Ch 18 Gene Regulation Consider: A multicellular organism (Pliny) Do each of his cells have the same genes? Yes, with an exception: germ cells are haploid Do each of his cells express the same genes? Examples to support your claim Do cells express the same genes at the same time and at the same level? Examples How are Genes Regulated? LE 18-20 How might genes be regulated? Regulation of enzyme activity Precursor Regulation of enzyme production Feedback inhibition Enzyme 1 Gene 1 Enzyme 2 Gene 2 Regulation of gene expression Enzyme 3 Gene 3 Enzyme 4 Gene 4 Enzyme 5 Gene 5 Tryptophan Give an example of how proteins are regulated. Bacterial Operons: Gene Regulation Model Genes grouped into operons - Promoter to help initiate transcription - Operator: DNA sequence acts as on-off switch - Genes encode metabolic enzymes Operon regulated by repressors and/or activators in response to environment. Trp Operon Group of genes that encode enzymes for tryptophan synthesis (an amino acid) RNA polymerase DNA mRNA proteins trpE trpD trpC trpB trpA E D C B A 5’ LE 18-21a Trp Operon ON most of the time TrpR gene also ON: makes inactive repressor protein trp operon Promoter Promoter Genes of operon DNA Regulatory gene mRNA trpE trpR 3 trpC trpB trpA C B A Operator Start codon Stop codon RNA polymerase mRNA 5 5 E Protein trpD Inactive repressor D Polypeptides that make up enzymes for tryptophan synthesis Tryptophan absent, repressor inactive, operon on If the cell is not synthesizing much protein (e.g. low nutrients), will it need to continue to make trp? How to shut off the trp operon? LE 18-21b_1 DNA mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off LE 18-21b_2 operator TrpR DNA No RNA made mRNA conformational Protein change Tryptophan (corepressor) Tryptophan present, repressor active, operon off Active repressor Is the trp operon repressible or inducible? ON unless excess trp binds and activates repressor protein-> Active TrpR binds operator Blocks transcription Trp operon OFF Do inducible operons exist? (usually OFF; need signal to turn ON Lac operon: group of genes involved in catabolism of lactose LE 18-22b Lac operon DNA lacZ lacl 3 mRNA 5 lacA Permease Transacetylase RNA polymerase mRNA 5 -Galactosidase Protein Allolactose (inducer) lacY Inactive repressor Lactose present, repressor inactive, operon on Enzymes facilitate lactose import and breakdown for cellular energy Let’s assume bacteria prefer glucose to lactose as a carbon source. If glucose is available in the surroundings, does it make sense for the lac operon to be ON? If it’s a waste of energy then how do bacteria repress (turn OFF) the Lac operon? LE 18-22a Promoter Regulatory gene Operator lacl DNA lacZ No RNA made 3 mRNA 5 Protein RNA polymerase Active repressor Lactose absent, repressor active, operon off • Inducible gene products – usually function in catabolic pathways (lactose metabolism) • Repressible gene products -products usually function in anabolic pathways (trp synthesis) Trp and lac operons (similarities): - Negatively controlled - Blocked by a repressor Positive Gene Regulation • Activator protein turns on Lac operon – catabolite activator protein (CAP) Glucose high cAMP low Glucose low cAMP high CAP-cAMP binds Lac promoter and induces transcription When would this occur, when glucose is high or low? Low LE 18-23a Promoter DNA lacl lacZ CAP-binding site Active CAP cAMP Inactive CAP RNA Operator polymerase can bind and transcribe Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized LE 18-23b Promoter DNA lacl CAP-binding site Inactive CAP lacZ Operator RNA polymerase can’t bind Inactive lac repressor Lactose present, glucose present (cAMP level low): little lac mRNA synthesized Oh gee, am I supposed to induce or repress? Do you have questions too? LE 18-21b_1 DNA mRNA Active repressor Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off LE 18-23a Promoter DNA lacl lacZ CAP-binding site Active CAP cAMP Inactive CAP RNA Operator polymerase can bind and transcribe Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized A mutation arises in the TrpR gene that inactivates the binding site for the co-repressor. How will the mutant phenotype differ from wildtype? If the operator is deleted from the lac operon predict how gene expression will be altered relative to wildtype under the following conditions a. lactose present, glucose absent b. lactose absent, glucose present c. lactose present, glucose present d. lactose absent, glucose absent