Chapter 16 Regulation of Gene Expression Summary Page 1 All genes are not turned on all the time Most DNA in the body have the same genes Prokaryotes regulate gene expression in response to environmental conditions Lactose Metabolism is E Coli is Regulated by an Inducible System o Adaptive enzymes are when bacteria can adapt to their environment producing certain enzymes only when specific chemical substrates are present o Constitutive Enzymes are enzymes that are produced continuously regardless of the chemical makeup of the environment o Inducible refers to the fact that the enzyme can be induced when certain environmental stimuli is present the enzyme will turn on gene expression regulated genes o Repressible refers to when a gene is always turned on the presence of a specific molecule o Negative control gene expression occurs unless it is shut off by some form of a regulator o Positive control transcription occurs only if a regulator molecule directly stimulates RNA inhibits gene expression molecule production o Cis acting site when the regulatory region is on the same strand as certain genes bind molecules that control transcription of the gene cluster o Trans acting element control transcription of the gene cluster determining whether the genes are transcribed into mRNA and thus whether the corresponding enzymes or other protein products may be synthesized from the genetic information in the mRNA Binding of a trans acting element at a cis acting site can regulate the gene cluster either negatively turning off transcription or positively turning on transcription o Lac operon constructed of three structural genes and an adjacent regulatory site the entire gene cluster functions in an integrated fashion to provide a rapid response to the presence or absence of lactose o Structural Genes genes that code for the primary structure of an enzyme genes used to make enzymes three in the lac operon o LacZ specifies for Beta Galactosidase an enzyme whose primary role is to convert the disaccharide lactose into the monosaccharaides glucose and galactose This is essential if lactose is to serve as the primary energy source in glycolysis o LacY specifies the primary structure of permease an enzyme that facilitates the entry of lactose into the bacterial cell o LacA codes for the enzyme transaceteylase which may be involved in the removal of a toxic by products of lactose digestion from the cell o Mutant cells that fail to produce lacZ or lacY are unable to use lactose as an energy source o These operons are closely linked on the chromosome Z Y A All three genes are transcribed as a single unit resulting in polycistronic mRNA resulting in the coordinated regulation of all three genes o Gratuitous inducers are chemical analogs of lactose Isopropylthiogalactoside IPTG behave like natural inducers but they do no serve as substrates for the enzymes that are subsequently synthesized provided strong evidence that the primary induction event does not depend on the interaction between the inducer and the enzyme o Constitutive mutations the lac operon is always on enzymes are produced regardless of the presence or absence of lactose Chapter 16 Regulation of Gene Expression Summary Page 2 Lac I mapped the mutation to a site on the bacterial chromosome close to but distinct fro the structural genes lacZ lacY and lacA Also called the repressor gene Lac OC produced effects identical of Lac I Also called the operator region Both types of constitutive mutants the enzymes are produced continuously inducibility is eliminated and gene regulation is lost o The Operon Model hypothetical mechanism involving negative control a group of genes is regulated and expressed together in a unit o Argues that the LacI gene regulates the transcription of the structural genes by producing a repressor molecule The repressor is allosteric the molecule reversibly interacts with another molecule undergoing both a conformational change in three dimensional shape and a change in chemical activity o Proposes that the wild type when no lactose is present repressor gene binds to the operator blocking transcription o When lactose is present the repressor gene is induced and doesn t block transcription lacOc o Is repressor is mutated it doesn t block transcription and it will run continuously lacI o Merozygote a cell that is diploid for a certain added gene but not for the rest of the chromosome made it possible to introduce an I gene into a host cell whose genotype is I which would restore inducibility o Trans Regulatory Element DNA sequence to which it binds to a cis regulatory element Diffusible trans protein I movable Fixed Cis next to not movable o Equilibrium Analysis extracts of Iq cells were placed in dialysis bag and allowed to attain equilibrium with an external solution of radioactive IPTG can diffuse freely in and out of bag at equilibrium the concentration of radioactive IPTG was higher in the bag then in the external solution showed that the IPTG binding or repressor protein only binds to DNA containing the lac region and does not bind to lac DNA containing and operator constitutive Oc mutation The Catabolite Activating Protein CAP Exerts positive control over the lac operon o CAP efficiently represses the expression of the lac operon when glucose is present The process of this inhibition is called catabolite repression o Cyclic Adenosine Monophosphate cAMP CAP binding is dependent on cAMP in order to bind the promoter CAP must be bound to cAMP the level of cAMP itself depends on an enzyme adenyl cyclase it catalyzes the conversion of ATP to cAMP o Glucose inhibits the activity of adenyl cyclase cause a decline in the level of cAMP in the cell in this condition CAP cannot for the CAP cAMP complex essential to the positive control of transcription o Cooperative binding occurs when cAMP CAP and RNA polymerase are in the presence of the lac promoter DNA and a tightly bound complex is formed o Repression loop occurs when binding of the repressor simultaneously at two operator sites distorts the conformation of DNA causing it to bend away from the repressor When the RNA polymerase is on the inside of the loop it prevents access by the polymerase Chapter 16 Regulation of Gene Expression Summary Page 3 It also positions the CAP binding site in a way that facilitates CAP interaction with RNA polymerase upon subsequent induction Tryptophan Operon is a Repressible gene o Tryptophan synthetase the enzyme that reacts to the