EXAM 3 STUDY GUIDE Chapter 16 Control of Gene Expression in Prokaryotes When energetic cost is Low regulatory speed is sLOW When energetic cost is high regulatory speed is fast Levels of gene control include o Alteration of DNA structure o Transcription o mRNA processing o RNA stability o Translation o Posttranslational modification Operon A group of structural genes plus sequences that control transcription o Contains promoter operator and structural genes as a part of a DNA sequence o Regulator proteins encoded from a regulator gene separate from the operon may bind to operator site to regulate mRNA transcription o Resulting proteins following translation catalyze reactions enzymes Binding of repressors inducers o Positive Activator o Negative Repressor o Inducible Gene is usually OFF must be turned on o Repressible Gene is usually ON must be turned off o EXAMPLE Negative inducible operon A repressor is usually bound to the operator preventing transcription of structural genes When an inducer ex Precursor V is present it binds to the regulator so that it cannot attach to the operator This allosteric shape change allows for transcription Negative repressible operon The repressor is inactive and unable to bind to the operator Transcription of structural genes takes place and proteins build up One of these protein products binds to the regulator protein repressor which activates it to bind to the operator This binding to the operator terminates transcription Positive inducible operon The activator is inactive and transcription is off When a substrate makes the activator active it can bind to the promoter and initiate transcription Positive repressible operon The activator is active and transcription occurs A product binds to the activator making it inactive and thus turning off transcription o Corepressor transcription product that activates the regulator protein to allow for negative repressible operon function The lac operon Lactose operon of E Coli o Permease actively transports lactose into the cell o B galactosidase breaks lactose into galactose and glucose B galactosidase ALSO converts lactose into allolactose B galactosidase then converts allolactose into galactose and glucose o Has 3 Structural genes Z gene B galactosidase Y gene permease A gene Transacetylase o In the absence of lactose the repressor is bound to the operator and inhibits transcription It is Normally repressed o In the presence of lactose some of it is converted by B galactosidase to allolactose Allolactose binds to the regulator protein preventing it from binding to the operator Transcription and translation occur Allolactose is the inducer o The repressor binding site overlaps the transcription start site o Contains multiple repressor binding sites that differ in gene expression o Lac Operon Problems B Galactosidase is active when lactose is present but not when lactose is absent because it is responsible for converting lactose to allolactose Constitutive Operator Oc is always on because a repressor cannot bind With no promoter P the inducer cannot bind LacZ is required for B galactosidase LacI is the repressor Partial Diploids o Chromosomal copy of the lac operon o Can have multiple genotypes two copies of genes because there are o Another copy is present that contains some or all of the lac genes on an F plasmid o Cis or Trans actions of genes can be examined in partial diploids Trans LacI is trans dominant and the repressor can bind to BOTH operators to repress transcription in absence of lactose In presence of lactose the repressor is inactivated For the wildtype genotype transcription produces a functional B galactosidase but for the mutant a nonfunctional version is transcribed o Super repressor Always binds to wildtype operator Transcription is prevented in both presence and absence of lactose Will NOT bind to a constitutive operator however Lactose binds to the repressor preventing it from binding to the promoter This will allow o Wildtype lac genes are necessary lac genes will result in non functional lac Second Mechanism of Control Attenuation of Transcription Catabolite Activator for transcription proteins Protein CAP o When Glucose is low cAMP is high cAMP binds CAP and this complex binds to the promoter of DNA This stimulates transcription at high rates and the conversion of glucose o Without CAP transcription rates are low RNA Polymerase doesn t bind to from lactose promoter as well CAP induces a bend in the DNA that promotes transcription o When cAMP is high ATP levels are low and energy levels are low The trp operon Tryptophan biosynthetic enzyme o Negative repression Repressor is normally inactive When there are high levels of tryptophan there is a lot of binding to the repressor so it can finally bind to the promoter and shut off transcription o Alternative secondary structures When there are high levels of tryptophan regions 1 2 fold and bind together and 3 4 also bind together This kink prevents transcription The ribosome does not stall at trp codons when there is already an abundance of tryptophan Low levels of tryptophan there is more time that the strands are dangling thus 2 3 find eachother and transcription can occur This is due to slow translation by ribosome Transcription occurs too quickly o High levels of trp results in attenuated mRNA and not all structural trp genes are encoded o Low levels of trp results in full trp mRNA because transcription is not stopped early and all structural genes are encoded Third Mechanism of Control Control of ompF by anti sense RNA o ompF A non specific transporter that is contained in the outer plasma membrane of a bacteria o LOW extracellular osmolarity OmpF protein is translated o HIGH extracellular osmolarity micF gene is activated which blocks ribosome binding site and ompF is NOT translated o Anti sense MicF binds from ribosome binding site all the way to start codon This prevents translation from ompF mRNA Riboswitch Acts like it is affected by a corepressor changing the secondary structure effectively masking the ribosome binding site Riboswitch is thus turned OFF o Without the regulatory molecule co repressor the riboswitch can assume the secondary structure that allows for ribosome binding and translation o The have BOTH transcriptional and translational control Ribozyme Following transcription the ribozyme degrades mRNA For proper function of lac operon Low glucose high lactose Chapter 17 Control of Gene Expression in Eukaryotes 1