Bacterial Operons

A model of gene expression regulation

Ch 18.4

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

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)

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

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

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.

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.

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