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Inducible Gene Expression: Diverse Regulatory Mechanisms

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Inducible Gene Expression: Diverse Regulatory Mechanisms REVIEWS MODES OF TRANSCRIPTIONAL REGULATION Inducible gene expression: diverse regulatory mechanisms Vikki M. Weake and Jerry L. Workman Abstract | The rapid activation of gene expression in response to stimuli occurs largely through the regulation of RNA polymerase II‑dependent transcription. In this Review, we discuss events that occur during the transcription cycle in eukaryotes that are important for the rapid and specific activation of gene expression in response to external stimuli. In addition to regulated recruitment of the transcription machinery to the promoter, it has now been shown that control steps can include chromatin remodelling and the release of paused polymerase. Recent work suggests that some components of signal transduction cascades also play an integral part in activating transcription at target genes. Chromatin Cells must be able to rapidly respond to changes in RNA polymerase II (Pol II)‑dependent transcription. A nucleoprotein structure their external environment — such as temperature or Although a traditional model of activator‑dependent formed of repeating nutrient availability — to exploit and survive in new recruitment of Pol II and the general transcription nucleosomal units in which conditions. Even cells in a multicellular organism need factors (GTFs) holds true for many inducible genes, 147 base pairs of DNA are wrapped around an octamer of to respond to developmental cues such as signalling recent studies suggest that Pol II is already present and 1–6 histone proteins consisting molecules to determine when to divide, migrate or poised for transcription at many inducible genes . of an H3–H4 tetramer flanked die. The production of new proteins in response to Therefore, it is becoming increasingly apparent that by two H2A–H2B dimers external stimuli results largely from rapid activation there is an additional level of regulation that occurs of gene transcription — this is known as inducible during the initial stages of transcription elongation Co-activator A protein that is recruited to gene expression. before Pol II is released into a productive transcription promoters through interactions Inducible gene expression has several features cycle. In addition, several recent studies suggest that with transcriptional activators, that distinguish it from the expression of genes that some components of signal transduction cascades and facilitates transcriptional are constitutively active (for example, housekeep‑ that lead to inducible gene expression that were once activation through the recruitment of RNA ing genes). Inducible genes are highly regulated and thought to function exclusively in the cytoplasm such polymerase II and the general must be able to be rapidly and specifically activated as mitogen‑activated protein kinases (MAPKs) are transcription factors. Many in response to stimuli. Once the stimulus is removed, recruited to chromatin and are integral components of co-activators also catalyse an inducible gene must quickly return to its basal, transcription complexes7. chromatin modifications that inactive state. Furthermore, multiple genes must We use three well‑characterized examples of induc‑ assist the kinetics of recruitment of the general often be synchronously activated in response to the ible gene expression to illustrate some of the key mecha‑ transcription machinery. same stimulus, such that the proteins required to nisms involved in transcription activation in response respond to the stimulus are produced simultaneously to stimuli: Gal gene induction in response to galactose at the appropriate relative levels. Similarly, multiple in Saccharomyces cerevisiae, heat‑shock gene induction in cells in an organism must respond to developmental Drosophila melanogaster and osmostress regulation cues in a coordinated fashion so that the appropriate in S. cerevisiae. We first discuss the initial steps of the morphogenetic process occurs over a broad region transcription cycle: activator‑dependent recruitment of co-activators Stowers Institute for Medical of cells. the transcriptional machinery and the role of Research, 1000 East 50th Here, we discuss mechanisms of inducible gene and nucleosome‑remodelling complexes in facilitat‑ Street, Kansas City, expression used by eukaryotic cells. Although proc‑ ing this recruitment. We then examine the events that Missouri 64110, USA. esses that occur following transcription such as protein occur following recruitment of the general transcription Correspondence to J.L.W. translation are also regulated as part of inducible gene machinery, including promoter clearance and release e-mail: [email protected] doi:10.1038/nrg2781 expression, we do not discuss them