1. Bacterial Operons A model of gene expression regulation Ch 18.4

2. Bacteria Organize their genes into operons Grouping genes with related functions into one transcriptional unit with a single on-off switch to control the group This is efficient because one switch and signal can control the entire group of genes needed to carry out a metabolic pathway

3. General Features of Operons The structural genes are located downstream of the promoter (lacZ in diagram below) Regulatory gene codes for a repressor or an activator (lacI in the diagram below) Operator, the place where the repressor binds, is in between the promoter and the structural genes (see diagram below) When the repressor or activator is active it binds to the DNA regulatory element and regulates transcription of the structural genes (see diagram below)

4. What is polycistronic mRNA? The mRNAs produced by operons are usually polycistronic. This is mRNA that encodes several genes linked together onto one mRNA molecule (note the proteins produced from this mRNA will ultimately be separate). This mechanism increases the regulatory efficiency of bacteria by allowing them to couple transcription and translation of groups of related genes. Polycistronic mRNA

5. Model Operons TRP operon- a repressible operon (tryptophan is the corepressor), negative regulation Lac Operon- Is both negatively regulated as an inducible operon (allolactose is the indicor), and positively regulated by an activator, cAMP. See your Prokaryotic Gene Expression packet or the following slides for diagrams of these operons

6. The TRP Operon is a repressible operon The trp operon encodes enzymes needed to produce tryptophan. Bacteria will not produce tryptophan if it already has some around, and will produce them if tryptophan is low Here is how the trp operon regulates this: The regulatory gene produces an inactive repressor protein. When activated by the presence of tryptophan (the corepressor) the repressor binds to the operator and RNA polymerase is blocked from accessing the structural genes of the trp operon.

7. The Lac operon is both positively and negatively regulated The lac operon structural genes encode enzymes needed to transport lactose into the cytoplasm and break it down. Bacterial cells will only produce these enzymes if both lactose is present and glucose is low. When lactose is present it binds to the active repressor, inactivating it. The inactive repressor can no longer bind to the operator and RNA polymerase is not blocked. The activator CAP binds to cAMP, a molecule that accumulates when glucose is low. CAP binding to the promoter activates gene expression by enabling RNA polymerase to bind.

8. Terms related to operons Repressor Activator Regulatory gene Regulatory element Promoter Operator Repressible operon Corepressor Inducible operon Inducer Positive regulation Activator Polycistronic mRNA