Gene regulation Two types of genes: 1)Structural genes – encode specific proteins 2)Regulatory genes – control the level of activity of structural genes ie. Control structural gene expression. Gene regulation in prokaryotes http://www.youtube.com/watch?v=oBwtxdI1zvk In prokaryotes, operons control the rate of transcription. An operon is a group of genes that work together and code for the enzymes regulating a particular metabolic pathway. OPERON Regulator gene Produces the repressor Promoter RNA polymerase binding site Operator Repressor binding site Structural gene A Structural gene B Structure of the operon The operon in prokaryotes comprises a number of different features: 1) Structural genes: code for particular enzymes in a metabolic pathway 2) Promoter gene: recognition site for the RNA polymerase to bind to. 3) Operator gene: controls the production of mRNA from structural genes. INDUCTION (initiation) If a substrate is uncommon the bacteria will not need the enzymes most of the time. So the repressor is usually attached. This prevents RNA polymerase from forming mRNA. Therefore: no enzymes. When the substrate molecule is present some of it acts as an inducer; it binds to the repressor, changing its shape so it can’t bind to the DNA. See this movie on the Transcription takes place. Lac operon in E. coli for more detail Example: ON Sugar lactose enters cell, genes coding for enzymes to break sugar down turned on Inducer R R RR P O SG1 SG2 REPRESSION When a substrate is normally present the enzyme should be normally operating. The only time this should stop is when the end product levels build up too much. The repressor cannot bind to the operator. Some of the excess product acts as an effector, which helps the repressor to bind. Transcription is stopped. See this movie on the Tryp operon in E. coli for more detail R RR P O SG1 SG2 http://www.sumanasinc.com/webcontent/anisamples/majorsbiolo gy/lacoperon.html Lac Operon - induction: Lac gene off (normal state) Repressor molecule binds to operator and prevents transcription by RNA polymerase Lactose presentacts as an inducer. Lactose binds to the repressor protein. Repressor can’t bind to the operator. Lactose all used up. RNA polymerase binds . Lac gene on. Structural proteins made. Tryptophan operon - repression Tryp gene on (normal state) Tryptophan accumulates in excess. Some of it acts as an effector and activates the repressor molecule. RNA polymerase binds Tryptophan doesn’t bind to the repressor which then can’t bind to the operator. Tryptophan levels in cells decrease, no excess. Effector and repressor molecule bind to the operator gene and prevent transcription by RNA polymerase. Gene regulation in prokaryotes - summary Genes for a metabolic pathway are linked together in operons with a common switch mechanism (operator). No introns – no RNA processing The structural genes undergo transcription and translation simultaneously. Regulation occurs by switching all genes of a pathway on or off. Gene regulation in eukaryotes Genes for metabolic pathways in eukaryotic cells are separated, not grouped as operons. The genes for a metabolic pathway are switched on separately. Genes have introns that are removed in RNA processing. Eukaryotic genes have a relatively large number of control elements. Regulatory DNA regions Eukaryotic genes have a promoter region upstream of the coding region, where RNA polymerase binds. There are 2 two types of regulatory sequences that effect transcription of the structural gene: 1) enhancer 2) silencer Enhancer sequences These are non-protein-coding sections of DNA that help regulate transcription by binding proteins called transcription factors (turn on). Silencer sequences These are non-protein-coding sections of DNA that help regulate transcription by binding proteins called repressors (decrease). Transcription factors Two types: 1) Activators – these are small proteins that bind to enhancer sequences or RNA polymerase. They cause an increase in transcription. 2) Repressors – these are small proteins that bind to silencer regulatory genes. They cause a decrease in transcription. Transcription factors (activators) that bind to the enhancer sequence RNA polymerase Transcription factors that bind to RNA polymerase Promoter region of DNA Enhancer sequence of DNA Coding region of gene Role of Transcription Factors Eukaryotic RNA polymerase cannot, on its own, initiate transcription. It depends on transcription factors to recognize and bind to the promoter. Transcription factors also bind to the enhancer sequence of DNA RNA polymerase Transcription factors (activators) that bind to the enhancer Transcription factors that bind to RNA polymerase Promoter region of DNA Enhancer sequence of DNA Coding region of gene Activating Transcription Transcription is activated when a hairpin loop in the DNA brings the transcription factors on the enhancer sequence (activators) in contact with the transcription factors bound to the RNA polymerase at the promoter. Protein-protein interactions are crucial to eukaryotic tanscription. The RNA polymerase can only produce a mRNA molecule once the complete initiation complex is assembled. Transcription factors bound to RNA polymerase Activators Enhancer Promoter RNA polymerase Initiation complex Transcription proceeds until a terminator sequence is encountered. Then transcription stops. http://highered.mcgrawhill.com/olc/dl/120080/bio28.swf Regulation Prokaryotes Eukaryotes 1. Genes for a metabolic pathway are linked together in operons with a common switch mechanism (operator). 1. Pathways are separated, not grouped by operons. 2. Genes are switched on separately. 3. Genes have introns that are removed in RNA processing. 4. Transcription and translation do not occur simultaneously. 5. Genes have a large number of control elements. 2. No introns-no RNA processing 3. The structural genes undergo transcription and translation simultaneously. 4. Regulation occurs by switching all genes of a pathway on or off.