Chapter 17 Gene Control in Bacteria 1 From DNA to RNA to Activation a DNA mRNA protein activated protein gene expression can be controlled at any point here 2 3 Mechanisms for cell to avoid producing unneeded proteins 1 avoid making mRNAs for certain enzymes known as Transcriptional Control occurs when regulatory proteins affect RNA polymerase s ability to bind to promoter 2 Cell might have way to prevent mRNA from being translated if it has been transcribed already known as Translational Control regulatory molecules shorted life span of mRNA so that it gets degraded by ribonucleases faster 3 Some proteins need to be phosphorylated to become active so this can be prevented in what is called Post Translational Control Note All of these occur in bacteria 3 Transcriptional Control a saves most energy for the cell b stops process at earliest possible point c slow reaction 4 Translational Control a allows a cell to make rapid changes in the proteins 5 Post Translational Control a provides most rapid response of all 3 mechanisms b spends a lot of energy 6 Gene expression a genes are not just on or off b they can vary in their level of expression 7 Metabolizing Lactose a Key model of gene regulation b E Coli can use many sugars for ATP production via cellular respiration and fermentation c To use lactose E coli must transport the sugar into the cell once lactose is inside enzyme Beta galactosidase catalyzes a reaction that breaks the sugar down into glucose and galactose Draw out experiment from p 309 d researchers found that lactose regulates gene for Beta galactosidase lactose acts as inducer a substrate in a reaction that stimulates expression of specific genes 8 Beta Galactosidase a produced only when lactose is present and glucose isnt 9 Identifying Genes Under Regulatory Control a To find mutants associated w particular trait there are 2 steps 1 Generate large number of individuals w mutations in random locations UV light X rays mutagens mutagens chemicals that damage DNA and increase mutation rate 2 Screen mutants to find individuals w defects you are looking for 10 Replica Plating to find Mutant Genes 1 when mutants w defects in lactose metabolism are desired mutagenized bacteria are spread on master plate filled w gelatinous agar containing glucose a Bacteria allowed to grow into colony 2 Block w sterilized velvet pressed to master plate a cells transferred to velvet 3 Velvet pressed onto plate containing a medium a medium a liquid or solid that supports growth must differ from master plate by a single component 4 After cells grow compare colonies that thrive on replica plate s medium with those on the master plate 11 Indicator Plates a Where mutants w metabolic deficiencies are observed directly b Compound used by Monod is acted on by Beta galactosidase compound acts as indicator molecule for functioning Beta galactosidase bec one of the molecules produced by this reaction is yellow c colonies w defect in Beta galactosidase enzyme or its production turn white 12 Different Classes of Lactose Metabolism Mutants a The initial mutant screen yielded three types of mutants b In one class the cells cannot cleave indicator molecule even if lactose is present as inducer interpretation No Beta galactosidase gene for it is defenctive c Second class the cells cannot accumulate lactose interpretation no membrane protein to import lactose gene for galactoside permease defective d Third Class the cells cleave indicator molecule w o lactose as inducer interpretation gene for regulatory protein that shuts down lacZ is defective 13 Constitutive Mutants a Abnormal cells that produce a product at all times b Mutated gene that produced constitutive Beta galactosidase was named lacI cells w mutant form of this gene expression occurs w or w o lactose c In these mutants gene remains on when it should be off 14 Putting Observations Together a hypothesized that normal product of lacI gene prevents transcription of lacZ and lacY when lactose is absent Figure 17 4 15 Several Genes Involved in Lactose Metabolism a 3 genes involved in lactose metabolism lacZ lacY and lacI b lacZ and lacY code for proteins req for metabolism and import of lactose lacI responsible for regulatory function c These 3 genes are close together suggests that lacZ and lacY might be controlled by lacI 16 Regulation of Transcription Figure 17 6 a 2 possible ways Negative Control and Positive Control b Negative control occurs when regulatory protein binds to DNA and shuts down transcription c Positive Control occurs when a regulatory protein binds to DNA and triggers transcription d lacZ and lacY under positive and negative control 17 Repressors Figure 17 7 a lacI gene produces an inhibitor exerts neg control over lacZ and lacY genes transcription inhibitor called a repressor binds directly to DNA or near promoter for the genes b Lactose interacts w repressor causes repressor to release from binding site c Repressor is parking brake Lactose releases parking brake 18 Constitutive Mutants a E coli cells w functioning copies of genes for B galactosidase and galactoside permease but lacks functional gene for repressor b Result They make B galactosidase all the time c When the cells are given a functional copy of repressor gene B galactosidase production declines and stops d So repressor codes for protein that shuts down transcription 19 So What a lacI gene codes for a repressor protein that exerts negative control on lacZ and lacY b Lactose is inducer by removing repressor and ending neg control 20 The lac Operon a Operon set of coordinately regulated bacterial genes that are transcribed together into one mRNA b Group of genes involved in lactose metabolism termed lac Operon c lacA gene codes for enzyme transacetylase catalyzes reactions that allow certain types of sugars to be exported from the cell when they are too abundant 21 3 Central Hypotheses to Jacob Monod model of lac operon regulation 1 lacZ lacY lacA are adjacent and transcribed into one mRNA initiated from single promoter of lac operon 2 repressor is a protein encoded by lacI that binds to DNA and prevents transcription of lacZ lacY and lacA Note section of DNA in lac operon where repressor binds is called the operator 3 Lactose binds to repressor repressor changes shape and causes repressor to drop from DNA strand This is another form of allosteric regulation 22 Catabolite Repression a Transcription of lac operon is drastically reduced when glucose is present when glucose
View Full Document
Unlocking...