in this Review. of paused Pol II into productive transcription elonga‑ Published online We focus on the events that are important for recruit‑ tion. Finally, we examine the role of signalling kinases 27 April 2010 ment of the transcription machinery and initiation of that seem to play an integral part in multiple aspects of 426 | JUNE 2010 | VOLUME 11 www.nature.com/reviews/genetics © 2010 Macmillan Publishers Limited. All rights reserved REVIEWS a Target gene Figure 1 | Early steps in the transcription cycle. a | Promoter selection is determined by the interaction of one or more transcriptional activator(s) with specific Activator DNA sequences (recognition sites) near target genes. Activators then recruit components of the transcription Recognition site TSS machinery to these genes through protein–protein interactions. b | Activation of gene expression is induced b by the sequential recruitment of large multi‑subunit protein co‑activator complexes (shown in purple and Co-activator Target gene pink) through binding to activators. Activators also recruit ATP‑dependent nucleosome‑remodelling Activator complexes, which move or displace histones at the ecruitment promoter, facilitating the rapid recruitment and assembly of co‑activators and the general transcription Nucleosome- remodelling complex machinery. c | Together, co‑activators and nucleosome remodellers facilitate the rapid recruitment of RNA polymerase II (Pol II) and the general transcription or-dependent r at factors (GTFs) TFIIA, TFIIB, TFIID, TFIIE, TFIIF and TFIIH c to form the pre‑initiation complex (PIC) on the core PIC Activ 9 GTFs promoter . These first three steps (a–c) constitute acti‑ Co-activator Target gene vator‑dependent recruitment. d | After PIC assembly, RNA polymerase II CDK7 in human TFIIH (Kin28 in yeast) phosphorylates Activator the serine 5 (S5) position of the Pol II carboxy‑terminal domain (CTD). At the same time, another subunit of TFIIH, the DNA helicase XPB (Rad25 in yeast), remodels Nucleosome- remodelling complex the PIC, and 11–15 bases of DNA at the transcription start site (TSS) is unwound to introduce a single‑stranded DNA template into the active site of Pol II83. Pol II then dissociates from some of the GTFs d GTFsCDK7 and transitions into an early elongation stage of Target gene e transcription83. This step is often referred to as promoter S5 RNA polymerase II P ranc escape or clearance but is not sufficient for efficient passage of Pol II into the remainder of the gene. e | Following promoter clearance, Pol II transcribes XPB Nucleosome- 20 – 40 nucleotides into the gene and halts at the remodelling complex GTFs clea omoter Pr promoter‑proximal pause site. Efficient elongation by Pol II requires a second phosphorylation event at the S2 position of the Pol II CTD by CDK9,a subunit of human 8,104 e P-TEFb Target gene P‑TEFb (Ctk1 in yeast) . Phosphorylation of the CTD creates binding sites for proteins that are important for mRNA processing and transcription through chromatin S5 S2 RNA polymerase II omoter- P P g such as the histone H3 lysine 36 (H3K36) methylase H3 H2A SET2 (REF. 104). Nucleosome remodellers also facilitate H4 H2B om pr passage of Pol II during the elongation phase of transcription. The transcription cycle continues with Nucleosome- remodelling complex pausin oximal elongation of the transcript by Pol II, followed by Release fr Release pr termination and re‑initiation of a new round of transcription (not shown). Nature Reviews | Genetics these initial stages of the transcription cycle. Although machinery to these genes through protein–protein the mechanisms involved in the chosen examples may interactions. These steps result in formation of the not always be observed in all other cases of inducible pre-initiation complex (PIC) on the promoter9,10. For General transcription machinery gene expression, we hope to provide a broad over‑ the purposes of this Review, these first three steps RNA polymerase II together view of the principles involved in inducible activation can be regarded as a single rate‑determining process, with the general transcription of transcription. which we refer to as activator‑dependent recruitment factors TFIIA, TFIIB, TFIID, (FIG. 1a–c). An additional level of regulation is required TFIIE, TFIIF and TFIIH. Activator-dependent recruitment for polymerase to proceed to productive transcription Transcriptional activator Gene activation involves a multistep recruitment elongation (FIG. 1d,e). Although all of the steps in the A sequence-specific process that consists of several potential rate‑limiting transcription cycle are subject to regulation11, we focus DNA-binding protein that steps during the initial stages of the transcription cycle in this Review on those steps that are most important increases the rate of (reviewed in REF. 8) (FIG. 1). During the initial steps
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