CHAPTER 18 Control of Gene Expression in Bacteria 3 21 2014 gene expression process of converting information that is archived in DNA into molecules that actually do things in the cell occurs when protein a gene product is synthesized and active o o most triggered by environment o o can be controlled at DNA mRNA protein and activated protein levels level of gene expression varies between on and off ability to regulate this allows cells to respond to environmental changes o o o o o o o forms of control that occur in bacteria affecting how much active protein is produced from a gene transcriptional control regulatory proteins affect RNA polymerase s ability to bind to a promoter and initiate transcription cell avoids making mRNAs for particular enzyme slow but most efficient saves most energy for cell bc stops processes of gene expression at earliest point translational control regulatory molecules alter lifespan of mRNA or affect translation initiation or elongation cell can prevent mRNA from being translated into a protein if the mRNA for an enzyme has been made allows a cell to make rapid changes in amounts of different proteins bc mRNA is present and ready to be translated post translational control regulating the addition of a phosphate group various chemical modifications to a protein provides fastest response bc only one step is needed to activate existing protein uses most energy trade off btwn speed of response and conservation of ATP amino acids and other resources constitutive genes being translated all the time to use lactose E coli must first transport the sugar into cell once lactose is inside cell enzyme beta galactosidase catalyzes rxn that breaks down disaccharide into glucose and galactose lactose acts as an inducer significant amounts of beta galactosidase produced only when lactose is present and glucose is not glucose prevents expression of gene for beta galactosidase inducer small molecule that triggers transcription of a specific gene 2 step process for finding mutants associated with a particular trait o generation of a large number of individuals w mutations at random locations in genomes expose populations to mutagens mutagens are any physical or chemical agent that increases the rate of mutation screen treated individuals for mutants with defects in the process or biochemical pathway in question genetic screen technique for selecting individuals w certain types of mutations out of a large population replica plating method of identifying bacterial colonies that have certain mutations by transferring cells from each colony on a master plate to a second replica plate and observing their growth when exposed to different conditions o lactose metabolism mutants yielded 3 mutant types 3 21 2014 Jacob Monod identified 3 genes involved in lactose metabolism lacZ lacY and lacI lacZ lacY code for proteins required for metabolism and import of lactose lacI responsible for regulatory function no lactose lacI shuts down expression of lacZ lacy lacI gene produces repressor protein that exerts negative control over lacZ and lacY gene transcription in lacI mutants parking brake broken lactose present transcription of lacZ lacY induced genes are close together suggests that lacZ and lacY might be transcribed together all 3 genes are under negative control operon set of coordinately regulated bacterial genes that are transcribed together into one mRNA an effective way to express sets of genes together limits to operon size negative control regulatory protein repressor binds to DNA to shut down transcription repressor acts as the parking brake in the lactose experiment lactose releases the brake by acting as an inducer and removing the repressor lac operon regulation group of genes involved in lactose metabolism hypotheses lacZ lacY lacA genes are adjacent and transcribed into 1 mRNA initiated from 1 promoter of lac operon cotranscription results in coordinated expression of the 3 genes repressor is a protein encoded by lacI that binds to DNA and prevents transcription of the lac operon genes o o o o o o o o 3 21 2014 operator section of DNA in the lac operon that the repressor binds to lacI expressed constitutively inducer lactose binds to repressor changing the shape of the repressor to make the repressor and DNA break apart allosteric regulation small molecule binds to protein to change shape activity repressor can t bind to DNA transcription can proceed lac operon is important because model introduced idea that bacterial genes and operons are under negative control lac operons showed that gene expression is regulated by physical contract btwn regulatory proteins and specific regulatory sites in DNA experiment an example of post translational control over gene expression positive control regulatory protein activator binds to DNA and triggers transcription ara operon regulation group of genes that allow E coli to use sugar arabinose o transcription of ara operon turned on by activator protein AraC AraC protein allosterically regulated by arabinose when bound to arabinose two copies of AraC protein attach to regulatory sequence of DNA called ara initiator that lies just upstream of promoter when AraC is bound to DNA it can bind to RNA polymerase interaction helps attach polymerase to promoter and accelerate initiation of transcription when no arabinose 2 copies of AraC protein remain together one binds to initiator other binds AraC is an activator and repressor to ara operator o o AraC is repressor that prevents transcription of ara operon and araC gene ara operon and adjacent gene araC code for araC activator global gene regulation the coordinated regulation of many genes o o regulon set of separate genes of operons that contain the same regulatory sequences and that are controlled by a single type of regulator protein allow bacteria to respond to environmental changes every gene in the regulon is transcribed when environmental change triggers removal of repressor protein uninduced state induced state may be under negative control by repressor protein or positive control by activator protein interactions among protein regulators and DNA sequences the bind produce finely tuned control over gene expression and regulate individual genes operons or large sets of genes 3 21 2014
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