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Chapter 15 Control of Gene Expression.docx

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BIOL 1500
Tamara Kelly

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Heena Loomba 1 Chapter 15: Control of Gene Expression 15.1 Regulation of Gene Expression in Prokaryotes  Prokaryotic cells undergo rapid and reversible alterations in biochemical pathways that allow them to adapt quickly to changes in their environment  Sugars might be more available in the water environment, and genes to coding for enzymes needed to metabolize this energy source need to be “turned on”  Other nutrients (ex. The amino acid tryptophan) may also be available in water. So genes coding for the enzyme needed to make the amino acid “from scratch” need to be “turned off”  Bacteria has a versatile and responsive control system allowing it to be efficient when using nutrients  Gene expression in prokaryotes is regulated at the transcription level with genes organized into functional units called operons  Operon function coordinates synthesis of proteins with related functions 15.2 Regulation of Transcription in Eukaryotes  Coordinated synthesis of proteins with related function occurs, but the genes are scattered around genomes (not organized into operons)  Individual eukaryotic genes also consist of protein-coding sequences and adjacent regulatory sequences  Short-term regulation: involves regulatory events in which gene sets are quickly turned on or off in response to changes in environmental or physiological conditions in the cell’s or organism’s environment  Long-term regulation: involves regulatory events required for an organism to develop and differentiate, occurs in multicellular eukaryotes (not unicellular eukaryotes)  Why is regulation of gene expression more complicated in eukaryotes?  Eukaryotic cells are more complex because nuclear DNA is organized with histones into chromatin  Multicellular eukaryotes produce many different types of cells  Eukaryotic nuclear envelop separates the processes of transcription and translation (do not happen simultaneously)  In eukaryotes there is transcriptional regulation, posttranscriptional regulation, translational regulation and posttranslational regulation  Why does chromatic structure play an important role in gene activation?  Genes in DNA that are tightly wound around histones in chromatin are inactive because their promoters are not accessible to the proteins that initiate transcription Heena Loomba 2 Chapter 15: Control of Gene Expression  Chromatin remodeling: changing the state of the chromatin so that the proteins that initiate transcription can bind to their promoters in order to activate a gene, opens the way for transcription to occur  Example of chromatin remodeling: an activator binds to a regulatory sequence upstream of the gene’s promoter and recruits a remodeling complex (protein complex that displaces a nucleosome from the chromatin) exposing the promoter  Organization of a eukaryotic protein-coding gene  The promoter is found upstream of the transcription unit and contains the TATA box  Transcription factors recognize and bind to the TATA box and recruit polymerase (RNA polymerase II cannot bind on its own) RNA polymerase II-transcription factor complex forms, the polymerase unwinds the DNA and transcription begins  Promoter proximal region contains promoter proximal elements (regulatory sequences) which increase the rate of transcription  Enhancer: contains regulatory sequences that determine whether the gene is transcribed at its maximum possible rate  Activation of transcription  General transcription factors: proteins which initiate transcription, bind to the promoter in the TATA box area, recruit the enzyme RNA polymerase II and orients the enzyme to start transcription at the correct place  Transcriptional initiation complex: combination of general transcription factors with RNA polymerase II, on its own it brings a low rate of transcription initiation (leading to a few mRNA transcripts)  Activators: regulatory proteins that control the expression of one or more genes, bind to the promoter proximal elements to increase rate of transcription, interact with the general transcription factors to stimulate transcription initiation  Housekeeping genes: genes expressed for basic cellular functions, have promoter proximal elements that are recognized by activators present in all cell types  Genes expressed in particular cell types or at particular times have promoter proximal elements that are recognized by activators found only in those cell types or at those times  Coactivator: large multiprotein complex, forms a bridge between the activators at the enhancer and the proteins at the promoter and promoter proximal region and causes the DNA to loop around on itself Heena Loomba 3 Chapter 15: Control of Gene Expression  Interactions between the coactivator, proteins at the promoter and RNA polymerase stimulate transcription to its maximal rate  Repression of transcription  Repressors oppose the effect of activators by blocking or reducing the rate of transcription  Some repressors bind to the same regulatory sequence to which activators bind (often in the enhancer) to prevent activators from binding to that site  Some repressors bind to their own specific site where the activator binds and interact with the activator so it cannot interact with the coactivator  Some repressors recruit histone deacetylation enzymes that modify histones causing chromatin compaction and making a gene’s promoter inaccessible  Combinatorial gene regulation  Characteristic of any given gene is the number and types of promoter proximal elements  Number and types of regulatory sequences in the enhancer are specific to each gene  Each different regulatory sequence in the promoter proximal region and enhancers binds a specific regulatory protein  Combinatorial gene regulation: combining a few regulatory proteins in particular ways so transcription of many genes can be controlled and many cell types can be specified  A relatively small number of regulatory proteins control transcription of all protein-coding g
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