2. INTRODUCTION
• Operon is defined as a group of structural genes that are transcribed together at the
same time.
• They are found only in prokaryotes.
• Function : Regulate the expression of structural genes by controlling transcription.
• Operon Structure :
Structural genes – one end of operon has set of structural genes namely gene a,
gene b and gene c transcribed into single MiRNA and translated to produce enzyme
a, b and c.
Promoter – lies upstream of first structural gene and RNA polymerase binds to it
and moves downward transcribing the structural genes.
3. Operator : Regulatory protein binds to a region of operon called operator and
controls transcription.
Regulatory gene : controls transcription of structural genes having its own promoter
and transcribed into short RNA and translated into small protein called regulatory
protein.
TYPES OF TRANSCRIPTIONAL CONTROL :
Positive Control : regulatory protein act as activator stimulating transcription.
Negative Control : regulatory protein act as repressor inhibiting transcription.
4. TYPES OF OPERON
Inducible Operons : transcription is normally off and some factor has to induce
transcription.
eg : Lac Operon
Repressible Operon : transcription is normally on and some factor has to inhibit
transcription and turn it off.
eg : Trp Operon
5. LAC OPERON
• It is a negative inducible repressor .
• Lactose operon is designated as Lac Operon.
• This operon codes for enzymes involved in the catabolism of lactose .
• In 1961, Francois Jacob and Jacques Monod described the lac operon model in E.
coli.
6. LACTOSE METABOLISM
Lactose is a major carbohydrate present in milk. E.coli living in the mammalian gut
and can metabolism lactose where it is broken down into glucose and galactose.
Lactose does not easily diffuse across the E. coli cell membrane and must be
actively transported into the cell by the protein permease
To utilize lactose as an energy source, E. coli must first break it into glucose and
galactose, a reaction catalyzed by the enzyme β-galactosidase.
This enzyme can also convert lactose into allolactose , a compound that plays an
important role in regulating lactose metabolism.
A third enzyme, thiogalactoside transacetylase , also is produced by the lac operon,
but its function in lactose metabolism is not yet known.
7.
8. LAC OPERON MODEL
It consists of :
• Structural genes –
a. lac Z codes for enzyme β-galactosidase – breaks down lactose into glucose and
galactose.
b. lac Y codes for galactose permease – actively transports lactose into E.coli cells.
c. lac A codes for thiogalactosidase transacetylase
• Operator ( lac O) – binding site for repressor.
• Promoter ( lac P ) – binding site for RNA polymerase.
• Repressor – gene coding for lac repressor protein that binds to operator and prevents RNA
polymerase from binding to promoter.
• Inhibitor ( lac I ) – controls production of repressor protein.
9. FUNCTIONING OF LAC OPERON
• In the absence of lactose (inducer) , the regulatory gene produce repressor protein
that binds to operator and prevents transcription.
10. • When lactose is introduced into the medium , some of it gets converted into
allolactose and binds to repressor preventing it from binding to operator.
• The repressor has two binding sites – one for allolactose and the other for DNA.
• Therefore the operator is set free and it induces the RNA polymerase to bind to the
initiation site of promoter.
• This stimulates the transcription and produce a polycistronic mRNA
• This miRNA codes for three enzymes that catabolize lactose .
