IκBα is required for full transcriptional induction of some NFκB-regulated genes in response to TNF in MCF-7 cells

Inflammatory stimuli triggers the degradation of three inhibitory κB (IκB) proteins, allowing for nuclear translocation of nuclear factor-κB (NFκB) for transcriptional induction of its target genes. Of these three, IκBα is a well-known negative feedback regulator that limits the duration of NFκB activity. We sought to determine whether IκBα’s role in enabling or limiting NFκB activation is important for tumor necrosis factor (TNF)-induced gene expression in human breast cancer cells (MCF-7). Contrary to our expectations, many more TNF-response genes showed reduced induction than enhanced induction in IκBα knockdown cells. Mathematical modeling was used to investigate the underlying mechanism. We found that the reduced activation of some NFκB target genes in IκBα-deficient cells could be explained by the incoherent feedforward loop (IFFL) model. In addition, for a subset of genes, prolonged NFκB activity due to loss of negative feedback control did not prolong their transient activation; this implied a multi-state transcription cycle control of gene induction. Genes encoding key inflammation-related transcription factors, such as JUNB and KLF10, were found to be best represented by a model that contained both the IFFL and the transcription cycle motif. Our analysis sheds light on the regulatory strategies that safeguard inflammatory gene expression from overproduction and repositions the function of IκBα not only as a negative feedback regulator of NFκB but also as an enabler of NFκB-regulated stimulus-responsive inflammatory gene expression. This study indicates the complex involvement of IκBα in the inflammatory response to TNF that is induced by radiation therapy in breast cancer.

Elevated pausing of RNA Polymerase II underlies acquired resistance to ionizing radiation

As the mainstay modality for many malignancies, ionizing radiation (IR) induces a variety of lesions in genomic DNA, evoking a multipronged DNA damage response to interrupt many cellular processes including transcription. How the global transcription cycle is altered by IR and whether it is contributing to the development of IR-resistance remain unaddressed. Here we report a genome-wide accumulation of paused RNA Polymerase II (RNAPII) after IR exposure. This increased pausing is partially maintained in cells acquired IR-resistance, notably on genes involved in radiation response and cell cycle, often leading to their downregulation. Individual knockdown some of these genes such as TP53 and NEK7 endows IR-sensitive cells with varying degrees of resistance, highlighting a novel link between elevated RNAPII pausing and the acquisition of IR-resistance. Accordingly, tuning-down the RNAPII pausing level by inhibiting CDK7 reverses IR-resistance both in cell culture and xenograft models. Our results suggest that modulation of the transcription cycle is a promising strategy to increase IR-sensitivity and thwart resistance.

Real-time single-cell characterization of the eukaryotic transcription cycle reveals correlations between RNA initiation, elongation, and cleavage

The eukaryotic transcription cycle consists of three main steps: initiation, elongation, and cleavage of the nascent RNA transcript. Although each of these steps can be regulated as well as coupled with each other, their in vivo dissection has remained challenging because available experimental readouts lack sufficient spatiotemporal resolution to separate the contributions from each of these steps. Here, we describe a novel application of Bayesian inference techniques to simultaneously infer the effective parameters of the transcription cycle in real time and at the single-cell level using a two-color MS2/PP7 reporter gene and the developing fruit fly embryo as a case study. Our method enables detailed investigations into cell-to-cell variability in transcription-cycle parameters as well as single-cell correlations between these parameters. These measurements, combined with theoretical modeling, suggest a substantial variability in the elongation rate of individual RNA polymerase molecules. We further illustrate the power of this technique by uncovering a novel mechanistic connection between RNA polymerase density and nascent RNA cleavage efficiency. Thus, our approach makes it possible to shed light on the regulatory mechanisms in play during each step of the transcription cycle in individual, living cells at high spatiotemporal resolution.

PAF1 facilitates RNA polymerase II ubiquitination by the Elongin A complex through phosphorylation by CDK12

ABSTRACTThe conserved Polymerase-Associated Factor 1 complex (PAF1C) regulates all stages of the RNA polymerase II (RNAPII) transcription cycle from the promoter to the 3’ end formation site of mRNA encoding genes and has been linked to numerous transcription related processes. Here, we show that PAF1 interacts with Elongin A, a transcription elongation factor as well as a component of a cullin-RING ligase that targets stalled RNAPII for ubiquitination and proteasome-dependent degradation in response to DNA damage or other stresses. We show that, in absence of any induced stress, PAF1 physically interacts with the E3 ubiquitin ligase form of the Elongin A complex and facilitates ubiquitination of RNAPII. We demonstrate that this ubiquitination is dependent of the Ser2 phosphorylation of the RNAPII carboxy-terminal domain (CTD) by CDK12. Our findings highlight a novel unexpected role of PAF1-CDK12 in RNAPII transcription cycle, raising the possibility that the Elongin A ubiquitin ligase plays a role in normal transcription process, and suggest a transcription surveillance mechanism ready to degrade RNAPII if needed.

Single-cell characterization of the eukaryotic transcription cycle using live imaging and statistical inference

The eukaryotic transcription cycle consists of three main steps: initiation, elongation, and cleavage of the nascent RNA transcript. Although each of these steps can be regulated as well as coupled with each other, their in vivo dissection has remained challenging because available experimental readouts lack sufficient spatiotemporal resolution to separate the contributions from each of these steps. Here, we describe a novel computational technique to simultaneously infer the effective parameters of the transcription cycle in real time and at the single-cell level using a two-color MS2/PP7 reporter gene and the developing fruit fly embryo as a case study. Our method enables detailed investigations into cell-to-cell variability in transcription-cycle parameters with high precision. These measurements, combined with theoretical modeling, reveal a significant variability in the elongation rate of individual RNA polymerase molecules. We further illustrate the power of this technique by uncovering a novel mechanistic connection between RNA polymerase density and nascent RNA cleavage efficiency. Thus, our approach makes it possible to shed light on the regulatory mechanisms in play during each step of the transcription cycle in individual, living cells at high spatiotemporal resolution.

A PP2A-Integrator complex fine-tunes transcription by opposing CDK9

SUMMARYGene expression is tightly controlled by Cyclin-dependent kinases (CDKs) which regulate the RNA Polymerase II (RNAPII) transcription cycle at discrete checkpoints. RNAPII pausing is a CDK9-controlled rate-limiting process that occurs shortly after initiation and is required for spatio-temporal control of transcription in multicellular organisms. We discovered that CDK9-mediated RNAPII pause-release is functionally opposed by a protein phosphatase 2A (PP2A) complex. PP2A dynamically competes for key CDK9 substrates, DSIF and RNAPII, and is recruited to transcription pausing sites by INTS6, a subunit of the Integrator complex. INTS6 depletion disrupts the Integrator-PP2A association and confers resistance to CDK9 inhibition. This results in unrestrained activity of CDK9 and dysregulation of acute transcriptional responses. Pharmacological PP2A activation amplifies RNAPII pausing mediated by CDK9 inhibitors and synergizes therapeutically in a model of MLL-rearranged leukemia. These data demonstrate that finely-tuned gene expression relies on the delicate balance of kinase and phosphatase activity throughout the transcription cycle.HIGHLIGHTSLoss of INTS6 confers resistance to CDK9 inhibitionINTS6 recruits PP2A to Integrator and chromatinPP2A/INTS6 complexes functionally oppose CDK9PP2A/INTS6 fine-tune acute transcriptional responsesSynergistic anti-cancer activity between PP2A activators and CDK9 inhibitors