Operons

Big picture Prokaryotic control of genome expression Prokaryotic control of genome expression 2 levels of control 2 levels of control  Change in # of enzyme molecules produced  Change in activity of enzymes present  Ex: feedback inhibition Genes turned on/off based on metabolic needs: operons Genes turned on/off based on metabolic needs: operons  Operator + promoter + group of genes  Often include several enzymes w/ in a pathway

* Regulator gene - a gene that codes for a repressor protein molecule. The repressor molecule binds to the operator and prevents RNA polymerase from binding to the promoter. * Regulator gene - a gene that codes for a repressor protein molecule. The repressor molecule binds to the operator and prevents RNA polymerase from binding to the promoter. * Promoter - a short sequence of DNA where RNA polymerase first attaches when a gene is to be transcribed. * Promoter - a short sequence of DNA where RNA polymerase first attaches when a gene is to be transcribed. * Operator - a short sequence of DNA where the repressor binds, preventing RNA polymerase from attaching to the promoter. * Operator - a short sequence of DNA where the repressor binds, preventing RNA polymerase from attaching to the promoter. * In addition, the part of chromosome (in eukaryotic cells) containing the gene must first decompact to form a "chromosome puffs" before the beginning of transcription. * In addition, the part of chromosome (in eukaryotic cells) containing the gene must first decompact to form a "chromosome puffs" before the beginning of transcription.

2 Types of operons Inducible: Inducible:  Default = off  Inducer turns transciption on  ex: an enzymatic pathway that allows the cell to break down lactose Repressible: Repressible:  Default = on  Corepressor turns transcription off  Ex: an enzymatic pathway that allows the cell to synthesize tryptophan

The Lac Operon Bacteria can use lactose if lactose is present and if there’s not enough glucose Bacteria can use lactose if lactose is present and if there’s not enough glucose B-galactosidase breaks down lactose B-galactosidase breaks down lactose Lactose present = transcription is on Lactose present = transcription is on  Allolactose (inducer) inactivates the repressor  Inactive repressor falls off of the operator  RNA polymerase binds

LAC OPERON

%7Esmalley/LacOperon.mov

The trp operon Cell can get tryptophan from environment or by synthesizing it Cell can get tryptophan from environment or by synthesizing it  Will only synthesize tryptophan if there isn’t any in the environment Tryptohan in the environment = no transcription Tryptohan in the environment = no transcription  Tryptophan activates repressor  Tryptophan is a corepressor  Repressor is from regulatory gene (trpR)  Repressor binds to operator  Blocks RNA polymerase

Positive gene regulation Ex: lac operon transcription also depends on [glucose] Ex: lac operon transcription also depends on [glucose] CRP: activates transcription (positive regulation) CRP: activates transcription (positive regulation)  glucose   cAMP  glucose   cAMP cAMP activates CRP cAMP activates CRP

Explain the events that would occur if... A cell was in an environment with plenty of glucose but no lactose, and lactose was added A cell was in an environment with plenty of glucose but no lactose, and lactose was added A cell was in an environment with tryptophan, then the tryptophan disappeared A cell was in an environment with tryptophan, then the tryptophan disappeared A cell was in an environment with lactose and plenty of glucose, then the glucose disappeared A cell was in an environment with lactose and plenty of glucose, then the glucose disappeared