Operon: groups of genes that are regulated together. Since a group of genes with a common
function can be transcribed together and a single signal can be used to control whether the
genes are actively transcribed or not.
Operon contains a promoter and an operator.
Operator is a region of DNA within the promoter or between the promoter and the genes to be
transcribed. When inhibitors are bound to the operator, RNA polymerase cannot transcribed the
genes. Operons are found mostly in prokaryotes.
When a repressor is attached to the promoter region of DNA, the structural genes needed to
synthesize lactose-digesting enzymes are inactive, preventing transcription. When the repressor
protein is bound to the DNA, RNA polymerase cannot bind to the DNA. So we need an inducible
operon to remove the repressor.
Inducible operon = lac operon in E.coli
The lac operon is an inducible operon that uses both negative and positive control. It is
inducible because the structural genes are normally inactive but the presence of lactose induces
them to become active.
- the repressor acts as a negative control mechanism  active repressor prevents transcription
- The activated CAP protein acts as a positive control mechanism  activated CAP promotes
transcription
A repressible operon is the opposite of inducible operon. Transcription occurs continuously and
the repressor protein (normally inactive) must be activated to stop transcription.
The trp operon is a repressible operon that uses negative control. It is repressible because the
structural genes are normally active but are repressed when tryptophan is present. It is negative
control because the presence of an active repressor prevents transcription.
Inducible operons  active  inhibits transcription
Repressible operons  active when tryptophan is present  inhibit transcription
Positive control mechanism requires the presence of an activator protein before RNA polymerase
will bind. The activator protein itself must be activated by binding to an inducer molecule. Once
activated, the activator binds to DNA and enables the binding of RNA polymerase.