Controlling gene expression

1. Controlling gene expression SBI 3U November 22nd, 2012

2. Prokaryotic gene control mechanisms Gene regulation regulated in response to 2 molecules: -lactose -tryptophan

3. The LAC OPERON • Lactose: sugar, potential source of energy for prokaryotes • Lac operon: a cluster of genes that contains the DNA sequences to regulate the metabolism of lactose

4. The lacoperon • Consists of a promoter and operator • Promoter: the site where DNA transcription begins • Operator: sequence of bases that control transcription • Repressor protein: binds to operator to repress gene transcription

5. The lacoperon • Absence of lactose: lac repressor protein is active and bound to the operator • Presence of lactose: some RNA polymerase binds to lac repressor protein, rendering it inactive • The rest of the RNA polymerase binds to the promoter region, and transcription of the lac genes begins

6. The lacoperon • As concentration of lactose decreases, the amount of inactivated lac repressor decreases  leading to the reactivation of the lac repressor • Lactose is a signal molecule  inducer • Inducer  signal molecule that triggers the expression of an operon’s genes

8. The trpoperon • Tryptophan: amino acid, use to build proteins • Trpoperon: regulates production of tryptophan • Trp repressor protein  activated by presence of tryptophan

9. The trpoperon • Inactive when trp is not present • RNA polymerase binds to the promoter region, transcription of genes that break down trp can begin • When tryptophan is available in the environment, the cell can use available tryptophan

10. The trpoperon • Tryptophan acts as a signal molecule and activates the repressor protein – corepressor • Corepressor: a signal molecule that binds to a regulatory protein to reduce the expression of an operon’s genes