Name Period The Control of Gene Expression in Prokaryotes Notes

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1 Bacterial DNA contains genes that encode for many different proteins (enzymes) so that many processes have the ability to occur -not all processes are carried out at any one time -what allows expression of some genes but not others? -Genes switch on and off as a response to environmental change (internal or external; usually intracellular) -Bacterial cells have 2 main ways of controlling metabolism 1. Regulation of Enzyme Activity (Enzyme level) -catalytic activity of enzymes increases or decreases in response to chemical cues. -ex: end product of a metabolic (anabolic) pathway may inhibit activity of an enzyme at the beginning of the pathway (feedback inhibition) -useful for a short-term and immediate response -making enzymes all the time is wasteful of an organism s resources 2. Regulation of gene expression (DNA level) -enzyme concentrations may rise and fall in response to cellular changes that switch genes on and off -ex: accumulation of product may trigger inhibition of mrna production by genes that code for an enzyme in pathway (gene repression) -slower to take effect than feedback inhibition, but more economical -prevents unneeded protein synthesis for enzymes, as well as unneeded pathway product Mechanisms for gene regulation were first discovered in E. coli by François Jacob and Jacques Monod -life span on average is 30 minutes -lives in cow gut -no lactose normally -but in calf gut aah milk Operons: the basic concept Sections of DNA that contain genes necessary to synthesize enzymes that direct certain pathways are called operons Components of on operon: Operons (string of genes in prokaryotes that control gene expression) regulated cluster of adjacent structural genes (gene that codes for a polypeptide) with related functions; eukaryotes (much more complex) -common in bacteria and phages -has a single promoter region, like our TATA box 1

2 -usually one per gene -where RNA polymerase binds to begin transcription -because only one, more of an all or nothing response -each structural gene encodes for a certain enzyme in a metabolic pathway Transcription produces a polycistronic mrna (a large mrna that is a transcript of several genes) with coding sequences for all enzymes in a metabolic pathway -is translated into separate polypeptides -contains start and stop codons for each -this grouping of structural genes into operons is beneficial because: -expression of genes can be coordinated -when cell needs the product of a pathway, all necessary enzymes are synthesized at once -entire operon is controlled by a single operator (a DNA segment between an operon s promoter and structural genes) 4 Important Parts of an Operon 1. Regulatory gene codes for the production of a repressor protein or regulator of other genes usually located some distance from the operon they control -involved in switching on or off the transcription of structural genes 2

3 specific protein that binds to an operator and blocks transcription -blocks attachment of RNA polymerase to promoter -has an active site with a specific conformation specific to a certain operator -most repressors have an allosteric site as well as their binding site -when another material binds to the allosteric site, it can inhibit the repressor, interrupting its bond to the operon and releasing it -when a repressor is bound to the operator, no transcription can take place -not all repressors are produced by regulatory genes-some are the end products of a reaction or something from cell s internal or external environment 2. Promoter region where RNA polymerase attaches to begin transcription 3. Operator region that can block the action of RNA polymerase if occupied by a repressor protein 4. Structural genes code for several related enzymes that direct the production of some particular end product Gene expression can be initiated by the operon to produce: 1. Inducible enzymes 2. Repressible enzymes 1. Inducible enzymes ex: lac operon has 3 structural genes: lac Z, lac Y, and lac A -controls the breakdown of lactose Lac Z codes for β-galactosidase which hydrolyzes lactose Lac Y codes for permease, a membrane protein that transports lactose into the cell Lac A codes for transacetylase, which has no known role in lactose metabolism Allolactose, an isomer of lactose, acts as an inducer to turn on the lac operon. -E. coli would rather use glucose; has to be all gone and lactose available to use lactose -regulatory gene produces an active repressor, under normal circumstances, that binds to operator region -operator region is occupied-no binding of RNA polymerase. No transcription, no producing an enzyme that will take part in the metabolic breakdown of lactose -when lactose is available, some of the lactose binds to the repressor -this makes repressor inactive -bond breaks between operator and repressor -at this point RNA polymerase can begin transcription of structural genes making enzymes necessary to carry out process of lactose hydrolysis -Lactose inhibits, binds to repressor inactivates repressor releases operator operon is transcribed enzymes for lactose metabolism are produced. 3

4 These enzymes are called inducible enzymes, because they can only be produced in the presence of a substance that turns on the operon Need to do this only when lactose is present lactose induces production of inducible enzyme 2. Repressible Enzymes -ex: trp operon -controls the metabolic pathway that synthesizes the amino acid tryptophan -regulatory gene produces an inactive repressor that does not bind to operator -therefore, synthesis of tryptophan occurs continuously -amino acid that is needed by the bacterium -RNA polymerase proceeds along structural genes and transcription occurs -When tryptophan is available in the environment, none has to be synthesized -therefore, rising levels of tryptophan cause tryptophan molecules to bind to the allosteric site in the inactive repressor -binding in this case makes the repressor active -trp acts as a corepressor-because both are needed together in order to inhibit (repress) the operon -with the trp molecule, the repressor binds to operator -prevents RNA polymerase from advancing -no transcription of structural genes -no enzymes can be produced that allow the synthesis of trp -enzymes are repressible, because the structural genes stop producing enzymes only in the presence of an active repressor -always need to produce trp until presence represses production of repressible enzymes 4

5 Differences between Repressible and Inducible Enzymes Repressible Inducible -switched on until specific metabolite activates repressor -anabolic pathway (build-up of tryptophan) -pathway product usually switches off its own production by repressing enzyme synthesis -ex: trp operon -switched off until specific metabolite inactivates repressor -catabolic pathway (breakdown of lactose -enzyme synthesis is switched on by a nutrient the pathway uses -ex: lac operon 5