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 Regulation of enzyme activity Regulation of enzyme production Enzyme 1 Regulation of gene expression Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 Gene 2 Gene 1 Gene 3 Gene 4 Gene 5 Tryptophan Precursor Feedback inhibition Give an example of how proteins are regulated. How might genes be 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) trpE trpD trpC trpBtrpA 5’ E D C B A mRNA DNA proteins RNA polymerase

LE 18-21a Promoter DNA trpR Regulatory gene RNA polymerase mRNA 3 5 Protein Inactive repressor Tryptophan absent, repressor inactive, operon on mRNA 5 trpE trpD trpC trpBtrpA Operator Start codon Stop codon trp operon Genes of operon E Polypeptides that make up enzymes for tryptophan synthesis D C B A Trp Operon ON most of the time TrpR gene also ON: makes inactive repressor protein 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 Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor

LE 18-21b_2 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor No RNA made operator TrpR conformational change

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 DNAlacl mRNA 5 3 Lac operon Lactose present, repressor inactive, operon on lacZ lacYlacA RNA polymerase mRNA 5 Protein Allolactose (inducer) Inactive repressor  -Galactosidase Permease Transacetylase 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 DNA lacl Regulatory gene mRNA 5 3 RNA polymerase Protein Active repressor No RNA made lacZ Promoter Operator 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 DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

LE 18-23b DNA lacl CAP-binding site Promoter RNA polymerase can’t bind Operator lacZ Inactive lac repressor Inactive CAP 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 Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor

LE 18-23a DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ 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