tissue. We generated a knock-in mouse line in which an MS2 binding site (MBS) cassette was targeted to the 3′UTR of the essential β-actin gene. As β-actin-MBS was ubiquitously expressed, we were able to uniquely address endogenous mRNA regulation in any tissue or cell type. We simultaneously followed transcription from the β-actin alleles in real-time and observed transcriptional bursting in response to serum stimulation with precise temporal resolution. We performed tracking of single endogenous labeled mRNA particles being transported in primary hippocampal neurons. The MBS also provided a means for high sensitivity Fluorescence In Situ Hybridization (FISH), allowing detection and localization of single β-actin mRNA molecules in various mouse tissues.

Synthesis of mRNA in eukaryotes involves the coordinated action of many enzymatic processes, including initiation, elongation, splicing, and cleavage. Kinetic competition between these processes has been proposed to determine RNA fate, yet such coupling has never been observed in vivo on single transcripts. In this study, we use dual-color single-molecule RNA imaging in living human cells to construct a complete kinetic profile of transcription and splicing of the β-globin gene. We find that kinetic competition results in multiple competing pathways for pre-mRNA splicing. Splicing of the terminal intron occurs stochastically both before and after transcript release, indicating there is not a strict quality control checkpoint. The majority of pre-mRNAs are spliced after release, while diffusing away from the site of transcription. A single missense point mutation (S34F) in the essential splicing factor U2AF1 which occurs in human cancers perturbs this kinetic balance and defers splicing to occur entirely post-release.DOI: http://dx.doi.org/10.7554/eLife.03939.001

Patient-derived induced pluripotent stem cells (iPSCs) provide an opportunity to study human diseases mainly in those cases for which no suitable model systems are available. Here, we have taken advantage of in vitro iPSCs derived from patients affected by amyotrophic lateral sclerosis (ALS) and carrying mutations in the RNA-binding protein FUS to study the cellular behavior of the mutant proteins in the appropriate genetic background. Moreover, the ability to differentiate iPSCs into spinal cord neural cells provides an in vitro model mimicking the physiological conditions. iPSCs were derived from FUSR514S and FUSR521C patient fibroblasts, whereas in the case of the severe FUSP525L mutation, in which fibroblasts were not available, a heterozygous and a homozygous iPSC line were raised by TALEN-directed mutagenesis. We show that aberrant localization and recruitment of FUS into stress granules (SGs) is a prerogative of the FUS mutant proteins and occurs only upon induction of stress in both undifferentiated iPSCs and spinal cord neural cells. Moreover, we show that the incorporation into SGs is proportional to the amount of cytoplasmic FUS, strongly correlating with the cytoplasmic delocalization phenotype of the different mutants. Therefore, the available iPSCs represent a very powerful system for understanding the correlation between FUS mutations, the molecular mechanisms of SG formation and ALS ethiopathogenesis.

SummaryThe FUS gene has been linked to amyotrophic lateral sclerosis (ALS). FUS is a ubiquitous RNA-binding protein, and the mechanisms leading to selective motoneuron loss downstream of ALS-linked mutations are largely unknown. We report the transcriptome analysis of human purified motoneurons, obtained from FUS wild-type or mutant isogenic induced pluripotent stem cells (iPSCs). Gene ontology analysis of differentially expressed genes identified significant enrichment of pathways previously associated to sporadic ALS and other neurological diseases. Several microRNAs (miRNAs) were also deregulated in FUS mutant motoneurons, including miR-375, involved in motoneuron survival. We report that relevant targets of miR-375, including the neural RNA-binding protein ELAVL4 and apoptotic factors, are aberrantly increased in FUS mutant motoneurons. Characterization of transcriptome changes in the cell type primarily affected by the disease contributes to the definition of the pathogenic mechanisms of FUS-linked ALS.