Investigating Transcriptional Responses to the Environment

尽管传统的基因调控模型假设启动子募集Pol II对于基因表达是必要和充分的，但我们最近发现，从启动子近端区域释放暂停的Pol II对于大量基因的表达是限速的。我们的初步工作调查了暂停型Pol II在果蝇中的流行，采用了全球定位分析（使用ChIP-chip和ChIP-seq技术）以及体内足迹分析的组合。令人惊讶的是，这些数据表明，Pol II暂停比之前认为的要广泛得多，发生在全基因组的数千个启动子中。我们和其他人最近将这些发现扩展到哺乳动物系统（小鼠和人类），证明暂停高等生物中普遍存在的基因调控策略。此外，我们的研究结果表明，在许多对环境或发育刺激作出反应的基因中，Pol II在非诱导状态下被预加载，这表明Pol II的存在，准备逃逸到基因中，促进了对动态变化的环境的有效、综合反应。

Whereas traditional models for gene regulation posit that recruitment of Pol II to the promoter is both necessary and sufficient for gene expression, we have recently found that release of paused Pol II from the promoter-proximal region is rate-limiting for expression of a large number of genes. Our initial work investigated the prevalence of paused Pol II in Drosophila, employing a combination of global location analysis (using techniques called ChIP-chip and ChIP-seq) as well as in vivo footprinting assays. Surprisingly, these data showed that Pol II pausing is much more widespread than previously appreciated, occurring at thousands of promoters genome-wide. We and others have recently extended these findings to mammalian systems (mouse and human), demonstrating that pausing a prevalent gene regulatory strategy in higher organisms. Moreover, our results reveal that Pol II is pre-loaded in the uninduced state at many genes that respond to environmental or developmental stimuli, suggesting that the presence of Pol II, poised for escape into the gene, facilitates efficient, integrated responses to a dynamically changing environment. Understanding the fundamental properties of paused Pol II, and the factors that govern maintenance vs. release of promoter-proximal Pol II into productive elongation are specific aims of research in the Adelman laboratory. In addition to providing crucial insight into stress-responses, this work is anticipated to elucidate gene expression during the development of cancer and AIDS, since similarly paused Pol II are observed at the mammalian promoters of proto-oncogenes like c-myc, c-fos and junB, as well as at the HIV promoter. As part of our efforts to better define the mechanisms underlying pausing, we have recently developed a novel technique for isolating the short RNA transcripts generated by paused Pol II, and analyzed them through massively-parallel sequencing of individual RNA molecules. This strategy allowed us to pinpoint both the locations of transcription initiation and pausing, at single-nucleotide resolution. Notably, this exciting new technique revealed a role for the DNA sequence within the initially transcribed region in specifying the efficiency of early elongation, providing insights into why polymerase pausing is more prominent at some genes than at others. In probing the molecular mechanisms governing Pol II stalling, the Negative ELongation Factor, or NELF complex, is of particular interest to the laboratory. NELF has been shown to establish paused Pol II at several genes to date, including the junB and HIV promoters. To globally identify targets of NELF, we have performed a microarray analysis on Drosophila cells that were depleted of NELF using RNA interference. We found that many NELF target genes are involved in stimulus-responsive pathways, with a particular enrichment in the innate immune response. To evaluate the physiological relevance of this finding, we have recently performed NELF depletion in the Drosophila fat body (the main immune responsive tissue), followed by microarray analysis of RNA levels to identify NELF target genes. This work confirms that NELF plays a key role in vivo in regulating expression of components of the innate immune system. Follow-up studies in both cells and flies revealed that NELF-mediated Pol II stalling is essential for an optimal immune response to bacterial challenge and indicated that polymerase stalling is necessary for either the binding or activity of the NF-kB transcription factor Relish (Rel). This result is consistent with our earlier suggestion that Pol II pausing helps establish a nucleosome-deprived, accessible chromatin architecture around gene promoters, facilitating future activation. In addition to our work in Drosophila, we are studying the role of polymerase pausing in the mammalian inflammatory response, using primary macrophages derived from mouse. These investigations reveal that many immediate early response genes, like TNF-alpha and junB, possess paused Pol II waiting in their promoter regions in resting, uninduced cells. In contrast, late primary and secondary response genes generally lack stalled Pol II and NELF prior to induction. Overall, we find that there is an interesting relationship between the presence of stalled Pol II and the kinetics of the immune response, in that paused genes exhibit much more transient bursts of transcription activity than do genes regulated through Pol II recruitment. This suggests that there could be a fundamental link between gene regulatory strategy (i.e. the step in the transcription cycle that is rate-limiting for gene expression) and the kinetics or magnitude of gene expression.